Science in Clothing Comfort (Woodhead Publishing India)

Science in Clothing Comfort Science in Clothing Comfort Apurba Das and R. Alagirusamy WOODHEAD PUBLISHING INDIA PVT...

Author: Apurba Das | R Alagirusamy

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Science in Clothing Comfort

Science in Clothing Comfort

Apurba Das and R. Alagirusamy

WOODHEAD PUBLISHING INDIA PVT LTD New Delhi

●

Cambridge

●

Oxford

Published by Woodhead Publishing India Pvt. Ltd. Woodhead Publishing India Pvt. Ltd., G-2, Vardaan House, 7/28, Ansari Road Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington Cambridge CB21 6AH, UK www.woodheadpublishing.com First published 2010, Woodhead Publishing India Pvt. Ltd. © Woodhead Publishing India Pvt. Ltd., 2010 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing India Pvt. Ltd. The consent of Woodhead Publishing India Pvt. Ltd. does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing India Pvt. Ltd. for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. Woodhead Publishing India Pvt. Ltd. ISBN 13: 978-81-908001-5-0 Woodhead Publishing India Pvt. Ltd. EAN: 9788190800150 Woodhead Publishing Ltd. ISBN 10: 1-84569-789-8 Woodhead Publishing Ltd. ISBN 13: 978-1-84569-789-1 Typeset by Sunshine Graphics, New Delhi Printed and bound by Replika Press, New Delhi

Contents

Preface Acknowledgements

vii ix

1

Introduction to clothing comfort

1

1.1 1.2 1.3 1.4 1.5

Need and selection of clothing Components of clothing comfort Clothing comfort and wearer’s attitude Human–clothing interactions Understanding clothing comfort

1 4 5 7 10

2

Psychology and comfort

13

2.1 2.2 2.3 2.4

Psycho-physiological factors of clothing comfort Psychophysics and clothing comfort Wear trial techniques Psychological aspects of aesthetic comfort

13 16 21 23

3

Neurophysiological processes in clothing comfort 31

3.1 3.2 3.3 3.4

Neurophysiological perceptions Mechanical and thermal receptors Sensory perceptions of human body Physiological requirements of the human body

31 36 45 48

4

Tactile aspects of clothing comfort

54

4.1 4.2 4.3 4.4

Tactile comfort sensations Fabric handle attributes for expressing tactile comfort Assessment of fabric handle characteristics Fabric parameters affecting tactile sensation

54 58 59 73

5

Thermal transmission

79

5.1 5.2 5.3

Introduction Thermo-regulation in human body Thermal distress

79 79 81

vi

Contents

5.4 5.5 5.6 5.7 5.8 5.9

Thermoregulation through clothing system Thermal comfort of clothing Transient heat flow and warm–cool touch of fabrics Measurement of thermal transmission characteristics Parameters for expressing thermal characteristics Thermal transmission characteristics of fabrics

6

Moisture transmission

6.1 6.2 6.3 6.4 6.5 6.6

Introduction Liquid water transfer: wicking and water absorption Principles of moisture vapour transfer Condensation of moisture vapour Evaluation of moisture vapour transmission Moisture sensation in clothing

7

Dynamic heat and mass transmission

7.1 7.2 7.3 7.4 7.5

Introduction Combined heat and moisture interactions with textile materials Factors affecting heat and mass transfer through fabrics Evaluation of heat and mass transmission Parameters expressing heat and mass transmission

8.

Garment fit and comfort

8.1 8.2 8.3 8.4 8.5

Introduction Body dimensions and pattern Garment fit and comfort relationship Factors related to garment fit Measurement of garment fit

Index

82 86 91 94 99 100

106 106 107 118 123 124 129

136 136 137 144 148 154

159 159 160 161 165 169 173

vii

Preface

There are many textbooks describing different aspects of clothing technology and sciences of human comfort. However, there are not many books giving a survey of the sciences of comfort and clothing technology in one volume. In this respect, this book would fulfill the need of undergraduate and postgraduate students who are studying various aspects of textiles and clothing and also the researchers who are working in the area of clothing comfort. The undergraduate and post graduate students of textiles, clothing and fashion or home science generally study about the basics of fibres, yarn formation, fabric formation, apparel production and their evaluation techniques. But the engineering of right type of garment/ clothing for any specific application is possible only when we understand the interrelationship between the clothing requirements and human comfort. The text in this book describes the aspects of science in perceiving the comfort by the human being and the science and technology of clothing that deals with the comfort. The first chapter consists of the details of criteria for the selection of clothing, components of clothing comfort, human clothing interaction and scientific understanding of clothing comfort. The second chapter provides the information on the perception of clothing comfort by human sensory system and psycho-physics involved in the perception and assessment of comfort, psychological aspects of clothing comfort, etc. In third chapter, the sensory systems of human, interpretation of the signals by nerve and brain for each sensation related to comfort and details of mechanical and thermal receptors present in the skin are described. The fourth chapter consists of the detailed survey of aspects of tactile comfort, fabric parameters that affect the tactile sensation and fabric handle and evaluation. In fifth chapter, the thermo-regulation system in human body and through clothing system, effect of thermal distress, transient heat flow for warm–cool sensation and evaluation of fabrics for thermal characteristics are detailed. The sixth chapter describes the liquid water transfer through fabrics, evaluation of liquid water transfer, principles of moisture vapour transfer and its evaluation by various methods. The seventh chapter consists of the survey of the combined heat and mass transfer through textile materials and its evaluation. The last

viii

Preface

chapter deals with the sciences of comfort with the size and fitness of garment. In this book, not only the scientific and technical information, but also other related basic information at each level is given to understand the concepts clearly. References to original sources have also been given to follow the literature that will be useful for the readers. The undergraduate, postgraduate and research students from textile engineering, fashion technology, clothing and apparel technology, home science will be benefited from this text book. This book also provides the guidelines such as comfort level of the person at various activity levels and at various climatic conditions, etc. which are needed to produce the functional garments for various applications. So, this book would be also useful for the industries which are involved in the production of functional garments. The authors are indebted to QIP/CEP of IIT Delhi and faculty board of the department of textile technology, IIT Delhi for giving necessary permission and providing financial support. The authors are thankful to their colleagues in the department for their support. They are also thankful to their students for all the help during literature search, writing and editing of the book. The authors would also like to express their appreciation to Woodhead Publishing India Ltd. for editing and publishing of this book. It is hoped that the students, teachers and researchers will be able to get the idea of science behind the clothing comfort with the help of this book. There may be some shortcomings in the book and the authors welcome the comments from readers and these constructive comments will be useful in bringing out the second edition of the book. Dr. Apurba Das Dr. R. Alagirusamy

Acknowledgements

ix

Acknowledgements

This book is the effort of many people in addition to the authors. We would like to take this opportunity to thank all the individuals. We are thankful to our family members for their faithful support in writing this book. We received great inspiration and constant encouragement from all the faculty members of our department and are highly acknowledged. We have been encouraged in this venture by many people and would like to acknowledge the considerable support we have received. We would like to thank Woodhead Publishing India Ltd. for recognizing the need for an up-to-date account of sciences of comfort in clothing, seeing it as an exciting field that has much to offer to the students and researchers. We would also like to thank the quality improvement program (QIP) of our institute for support. We are also indebted to our students for their excellent coordination, organizational skills and effective liaison during our manuscript preparation. We are grateful to many of our teachers and mentors for their education and endorsement. Dr. Apurba Das Dr. R. Alagirusamy

1 Introduction to clothing comfort

1.1

Need and selection of clothing

The basic needs of human are food, clothing and shelter. After fulfilling the first need of food, a person looks for the second important need, i.e. clothing. In the present day society, we expect much more from clothing than to satisfy our basic need. In most societies the clothing is for the purpose of expressing wealth, status, occupation, age, occasion, gender, etc [1]. There are various factors which influence the selection of clothing type. Figure 1.1 illustrates the important factors which influence the selection of clothing. It is evident from Fig. 1.1 that the factors which influence the selection of clothing can be divided broadly into four major groups, i.e. social factor, economic factor, environmental factor and physical factor. All these factors play significant roles in selection of clothing of a person. The social factors include the place where a person lives (urban or rural area), cultural background of person, gender, occupation, occasion, social status, etc. Depending on the place where a person lives, the clothing pattern changes. In urban area, due to close cultural interactions between the various sections of people, the clothing pattern becomes more cosmopolitan in nature. But on the other hand the rural clothing is more influenced by the regional factors. Similarly, clothing is also influenced by cultural background and upbringing of a person. The upbringing influences the taste of a person toward the clothing significantly. The modern society does not believe in gender biasness and strongly oppose this. But, are we ready to accept this to be applied while selecting clothing? Except few exceptions, we are still comfortable in maintaining differences in male and female clothing. In some cases a person selects his clothing depending on the occupational requirement. For example, one can easily make out the difference between a police and a common man depending on his clothing, or in a hospital a nurse can be easily identified based on her clothing. We generally prefer to wear different clothing depending on the occasion, namely formal wear, casual wear, etc. A person generally prefers

1

2

Science in clothing comfort Factors for clothing selection

Social factors

Socioeconomic condition

Economic factors

Environmental factors

Economic status of an individual

Availability of technology / raw materials

Climatic condition

Age of a person

Rural / Urban

1.1

Cultural background

Gender

Physiological factors

Protection from extreme conditions

Health condition of a person

Occupation

Physical structure of body

Occasion

Unusual places (deep sea, space, etc.)

Thermophysiological responses of body

Activity level

Social status

Factors affecting the clothing selection.

to wear formal clothing in office, but the same person prefers casual wear in leisure trip. It is also very common that a person tries to show his social status through clothing, this trend prevails in every society since the beginning of the civilization. The kings always tried to differentiate themselves from the common man by wearing royal clothing. Among the economic factors the important components are economic condition of society, economic status of individual and availability of technology or raw material. When the economic condition of society changes that also reflects through clothing. It is well-known fact that the general clothing pattern of rich and poor sectors of society differs and it is obvious. This is also true for individual. Each individual selects clothing


3

depending on the affordability. Person also selects clothing to show his economic status. The availability of a particular type of clothing depends mainly on the availability of technology and raw materials. These two factors are directly or indirectly dependent on the economic situation and affordability of the society. The environmental factors include climatic conditions (too cold, too hot, raining, chilled wind, etc.), protection from extreme environment, unusual places (space or under water), etc. Depending on the environmental conditions the clothing need changes. Here, the performance factors are the dominating parameters. One requires different clothing for different climatic conditions. A person, going to extreme cold place, will definitely like to protect himself from extreme cold by wearing extreme cold protecting clothing. But, the same person will not use the same clothing in normal environment. Depending on the climatic temperature the garments are broadly divided into two categories, namely winter wear and summer wear. Similarly, in rainy days we require clothing which is waterproof. Clothing pattern also changes depending on the environmental threat, like explosives, poisons, biological attacks, fire, radioactive or ultraviolet rays, etc. Clothing also has to withstand falling and flying objects in certain circumstances. Depending on the needs of unusual places, like deep under sea, space etc., the type of clothing changes. In these places the special type clothing are required for protection and specific performance. The last and very important factor is physical conditions of a person, which include age, condition of health of person, body structure, physiological response of body, activity level, etc. The clothing pattern changes with the age of person due to the psychological and physiological changes with time. A child needs different type of clothing than an aged person. Similarly the clothing need also changes with the physical health of a person. Someone with specific problem with a particular fibre, like allergy, irritation, would like to avoid wearing that particular clothing made with these fibres. Clothing selection also depends on the physical built of body, i.e. whether fat or thin, tall or short, etc. Person with special physical need may require specific clothing. Physiological response of body varies widely from person to person and so does the clothing need. In a given environmental condition a particular person may feel more cold or heat or sweat than others. This is due to the fact that the thermo-physiological responses are different for different persons. The selection of clothing also depends on the level of activity of a person. Under heavy activity the human body generates more heat and sweat. The clothing, he wears, should be able to dissipate and transmit the heat and sweat quickly to keep the body heat under control. A sports person needs special sportswear depending on the type of sports or a worker needs specific work wear depending on his activity. People in challenging activities and sports could

4

Science in clothing comfort

use smart clothing, that is, clothing that can sense the wearer’s condition or situation and, in turn, modify its own structure to protect him or her, for example to keep the body warm or cool. A very well-known proverb says that “There is no such thing as bad weather, only bad clothing”. Textiles always have played important roles in well-being of a human being by protecting it from different adverse environmental conditions and making him feel comfortable. Comfort characteristic is an important functionality of clothing. Human thermophysiological comfort is associated with the thermal balance of human body, which is highly dependent on metabolism rate, physical activities, ambient temperature, and thermal and moisture transmission behaviour of the worn clothing [2]. Clothing creates a microclimate between the skin and the environment, which supports the body’s thermoregulatory system to keep its temperature within a safe range, even when the external environment temperature and humidity changes to quite an extent.

1.2

Components of clothing comfort

Comfort is one of the most important aspects of clothing. Many attempts have been made to define comfort, but a satisfactory definition is yet to be obtained [3]. Comfort has been defined by many researchers in different ways [4–6]. ● ● ● ●

Comfort is influenced by the physiological reaction of the wearer. Comfort is temperature regulation of the body. Comfort is the absence of unpleasantness or discomfort. Comfort is a state of pleasant psychological, physiological and physical harmony between a human being and the environment. All three aspects are equally important, since people feel uncomfortable if any one of them is absent.

So, to know about the comfort characteristics of any particular fabric or clothing, it is required to determine the different properties of the fabric which have direct effects on the comfort. Broadly there are four basic elements of clothing comfort, namely thermo-physiological aspect, sensorial or tactile aspect, physiological aspect and fitting comfort. The thermo-physiological comfort concerns about the heat and moisture transmission characteristics through clothing, i.e. transmission of heat, air, and moisture (liquid and vapour). The sensorial or tactile comfort is related with the mechanical contact of the fabric with skin, i.e. how a fabric or garment feels when it is worn next to the skin. These are fabric handle or feel, softness, fullness, warm–cool touch, static charge generation, flexing, pricking, itching, etc. The physiological comfort


5

depends on the aesthetic properties of fabric, i.e. drape, luster, colour, crease, pilling, staining, etc. The fitting comfort deals with the size and fit of clothing. All the above comfort aspects are strongly correlated between them. In clothing comfort, the most important factor is the movement of heat and moisture (liquid and vapour) through clothing to maintain the thermal equilibrium between human body and the environment. According to Goldman [7] there are four primary factors in clothing comfort, i.e. function, feel, fit and fashion. The function related to clothing comfort parameters are thermal and moisture (liquid and vapour) transmission, water absorbency, drying behaviour, etc. The thermal transmission is a linear function of fabric thickness and relatively independent of fibre characteristics. Thus the thermal transmission can be controlled by the modification of yarn and fabric structures. Moisture vapour permeability also controls thermal characteristics by evaporative cooling phenomenon. The water transmission in liquid form, i.e. wicking, depends mainly on the type of fibre, weave structure of fabric and the finishes applied to the fabrics. The absorbency of water depends on fibre type, finishes, weave and design of fabric. Although the wicking is important, the amount of liquid that can be blotted away from the skin is also very important. The drying behaviour depends on the type of fibre, fabric and design of fabric. It is important because the ability of the body heat to rapidly dry clothing and restore insulation is a critical factor for survival. The clothing comfort related to feel are broadly divided into two distinct areas, namely the feel of clothing when held between the thumb and the fingers and the feel of clothing by the wearer when worn in contact with skin. Fit may incorporate factors from fashion, including concepts that may be diametrically opposed to comfort [7]. The clothing fashion is related with the psychological comfort.

1.3

Clothing comfort and wearer’s attitude

Comfort and satisfaction with clothing are influenced by both characteristics of clothing as well as by attitudinal and psychological perceptions of the wearer. The clothing characteristics include the physical characteristics of the fibres and materials from which the clothing is made, its tactile characteristics, design features of the clothing, brand labels, information on fabric/garment care, price, etc [8]. The wearer’s attitudes towards clothing are influenced by the sensory attributes of the clothing (softness/harshness, warm/cool touch etc.), serviceability characteristic (e.g., durability, creasing, pilling) and most importantly by its expected comfort and satisfaction related attributes.

6


These attitudes may be gathered either through prior experiences with the exactly same or similar type of clothing, or from information obtained about the clothing through interpersonal, advertising or retail channels. These attitudes toward either fabrics or items of clothing can significantly affect the actual physiological comfort and other performance properties of the clothing and can become the primary determinant of consumer behaviour through their influence on behavioural intentions [9–11]. A large number of studies have been reported on attitudes toward clothing [12–15]. DeLong et al. [12] in their study to evaluate the consumer response to apparels asked consumers to complete the sentence “When I think about sweaters, I think about” without presenting any fabric samples or items of clothing. They performed a content analysis of the words that consumers used in order to assess the factors underlying the concept of “sweater”. Byrne et al. [13], in their perception study on fibre types and end use, used semantic differential grids to study consumer attitudes toward silk, cotton, polyester and nylon for use in sport shirts and undershirts. They have concluded that the consumer attitudes toward the names of different fabrics were distinct and that the intended enduse greatly influenced perceptions of the adequacy of the fabric. During the study on consumer preferences for natural, synthetic and blended fibres, Forsythe and Thomas [14] observed that consumers have welldefined attitudes toward fibres and, with the exception of polyester/cotton blends, these attitudes are consistent across demographic variables. The attitudes of consumer about fabrics and clothing can be reliably assessed with appropriate psychometric techniques applied to fabric or clothing names. Conjoint analysis technique is widely used by researcher for assessing consumer attitudes toward clothing. This technique deals with the factors related to the consumer attitudes and behavioural intentions by using multiattribute choice alternatives within a specified experimental design [8, 15, 16]. Using this technique, a survey had been conducted where consumers are given a large set of multi-attribute choice alternatives. Consumers choose or rate each combination of product variables on attitudinal or behavioural dimensions of interest. The product attributes are the dependent variable and by varying the attributes and their levels according to a statistically determined experimental design, conjoint analysis enables the researcher to “work backwards” from the choices/ ratings to uncover the relative importance of each factor to the consumer’s decision process. Conjoint analysis has been used in clothing research to study the relative importance of attributes related to the aesthetic perceptions of garments [17].


1.4

Human–clothing interactions

1.4.1

Clothing as thermal barrier

7

Hindrance to the release of body heat Fourt and Hollies [18] have described the clothing system as “a quasiphysiological system interacting with the body”. This means the relationship between human body and clothing is a two-way process. Both the clothing and the wearer perform their specific activities for others. The clothing protects the wearer from the environmental hazards for which it has been designed, whether they are heat, cold, fire, toxic agents or any other thing. At the same time the clothing does some adverse things to the wearer, e.g. by unwanted thermal insulation when it is not required, or by hindering the free evaporation of sweat from skin. Presence of clothing layer(s) prevents the efficient evaporative cooling of human body, which is his sole defence against severe heat. Thus the wearer faces the unbearable and dangerous conditions when he or she works near fire, like overheating, dehydration, and sometime may also collapses. In normal conditions, without any activity, the metabolic heat produced by a normal person is nearly about 80 watts (same as an electric light bulb!) and in the condition of high activity it can rapidly rise to more than a kilowatt [19]. So, the human body requires an effective cooling system, and physiological system of the body provides this cooling effect. This metabolic heat load, mainly during high activity, poses a consistent threat of overheating and the presence of clothing makes the threat even worse. During high activity in extremely hot environment, e.g. worker in furnace, firefighter, etc. gains hundreds of watts more from the surroundings in addition to the metabolic heat generation. Sweating, which is an excellent mechanism for cooling the skin by evaporating water from it, is the only mechanism to reduce these great heat loads. On the other hand, the excessive sweating may also results dehydration. During high activity condition, in hot environment, a normal person can release sweat at the rate of about 1 litre/hour. There are various linked mechanisms within the human–clothing system which are essential to maintain the correct body temperature and the failure of this link of heat transfer in any form causes increase in body temperature and the person may feel sick or dizzy. The most important mechanisms for effective heat transmission are: ●

● ●

all the metabolic heat produced should be carried to the inner body surface (inner layer of skin) by the effective circulation of sweat; the skin should be able to generate the necessary amount of sweat; the generated sweat should get transmitted effectively (in liquid as well as in vapour form) through clothing ensemble.

8


One cannot adjust or change the first two mechanisms, but can definitely control the third mechanism by proper clothing. When someone wears excess number of clothing than what is required, he may feel overstressed or overheated with normal activity. Helps to retain body heat Except very hot environmental conditions and at very high activity levels, most of the environmental temperatures are below the human body temperature and clothing is required to hinder the flow of body heat to the atmosphere. So, in all these environmental conditions the heat flows out from the human body to the atmosphere due to the temperature difference, i.e. human body temperature is higher than the environment. In normal room temperature, i.e. approximately 27±2°C, the wearer requires minimum clothing layers to maintain the heat balance. The wearer does not require too much thermal insulation in clothing as the temperature difference between skin and the normal environment is low. The heat, generated in the body, gets transmitted slowly through the clothing and the open body surfaces (hands, arms, face, palms, etc.). As the temperature of the atmosphere drops further (say below 10°C) the rate of heat loss from body to atmosphere increases rapidly and the wearer feels cold due to thermal imbalance. The best and easiest way to prevent this body heat loss is to have certain insulating layer around the body, and that is done by wearing some additional layers of clothing (which also provide insulating still air layer). Under this condition, loss of body heat through clothing drops significantly and little amount of heat loss still takes place through some opening of body surface. In extreme cold conditions (say below –20°C) the loss of body heat is prevented by enhancing the thermal insulation of clothing and covering all the body parts.

1.4.2

Mechanisms of enhancement of body heat release

The symptoms of overheating or overstress due to excess number of clothing rapidly disappear when the excess clothing is removed. The transmission of body heat through clothing ensemble changes automatically by different mechanisms. Activity of the wearer influences the heat transmission characteristics of clothing. As soon as the wearer starts moving or walking or running the thermal insulation of clothing reduces because of a combination of forced air circulation between and through the layers of clothing. This reduction in thermal transmission is further enhanced by the typical bellows effect at various openings and also due to movement


9

the thermal insulation of the surrounding air reduces. During activity the clothing gets wet from sweat which also causes the drop in the thermal insulation. This automatic reduction in thermal insulation of clothing during activity level may not be always sufficient and in those cases the wearer becomes over-heated and sweats. This is due to the fact that the clothing layers actually hinder evaporation of sweat. Majority of the generated sweat wets the clothing in normal environment or in cold environment condenses in the outer layers. In either case the sweat removes less heat from the body than it does when it is able to evaporate from the skin, and additional sweat therefore has to be secreted to maintain the heat balance. Consequently the wearer is too hot while he is active, and when he later rests he becomes chilled because of the reduced insulation of wet clothing and the continuing evaporation of water from it [19]. The over-heating of body can also be reduced by proper clothing design, i.e. by providing effective ventilation in the clothing. The changes in clothing design may be effected by: (i) creating openings, to allow natural convection by chimney effect, at various places in the clothing, e.g. neck, wrists, ankle and waist. (ii) designing loose fit clothing to have free convection of air and free interchange with outside air by means of a bellows effect. (iii) providing full-length zippers in the clothing for specific applications. (iv) avoiding the use of impermeable materials, whenever possible, can further facilitate evaporative cooling.

1.4.3 Multilayer clothing system Most of the performance clothing assemblies are generally not a single layer system. These generally consist of a number of layers and each layer performs its specific function. These layers are generally of three types, i.e. inner layer, middle layer(s) and outer layer. A clothing ensemble that should function with high requirements to comfort and protection must be put together methodically from the inside out [20]. Figure 1.2 shows the typical functions of individual layers of a three layer clothing system, where the inner layer is generally an underwear which performs mainly the sweat absorption, direct cooling of the skin, transmission and tactile functions; the middle layers are generally shirt or sweater which helps still-air entrapment to provide insulation, transmission etc.; and the outer is primarily a shell layer for protection from extreme environmental factors, like rain, wind, chemical, heat, radiation, etc.

10


Thermal protection

Water repellency Wind proof Outer layer Middle layer Inner layer Heat or Sweat (liquid or vapour)

1.2 Three-layer clothing system [21].

1.5

Understanding clothing comfort

1.5.1

Need and consumer trends

The basic and universal need of consumers in clothing is comfort and they look for good feel and comfort when they buy clothing and other textile materials. Clothing is very important in our life that we use everyday to obtain physiological and psychological comfort and also to ensure physical conditions around our body suitable for survival. Therefore, it is extremely important for the survival of human beings and improvement of the quality of our life to have good understanding of the fundamentals of clothing comfort. From the viewpoint of the manufacturers of clothing and textile materials, understanding of clothing comfort has substantial financial implications in the effort to satisfy the needs and wants of consumers in order to obtain sustainable competitive advantages in modern consumer markets. Consumer always expects some additional functional qualities from the clothes they purchase. Clothing is manufactured in a wide range of thermal, tactile and physical properties to meet consumer needs. Depending on the needs and expectations of the consumers, the clothing and textile manufacturers provide wide range of options to enhance human comfort. For example, clothing made from blends and natural fibres are preferred to man-made fibres for all comfort attributes except smoothness, or woven fabrics are preferred to knits for smoothness, thickness and openness. To understand the basics of clothing comfort, sensory tools as well as the equipments to evaluate the comfort related characteristics of textile materials have been developed. Large number of studies has been carried out and many equipment are developed in the textile and clothing area such as mechanical, thermal and surface testing, so as to evaluate the related physical properties, but the links between measurement and the consumer feeling of comfort are still difficult to establish.


11

Consumers want everything from the clothing, i.e. it should look good, feel good, perform well, would like their clothing to match with their chosen attitudes, roles and images. Consumers are now allowing touch, smell, intuition, and emotion to influence their decision on clothing selection more than their aesthetic sense. As a result, great importance is being attributed to the wearing experience and thus comfort is being reinforced as a key parameter in clothing. It is also true that requirements of consumers on comfort changes with products and situations. Clearly, understanding and satisfying the needs of consumer towards clothing products are crucial for the long-term survival and growth of clothing and textile demand. Understanding and enhancement of clothing comfort is definitely one of the important issues.

1.5.2

Scientific approaches

To have proper understanding of the clothing comfort and to predict comfort performance of clothing during wear, one needs integrated scientific knowledge of physics, physiology, neurophysiology, and psychology of comfort. In long-term perspective, it is very important to have proper knowledge on clothing comfort to improve the quality of life and the survival of human beings. The clothing and textile industries should take necessary initiative in this area to achieve market leadership. Researchers identified the psychological sensory attributes what consumer desire, which is correlated with the technical parameters of clothing through psychophysical perceptual trials. The clothing can be developed with specified technical parameters to achieve certain level of psychophysical comfort. Li [22] reported that there are five levels of understanding clothing comfort. The important steps for scientific understanding of clothing comfort are market research, wear trials, objective evaluation of clothing characteristics and objective evaluation of fabric characteristics. The market research is generally carried out by identification of target group, personal interviews and consumer surveys to gather market information on the products. The wear trials can be conducted either in the field in which the clothing are used or in climatic chambers for psychological sensory study, consumer focus group study and subjective evaluation of clothing. The objective evaluation of clothing characteristics, e.g. thermal and moisture transmission are generally done either on human subjects or thermal manikins. The objective evaluation of fabric characteristics are carried out by testing transmission (moisture, heat), handle, tactile and aesthetic characteristics of fabrics. The information on clothing comfort requirements should flow from customer to technical specifications of fabrics and clothing to have a new product that can satisfy the requirements of consumers. On the other hand, one can predict the consumer acceptability of particular clothing by proper understanding of

12


fabric and clothing characteristics, physical and psychophysical mechanisms. Using statistical and mathematical tools one can easily optimize the clothing parameters as per the identified consumer’s requirements even before actual production.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

COATES J . F ., ‘From my perspective: The future of clothing’, Technological Forecasting & Social Change 72, 101–110, 2005. OLSCHEWSKI H . and BRUCK K ., ‘Cardiovascular and muscular factors related to exercise after pre-cooling’, J. Appl. Physiol. 64, 803–811, 1988. SLATER K ., ‘Comfort properties of textiles’, Text. Prog. 9(4), 1–42, 1977. HATCH K . L ., Textile Science, West Publishing Company, New York, 1993. SLATER K ., Human Comfort, Thomas Springfield, Illinois, 1985. SLATER K ., ‘The assessment of comfort’, J. Text. Inst., 77, 157–171, 1986. GOLDMAN R . F ., ‘The four ‘Fs’ of clothing comfort’, Elsevier Ergonomics Book Series: Environmental Ergonomics - The Ergonomics of Human Comfort, Health and Performance in the Thermal Environment 3, 315–319, 2005. SCHUTZ HOWARD G ., CARDELLO ARMAND V . and WINTERHALTER C ., ‘Perceptions of fiber and fabric uses and the factors contributing to military clothing comfort and satisfaction’, Textile Res. J. 75(3), 223–232, 2005. SMITH J ., ‘Perceived comfort’, Text. Horiz. 9, 44–45, 1986. SHIM S . and DRAKE M. F ., ‘Consumer intention to purchase apparel by mail order: beliefs, attitude, and decision process variables’, Cloth. Textiles Res. J. 9(1), 18–26, 1990. WITTER B . S . and NOEL C ., ‘Apparel advertising: a study in consumer attitude change, cloth’, Textile Res. J. 3(1), 34–40, 1984–85. DELONG M . R ., MINSHALL B . C . and LARNTZ K ., ‘Use of schema for evaluating consumer response to an apparel product’, Cloth. Textiles Res. J. 5, 17–26, 1986. BYRNE M . S ., GARDNER , A . D . W. and FRITZ , A . M ., ‘Fiber types and end-uses: A perceptual study’, J. Textile Inst. 84(2), 275–288, 1993. FORSYTHE S . M. and THOMAS , J . B ., ‘Natural, synthetic and blended fabric contents: an investigation of consumer preferences and perceptions’, Cloth. Textiles Res. J., 8(3) 60–64, 1989. GREEN P . E . and SRINIVASAN V ., ‘Conjoint analysis in consumer research: issues and outlook’, J. Customer Res., 5 103–124, 1978. GREEN P . E . and SRINIVASAN V ., ‘A general approach to product design optimization via conjoint analysis’, J. Marketing 45, 17–37, 1978. ECKMAN M ., ‘Attractiveness of men’s suits: the effect of aesthetic attributes and consumer characteristics’, Cloth. Textiles Res. J. 15 (4), 193–202, 1997. FOURT L . and HOLLIES N . R . S ., Clothing: Comfort and Function, Dekker, New York, 1970. BUDD G . M ., ‘Clothing physiology’, Fire Safety Journal 4, 77–81, 1981. RUTH N ., Work clothing, International Journal of Industrial Ergonomics, 7, 77– 85, 1991. DAS A., Objective evaluation of comfort characteristics of textiles, Seminar on comfort in textiles, Department of Textile Technology, IIT Delhi, October 16, 2004. LI Y ., The science of clothing comfort, Textile Progress 31(1/2), 2001.

2 Psychology and comfort

2.1

Psycho-physiological factors of clothing comfort

The physiological factors of human body for expressing the human comfort are average skin temperature, degree of skin wetness (indicated by electrical conductivity at the body surface), rate of sweating, the amount of sweat, sweat absorbed by clothing, and rate of heart beat. It is important to correlate all the physiological parameters with contributing psychological factors to predict the perceptions of comfort. Thermal effects contribute extensively to the ‘comfort’ of an individual, complex physiological and psychological factors collectively play an important role in defining this complex quality with reference to clothing [1]. In fact, clothing comfort is the psychological feeling of wearer who wears the clothing under different environmental conditions. The factors influencing the clothing comfort sensations of wearer can be divided broadly into three groups: (i) physical factors (deals with the human–clothing–environment system); (ii) psychophysiological factors of the wearer; and (iii) psychological filters of the brain. The comfort status of wearer depends on all these factors and their complex interactions and synchronizations. Figure 2.1 shows the interrelationships between the important physical and physiological factors those control the clothing comfort. The figure illustrates the process of how the subjective perception of overall comfort is formulated. The physical processes provide different signals or stimuli (e.g., warm/cool, touch, prick, pressure, wetness, etc.) to the sensory organs of the human body. The human body receives all these stimuli and subsequently generates neurophysiologic impulses. The neurophysiologic impulses are then send to the brain to take corrective actions to adjust the sweating rate, blood flow, and sometimes heat production, shivering, etc. [2]. The brain, after receiving the sensory impulses, processes all these impulses to generate the human subjective perception of various individual sensations, and further evaluate and weigh them based on the past experiences. The processes of evaluation and weighing are influenced by

13

14


many factors such as physical, environmental, social, cultural, etc. The clothing comfort is a human psychological perception related with clothing ensemble, which is an outcome of complex linkages between individual sensory stimuli received by brain, evaluation and weighing of all these stimuli to formulate subjective perception of overall comfort based on wear experience.

Psychophysiological process

Heat, liquid/vapour transmission of clothing

Psychological process Appearance & feel

Thermoregulatory response of body Physiological process

Skin sensory responses

Human interaction / activity in (with) environment

Environmental /physical factors

Perception of brain Comfort

Fitness Environmental condition (hot / cold, RH %)

Psychological process

2.1 Important physical and physiological factors controlling the clothing comfort.

2.1.1

Psychological perceptions of clothing comfort

The wearers consider the comfort as one of the most important attributes in their clothing ensembles, so there is a need to develop an in-depth scientific understanding of the psychological perception of clothing comfort sensations. The physical comfort is greatly influenced by tactile and thermal sensations arising from contact between skin and the immediate environment [3]. Comfort may be defined as pleasant state of physiological, psychological and physical harmony between a human being and the environment [4]. Comfort can also be defined as a holistic concept, which is a state of multiple interactions of physical, physiological, and psychological factors [5]. All these definitions only identify the factors influencing the human psychological perceptions. Wong et al. [6] developed a linear model based on artificial neural network predictions using three major factors which affect the comfort perceptions, namely moisture related factor, tactile sensations and thermal-fit comfort, and their relative weights


15

to predict overall comfort perceptions. They have developed feed-forward back-propagation neural network models to predict an overall comfort perception from ten individual sensory perceptions (clammy, clingy, damp, sticky, heavy, prickly, scratchy, fit, breathable and thermal). They have reported a good agreement between predicted and actual clothing comfort perceptions, which indicated that the neural network is an effective technique for modelling the psychological perceptions of clothing sensory comfort. They have further reported that the functions and interrelationships of individual sensory perceptions and comfort are unknown.

2.1.2

Sensory perceptions of clothing comfort

Different types of sensations generated from clothing ensemble depend mainly on the various combinations of type of clothing, type and level of activities and the environmental conditions experienced by the wearer during the activities. The most common clothing comfort related sensory attributes are thermal, moisture, tactile, hand, and aesthetic experiences. The experts in the field of sensory attributes can easily identify the difference between the above attributes and suggest accordingly. But, it is very important to identify some commonly recognized comfort attributes of clothing among ordinary wearers, and what they are if they exist. When the level of human activity or the temperature and humidity of microclimate change the changes in various sensory perceptions, like warmth, chilliness, scratchiness, dampness etc., can be very easily detected [7]. Strong sensations can also be experienced, both indoors and outdoors, when mild or heavy sweating occurred, and during modest excursions of warming or chilling following the inception of sweating. There are many attributes which describe the clothing comfort sensory perceptions of human. Some of the important attributes are loose or tight, heavy or light, stiff or pliable, sticky or non-sticky, absorbent or non- absorbent, cold or warm, pleasant or clammy, dry or damp, pricky or non-pricky, rough or smooth and scratchy or non-scratchy, etc. Some of these attributes do not give useful contribution in prediction of clothing comforts. So, most important and established attributes for subjective evaluation of sensory perceptions of clothing comfort are course–fine, rough–smooth, stiff–pliable, harsh–soft, cool– warm, hard–soft, and rustle–quiet, for expressing sensorial comfort. In developing methodology for evaluation of fabric handle, Kawabata [8] generated sensory attributes by letting a panel of expert judges (the Hand Evaluation and Standardization Committee) judge the fabric handle and asking them the reasons for their decisions. They identified terms such as KOSHI (stiffness), NUMERI (smoothness), and SHARI (crispness) as ‘primary hand’ expressions.

16


The investigation on physiological sensory responses to clothing, of consumers living in different countries, revealed that the ratings of most of the sensory attributes are significantly different between three types of clothing, i.e. summer wear, winter wear, and sportswear. No significant differences in rating of the sensory descriptors were found between male and female respondents. The physiological sensory responses to clothing which were considered in the study are snug, loose, stiff, lightweight, staticky, non-absorbent, sticky, heavy, cold, damp, clammy, clingy, rough, cool, hot, soft, warm, wet, prickly, itchy, chill, sultry, tickling, and raggy [2]. The wearers themselves know best and they are capable of making objective, quantitative and repeatable assessments of their sensations of their clothing. Therefore, sensory attributes should come from the wearers instead of experts or researchers.

2.2

Psychophysics and clothing comfort

2.2.1

Laws of psychophysics

Fechner, in 1860, originated psychophysics to describe the mathematical relationship between the conscious experience of a sensation and an external physical attributes [9]. According to his theory, if one knows the mathematical form of the psychophysical relation between a physical attribute and its corresponding sensation, he can measure psychological attributes by measuring their physical factors. Therefore, psychophysics is about the quantification of the strength of internal sensations, which can be broadly defined as the quantification of sensory experience. The strength of internal sensations has two aspects of indication, i.e. (i) the assessment of human powers of signal identification and sensory discrimination, and (ii) the calibration of subjectively perceived intensities and other parameters of stimulation. Weber, in 1834, [10] proposed that the threshold (i.e. the just noticeable difference) of stimulus (ΔSp) are proportional to the magnitude of stimulus Sp. This is known as Weber’s law and can be expressed as: ΔSp/S p = K (2.1) where K is a constant indicating the power of a human being to detect signals and discriminate sensations. This law holds good for many stimulus attributes down to about the absolute threshold which is the smallest magnitude of stimulus that can be perceived. Fechner, in 1860, [9, 10] proposed using “just noticeable deference” as a unit to measure internal sensation. Fechner assumed that sensation Rs increases as the logarithm of physical stimulus magnitude Sp; this is called Fechner’s law and can be described as:


R s = K´logSp

17 (2.2)

where K´ is the constant determined by the stimulus threshold which represents the lowest physical value evoking sensation and the deferential threshold providing a subjective unit of sensory intensity. This law states that sensation increases in arithmetic steps as the physical stimulus is increased in logarithmic steps. Both Fechner’s law and Weber’s law of psychophysics are related to each other. Stevens, in 1953, [10] developed a method of estimation of the relationship between subjectively perceived intensity and physical stimulus strength. This method was applied to a large number of different stimulus attributes. The results from each stimulus attribute generally follow the following relationship, R s = aS pb

(2.3)

where, ‘a’ is a scale factor and ‘b’ an exponent characteristics of the attribute. This equation is known as Stevens’ power law. All these laws of psychophysics indicate that there are fundamental differences between the physical stimulus and the sensation that one experiences. Weber’s law and Fechner’s law play some fundamental role in sensory discrimination in terms of the ability to distinguish one stimulus from another, but fail to provide a basis for measuring sensation. Stevens’ law proposes a power relation between physical stimulus magnitude and internal sensation which provides a ‘direct’ measurement of sensation in sensory judgment process.

2.2.2

Types of psychophysical scaling

Psychological scaling is a process of assigning numbers to characteristics of objects or events, according to rules which reflects some aspects of reality. Psychological scaling has been widely used in marketing research to obtain consumers opinions and study their attitudes and preferences. Assigning numbers does not always correspond to the real numbers that are obtained from objective measurement in physical means. The numbers cannot necessarily be added, subtracted, divided or multiplied. The numbers are used as a symbol to represent certain characteristics and the rules specifying how numbers are assigned to the characteristics to measure. These rules may be arbitrary and changes as per the specific condition. The rules governing how to assign numbers constitute the essential criteria defining each scale. There are four types of scale of measurement: nominal scale, ordinal scale, interval scale and ratio scale [11]. Moving from nominal scale to ratio scales, the rules become more complex and the kinds of arithmetic operations for which the numbers can be used are

18


increased. Each scale exhibits symmetries and hence corresponds to a type of symmetry group [12]. In fact, the four scales correspond to a descending sequence of subgroups, e.g. the group for the nominal scale containing the group for the next scale, i.e. the ordinal scale. Similarly, the group for the ordinal scale contains the group for the interval scale, and the group for the interval scale contains the group for the ratio scale. In other words, the symmetries which are members of the set corresponding to the ratio scale are also members of the set corresponding to the interval scale although the latter contains additional symmetries as well, and so on, the set corresponding to the nominal scale having the greatest number of symmetries as members [13]. Nominal scales consist of numbers used to categorize objects. A nominal number serves as a label for a class category. In a nominal scale, items are sorted into classes with no quantitative information conveyed. Numbers may be used in a nominal scale, but they are only used to indicate group membership, e.g., the numbers on the jersey of a player in a hockey team or one can assign 0 to male and 1 to female. The number 1 does not imply superior position to number 0 or number on the jersey of a player generally does not indicate the performance of player. The rules for nominal scales are that all numbers of a class have the equal value. The only arithmetic operation that can be performed on nominal data is the count in each category. Nominal numbers cannot be added, subtracted, multiplied and divided. The nominal scales only distinguish the objects or events on the scale from things that are not on it. Due to its high degree of symmetry, it conveys little information and is hence the weakest form of measurement. For example, grading the students only in the form of pass or fail does not convey much information about student performance than assigning grades or exact percentile. Ordinal scales comprise numbers or other symbols used to rank the events or objects according to their characteristics and their relative position in the characteristics. Ordinal data indicate the relative position of objects on certain characteristics scales but not the magnitude of the differences between the objects. A mode or median may be used, but not a mean. Nonparametric statistics can be applied to ordinal data. Some of the symmetries in nominal scales disappear during the shifting of events or objects from nominal to ordinal scales. This is because the ordinal scale is less symmetrical than a nominal scale. For example, generally the top person in an organization gets highest salary and the salary reduces according to the hierarchy in the organization. But as long as the hierarchy is preserved, there is no social significance in varying the salary in each level. Hence, there are symmetries here as well, transformations which would make no social difference. But there is less symmetry here than in nominal scales,


19

since some transformations that would not socially matter in ordinal scales do matter in nominal. For example, giving more salary to manager than the president would be forbidden, or it would indicate a change in status. The transition from ordinal scale to interval scale results in a reduction of symmetries. In the interval scales the numbers are used to rank the objects or events in such a way that numerically equal distances on the interval scale represent equal distances in the characteristics of the objects or event being measured. But both zero and their unit of measurement are not fixed and are arbitrary. Therefore, interval data can indicate both relative position of objects and the magnitudes of differences between the objects on the characteristics being measured. The entire range of statistics can be applied to interval scales. On an interval scale, one unit represents the same magnitude as any other. For example, in Box and Behnken [14] three-factors and three-level model the factors (independent variables) are coded with –1, 0 and +1 for their three levels. In the actual data the intervals in the factors are numerically same. Another example is the measurement of temperature in centigrade scale. One degree centigrade is warmer than 0°C to the same extent as 2°C is warmer than 1°C. Fiske [11] stated that “Equality matching relationships resemble an interval scale in that people can not only specify who owes what to whom, but also how much they owe”. On the basis of ordinal scale, people keep track of imbalances or differences between each other and try to maintain balance. Equality, following particular turn, strict reciprocity is maintained strictly. Examples are voting, games that involve equal turntaking, and so on. There is less symmetry here than in the case of ordinal scale. In interval scale, one must make sure that everyone has the same thing, however, sameness is defined. This degree of precision is lacking in ordinal scale [13]. A ratio scale is exactly like an interval scale, except that it has an absolute 0 point. For example, the kelvin temperature scale has absolute zero point but the centigrade temperature scale measures the freezing point of water defined as zero degrees Celsius and does not have absolute zero scale. Ratio scales represent the numbers used to rank objects such that numerically equal distances on the scale represent equal distances of the characteristics measured and have a meaningful zero. Like interval scales, entire range of statistics can be applied to ratio data. Ten degrees centigrade is not twice as hot as 5°C, but 10 K is twice as hot as 5 K. In ratio scale, people order their interactions according to a system of ratios and proportions such as salary, rents, taxes, etc. This allows each individual or group of like-minded people to decide how to act and evaluate actions according to cost-benefit analysis [13]. All the above four types of psychological scales are important for better understanding of psychology of clothing comfort. The nominal scales

20


determine quality and have been used for categorization and classification such as gender, age, and place of living. Ordinal scales determine equality and relative position and have been used to obtain the rankings of fabrics or clothing in consideration. The most frequently used scales are the interval scales, which determine equality, relative position, magnitude of differences and have been widely used to obtain the perception of various attributes of clothing. The ratio scales are mainly applicable to the data generated from physical instruments, which determine equality, relative position and magnitude of difference with a meaningful zero.

2.2.3

Psychophysical scaling of clothing comfort

The tactile parameters, like prickliness, fabric itchiness, fabric stiffness, fabric softness, fabric smoothness, roughness, scratchiness, etc., are basically sensory comfort attributes, but the psychological comfort is not a sensory attribute, because it is not associated directly with any single human sense organ. The psychological clothing comfort is characterized by emotion and affection, which is related with the liking towards particular clothing. Thus, there is no underlying physical dimension of the stimulus that varies continuously and is monotonic with the perception of comfort. The same stimulus can generate altogether different comfort responses from different individuals. As a result, it is not possible to define a comfort scale based on physical standards that is valid for all users [15]. In general the clothing comfort is characterized by emotional attributes, so the judgment can be done effectively by untrained consumers instead of experts. This requires a method for psychophysical scaling of clothing comfort that is simple and easy to understand which do not require any training or complex instructions. The ‘category scale’ is the most commonly used subjective scale for rating comfort. This is characterized by a series of verbally and/or number labelled points or descriptive categories, like ‘extremely comfortable’, ‘moderately comfortable’, ‘slightly comfortable’, etc. In this type of scaling, a person can rate his subjective comfort sensations by placing them into one of several descriptive categories. Since less than five categories can result in a loss of discrimination sensitivity, the number of categories is typically around seven to nine, or sometime it can also be more [16, 17]. Due to the simplicity, versatility, and high reliability the ‘category scales’ are widely used for measurement of subjective clothing comfort and other psychological attributes. Although there are many advantages, still there exist some critical problems associated with the use of ‘category scales’. In case of a numbered category scale, the numbers with equal intervals do not represent equal subjective intervals [18]. In the labelled category scales, subjects attend primarily to the word labels and not to the numbers [19]. In these


21

cases, unless the verbal labels are chosen on the basis of extensive evaluation process to verify that the differences between ‘slightly comfortable’ and ‘moderately comfortable’ are the same as those between ‘moderately comfortable’ and ‘extremely comfortable’ the scale cannot be considered to be an interval scale, but merely an order of comfort sensation. Another common problem with category scales is that the normal tendency of a person is to avoid the end categories; this is called “category end effect”. This “category end effect” results in seven-point category scales being functionally reduced to five-point scales after eliminating two end points; and similarly the five-point scales is reduced to three-point scales, and so on. The recent developments in psychophysical methodology that enable better quantification of both the descriptive aspects of tactile sensations and the scaling of the emotional attributes of handle helped the researchers in applying well-established psychophysical approaches to study the sensory and comfort characteristics of clothing. After the development of Kawabata instrumental evaluation systems [8, 20, 21] for measuring lowstress mechanical characteristics of fabrics, it has now become relatively simple to measure many subjective attributes objectively. Combining the psychophysical sensory methodology with the established instrumental methods of fabric characterization now makes it possible to develop better predictive relationships among sensory, instrumental and comfort characteristics of clothing.

2.3

Wear trial techniques

Human being often uses hands to obtain tactile information, but much of the tactile sensations come from parts of the body other than hands. This suggests the necessity of study of the perception of clothing comfort in actual wear situations. Therefore the wear trialing is an important technique for clothing comfort research. Sensory clothing comfort perceptions are primarily associated with skin sensory systems. In addition to this the clothing comfort sensations involve various sensory channels from all the five senses: visual, auditory, smell, taste and touch. A certain type of clothing comfort sensation is generated under certain wear conditions with a particular type of external stimuli and physical activity. The external stimuli (heat, moisture, wind, etc.) and mechanical stimulation from fabric to the skin (softness, scratchy, pricky, etc.) are normally generated under specific combinations of physiological states (e.g. sweating rate), materials used in the clothing, fitness of clothing and environmental conditions (e.g., temperature, humidity and air velocity). Hollies et al. [22, 23, 25] proposed the wear trial technique to generate reactions of wearer to any perceived discomfort sensations produced by

22


different climatic conditions, and by alternating sweating and cooling off conditions that might be encountered in actual wear of clothing. In the wear trial experimental technique they also characterized the sensory comfort of clothing, which included a number of components: (i) Generation of sensory attributes with wearers. (ii) Selection of particular testing conditions for effective analysis of the perception of various sensations. (iii) Designing attitude scales in the way of subjective rating sheets to obtain various sensory responses to particular garments. The sheets contain different comfort attributes (e.g., stiff, sticky, non-absorbent, cold, damp, clammy, clingy, rough, scratchy, etc.) at different time interval in particular environmental condition. The comfort intensity was scaled at five different scales, i.e. 1 is totally uncomfortable and 5 is completely comfortable. (iv) The wear trial was then conducted in controlled environments chambers according to predetermined protocol. (v) The wearers rated the garments for each comfort attribute separately for different time interval. (vi) All collected data were analyzed and the results were interpreted. In a recent research study [24] warm and humid climatic conditions were produced using a climatic chamber with precise control of air temperature and humidity. In this study, different varieties of garments were worn with six coverall types. Each test session was made up of five individual evaluation periods, which yielded approximately 900 individual evaluations. Comfort ratings were assessed on all test garments in each of the five rating periods of the protocol. An evaluation form was designed to record ratings of comfort and sensory properties for each of the five periods. The scales used to assess overall comfort, thermal sensation, and contact comfort sensations were recorded. During each evaluation period, wearers were asked to indicate the number, on the designated rating scale, that best described their perceived sensations. The wearers and the garments were precisely weighed before and after completion of the wear trial protocol to estimate the moisture loss from the body, and to determine the amount of moisture accumulated in the test garments. During the wear trial study the overall comfort sensations, thermal sensation and skin contact comfort sensations of garments were rated by the wearers with different terms and scales. The overall comfort sensations were expressed in seven scales, i.e. 1 – Very uncomfortable, 2 – Uncomfortable, 3 – Slightly uncomfortable, 4 – Neither comfortable nor uncomfortable, 5 – Slightly comfortable, 6 – Comfortable and 7 – Very comfortable. Statistical techniques have been adopted for data analysis.


23

Wong et al. [26] used neural network technique in wear trial to predict the human psychological perceptions of clothing sensory comfort. They have selected twenty-two professional athletes as subjects to take part in the psychological sensory cycling trial. The wear trials were conducted in an environmentally controlled laboratory. Before the trial, athletes were subjected to medical fitness examinations to ensure that they were able to complete the experiment. They have chosen different commercial sportswear in their study. Initially they invited the professional athletes for a pre-trial before the formal trials to obtain training and understanding of the questions and procedures involved. During each trial, each athlete was required to shower upon arriving at the laboratory, then change into a test garment and a pair of nylon shorts, and rest to equilibrium for 20 minutes. During the wear trial the laboratory conditions were controlled at 15°C, 65% RH, and an air velocity varying between 0.15 and 1.50 m/s. At the end of the equilibrium period the athletes were asked to ride ergonomic bikes for 90 minutes under work loads maintaining their heart rates at 70% of their estimated maxima. The athletes were asked to rate the sensory perceptions (e.g., clammy, clingy, sticky, damp, heavy, prickly, scratchy, fit, breathable and thermal) of the sportswear at different time interval, i.e. at the beginning, after 30 minutes, after 60 minutes and after 90 minutes. The ratings by the athletes were subsequently converted into 0–100 scales for all the sensory perceptions except fit and thermal sensations. The fit and thermal sensations were rescaled to the range from –50 to +50 because in these two perceptions the wordings used in the scale’s two ends to describe the perception of fit (from too loose to too tight) and thermal (from too cold to too hot) were different from the other sensory perceptions such as damp (from not at all to extremely). In their study, Wong et al. [26] developed the neural network prediction model on the basis of a feed-forward back-propagation network. The network model consisted of three layers, i.e. input layer, hidden layer and output layer. A good agreement between predicted and actual clothing comfort perceptions have been observed, which proved that the wear trial technique is an effective technique for predicting the psychological perceptions of clothing sensory comfort.

2.4

Psychological aspects of aesthetic comfort

The physical attributes of the human body is directly related to the aesthetic comfort characteristics of clothing. A large number of researchers [27– 32] have studied the complex interplay between clothing aesthetics and body attributes and the human body has been designated as the central element in the aesthetic experience of clothing. The relationships between the aesthetics of clothing and the physical attributes of the body is not the matter of only textile and clothing discipline but many other fields of

24


research, like physical and demographic attributes affecting aesthetics, social aspect, psychological, cultural aspects that influence the aesthetic experience, etc. The researchers have taken into account of all these factors in their studies on clothing aesthetics. Human body imaging technique may be adopted in the study of clothing aesthetics. The clothing not only creates a person’s appearance but also provide aesthetic pleasure to the person through the wearing experience. The wearers generally try to achieve the aesthetic pleasure through their clothing by emphasizing certain positive features of their bodies through their clothing and hiding other negative features. Therefore, aesthetic attributes in clothing helps to minimize the differences between cultural beauty concepts and their perceived appearance, which helps to improve self-image and have stronger self-esteem of a person [33].

2.4.1

Evaluation of clothing aesthetics

Clothing comfort related to aesthetics is a complex interrelationship between the following concepts: ● ● ● ● ●

Style of the clothing adopted Surface texture of clothing Drape of fabric used Cover Creasing and resilience characteristics of fabrics.

All these concepts are generally described by how they are subjectively perceived by common word pairs used to communicate their values (e.g., thick–thin, rough–smooth, etc.). The physical or transmission characteristics of fabrics, namely mass per unit area, thickness, thread density, air permeability, thermal transmission, wicking, etc., can be easily measured by objective test methods. But, due to significant subjectivity the aesthetic characteristics cannot be measured accurately and there is no standard method of measuring aesthetic characteristics of clothing. The fabric aesthetics is entirely subjective and different people can rate same fabric in different scales based on their own perceptions. The main problem with the measurement of aesthetic attributes of clothing is to gather useful and consistent information by questioning people about the clothing or fabric. If this is done properly, then the numerical data can be obtained using different mathematical techniques and subjective test methods. The possible steps to measure the fabric aesthetics are [34] as follows: ●

Definition of fabric aesthetics in terms of basic elements having the form of common words.

Psychology and comfort ●

●

25

Identification of a system for selecting rating scales. This system involves questions for subjective measurement of these basic elements. Transformation of data from rating scales to numerical definition of a specific aesthetic property.

By the term ‘aesthetics of clothing’ we generally mean the appearance and handle of fabrics, which are mainly perceived by the senses. It has been already mentioned that the perceptions are relative attributes, i.e. it can be scaled based on their relative position on scales involving contrasts (warm–cool, good–bad, soft–hard, beautiful–ugly). Clothing aesthetic perceptions are the combinations and interrelations of measurable physical data (rigid–flexible, soft–hard) and values, which are psychological factors (good–bad, beautiful–ugly, fashionable–unfashionable). Some terminologies associated with the aesthetics of clothing are really conceptual, for example hand, cover and body do not have value polarity, i.e. they need not be good or bad, desirable or undesirable. Moreover, all these terminologies do not have a simple measurable physical reality. However, sometimes these parameters are defined to represent sense data only. For example, ‘surface texture’ is a fabric parameter which can be measured subjectively, but still this is merely a terminology which represents the skin sensorial as well as visual sensations of fabric. Limiting the meaning in this way often restricts communicative value. The surface texture of fabrics is evaluated in relation to the aesthetic comfort characteristics of clothing. To evaluate is to judge something on a scale that has opposite poles or a gradation. For example, a scale for evaluating comfort concepts is the pain–pleasure polar scale. It is a psycho-physical scale. Aesthetic concepts are not physical attributes. These can be expressed in terms of psycho–cultural scales and can be evaluated on polar scales such as beautiful–ugly, good–bad, etc. The surface texture of a specific type of fabric can be evaluated by the simple polar word scale, like soft– harsh or rough–smooth. It is quite possible that a fabric is aesthetically very beautiful, but painful from the skin sensory comfort point of view. For example, a tweed fabric may be unpleasant to the skin but pleasant to the eye or an aesthetically beautiful winter garment may be thermophysiologically extremely uncomfortable in warm and humid conditions. The aesthetic character of fabrics is primarily defined by subjective methods. The wearers are asked to evaluate qualities identified by simple word pairs whose meanings can be easily recognized as polar opposite words, like smooth–rough, soft–hard, flexible–rigid. Some of the confusing clothing terms should be avoided in evaluating the fabric aesthetics. For example, for evaluating the drape behaviour of fabric, if one wants to decide the fact that whether the fabric is good or bad the answer will create

26


confusion. He should know the exact application of fabric, e.g. whether the fabric will be used for skirts or for window coverings. A fabric drape may be good for window covering, but the similar drape may not be acceptable for skirts. So, simpler attributes signified by polar words, like flexible–stiff, which are related to fabric drape characteristics, need to be evaluated.

2.4.2

Aesthetic concepts of clothing

Clothing aesthetics can be divided into different aesthetic concepts. Figure 2.2 shows the interrelationships among physical parameters, psychological attributes, subjective evaluation and aesthetic comfort of clothing. Physiological sensations (visual, tactile, kinesthetic)

Objective evaluation (physical or tensile testing data)

Clothing aesthetic concepts

Subjective evaluations expressed by words (soft, harsh)

Simplified polar scale (beautiful–ugly, good–bad)

2.2 Components of clothing aesthetics.

Following are the guidelines for setting aesthetic concepts related to psychological clothing comfort: ●

●

The concept must be related to at least one of three main physiological sensations, i.e. visual sensation, tactile sensation, or kinesthetic sensation. The concept can be a combination of sub-concepts, expressed by words which are more explicit. For example, ‘resilience’ is less explicit than its component sub-concepts ‘compressional resilience’ and ‘liveliness’. Similarly, the term cloth cover cannot communicate the aesthetic concept completely, so its sub-concepts ‘top cover’ and ‘bottom cover’ are used.

Psychology and comfort ●

●

27

The concepts may be made technically explicit by physical measurements. These measurements attempt to quantify objectively to replace the sense data. Aesthetic, concepts or sub-concepts can always be evaluated subjectively. Subjective evaluation scales are represented by common words (quality words) which express the psychological value of the sense data associated with the concept.

The most commonly used concepts related to clothing aesthetic attributes are clothing cover, drape, body, style, surface texture and resilience [34]. Cover

Drape

Body

– The cover can be sub-divided into ‘top cover’ and ‘bottom cover’. The ‘top cover’ is the apparent continuity of the surface of fabric, i.e. degree of obscurity of the fabric weave pattern due to surface fuzziness. On the other hand the ‘bottom cover’ is the degree of obscurity of the fabric weave pattern due to fabric sub-layer. The cover can be expressed by dense– open, fuzzy–clean, smooth–rough, full–lean, etc. The fabric cover is objectively measured by streak meter, light transmission, surface contact area, air permeability, etc. – The fabric drape is generally sub-divided into ‘liveliness’ and ‘fit’. The term drape means the form a fabric will assume due to its own weight when hung freely. The aesthetic perception of fabric drape depends on the behaviour of fabric under static and dynamic states. The fabric drape is expressed clinging–flowing, dead–lively, limp–crisp, sleazy– full, etc. Drape of fabric mainly depends on the bending and the shear characteristics and can be subjectively evaluated by measuring bending rigidity (cantilever or loop method), drape coefficient by drape meter. – The body of a fabric means the overall substance between the edges of fabric, i.e. the perception of the total substance of fabric during use. The body of fabric is expressed by light–heavy, lofty–thin, bulky– sleazy, full–lean, etc. The fabric body is subjectively evaluated by measuring mass per unit area, thickness, porosity, density, etc.

28 Style


– Style is basically a visual aesthetic perception and can be perceived through colour, pattern and type of clothing. Clothing style is evolved from the combined perceptions of the textile arts and technology. The style of fabric is subjectively evaluated by measuring the colour value, depth of shade, weave structure, yarn structure, clothing pattern, fit of garment, etc. Surface texture – The surface texture of fabric means the tactility, surface roughness and pattern of fabric. This is basically tactile and visual perceptions of fabrics. It is generally expressed by the terms smooth–rough, dry–clammy, grainy–plain, slippery–sticky, slick– greasy, fuzzy or hairy–clean, soft–hard, pricky–soft, warm–cool, dull–lustrous, etc. Subjectively the surface texture can be evaluated by measuring surface roughness of fabric, fabric–fabric or fabric– other surface friction both static and dynamic conditions, optical reflectance of fabric surface, contact point or contact area at the fabric surface, surface fuzziness, etc. Resilience – It is the ability of a fabric to return to its previous position after deformation force is released. Generally resilience can be of different type, i.e. resilience from wrinkle or crease, compressional resilience, extensional resilience, liveliness, etc. The perception of wrinkle or crease resilience is the ability of fabric to recover from wrinkle or crease. Similarly, compressional resilience and extensional resilience are the perceptions of the resistance to and recovery from transverse compression and planer extension of the fabric respectively. The liveliness is the perception of the rate of recovery from small deformations. The resilience is generally expressed in terms of bounce–limp, lively–rubbery, lofty– mushy, snappy–stiff, nervous–dead, etc. The subjective evaluation of resilience characteristics of fabric is done by measuring compressional, tensile or bending characteristics of fabrics (e.g. Kawabata evaluation system), vibration damping, crease recovery angle, etc.


29

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17. 18. 19. 20.

21. 22.

ANDREEN J . H ., GIBSON J . W .

and WETMORE O. C ., ‘Fabric evaluations based on physiological measurements of comfort’, Textile Res. J. 23, 11–22, 1953. LI Y ., ‘The science of clothing comfort’, Text. Prog. 31(1/2), 2001. LAMOTTE R . H ., HOLLIES N . R. S. and GOLDMAN R. F ., Clothing Comfort, Ann Arbor Science, Michigan, 1977, 85–105. SLATER . K ., The assessment of comfort, J. Text. Inst. 77, 157–171, 1986. LI , Y ., Clothing, comfort and its application, Text. Asia 279, 29–33, 1998. WONG A . S . W., LI Y ., YEUNG P. K . W . and LEE P. W . H ., Neural network predictions of human psychological perceptions of clothing sensory comfort, Textile Res. J. 73, 31–37, 2003. LAMOTTE R . H ., Psychophysical and Neurophysiological Studies of Tactile Sensibility, in: Clothing Comfort, Ann Arbor Science, Ann Arbor, 1977. KAWABATA S., The Standardization and Analysis of Hand Evaluation, 2nd edition, The Textile Machinery Society of Japan, Osaka, 1980. ENGEN T ., Psychophysics in Sensory Systems II: Senses Other than Vision, editor WOLFE J . M., A Pro Scientia Viva Title, Boston, 1988, 104–106. LAMING D ., Psychophysics in Sensation and Perception, editors Gregory R. L. and Colman A. M., Longman, London, 1996, 97–123. FISKE A . P ., Structures of Social Life: The Four Elementary Forms of Human Relations. The Free Press, New York, 1991. STEVENS S . S ., ‘On the theory of scales of measurement’, Science 103, 677–680, 1946. BOLENDER J ., ‘Hints of beauty in social cognition: Broken symmetries in mental dynamics’, New Ideas in Psychology, 26, 1–22, 2008. BOX G . E . P . and BEHNKEN D . W ., ‘Some new three level designs for the study of quantitative variables’, Technometrics 2, 455–476, 1960. CARDELLO A . V ., WINTERHALTER C. and SCHUTZ H . G ., ‘Predicting the handle and comfort of military clothing fabrics from sensory and instrumental data: Development and application of new psychophysical methods’, Textile Res. J. 73, 221–237, 2003. BURGARD D . R. and KUZNICKI J . T ., Chemometrics: Chemical and Sensory Data, CRC Press, Boca Raton, 1990. MEILGAARD M. C ., CIVILLE , G . V . and CARR B . T ., Sensory Evaluation Techniques, 2nd edition, CRC Press, Boca Raton, 1991. STEVENS S . S . and GALANTER E . H ., ‘Ratio scales and category scales for a dozen perceptual continua’, J. Exp. Psychol. 54, 377–411, 1957. GREEN B. G ., DALTON P., COWART B., SHAFFER G ., RANKIN K . and HIGGINS J ., ‘Evaluating the labeled magnitude scale for measuring sensations of taste and smell’, Chem. Senses 21(3), 323–335, 1996. KAWABATA S . and NIWA M., Analysis of hand evaluation of wool fabrics for men’s suits using data of a thousand samples and computation of hand from the physical properties, in Proceedings of 5th International Wool Textile Research Conference 5, 413–424, 1975. KAWABATA S ., POSTLE R . and NIWA M ., Objective Specification of Fabric Quality, Mechanical Properties and Performance, The Textile Machinery Society of Japan, Osaka, 1982. HOLLIES N . R. S ., CUSTER A . G ., MOREN C. J . and HOWARD M . E ., ‘A human perception analysis approach to clothing comfort’, Textile Res. J. 49, 557–564, 1979.

30 23.

Science in clothing comfort HOLLIES N . R . S ., DEMARTINO R . N ., YOON H . N ., BUCKLEY A ., BECKER C . L .

and JACKSON ‘Improved comfort polyester, Part IV: Analysis of the four wearer trials’, Textile Res. J. 54, 544–548, 1984. YOO S. and BARKER R . L ., ‘Comfort properties of heat resistant protective work wear in varying conditions of physical activity and environment. Part II: Perceived comfort response to garments and its relationship to fabric properties’, Textile Res. J. 75, 531–539, 2005. HOLLIES N . R. S ., ‘Psychological scaling in comfort assessment’, In Clothing Comfort, editors Hollies N. R. S. and Goldman R. F.) Ann Arbor Science Publishers Inc., Michigan, 1977. WONG A . S. W ., LI Y ., YEUNG P . K . W . and LEE P . W . H ., ‘Neural network predictions of human psychological perceptions of clothing sensory comfort’, Textile Res. J. 73, 31–37, 2003. RUDD N . A ., LENNON S . J ., ‘Body image: linking aesthetics and social psychology of appearance, Clothing and Text. Res. J. 19(3), 120–133, 2001. DELONG M., The Way We Look: Dress and Aesthetics, Fairchild Publications, New York, 1998. HILLESTAD R., ‘The underlying structure of appearance’, Dress 5, 117–125, 1980. DELONG M. R. and LARNTZ K ., ‘Measuring visual response to clothing’, Home Economics Res. J. 8(4), 281–293, 1980. FIORE A . M., MORENO J . M. and KIMLE P . A ., ‘Aesthetics: A comparison of the state of the art outside and inside the field of textiles and clothing. Part III: Appreciation process, appreciator and summary comparisons’, Clothing and Text. Res. J., 14(3), 169–184, 1996. KUPFER J ., Clothing and aesthetic experience, editors DELONG M. and FIORE A . M ., Aesthetics of textiles and clothing: Advancing multi-disciplinary perspective, Monument, Intl Text. and Apparel Assoc. 97–104, 1994. CHATTARAMAN V . and RUDD N . A ., ‘Preferences for aesthetic attributes in clothing as a function of body image, body cathexis and body size’, Clothing and Text. Res. J. 24, 46–61, 2006. B R A N D R . H ., ‘Measurement of fabric aesthetics: Analysis of aesthetic components’, Textile Res. J. 34, 791–801, 1964. W.,

24.

25. 26. 27. 28. 29. 30. 31.

32. 33. 34.

3 Neurophysiological processes in clothing comfort

3.1

Neurophysiological perceptions

3.1.1

Sensory system of human skin

The structure of human skin is very complex. Figure 3.1 shows the structure of hairy skin which covers most of the human body. The skin has several layers. The overlaying outer layer is called epidermis, which consists of several layers of dead cells on top of a single living cell. The layer below epidermis is called dermis, which contains a network of blood vessels, hair follicle, sweat gland and sebaceous gland. Beneath the dermis are subcutaneous fatty tissues. The layers of epidermis are as follows: Stratum Germinativum, i.e. growing layer; Malpighion layer, i.e. pigment layer; Stratum Spinosum, i.e. prickly cell layer; Stratum Granulosum, i.e. granular layer; Stratum Lucidum and Stratum Corneum, i.e. horny layer [1]. The basic functions of human skin are [2] ● ● ● ● ● ● ● ● ● ●

To protect from external stimuli like light, heat, cold and radiation; To check of body fluids and tissues; Reception of stimuli like pressure, heat, pain, etc.; Biochemical synthesis; Metabolism and disposal of biochemical wastes; Regulation of body temperature; Controlling of blood pressure; Prevent penetration of noxious foreign material and radiation; Cushions against mechanical shock; Interspecies identification.

Skin is the interface between the body and its environment and it is highly stimulated and contains specialized sensory receptors to sense different external stimuli. There are mainly three types of stimuli, i.e. mechanical interactions with external objects, thermal interactions due to heat flow to or from the body surface, and damaging (traumatic and chemical) insults. In responding to these stimuli, the skin sensors generate different sensations, like touch, pressure, pain, warm, cold, etc.

31

Neurophysiological processes in clothing comfort

33

The Pacini’s corpuscle detects rapid vibration (200–300 Hz). Ruffini’s endings are responsible for detecting tension deep in the skin. The main functions of Meissner’s corpuscles are to detect and adapt the changes in texture. Merkle’s nerve endings detect sustained touch and pressure [3, 4]. Hair follcle’s nerve ends and free nerve ends are also mechanoreceptors. Hair follcle’s nerve ends sense the changes in position of hairs and the free nerve ends sense touch, pressure and stretch. The thermoreceptors are the sensory receptors which code the absolute and the relative changes in temperature, primarily within the safe temperature range and also respond to both constant and fluctuating skin temperatures. There are two types of thermoreceptors: cold receptors and warm receptors. The cold receptors have a peak sensitivity of around 25– 30°C and are excited by reduction in temperature. The warm receptors have a peak sensitivity of around 39–40°C and are sensitive to increase in skin temperature [4, 5]. The nociceptors are the sensory receptors which are responsible for sensing the pain, like heating the skin, strong pressure, or contact with sharp or damaging objects. These receptors have relatively high thresholds to act as warning devices that enable the organism to take protective action in time [5]. They react to potentially damaging stimuli by sending nerve signals to the spinal cord and brain.

3.1.2

Nerve endings in human skin

The sensory perceptions of human skin are governed by mainly two types of nerve endings in the skin layers, i.e. corpuscular endings and free nerve endings. Figure 3.3 illustrates the different types of nerve endings and nerve fibres in the skin layers. Corpuscular endings have small swelling on the nerve fibres and are responsible for different type of sensations, like touch, pressure, cold, heat, etc. The different types of nerve endings are Pacini’s corpuscles, Meissner corpuscles, Merkle’s nerve ending, Krause’s end bulb, Ruffini endings, hair follicle nerve ends and free nerve ends. The free nerve endings in subcutaneous fat are associated with pain fibre, and those projecting in to the epidermis may be associated with cold fibres or pain fibres [6]. Pacini’s corpuscles These mechanoreceptor nerve endings are responsible for pain and pressure sensations and detect gross pressure changes and vibrations. These are rapidly adapting receptors in the human skin. Due to any deformation in the corpuscles the pressure sensitive sodium ion channels are opened, which allow the sodium ions inflow and create a receptor potential. Pacini’s

34


Pacini’s corpuscles

Ruffini’s endings

Meissner’s corpuscles

Merkle nerve ending

Hair follicle nerve ends

Krause’s end bulb

Free nerve endings

3.3 Different nerve endings in the skin layers.

corpuscles are capable to vibration and can sense any vibration even from few centimeters. Their optimal sensitivity is 250 Hz and this is the frequency range generated at the finger tips by textures of size less than 200 μms [7]. These nerve endings respond when the skin is rapidly indented, but do not respond when the pressure is steady [8]. Meissner’s or tactile corpuscles These mechanoreceptor nerve endings are responsible for light touch. These rapidly adaptive receptors have highest sensitivity (lowest threshold) when sensing vibration of lower frequency. The tactile corpuscles are distributed throughout the skin, but the concentration is very high to those places where the sensitivity is high at light touch, e.g. palms, lips, face, tongue, fingertips, etc. In case of any deformation, the Meissner’s corpuscles cause an action potential in the nerve. As these are quickly adapting mechanoreceptors, the action potential generated in the nerves decreases rapidly and ultimately ceases. Due to this action the wearer stops feeling his clothing after certain time. Due to their superficial location in the dermis these mechanoreceptors are particularly sensitive to touch and vibrations, but they cannot detect properly because they can only sense that something is touching the skin [9].


35

Merkel nerve endings These mechanoreceptor nerve endings are responsible for providing information regarding pressure and texture and are classified as slowly adaptive type of mechanoreceptors. These nerve endings also have wide distribution in the human skin. These nerve endings are structurally rigid and are not encapsulated, which causes them to have a sustained response to mechanical deflection of the tissue of less than 1μm. Due to the sustained response to pressure these nerve endings are classified as slowly adapting. Merkel nerve ending is the most sensitive mechanoreceptor to vibrate at low frequency (within 5–15 Hz) [7]. Krause’s end bulbs The Krause’s end bulbs are the mechanoreceptors in the human skin and have the ability to detect low-frequency vibration. These can be found in some specific parts of human body, e.g. in the transparent lubricating mucous membrane that covers the eyeball and the area under the surface of the eyelid (conjunctiva), in the mucous membrane of the lips and tongue, etc. Ruffini endings This is a class of slowly adapting spindle-shaped mechanoreceptor and can be found only in the glabrous dermis and subcutaneous tissue of humans [10]. It is sensitive to skin stretch and contributes to the kinesthetic sense, e.g. control of finger position and movement. These mechanoreceptors sense and monitor the slipping of objects along the surface of the skin, e.g. slipping of garment on one’s body. These are located in the deep layers of the skin and register mechanical deformation within joints more specifically very small angle change (up to 2 degree) at continuous pressure state [11]. Hair follicle nerve ends These are the mechanoreceptors in the human skin present at the base of the hair follicle. These are sensory nerve fibres that wrap around each hair bulb. During bending or pulling of the hair these stimulate the nerve endings allowing a person to feel that the hair has been moved or pulled. One of the main functions of hair is to act as a sensitive touch receptor and it is done through this receptor. Greasy or oily glands are also associated with each hair follicle and these glands produce an oily secretion to help condition the hair and surrounding skin.

36


Free nerve endings The free nerve endings are the most common type of nerve ending and are most frequently found in the skin. These nerve endings are responsible for conveying sensory information from the periphery of body to the brain. The free nerve endings act as skin sensory receptors and are mainly used to detect pain sensations. Unlike those found in Meissner’s or Pacini’s corpuscles, the free nerve endings are un-capsulated and have no complex sensory structures. They penetrate the epidermis and end in the stratum granulosum [12] and can have different rate of adaptation, i.e. rapidly adapting, intermediate adapting and slowly adapting. Different free nerve endings work as thermoreceptors, mechanoreceptors and nociceptors. In other words, they express polymodality, i.e. having multiple stimulus modalities. They are responsible for sensing mechanical stimuli (e.g., touch, pressure, prick, stretch, etc.), temperature, or pain.

3.2

Mechanical and thermal receptors

Various sensory receptors are responded by mechanical and thermal stimuli generated by surroundings, results in different perceptions, like touch, pressure, tactile, warm, cold, humid, prickle, itch, etc., which affect the comfort sensations of wearer.

3.2.1

Sensations related to mechanical stimuli

The mechanical sensory perceptions of clothing are mainly of four types, namely wear sensations during activity; prickle, itch and rashes; touch and pressure sensations; and roughness and scratchiness sensation [5]. Wear sensations during activity Clothing is continuously in contact with the skin of most part of the human body and this contact may be dynamic as well as static. During activity or movement of a clothed person the dynamic wear sensation changes rapidly depending on the situations. When a person moves, the cloth touches different parts of the body which results different dynamic sensations as the area of contact is large and spreads over parts with different sensitivity. Due to the change in activity level the human body frequently changes its physiological parameters such as rate of sweating, temperature of skin and moisture at the skin surface, which generates various new thermal stimuli. Depending on the interactive phenomenon of clothing with the skin in terms of heat and moisture the mechanical stimuli also changes. Clothing also moves towards and away from the


37

skin frequently due to the body movement, which also induces new mechanical stimuli frequently. Prickle, itch and rashes One of the very common types of discomfort sensations related to mechanical stimuli is prickle. The wearers always complain about the prickle for those clothing which are used next to skin. For example, a person feels prickle sensation when he wears a woollen inner garment, especially with coarser wool in hot and humid condition. Prickle is usually described as the sensation of many gentle pinpricks. It’s a common perception that the prickle sensation associated with wool is related with skin allergic response. The degree of discomfort caused by prickle varies with person, skin type, humidity and temperature of atmosphere as well as in the microclimate, type of fibre used in clothing, etc. The relationship between prickle and itch sensation and human cutaneous small nerves have been studied [13], where skin sensations were tested on the forearms of different volunteers, in whom anoxia nerves blocks of the forearms were produced by inflating a blood pressure cuff (about 270 mm Hg) on the upper forearm, as shown in Fig. 3.4.

Pressure exerted by blood pressure cuff

Sensation detection point

3.4 Prickle and itch sensation test setup [5].

The above study [13] reported that both touch and prickle sensations change with time (Fig. 3.5) and the prickle sensations are associated with small nerve fibres. It is observed from Fig. 3.5 that touch sensation dropped consistently, initially at slower rate and then at very fast rate. On the other hand the prickle sensation, evoked by pain, temperature and fabric, initially increases and then drops rapidly. The touch sensation is completely lost after about 20 min, but prickle sensation remained until about 40 min and the point of complete anesthesia starts [5, 13].

38


Sensitivity Level

Touch sensastion

Prickle sensation

Loss of skin sensation Time 3.5 Time course of loss of prickle and touch sensations [5].

A number of studies [14, 15] have been carried out to understand the prickle characteristics of fabrics. Wills [14] reported that for the initiation of pain sensation the summation of responses from the pain group of nerves is necessary. Gransworthy et al. [15] carried out extensive study to understand the causes of prickle and itch from the skin contact of fabrics. They conclude from their results that fabric-evoked prickle is the result of low-grade activity in nociceptors and that the stimuli are protruding fibre ends exerting loads of approximately 75 mgf or more against the skin. They have also observed that to have prickle sensation a minimum number of high-load bearing fibre ends or certain minimum skin contact area with fabric is required. Prickles from the fabrics could not be perceived if the density of high load bearing fibre ends is less than 3 per 10 cm2 of the fabric, or the skin contact area is below 5 cm2 [5, 15]. The fabric prickle sensation depends mainly on the following important parameters [5]: ●

●

●

Males have higher thresholds and more variations to sensitivity to prickles than females. This means that female skin is more sensitive towards prickle sensation, which is due to differences in features of skin sensory nerves among male and female. With the increase of age of a person the hardness of his outermost skin layer increases, thus the prickle sensitivity decreases gradually with age. Due to the difference in skin hardness a kid may feel more prickle sensation than an old person with a particular type fabric touching the skin. The free nerve endings, which sense pain (nociceptors), are generally located very close to surface of hairy skin, but not in glabrous skin. On the human body, glabrous skin is skin that is hairless. It is found


●

39

on fingers, palm of the hand or the sole of the foot. Due to absence of nociceptors close to the skin surface the prickle sensation due to fabric is not felt with figures, palms or feet. It has been observed that in hot and humid environment a person feels more prickle sensation. The outermost layer of the epidermis consists of dead cells, which is known as stratum corneum. The stratum corneum becomes soft in humid condition and the protruding fibres from garments can easily penetrate through it, which results prickle sensation. It has also been reported [15] that for constant humidity the prickle sensitivity increased with the increase in ambient temperature in the range of 12–32°C. This is due to increase in the skin moisture content due to perspiration in hot and humid conditions, which result in the increase in softness of stratum corneum.

In the classical definition, ‘Itching is an unpleasant cutaneous sensation which provoke desire to scratch’ [16]. Itch sensation can originate in the peripheral nervous system or in the central nervous system [17] and the itch receptors are only found on the top two skin layers, the epidermis and the epidermal/dermal transition layers. Itch originating in the skin can be induced by a variety of stimuli, including mechanical, chemical, thermal and electrical stimulation. Normally the itch sensation develops the activation of some superficial skin pain receptors. The pain receptors responsible for itch may be of a different type to those responsible for prickle sensation. The sensations of itch and pain are generally considered to be dependent of each other, but in a recent study [18] it was found that itch has several features in common with pain, but exhibits notable differences. The prickle and itch can be a major detrimental factor of whether a person can wear the garment made of wool, acrylic or some other fibres comfortably. The skin irritation on wearing wool garment directly on the skin has long been thought to be due to the result of an allergy to the wool itself. But, researches in this area revealed that very few people have a true allergy to wool, and that it’s the mechanics of the fibre itself that irritate the skin. People with sensitizing skin conditions, such as eczema or atopic dermatitis, who are already more susceptible to discomfort from wearing clothing against their skin made with these materials. The pain receptors in the skin are mainly responsible for the prickle sensation during wearing of garments made of wool and other fibres. Prickle and itch sensations, on contact with natural and synthetic fibres, vary from person to person, and for some sensitive person the skin can even redden from the constant touch with these fibres. The itch and prickle sensation can also depend on individual factors, like skin thickness, age, temperature and moisture on the skin. The thinner skin is more sensitive. Younger

40


persons have thinner skin than an old person, so generally the skin sensitivity towards itching and prickle is more for younger person. The itch sensation increases with temperature of the atmosphere and humidity of the skin. Apart from the type of fibre, the physical characteristics fibres, such as length and diameter, play role in causing skin discomfort. For example, shorter fibres generates the perception of prickle, as there are more fibre ends to be felt in any given surface area of a garment. Similarly, coarser fibres are likely to intensify the prickling sensation than their finer counterparts due to higher bending rigidity. So, a person can try to prevent or reduce the prickle and itch sensations in their garments by proper selection of materials. Skin rashes or localized skin reddening or localized skin irritation occurs in the small proportion of the skin. There are different causes of skin rashes, and the garments with prickle and itch sensations are one of the causes of generation of skin rashes. The mechanical stimulation of skin pain receptors from prickly fabrics are the main causes of garment related skin rashes. The probable mechanism of skin rashes due to prickle and itch is known is axon reflex [19, 20], which is a response brought on by peripheral nerve stimulation. Rashes due to clothing may occur very rapidly within minutes or very slowly in hours and can be relieved quickly after fabric is removed from the skin. But, in case the garment is in contact with skin for very long time it may result a severe reaction. Touch and pressure sensations Human body can evoke the sensation of touch and pressure at any point on the surface, but the sensitivity varies from one region of body to another. In the process of fabric–skin contact and mechanical interaction during wear, clothing exerts pressure and dynamic mechanical stimulation to the skin which in turn triggers various mechanoreceptors and generates a wide range of touch and pressure sensations. The mechanoreceptors, responsible for touch and pressure sensations, are sensitive to stimuli that distort their cell membranes. The receptors, responsible for touch and pressure sensations, contain mechanically regulated ion channels, which open and close in response to mechanical actions on the skin surface. The receptors responsible for touch and pressure sensations are mainly tactile receptors (i.e., free nerve endings, root hair plexus, Merkel’s discs, Meissner’s corpuscles, Pacinian corpuscles and Ruffini corpusles) and baroreceptors. The tactile receptors provide the sensations of touch, pressure and vibration. Distinctions between all these sensations are not very well defined. Fine touch and pressure receptors are extremely sensitive and have relatively narrow receptive fields and provide detailed information about a source of stimulation, including the


41

exact location, shape, size, texture and movement. On the other hand, the receptors for rough touch and pressure provide poor localization and information. The baroreceptors monitor changes in pressure exerted on the body by clothing due to movement of a clothed person. The baroreceptors consist of free nerve endings that branch within the elastic tissues in the walls of organs. During activity the clothing exert different levels of pressure on the skin which stretch or recoil tissues in the walls, and the information is then passed on to centres in the brain. Roughness and scratchiness sensations Roughness and scratchiness sensations depend on the surface texture of fabrics and the way the fabrics move over the human skin surface. During activity of a clothed person, the fabric moves across the underlying skin and the fabric to skin friction (both static and kinetic friction) resisting that movement forces the skin to displace. This displacement of skin stimulates the sensory receptors that are responsible for touch sensation. As the surface roughness of fabrics increases, this displacement of skin also become more and the sensory receptors detect this difference in sensation. Higher fabric to skin friction results more skin abrasion [5, 21].

3.2.2

Sensations related to thermal stimuli

The skin acts as a barrier between the organism and its environment. For effective designing of comfortable clothing, it is essential to understand the human thermal physiology, heat and moisture transfer from the skin surface, and human thermal comfort. Humans maintain their core temperatures within a small range, between 36 and 38°C. Skin is the most important organ of human body through which heat and moisture flow to and from the surrounding environment to maintain the heat balance. The skin also contains thermal sensors that take part in the thermoregulatory control, and these affect the person’s thermal sensation and comfort [22]. The complex vascular systems and sweat glands in the skin help to change the conductance of skin in response to thermoregulatory demands of the body. Many researchers have established that when the skin is touched with small warmth and cold stimulators, some spots on the skin feel warm and/or cold, others do not. The human skin contains four types of thermally sensitive nerve endings (thermoreceptors), namely cold, warmth, hot pain and cold pain [23, 24] and each thermoreceptor is activated in a specific temperature range (Fig. 3.6).

42

Science in clothing comfort Warmth receptors

Discharge frequency (per second)

10

8

Hot pain receptors Cold receptors

6

Cold pain receptors

4

2

0 5

15

25

35

45

55

65

Temperature (°C) 3.6 Discharge frequencies of a cold receptor, a warmth receptor, and cold and hot pain receptors at different temperatures [22].

All these nerve endings sense the temperature of skin and transmit the information to the brain. Cold and warmth receptors in the human skin are responsible for sensing normal environmental temperatures which are not harmful to human body. The harmful temperatures (i.e. too hot or too cold), which are likely to damage an organism are sensed by sub-categories of nociceptors (i.e. cold pain and hot pain receptors) that may respond to extreme cold or heat [25]. It can be observed from Fig. 3.6 that the warmth receptors start sensing at the temperature around 30°C and become inactive at around 50°C with highest impulse frequency at around 45°C. Normally the hot pain receptors are active at temperature beyond 50°C and at that time the warmth receptors are inactive. Similarly, the cold receptors are active within the temperature range from 7°C to 42°C with highest impulse frequency at around 25°C. Normally the cold pain receptors are active at temperature below 10°C and at that time the cold receptors are inactive. The warmth and cold receptors in the human skin are distributed in different concentrations in the different parts of the body. In general the numbers of warmth thermoreceptors are much less than the cold receptors. Figure 3.7 shows the distribution of the cold and warm receptors in the different parts of the human skin [22, 26–28]. The cold thermoreceptors are located at upper layer of dermis, i.e. immediately beneath the epidermis, at an average depth of 0.15–0.17 mm, whereas the warmth thermoreceptors are located in the dermis and below cold thermoreceptors. The location of the warmth thermoreceptors is within the upper layer of the dermis at an average depth of 0.3–0.6 mm [26, 29, 30]. Due to the presence of higher numbers and shallower depth of cold thermoreceptors as compared to that of warmth thermoreceptors, the


43

Number of cold receptors (per cm2) Number of warmth receptors (per cm2) 10 Thigh

Nose Other parts of face

8 6 4

Finger (inner part)

2

Chest

0

Finger (outer part)

Back of hand

Palm of hand

Forearm

3.7 Typical number of cold and warm thermoreceptors in human skin.

Impulse frequency (per second)

humans are more sensitive to danger from cold than from heat [22]. The thermoreceptors are in general dynamic in nature which helps in adaptation with different climatic conditions and determine the thermal sensation and comfort responses of a person. Figures 3.8 and 3.9 show the static and dynamic responses respectively of thermoreceptors (warmth and cold), i.e. in constant temperature condition and under the abruptly changing temperature conditions.

Cold receptors

Warmth receptors

5

15

25

35

45

55

65

Temperature (°c) 3.8 Responses of thermoreceptors under constant temperature [22].

44


(c)

Impulse frequency of warmth thermoreceptors

(b)

Impulse frequency of cold thermoreceptors

Dynamic temperature profile

(a)

Time 3.9 Responses of thermoreceptors under changing temperature [22].

The static temperature responses of cold and warmth thermoreceptors have been discussed earlier also. It is observed from Fig. 3.8, the sensitivity of cold thermoreceptor gradually increases with the increase in temperature and after certain point it starts reducing. Figure 3.9(a) shows the dynamic changes in temperature profile with time, where initially for some time the temperature remains at lower level. After certain time the temperature is increased abruptly to a high level and then kept at high level for some time. After then the temperature is again dropped abruptly to a lower level and kept at that level for remaining time. The thermoreceptors are capable to adapt the changing temperature conditions and react accordingly. Figure 3.9(b) shows that in case of an abrupt increase in temperature, the cold thermoreceptors are strongly stimulated at first, sending impulses at a high frequency, but this stimulation fades rapidly during the first minute following the increase in temperature, and then progressively more slowly until it reaches a steady level. On the other hand, in the case of warmth thermoreceptors (Figure 3.9c) the impulse frequency increases abruptly with the sudden drop in temperature and again this stimulation fades rapidly during the first minute following the drop in temperature. The thermoreceptors respond to steady temperature states at this lower rate. We generally feel much colder or warmer when the temperature of the skin changes abruptly


45

than when the temperature remains at the same level. For example, the stronger sensation of cool or warmth felt upon entering a cold pool or a hot tub [22]. The temperature of skin plays an important role for any thermal sensation and a person feels comfortable within a narrow range of skin temperatures. Figure 3.10 shows that different body parts have different ranges of comfortable temperatures. Lower comfortable temperature (°C) Higher comfortable temperature (°C) Foot

Head 40

Face

30 Lower leg

Breath

20 10

Thigh

Front neck

0

Hand

Chest Back

Lower arm Upper arm

3.10 Ranges of comfortable skin temperature by body part [22].

3.2.3

Sensations related to humidity stimuli

There are different types of receptors in the human skin, which sense different types of physical stimuli including touch, pressure, thermal, cold and pain. However, there is no receptor in the skin that responds for moisture or dampness sensation [31].

3.3

Sensory perceptions of human body

Mechanoreceptors are responsible for conveying stimuli of touch or pressure from skin to the dermal and subdermal receptor sites at which they are transduced into neural signals, and through the tactile receptors that perform the transduction. The sensory perception of human skin is commonly measured by the two-point threshold method. This is the minimum distance between two point-like indentations applied to the skin below which only a single point of contact is detected with the senses (Fig. 3.11).

46


Two point-like indentations

Two point-like indentations

3.11 Measurement of sensory perception by two-point threshold method.

The pressure applied below the two-point threshold distance feels like one point, and beyond that one can feel distinct differences in pressure. This value varies from 2.5 mm in the fingers, up to as much as 50 mm for other body regions [37] as shown in Fig. 3.12 and Table 3.1 [32]. Little finger Hallux 50 Ring finger Middle finger Sole 40 Calf

30 20

Thigh

10

Belly

0

Back

Index finger Thumb Palm Forearm Upper Arm

Breast Upper Lip Nose

Cheek

Shoulder Forehead

3.12 Mean two-point threshold distance (mm) at different body parts [32].

The two-point threshold for any part of the body is determined by the size of the receptive fields and the extent of overlap. Tactile sensation varies widely from person to person and depends very much on the nature of the stimulus that is used and properties of the stimulus, such as frequency, duration and amplitude [33, 34]. The two-point tactile sensations change with the frequencies and amplitude of vibration [35]. In addition to twotype of stimulation can be measured by the minimum noticeable intensity point threshold distance, the psychophysical thresholds for a particular


47

Table 3.1 Mean two-point threshold distance at various body parts [32] Body parts

Mean threshold distance (mm)

Little finger Ring finger Middle finger Index finger Thumb Palm Forearm Upper arm Shoulder Forehead Cheek Nose Upper lip Breast Back Belly Thigh Calf Sole Hallux

4 2.5 2.5 2.5 3 10 38 43 36 17 7 7 5 32 40 35 45 50 21 9

of stimulation (Imin). It is the minimum amplitude of vibration of stimulation, keeping other stimulus properties constant, when a person starts sensing it. On the other hand, the maximum stimulus intensity (Imax) is typically taken to be the threshold amplitude for pain perception. The maximum dynamic range R (decibels), attainable by stimulation with a particular stimulus type, is expressed as [32] R (dB) = 10 log10 (I max/Imin)

(3.1)

The maximum dynamic range (R) depends on the type of stimulus, frequency of stimulation, location on the body, etc. There are different types of qualitative tactile and vibrotactile sensations in our daily activities. The tactile sensations include pressure, texture, prick, puncture, thermal properties, softness, wetness, slip, adhesion, friction, dynamic contact and release), pain, object features (shape, edges, embossing), etc. Some of the vibrotactile sensations are tickling, itch, vibration, and buzzing sound [38]. Most of these sensations are linked with different tactile stimulations of a person.

3.3.1

Transmission of neurophysiological sensations

It is important to identify the specific actuators required to produce the different types of human neurophysiological sensations with different

48


tactile displays, like pressure, touch, prickle, etc. Basically the different types of tactile displays transmit energy to the skin surface and then from skin to different tactile receptors in the skin. There are different methods of flow of tactile responses, namely (i) low frequency and low amplitude mechanical deformation; (ii) vibrotactile stimulation; (iii) electrotactile stimulation; (iv) force feedback displays; (v) thermal sensation; (vi) air or liquid jets or currents [32]. The low frequency and low amplitude mechanical deformation is responsible for sensing contact of any object with our skin, which may be continuous or intermittent. By this method human is able to distinguish between continuous contact with an object, and the intermittent contact, in which an object is brought in and out of contact with the body part. The human skin has high sensitivity to the intermittent contact [39]. The vibrotactile stimulation senses the matters when they are vibrating against the skin, and can sense a frequency of about 250 Hz [32]. Vibrations may be effectively transmitted through an air gap, again due to the high intermittent contact sensitivity. In the electrotactile stimulation, currents are passed through the skin, which excite the sensory systems directly rather than the tactile receptors themselves. Current may be supplied by electrodes of different types, or by fine wires inserted into the skin. Different nerves can be excited differently through the design of the drive signal and electrical contacts. The force feedback displays are the kinesthetic tactile sensory systems and they interact with the different types of sensations, like friction, vibration, etc. The thermal sensations are related to inward or outward heat flow through skin by conduction via a medium, convection, or radiation. Heat is transferred to heat-sensitive receptors by conduction through tissues [32]. The air or liquid in the form of jets or currents stimulate either hair follicle receptors by moving hairs or by different types of mechanoreceptors by exciting with forces or vibrating skin [40].

3.4

Physiological requirements of the human body

3.4.1

Metabolic heat and body temperature

In unusual cases, if the core body temperatures drop below 32°C or raise above 43°C there is a definite risk of life, but for normal activity of the body still a narrow range, i.e. between 36°C and 38°C is required [41]. Human skin acts as the barrier between the internal body organs and the external environment and it has important functions in keeping the body temperature nearly constant. This is done mainly by controlling thermal radiation, by adjusting the diameter of peripheral blood vessels and by


49

sweating. The distribution of skin temperature throughout the body is not uniform (i.e. lower in the extremities than in the trunk) and it also changes with environmental temperature (i.e. it is higher in summer than in winter). The disturbance in stability of body temperature due to heat (in summer) or cold (in winter) is controlled by internal physiological mechanisms of body. In winter the reduced temperature is controlled by increased metabolic rate and constriction of the peripheral blood vessels, whereas in summer the sweating process is activated to reduce the body temperature [42]. The blood circulation through the vascular system in the skin (Fig. 3.13) assists to the principal mechanism of thermal equilibrium. In winter, the heat is retained within the body by reducing blood circulation (vasoconstriction) to the skin. In case the vasoconstriction is insufficient in restricting the body heat, the body starts producing additional metabolic heat through tensioning the muscles, starting with ‘muscle tone’ in the skin, and then leading to shivering which begins in trunk region and transmitted to the limbs [43]. A person stops shivering when he wears cloth, which is due to the insulation provided by the clothing. On the other hand, in case of increased body heat, the vascular system in the skin enhances the release of body heat through the skin (vasodilatation). The physiological mechanisms are incapable to completely control the body temperature in a cold climate. So, additional clothing or external heating are required for maintaining the body temperature. Hair Epidermis

Dermis Artery Veins Subcutaneous tissue 3.13 Vascular system in the skin [1, 22].

The human body requires about 40 kcal/h/m 2 body area for basic activities. The heat production rate increases rapidly during heavy activity and the produced heat has to be dissipated effectively. Figure 3.14 shows the changes in body core temperature with time at different levels of activity.

50

Heavy activity

32

34

36

Walking Resting

30

Body core temperature (°C)

38

40


Time 3.14 Changes in body core temperature with time at different levels of activities [44].

It can be observed from Fig. 3.14 that during rest the body core temperature remains almost constant, i.e. approximately around 37°C. In case when a person is walking the body temperature initially increases and after that in remains constant. But, in case of heavy activity the body temperature increases very rapidly. The heat balancing is done by the exchange of heat with the environment mainly by radiation and to some extent by conduction [44]. Depending on the extent and direction of the temperature gradient between the body and the surrounding object or environment, a person may either gain or lose heat by radiation. The heat exchange between the human body and the surrounding object or environment by conduction is through a surrounding medium in contact with the skin. Due to the lower thermal conductivity of air and most of the clothing materials, the conductive heat flow is usually of rather small quantitative importance. On the other hand, the thermal conductivity of the liquid medium (e.g. water) is very high. Due to this the insulation characteristics is almost lost when the garment becomes wet.

3.4.2

Metabolic heat loss and sweating

The release of excess metabolic heat happens through the secretion of sweat through sweat glands onto the surface of skin. The sweating rate of a person can go up to maximum of 4 l/h [45]. The cooling of human body, in hot environment, is achieved not by sweating but by the evaporation of sweat. In very hot and humid condition, although we sweat heavily, but we normally do not feel cool. This is due to the fact that the sweat cannot evaporate and take latent heat from our body at high humidity condition and it simply drips of the body. In dry climate the sweat generally gets


51

evaporated, without wetting the skin surface, by the heat supplied by the skin surface (insensible evaporation). No cooling effect is achieved and the body temperature rises steeply. In case there is little wind blowing, that helps in evaporation of sweat even in hot and humid climate. The evaporative cooling in a given climatic condition depends on the fact that whether a person gets used to that climate, which is known as acclimatization. The higher temperature of the surrounding environment does not ensure that the sweat will always evaporate. The sweat may start dripping for a person who is not used to the hot climate and the body heat transmission through evaporation becomes ineffective. On the other hand, if the same person gets used to the same hot climate he will look drier and feel cooler due to evaporative cooling. The rate of sweating depends on the number of participating sweat glands and the output of each active gland. The evaporative heat loss becomes more effective if the sweat, coming out from the active sweat glands, covers the body evenly. The number of sweat glands per unit area is different at different parts of the body, e.g. very high concentration is in the front and back of trunk, back of hand, forearm, upper arm, forehead; medium concentration in arms, legs, cheeks; and very low concentration in soles, palms, armpits, inside of thighs [22, 46]. The distribution of number active sweat glands/cm 2 in some of the human body segments is shown in Fig. 3.15 [47]. Forearm 400 Cheek

350

Upper arm

300 250 200

Temple

150

Upper back of hand

100 50 0

Forehead

Trunk-anterior chest

Trunk-scapular region of back

Thenar eminence Leg

3.15 Distribution of sweat gland (glands/cm 2) in human body [32].

52


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4 Tactile aspects of clothing comfort

4.1

Tactile comfort sensations

4.1.1

Human tactile responses

The tactile sensation of clothing is very important for clothing comfort. Tactile comfort of clothing is based on the human sensory response to clothing materials and is sensed by a variety of thermal, physiological and mechanical stimuli [1, 2]. During wearing of clothing, the clothing materials make contacts with the skin where numerous receptors are located which give rise to various sensations felt by the wearer. Touch and tactile properties are very important parameters to be considered for fabrics that come into direct contact with the human skin. On wearing clothing, the tactile sensations are mainly produced when clothing contacts and impacts local skin of human body [3, 4]. The tactile parameters of all these fabrics affect the clothing comfort, because the sensory feel of clothing materials is dependent on the mechanical stimuli due to pressure and frictional forces [2, 5]. When the clothing materials come into contact with the human skin they stimulate the various mechanoreceptors (e.g. free nerve endings, root hair plexus, Merkel’s discs, Meissner’s corpuscles, Pacinian corpuscles and Ruffini corpusles) present in the different layers of skin (i.e. epidermis, dermis and subcutaneous zone). The face, the torso and the hand are the most tactile sensitive skin areas of human body [6]. The textile fabrics are the thin sheet materials and the typical tactile characteristics of textile materials are the flexibility, compressibility, surface texture, extensibility, friction, etc. The surface and bulk properties of textile fabrics are one of the most frequently used tactile characteristics of fabrics. Fabric softness is a complex tactile sensation which determines the initial tactile perception of a wearer towards the clothing even before the wearing of the clothing. The fabric softness is commonly perceived by pressing or squeezing with fingers. The softness or fullness sensations of fabrics are objectively expressed by compressibility and resilience characteristics. The fabric frictional characteristics influence the tactile

54


55

sensations of clothing to a great extent. The Merkel’s discs and the Ruffini endings play are mainly responsible for tactile sensations related to surface texture and touch of fabric [7, 8]. Due to their location, receptive field and response type, the Pacinian, the Merkel and the Meissner mechanoreceptors can characterize the roughness of fabrics, whereas the Ruffini and Meissner mechanoreceptors can characterize the friction between skin and fabric [9]. The causes of the prickle from the skin contact of fabric can be detected by the pain receptors [10].

4.1.2

Tactile characteristics of clothing

The low stress mechanical properties of fabrics (e.g., bending, shear, extension) are objectively measured to assess the tactile characteristics of fabrics. Kawabata Evaluation Systems for Fabrics (KES-F) and FAST (Fabrics Analysis by Simple Tests) systems are available for measuring the fabric handle related characteristics. But, as far as the tactile responses are concerned, all the low stress mechanical characteristics directly or indirectly stimulate the touch, pressure, roughness and other mechanoreceptors of human skin. The prickle sensation due to clothing is one of the most irritating discomfort sensations for clothing wear next to skin. A special type of pain nerve is responsible for prickle sensation. Individual protruding fibre ends from a fabric surface are responsible for triggering the pain nerve endings, when contacting the skin. Perceptions of prickle sensations require combined responses from a group of pain nerves. Fabric prickliness can be measured by using low pressure compression testing, laser counting of protruding fibres and a modified audio pickup method [11]. Matsudaira et al. [11] modified Kawabata compression tester (KESF-3) to measure the relationship between applied pressure and fabric thickness at the initial stage of fabric compression, i.e. when bending of fibres protruding from fabric surface takes place during compression. WRONZ developed laser hairiness meter where the fibres protruding from the fabric surface are counted by laser beam. This gives fairly good indication of prickliness of fabric. The sensitivity of the instrument was found inadequate for the detection of all the fabric surface hairs, where the coarser and stiffer hairs are preferentially detected [2]. Matsudaira et al. [11] also used a modified audio pick-up technique (Fig. 4.1) for measuring the mean force per contact with the protruding fibre. They have observed that this technique is the most effective measure of fabric prickle and the result obtained from this instrument correlated well with the subjective perception of fabric prickle.


57

Subjective scratchiness rating

Itchiness is also found to result from activation of some superficial pain receptors. The perception of itchiness in clothing is highly correlated with perception of prickliness and both the sensations are considered as important tactile sensory components [12, 13]. The sensation of fabric itchiness depends on the fibre diameter, fabric thickness at low and high pressures, and fabric surface roughness [2]. The frictional interaction between fabric and skin during contact are the key factors determining some of the important tactile sensations, i.e. perceptions of roughness, smoothness and scratchiness. Presence of moisture at the skin surface alters the intensity of fabric roughness perceptions due to change in friction. As moisture content increases the friction between skin and fabric surface increases, which results in displacement of skin and thus more and more touch receptors are stimulated. This is the reason for a fabric that is perceived to be comfortable when a person is not sweating may be perceived to be uncomfortable under sweating conditions [2]. Mehrtens and McAlister [14] reported that the scratchiness was the greatest source of discomfort and in terms of scratchiness and clinginess men are slightly more critical than women, which is due to the fact that generally men perspire more than women. They have developed an objective method to test the scratchiness of fabrics in which a fabric was passed over a microphone yielded a scratchiness sound. In that method, the fabric was moved at the speed of 7 yd/min across a brass sheet above a microphone and the signal from microphone was then sent through an integrating amplifier into a recorder. They have observed good correlation between subjective sensation and objectively measured values of fabric scratchiness, as shown in Fig. 4.3.

Objective scratchiness rating 4.3 Subjective and objective scratchiness ratings [14].

58


4.2

Fabric handle attributes for expressing tactile comfort

The quality of fabric is generally perceived through tactile sensations. Consumers of textile products always touch the product before buying them and the majority of rejections are due to poor hand. Improved finishing, new fibres, speciality textile products are successful in clothing industries mainly because of improvement in fabric handle. Fabric handle may be defined as the human tactile sensory response towards fabric, which involves not only physical but also physiological, perceptional and social factors; this very fact complicates the process of fabric hand evaluation tremendously [15–20]. The tactile comfort of clothing is the sense of touch of fabric which is directly related to the fabric handle characteristics. Fabric handle has been defined by the subjective assessment of a textile obtained from the sense of touch [21] and can be assessed by measuring the low stress mechanical characteristics and surface characteristics of fabrics. The ease of motion and the level of pressure generated by the clothing on to the human body are directly related to the tactile comfort characteristics of clothing. Traditionally, the fabric handle characteristics are evaluated subjectively by sensing the roughness, smoothness, softness, harshness, flexibility, thickness, scratchiness, prickle, etc. against the skin. Objective measurement of fabric handle may be of major commercial significance if they, for example, assist in explaining handle assessment or provide some information about the tactile sensations of fabrics. It is also necessary to examine the subjective assessment of handle before examining its relationship to fabric mechanical and surface properties. In textile and garment industries the fabric handle is traditionally assessed subjectively. Peirce [16] obtained objective measures of fabric handle, identified the importance of bending, compression, specific volume, and surface properties. Howorth and Oliver [22], for the first time, identified the three quality attributes which directly affect the handle of suiting fabrics. These quality attributes are surface smoothness, fabric stiffness and thickness. A series of fabric handle attributes have been proposed by the “Japanese hand evaluation and standardization committee for men’s clothing for different climatic conditions” [23]. For men’s summer clothing, they have identified four primary fabric handle related quality attributes, namely fullness, springiness/stiffness, crispness and hardness. On the other hand, for men’s winter suiting fabrics they proposed three main fabric handle related quality attributes, namely smoothness, fullness, and springiness/ stiffness. They have also established standards to describe the intensity of the above primary hand attributes on a 10 point scale. Studies [24–26] have been done on the objective measurement of fabric handles by measuring fabric low-stress mechanical and surface properties. The KES-


59

F instruments were used to measure fabric tensile, shear, bending, compression, and surface properties. Winakor et al. [27] have considered the different physical characteristics of fabrics, like stiffness, roughness, and thickness which represent the bending, frictional, and compressional deformations, respectively, that occur in handling a fabric. They have selected nine pairs of polar adjectives (i.e. limp–crisp, flexible–stiff, firm– sleazy, scratchy–silky, fine–coarse, smooth–rough, soft–hard, light–heavy, thin–thick) to express the tactile sensory attributes of fabrics which are related with the fabric characteristics, i.e. low stress mechanical and other physical properties. Figure 4.4 shows the interrelationships between fabric characteristics and pairs of polar adjectives of tactile sensations. Firm–sleazy Limp– crisp

Smooth– rough Flexible– stiff

Scratchy –silky

Bending

Fine–coarse Friction

Fabric characteristics Thickness

Compression Soft– hard

Mass per unit area

Thin–thick

Light– heavy

4.4 Fabric characteristics and tactile attributes.

Kawabata [28] related the different fabric properties with the individual hand expressions. Figure 4.5 shows the different hand expressions (given within oval) proposed by Kawabata [28] and related fabric characteristics (given within the rectangles). The hand expressions proposed by Kawabata are in Japanese terms. The equivalent English terms are given in parenthesis.

4.3

Assessment of fabric handle characteristics

4.3.1

Subjective assessment

Traditionally the handle characteristics of fabric are assessed subjectively. The psychologically based subjective assessment of fabric handle is first reported by Binns [29, 30]. He has proposed an early analysis of the mechanism by which the fabric handle was assessed by judges. He found that the judgment on fabric handle of a number of experienced judges

Hand expression

Hari (Anti-drape stiffness ) Fukurami (Fullness and softness)

A feeling coming from a combination of bulky, rich, and well-informed impressions. A springy property in compression and thickness, accompanied by a warm feeling reflects this quality.

4.5 Primary hand expressions and related fabric properties.

Influenced by bending, shear, and compression properties.

A feeling coming from a crisp and ridged fabric surface. Observed in density woven fabrics made from hard twisted yarns. The fabric gives a cool touch feeling.

Influenced by fabric thickness, weight, tensile, shear, and surface properties.

Kishimi (Scroopy and silk-like feeling) Shari Shinayakasa ( Crispness) (Soft, flexible, and smooth feeling)

Koshi (Stiffness)

Primarily relates to bending stiffness. Fabric with a compact weave structure and made from springy and elastic yarn results a high koshi value.

The opposite of limp conformability

60 Science in clothing comfort


61

gives a more reliable and accurate objective grading of fabric handle than is possible from an individual judge’s grading. For the assessment of fabric handle, researchers have adopted the factor analysis technique, where they attempted to identify the underlying interrelationships in the handle assessments of a range of fabrics. In the factor analysis the researchers have isolated three important factors responsible for fabric handle, namely smoothness, stiffness and bulk, where the fabric bulk is directly proportional to area density and thickness of the fabric [31]. Attempts have been made to assess the fabric handle characteristics subjectively based on psychophysical concepts [32–34]. Psychophysical concepts from both decision theory and information theory have been used and proposed the four different sensory attributes which correspond to the four fabric characteristics, namely smoothness, stiffness, bulk properties and warmth. This psychophysical concepts theory is analogous to the Young-Helmholtz theory [35], which proposes three colour receptors in the eye, one each for red, green, and blue, for perceiving colour [36]. Kawabata [37, 38] developed subjective assessment technique of fabric handle characteristics based on two assumptions, i.e. (i) the assessment of fabric handle characteristics was based on tactile sensations caused by fabric mechanical and surface properties; and (ii) the final judgment of handle is based on the suitability of the mechanical and surface properties for the particular end use of the fabric. Kawabata subsequently developed a series of instruments (KES-FB) for evaluating the objective assessment of fabric handle characteristics by measuring various low stress mechanical properties of fabrics. The Weber–Fechner law of psychophysics has been adopted by Matsuo [32] during the subjective analysis of fabric handle characteristics. The Weber–Fechner law intends to describe the relationship between the physical magnitudes of stimuli and the perceived intensity of the stimuli. Matsuo assumed that the Weber–Fechner law is applicable when the fabric mechanical properties were used as the handle stimuli and used a nonlinear combination of mechanical properties to explain fabric handle assessments. In practice, the results obtained from this model depend strongly on the values assigned to the minimum sensibility which a judge can discriminate for each mechanical property.

4.3.2

Objective assessment

Researchers have been attempting to objectively measure the fabric hand characteristics since 1970s. The outcomes of the intense researches are the two instrumental approaches to measure the low stress mechanical and surface characteristics of fabrics, i.e., the Kawabata Evaluation System (KES) and Fabric Assurance by Simple Testing (FAST). These methods

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are based on correlations between a number of subjectively assessed discrete fabric sensory attributes such as smoothness, firmness, fullness, crispness and hardness, and corresponding mechanically measurable fabric properties, i.e., low mechanical stress tensile, shear, bending, compression, and surface friction [38, 39]. Kawabata evaluation system of fabric (KES-F) Kawabata Evaluation System of Fabric (KES-F) has the following four modules for measuring low stress and surface characteristics of fabrics: (i) (ii) (iii) (iv)

KES-F1 for measurement of tensile and shearing characteristics; KES-F2 for measurement of bending characteristics; KES-F3 for measurement of compressional characteristics; KES-F4 for measurement of surface frication and roughness.

Figure 4.6 shows the principle of KESF-1 system. The fabric specimen is clamped between two jaws (one attached with the drum for tensile force application and other is attached with slide for shear force application) and subjected to a constant tension of 10 gf/cm by a weight attached to the drum on which one jaw is mounted. Tensile force is applied by allowing the drum to rotate freely. The tensile force is measured by the tensile force detector by measuring the torque and the tensile strain is measured by tensile strain detector from the data of angle of rotation of the drum. The shear force is measured by a transducer connected to the other jaw (attached with slide) which moved sideways to apply the shear deformation. The shear force is measured by the shear force detector by measuring the force required to slide and the shear strain is measured by shear strain detector from the data of the displacement of the slide.

Tensile direction

Shear force detector Shear direction

Shear strain detector Fabric specimen

Slide for shear force application Tensile strain detector Drum for tensile force application Tensile force detector 4.6 Working principle of KESF-1 [40].


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The typical instrument settings, loading conditions and parameters measured for tensile and shear characteristics in KESF-1 system are [41] Tensile characteristics Settings and loading conditions: Rate of extension – 0.1 mm/s; Sample size (L×W) – 5 × 20 cm; Maximum tensile force – 5 N/cm. Test parameters and units: ● ●

● ●

Elongation at 5 N/cm tension (EM) is expressed in percentage; Energy required to extend the fabric specimen to 5 N/cm tension (WT) is expressed in J/m2; Linearity of stress-strain curve (LT) is unitless; Tensile resilience (RT) is expressed in percentage.

Shear characteristics Settings and loading conditions: Speed of shearing – Sample size (L×W) – Maximum shear angle – Constant sample tension –

0.417 mm/s; 5 × 20 cm; ±140 mrad; 0.1 N/cm.

Test parameters and units: ● ● ●

Shear rigidity at 39.4 mrad shear strain (G) is expressed in N/m; Shear hysteresis at 8.7 mrad shear strain (2HG) is expressed in N/m; Shear hysteresis at 87 mrad shear strain (2HG5) is expressed in N/m.

The principle of KESF-2 system is shown in Fig. 4.7. The fabric specimen is gripped by two jaws. One jaw is attached with the bending arrangement which moves in circular direction to apply bending force. The other jaw is connected with torque sensor which detects torque value of steel wire during bending of specimen. The curvature of bending is obtained from the drive to the bending arrangement. The fabric specimen is bent with the help of bending arrangement between the curvatures of –2.5 cm–1 and +2.5 cm–1. The typical instrument settings, loading conditions and parameters measured for bending characteristics in KESF-2 system are [41]

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Steel wire Torque sensor Fabric specimen Bending arrangement

Drive system of bending arrangement 4.7 Working principle of KESF-2 [40].

Settings and loading conditions: Rate of bending – 0.5 cm–1/s; Sample size (L×W) – 20 × 1 cm; Maximum curvature – ±2.5 cm–1. Test parameters and units: ● ●

Bending rigidity at 1.5 cm–1 curvature (B) is expressed in μNm; Bending hysteresis at ±0.5 cm–1 curvature (2HB) is expressed in mN.

Figure 4.8 shows the working principle of KESF-3 system. The fabric specimen is compressed between two plates, i.e. anvil and the pressure foot. The fabric specimen is placed on the anvil and pressure is increased with the help of pressure foot, while continuously monitoring the sample thickness by thickness detector. The compressive pressure is detected by the compressive force detector. The pressure foot gets drive from the drive arrangement. The typical instrument settings, loading conditions and parameters measured for compression characteristics in KESF-3 system are [41] Settings and loading conditions: Rate of compression – 0.02 mm/s Area of circular pressure foot – 2.0 cm2 Maximum compressive pressure – 5 kPa


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Thickness detector Drive of pressure foot

Pressure foot Fabric specimen Anvil

Compressive force detector

4.8 Working principle of KESF-3 [40].

Test parameters and units: ●

● ● ● ●

Thickness compression as a proportion of original fabric thickness (EMC) is expressed in %; Fabric thickness at 5 Pa pressure (TO) is expressed in mm; Compression energy at 5 kPa pressure (WC) is expressed in J/m 2; Linearity of compression curve (LC) is unit less; Compressional resilience (RC) is expressed in %.

Friction and surface roughness characteristics of fabrics are measured by KESF-4 system. The working principle is shown in Fig. 4.9, where the surface roughness and surface friction of fabric specimen are being measured at same time. The fabric specimen, kept at constant tension by hanging dead weight, gets to-and-fro motion from a drum which rotates intermittently in clockwise and anti-clockwise directions. The frictional force between fabric specimen and the friction surface at the friction point is detected by frictional force detector. The surface roughness of the fabric is measured by surface roughness detection system, which is basically a displacement sensor. The probe of the displacement sensor is in touch with the fabric surface. When the fabric moves in the horizontal plane, due to the surface roughness the probe deflects vertically. This vertical deflection of the probe is the measure of surface roughness of fabric.

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Science in clothing comfort Surface roughness detection system Fabric specimen Friction point Drum for fabric motion

Frictional force detector

Dead weight 4.9 Working principle of KESF-4 [40].

The typical instrument settings, loading conditions and parameters measured for friction and surface roughness characteristics in KESF-4 system are [41] Frictional characteristics Settings and loading conditions: Traverse rate of fabric Constant tension on fabric Normal load Maximum fabric movement

– – – –

1.0 mm/s 0.1 N/cm 0.5N 3 cm

Test parameters and units: ● ●

Coefficient of surface friction (MIU) is unitless; Mean deviation of MIU (MMD) is unitless;

Surface roughness Settings and loading conditions: Traverse rate of fabric Constant tension on fabric Contact force Maximum fabric movement

– – – –

1.0 mm/s 0.1 N/cm 0.1N 3 cm

Test parameters and units: ●

Mean deviation of fabric surface profile or geometrical surface roughness (SMD) is expressed in μm.


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From all the above parameters, which are obtained from KESF instruments the total handle value (THV) can be calculated, which is the indicator of fabric handle behaviour of fabric. Fabric assurance by simple testing (FAST) The FAST system has been developed by CSIRO (Australia) primarily for quality control and assurance of fabrics [42–44]. It also gives the objective indication of fabric handle characteristics. It consists of a series of three instruments (i.e. FAST-1: Compression meter; FAST-2: Bending meter; and FAST-3: Extension meter) and a test method (FAST-4: Dimensional stability test) which are inexpensive, simple to use and robust in construction. It measures properties which are closely related to the ease of garment manufacturing, handle characteristics and the durability of surface finishing. FAST-1 Figure 4.10 shows the schematic diagram of FAST-1system, i.e. compression meter. It measures the fabric thickness over a range of loads, the variability and the durability of the thickness of the fabric surface layer. It can measure fabric thickness to micrometer resolution at two predetermined loads, and thereby enables the accurate measurement of surface layer thickness. The fabric thickness (T) is measured at a pressure of 2 gf/cm2. Surface thickness (ST) is the difference in thickness of a fabric measured at pressures of 2 gf/cm2 and 100 gf/cm2. This gives information about the hairiness or surface bulk of the fabric (closely related to surface treatment like brushing, singeing, finishing etc.). Released surface thickness (STR) is the measure of the surface thickness after the fabric is exposed to steam or water. The increase in fabric surface thickness obtained by this steaming process simulates the actual changes in surface characteristics that occur during the actual use of garment. Pressure foot Normal load

Surface thickness Thickness Fabric specimen Support plate 4.10 Principle of FAST-1system.

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FAST-2 The schematic diagram of FAST-2 system, i.e. bending meter, which measures the bending length (BL) and bending rigidity (B) of fabric, is shown in Fig. 4.11. The instrument measures the fabric bending length according to BS 3356-1961. The fabric bending length simulates the draping behaviour of fabric and the bending rigidity is related to the quality of stiffness when a fabric is handled. The bending rigidity is particularly crucial in the tailoring of lightweight fabrics as a very flexible fabric (low bending rigidity) may cause seam puckering while a high bending rigidity fabric can be more manageable in sewing and so produce a flat seam. The operator error in aligning the sample is eliminated with the use of an optical sensor. The bending length is displayed automatically, so chances of the error due to the operator’s judgment are not there. Before start of the test

During rest

Fabric specimen

Support plate 41.5°

Fabric specimen Bending length

Support plate 41.5°

4.11 Principle of FAST-2 system.

FAST-3 Figure 4.12 shows the schematic diagram principle of FAST-3 system, i.e. extension meter, for measuring fabric extension. It measures extensibility of fabric at various loads as well as its shear rigidity. It is capable of measuring the fabric extensibility in warp, weft and bias directions over a range of loads, with direct reading of extension as a percentage of the initial gauge length. The fabric extension is displayed as a percentage with a 0.1% resolution. Extensibility is measured at three loads 5 gf/cm (E5), 20 gf/cm (E20) and 100 gf/cm (E100). The difference in fabric extensibilities at between E5 and E20 is used to calculate fabric formability, which is a parameter related to the incidence of seam pucker. Fabric extensibility is combined with bending rigidity to calculate the fabric formability (F), which is a measure of the ability of a fabric to absorb compression in its own plane without buckling. E100 is used in FAST control chart (FAST fabric fingerprint) as the measure of fabric extensibility. If the value is below approximately 2%, then the fabric will be difficult to extend during seam overfeed.


69

Fixed jaw

Before loading

After loading

Fabric extension

Load 4.12 Principle of extension measurement in FAST-3 system.

Apart from the fabric extensibility, another important fabric handle related parameter, i.e. shear rigidity (G), is measured in FAST-3 system (Fig. 4.13). The bias extension is converted to shear rigidity, which is directly related to fabric looseness. For shear rigidity below 30 N/m, the fabric deforms so easily that it may give problems in handling, laying-up and sewing. Conversely if it is above 80 N/m then the fabric can be difficult to overfeed, mould, etc. [40]. Before loading

After loading Fixed jaw

Biased extension

Loading in biased direction 4.13 Principle of shear measurement in FAST-3 system.

FAST-4 Poor dimensional stability of fabrics is one of the main causes of poor appearance in garments. This is a test method for measurement of Relaxation

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Shrinkage (RS) and Hygral Expansion (HE), which can test these parameters in less than an hour as compared to the conventional one-day test. The drying is done in a forced convection oven. A template and a ruler are the only equipments required to do the test. The results from this method simulates the change in fabric dimensions that may occur during the actual wear as the fabric is subjected to washing and changing humidity conditions. The relaxation shrinkage is mainly due to the recovery of fabric structure which got strained during manufacturing, while hygral expansion or contraction is caused by the swelling or deswelling of hygroscopic fibres. Very high relaxation shrinkage results in problem of change of garment size, puckering, etc. Similarly, higher hygral expansion may result in seam pucker, fabric waviness, buckling and overall poor garment appearance. The testing is completed in following three different steps (Fig. 4.14), ●

●

●

Step-I

– the fabric specimen is first over dried (up to 0% moisture) to measure its dry dimensions (l1); Step-II – soaked in water to measure its wet relaxed dimensions (l2); Step-III – the specimen is dried again to measure its final dry dimensions (l3).

The relaxation shrinkage (RS) and the hygral expansion (HE) are calculated from the above dimensions, using following relationships: Relaxation shrinkage (RS) = [(l 1 – l 2) × 100] ÷ l1 Hygral expansion (HE) = [(l 2 – l 3) × 100] ÷ l3

Fabric dimensions

Soaking in water

Re-drying

Oven drying l2

l1 Step-I

Step-II

Step-III l3

Time required 4.14 Steps to test the dimensional stability in FAST-4 [43].

(4.2) (4.3)

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The FAST data analysis software, which is included as part of the FAST System package, automatically plots the appropriate values and joins the various plotted points together to form a fabric ‘FAST control chart’ or ‘FAST fingerprint’, which is unique to each particular fabric. Figure 4.15 shows a typical FAST control chart. Each value has a separate scale showing a graphical representation of the range of values in the appropriate units that we expect for each of the various measurements. For example relaxation shrinkage ‘RS-1’ represents the warp value and ‘RS-2’ that of the weft. In addition to the values for the various measurements, each scale contains one or more shaded zones. If the fingerprint falls into one of these zones, a potential problem with the particular aspect(s) of fabric performance influenced by that property is indicated. Lower value RS-1

Relaxation shrinkage

RS-2 HE-1

Hygral expansion

Formability

Higher value Fusing Pleating

Sizing

Units %

Pleating-puckering %

HE-2 Pucker

F-1

mm2

F-2 Overfeed moulding

Check matching laying up

E-100-1

%

E-100-2

%

Extensibility

B-1

Bending rigidity

Outting

μNm

B-2 Laying-up

Shear rigidity Thickness Surface thickness

Stiff

Moulding sleeve insertion N/m

G

T ST

Lean

Full

Smooth

mm

Released surface thickness STR Weight

W

mm mm

Light

Heavy

g/m2

4.15 Typical FAST control chart.

Fabric extraction principle The fabric extraction principle is not a new idea at all; it has been a common practice for many years by ladies in certain parts of the world when searching for a desired scarf at a market. They would take off their rings and pull out a scarf through the ring, judging the overall quality of the scarf based on the resistance during the pulling out process [17]. The fabric

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is extracted through a specially designed nozzle and the force required to extract the fabric through the nozzle is measured. During this extraction process the sample is deformed under a very complex yet low stress state including tensile, shearing and bending as well as frictional actions, similar to the stress state when we handle a fabric. The fabric specimen gets folded, sheared, rubbed, compressed and bent during extraction. A large number of studies [17, 45–51] have been reported where fabric extraction technique is used for evaluation of fabric hand. In one of these studies [50] a circular fabric specimen, 250 mm in diameter held by a pin, is drawn through a cylindrical nozzle of highly polished steel, 20 mm in diameter and 20 mm in height, as shown in Fig. 4.16. Load cell

No Nozzle Fabric specimen

4.16 Fabric extraction technique.

The fabric sample should be free from wrinkles and creases. As the top jaw, with which the connecting pin is attached, moves upward, it extracts the circular fabric specimens through the nozzle. The force required to extract the fabric specimen through the nozzle changes as more and more of the specimen is introduced into the nozzle. The extraction force can be recorded by the instrument. A typical extraction force–displacement curve is shown in Fig. 4.17. The fabric handle behaviour has been defined by two parameters: (i) peak extraction force and (ii) traverse at peak extraction force. Traverse at peak extraction force is the movement of the cross-head from where the fabric sample starts exerting resistance to extraction till the force reaches to its maximum. Both these parameters can be obtained from force– displacement curves. Higher peak extraction force indicates stiffer fabric and higher traverse at peak extraction force value shows the fabric surface become smoother. The extraction force is the combination of fabric resistance to bending, compression, shear, extension and sliding.

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Traverse at peak extraction force

Extraction force


Peak extraction force

Displacement 4.17 Typical extraction force–displacement curve.

4.4

Fabric parameters affecting tactile sensation

Fabric roughness and scratchiness are the important characteristics which directly affect the tactile sensation of clothing. It has been reported [2] that the sensation of roughness correlated with fabric surface roughness characteristics (frictional force, mean surface roughness coefficient, and deviation of surface roughness coefficient), compressional characteristics of fabrics (compressibility and compressional energy), fibre diameter, tensile characteristics of fibre (breaking load and breaking elongation), and tensile characteristics of fabric (breaking elongation, elastic recovery). The sensation of scratchiness, on the other hand, is related to fabric tensile characteristics (breaking elongation, work of rupture and the modulus), fabric surface roughness (frictional force, mean surface roughness coefficient, and deviation of surface roughness coefficient), compressional characteristics of fabrics (compressibility, linearity of the compression curve, compressional energy and slope of the compression-thickness curve). In a study on roughness sensation of woven and knitted fabrics made from nylon monofilaments of different diameters [52], it has been reported that the perception of roughness increases with the increase in filament diameter and knitted fabric is rougher than the woven fabric made from the same diameter filament (Fig. 4.18). It has been reported that prickle sensation of fabric is directly correlated with fibre diameter, fabric thickness at low loading, and fabric surface roughness [53]. It has also been reported [54] that the prickle sensation of the fabrics could be predicted from the density of coarse fibre ends per unit area of fabric and the variations in diameter distribution in individual fibre mass, especially the percentage of coarse fibre ends, influence the fabric prickle sensation significantly.

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Roughness sensation

Knitted fabric

Woven fabric

Filament diameter 4.18 Effect of filament diameter and fabric type on roughness sensation [52].


Mehrtens and McAlister [14] have reported that the fabric scratchiness sensation was influenced by the flexural rigidity of fibre and the friction characteristics of fabrics. Figure 4.19 shows that there is very good correlation between fibre flexural rigidity and fabric scratchiness sensation. The flexural rigidity of fibre is dependent on fibre modulus, cross-sectional shape, denier and density, which are influenced by polymer structure and orientation. Fibre surface structure also affects the fabric scratchiness through an effect on friction, i.e. higher friction means higher scratchiness. They have also reported that fabrics made from fibres with lower friction (e.g. nylon and rayon) show lower scratchiness than predicted from the flexural rigidity.

Fibre bending rigidity

4.19 Fibre flexural rigidity and subjective ratings [14].

The clinginess of fabrics during wear is also a cause of tactile discomfort. Under conditions of profuse sweating, such as that occurs during exercise in hot surroundings, it is expected that high wickability of fabric is to avoid clinging of garments. One of the test methods for prediction clinging


75


characteristics of fabrics is clinging tension test. There is fair correlation of clinginess sensation with a fabric clinging tension test [14], as shown in Fig. 4.20. The clinging tension is the tension required to remove a fabric from a wet surface and it is measured as a function of the water content of the fabric. The wet fabric is placed against a porous, water-soaked surface (e.g. pressed asbestos) and is pulled nearly parallel to the surface with a tensile tester. The maximum tension, which occurred as the fabric began to slide, is then plotted against the water content of the fabric.

Fibre bending rigidity

4.20 Relation between clinginess sensation and clinging tension [14].

Clinging sensation

It has also been reported that the clinging sensation is also a function of flexural rigidity of fibre [14]. The lower the flexural rigidity the greater is the clinginess (Fig. 4.21), which may be due to a lesser ability of the fibres to break up the water film causing adhesion between fabric and skin.

Flexural rigidity of fibre

4.21 Relation between clinging sensation and fibre flexural rigidity [14].

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Fabric warmness and heaviness also cause discomfort. The rating of the tactile sensation of heaviness alone is very low. Since heaviness is related to warmness, the two sensations have been combined by Mehrtens and McAlister [14]. They have reported that warmness might be more dependent on fabric thickness than on weight, and that heaviness might be more dependent on weight than on thickness. It has been reported that the product of fabric weight and fabric thickness was a better objective measurement for correlation with warmness and heaviness than either weight or thickness alone [14]. The tactile comfort sensations can be greatly improved by different types of finishing treatments at fabric as well as in the garment stage. There are many finishing treatments, namely silicon finish, nano-finish, brushing, etc., for improvement of tactile sensation of fabrics. It may be said that “cloth is made in the finishing”. The treatments are performed on fabrics with the help of chemical or mechanical treatments at the last step in the textile production before the clothing operations or even after garmenting. The type of finishing treatment and different process parameters affect the tactile sensation in a significant way.

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‘From fabric hand to thermal comfort: The evolving role of objective measurements in explaining human comfort response to textiles, Int. J. Clothing Sci. Technol. 14(3/4), 181–200, 2002. LI Y ., ‘The science of clothing comfort’, Textile Progress, editor LAYTON J . M ., The Textile Institute, Manchester, Vol. 31, 2001. HOLLINS M ., BENSMAIA S ., KARLOF K . and YOUNG F ., Perception Psychophysics 62(2000), 1534. PAN N ., KAISER S ., CHANDLER J ., DALLAS M . J ., BRANDT B ., CAMERON B ., BROWN D . and BURNS L ., Fabric hand evaluation: perception and instrumentation, in Proceedings of the International Textile and Apparel Association Annual Meeting, Dallas, Nov. 1998. GEE M. G ., TOMLINS P ., CALVER A ., DARLING R . H ., and RIDES M ., ‘A new friction measurement system for the frictional component of touch’, Wear 259(7–12), 1437–1442, 2005. LEDERMAN S . J ., Skin and Touch: Encyclopedia of Human Biology, 2nd edition, Vol. 8, Academic Press, San Diego, 1997. JIANG W ., TREMBLAY F ., and CHAPMAN C . E ., ‘Neural encoding of texture changes in the primary and the secondary cortical areas of monkeys during passive texture discrimination’, J. Neurophysiol. 77(3), 1656–1662, 1997. CONNOR C . E ., HSIAO S . S ., PHILLIPS J . R ., and JOHNSON K . O ., ‘Tactile roughness: neural codes that account for psychophysical magnitude estimates’, J. Neurosci. 10, 3823–3836, 1990. BUENO M. A. and RENNER M., ‘De l’Etoffe à la Main’, Pour la Science 266, 144–149, 1999. GARNSWORTHY R . K ., GULLY R . L ., KANDIAH R . P ., KENINS P ., MAYFIELD R . J ., and


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WESTERMAN R . A ., Understanding the causes of prickle and itch from the skin contact of fabrics, Report No G64, Australasian Textiles 8(4), 26–29, 1988. MATSUDAIRA M ., WATT J . D . and CAMAB, G . A ., ‘Measurement of the surface prickle of fabrics. Part I: The evaluation of potential objective methods, J Text. Inst. 81, 288–299, 1990. LI Y . Dimensions of comfort sensations during wear in a hot condition, Submitted to J. Federation of Asian Textile Associations. LI Y . ‘Dimensions of sensory perceptions in a cold condition’, J. Chitta Text. University 15(3), 50–53, 1998. MEHRTENS D . G . and MCALISTER K . C ., ‘Fiber properties responsible for garment comfort’, Textile Res. J. 32, 658–665, 1962. BINNS H ., ‘The discrimination of wool fabrics by the sense of touch’, Br. J. Psychiatry 16, pp. 237–247, 1926. PEIRCE F . T ., ‘The ‘handle’ of cloth as a measurable quantity’, Journal of the Textile Institute 21, pp. T377–416, 1930. B R A N D R . H ., ‘Measurement of fabric aesthetics: analysis of aesthetic components’, Textile Research Journal 34, p. 791, 1964. VAUGHN E . A . and KIM C . J ., Study on Fabric Hand, Parts I, II, and III, Technical Conference, American Association of Textile Chemists and Colorists, 1975. ELKS B . C . and CARNSWORRTHY R . K ., ‘A review of techniques for the assessment of hand’, Textile Research Journal, 50, p. 231, 1980. PAN N ., ‘Quantification and evaluation of human tactile sense towards fabrics’, Int. Journal of Design & Nature, 1(1), 1–13, 2006. Textile Terms and Definitions, 8th edition, The Textile Institute, Manchester, 1986. HOWORTH W. S ., and OLIVER P . H ., ‘The application of multiple factor analysis to the assessment of fabric handle’, J. Textile Inst., 49(T540-T553), 1958. KAWABATA S ., HESC Standard of Hand Evaluation, Vol. 2, The Textile Machinery Society of Japan, 1980. MAHAR T . J ., DHINGRA R . C ., and POSTLE R . Measurement and interpretation of lowstress fabric mechanical and surface properties, Part I: Precision of measurement, Textile Res. J. 57, 357–369, 1987. DHINGRA R . C ., LIU D . and POSTLE R ., Measuring and interpreting low-stress fabric mechanical and surface properties, Part II: Application to finishing, dry-cleaning and photo degradation of wool fabrics, Textile Res. J. 59, 357–368, 1989. POSTLE R . and DHINGRA R . C ., Measuring and interpreting low-stress fabric mechanical and surface properties: Part III: Optimization of fabric properties for men’s suiting materials, Textile Res. J. 59, 448–459, 1989. WINAKOR G ., KIM C . J . and WOLINS L ., Fabric hand: tactile sensory assessment, Textile Res. J. 50, 601–610, 1980. KAWABATA S . editor, ‘The standardization and analysis of hand evaluation’, 2nd edition, HESC, The Textile Machinery Society of Japan, Osaka, Japan, 1980. BINNS H ., ‘A comparison between the judgments of individuals skilled in the textile trade and the natural judgments of untrained adults and children’, J. Textile Inst. 17, T615, 1926. BINNS H ., ‘The discrimination of wool fabrics by sense of touch’, J. Psychol. 1263, 7, 1926. HOWORTH W. S . and OLIVER P . H ., ‘The application of multiple factor analysis to the assessment of fabric handle’, J. Textile Inst. 49, T540, 1958. MATSUO T ., NASU N. and SAITO M., ‘Study of hand. Part 2: The method of measuring hand’, J. Textile Mach. 17(03), 92, 1971.

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5 Thermal transmission

5.1

Introduction

Thermal comfort of a clothing system is associated with the thermal balance of the body and its thermoregulatory responses to the dynamic interactions with the clothing and the environment. Not only the heat but also moisture transmission behaviour of a fabric plays a very important role in maintaining thermo-physiological comfort. The fabrics should allow moisture, both in the form of sensible and insensible perspiration, to be transmitted from the body to the environment in order to cool the body and reduce the chances of drop in thermal insulation of the fabric due to accumulation of moisture within the micro-climate region. If the clothings those are in contact with the skin are not dry, then the heat flow from the body increases, which results in unwanted loss in body heat. This also results in a clammy feel. It is necessary to design fabrics with required moisture transmission properties for a specific end-use. In actual wear conditions the transmission of moisture and heat through the clothing system takes place in steady state as well as transient conditions. The human body is rarely in a thermal steady state, but is continuously exposed to transients in physical activity and environmental conditions [1]. Comfort is a pleasant state of psychological, physiological and physical harmony between the human being and the environment [2]. The processes involved in human comfort are physical, thermo-physiological, neurophysiological and psychological. Thermo-physiological comfort is associated with the thermal balance of the human body, which strives to maintain a constant body core temperature of about 37°C and a rise or fall of ±5°C can be fatal. Hypothermia and hyperthermia may result, respectively, due to the deficiency or excess of heat in the body, which is considered to be a significant factor in limiting work performance.

5.2

Thermo-regulation in human body

The core temperature of the human body is biologically maintained within a narrow range, close to 37°C during rest. It is independent of even large

79

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variations in environmental temperature. The core temperature is usually measured as rectal temperature. However, the temperature in the core is not uniform, but varies among the different body organs depending on the local heat balance; some organs are heated by the blood flow and others cooled by the blood flowing through them. During physical activity the core temperature rises proportionally with the intensity of the activity level [3]. The rise in temperature varies from person to person and is proportional to their maximum oxygen uptake. The temperature of skin surface of a naked person varies widely between different locations. The body parts situated farthest from point of origin of heat, for example, hands and feet, are generally cooler than the temperatures on other parts of the body. The average skin temperature is an area weighted mean which can be calculated from several surface temperature measurements and it varies with the ambient temperature, but it is independent of work intensity if heat balance can be maintained. Temperature regulation is achieved by an autonomic regulation which matches the rate of heat loss to that of heat production [4]. The centres for this regulation are located in the hypothalamus, which is an important part governing autonomic nervous system. Neuro-physiological studies have shown that temperature sensors in the brain respond to local thermal stimulation. Other cells in the hypothalamus receive information from skin thermo-receptors or other brain areas and interact by modifying the signal from the first-mentioned sensors and control the activity of the thermal effectors. Other regions of the body, such as the spinal cord and the abdomen, contain thermal sensors responding to the local temperatures [5]. Many models for the thermoregulation system have been suggested which describe the interaction between core and surface temperature signals and their relative importance. Most models imply a thermostat principle, with a reference set-point. A deviation from this reference point drives the appropriate thermal response (sweating, heavy breathing, constriction, shivering and non-shivering). The sensitivity of such proportional control system in the hypothalamus may be controlled by skin sensors, physical activity, etc. Some of the non-thermal factors, such as electrolyte balance and hydration state, also have influence on temperature regulation and on the sensitivity of the regulation mechanisms. Increased sweat secretion and diminished electrolyte loss in the sweat are the most important mechanisms in temperature regulation. As far as the acclimatization of human body to cold environment is concerned only insignificant changes occur. The most important is a local adaptation due to cold involving vasodilatation in the hands and face [6].


5.3

81

Thermal distress

When a person is exposed to extreme environments (too hot or too cold) the threshold limits for the thermoregulation system may reach quickly, and thus control over the thermoregulatory system is lost [7]. During activity in extreme hot environments, the rate of sweating and the rate of evaporation of sweat controls the heat loss from human body. If the heat loss is insufficient, the core temperature will increase above the controlled level (i.e. hyperthermia). This, in combination with dehydration due to sweat loss, may lead to heat exhaustion, when the person cannot continue the activity, blood pressure drops and he may faint. This condition may eventually lead to heat stroke, where sweating stops and core temperature rises to very high levels. Other acute effects of thermal distress are heat cramps, dehydration and heat edema. In addition, thermal distress of skin, namely heat rash and failure of the sweat glands may also appear. On the other hand, in extreme cold environments, the heat loss is greater than the production which causes the body core temperature to fall. This condition is known as hypothermia and it becomes increasingly severe as the body temperature falls. At a core temperature of 35–36°C the person becomes confused, shivering stops and the decline of core temperature occurs at a faster rate. At about 30°C, the person becomes unconscious, and the heart will stop beating at about 27°C. However, people can be brought to consciousness by proper re-warming, even from very low core temperatures [8, 9, 10]. Acute cold injuries also include frostbite and other frost injuries. Short-term heat and cold hazards may be experienced in indoor situations, for example in industrial work. In steel plants and the glass industry etc. the workers may be exposed to heat stress. Cold store rooms and freeze rooms in the food producing industries may cause cold discomfort and hypothermia. Cases of hypothermia have been reported in elderly persons even in normal environments, where they have been too weak or too poor to protect themselves against the cold in their homes. Heat load causes annoyance at work, increases strain on the cardiovascular system, and evokes other thermoregulatory adjustments [1]. Chronic after-effects of acute heat illnesses are, among others, reduced heat tolerance, malfunctioning of sweat glands, reduced sweating capacity, muscle soreness and stiffness, reduced mobility, chronic heat exhaustion, and cellular damage in different organs, particularly in the central nervous system, heart, kidneys and liver. Cumulative effects of long-term exposures to work in hot environments results in chronic heat exhaustion; the illness being characterized by a set of symptoms including headache, gastric pain, sleep disturbance, irritability, abnormally rapid heartbeat, vertigo and nausea. After many years of work in the heat, the symptoms gradually become worse and there can be observed hypertension, reduced libido,

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sexual impotency, myocardial damage, non-malignant diseases of the digestive organs, and anaemia [11, 12]. Long winters and long periods of exposure to cold have harmful human effects such as an increased rate of cardiovascular disease mortality. The anginal pain typical of coronary heart disease can be provoked by different types of cold stimulation, and changes appear in the electrocardiogram. Both systolic and diastolic blood pressures are higher in the cold than at room temperature. Furthermore, it has been observed that acute cold increases the systolic blood pressure, even in healthy people who are in generally good physical condition. Together, cold weather and physical strain are considered risk factors, especially for those middle-aged and elderly persons who suffer from latent or manifest disease of the circulatory or respiratory systems [9]. Besides coronary heart disease, exposure to cold increases the symptoms of other diseases such as rheumatoid arthritis, some respiratory diseases (asthma, chronic bronchitis), and various skin diseases. Very common are the symptoms and signs of dry skin and nasal mucosa, which are indications of repeated exposure to cold.

5.4

Thermoregulation through clothing system

The human body continuously generates heat by its metabolic processes. The heat is lost from the surface of the body by convection, radiation, evaporation and respiration. In a steady-state situation, the heat produced by the body is balanced by the heat loss to the environment. The human body has a very intelligent thermoregulatory system to ensure the body core temperature is maintained around 37°C. When the body temperature increases higher than the set value, vasodilatations of blood vessels are activated to increase the blood flow to the skin for the purpose of increasing heat loss. If the body temperature continues to rise, the sweating mechanisms will be activated to accelerate heat loss by evaporation of the liquid sweat. In contrast, when the body detects its temperature decreased lower than the set value, vasoconstriction of blood vessels will be activated to decrease the blood flow to the skin to reduce heat loss, and the metabolic rate will be increased by stimulating the muscles, which results in shivering [13]. The thermoregulation system of human is schematically shown in Fig. 5.1. A person can live comfortably only in a very narrow thermal environment from 26 to 30°C without wearing clothing [14]. With clothing, human beings can live and perform various physical activities comfortable in a wide range of thermal environments from -40 to 40°C. So, clothing plays an important role in providing thermal protection for the human body and creates a comfortable thermal microclimate so that one can survive and


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Human brain Integration and control of senses

Cold or hot sensations Skin thermoreceptors

Shivering Sweating Construction or dilation of blood vessels Human body responses

5.1 The human body thermoregulation system.

live in the thermal environments in which our body cannot cope up alone. Therefore, thermal functional design of clothing is critically important for human health and comfort, and in extreme cases, it can be a matter of life and death. The body continuously produces heat which must be transferred to the environment. The release of body heat mainly takes place through the skin. The body heat is transported to the skin through the circulation of blood and from skin it is transferred to the environment. Our body always generates metabolic heat due to the internal biological and physical activities of the muscles and organs, the amount of which depends on the intensity of the activities. Figure 5.2 shows that the thermal exchange between body and environment takes place through heat conduction, convection and radiation. In addition to heat transfer the exchange of body moisture takes place through perspiration and sweating. The thermal protection characteristics of clothing are essential function in most of the environmental conditions in various parts on the earth. The general clothing assemblies approximately covers around 90% of a human body. Therefore, the thermal transmission characteristics of clothing are extremely important, as our body responds to the external thermal environment through clothing. The thermoregulation mechanism through clothing depends primarily on the thermal behaviours in human body and transmission characteristics of clothing and can be summarized as [15, 16], ●

The biological thermal activities in the human body, namely metabolic

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Science in clothing comfort Perspiration (vapour)

Heat flow by convection Heat flow by radiation

Sweating (liquid)

Heat flow by conduction 5.2 Thermoregulation through clothing.

●

●

●

heat generation human body, blood circulations between different parts of the body to transfer energy, sweating, shivering, the interactive thermal activities between the human body and the external environment through the skin (by conduction, convection and radiation), heat loss due to evaporation of moisture through perspiration and sweating. The heat transmission through clothing ensemble (by conduction, convection and radiation) and the latent heat of various phase changes in clothing materials, such as the heat transferred by the processes of condensation or evaporation and freezing or melting. The moisture transfer process in clothing, involving water vapour diffusion and convection in the void space within the textile structure, moisture diffusion in fibres, liquid water diffusion through capillary channels in textile materials, moisture condensation / evaporation and freezing/melting etc. The thermal barrier between the human body and the environment formed by the clothing ensemble and entrapped still air influence the heat and mass transmission from human body to the environment or vice versa in the form of heat and moisture (both liquid and vapour) transfer processes. In fact the heat and moisture transfer processes are normally coupled under transient situations.

The metabolic heat generation is determined by metabolic activity. Table 5.1 shows the approximate range of body heat production for an average male for different types of activity.


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Table 5.1 Typical range of metabolic heat generation for various activities [17].

Activities

5.4.1

Metabolic heat generation (W/m2)

Resting Sleeping Seated quietly Standing

35–35 55–65 65–75

Normal walking on the level 3 km/h 5 km/h 7 km/h

110–120 150–160 210–220

Indoor activities Reading Writing Working on computer Filing, seated Filing, standing Lifting/packing

50–60 55–65 60–70 65–75 75–85 120–130

Miscellaneous work Cooking Dancing Playing tennis Playing basketball

90–110 140–200 200–300 300–450

Human as blackbody

In physics, a black body is an object that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none are reflected. Because no visible electromagnetic radiation (i.e. light) is reflected or transmitted, the object appears black when it is cold. If the black body is hot, these properties make it an ideal source of thermal radiation. If a perfect black body at a certain temperature is surrounded by other objects in thermal equilibrium at the same temperature, it will on average emit exactly as much as it absorbs, at every wavelength [18]. A black body at certain temperature (T) emits exactly the same wavelengths and intensities which would be present in an environment at equilibrium at temperature (T), and which would be absorbed by the body. Since the radiation in such an environment has a spectrum that depends only on temperature, the temperature of the object is directly related to the wavelengths of the light that it emits. At normal room temperature the black bodies generally emit mostly infrared light, with the increase in temperature these start to emit visible lights.

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Human body can also be considered as black body. For example, some of the person’s energy is radiated away in the form of electromagnetic radiation, most of which is infrared. The net power radiated is the difference between the power emitted and the power absorbed [18]: Pnet = Pemit – Pabsorb

(5.1)

The total energy radiated by an adult male in one day is about 2000 kcal (food calories). Primary metabolic rate for a 40-year-old male is about 35 kcal/(m2·h), which is equivalent to 1700 kcal per day assuming the 2 m2 area. However, the mean metabolic rate of an adult without any activity is about 50–70% greater than their basal rate [19]. There are other important thermal loss mechanisms, including convection and evaporation. Conduction is negligible since the Nusselt number (i.e. the ratio of convective to conductive heat transfer) is much greater than unity. Evaporation (perspiration) is only required if radiation and convection are insufficient to maintain a steady state temperature. The heat loss by radiation is 2/3 of thermal energy loss in cool, still air. Given the approximate nature of many of the assumptions, this can only be taken as a crude estimate. Ambient air motion, causing forced convection, or evaporation reduces the relative importance of radiation as a thermal loss mechanism [20].

5.5

Thermal comfort of clothing

The human body is affected by various external conditions in the winter and summer seasons. These external conditions can include changes in ambient temperature, vapour pressure, air velocity, and clothing insulation, among other factors that affect skin temperature [21]. On the other hand, the body continuously produces heat, which must be transferred to the environment. People sometimes feel uncomfortable as a result of some of their physical activities due to the change of external conditions. The temperature and humidity of the environment may profoundly influence the body’s skin and interior temperature [22]. The human body is adapted to function within a narrow temperature range. Generally, the human body keeps its body temperature constant at 37 ± 0.5°C under different climatic conditions. Human thermal comfort depends on combinations of clothing, climate and physical activity [23]. The human body converts the chemical energy of its food into work and heat. The amount of heat generated and lost varies markedly with activity and clothing levels [24]. The heat produced is transferred from the body’s skin to the environment. In a steady-state heat balance, the heat energy produced by the metabolism equals the rate of heat


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transfer from the body by conduction, convection, radiation, evaporation and respiration [25]. Therefore, clothing is needed to protect the body against climatic influence and to assist its own thermal control functions under various combinations of environmental conditions and physical activities [23]. The heat loss from the body and the feeling of individual comfort in a given environment is much affected by the clothing worn [26].

5.5.1

Heat exchange through clothing

In case of a steady-state heat balance condition of human metabolism system the heat energy produced by metabolism should be equal to the rate of heat transferred from the body. The body heat is transmitted through conduction, convection, radiation evaporation and respiration. The basic thermodynamic process in heat exchange per unit body surface area (W/ m2) between human and the environment is expressed by the general heat balance equation [16, 17, 27, 28]; M – W = C + Ck + Cres + R + Eres + Esk

(5.2)

where M is metabolic rate, i.e. internal energy production; W is external work; C is heat loss by convection; Ck is heat loss by conduction; Cres is sensible heat loss due to respiration; R is heat loss by radiation; Eres is evaporative heat loss due to respiration; and Esk is heat loss by evaporation from the skin. The external work (W) in the equation is small, and is generally ignored under most situations. The internal energy production (metabolic rate) is determined by metabolic activity. The typical rate of body heat production for an average male for different types of activity is given in Table 5.1. Due to presence of clothing the rate of conduction of heat from our body slows down. The nature of the clothing influences the rate of heat loss by conduction (Ck). The conduction heat loss is usually insignificant. Also, the rate of change of heat stored in the body is neglected in a steadystate heat transfer with its environment. The exchange of heat is always related in some way to the body surface area, irrespective of the fact whether it is by radiation, convection or vaporisation. The transfer of heat from the human body to the environment through convection process is expressed as [16, 28, 29] C = f cl ⋅ hc ⋅ (Tcl − Ta )

(5.3)

where f cl, clothing area factor (clo); hc, coefficient of convection heat transfer (W/m2K); Tcl, clothing surface temperature (°C) and Ta, ambient air temperature (°C).

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The heat transfer coefficient (hc) depends on the air velocity across the body and also upon the position of the person and orientation to the air current [25]. An approximate value of hc during forced convection can be evaluated from the following empirical equation [27] hc = 12.1⋅Va 0.5

(5.4)

where Va is the air velocity (m/s). The clothing area factor (fcl) can be evaluated by the following empirical equation [30]: f cl = 1.05 + 0.1 Icl

(5.5)

where Icl is the thermal insulation of clothing (clo). The rate of heat transfer through clothing by radiation depends on the mean temperature of surrounding environment, temperature of clothing surface and characteristics of clothing and environment. The heat transmission by radiation between the body and surrounding environment can be expressed by the following empirical equation [16, 28, 29]:

)

⎡ ⎣

4 R = σ ⋅ ε cl ⋅ f cl ⋅ Fvf ⎡(Tcl + 273.15 ) + (Tr + 273.15 ⎣

4

(5.6)

where σ is the Stefan-Boltzmann constant, which has the numerical value of 5.67×10–8 W/m2K4, εcl is emissivity of the clothing and Tr is radiant temperature. The emissivity of the clothing and skin is very close to that of a black body, and thus has a value of nearly 1. The effective area of the body for radiation is consequently less than the total surface area, usually about 75 percent of the total [25]. The effective area is determined with the view factor between the body and surrounding surface (F vf). The surrounding surface temperature is usually at a low temperature level. Thus the temperatures of the surrounding surfaces can be taken as approximately ambient air temperature [16]. Apart from all these mechanisms of heat loss from human body there are other types of heat loss mechanisms which are not directly related to clothing characteristics. These are heat loss due to respiration and heat loss due to evaporation from skin. The heat loss due to respiration can be divided into two components, namely evaporative heat loss (latent heat) and sensible heat loss. The rate of heat loss by evaporation is the removal of heat from the body by the evaporation of perspiration from the skin. Evaporation always constitutes a rejection of heat from the body [25]. The evaporation loss is dependent upon the mass transfer coefficient and the air humidity ratio for a given body surface temperature.


5.5.2

89

Heat exchange and Newton’s Law of cooling

Newton’s Law of Cooling states that the rate of change of the temperature of an object is proportional to the difference between its own temperature and the ambient temperature (i.e. the temperature of its surroundings). Newton’s Law makes a statement about an instantaneous rate of change of the temperature. It states that the rate of change of the temperature (dT/dt) is proportional to the difference between the temperature of the material (Tt) and the ambient temperature (Ta) [31]. This means that dT / dt is proportional to (T – Ta)

(5.7)

T(t) = Temperature of any material at time t. T(0) = Initial temperature Ta = Ambient temperature Clearly, if the material is hotter than the ambient temperature, i.e. (Tt – Ta) > 0, then the material cools down, which means that the derivative dT/dt should be negative. This means that the equation we need has to have the following sign pattern, dT / dt = –k (T – Ta)

(5.8)

where k is a positive constant. y(t)= T(t) – Ta = Difference between material and ambient temperatures at time t y0 = T(0) – Ta = Initial temperature difference at time t=0 Now, the derivative of y(t), and use the Newton’s law of cooling, we arrive at dy d dT dTa dT = (T (t ) − Ta ) = − = = − k (T − Ta ) = − ky dt dt dt dt dt

(5.9)

The solution of the differential equation is: y(t) = yoe–kt

(5.10)

Therefore, T(t) – Ta = (To – Ta)e–kt

(5.11)

T(t) = Ta + (To – Ta)e–kt

(5.12)

or,

By rearranging the Newton’s equation for clothing assembly it becomes,

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(T– Tamb) = (T0 – Tamb)e–(1/RC)t

(5.13)

where T is the temperature of the body at a particular moment, T0 the initial temperature of the body and Tamb the ambient temperature, t is the cooling time (s). The cooling constant has the form [32]:

K1 =

1 1 = R1C ( R0 + R) C

(5.14)

where C is the thermal capacity of the system (J/K), R1 is the thermal resistance of both the fabric medium and the air medium around the fabric sample (K/W), R is the thermal resistance of the fabric layer (K/W), R0 is the thermal resistance of the air layer (K/W). The thermal resistance of fabric could be calculated by the relation [32]: R=

1 C

⎛ 1 1 ⎞ − ⎟ ⎜ ⎝ K1 K 0 ⎠

(5.15)

where K0 is the cooling constant of the system without fabric. Knowing the thermal resistance as well as the fabric sample surface area S (m2) and fabric thickness d (m), the fabric thermal conductivity λ (W/m K) can be calculated as [32]:

λ=

1 d × R S

(5.16)

The thermal characteristics of fabrics are influenced by ambient temperature. The heat transfer coefficient h (W/m 2 K), consisting of conduction, convection and radiation mechanisms, is calculated by the relation: h=

1 dQ × S ΔT dt

(5.17)

where dQ/dt (W) is the measured heat transfer rate from the system through the fabric to the ambient surroundings, S (m 2) is the area of the fabric sample and ΔT (K) is the difference between the average temperature of the system and the ambient temperature [32]. Newton’s cooling rate law is applied in order to evaluate the thermal properties of textile fabrics. The procedure of thermal resistance determination is based on Newton’s cooling rate law [32]:

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1 dQ Q =T = − R C dt

(5.18)

where C (J/K) is the thermal capacity of the body, Q (J) is amount of heat, T (K) is the temperature of the body, dQ/dt is the amount of heat passing through the body per unit time and R (K/W) is the thermal resistance of the body.

5.6

Transient heat flow and warm–cool touch of fabrics

The transient heat conduction phenomenon is responsible for warm or cool sensation when human skin touches another body. When skin comes in contact with the clothing surface, which is normally at a lower temperature than the skin surface, the heat flows away from the skin. Loss of heat causes the temperature of the skin to fall, which is sensed by thermoreceptors located within the skin’s dermal layer. The higher the rate of the heat flow, the more rapid the temperature drops near the thermoreceptors and the more intense is the feeling of coolness [5]. The rate of change in temperature, resulting from heat flow from the skin to a clothing material at a lower temperature when brought into contact with it, is determined by the thermal inertia of the material. The thermal inertia is the function of density, specific heat and thermal conductivity of material. Any material that can absorb and conduct heat well will easily draw heat away from the skin and feel cool, i.e. the higher the thermal inertia the cooler it will feel to the touch. The fabric structural features, particularly surface properties, have a great influence on cool–warm feel, and a threelayered model based on inner, middle and outer layers with different thermal properties would be more appropriate. The rates of change of skin temperature detected by the thermoreceptors that determine warm or cool sensations varied quite significantly for different types of fabrics. Typical values of rate of temperature changes for warm and cool fabrics are shown in Fig. 5.3. The maximum rate of heat flow occurs within about 0.2 s after an object is touched, and this initial sensation is most important for the perception of warmth or coolness to the touch. The coolness sensation is estimated by measuring the initial rate of temperature change, the higher the initial rate of temperature change higher will be coolness in touch. Figure 5.3 includes the measured responses of four fabrics, i.e. cool fabric; fabric with smooth surface and two sides of the same fabric brushed; and warm fabric. There were substantial differences in the rates of change of temperature in all the fabrics. This suggests that sensations of warmth or

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Skin Temperature (°F)

95.0

94.8

Cool fabric Smooth fabric Brushed fabric Warm fabric

94.6

94.4

0

1 2 Contact Time (s)

3

4

5.3 Rate of change of skin temperature for different fabrics.

coolness to the touch are related to the surface properties of fabrics; in this case, surface hairiness, in addition to the materials [33]. In addition to the fabric surface characteristics the type of materials plays an important role in determining the fabric warm or cool feeling of fabrics. Warm/cool feeling felt by man’s skin contacting an object is related to the heat flow between man’s skin and the contacted object. It has been reported that the maximum heat flux which is observed shortly after the contact of the heated plate to fabric correlated well with warm/cool feeling of human subjects to make it a convenient measure of warm/cool feeling of fabrics. This maximum heat flux is named as qmax, as shown in Fig. 5.4.

q (W/m2K)

qmax

qmax

Summer clothing Winter Clothing Time (sec)

5.4 Typical heat flux with respect of time.


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The qmax value was introduced as a measure of predicting warm/cool feeling of fabrics by Kawabata and his team [34, 35], where qmax is the peak value of heat flux which flows out of a copper plate having a finite amount of heat into surface of fabric after the plate contacts the fabric surface. At first, a theoretical prediction of qmax was given as transient heat conduction within homogeneous body and the predicted qmax was interpreted with respect to the effects of test condition and the thermal properties of test specimen. The value qmax is a measure of warm/cool feeling. It was found that warm/cool feeling correlates well with transient heat conduction, especially with qmax, the peak value of heat flux which arises for a very short time after the touching of the skin to the fabric surface. The physical meaning of qmax is discussed on the basis of theoretical analysis of transient heat conduction from outside object into human skin. Figure 5.5 shows how the qmax of wide range of fabrics vary with moisture content of fabrics. It is clear that for all the fabrics the qmax value increases with the increase in moisture content, which means that the fabric feels cool at higher moisture content. It is also clear that the linen fabric has maximum qmax and wool fabric have minimum qmax up to certain level of moisture content. This is the reason why linen feels cool and wool feels warm when they are touched. Kawabata and Akagi [34] reported the following experimental results in regards to warm–cool feel of fabrics (qmax): ●

●

●

●

A high correlation was obtained between physically measured qmax and fabric warm/cool feeing data gathered in human sensory tests. A high qmax value corresponds to the cool feeling and a low qmax value to warm feeling. The value qmax depends on fabric surface condition and not on the number of fabric layers or fabric thickness. The value q max is sensitive to fabric water content and surface geometry.

When skin is touched by an object whose temperature is lower than skin, heat flows from the skin to the object, and it is considered that the cool feeling arises from this short term transient heat conduction. The heat conduction properties of materials have direct influence on qmax. The heat conduction is an important parameter governing the transient heat conduction (heat flux) after the contact with specimen. In other words, it is the heat diffusion rate which dominates when heat flows from heat source (e.g. human skin) to material. The heat diffusion rate depends on area of contact, density of material, thermal conductivity, thermal diffusivity and heat capacity [34]. When skin is touched by an object different in temperature, the steady temperature distribution in skin is disturbed making the thermoreceptor in skin to develop warm/cool signals.

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5.7

Measurement of thermal transmission characteristics

The transmission of heat through a fabric occurs both by conduction through the fibre and entrapped air and by radiation. The thermal conductivity of the fabric is measured by the total heat transmitted through fabric per unit time with unit temperature difference. The insulation value of fabric is measured by its thermal resistance, which is the reciprocal of thermal conductivity. In practice it is very difficult to measure the rate of heat flow in a particular direction, as the heater dissipates heat in all directions. Two methods are used to overcome this problem: ● To compare with a sample with known thermal conductivity value (Togmeter) ● To reduce the heat loss (Guarded hot plate method)

5.7.1

Togmeter

Togmeter avoids the problem of measuring heat flow by placing a material of known thermal resistance in series with the material under test so that the heat flow is same through both materials [36]. The internal resistance of test fabric can be calculated by comparing the temperature drop across test fabric with the temperature drop across the standard material. There are two methods of test that can be done with togmeter. Two-plate togmeter In this method the specimen under test is placed between heated lower plate (standard) and an insulated upper plate as shown in Fig. 5.5. The upper plate has low mass, so that it does not compress the fabric. The temperature is measured at the heater (T1), between the standard and test fabric (T2) and between the test fabric and upper plate (T3). The heater is adjusted so that the temperature of the upper face of the standard is at skin temperature (31–35°C). A small airflow is maintained over the apparatus. Top plate

T3

Fabric specimen T2

Bottom plate T1 Heater Flow of Heat

5.5 Schematic diagram of two-plate togmeter.


95

Single-plate togmeter In this method the specimen under test is placed on the heated lower plate as above, but it is left uncovered as shown in Fig. 5.6. In place of top plate, the air temperature (T3) is measured. The air above the test specimen has a considerable thermal resistance itself so that the method is in fact measuring the sum of the specimen and air thermal resistance. A separate experiment is therefore performed without the specimen (i.e. a bare plate test) to measure the resistance of the air Rair. T3

Fabric specimen T2

Standard plate T1 Heater Flow of Heat

5.6 Schematic diagram of single-plate togmeter.

Rair = Rstand × (T2 – T3) / (T1 – T2),

(5.19)

where R air = Thermal resistance of the air R stand = Thermal resistance of the standard material. To determine the sample resistance, the above experiment is repeated with the sample placed on the bottom plate and the apparatus is again allowed to reach the equilibrium. The thermal resistance of the sample: (Rsample) = Rstand × (T2 – T3) / (T1 – T2) – Rair

5.7.2

(5.20)

Guarded hot plate

In this method thermal transmittance of the fabric, which is the reciprocal of the thermal resistance, is measured [37]. The apparatus consists of a heated test plate surrounded by a guard ring and with a bottom plate underneath as shown in Fig. 5.7. A constant temperature in the range of human skin temperature (33–36°C) is maintained in all the plates. The whole of the surroundings of the apparatus is maintained at fixed conditions from 4.5 to 21.2°C temperature and 20 to 80% R.H. With test fabric in

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Guard ring Test Plate

Test Sample

Heater

Insulation Top view Top view

Side Sideview view

5.7 Schematic diagram of guarded hot plate.

place the instrument is allowed to reach the equilibrium. In this method the amount of heat passing through the samples in watts per square meter is measured from the power consumption of the test plate heater. Combined transmittance of specimen and air, U1 U 1 = P/[A(TP – Ta)] W/(m2K)

(5.21)

where TP and Ta are temperature of test plate and air, respectively P = power loss from test plate (W) A = area of the test plate (m2) The bare plate transmittance Ubp is calculated similarly. The intrinsic transmittance of the fabric alone, U2, is calculated as, 1/U 2 = 1/U 1 – 1/Ubp

(5.22)

The thermal resistance of the textile materials is measured in terms of S.I. unit Km2/W. It is defined as the ratio of the temperature difference between the two faces of the material to the rate of heat transfer per unit area of the material to the faces. A practical unit of thermal resistance widely used is Tog, which is one tenth of the S.I. unit. Another common unit is Clo, approximately equal to 1.55 × Tog. It is defined as the resistance of a clothing assembly which provides comfort to a sitting / resting subject in a normally ventilated room.

5.7.3

KES-F Thermo Labo-II

It was developed for evaluating the thermal transmission characteristics of textile fabrics [38]. This test is used to measure the power loss (Watt)


97

from BT-Box to the Water Box through the fabric samples. The sample is kept on the Water Box at the room temperature (20°C). The temperatures of the BT-Box and Guard are kept at 30°C. The amount of heat flowing through the fabric sample per unit area (W/m2) is measured from the power consumption of the test plate heater. The thermal conductivity (W/mK) of fabrics can be calculated using the following equation [38, 39]:

Thermal conductivity (k ) =

k =

Heat flow rate × distance area × temperature difference

Q L × t A × ΔT

(5.23)

(5.24)

where, Q is the quantity of heat; t is time; L is the fabric thickness; A is test area of fabrics and ΔT is temperature difference. Alambeta instrument is used to measure thermal conductivity, thermal resistance and thermal absorptivity values [40].

5.7.4

Thermal manikin

Thermal manikins are traditionally used by research institutes for climate research. In recent years manikins are increasingly used in many practical applications. Manikins are nowadays frequently used for testing and product development by the building and the automobile industry for evaluation of the performance of heating and ventilation systems. The clothing industry uses manikins for development of clothing systems with improved thermal properties. Test houses perform tests on protective clothing according to specific international standards. This kind of work results in continuous improvement of environments and products of importance for comfort, health and safety in working life. Thermal manikins are complex instruments for measurement of thermal transmission behaviour of clothing in actual wear conditions. A humanshaped thermal manikin measures the heat losses due to conduction, convection and radiation losses over the whole surface of the manikin body and in all directions. Depending on number of individually regulated body segments of the manikins surface the spatial resolution can be high. The number of individually regulated segments can be even more than 30. By summing up the area weighted values, a value for whole body heat loss is determined. For the same exposure conditions, a thermal manikin measures heat losses in a relevant, reliable and accurate way. The method is quick and easily standardized and repeatable.

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The salient features of thermal manikins are as follows: ●

● ●

●

It can simulate the human body (the whole body and local) heat exchange. It can measure the 3-dimensional heat exchange from human body. It can integrate the dry heat losses from human body in a realistic manner. It can measure the clothing thermal insulation objectively.

Thermal manikin such as the one shown in Fig. 5.8 provides a useful and valuable complement to direct experiments with human subjects. In situations where the heat exchange is complex and transient, the measurements with a thermal manikin produce relevant, reliable and accurate objective values for whole body as well as local heat exchange. Such values are useful for: ● ●

●

●

●

evaluation of thermal stress in environments with human body, determination of heat transfer and thermal properties of clothing assemblies, prediction of human responses to extreme or complex thermal conditions, validation of results from human experiments regarding thermal stress, and simulation of responses in humans exposed to thermal environments.

5.8 Photograph of thermal manikin.


5.8

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Parameters for expressing thermal characteristics

The thermal transmission characteristics of textile materials are expressed by several parameters, namely Met, clo, tog, permeability index, evaporative transmissibility, permeation efficiency factor, index of water permeability. The main applications of all these parameters are mainly to express the heat exchange between human body and its environment, and for the prediction of the physiological variables under heat stress conditions [41].

5.8.1

Met, Clo and tog

The heat exchange between human body and the environment can be quantified in terms of Met and clo units [42, 43]. One Met is used to quantify the metabolism of a man resting in a sitting position under conditions of thermal comfort [42]. One Met is equivalent to 50 kcal/m2h (i.e., 58.2 W/m 2). The term ‘clo’ is the measure of clothing insulation and one clo is defined as the insulation of a clothing system that maintained a sitting–resting average male comfortable in a normally ventilated room (0.1 m/s air velocity) at the air temperature of 21°C and relative humidity less than 50%. It is assumed that on an average 24% of the metabolic heat is lost through evaporation from the skin and remaining 38 kcal/m2h should be transmitted through the clothing assembly by conduction, convection and radiation. The comfortable mean skin temperature is 33°C. The total insulation of the clothing plus the ambient air layer is given by [41]: It =

33 − 21 = 0.32 m 2 °C h/kcal 38

(5.25)

The insulation of air at the above specific condition is 0.14 m2 °C h/kcal, the insulation of the clothing is found to be 0.18 m 2 °C h/kcal, which is the difference between the total insulation of the clothing plus the ambient air layer (i.e. 0.32 m2 °C h/kcal) and insulation of only the air layer (0.14 m2 °C h/kcal). Thus, 1 clo unit is defined as 0.18 m2 °C h/kcal, which is equal to 0.155 m2 °C/W [41]. This insulation of clothing is known as effective insulation. A warm clothing assembly (business suit ensemble) provides approximately 1 clo of insulation for the whole body [44]. Tog, a unit of thermal resistance, is defined as the thermal resistance that is able to maintain a temperature gradient of 0.1 °C with a heat flux of 1 W/m2 [45]. A light summer suit offers 1 tog insulation. When the thermal insulation is expressed in terms of tog, the temperature drop through clothing assembly can be calculated by one-tenth of the product

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of tog value and the heat flux. Also, the heat flux can be calculated by dividing the temperature drop by one-tenth of the tog value, if the temperature drop is known.

5.8.2

Permeability index

The permeability index developed by Woodcock [46] is an indicator of the evaporative performance of clothing. The permeability index (i m) can be expressed by the following equation:

im =

Rt LR × Ret

(5.26)

where Rt is the total thermal resistance of the clothing plus surface air layer (m2 °C/W), and Ret is the total evaporative resistance of the clothing plus the air layer (m2 kPa/W). The ratio Rt/Ret represents the effectiveness in transmitting evaporative heat as compared to the dry heat transmitted. Lewis Relation (LR) is the ratio of evaporative mass transfer coefficient to convective heat transfer coefficient. For typical applications it can be treated as a constant equivalent to 16.65°C/kPa [47]. Theoretically the value of permeability index ranges from 0 (i.e., completely water vapour impermeable) to 1 (completely water vapour permeable). As the permeability index is a dimensionless quantity, it offers the same value no matter what unit is used.

5.9

Thermal transmission characteristics of fabrics

Thermal transmission characteristics of fabrics depend on various factors, namely the morphological characteristics of component fibres, internal structure of yarn and physical and structural characteristics of fabrics. The thermal conductivity of textile fibres is dependent on various molecular structural parameters, like molecular structure, density, crystallization level, crystal orientation angle and mobility of molecular chains in amorphous regions, etc. For example, the specific heat of cellulose is 1.25 kJ/kg K, but the morphological structure of cellulose fibres, either natural or regenerated, is responsible for their thermal capacity and thermal conductivity of various cellulosic fibres. The thermal transmission behaviour of textile fabrics is also influenced, to a great extent, by fibre arrangement within yarns. The packing density of staple yarns varies widely depending on the fibre arrangement. Irrespective of the production techniques, textile fabrics are porous materials consisting of a solid matrix with an interconnected void and the thermal transmission characteristics


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depend on primarily on the porosity of fabrics. So, it is obvious that the parameters which affect the fabric porosity also affect its thermal transmission behaviour. As far as the geometrical characteristics of textile fabrics are concerned the fabric thickness has the most significant influence on thermal behaviour, explaining more than 90% of the phenomenon. This is due to the fact that the increase in thickness of fabric affects the fabric porosity due to the corresponding increases of fabric volume [32]. The thermal insulation characteristics of textile assemblies depend on the randomness of fibre arrangement in fabrics. Fibre arrangement as well as fabric thickness determines fabric insulation. Thus there seem to be many individual and combined factors involving fibre, yarn, and fabric characteristics and manner of applying fabric to the body which affect human thermal, physical, and psychological comfort. Many of the effects are so small that when coupled with the ability of the body to make compensating physiological adjustments they are most difficult to isolate and measure, either subjectively or objectively [48]. Yarn structural parameters influence the thermal transmission characteristics of fabrics due to presence of air pockets within the yarn body. The bulked yarns produced by shrinkage of one fibre component in the yarn not only differ from other yarns in structure but also in their bulk, mechanical and surface properties. The thermal transmission properties of fabrics produced from these yarns are affected by the bulkiness of yarns. The properties of fabrics made of bulked yarns are different from the normal yarn fabrics in all respect. The bulking of yarn gives voluminous textile product having good thermal insulating properties. In a study by Das et al. [49] the bulked yarn fabrics show lower thermal conductivity than 100% cotton fabric, which may be attributed to the very bulky structure of the yarns working as an insulating medium. The entrapped air in the loose fibrous assembly spaces does not allow heat of inner layer to transmit to outer layer. The structural modification of yarns can also be done by incorporating additional micro-pores inside the yarn body in addition to the existing micro-pores. This increases the porosity of the yarn and hence influences the thermal characteristics of fabrics. The fabrics with micro-pores content in the yarn have lower thermal conductivity as compared to 100% cotton reference fabric sample. This is due to the fact that removal of PVA fibres creates micro-pores within the yarn structure resulting in more entrapped air. Since the air is a poor conductor of heat as compared to fibre, thus resists transmission of heat through fabric [50]. The woven fabrics made out of staple twistless and hollow yarns have great impact on the comfort related properties, i.e. air permeability, thermal conductivity, percentage water vapour permeability, wicking and water

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absorbency. The fabric made of parent yarn (normal yarn) shows the maximum thermal conductivity and fabric with hollow yarn shows minimum thermal conductivity values (Fig. 5.9). The fabric with twistless yarn shows intermediate thermal conductivity value. The minimum thermal conductivity of fabric with hollow yarn is due to very bulky structure of hollow fibrous assembly in weft works as an insulating medium. It entraps air in the hollow spaces and does not allow heat of inner layer to transmit to outer layer [51].

Thermal resistance (tog)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Parent yarn

Twistless yarn

Hollow yarn

Type of yarn in fabric

5.9 Thermal resistance of fabrics made of speciality yarns.

It has been reported that as the yarns in knitted fabrics become finer the thermal resistance and thermal conductivity decrease. In fact there is an inverse relationship between thermal resistance and thermal conductivity (R = h/λ; R is thermal resistance, h is fabric thickness and λ is thermal conductivity). However, the study revealed that as the thermal resistance decreases the thermal conductivity decreases as well. This contradiction has been explained by the fabric thickness. When finer yarn was used in fabric, yarn diameter and therefore fabric thickness was lower. If the amount of decrease in thickness is more than the amount of decrease in thermal conductivity, thermal resistance also decreases. Thermal absorptivity value decreases while the yarn is getting finer. An increase in yarn twist coefficient results in decrease in thermal resistance. This may be due to the fact that as the twist coefficient increases the yarn becomes finer, as a result the fabric thickness decreases [52]. The decrease in hairiness increases the surface area between the fabric and skin; this causes cooler feeling. The thermal resistance of the fabrics made of carded yarns is higher than the fabrics from combed yarns. This is due to the fact that the fabrics produced from carded yarns have more hairiness. As the yarn hairiness increase, the amount of static air that prevents the passage of heat also increases. Another reason for this is fabric thickness [52].


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With the increase in microclimate thickness the total heat flux from human body decreases. This is due to increase in air layer which behaves like an insulating material. The contribution of radiation in total heat flow increases with the increase in microclimate thickness. This is due to the fact that the heat transmission due to radiation is independent of microclimate thickness. The lesser effect of fabric thickness variation compared to the variation in microclimate thickness has been reported. This is mainly due to the fact that the thermal conductivity of fabric is more than the microclimate, and hence, the result is less sensitive to fabric thickness than microclimate thickness. The effect of fabric thickness will be larger when the thickness of microclimate is smaller [53].

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17.

NIELSEN R .,

‘Work clothing’, Intl J Ind Ergonomics 7, 77–85, 1991. Human Comfort, C.C. Thomas, Springfield, 1985. GISOLFI C . V . and WENGER C . B ., ‘Temperature regulation during exercise: old concepts, new ideas’, Exerc Sport Sci Rev 12, 339–372, 1984. CABANAC M ., ‘Regulation and modulation in biology – A re-examination of temperature regulation’, Ann NY Acad Sci 813, 21–31, 1997. HENSEL H., Thermoreception and Temperature Regulation, Academic Press, New York, 1981. SENDOWSKI I ., SAVOUREY G ., BESNARD Y . and BITTEL J ., ‘Cold induced vasodilatation and cardiovascular responses in humans during cold water immersion of various upper limb areas’, Eur. J. Appl Physiol Occup Physiol 75, 471–477, 1997. KING J ., ‘Thermoregulation: Physiological responses and adaptations to exercise in hot and cold environments’, J. Hyperplasia Res 4(3), 2004. KEIM S . M., GUISTO J . A . and SULLIVAN J . B ., ‘Environmental thermal stress’, Ann Agric Environ Med 9, 1–15, 2002. PUGH L ., ‘Cold stress and muscular exercise with special reference to accidental hypothermia’, Br Med J 2, 333–337, 1967. REULER J . B ., ‘Hypothermia. Pathophysiology, clinical settings and management’, Ann Int Med 89, 519, 1978. DUKES- DUBOS F . N ., ‘Hazards of heat exposure: a review’, Scand. J. Work Environ. Health 7, 73–83, 1981. http://www.cdc.gov/niosh/hhe/reports/pdfs/1986-0422-1891.pdf LI Y., AIHUA M., RUOMEI W., XIAONAN L ., ZHONG W., WENBANG H ., LIYA Z . and YUBEI L ., ‘P-smart—a virtual system for clothing thermal functional design’, ComputerAided Design 38, 726–739, 2006. FANGER P . O ., ‘Thermal environment-human requirements’, Sulzer Technical Rev 67, 3–6, 1985. AIHUA M ., YI L ., XIAONAN L ., RUOMEI W. and SHUXIAO W ., ‘A CAD system for multistyle thermal functional design of clothing’, Computer-Aided Design 40, 916– 930, 2008. TUĞRUL O . R ., ‘The effect of thermal insulation of clothing on human thermal comfort’, Fibres & Textiles in Eastern Europe 15(2), 61, 2007. BUTERA F . M ., ‘Principles of thermal comfort’, Renewable & Sustainable Energy Reviews 2, 39–66, 1988. SLATER K .,

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18. http://en.wikipedia.org/wiki/Black_body 19. http://www.complore.com/black-body 20. http://en.wikipedia.org/wiki/Black_body#column-one 21. Thermoregulation, 2005. Harvard University, Division of Engineering and Applied Sciences: web site, http://www.deas.harvard.edu/courses/es96/ spring1997/web_page/ health/thermreg.htm, 20.07.2005. 22. THRELKELD J . L ., Thermal Environmental Engineering, 2nd edition, PrenticeHall Inc., New Jersey, 1970. 23. LAYTON J . M ., ‘The science of clothing comfort’, Textile Progress 31(1/2), 2001. 24. Thermal comfort (2005a). Environmental Engineering Science 1, http:// www.esru.strath.ac.uk/Courseware/Class-16293/6-Comfort.pdf, 21.07.2005. 25. STOECKER W. F. and JONES J . W., Refrigeration and Air Conditioning, 2nd edition, McGraw-Hill, Singapore, 1982. 26. OGULATA R. T., ‘The effect of garment on man’s thermal comfort’, Association for the Advancement of Modeling & Simulation Techniques in Enterprises 70(1,2), 15–26, 2001. 27. ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc, New York, 1977. 28. HAN T. and HUANG L., ‘A model for relating a thermal comfort scale to EHT comfort index’, SAE Technical Paper Series, 2004. 29. ENGLISH M . J. M ., ‘Physical principles of heat transfer’, Current Anesthesia & Critical Care 12, 66–71, 2001. 30. ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc, New York, 1993. 31. http://www.ugrad.math.ubc.ca/coursedoc/math100/notes/diffeqs/cool.html. 32. SNEŽANA B STANKOVIĆ, DUŠAN POPOVIĆ and GORAN B POPARIĆ ‘Thermal properties of textile fabrics made of natural and regenerated cellulose fibers’, Polymer Testing 27, 41–48, 2008. 33. SCHNEIDER A. M . and HOLCOMBE B. V., ‘Properties influencing coolness to the touch of fabrics’, Text Res J 61, 488, 1991. 34. KAWABATA S . and AKAGI Y., J. Text. Mach. Soc. Japan 30, T13, 1977. 35. YONEDA M ., KAWABATA S., ‘Analysis of transient heat conduction in textiles and its applications, Part II’, J Text. Mach. Soc. Japan 31, 73–81, 1983. 36. BS 4745 Method for the determination of thermal resistance of textiles. 37. ASTM D 1518 Thermal Transmittance of textile materials. 38. KAWABATA S. and NIWA M ., Modern Textile Characterization Methods, Marcel Dekker, New York, 329–354, 1996. 39. YIP J . and NG S.-P., ‘Study of three-dimensional spacer fabrics: Physical and mechanical properties’, J Matl Processing Tech 206, 359–364, 2008. 40. HES L., Thermal properties of nonwovens, in Proceedings of Congress Index 87, Geneva, 1987. 41. HUANG J ., ‘Thermal parameters for assessing thermal properties of clothing’, Journal of Thermal Biology 31, 461–466, 2006. 42. GAGGE A. P., BURTON A. C. and BAZETT H. C., ‘A practical system of units for the description of heat exchange of man with his environment’, Science 94, 428– 430, 1941. 43. GOLDMAN R. F ., Thermal comfort factor: concepts and definitions, editors Hollies N R S and Goldman R F. ‘Clothing comfort: interaction of thermal, ventilation, construction and assessment factors’, Ann Arbor Science, 1977.

Thermal transmission 44. 45. 46. 47. 48. 49. 50. 51. 52. 53.

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ASHRAE, ASHRAE Standard 55-1992 Thermal environmental conditions for human occupancy. American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc., Atlanta, 1992. PEIRCE F . T . and REES W. H ., ‘The transmission of heat through textile fabrics’, Part 2, J. Text Inst, T181–T204, 1946. WOODCOCK A . H ., ‘Moisture transfer in textile systems’, Part 1, Text Res J 32, 628–633, 1962. MCCULLOUGH E . A ., ‘Factors affecting the resistance to heat transfer provided by clothing’, J Therm Biol 18, 405–407, 1993. WERDEN J . E ., FAHNESTOCK M . K . and RUTH L ., ‘Thermal comfort of clothing of varying fiber content’, Text Res J 29, 640–651, 1959.Galbraith DAS A ., KOTHARI V . K . and BALAJI M., ‘Studies on cotton-acrylic bulked yarns and fabrics: Part II – Fabric characteristics’, J. Text Inst 98, 363– 375, 2007. DAS A ., ISHTIAQUE S . M . and SINGH R . P ., ‘Packing of micro-porous yarns. Part II – Optimization of fabric characteristics’, J. Text Inst 100(3), 207–217, 2009. DAS A . and ISHTIAQUE S . M ., ‘Comfort characteristics of fabrics containing twistless and hollow fibrous assemblies in weft’, Journal of Textile and Apparel, Technology and Management 3(4), 1–7, 2004. NILGÜN ÖZDIL , ARZU MARMARALI , SERAP DÖNMEZ KRETZSCHMAR , ‘Effect of yarn properties on thermal comfort of knitted fabrics’, International Journal of Thermal Sciences 46, 1318–1322, 2007. MIN K ., SON Y ., KIM C ., LEE Y . and HONG K ., ‘Heat and moisture transfer from skin to environment through fabrics: A mathematical model’, International Journal of Heat and Mass Transfer 50, 5292–5304, 2007.

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6 Moisture transmission

6.1

Introduction

Moisture transmission through textiles has a great influence on the thermophysiological comfort of the human body which is carried out through perspiration both in vapour and liquid form. The clothing to be worn should allow this perspiration to be transferred to the atmosphere in order to maintain the thermal balance of the body. Diffusion, absorption–desorption and convection of vapour perspiration along with wetting and wicking of liquid perspiration play a significant role in maintaining thermophysiological comfort. The scientific understanding of the processes involved in moisture transmission through textiles and the factors affecting these processes are important to designing fabrics and clothing assemblies with efficient moisture transfer in different environment and workload conditions. The processes which play the major role in moisture transmission in a particular situation are dependant on the moisture content of the fabric, the type of material used, the perspiration rate and the atmospheric conditions, such as humidity, temperature and wind speed. In a regular atmospheric condition and during normal activity level, the heat produced by the metabolism is liberated from body to atmosphere by conduction, convection and radiation and body perspires in vapour form to maintain the body temperature. At higher activity levels and/or at higher atmospheric temperatures, the production of heat is very high, which activates the sweat glands to produce liquid perspiration as well [1]. The vapour form of perspiration is known as insensible perspiration and the liquid form as sensible perspiration. When the perspiration is transferred to the atmosphere, it carries out heat (latent as well as sensible) thus reducing the body temperature. The fabric being worn should allow the perspiration to pass through; otherwise it will result discomfort. The perception of discomfort in the active case depends on the degree of skin wetness. During sweating, if the clothing moisture transfer rate is slow, the relative and absolute humidity levels of the clothing microclimate will increase suppressing the evaporation of sweat. This will increase rectal and skin temperatures, resulting in heat stress. It is also important to reduce the degradation of thermal insulation caused by moisture build-up. If the

106


107

ratio of evaporated sweat and produced sweat is very low, moisture will be accumulated in the inner layer of the fabric system, thus reducing the thermal insulation of clothing [2] and causing unwanted loss in body heat. Therefore, both in hot and cold weather and during normal and high activity levels, moisture transmission through fabrics plays a major role in maintaining the wearer’s body at comfort. Hence, a clear understanding of the role of moisture transmission through clothing in relation to body comfort is important [3]. The textile parameters which are affecting the clothing’s moisture transmission properties should be identified to achieve the required functionality of the clothing by engineering the fabric. The concept of clothing comfort and the factors influencing the same have been investigated by various researchers since 1930s till present date. Researchers have worked to understand the effect of environmental conditions on the moisture transmission properties of textile assemblies. Moisture transmission through textile materials can be divided into two basic principles, namely moisture vapour transmission and liquid water transmission. The mechanisms involved in transmission of vapour and liquid moisture through textile materials are quite different. Experimental work has been conducted by number of researchers to determine the influence of different material parameters, i.e., surface modification, fibre and fabric finish, fibre type, thickness and porosity of the material as well as ambient temperature and pressure on moisture vapour transmission characteristic of textile materials [4–9]. The liquid moisture transmission behaviour of a textile material is generally characterized by different methods, namely horizontal wicking, transplanar or transverse wicking and vertical wicking [10, 11]. The effects of different textile parameters, namely yarn twist, yarn tension, fibre cross-sectional shape, spinning technologies and texturing on the wicking behaviour of yarn have been reported by different researchers [12–16].

6.2

Liquid water transfer: wicking and water absorption

The transmission of moisture through textile materials in liquid form is mainly due to the fibre–water molecular attraction at the surface of the fibre materials, which is mainly determined by the surface tension and the effective capillary pore distribution. Liquid transfer through a porous structure involves two-stage process, i.e. initially wetting and then wicking [17]. Wetting is the initial process involved in fluid spreading. In this process the fibre–air interface is replaced with a fibre–liquid interface as shown in Fig. 6.1(a) [18]. The forces acting at a solid–liquid boundary under equilibrium are generally expressed by the following Young-Dupre equation,


109

In sweating conditions, wicking is the most effective process to maintain a feel of comfort. In the case of clothing with high wickability the moisture coming from the skin spreads throughout the fabric offers a dry feeling, and the spreading of the liquid enables moisture to evaporate quickly from larger surface. When the liquid wets the fibres, it reaches the spaces between the fibres and produces a capillary pressure. The liquid is forced by this pressure and is dragged along the capillary due to the curvature of the meniscus in the narrow confines of the pores as shown in the Fig. 6.1(b) [18]. The magnitude of the capillary pressure is given by the Laplace equation:

P=

2γ LV cos θ Rc

(6.2)

where P is the capillary pressure developed in a capillary tube of radius Rc. The difference in the capillary pressure in the pores causes the fluid to spread in the media. Therefore a liquid that does not wet the fibres cannot wick into the yarn or fabric [19]. The ability to sustain the capillary flow is known as wickability [20]. The distance travelled by a liquid flowing under capillary pressure, in horizontal capillaries, is given by the WashburnLukas equation:

L=

Rcγ cos θ 1 2 t 2η

(6.3)

Where, L is the capillary rise of the liquid in time t and η is the viscosity of the liquid. The amount of water that wicks through the channel is directly proportional to the pressure gradient. The capillary pressure increases as both the surface tension in the solid–liquid interface and the capillary radius decrease. A textile material consists of open capillaries, formed by the fibre walls [21]. From the Washburn-Lukas equation, it is expected that capillary rise at a specific time will be faster in a medium with larger pore size. However, Miller [22], using a comparative wicking study, showed that this is not always the case. He found that higher initial wicking through the capillaries with bigger diameter has been overtaken with time by the capillaries with smaller diameter. A larger amount of liquid mass can be retained in larger pores but the distance of liquid advancement is limited. This may be explained by the Laplace equation, as the radius of the capillary decreases, the pressure generated in the capillary will be higher, causing faster flow through the capillary. The model developed by Rajagopalan and Aneja [23] also predicts that at a constant void area increasing the perimeter of the filaments increases the maximum height attained by the liquid. Conversely, increasing the void area at a constant perimeter

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decreases the final height attained but increases the initial rate of liquid penetration. With the increase in the packing coefficient of the yarn, the fibres come closer to each other introducing a greater number of capillaries with smaller diameter likely to promote liquid flow. In any system where capillarity causes relative motion between a solid and a liquid, the shape of the solid surfaces is an important factor, which governs the rate and direction of liquid flow [24]. The shape of the fibres in an assembly changes the size and geometry of the capillary spaces between the fibres and consequently the wicking rate. With an increase in the non-roundness of a fibre, the specific area increases, thus increasing the proportion of capillary wall that drags the liquid. The tortuosity [25] of the pores has a great influence on the wicking process. It depends on the alignment of the fibres as well as on irregularities in the fibre diameter or shape along the pores. With the increase in the tortuosity of the pores the wickability reduces [15, 16, 26]. For instance, yarns spun with natural fibres have very irregular capillaries due to various factors such as fibre roughness, cross-sectional shape and limited length, which interrupt the flow along the length of the yarn. In the case of textured filament yarns, as the number of loops in the yarn increases, the continuity of the capillaries formed by the filaments decreases as the filament arrangement becomes more random. Under these conditions wicking is reduced. The same explanation is also applicable to the slower wicking found in twisted yarns. During the spinning process, at higher twist levels, slow migration of fibres takes place along the yam structure, changing the packing density and resulting in disruption of the continuity, length and orientation of the capillaries. The twist direction has no significant effect on the yarn wicking performance. The presence of wrapper filament retards the wicking as the volume of liquid in the capillaries reduces [13]. The density and geometry of fabric pores, which can be varied according to woven fabric structure, has a significant influence on the liquid flow pattern, both in the interstices and downstream. Darcy’s law is used to describe a linear and slow steady state flow through a porous media and is expressed by the following equation [18].

γ SV − γ SL = γ LV cos θ

(6.4)

The rate of flow (Q) changes directly with the pressure head (ΔP) and is inversely proportional to the length of the sample (L0) in the direction of flow. K is the proportionality constant, known as the flow conductivity of the porous medium with respect to the fluid. K is dependent on the properties of the fluid and on the pore structure of the medium [27]. Hydraulic conductivity can be written more specifically in terms of permeability and the properties of the fluids:


K=

k

η

111

(6.5)

where k is the permeability of the porous medium and is normally a function of the pore structure and η is the viscosity of the liquid. Capillary pressure and permeability are the two fundamental properties used to predict the overall wicking performance of a fabric. The capillary pressure decreases with an increase in the saturation as the pores fill with liquid and decreases to zero for a completely saturated media. The permeability of the media increases with an increase in the saturation, due to the higher cross-sectional area of the absorbed water film to flow [28]. At low saturation level, smaller pores in the media fill up first than larger pores. Wicking can not begin until the moisture content is very high [29]. Initially the liquid spreading in a fibrous material is facilitated by small, uniformly distributed and interconnected pores. On the other hand, high liquid retention can be achieved by having a large number of pores or a high total pore volume. The dynamic surface wetness of fabrics is an important parameter influencing the skin contact comfort in actual wear, as it is influenced by both the collection and the passage of moisture along the fabric [30]. The dynamic surface wetness of fabrics has been found to correlate with the skin contact comfort in wear for a variety of types of fabrics, suggesting that the mobility of thin films of condensed moisture is an important element of wearing comfort. Under normal unstressed condition the perspiration of a resting person amounts to about 15 g/m2h and under conditions of exertion or in a hot environment, the perspiration increases to a value that may exceed 100 g/m2h. Perspiration rate increases with the level of activity. The higher moisture collection by clothing after exercise, even in cold weather, creates a surprisingly discomfort sensation due to the presence of a certain amount of water in the skin clothing interface. Even very low moisture, as little as 3–5% moisture content in the garment, creates sufficient discomfort. Clothing thermal insulation also decreases due to the moisture accumulation, and the amount of reduction varies from 2 to 8%, as related to moisture collection within the clothing assemblies [31]. Thus, in case of those activities where production of sensible perspiration is very high, dynamic surface wetness is a very important factor. In the case of a cotton fabric, even though the moisture uptake from the skin is high due to high moisture regain, the dynamic surface wetness is not very good. Due to low capillarity the transfer of liquid moisture is not spontaneous. It collects moisture in stead of flowing it out. As a result, it creates a clammy feeling in high sweating condition. In the case of normal polyester fibre fabrics, even though capillarity is good, due to poor wettability they are not comfortable to wear. In the case of polyester

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microdenier fibre fabrics, the water up take is high and due to the high number of capillaries a large amount of moisture can pass very quickly through them to the atmosphere, thus providing a dry and comfortable feeling to the wearer [18].

6.2.1

Evaluation of liquid water transfer

Wettability The wettability of textile materials is tested to evaluate the wetting performance. This can be measured by the following methods. (i) Tensiometry – Tensiometer is an instrument used to measure the wettability of the fabric by measuring the wetting force by Wilhelmy method. In this method the wetting force (force applied by the surface, when liquid comes in contact with the surface) is measured. The contact angles are calculated indirectly from the wetting force when a solid is brought in contact with the test liquid using Wilhelmy principle [11]. (ii) Goniometry – In this method the wettability of a material is measured by measuring the contact angle between the liquid and the fabric by image processing method [32]. Automated Contact Angle Tester (ASTM D 572599), HTHP contact angle tester, drop analyzer tester have been developed based on this principle. Two processes are used, namely static wetting angle measurement and dynamic wetting angle measurement [33]. The dynamic contact angle is required as a boundary condition for modelling problems in capillary hydrodynamics, including certain stages of the droplet impact problem. The dynamic contact angle differs appreciably from the static advancing or receding values, even at low velocities. The dynamic contact angle can also be measured directly through low-power optics, but it leads to manual error. The dynamic contact angle depends on the spreading velocity of the contact line. To investigate the dynamic contact angle of impacting liquid droplets, a series of experiments were conducted by S¡ikalo et al. with individual droplets impacting onto dry and smooth solid surfaces [34]. To observe the spreading of a droplet, high resolution CCD camera (Sensicam PCO 1240 ×1024 pixels) equipped with a magnifying zoom lens was used. The magnification can be manipulated so that the image can accommodate the maximum spread of droplet [34]. Kamath et al. [35] have developed an apparatus to measure wettability of filament specimen using liquid membrane technique. The force exerted by the liquid membrane on the filament specimen as the ring with liquid membrane moves up or down the filament specimen is measured in this instrument, thus measures the wetting force. Manchester University developed UMIST wettability tester which gives the idea of wettability as well as initial wicking rate of the fabric.


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Skin dynamic wetness is a very important factor determining the contact comfort feeling of the skin. Clothing vapour resistance has been related with skin wetness and metabolic rate by the following equation [36]: w=

E sw E max + 0.06

(6.6)

where Esw is the regulatory sweat evaporation rate, Emax is the maximal evaporation rate possible in the ambient climate with the present clothing and skin temperature for a totally wet skin and 0.06 being the minimal skin wetness (or moisture evaporation) due to diffusion through the skin. ISO 7730 is used to determine skin temperature, sweat rates and ambient temperatures for comfort at various metabolic rates. In ISO 7730, required sweat evaporation at comfort is given as a function of metabolic rate [37]:

(

)

E sw Wm −2 = 0.42 (M − 58)

(6.7)

where M is the metabolic rate and the sweat evaporation (W/m 2). Scheurell et al. [30] have developed a technique to measure the fabric dynamic wetness. In that method they made it possible to observe the dynamic moisture change in the fabric by treating the fabric with cobaltous chloride before the experiment and to observe the change in the colour due to the absorption of moisture during the test. The general terms and units used for measuring absorption of fabrics are as follows: (a) Bulk Material Absorption (BMA) (g g –1 ) – it records the total absorption capacity of the fabric. (b) Bulk Absorption Rate (BAR) (g g–1s–1) – it calculates the amount of water absorbed vertically by 1 gm of fabric. (c) Bulk Absorption Time (BAT) (s) – it records the time in seconds it takes for the water to be absorbed vertically into the fabric. Wicking Liquid used for wicking test The liquid which generally used for testing the wicking (for fabric comfort evaluation purpose) of a fabric or yarn should represent close to human sweat. Research suggests that for clothing physiology studies, the test should employ with a liquid with surface energy properties similar to human perspiration and heated to human skin temperature of around 35°C. Literature says [38] that the principle electrolytes in sweat include sodium, chloride (sodium chloride is table salt) and potassium, potassium chloride

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(is often marked as a table salt substitute). Most human sweat contains at least 700 mg of sodium per liter, and probably averages around 1000 mg of sodium per litre. In an experiment, conducted by Simile [39], saline solution was produced using sodium chloride and water in order to simulate sweat. Sodium chloride (NaCl) has an atomic mass of 58 g/mol with the sodium atom occupying 40% of that mass; therefore, 1 g of sodium per litre equals 2.5 g of NaCl per litre. Converting volume to millilitres, the solution becomes 0.0025 g NaCl/ml or a 0.25% solution. This solution was used in a horizontal-downward wicking test with a fabric sample (7 × 1.5 cm). Comparing the results obtained by the perspiration simulant, distilled water and tap water, using the same test method, it has been observed that testing with distilled water can give a good indication of how a fabric would act when in contact with liquid perspiration [39]. Hernett and Mehta also found very minor differences in their results comparing heated human perspiration and distilled water [20]. Methods of measurement After wetting of the fibre, when the liquid reaches in the capillary, a pressure is developed which forces the liquid to wick along the capillary. Capillary penetration of a liquid can occur from an infinite (unlimited) or finite (limited) reservoir [40]. The different forms of wicking from an infinite reservoir are transplanar or transverse wicking, in-plane wicking and vertical or longitudinal wicking. A spot test is a form of wicking from a limited reservoir [11]. In case of vertical capillary rise gravity acts to slow down the flow rate before the equilibrium is reached [41], this phenomenon is not there in case of in-plane wicking. Different published standards propose wide range of test conditions for evaluation of a particular parameter. For example, BS 3424:1996, Method 21, specifies a very long time period (24 hours) and is intended for coated fabrics with very slow wicking properties. Whereas DIN 53924, 1978, specifies much shorter time (5 minutes maximum) and is therefore more relevant to the studies of clothing comfort involving the transfer of perspiration. The terms and units generally used for measuring wicking of fabrics are as follows: (a) Amount of Water Wicked (AWW) (g g–1) – it determines the wicking capacity of the fabric away from the absorption zone. (b) Surface–Water Transport Rate (SWTR) (gg–1s–1) – it calculates the amount of water wicked by 1 g of fabric per second. (c) Wicking Time (WT) (s) – it is the time in seconds for water to wick across a specified distance (3.25 cm).


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The terms spontaneous transplanar [40] or transverse wicking [3, 20] are used when the transmission of a liquid is through the thickness of the fabric, i.e. perpendicular to the plane of the fabric. Several techniques have been developed to measure transplanar liquid transport into fabrics. One of the test methods to measure the moisture accumulation associated with the wicking of liquid moisture from sweating skin [42]. The apparatus consists of a horizontal sintered glass plate kept moist by a water supply whose height can be adjusted so as to keep the water level at the upper surface of the plate. The specimen is placed onto the porous glass plate, and the uptake of water is measured by timing the movement of the meniscus along the long horizontal capillary tube. These tests have been used as simulation of a sweating skin surface. In case of in-plane wicking, the fabric surface stay in contact with the wetting liquid at a point and from there liquid flows through the capillaries along the fibre axis. An instrument has been developed by IIT Delhi in order to measure in-plane wicking of the fabric (Fig. 6.2). The instrument works on the siphonic principle and the water uptake by fabric sample with time is recorded. The fabric sample is placed on a horizontal base plate which is connected to the liquid reservoir by means of a siphon tube. The fabric is covered by a glass top plate so as to ensure intimate contact between the base plate and the fabric [43]. Similar instrument has been developed by Adams and Rebenfeld [27] to measure the in-plane flow of fluids in a fibrous mass, but they have used image analysis technique to obtain the shape and position of a radially advancing fluid front, which can define the directional permeability in the plane. There is a limitation with this type of instrument due to the use of the upper plate. The possibility arises that air bubble might be trapped in the fabric or between the plates and fabric, as there is no other path for the air bubble to escape other than the edges of the fabric. This could introduce an uncontrollable error. Another source of error with this method is that as the plates, both bottom as well as top, stay in contact of the fabric two extra capillaries are formed. One is between the bottom plate and the fabric and another one is between the fabric and the top plate. As a result it does not become possible to get actual wicking of the fabric. The alternative method of measuring the transverse wicking characteristics of fabrics is shown in Fig. 6.3. In this method also the transmission of water happens through the thickness of the fabric, i.e. perpendicular to the plane of fabric. In this test the fabric sample is placed on the top of the moist glass plate as shown in the Fig. 6.3. The sintered glass plate can draw water depending on the wicking power. The water level should just touch the bottom surface of the fabric, but not flood it. The rate of water absorption is measured by the movement of the meniscus along the long horizontal capillary tube.


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been mounted above the plate to record the spreading of the liquid. This instrument is placed on a compression load cell and is connected to the bottom of a plate using a plastic tube. This instrument replaces the electronic controls with an A/D interface card and appropriate software, which automatically maintains the platform height at the same level, maintaining a constant pressure head for the testing. They have developed new types of plates to eliminate the extra capillaries and determine intrinsic wicking ability of the fabric. In the middle of the plate there is a cylinder where the liquid enters the system and that is the initial point of absorption/ wicking and is also the only point at which the fabric is touching the plate. Liquid spreading distribution was determined by analyzing the image captured by attached camera. Several techniques have been developed for measurement of vertical wicking characteristics. In visual observation method the movement of the liquid along the sample is observed (Fig. 6.4). Sample is hung from a horizontal fabric hanging attachment and lowered vertically into a reservoir. The sample comes into contact with the contents of the reservoir at a perpendicular direction. A little amount of dye is added in the water, which can enhance the clear observation of the liquid. The movement of the liquid, in terms of height wicked by the water, is measured with time. Microscopic observation has also been used by some researchers [25, 46, 47]. A certain amount of load should be hung at the lower end of the sample to keep it straight.

Scale Scale

Fabric Fabric

Clamp Clamp

Reservoir 6.4 Fabric vertical wicking tester.

Ansari and Haghighat [48] have developed an apparatus to study water transport behaviour along yarn, using electrical resistance technique. This technique is based on the phenomenon of the difference in electrical

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conductivity of air and water. Electrical conductivity of water is 18 times that of air; so as the liquid wicks along the sample, electrical resistance get reduced. The rise of the liquid water in the sample triggered an electrical circuit which was coupled with a personal computer, so the distance of rise as a function of time is determined. Electrical resistance technique for fabrics was applied by many other authors as well [16, 49]. Hu et al. [50] have developed a moisture management tester to characterize fabric liquid moisture management properties, based on the same principle. So measuring the electrical resistance of the fabric by this tester, it has been correlated with the fabric moisture content. Ten indices have been introduced to characterize the liquid moisture transmission. This method is also used to measure the liquid water transfer in one step in a fabric in multi-direction, as the liquid moisture spreads on both surface of the fabric and transfer from one surface to the opposite. Water transport along textile fibres has also been studied by various scientists using electrical capacitance technique. Ito and Muraoka [12] have developed an apparatus based on the electrical capacitance technique. Similar apparatus was also developed by Tagaya et al. [51]. The dielectric constant of water is 80 times more than that of air, so the water transport can be accurately detected by measuring the change in electrical capacitance between the condensers. The electric current generated by the change in electric capacitance, which is caused by the transport of water, is converted into voltage by a current to voltage converter circuit.

6.3

Principles of moisture vapour transfer

The moisture vapour transmission property of a fabric is essentially governed by inter-yarn or inter-fibre spaces. The vapour diffuses through the air spaces between the fibrous materials. The relatively open fabric structure promotes the diffusion process. From Fig. 6.4 it can be observed that during the diffusion of moisture vapour through textiles materials the resistance to the moisture vapour diffusion comes in different layers. These different layers are (i) evaporating fluid layer (which remains full of water saturated vapour), (ii) confined air layer (between the skin and fabric), (iii) boundary air layer, and (iv) ambient air layer. Moisture vapour resistance mainly depends on the air permeability of the fabric and represents its ability to transfer the perspiration coming out of the human skin. The resistance provided by the fabric is lower than that of the external boundary layer and often much lower than the inner confined air layer between skin and fabric.


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Ambientair airlayer layer Ambient Boundaryair air layer Boundary layer Fabriclayer layer Fabric

Evaporating Evaporatingfluid fluid layer layer Human skin Human skin 6.5 Different layers through which moisture vapour transports.

Moisture in vapour form transmit through textile materials by the following mechanisms: ●

●

● ●

Diffusion of the water vapour through the air spaces between the fibres. Absorption, transmission and desorption of the water vapour by the fibres. Adsorption and migration of the water vapour along the fibre surface. Transmission of water vapour by forced convection.

6.3.1

Diffusion

In the diffusion process, the vapour pressure gradient acts as the driving force in the transmission of moisture from one side of a textile layer to the other. The diffusion of moisture vapour through the fibrous assembly is a mass transfer phenomenon which occurs on a molecular level at lower speed. The moisture vapour is transported from the higher concentration zone to the lower concentration zone. Fick’s law [52] proposed the relation between the flux of the diffusing substance and the concentration gradient with the help of following relationship:

J Ax = DAB

dC A dx

(6.8)

Where, JAx is the rate of moisture flux; dCA/dx is the concentration gradient; and D AB is the diffusion coefficient or mass diffusivity of one component diffusing through another media. The diffusion which follows Fick’s law is called Fickian diffusion. In this case the diffusion coefficient does not alter with the changes in the moisture vapour concentration within the material or with the changes in temperature. In case of air permeable fabrics and micro-porous polymers this type of diffusion takes place. The diffusion which does not follow this law is called non-Fickian diffusion.

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Hydrophilic polymers transfer water vapour according to non-Fickian diffusion. The water vapour transmission rate of the hydrophilic polymers conforms to the following relationship:

WVT = DS

( p1 − p2 ) / l

(6.9)

where, (p 1 – p 2) = partial pressure gradient between the two surfaces; l = thickness of the polymer; D = diffusion constant; and S = solubility coefficient. Moisture vapour can diffuse through a textile medium by two principles, namely simple diffusion through the air spaces within the fibrous structure and diffusion along the fibre itself [53, 54]. In the case of diffusion along the fibre, moisture vapour diffuses from one surface of the fabric to the surface of fibre and then travels along the interior of the fibres and its surface and finally reaches the other surface of fabric. At a specific concentration gradient the diffusion rate along the textile material depends on the porosity of the material and also on the water vapour diffusivity of the fibre. The diffusion coefficient of water vapour through air is 0.239 cm 2s –1 and through cotton fabric is around 10–7 cm 2 s –1. The moisture diffusion through the air portion of the fabric is almost instantaneous whereas through a fabric system it is limited by the rate at which moisture can diffuse into and out of the fibres, which is due to the lower moisture diffusivity of the textile material [55]. In the case of hydrophilic fibre assemblies, vapour diffusion does not obey Fick’s law. It is governed by a non-Fickian, anomalous diffusion [56, 57]. In this case the diffusion process completes in two stages. The first stage corresponds to Fickian diffusion but the second stage is much slower which follows an exponential relationship between the concentration gradient and the vapour flux [58–60]. This diffusion process can be explained by swelling of the fibres. Due to the affinity of the hydrophilic fibre molecules to water vapour, as it diffuses through the fibrous system, it is absorbed by the fibres causing fibre swelling and reducing the size of the air spaces, thus delaying the diffusion process [61]. According to Li et al. [62] this phenomenon is mainly due to the fact that the heat of sorption, produced during absorption of moisture vapour, increases the temperature of the fibrous assemblies, which in turn affects the rate of moisture transmission. The moisture diffusivity of a textile material is influenced by a number of factors. It decreases with an increase in the fibre volume fraction of the material. With the increase in fibre volume fraction the proportion of air within the fibrous assembly decreases, this in turn reduces the total diffusivity. The moisture diffusivity through the fabric decreases with an increase in the flatness of the fibre cross-section [63]. With an increase in fabric thickness, the porosity of the material is reduced, thus reducing the diffusion rate.


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Water vapour diffusion is directly correlated with the air permeability of the fabric. As the fabric porosity increases the air permeability increases, which also results in higher moisture transmission through the air spaces within the fabric. The type of finish applied (i.e. hydrophilic or hydrophobic) to a fabric surface has no significant effect on the diffusion process [4]. The diffusion co-efficient of moisture vapour in air can be given as a function of temperature and pressure by the following equation [64]: 2

⎡θ ⎤ ⎡P ⎤ D = 2.20 ×10 ⎢ ⎥ ⎢ 0 ⎥ ⎣θ0 ⎦ ⎣ P ⎦ −5

(6.10)

where D is the diffusion co-efficient of water vapour in air (m 2/sec), θ is the absolute temperature (K), θ0 is the standard temperature of 273.15 K, P is the atmospheric pressure and P0 is the standard pressure (bar). In general, the diffusion co-efficient of fibres increases with the increase in the concentration of water in the fibres; an exception to this behaviour is shown by polypropylene due to its hydrophobicity [56]. The water vapour transmission through fabrics increases with the increase in the moisture content and in the condensation of moisture vapour within the fabric [28].

6.3.2

Sorption–desorption

Sorption–desorption is an important phenomenon of moisture vapour transmission which is responsible for maintaining the microclimate during transient conditions. The hygroscopic fibrous materials absorb moisture from the humid air close to it and release this absorbed moisture in dry air. This process enhances the transmission of moisture vapour from the human skin to the environment. The transmission of moisture vapour in case of hygroscopic materials is higher than materials which do not absorb moisture and thus reduce the moisture built up in the microclimate [65, 66]. During absorption–desorption process the absorbing fabric works as a moisture source to the atmosphere [67]. It also works as a buffer by maintaining a constant vapour concentration in the air immediately surrounding it, i.e. a constant humidity is maintained in the adjoining air, though temperature changes due to the heat of sorption. Adsorption of water molecules takes place below a critical temperature, due to the van der Waal’s forces between the moisture vapour molecules and the solid surface of the textile materials. The higher the vapour pressure and the lower the temperature, the higher is the amount absorbed [65]. The amount of moisture vapour absorbed by the textile materials depends on the moisture regain of the fibre and the relative humidity of the atmosphere. In the case of hygroscopic fibres (e.g.

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cotton, viscose, wool) the moisture sorption is dependent not only on the moisture regain and environmental humidity, but also on the phenomena associated with sorption hysteresis, temperature, dimensional changes, elastic recovery, swelling of the fibres etc. When fibre absorbs moisture the fibre macromolecules or microfibrils are normally pushed apart by the absorbed water molecules and fibre swelling takes place. This reduces the inter-fibre as well as inter-yarns pores, thus reduces the water vapour transmission through the fabric. With the increase in fibre swelling the capillary channels between the fibres get blocked which results lower wicking. Moreover, the distortion caused by the fibre swelling results in built up of internal stresses which affects the moisture sorption process. The mechanical hysteresis of the fibres enhances the adsorption hysteresis [10]. The adsorption hysteresis increases with the increase in the hydrophilicity of fibre.

6.3.3

Forced convection

The transmission of moisture vapour that takes place while air is flowing over a moisture layer is known as forced convection. The amount of moisture transmission in this process is governed by the difference in moisture concentration between the surrounding atmosphere and the source of moisture vapour. The process is governed by the following equation [68]:

Qm = − A hm (Ca − Cα )

(6.11)

where Q m is the mass of moisture vapour transmitted by convection through the fabric area of A along the direction of the flow, C a is the moisture vapour concentration on the fabric surface and C α is the vapour concentration in the air. The rate of moisture transmission can be controlled by the difference in vapour concentration, (C a – C α), and the convective mass transfer coefficient h m, which depends on the fluid properties as well as on its velocity. In a windy atmosphere the convection method plays a very significant role in transmitting moisture from the skin to the atmosphere through clothing [69, 70]. Evaporation and condensation also have significant effects on moisture vapour transmission through porous textile materials. These depend on the temperature and moisture distribution in porous textile materials during the transmission of moisture vapour transmission [71]. During the evaporation of body perspiration in liquid form the latent heat is taken away from the body and the body cools down. The importance of evaporative heat transfer in maintaining thermal balance becomes more crucial with the increase in the surrounding atmospheric temperature. In this case, due to the low temperature difference between the human body


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and the environment the heat transmission through conduction and convection reduces [72]. When a negative temperature gradient exists between the skin and the environment, evaporative heat transfer becomes the only way to cool down the body temperature. As the latent heat of evaporation of water is vary large (about 2300 kJ/kg) a small amount of evaporation of perspiration results in significant amount of heat flow [73]. The presence of wind enhances the evaporative heat transfer due to enhanced evaporation rate and results in additional cooling that is desirable in periods of peak performance.

6.4

Condensation of moisture vapour

Condensation of moisture vapour is a direct result of a fabric being saturated by liquid perspiration and it generally occurs within the fabric whenever the local vapour pressure increases to saturation vapour pressure at certain temperature [74]. Condensation normally occurs when the atmospheric temperature is very low. When the relatively warmer and moist air from the skin comes into contact with relatively cooler fabric surface, the fabric surface works as a cold wall and condensation occurs. It has been reported that condensation can occur at atmospheric temperatures below 10°C [75]. In the case of water proof fabrics, where the water vapour can diffuse from the skin to the fabric layer more easily than from the fabric layer to the atmosphere, the chances of occurrence of condensation is very high. In most of the cases the condensation of moisture vapour in an initially dry porous fibrous material takes place in three stages. At the first stage velocity, temperature and vapour concentration fields are developed within the material and condensation begins. In the second stage, the liquid content increases gradually, but it is still too low to move and finally, as the liquid content increases further and goes beyond certain threshold value, the pendulum-like drops of condensate coalesce and begin to move under surface tension and gravity [76]. When the vapour concentrations in both the surfaces of the fabric are at the saturation level, condensation of moisture vapour occurs in the entire thickness of the fabric. If the moisture vapour concentration at the two faces is below saturation level at a specific atmospheric temperature, condensation occurs only over certain region within the fabric. In this case, condensation of moisture vapour occurs in the fabric, which forms a wet zone separated by two dry zones [75, 76]. The proportion of the wet region increases with the increase in condensation of moisture vapour. The progress of condensation process of moisture vapour takes place mainly in the direction of the warmer side rather than that of the cooler side [77].

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6.5

Evaluation of moisture vapour transmission

The measurement of moisture vapour transmission through fabrics is a slow and somewhat delicate operation but can be carried out very effectively. Following are the different standard methods used for determining the moisture vapour transmission characteristics of textile materials. (i) Evaporative dish method or control dish method (BS 7209); (ii) Upright cup method or Gore cup method (ASTM E 96-66); (iii) Inverted cup method and the desiccant inverted cup method (ASTM F 2298); (iv) The dynamic moisture permeable cell (ASTM F 2298); and (v) The sweating guarded hot plate method, skin model (ISO 11092). Different terms are used to express the moisture vapour transmission characteristics of textile materials. Results obtained from different available methods are not always comparable due to the different testing conditions and the units of measurement. Following terms and related units are used for expressing the moisture vapour permeability of the fabrics [28, 54, 77]: ●

●

●

●

The percentage water vapour permeability index, WVP (%), is used in the evaporative disc method (BS 7209). This method uses water at 20°C and an atmospheric condition of 20 ± 2°C and 65 ± 2% relative humidity. This standard is based on the control dish method (CAN2-4.2-M77) and the Gore modified disc method (BPI 1.4). The moisture vapour transmission rate (g/m2/Day) is used in the cup method (ASTM E96-66). The resistance to evaporative heat transfer, Ret (m2Pa/W), is used in the sweating guarded hot plate (ISO 11092:1993, EN 31092). It is an indirect method of measuring the vapour transmission property of a fabric. In this test method, the experiment is carried out in an isothermal condition at the standard atmospheric condition. The resistance of equivalent standard still air (cm) is used in the holographic visualization method. In this method it is possible to measure the resistance offered by the fabric layer and the air layer separately. The resistance of the fabric can be expressed in terms of the standard still air (cm) providing the same vapour resistance.

The different methods used for determining the moisture vapour transmission characteristics of textile materials are given below.

6.5.1

Evaporative dish method

In this method water vapour transmission rate through fabric is measured according to BS 7209 standard (Fig. 6.6). This method works on the basis


125

of gravimetric measurement. The specimen under test is sealed over the open mouth of a dish containing water and placed in the standard atmosphere for testing. After a period of time the total system reaches to equilibrium. The successive weighing of the dish is made and the rate of water vapour transfer through the specimen is calculated. The steady state water vapour permeability is measured in this method. The relative water vapour permeability of the sample is calculated by comparing the result with a reference fabric. Moisture Vapour

Fabric specimen

Dish containing water

6.6 Moisture vapour permeability tester.

Water vapour permeability (WVP) = 24 M/A. t (g/ m 2/day); and Relative water vapour permeability index (%) = (WVP)f × 100 / (WVP)r where M is the loss in mass (g) of water vapour through the fabric specimen, t is the time between weighing (h), A is the internal area of the dish (m2), (WVP)f and (WVP)r are the water vapour permeability of the test fabric and reference fabric, respectively.

6.5.2

Upright cup method

This method is similar to that of evaporative dish method. In this method the water vapour transmission rate of fabric is measured according to ASTM E 96-80 procedure B. A shallow cup is filled with 100 ml distilled water and a circular sample with a diameter of 74 mm is mounted on the cup by covering with a gasket and clamping into its position (Fig. 6.7). The cup assembly is housed in an environmental chamber. The air temperature in the chamber is set to 23°C, and the relative humidity is controlled at 50%. The air velocity in the chamber is maintained at 2.8 m/s. The cup assembly is weighed to the nearest 0.001 g on a top loading balance periodically throughout one day. The water vapour transmission rate is calculated as,

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WVT =

24 × G ; g / m2 / 24h A×T

(6.12)

where WVT = water vapour transmission rate (g/m2/day); G = change in mass (g); T = testing time (h); A = test area (m 2). Fabric specimen

Ring Air layer Water Upright cup

6.7 Upright cup mthod.

6.5.3

Inverted cup test method

In this method the water vapour transmission rates of fabrics are measured according to ASTM E96, Procedure BW. To prevent the water in the cup from wetting the specimen in the inverted test, a piece of hydrophobic PTFE membrane is used to seal over the mouth of the cup. The test specimen is placed over the membrane. The cup assembly, as shown in Fig. 6.8, is placed in an inverted position on the upper deck. The cup assembly is weighed periodically throughout one day. The calculations are the same as that for the upright cup test. The inverted cup method is designed mainly for use with waterproof samples, because the fabrics which allow the passage of liquid water may not be inverted as they will leak.

6.5.4

Desiccant inverted cup method

This method for measuring water vapour permeability of fabrics works as per ISO 15496 2004 standard [78]. In this method water vapour transmission rates of fabrics are measured in the same way as inverted cup test method. Only difference is that in this method the cup used in this method is partly filled with desiccant such as potassium acetate, calcium


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Water Inverted cup

Ring

Fabric specimen

6.8 Inverted cup method.

chloride, anhydrous CaSO4 or anhydrous MgClO4. The drying agent stays in direct contact with fabric minimizing the path of water vapour. The inverted cup is covered by the specimen and the specimen is covered by another piece of waterproof and vapour permeable membrane. The inverted cup along with specimen is immersed into the water bath filled with distilled water with the help of specimen holder. The measuring cup initially is weighed by means of a balance then inverted and inserted into the specimen holder. After certain time (t), the measuring cup is removed and reweighed. The water vapour permeability of the specimen is then calculated by using the following equation:

WVT = t × ( w2 − w1 ) / a

(6.13)

Where WVT is water vapour transmission rate; w2 = mass of cup assembly after test; w1 = mass of test cup assembly body before test; a = test area.

6.5.5

Sweating guarded hot plate method

The sweating guarded hot plate apparatus or “Hohenstein” skin model [72, 80] is used to measure the thermophysiological comfort of clothing. It works as per ISO 11092 standards. It simulates the moisture transport through textiles and clothing assemblies when worn next to the human skin. This model measures the water vapour resistance of the fabric by measuring the evaporative heat loss in the steady state condition. The temperature of the guarded hot plate is kept at 35°C (i.e. the temperature of the human skin) and the standard atmospheric condition for testing (65% R.H. and 20°C) is used. In this skin model, A is the test area; Pm is the saturation water vapour partial pressure at the surface of the measuring unit; Pa is the water vapour partial pressure of the air in the test chamber;

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H is the amount of heat supplied to the measuring unit; ΔH C is a correction factor and Ret0 is the apparatus constant. The water vapour resistance of the fabric (Ret) may be calculated as follows:

Ret =

6.5.6

A(Pm − Pa ) − Ret 0 (m 2Pa/ W) H − ΔH c

(6.14)

The PERMETEST apparatus

This fast response measuring instrument (skin simulator) has been developed by Hes [82] for the measurement of the water vapour permeability of textile fabrics, garments, nonwoven webs and soft polymer foils. It works on the principle of heat flux sensing, i.e. by measuring the evaporative heat resistance. The temperature of the measuring head is maintained at room temperature for isothermal conditions. The heat supplied to maintain the temperature of the measuring head, from where the supplied water gets evaporated, is measured. The heat supplied to maintain a constant temperature with and without the fabric mounted on the plate is measured. This instrument measures the relative water vapour permeability (%) of the fabric in the steady state isothermal condition using the following equation: Relative water vapour permeability (%) =

Heat lost when the fabric is placed on the measuring head ×100 (6.15) Heat lost from the bare measuring head

The PERMETEST can be used according to both BS 7209 and ISO 9920 standards. If the ring above the measuring head is used, a separating air layer will be created between the measuring head (simulated skin) and the fabric layer, thus providing the measuring condition according to BS 7209. On the other hand, if the ring above the measuring head is not used, the fabric will be in direct contact with the measuring head, i.e. according to the conditions used for the ISO 9920 standard.

6.5.7

Moisture vapour transmission cell

This is a faster and more simplified method for measuring the water vapour transmission behaviour of fabrics. In principle, the cell measures the humidity generated under controlled conditions as a function of time. There are two cells, namely lower and upper cells. The cells are separated by the test specimen. The lower cell is partially filled with water and the upper cell is almost dry at the start of the test. As the moisture vapour is


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transmitted through the fabric sample the relative humidity of the upper cell increases with the time. The change in humidity at a given time interval represents the moisture vapour transmission rate (T) of the fabric. The standard relationship is, ⎛ 1440 T = 269 × 10 − 7 ⎜⎜ Δ % RH × Time Interval ⎝

(

6.5.8

)

⎞ ⎟⎟ g/in2/day ⎠

(6.16)

Dynamic moisture permeable cell

The dynamic moisture permeable cell (DMPC) method is capable of evaluating the moisture transmission properties of textiles under various conditions, i.e. pure diffusion, combined diffusion and convection and pure convection [69, 82]. The convective flow is evaluated by measuring the relative pressure drop at the bottom outlet. The convective flow through hygroscopic porous materials is complicated, due to the tendency of the fibres to take up water vapour and experience fibre swelling. The change in the fabric connective flow properties has been taken as a function of relative humidity. The DMPC can be used to obtain both steady state and dynamic state data.

6.5.9

Holographic bench technique

Holographic bench technique has been developed by Berger and Sari [83]. In this method the mass flow is measured with high accuracy using a microweighing technique. The resistance to the water vapour transfer depends on the resistance of the air layer and the outer clothing. The resistance offered by the fabric layer in vapour transmission from the skin to the atmosphere is much lower than that offered by the external boundary air layer and often much lower than that of the inner confined air layer between the skin and the fabric. So, in order to measure the flow resistance of a textile, one also needs a precise determination of the surrounded air layers. Holographic bench technique separately measures the water vapour flow resistance offered by different air layers; thus it provides the precise vapour resistance value of the textile layer.

6.6

Moisture sensation in clothing

The moisture sensation of a clothing ensemble is the primary reason for wearers’ dissatisfaction with the comfort properties of clothing. The problem is intensified further for functional apparel because this sort of clothing is frequently worn under stressful environmental conditions in

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which moisture accumulates on the skin and within the clothing layers and contributes to wearer discomfort. While efforts are being made in the objective measurement of fabric moisture properties with the anticipation of relating them to actual wearing conditions, there has been little refinement of the subjective measurement of such factors. Moisture sensation of clothing can be expressed either in terms of absolute threshold or in terms of difference threshold.

6.6.1

Absolute threshold

The absolute threshold is defined as the minimum value of a physical stimulus that will evoke a sensation. There may be some areas on the surface of the body that are not as sensitive to moisture as others, and they may differ greatly from person to person. The overall percentage of moisture in clothing related it to sensations of comfort /discomfort, but the amount of moisture that must accumulate in the first place before one even detects has not been considered.

6.6.2

Difference threshold

The difference threshold is the minimum amount of stimulus change required to produce a sensation difference, referred to as the just noticeable difference on the psychological continuum. E. H. Weber, a 19th century physiologist, determined that physical stimulus intensity must be increased by a constant fraction of its starting value in order to be just noticeably different. Weber’s law is written as

∆Φ/Φ = c

(6.17)

where Δ Φ is the change in stimulus intensity required to be just noticeably different; c is a constant fraction of the starting stimulus intensity. Weber’s prediction has been confirmed for a wide range of stimulus intensities and sensory modalities and is shown to be an extremely useful calculation providing an index of sensory discrimination that can be compared across different conditions and modalities. Because of the fact that the Weber’s fraction is a unit-less measure, it serves as an index of sensory discrimination that can be compared across different conditions. For evaluating the moisture sensitivity using different fabric stimuli, one should compare the values of the Weber fractions to examine the effect of fabric stimulus on moisture sensitivity. Weber’s fraction should only be considered as an approximation of differential sensitivity; however, since it increases dramatically at levels of stimulus intensities near the absolute threshold.


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It may be noted that the moisture sensation is only one of the many sensations contributing to clothing comfort. Future investigations of moisture sensation or other sensorial comfort variables could examine the effects of various levels of these factors on subject sensitivity [84].

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MCARDLE W., KATCH F ., KATCH V ., Exercise Physiology-Energy, Nutrition and Human Performance, 4th edition, Williams and Wilkins, Baltimore, 1996. SIMILE C . B., Critical evaluation of wicking in performance fabrics, Master thesis, School of Polymer, Textile, and Fiber Engineering, Georgia Institute of Technology, 2004. KISSA E ., ‘Wetting and wicking’, Text Res J 66(10), 660–668, 1996. D ´ SILVA A . P ., GREENHOOD C ., ANAND S . C ., HOLMES D . H . and WHATMOUGH N ., ‘Concurrent determination of absorption and wickability of fabrics: A new test method’, J Text Inst 91(3), 383–396, 2000. YOO S. and BARKER R. L ., ‘Comfort properties of heat-resistant protective workwear in varying conditions of physical activity and environment. Part I: Thermophysical and Sensorial Properties of Fabrics’, Text Res J 75(7), 523–530, 2005. CHATTOPADHYAY R . and CHAUHAN A ., ‘Wicking behavior of compact and ring spun yarns and fabrics’, in One-day Seminar on Comfort in Textiles’, Indian Institute of Technology, Delhi, 2004, October 16, 20–30. MILLER B . and TYOMKIN I ., ‘Spontaneous transplaner uptake of liquids by fabric’, Text Res J 54, 706–712, 1984. KONOPKA A. and POURDEYHIMI B., ‘In-plane liquid distribution of nonwoven fabrics. Part 1– Experimental observations’, INJ Winter 22–27, 2002. SENGUPTA A . K . and SHREENIVASA MURTHY H . V ., ‘Wicking in ring spun vis-à-vis rotor spun yarns’, Text Res J 55, 155–157, 1985. NYONI A. B. and BROOK D., .Wicking mechanisms in yarns-the key to fabric wicking performance’, Text Res J 97(2), 119–128, 2006. ANSARI N . and HAGHIGHAT K ., ‘The wicking of water in yarn as measured by an electrical resistance technique’, J Text Inst 91(3), 401–419, 2000. KAMATH Y . K., HORNBY S., WEIGMAN H. D. and WILDE M. F ., ‘Wicking of spin finishes and related liquids into continuous filament yarns’, Text Res J 64(1), 33–40, 1994. HU J ., LI Y ., YEUNG A . K . W., WONG S . W . and XU W., ‘Moisture management tester: A method to characterize fabric liquid moisture management properties’, Text Res J 75(1), 57–62, 2005. TAGAYA H ., HAIKATA J ., NAKATA K . and NISHIZAWA K ., ‘Measurement of capillary rise in fabrics by electric capacitance method’, Sen-I Gakkaishi 47, 422–430, 1987. SACHDEVA R . C ., Fundamentals of Engineering Heat and Mass Transfer, 2nd edition, New Age International (P) Ltd., India, 2005. FOHR J . P ., ‘Dynamic heat and water transfer through layered fabrics’, Text Res J 72(1), 1–12, 2002. LOMAX G . R ., ‘The design of waterproof, water vapour-permeable fabrics’, J. Coated Fabrics 15(7), 40–49, 1985. KOTHARI V . K ., Quality Control: Fabric Comfort, Indian Institute of Technology, Delhi, 2000. REN Y . J . and RUCKMAN J . E ., ‘Water vapour transfer in wet waterproof breathable fabrics’, J Indus Text 32(3/1), 165–175, 2003. MORTON W. E . and HEARLE J . W. S ., Physical Properties of Textile Fibres, Textile Institute, Manchester, 1993. NORDON P ., MACKAY B . H ., DOWNES J . G . and MCMAHON G . B ., ‘Sorption kinetics of water vapour in wool fibres: Evaluation of diffusion coefficients and analysis of integral sorption’, Text Res J 30, 761–770, 1960. LI Y . and HOLCOMBE B . V ., ‘A two-stage sorption model of the coupled diffusion of moisture and heat in wool fabrics’, Text Res J 62(4), 211–217, 1992.

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and DAVID H. G., Int J Heat Mass Transfer 10, 1967. ‘Measuring the water vapor permeability of coated fabrics and laminates’, J Coated Fabrics 25(4), 311–320, 1996. LI Y ., HOLCOMBE B . V ., SCHEIDER A . M . and APCAR F ., ‘Mathematical modelling of the coolness to the touch of hygroscopic fabrics’, J Text Inst 84(2), 267–273, 1993. WOO S . S ., SHALEV I. and BARKER L ., ‘Heat and moisture transfer through nonwoven fabrics. Part II: Moisture diffusivity’, Text Res J 64(4), 190–197, 1994. JOST W., Diffusion in Solid, Liquids and Gases, Academic Press, New York, 1960. BARNES J . C . and HOLCOMBE B . V ., ‘Moisture sorption and transport in clothing during wear’, Text Res J 66(12), 777–786, 1996. HONG K ., HOLLIES N . R . S . and SPIVAK S . M ., ‘Dynamic moisture vapour transfer through textiles’, Text Res J 68(12), 697–706, 1988. SUPRUN N ., ‘Dynamics of moisture vapour and liquid water transfer through composite textile structures’, Int J Clothing Sci & Tech 15(3/4), 218–223, 2003. INCROPERA F . P ., and DEWITT D . P ., Fundamentals of Heat of Mass Transfer, 4th edition, John Wiley and Sons, New York, 1996. GIBSON P ., KENDRICK C ., RIVIN D . and SICURANZA L ., ‘An automated water vapour diffusion test method for fabrics, laminates and films’, J Coated Fabrics 24(4) 322–345, 1995. GIBSON P . W. and CHARMCHI M., ‘Modelling convection/diffusion processes in porous textiles with inclusion of humidity-dependent air permeability’, Int. Comm. Heat Mass Transfer 24(5), 709–724, 1997. LI Y . and ZHU Q ., ‘Simultaneous heat and moisture transfer with moisture sorption, condensation and capillary liquid diffusion in porous textiles’, Text Res J 73(6), 515–524, 2003. HAVENITH G ., HOLMER I., HARTOG E . A . D . and PARSONS K . C ., ‘Clothing evaporative heat resistance – proposal for improved representation in standards and models’, Ann Occup Hyg 43(5), 339–346, 1999. SCHNEIDER A . M . and HOSCHKE B . N ., ‘Heat transfer through moist fabrics’, Text Res J 62(2), 61–66, 1992. RUCKMAN J . E ., ‘Analysis of simultaneous heat and water vapour transfer through waterproof breathable fabrics’, J Coated Fabrics 26, 293–307, 1997. HOLMER I ., ‘Protection against cold’, Textiles in Sport, editor Shishoo R., Woodhead Publishing Limited, Cambridge, 2005. MURATA K ., ‘Heat and mass transfer with condensation in a fibrous insulation slab bounded on one side by a cold surface’, Int J Heat Mass Transfer 17(38), 3253–3262, 1995. REN Y . J . and RUCKMAN J . E ., ‘Condensation in three-layer waterproof breathable fabrics for clothing’, Int J Clothing Sci and Tech 16(3), 335–347, 2004. ISO 15496 2004 Textiles – Measurement of Water Vapour Permeability of Textiles for the Purpose of Quality Control, International Organization for Standardization, Geneva, Switzerland, 2004. HUANG J . and QIAN X ., ‘Comparison of test methods for measuring water vapor permeability of fabrics’, Text Res J 78, 342, 2008. CONGALTON D ., ‘Heat and moisture transport through textiles and clothing ensembles utilizing the “Hohenstein” skin model’, J Coated Fabrics 28(1), 183– 196, 1999. PAUSE B .,

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7 Dynamic heat and mass transmission

7.1

Introduction

Dynamic transmissions of dry and wet heat are the important characteristics of a clothing assembly to characterize its thermophysiological behaviour. Each fabric layer is characterized by the two types of resistances, namely the resistance to dry heat (insulation) and moisture vapour resistance. The dry resistance is proportional to the thickness of the fabric layer, whereas for moisture vapour permeability along with other parameters the construction of the fabric layer is also important. Both for the insulation and the moisture vapour permeability the thickness of the enclosed air layers in an ensemble is of major importance. Insulation is increased more by a non-moving air layer than by a textile layer. The moisture vapour permeability is generally more dependent on the fabric parameters, but it decreases with increasing thickness of air layers. When a person is in dynamic state the air enclosed within the clothing also comes into dynamic state. This result in disturbance in the thermal gradients by forced convective movements of air through openings of clothing and the enclosed air transmits directly through fabric layers to the environment. This air exchange reduces the insulation and increases the moisture vapour permeability of the clothing ensemble considerably. The final exchange of heat and moisture vapour is dependent on level and pattern of activity and on the size of the openings. Wind and physical movements decrease the insulating air layer sticking to the outside surface of the clothing assembly. With large openings in the clothing structure the wind penetrates into the clothing thus increases the heat loss. In strong windy situation the high air velocity may compress the clothing and thus reduce its insulation as the enclosed air layers are reduced. Insulation of clothing also reduces if the clothing gets soaking wet with sweat or water. The evaporative heat dissipation from wet clothing can be of significant extent especially when the air velocity is high. Spencer-Smith [1] proposed to quantify the wind induced reduction in thermal insulation of clothing by a reduction factor, which was proportional to the wind velocity and the square root of the fabric air permeability. The thermal insulation of a clothing

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ensemble can be estimated from the component garments and the fabric thickness. In addition to the normal modes of heat transmission, namely conduction, convection and radiation, the transmission of heat through the clothing system involves the latent heat of various phase changes within fibrous materials. The processes of transmission of heat and moisture vapour are coupled by the exchange of the latent heat during the phase changes such as evaporation or condensation, absorption or desorption and freeze or melting. The thermal characteristics of clothing assemblies are largely determined by these dynamic heat and moisture transfer processes. So, dynamic heat and moisture transmission characteristics of clothing are extremely important phenomena that control the thermophysiological comfort of a person. During high activity when a clothed person sweats, the sweat accumulates within the clothing ensemble. After the person comes to the resting state and the metabolic rate decreases, the body then does not need evaporative cooling any more. At that time sweating stops, but the evaporation of moisture from the clothing ensembles continues, which provides unwanted cooling. This is the dynamic situation of heat and moisture vapour transmission. The dynamic heat and moisture vapour transmission characteristics of clothing cannot be expressed by the parameters used for steady-state conditions. The clothed human body is, in most of the time, under dynamic state, i.e. it is subjected to changes in environmental variables, clothing and levels of activity. Clothing, particularly those made from hygroscopic fiber, e.g., cotton and wool, plays a major role during dynamic state. The heat transmission between the body and the environment may be affected significantly by the dynamic response of the clothing. Heat can bring prevailing impact on clothing, when moisture is absorbed by the clothing. This phenomenon is greatly affected by either the change of environmental conditions or physiological changes in the body such as the metabolic heat or rate of sweating.

7.2

Combined heat and moisture interactions with textile materials

Moisture transmission through a textile material is not only associated with the mass transmission processes, but the transmission of heat must also be taken into account. Heat and moisture absorption in hygroscopic materials are inseparably interrelated. During the transmission of water molecules through textile materials, they are absorbed by fibre molecules due to their chemical nature and structure. Absorption of water is followed by the liberation of heat, known as heat of absorption. The amount of heat produced

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is dependant on the absorption capacity of the material. Due to the production of heat, as the temperature is increased on the surface of the material, the rate of moisture vapour transmission is reduced [2, 3, 4, 5]. During the investigation of the wool-water system, Cassie et al [6] observed that in a textile material, placed in a humid atmosphere, the time required for the fibres to come to equilibrium with the atmosphere is negligible compared to the time required for the dissipation of heat generated and absorbed by the fibres when regain changes. With the increase in humidity, the heat transfer efficiency of the material increases. The heat transfer process also comes into play during the moisture transportation, under dynamic conditions, due to phase change of the water molecules. Thus, during the transient stages of moisture sorption and diffusion, the heat transfer process is coupled with four different forms of moisture transfer due to the heat released or absorbed during sorption/desorption and evaporation/condensation which in turn are affected by the efficiency of heat transfer and the length of the transient stage is dependant on the heat transfer process [7]. The coupling effect between moisture diffusion and heat transfer depends on a number of properties, such as the moisture of sorption capacities (isotherm), the fibre diameter, the water vapour diffusion coefficient, the density and the heat of sorption [8]. The heat of wetting of cellulosic fibres depends to some extent on the moisture regain and the crystalline structure, and it decreases proportionally with an increase in the degree of crystallinity of the fibres [9]. Two transient phenomena, buffering and chilling, are associated with the simultaneous heat and moisture vapour transport through fibre assemblies [7]. The cooling effect or buffering effect is experienced due to perspiration in hot climates and the chilling effect is associated with the after-exercise sweating in cool climates. At a sudden increase in relative humidity in the climate, fabrics absorb moisture maintaining a microclimatic condition and generate heat. This gives rise to a thermostatic or buffering action for the person wearing the fabric in clothing [10]. There would be a cooling effect at the onset of perspiration in hot climates, whereas in the case of cold climates it would result in a ‘post exercise chilling effect’ [11]. It reduces the working performance, even causing hypothermia. When water vapour (vapour perspiration) comes into contact with a cold wall (clothing) then it condensates thus reduces the thermal insulation of clothing. Both these phenomena are extremely dependent on atmospheric temperature and humidity conditions.

7.2.1

Clothing thermal insulation during sweating

Clothing thermal insulation is an important parameter in thermal comfort. It is used to determine the heat stress of a clothed person in a hot


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environment, which depends on the amount of moisture vapour evaporation to maintain the thermal equilibrium of body, rate of body sweating and skin wetness. It also depends on the extent of cold stress in a cold environment. It is also an important measure of the effectiveness of clothing functional design and suitability of clothing systems for specific end uses. Heat transfer through clothing is an important topic related to thermal comfort in environmental engineering and functional clothing design. The total heat transmitted through clothing is commonly considered as the sum of the dry heat transfer and the evaporative heat transfer. The difference between clothing thermal insulation in non-perspiring and perspiring conditions may cause errors when calculating dry heat loss during perspiration, and consequently errors in evaporative heat loss and measured water vapour resistance. It is generally believed that perspiration reduces clothing thermal insulation as a result of greater effective thermal conductivity, liquid water transport, and evaporation within wet clothing assemblies. Total heat loss greatly increases with sweating due to evaporative heat loss. Clothing thermal insulation decreases during perspiration, and the amount of reduction varies from 2 to 8%, as related to water accumulation within clothing ensembles [7].

7.2.2

Dampness

Moisture in clothing has been widely recognized as one of the most important factors contributing to discomfort sensations. The skin wetness contributes to the sensation of humidity, and that the sensation of dampness is related to the amount of sweat accumulated in clothing. The subjective sensations of skin and clothing wetness are considered as sensitive criteria for evaluation of the thermal function of clothing. The moisture in clothing contributes significantly to comfort perceptions during actual wear conditions [12–15]. A chilling sensation is produced when damp fabrics are placed on the specific part of the body, which is due to the temperature drop at the skin in contact with the moist fabrics. Also, the temperature drop is influenced significantly by the degree of the fabric skin contact that is associated with fabric construction and surface hairiness. As fibre hygroscopicity is a critical factor determining the coupled heat and moisture transfer behaviour in fabric, it has a significant impact on the skin temperature drop during the contact. Comparing fabrics with different degree of hygroscopicity, the skin temperature drop increases with the level of excess moisture as the degree of fibre hygroscopicity increases. Ambient conditions, such as temperature and relative humidity, influence the skin temperature drop significantly. The skin temperature drop

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decreases as ambient temperature increases, because of the decrease in temperature difference prior to the contact. However, ambient temperature has negligible influence on the differences of the skin temperature drop among different types of fibres because ambient temperature mainly influences the dry heat transfer process, not the moisture exchange process. Ambient relative humidity, on the other hand, shows significant impact on both the skin temperature drop of all fibres and the differences between the fibres. When ambient relative humidity increases, the difference in moisture concentration between the fabric and the environment decreases, resulting in a smaller temperature gradient between the skin and fabric, hence a smaller skin temperature drop during the skin–fabric contact. The differences between various types of fibres are much greater when ambient relative humidity is low. When the relative humidity approaches saturation, the difference between fibres becomes negligible.

7.2.3

Clamminess and heat loss during high activity

The absorption of moisture by hygroscopic fibres in clothing releases heat, which has significant impact on the heat balance and thermal perceptions of wearer experiencing a sudden change from a warm and dry atmosphere to a cold and humid environment. Hygroscopic fibres have the ability to absorb a considerable amount of moisture from the surrounding atmosphere. In transient humidity conditions, hygroscopic fibres can absorb or desorb moisture from, or to, the surrounding environment, which can delay the moisture change in the clothing microclimate. Theoretically, this effect often acts as a buffer against sudden humidity changes in favour of the wearer. The moisture build-up at the inner fabric surface facing the sweating skin is generally very slow in case of fabrics made of hydrophilic fibres like cotton and the moisture build-up is very fast in case of fabrics made of hydrophobic fibres like polyester. Figures 7.1 and 7.2 show the accumulations of moisture within the microclimate region of fabrics made of hydrophilic and hydrophobic fibres respectively. It is evident from Figs. 7.1 and 7.2 that the moisture buffering by hygroscopic fibres could be effective during a certain period after exercise. The length of this buffering period and the magnitude of delay of humidity rise depend on the ability of the fabric to remove moisture relative to the speed of moisture build-up in the clothing microclimate, which is related to ambient conditions, clothing material and style, and exercise intensity of the subjects. Therefore, the apparent contradiction on clothing buffering effect can be largely attributed to the differences in the climatic and exercise condition used.

Dynamic heat and mass transmission Release of heat

141

Sweat evaporation

Liquid Moisture vapour

Clothing

Microclimate region

Metabolic heat

Body sweat (liquid and vapour)

Human skin

7.1 Low moisture build-up in case of hydrophilic textiles.

Release of heat

Sweat evaporation

Liquid Moisture vapour

Clothing

Microclimate region

Metabolic heat

Human skin Body sweat (liquid and vapour)

7.2 High moisture build-up in case of hydrophobic textiles.

By comparing fabrics with different level of hygroscopicity, the duration of the transient behaviour depended strongly on the moisture sorption capacity of the fabric. The moisture flux across an inert porous barrier can reach a steady state within seconds, while non-steady condition may last for more than an hour when a wool fabric is exposed to a humidity gradient. During the transient period, the total amount of moisture removed from a high humidity environment is greater with a highly hygroscopic fabric such as cotton than with a weakly hygroscopic fabric such as polyester. In comparing wool and polyamide clothing, the evaporation limit was reached later when the subject wore a wool garment, and the perception of sweating and clinging sensations were delayed. To study the effect of moisture

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absorbency of fibres in hot environment on the sweating rates of secondary subjects, it was found that subjects wearing polyester lost greater amounts of sweat, and the humidity in the microclimate was significantly higher when wearing cotton [12]. Li and Holcombe [13] calculated the dry and evaporative heat fluxes during exercise at the outer surface of the clothing by measuring the temperature and moisture gradients. It can be observed from Fig. 7.3 that there is no difference between dry and evaporative heat flux before sweating. After sweating, the dry heat flux at the outer surface of the garment was significantly higher when wearing wool than polyester. No significant difference in evaporative heat flux was found between wool and polyester. Their results also indicated that fibre hygroscopicity has a significant impact on the thermal response of the body and the heat balance of the body–clothing system during the transient period of exercise. When sweating starts, highly hygroscopic fibres absorb considerable amounts of sweat and their temperature rises due to the heat of sorption released. The elevated fabric temperature interacts with the body, stimulating higher skin temperature and raising sweat rate. Some of the sweat is further absorbed by the fabric, adding to the release of sorption heat and increasing the dry heat loss at the outer surface of the garment. Hence, the body is able to shed more heat during exercise. The sorption of moisture and the

Sweating starts

Wool Heat flux (W/m2)

Polyester

Exercise time

7.3 Heat flux at the outer clothing surface during exercise. [12]


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released sorption heat by weakly hygroscopic fibres such as polyester are very low. Most of the sweat in the garment was present as liquid and it had a smaller influence on the dry heat loss at the outer surface of the garments. Therefore, the role of clothing made from weakly hygroscopic fibres is more passive and its enhancement of heat loss during exercise is smaller.

7.2.4 Buffering effect of clothing The buffering effect of clothing made of hygroscopic materials has significant impact on the thermal balance and comfort of the wearer during the change in humidity due to environmental changes. Wearers frequently experience various sudden and large changes in the external environmental conditions. For instance, a wearer may be exposed to differences in temperature and humidity greater than 10°C and 30% RH when walking from an air-conditioned indoor environment to a hot and humid summer outdoor environment. The difference in temperature between an airconditioned indoor environment and an outdoor winter environment in the cold regions can be greater than 20°C. Clothing is an extremely important barrier to protect the body against such sudden environmental changes [12]. The magnitude of the buffering effect for a single-layer garment made from wool and polyester was estimated by using a model simulating the heat and moisture transport processes in clothing and their interaction with the thermoregulatory system [14]. The temperature changes at the skin surface, when a person wears wool or polyester garments in an ambient temperature of 25°C and the RH varying from 30 to 90% and is suddenly caught in the rain, were predicted. The skin temperature changes when wearing wool were predicted to be smaller than those when wearing polyester. Also, the difference in skin temperature changes between wool and polyester decrease with increasing ambient relative humidity. These predictions suggest that there are differences between wool and polyester fibres in buffering against rain-chill, which represent the extremes of hygroscopicity. For other textile fibres such as cotton and acrylic, whose hygroscopicities fall between these extremes, the effectiveness of buffering is expected to be related to the extent of their hygroscopicity. The more hygroscopic the fibre, the stronger the buffering effect. When a subject was wearing acrylic, the skin temperature decreased more than when he was wearing wool. When wearing acrylic, the increase in humidity was greater and faster than when wearing wool. These experimental results confirmed the predictions that highly hygroscopic fibres such as wool could buffer the body against sudden changes in temperature and humidity more effectively [12].

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7.3

Factors affecting heat and mass transfer through fabrics

The transmission of heat and moisture vapour through textile materials is one of the major concerns in the design of high performance clothing such as work wears, sports wears and military uniforms for extremely cold or hot conditions. For better understanding of the heat and moisture transmission behaviour of clothing systems, proper mechanisms for heat and moisture transmission from skin to environment through the fabrics need to be understand. Min et al. [16] concluded that the microclimate plays the most significant role in the heat and moisture transfer from skin to environment. Following are some of the important parameters which affect the heat and moisture transmission characteristics of fabrics.

7.3.1

Yarn characteristics

In the studies conducted by Das et al. [17, 18], the bulking treatment of ring spun cotton yarn reduces the thermal conductivity of fabrics as compared to 100% cotton fabric, which may be attributed to very bulky structure of the weft which works as an insulating medium. It entrapped air in the loose fibrous assembly spaces and does not allow heat of inner layer to transmit to outer layer. Moisture vapour transmission is an important parameter in evaluating comfort characteristics of a fabric, as it represents the ability to transfer perspiration coming out of the body. The MVTR values of the bulked yarn fabrics are more than that of 100% cotton fabric. Since yarn structure plays important role in transmission of water vapour, the open structure of bulked yarn have better cover factor which allows water vapour to transfer from inside to outside through diffusion. Similarly the fabrics produced from yarns with micro-pores within the yarn structure show higher MVTR value than the reference fabric from 100% cotton normal yarn [19]. The increase in the moisture vapour transmission rate with increase in the micro-pores is due to better exchange of water molecules in vapour form between two faces of the fabric. The micro-pores assist in transfer of water particles in vapour form from one surface to the other surface by diffusion through them. The fabrics with micro-pores content in the yarn have lower thermal conductivity as compare to 100% cotton reference fabric sample [19]. This is due to the fact that the micro-pores within the yarn structure entrap more air. Since the air is a poor conductor of heat as compared to fibre, thus resists transmission of heat through fabric.


7.3.2

145

Effect of environment temperature

It has been reported [16] that all the individual mechanisms of heat flux (W/m2), i.e. total heat flux, conduction, radiation and moisture diffusion, decrease with the increase in environment temperature, hence heat flux decreases with the decrease in the driving force. In the case of fabric surface temperatures, the inner surface is less sensitive than the outer surface.

7.3.3

Effect of microclimate thickness

The total heat flux and individual heat transfer mechanisms, moisture fraction at the inner and outer fabric surfaces, and temperature at the inner and outer surfaces of fabrics decrease with the increase in microclimate thickness [16]. The decrease in heat flux is the result of increased air layer which behaves like an insulating material. The contribution of radiation increases with the increase in microclimate thickness, since radiation is independent of microclimate thickness while the fabric surface temperature is lowered.

7.3.4

Effect of fabric thickness

The total heat flux varies about 20% when fabric thickness changes from 0.5 to 5 mm [16]. The lower variation in total heat flux compared to the variation in microclimate thickness is due to the fact that the thermal conductivity of fabric is larger than the air layer between skin and clothing (i.e. microclimate) and, hence, the result is less sensitive to fabric thickness than microclimate thickness. The effect of fabric thickness is larger when the thickness of microclimate is smaller.

7.3.5

Effect of layering of fabrics

Resistances offered by clothing during transmission of heat and moisture vapour are two most important characteristics of clothing which control the thermal comfort. Proper understanding of dynamic transmission behaviour of these two clothing properties is very important during the selection of clothing for specific end uses and designing of functional clothing assemblies. Clothing can be of two types: tight-fitting inner garment and a loose-fitting outer garment as shown in Fig. 7.4. When a clothed person walks through a windy environment the loose outer garment generally flaps, pumping out warm air and moisture vapour from the air gap between the tight-fitting inner garment and the loosefitting outer garment and replacing it with cooler air from the surrounding environment, and at the same time wind may penetrate through the pores

146

Science in clothing comfort Air gap between loose-fit and tight-fit garment Human body

Heat and mass flow from human body

Heat flow without mass transmission (conduction, convection, radiation)

Skin Tight-fit inner garment

Heat flow with mass transmission (moisture evaporation)

Surface air layer Loose-fit outer garment

7.4 Heat and mass transfer from human body covered with tight-fit and loose-fit garments.

of outer garment to create dynamic heat and mass exchange. The actual mechanism of dynamic heat and mass transfer through clothing system is generally very complicated. In order to simplify the analysis under steady state, one can consider the dry heat flow through clothing as consisting of two parts: the first part is induced by conduction, convection and radiation, and the other part induced by air ventilation and wind penetration. Similarly the heat flow due to moisture evaporation can also be regarded as consisting of two parts: the part induced by diffusion and convection and the other part induced by air ventilation and wind penetration [15]. It is evident from Fig. 7.4 that the entire dry heat (H dryt) generated by human body transmits initially through the tight-fit inner garment (H ig) and then divided into two components, i.e. heat flow without mass transmission and heat flow with mass transmission, while transmitting through the loose-fit outer garment. The heat flow without mass transmission through tight-fit inner garment (H di) is governed by conduction (H cn), convection (H cv) and radiation (H rad). The heat flow with mass transmission is governed by moisture evaporation (H mev), i.e.


147

by air ventilation and/or wind penetration directly into the environment [13]. The relationship is as follows:

H dryt = H ig = ( H di ) + ( H mev ) = H cn + H cv + H rad + H mev

(7.1)

Since the total dry heat transmitted through tight-fit inner garment (Hdi) must transmit through the loose-fit outer garments and the outer surface of the clothing ensemble, we have: Hdi = Hdo = H dos

(7.2)

where H do is the dry heat loss through the outer garments and H dos is the dry heat loss from the outer surface of clothing ensemble. The evaporative heat loss by moisture transfer (H mev) must transmit through the outer garments and the outer surface of the ensemble. The evaporative or latent heat transfer through outer garments should be equal to the evaporative or latent heat loss from the outer surface of the clothing ensemble. After passing through the tight-fit inner garment, the total evaporative heat generated by sweat evaporation is divided into two components: evaporative heat loss through the outer garments (H evo) and evaporative heat loss directly into the environment by air ventilation and/or wind penetration (H veno). The relationship can be written as: Hmev = Hevo = Hveno

(7.3)

The temperature of air gap between two layers of fabrics increases when water vapour transmission takes place and the increase in temperature is almost proportional to the water vapour absorption rate of the fabric [20]. The dynamic thermal response of different types of clothing ensemble is predominantly governed by the moisture sorption and desorption in hygroscopic fabrics [21]. The thermal parameters that describe the dynamic response of fabrics due to the changes in physiological and environmental conditions are more important than those under steady-state conditions. Faster the vapour pressure build-up at the inner fabric surface as well as in the microclimate, the stronger the discomfort sensations will occur. Higher moisture vapour pressure at the inner fabric surface and microclimate results in more intense sensation of discomfort. The surface temperature of clothing changes during the dynamic transmission of moisture vapour within the clothing. In case of hygroscopic fibres the increase in the surface temperature is due to release of more heat from moisture sorption. The more heat loss from the body during transient period, the better the heat and moisture transfer property of the fabric [22].

148


7.4

Evaluation of heat and mass transmission

The evaluation methods of combined heat and moisture comfort of clothing are more complex than the testing methods for other properties of fabrics. To evaluate the heat and moisture comfort of clothing ensemble, it is required to consider human body, clothing, environment, and the other factors. Three methods are available for evaluating the combined heat and moisture comfort characteristics of clothing ensemble – microclimate method, thermal manikin with sweating skin, and wear trial method [23]. Microclimate method is used to evaluate the heat and moisture comfort of fabric, where as for thermal manikin with sweating skin clothing is required and wear trial method is a subjective measurement. Thermal manikin with sweating skin has been developed to present the human body. Manikin acts as a heat and moisture transfer sensor that mimics a real threedimensional body. It senses the difficult-to-model local sweat evaporation, convection and radiation processes that are highly dependent on local microclimate. Manikin is also used to be clothed to accurately depict the sweat transport of a clothed human and analyze other clothing effects. In order to ascertain how perspiration or sweating affects clothing thermal insulation with dry heat transfer, a special type of perspiring fabric, covering the manikin surface to simulate the sweating skin, is used to measure clothing thermal insulation when there is little perspiration and when there is heavy perspiration. The skin of the manikin is generally made of a strong, breathable (i.e., water proof, but moisture permeable) fabric with completely sealed seams. It is filled with water to create a soft body similar to a human body. Since the breathable fabric is permeable to moisture vapour, gaseous “perspiration” can be simulated depending on the breathability of the skin. The skin of manikin is generally designed with a long zipper at the back, so it can be replaced after prolonged use or interchanged with another skin made from fabric of different specifications. The manikin is generally heated by heaters within the trunk, and its core temperature is controlled approximately close to the body core temperature, i.e. at 37°C. Water within the manikin is circulated by a pump and a piping system, which distributes heat to the head, arms, and legs by pumping the warm water to the extremities. The skin temperature distribution depends on the output of the pump and the opening of the valves, which is preadjusted to an appropriate setting. Due to its design and construction, the manikin is inexpensive, especially compared with other expensive sweating manikins. Normally two types of manikins are available: sweating manikin and dry manikin. Using dry manikin it is only possible to measure dry heat flow both in transient and steady condition, whether using sweating manikin it is possible to measure evaporated heat loss as well as dry heat loss in transient and steady state conditions [24].

150


relationship between the microenvironment and the dynamic heat and moisture comfort of fabric and their characteristics. PERMETEST [26] instrument is used to determine the relative water vapour permeability as well as the thermal transmission characteristics. The outer surface of the tested sample is exposed to a parallel air flow and the other sample side faces a porous humid layer (0.2 ml of water are injected into the layer), which simulates any underwear filled with liquid sweat. A space of 1 mm between the sample and this layer separates the liquid and vapour phase of the water. The working principle of the instrument consists in measuring the dynamic heat flow caused by the evaporation of water passing through the tested specimen. Relative water vapour permeability is defined as the ratio of the heat loss measured with sample and the heat loss measured without sample. Along with water vapour resistance of the fabric (Hohenstein Skin Model [27]), PERMETEST can also measure the dry heat transfer through the fabric. Under the steady state condition it measures the thermal resistance of the fabric by measuring the dry heat transfer to the guarded hot plate through the fabric by the following equation: Rt =

A(Tm − Ta ) − Ra 0 (m2K/W) H − ΔH c

(7.7)

where, Ra is the thermal resistance of the fabric and Ra0 is the intrinsic thermal resistance of the instrument. Tm and Ta are the temperatures of the guarded plate and air consecutively. The dry heat resistance Rt of the fabric is measured in PERMETEST by measuring the difference between the heat flow with and without sample,

⎡ 1 1 ⎤ Rt = (T1 − T0 )× ⎢ − ⎥ ⎣ S .u1 S .u 0 ⎦

(7.8)

T1, T0 are the temperatures with and without samples respectively; u1 and u0 are output voltages with and without sample. S is the sensitivity of instrument. The sweating guarded hot plate simulates both heat and moisture vapour transfer from the body surface through the clothing layers to the environment. It measures both the thermal resistance and water vapour resistance of fabrics [28]. A sweating guarded hot plate (Fig. 7.5) consists of test plate, guard pate, temperature controller and water supply unit [20]. The test plate, fixed to a metal block with heating element, is a square porous metal plate. The square test section in the centre of the plate is


Water source

151

Hood for air flow

Fabric

Guard plate

Porous test plate Bottom plate

Guard plate

7.5 Schematic diagram of sweating guarded hot plate.

surrounded by the guar plate, which prevents lateral heat leakage from the edges of the specimen. The bottom plate beneath the test section prevents the downward heat loss from the test plate and guard plate. This arrangement ensures that both the heat and/or moisture transmit in upward direction only, i.e. along the specimen thickness direction. A square fabric specimen is mounted on the square porous test plate that is heated to a constant temperature that approximates body skin temperature (35°C). The plate temperature is measured by the sensor sandwiched directly underneath the plate surface. The electrical power to maintain the constant temperature is recorded. The whole apparatus is housed in a chamber so that the environmental conditions can be carefully controlled. For the determination of thermal resistance of the sample, the air temperature, relative humidity and the air speed generated by the air flow hood are controlled as per the standard specifications. The thermal resistance of the fabric is measured in similar way as that of normal guarded hot plate system, i.e. without any moisture vapour. Total thermal resistance (Rh) of the fabric under the steady state condition is given by [22] Rh = A(Ts – Ta)/H m2 °C/W

(7.9)

where A the area of the test section (m2), Ts the surface temperature of the plate (°C), Ta the temperature of the ambient air (°C), and H the electrical power (W). To measure the moisture transmission characteristics of the fabrics, distilled water is supplied to the surface of the porous plate from a dosing device. The dosing device is normally activated when the water level in the plate is about 1 mm below the plate surface. The water entering the measuring unit is preheated. A level switch is connected to the measuring unit to maintain a constant rate of evaporation. A water vapour permeable and liquid water impermeable membrane is fitted over the plate. The test fabric is placed above the membrane. The electrical power to maintain the

152


plate at a constant temperature of 35°C is an indicator of water evaporation rate. Air temperature is set at 35°C and relative humidity is controlled at 40%. After a steady state is reached, the total evaporative resistance (Rv) of the fabric is calculated by using the following formula [29]: Rv = A(Ps – Pa)/H m2kPa/W

(7.10)

where Ps the water vapour pressure at the plate surface (kPa), Pa the water vapour pressure of the air (kPa), and H the electrical power (W).

7.4.1

Measurement of condensation in the clothing

Condensation takes place in presence of very high gradient of vapour pressure as well as temperature. It is a direct result of a fabric being saturated by liquid perspiration [30]. It occurs within the fabric whenever the local vapour pressure rises to the saturation vapour pressure at the local temperature [31]. Condensation normally occurs when the atmospheric temperature is very low; when the warm and moist air from the body meets the fabric, it works as a cold wall and condensation occurs. Sweating skin model is used to determine the condensation in the fabric for both in steady and transient state conditions [32, 33]. In this model two different procedures are used to analyze unsteady heat and moisture processes. In one of the procedures the liquid water is injected close to the hot side of the sample; it evaporates there and re-condenses toward the cold impermeable side of the slab. In other procedure the hot plate is directly exposed to the moist air flow. Transient temperature changes are monitored and the total amount of absorption and condensation is measured after a specific time. Based on the sweating skin model, Xiaohong et al. [34] proposed an apparatus to investigate whether condensation occurs on the fabrics or not. Sweating skin within the microclimate is simulated by the use of a fully wetted qualitative filter paper heated to skin temperature (32°C) using a hot plate. Two sensors are used to record and display temperature and relative humidity simultaneously. Dynamic vapour pressure is calculated using the data of relative humidity and temperature by the following equations:

φ=

P × 100% Ps

(7.11)

and

⎡ ⎛ T − 273.15 ⎞⎤ Ps = 4.607 exp ⎢17.06⎜ ⎟⎥ ⎝ T − 40.25 ⎠⎦ ⎣

(7.12)


153

where P is moisture vapour pressure (mmHg) at temperature T (K); Ps is saturation vapour pressure (mmHg) at same temperature and φ is the relative humidity (%). The relationship between the water vapour pressure and radius of water droplet when condensation occurs is given by the following equation: ln

2σ 1 P = ⋅ Ps ρ w RT r

(7.13)

where r is radius of water droplet, σ is surface force of water, ρw is density of water, R is constant.

Moisture vapour pressure (mmHg)

Pressure (P)–temperature (T) diagram of wetted air is used to record the condition of microclimate and study whether condensation is formed on the inner surface of a fabric. In theory, condensation occurs under the conditions of water vapour pressure present exceeding the saturation vapour pressure. The saturation line is described as the water vapour pressure giving rise to 100% relative humidity at a specific temperature. A typical saturation line has been shown in the Fig. 7.6. Keighley [35] and Ruckman [36] also suggested that the condensation occurring on the fabrics may be predicted if a saturation line and water vapour concentration line are utilized. Keighley [35] developed a method that involved measurement of water vapour concentration utilizing infrared absorption at the specific frequency of strong water vapour absorption. Ruckman [36] provided a solution to the problem of condensation on the inner surface of fabric by perforating metal cylinder simulating the perspiring human body to investigate the couple mechanisms of water vapour transfer and heat transfer.

80 70

Saturation line

60 50 40 30 20 10 0

50

40

30

20

Temperature (°C) 7.6 The saturation line. [23]

10

0

154


Fan et al. [37] and Murata [38] have also set experimental apparatuses to measure the condensation in the fibrous material maintaining a hot and humid surface and another cold wall. Fukazawa et al. [39] have developed an apparatus to measure the water vapour resistance of the textiles with and without temperature and pressure differences imposed on both sides of the fabric, and also to see the effect of the temperature and pressure difference on the condensation in the textile material.

7.5

Parameters expressing heat and mass transmission

7.5.1

Evaporative transmissibility

The evaporative transmissibility is the ratio of moisture vapour permeability index (im) to total insulation (im/clo) [40]. It is an indicator of the proportion of the maximum evaporative cooling of sweat generated in a specific environment. The effect of presence of air current is neglected here. Thus, evaluation of the insulation and moisture evaporative capacity of a clothing system is able to accurately estimate the relative advantages of the clothing as compared with another, with regard to the thermal protection or strain when the clothing is worn. The evaporative transmissibility (im/clo) is helpful to compare the ensembles with different insulation values. Two clothing ensembles with different thermal insulation characteristics but same evaporative transmissibility would exchange same heat between the body and the surrounding in the same environments at the same activity levels. It is easy for clothing materials with high evaporative transmissibility value (im/clo) to transport heat by means of both convective heat transfer and evaporative cooling. But in case of environment with high humidity and at low air speed, the evaporative cooling is less important and the thermal transmission characteristics become the most important factor.

7.5.2

Permeation efficiency factor

Permeation efficiency factor (f pcl) describes the cooling efficiency of sweating on the skin surface for a clothed human body. The permeation efficiency factor (fpcl) ranges from zero, when a person wearing a completely impermeable clothing, to unity, for a nude subject [41]. It is calculated in terms of the convective heat transfer coefficient between the human body surface to the environment and the intrinsic insulation of clothing. This can be expressed by the following equation [42]: f pcl = 1/ (1 + 0.143 × hc × I cl )

(7.14)


155

The relationship between permeation efficiency factor (fpcl) and moisture vapour permeability index (im) is governed by the following equation [41]: f pcl =

im hc × ( I a + I cl )

(7.15)

where h c is the convective heat transfer coefficient (W/m2 °C), I cl the intrinsic insulation of clothing (clo), and Ia the insulation of boundary air layer (clo). Moisture vapour permeability index (im) is an indicator of the evaporative performance of clothing.

7.5.3

Heat lost to the environment by evaporation

The heat lost to the environment by evaporation (E) is calculated using an evaporative heat exchange coefficient and the water vapour pressure difference between the skin and the ambient air. The equation is analogous to the convective heat transfer equation: E = heWs (Pss – Pa) W/m2

(7.16)

where he is evaporative heat transfer coefficient (W/m kPa); Ws is skin wettedness (it is a dimensionless parameter defined as the ratio of evaporative rate from skin, e, and the maximum evaporative rate of skin, emax); Pss is water vapour pressure at the skin surface, i.e. the pressure of saturated air at the skin temperature (kPa); Pa is water vapour pressure of the ambient air (kPa). The evaporative heat transfer coefficient H e for the outer air layer of a nude person can be estimated from the convective heat transfer coefficient (hc) using the Lewis ratio (LR), which describes the relationship between convective heat transfer and mass transfer coefficients for a surface. Lewis relation (LR) is the ratio of evaporative mass transfer coefficient to convective heat transfer coefficient (hc). 2

LR = he / hc

(7.17)

The Lewis ratio equals approximately 16.5 K/kPa for typical indoor conditions [43]. Evaporative heat loss from the skin depends on the amount of moisture on the skin and the difference between the water vapour pressure at the skin and in the ambient environment. Skin wettedness is the ratio of the actual evaporative heat loss to the maximum possible evaporative heat loss, emax, under the same environmental conditions and for a completely wet skin the value of Ws is 1. Evaporative heat loss from the skin is a combination of the evaporation of sweat secreted because of thermoregulatory control mechanisms and the natural diffusion of water through the skin. With no regulatory sweating, skin wettedness caused by

156


diffusion is approximately 0.06 for normal conditions. For large values of emax or long exposures to low humidities, the value may drop to as little as 0.02, because dehydration of the outer skin layers alters its diffusive characteristics. Skin wettedness is strongly correlated with warm discomfort. For clothed subjects, w > 0.2 is perceived as uncomfortable. Skin wettedness can theoretically approach 1.0 while the body still maintains thermoregulatory control, but in practice it is difficult to exceed 0.8 [44].

7.5.4

Clothing ventilation

The Clothing Ventilation Index is a quantitative which predicts the effectiveness, preference and suitability of clothing assemblies; firstly, to ensure that the clothing is worn and used correctly and, secondly, to improve performance by minimising heat strain, sweat retention and thermal discomfort. Ventilation is vital to the removal of sensible and insensible heat and, therefore, an important determinant of thermal comfort. Thermal comfort and heat loss from the body by convection and evaporation of sweat can be measured using Fanger’s model of thermal comfort [45]. As the thermoregulatory system is only able to control the loss of sensible and insensible heat if there is sufficient quantity of air flowing through the micro-environment, the clothing ventilation technique became important in determining the performance of all clothing ensembles. The factors influencing air exchange are air permeability of fabric, design, sizing (fit), wind and the resilience of fabrics. The clothing ventilation is an evaluation method of microclimate air exchange, not the physiological response of human body. The fit of the clothing has a direct effect on the ventilation and there is a need for measuring standard sizes and fit for comparison purpose and repeatability of results [46].

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Science in clothing comfort MCCULLOUGH E . A ., HUANG J . and KIM C . S ., ‘An explanation and discussion of sweating hot plate standards’, J ASTM Int 1(7), 13, 2004. HUANG J ., ‘Sweating guarded hot plate test method’, Polymer Testing 25, 709– 716, 2006. REN Y . J . and RUCKMAN J . E ., ‘Water vapour transfer in wet waterproof breathable fabrics’, J Indus Text 32(3/1), 165–175, 2003. RUCKMAN J . E ., ‘Water resistance and water vapour transfer’, Textiles in Sport, editor Shishoo R., Woodhead Publishing Limited, Cambridge, 2005. MOTAKEF S . and EL - MAHER M . A ., ‘Simultaneous heat and mass transfer with phase change in a porous slab’, Int J Heat Mass Transfer 29(10), 1503–1512, 1986. WIJEYSUNDERA N . E ., HAWLADER M . N . and TAN Y . T ., ‘Water vapour diffusion and condensation in fibrous insulation’, Int J Heat Mass Transfer 32(10), 1865– 1878, 1989. XIAOHONG Z ., SHANYUAN W. and GUANLU Y ., ‘An apparatus used to investigate condensation for fabrics, laminates and films’, J Indus Text 32(3), 177–186, 2003. KEIGHLEY J . H ., ‘Breathable fabrics and comfort in clothing’, J Coated Fabrics 15(10), 89–104, 1985. RUCKMAN J . E ., ‘Analysis of simultaneous heat and water vapour transfer through waterproof breathable fabrics’, J Coated Fabrics 26, 293–307, 1997. FAN J . and CHENG X . Y ., ‘Heat and moisture transfer with sorption and phase change through clothing assemblies, Part II: Theoretical modelling, simulation and comparison with experimental results’, Int J Heat Mass Transfer 75(3), 187–196, 2005. MURATA K ., ‘Heat and mass transfer with condensation in a fibrous insulation slab bounded on one side by a cold surface’, Int J Heat Mass Transfer 38(17), 3253–3262, 1995. FUKAZAWA T ., KAWAMURA Y ., TOCHIHARA Y . and TAMURA T ., ‘Water vapour transport through textiles and condensation in clothes at high altitudes – combined influence of temperature and pressure simulating altitude’, Text Res J 73(8), 657–663, 2003. GOLDMAN R . F ., Evaluating the Physiologic Effects of Clothing on the Wearer, US Army Research Institute of Environmental Medicine, Natick, 1978, 1–21. NISHI Y ., GONZALEZ R . R . and GAGGE A . P ., ‘Direct measurement of clothing heat transfer properties during sensible and insensible heat exchange with thermal environment’. ASHRAE Trans 81, 183–199, 1975. NISHI Y . and GAGGE A . P ., ‘Moisture permeation of clothing - A factor governing thermal equilibrium and comfort’, ASHRAE Trans 76, 137–145, 1970. MCCULLOUGH E . A ., ‘Factors affecting the resistance to heat transfer provided by clothing’, J Therm Biol 18, 405–407. ARENS E . A . and ZHANG H ., The Skin’s Role in Human Thermoregulation and Comfort, Center for Environmental Design Research, University of California, Berkeley, 2006. FANGER P . O ., Thermal Comfort, McGraw–Hill, New York, 1972. LUMLEY S . H ., STORYT D . L . and THOMAS N . T ., ‘Clothing ventilation-update and applications’, Applied Ergonomics 22(6), 390–394, 1991.

8 Garment fit and comfort

8.1

Introduction

Modern consumers demand apparel products with superior multifunctional and comfort performance to satisfy their physiological and psychological needs. Garment size, fit and pressure comfort have been identified as important attributes. No matter how well a fabric is engineered to have optimum value of heat, moisture or air transmission, the clothing produced from this fabric cannot be regarded as comfortable if it does not fit properly. While the clothing comfort related to tactile aspects, thermal and moisture transmission and aesthetic appearance depend on the type of materials used and the design of the clothing, the comfort related to garment fit mainly depends on the size and design of garments. The relationship of size of garment to the size of body is very complex and requires detailed analysis of many complex factors. The clothing is expected to conform to our body shape and is our nearest environment, so it is expected to fit closely and deform in synchronization with our body movement. Individuals have apparel fit preferences based upon aesthetic and functional expectations. When we judge the fit of a garment, the judgment is mainly based on both visual and tactile sensations. The comfort level of the garment is judged based on both tactile and aesthetic responses. The concept of comfort encompasses many dimensions, including physical, psychological, and social comfort. In order to understand about comfort sensations related to garment fit, the responses related to tactile and aesthetic aspects also need to be considered. The clothing comfort related to fit depends on the mass of clothing ensemble, ease of movement of clothing over skin, pressure applied on the body surface and ventilation provided by the clothing. The sensory responses of clothing comfort can be divided into two categories: one is the feel of the fabric on the surface of the skin and the other garment fit sensations. The feel on the skin surface of the fabric is further subdivided into sensations of tickle, prickle, allergic reactions, contact thermal sensations, perception of moisture, and abrasion. Garment fit sensations are subdivided into general pressure sensations and localized pressure sensations [1]. A badly fitting garment can restrict cardio-

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vascular flow, causes skin irritation, results unpleasant thermal or moisture conditions and ultimately results in discomfort to the wearer. The garment fit related comfort is very crucial for performance clothing. The performance clothing should fit better and not limit job performance. They should provide comfort fit design which enable a greater range of movement while stretching and bending, improve mobility with more room throughout the body, provide a more tailored fit and better overall comfort and be easier to wear and take off.

8.2

Body dimensions and pattern

Relationship between the size and design of clothing with the form of the body is a complex problem of ergonomics engineering. Determination of even the most basic pattern shapes requires complex decisions related to dimension, all of which are related to the dimensions of the body for which the clothing is produced, but not necessarily are equal to the body size. Designing clothing for a specific individual body size also poses tremendous challenges when one wants to consider all the aspects, like fit, design, aesthetic or other comfort related aspects together. Extensive use of computer software helps in the development of patterns by measuring the body, analyzing anthropometric data, drafting clothing patterns, and designing and manufacturing clothing. Such technology facilitates largescale anthropometric studies and the development of improved-fit models and sizing standards for mass production. It also encourages the expansion of custom clothing production by providing a more accurate and costeffective means for fitting the individual. Essentially, the problem of fitting a garment to the human body involves the spatial relationship of the twodimensional garment plane to the body surface [2]. The dimensions of pattern of a garment are not identical to the corresponding dimensions across the body surface. Therefore, the process of determining pattern dimensions from body dimensions must ultimately be evaluated as the three-dimensional correspondence of the resulting garment to the body form. Pattern dimensions have often been determined by a process of (1) taking linear (length and circumference) measurements over the body surface with measuring tape and then (2) applying those measurements in some predetermined manner to the pattern draft. This process of garment sizing may results in improper fit and needs repeated trials and fittings of the garment by a skilled technician after the pattern is cut from cloth. Experience, therefore, demonstrates that these linear bodysurface measurements are not directly applicable to pattern dimensions and are useful primarily as approximations. Another type of data traditionally used in pattern development is the visual assessment of body configuration by the expert eye of the tailor.


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Gazzuolo et al. [2] compared the photographic body data with linear anthropometric data and subsequently predicted pattern dimensions. Their work involved taking traditional linear measurements of body, photographing the body and measuring the photographs. They determined the pattern shapes by a methodology which located fabric planes directly on the body. They have (i) developed statistical models predicting key measurements of a garment pattern from standard linear measurements; (ii) developed comparable models for predicting garment pattern measurements from quantities measured from the photographs; and (iii) compared these models to determine whether linear or photographic measurements had greater predictive power for determining garment patterns. They have observed that the photographic model has predictive power in the determination of pattern dimensions; there are several advantages to this method of anthropometric data collection. Photographic techniques are far less intrusive and more efficient with regard to time, effort and cost than are manual tape measure techniques.

8.3

Garment fit and comfort relationship

8.3.1

Tight-fit and loose-fit

The size and fit of a garment has direct influence on the comfort characteristics. In a loose-fitting garment the volume of entrapped still air is more and has larger openings at places like the neck, waist, wrist, and ankles, which cause greater reductions in their thermal insulation and moisture vapour resistance during windy conditions and body movement. Figures 8.1a and 8.1b show the typical shapes of loose-fit and tight-fit garments [3]. The tight-fit garment is designed to fit tight to the skin in order to allow the technical aspects of the garment to work effectively. However, no such garment should be so tight as to restrict freedom of movement. Generally the tight-fit garment is designed to fit all contours of the body and compress core muscle groups. The normal-fit garment, on the other hand, provides a less tight-fit but still provides all the benefits of moisture transmission and thermal management in appropriate locations. The loose-fit garments are designed as a comfort fit with all the moisture management benefits and generally worn as a casual garment or an outer garment. In a research work conducted by McCullough et al. [4], the thermal insulation of two pairs of loose-fitting and two pairs of corresponding tight-fitting long trousers were measured, using a standing manikin in little wind (air velocity in the chamber was less than 0.11 m/s). They have observed that the loose-fitting trousers have much greater thermal insulation than the corresponding tight-fitting trousers (33% more for the tweed trousers and 48% more for the denim trousers). In another

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study conducted by Havenith et al. [5] the thermal insulation of three clothing ensembles on four combinations of body postures and movements (two with loose fit and two with tight fit) when the subjects were sitting and walking at two speeds and at three wind speeds were determined. They have concluded that the tight-fit clothing had 6–31% less insulation than loose-fit clothing. The difference in thermal insulation values between tight-fit and loose-fit clothing was found to be maximum in sitting condition. They have also observed that the difference in thermal insulation value was lower in presence of wind.

8.1a Typical loose-fit garment.

8.3.2

8.1b Typical tight-fit garment. [3]

Garment fit and pressure

The comfort related to garment pressure and fit mainly depend on the tactile responses of human body which include thermal and moisture perceptions, prickle related to allergic reactions and reactions to the surface texture of materials, friction between clothing and skin surface and pressure sensations. Two categories of tactile sensations are somesthetic sensations and kinesthetic sensations. The somesthetic sensations are touch response from the nerves in the surface of the skin. Tickle, prickle and abrasion are somesthetic responses to clothing. The kinesthetic sensations or the deep pressure sensations are felt by the nerves in the muscles and the joints [6]. Pressure sensations created by the resistance of the garment to movement and the weight of the garment in response to movement are kinesthetic responses to clothing fit [7]. The multidirectional and random forces generated during dynamic interactions between a garment and a moving human body generates pressure sensations. The discomfort level of clothing pressure was found to be between 60 and 100 g/cm2, depending on the individual and the part of the body concerned, which is similar to blood pressure in the capillary


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blood vessels near the skin surface [8]. So, the pressure exerted by a garment is an important design criterion and is affected by its style, fit and mechanical properties. It is directly related to the degree of space allowance (F, the difference between the surface areas of garment and body) between the body and the garment during body movement. According to the degree of space allowance (F), garments can be classified into three types: foundation garments (F < 0), perfectly fitting garments (F = 0) and loose fitting garments (F > 0). In foundation garment the area of garment is less than the body area (such as women’s close-fitting foundation garment, pressure garment, etc.). These are mainly designed to apply a certain level of pressure on the body part concerned when the body is in active condition or at rest. The perfectly fitting garments are those where the garment area is exactly equal to the body area (such as tights, socks, body stockings, etc.) and have a figure-shaping function but are not designed to apply pressure to the body. Therefore, a perfectly fitting garment only restricts body movement as a result of garment pressure, but no pressure is applied when the body rests. In loose-fitting garments the area of the garment is larger than the body area (such as loose-fit outer wear, casuals etc.). As the movement of body can reduce the space allowance, loose-fitting garments may also sometime exert pressure on the body at contact areas. Therefore, the level of garment pressure varies significantly for different wear situations, depending on four factors namely, design and fit of the garment, shape of the body part, mechanical properties of the underlying tissue, and mechanical properties of the fabrics [9]. The evaluation of human comfort due to garment fit and subjective garment pressure sensations are generally done by conducting wear trials [10–12]. In the study conducted by Makabe et al. [11] the garment pressure was measured on the covered area at the waist for corsets and waistbands and conducted a sensory test for garment pressure. Their results indicate that the garment pressure at the waist is influenced by the area covered, respiration and the ability of the garment to assume the complex body curvature during the body movement. During subjective evaluation of garment pressure and comfort sensations at the waist, they have observed that in the lower garment pressure range (0–15 gf/cm 2) no sense of discomfort is there. In the medium range of garment pressure (15–25 gf/cm2) negligible or only slight discomfort is perceived. But in higher pressure range, i.e. when the garment pressure exceeds 25 gf/cm 2, extreme discomfort is perceived. Denton [8] pointed out that as the body curvature increases the garment pressure on the body increases. The body curvature of average women’s waist at the sides is roughly 3.5 times greater than that at the front, so unwanted pressure on the sides of the waist is 3.5 times greater than the desired figure flattening pressure on the front [9].

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Similar finding has also been reported during the measurement of internal pressure exerted by pressure bandage using Laplace equation [13]. As per the Laplace equation it is expected that the exerted internal pressure (P) would increase with increase in curvature of the limb. The Laplace equation is given below, P = T × n ×w r

(8.1)

where T is tension of bandage during wrapping; n is number of wraps; r is radius of curvature; and w is width of bandage.

8.3.3

Evaluation of tactile perception to fit

Various methods have been developed to measure the human perceptions, like odours, tastes etc. Ashdown and DeLong [7] adapted the test method for measuring tactile perception to fit, which was developed by the ASTM Committee E-18 for sensory evaluation of products. Constant stimulus difference (i.e. difference from control value) test was used as the model in the development of the perception of fit tests. Two types of thresholds were identified by this test: the difference threshold (or) the extent of change in the stimulus that produces a noticeable difference; and the recognition threshold (or) the extent of change in a stimulus necessary for positive identification of the difference. In the first part of this test a series of garments (pants) along with their corresponding control (garments) were presented to a panel of experts. The experts then rated the difference of the sample from the control. The scale for pants provided the response choices of ‘looser’, ‘a little looser’, ‘the same’, ‘a little tighter’ and ‘tighter’ for the waist, hips and crotch. The second part of the perception test was designed to address the large number of interactions that occurred in the first part of the perception test. In order to focus the subjects’ attention on one area of concern, they were told what area of the test samples (pants) would vary from the control sample. The second test was therefore performed with variations at a single location. Two experts were asked to respond to test samples (pants) with hip or crotch variations. In the second type of perception test, the experts were informed that the variations were all at this specific location. The final perception test was carried out to determine the significance of the thresholds perceived in relation to comfort or discomfort perceptions with garment fit. The participants were given only the test garment that they had perceived as different from the control to try on in a preference test. A mirror was provided and subjects were asked to indicate whether they found the garments are comfortable or not, even though they perceived differences from the control garment.


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Psychophysical scaling technique was used by You et al. [14] to assess the pressure perception and other relative wearing sensations during wearer trials of tight-fitting garments. The sense of pressure was recorded on a scale of 0–10. One tight-fitting pant, with very high clothing pressure, as presented to subjects as the standard for the maximum degree of pressure, was indexed as 10. On the other hand, when the subject was naked, i.e. at the minimum degree of pressure, the scale was indexed as 0. The subjects were asked to rate the sense on a scale 0–10. Every subject was asked to assess the pressure sensation of each area while wearing the garments.

8.4

Factors related to garment fit

8.4.1

Air gap thickness

Chen et al. [15] reported that, in case of smaller air gap between skin and the garment (for medium fit garment), both the thermal insulation and moisture vapour resistance increases at higher rate with the increase in the thickness of the air gap between the garment and the body. The rate of increase gradually decreases as the air gap becomes higher (as the garment becomes loose-fit), and the rate of increase is much less than that of the theoretically ideal still air due to natural and forced convection. When the air gap exceeds a certain value (in case of very loose-fit garment), possibility of drop in thermal insulation and moisture vapour resistance is there with the increase in the air gap. This is due to the fact that in loose-fit garments, there are easy passages of adjacent atmospheric air to penetrate through the openings and interfere with the still air of the microclimate. Also the air gap results in greater natural convection. Thermal insulation and moisture vapour resistance reach a maximum at a certain air gap thickness depending on fabric properties, wind conditions and garment fit. Tighter fitting garments are preferable to keep the body warm in windy. Chen et al. [15], during their study with open-structured knitted garment, reported that in absence of wind the maximum thermal insulation was reached with air gap thickness of approximately 1 cm, corresponding to a difference of 7.5 cm in girth between the garment and the body. On the other hand, in windy conditions the maximum thermal insulation reached at lower air gap thickness (i.e. approximately 0.6 cm thick), corresponding to a difference of 5 cm in girth between the garment and the body. More natural and forced convection is believed to cause the slower increase of thermal insulation and vapour resistance with the increase in air gap thickness.

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8.4.2

Garment ventilation

Most of the garments, especially the protective clothing, are designed to protect the human being from hostile thermal, biological, chemical environments, etc. The comfort properties of these garments are of considerable interest for their satisfactory performance. In addition to the selection of raw material, constructional parameters and finish, garment fit also gains its importance in deciding the transfer of heat and moisture to the environment. Numbers of literatures [16–18] are available which reveal the effect of ventilation and fit on the comfort properties of protective clothing. Daanen et al. [16] investigated the ventilation rate by changing the fit of the garment (jackets). The jackets were made from normal fit to oversize by using metal rings inside. These rings enlarged the air volume between skin and clothing by about 60%. Nine male subjects participated in the study. The 3D scanner method was used for measuring the volume of trapped air. The volume also varied due to the variations in body dimensions of the subject. Ventilation rate was measured during standing, walking, swinging arms and bending arms by TNO tracer gas method [17]. The ventilation rate was found to be higher (200 litre/min.) in loose-fit garment than normal-fit garment (120 litre/min.). So, looser the garment more the heat loss was observed. Another way to enhance the heat loss is to make clothing more air permeable. Havenith et al. [18] investigated the effect of air permeability on heat strain of chemical protective clothing. He has observed that the tolerance time when walking on a treadmill has increased from 174 ± 42 to 203 ± 56 min by increasing the ventilation through the material from 186 to 365 litre/m2. It can be concluded that fit of the garment and air permeability are the two important factors in deciding the heat and moisture transfer of clothing.

8.4.3

Fluctuating microclimate in loose-fit garment

Due to metabolic action, the human body continually produces heat. The clothing system contributes greatly to thermal comfort through the regulation of heat balance. The size and fit of a garment influences the thickness of microclimate. The fluctuation of microclimate occurs very frequently due to activity and body movement. This phenomenon is very significant in case of loose-fit garments. Shigaki et al. [19] have reported the changes in garment surface temperature on a garment made of cotton and polyester fabrics, due to fluctuation of relative humidity of microclimate. They have reported that with the rapid fluctuation of relative humidity of microclimate the surface temperature of cotton garment fluctuates significantly, whereas in case of polyester garment the fluctuation was smaller than cotton. Figure 8.2 shows the typical changes in the surface


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8.2 Typical garment surface temperature (ST) response with rapid change in relative humidity (RH).

temperature of cotton and polyester garments with the humidity change in the microclimate. The quick rises and falls in the temperature were observed for cotton garments. It corresponded to the rapid humidification and dehumidification of the microclimate. The similar results were obtained in the case of polyester fabric. However, the temperature changes for polyester were smaller than for cotton. This tendency shows that the hygroscopicities of fabrics correspond directly to their heat of absorption for moisture. Figure 8.3 shows the typical changes in surface temperature under the increasing and decreasing humidity at very low rate. It has been reported that under this condition, the surface temperature changes were very small [18]. The rate of the temperature change was found to increase with the increase in the rate of relative humidity. The increase in surface temperature for cotton fabric was higher than for polyester. Regardless of the increasing rate of the humidity, the hygroscopic fabric produces the adsorption heat. The rapid and large changes in the fabric temperature against the skin due to fluctuation of relative humidity of microclimate, which have been observed especially for hygroscopic cotton garment, affect the thermal comfort of clothing.

8.4.4

Garment fit and pressure sensation

The sensation of pressure during wearing of tight-fit garment is a major component of comfort and the pressure applied by a garment mainly depends on the extensibility of fabrics, the fitness of garments and the

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Science in clothing comfort Slow change in relative humidity

Surface temperature of cotton garment Surface temperature of polyester garment

Time

8.3 Typical garment surface temperature (ST) response with slow change in relative humidity (RH).

style of garments. The pressure comfort is one of the most important factors influencing a wearer’s sensation of comfort in tight-fitting garments. You et al. [14] developed a wearing experimental procedure and investigated the relationship between different subjective wearing sensations using factor analysis. They have also reported that fitness of garment and extensibility of fabric has great predictive power for the subjective measurement of pressure when the style of garments remains the same. Zhang et al. [20, 21] reported that the extensibility of fabrics has the main contribution to the garment pressure. They have developed a mathematical model to analyze the stress distributions of fabrics using membrane theory. The fitness of the garments can be defined in the following way [14]: The fitness of garment =

the girth of garment − the girth of naked body the girth of naked body

(8.2) The clothing pressure can be measured using pressure sensors. You et al. [14] selected four areas of body (i.e. hip, shank, thigh and knee) where the clothing pressure on the body was measured. They have used GYG-06 pressure sensor which was primarily designed to measure the application in which the pressure range can be expected to 0–10 kPa. The sensor cell was small and flexible, and so could be easily inserted between the pants and skin without affecting the accuracy of the pressure measurement. The sensors were put to the four parts between clothing and body to measure clothing pressure. Garment strains around the above four areas were also


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measured. This was done by drawing a series of lines on the thigh-fit at regular intervals and measuring the changes in dimension that took place when subjects were wearing the garment. They have reported that the garment comfort during wearing has a negative correlation with the feeling of fetter, scratchy, heavy and the sensation of pressure, and has a poor correlation with the feeling of softness and smoothness. The pressure sensation, wearing comfort of pressure and fettered feelings, are directly related to the garment fit. They have also reported that the garment fit and the fabric extensibility have good ability to predict pressure sensation for clothing, when garments have the same style and the skin stretch of subjects is not high. The garment fit is a problem of pressure against the skin reaching unpleasant levels or restricting body motion and increasing the energy cost of movement. Individuals vary significantly in their sensitivity to garment pressure. The acceptance of a normal garment can be increased by designing the garment for optimum pressure, i.e. reduction in the pressure. Perfectly fitting garments are required to be designed to fit the body shape without exerting any specific shaping pressure. The garments should be more or less skin-tight to accommodate body movement and provide comfort. A fabric should have lower tensile modulus in multidirections and effective elastic recovery to ensure that the garment does not become loose and buckled on the body parts and to accommodate body movement [9]. The garment only constrains body movement as result of garment pressure, but no pressure is applied when the body rests. The external forces exerted by human body are balanced by fabric internal stresses (tensile, shear and bending) and the inertia force of the garment during body movement.

8.5

Measurement of garment fit

Croney [22] defined anthropometry as ‘the practice of measuring the human body’. He also recommended that the static and dynamic anthropometric data will provide the designer with an armature of dimensions around which ideas can grow. Pheasant [23] expands this definition to ‘applied anthropometrics’, which include numerical data concerning size, shape and other physical characteristics of human beings that could be applied in the design context. Traditionally tailors have accurately measured the human body size for garment making with measuring tape based on their experience. They recognized similarities between the garments they made for individuals and began to think in terms of proportionally scaled patterns for people of different sizes, known as “graded” sets of clothing sizes [24]. As a

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result ready-to-wear is now the principle source of clothing production available to the global mass market. Efficient mass production of clothing requires a method of producing accurate size of garment which will fit properly. The traditional methods of body size measurement are timeconsuming, chances of human error, inaccuracies of measurement and most important is that one has to touch the body physically during body size measurement. Fan et al. [25] mentioned that each size has a distinct code (e.g. small, medium, large or 10, 12, 14) to guide consumers to choose apparel which fits their body properly. They have also emphasized the importance of a 3D digitization of body form and clothing surface to assist the spatial analysis of clothing appearance, body measurement and garment fit. 3D laser scanning is a process used to build a digital 3D copy of a physical body surface very accurately without touching. The development of 3D laser scanning technology has solved many of the problems associated with the traditional body size measurement systems. This system generates the accurate body size of a person in seconds. At present this technology is used for different purposes, namely apparel design (protective wear, wearable technology, thermal comfort, athletic equipment/uniform, mass communication); ergonomics (validation of models, seat design); reverse engineering (finite element analysis solution, rapid prototyping, standard/tolerance); and biomedical applications (obesity determination, body asymmetries, rehabilitation engineering) [26]. Figure 8.4 shows the principle of 3D laser scanning system of human body size measurement. The system consists of two primary components: (i) A hardware system

Enlarged view of scanning assembly

Scanning assembly

Scanning assembly

Scanning assembly

CCD Cameras Scanning assembly

Laser source

8.4 Schematic diagram of 3D laser scanning system of body size measurement.


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which consists of charge coupled device (CCD) camera, laser source and computer, and (ii) image recognition software. It can be observed from Fig. 8.4 that the scanning assembly consists of a structural frame to keep the scanning devices in their required positions. Curtains are generally hung from the frame to minimize the interferences of outside light. The vertical columns, located in the four corners, are containing the scanning assemblies. The scanning assembly (shown in enlarged view) consists of a laser and two CCD cameras. All the four scanning assemblies are connected with an elevator assembly that travels up and down in the vertical columns. After the proper calibration, all the four elevator assemblies travel downward direction in unison and sweep the scanning zone with a horizontal plane of laser light. During this process the laser light illuminates the contour of the human body standing within the scanning area and the CCD cameras record discrete points on these contours at each horizontal point [26]. The total scanning process takes few seconds. The data from the CCD cameras are then transmitted to the computer through the A/D converter and the image recognition software finally creates a point cloud representation of the body contour. The point cloud data are then used for the representation of human body size.

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‘Clothing’, Textiles 16(1), 23–27, 1986. and BYE E., ‘Predicting garment pattern dimensions from photographic and anthropometric data’, Applied Ergonomics 23(3), 1992. http://www.baselayer.co.uk/fit-and-size-guide.php MCCULLOUGH E . A ., JONES B . W. and HUCK J . A ., ‘Comprehensive data base for estimating clothing insulation’, ASHRAE Trans 91, 29–47, 1985. HAVENITH G ., HEUS R . and LOTENS W. A ., ‘Resultant clothing insulation: A function of body movement, posture, wind, clothing fit and ensemble thickness’, Ergonomics 33(1), 67–84, 1990. MCBUMEY D . and COLLINS V . B ., Introduction to Sensation/Perception, PrenticeHall Inc, Englewood Cliffs, 1977. ASHDOWN S . P . and DELONG M ., ‘Perception testing of apparel ease, Variation’, Applied Ergonomics26(1), 47–54, 1995. DENTON M . J ., ‘Fit, stretch and comfort’, Textiles 12–17, 1972. ZHANG X ., YEUNG K . W. and LI Y ., ‘Numerical simulation of 3D dynamic garment pressure’, Text Res J 72, 245–252, 2002. MAKABE H ., MOMOTA H ., MITSUNO T . and UEDA K ., ‘A study of clothing pressure developed by the girdle’, J Jpn Res Assoc Text End-uses 392, 424–438, 1991. MAKABE H ., MOMOTA H ., MITSUNO T . and UEDA K ., ‘Effect of covered area at the waist on clothing pressure’, Sen-iGakkaishi 4109, 513–521, 1993. MAKABE H ., MOMOTA H ., MITSUNO T . and UEDA K ., A study of clothing pressure developed by the brassiere, J Jpn Res Assoc Text End-uses 392, 416–423, 1991. GAZZUOLO E ., DELONG M., LOHRT M., LABAT K .

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Index

3D scanning, 170 Absolute threshold, 130 Acclimatization, 51, 80 Aesthetic attributes Cover, 27 Drape, 27 Body, 27 Style, 28 Surface texture, 25, 28, 41, 55, 162 Resilience, 28 Aesthetic concepts, 26 Air gap, 165 Alambeta, 97 Ambient conditions, 139 Anthropometric data, 160, 169 Baroreceptors, 40 Bending length, 67 Bending rigidity, 67 Black body, 85 Body attributes, 23 Body temperature, 48, 86 Buffering effect, 143 Capillarity, 110 Category end effect, 21 Category scale, 20, Clo, 99 Clothing aesthetic, 5, 23, 159 Clothing assemblies, 9, 83, 101, 136 Clothing Ventilation Index, 156 Cold injuries, 81 Cold thermoreceptors, 41, 91 Comfort perception, 13, 20, 36 Comfort sensations, 14 Comfort definition, 4, 14 Condensation, 123, 152 Contact angle, 112

173

Convection, 87 Core temperature, 49, 79, 148 Cutaneous sensation, 39 Desiccant inverted cup method, 126 Difference threshold, 130 Diffusion coefficient, 120 Diffusion rate, 93, 120 Diffusion, 119 Discomfort, 4, 21, 37, 40, 55, 76, 81, 106, 130, 139, 147, 156, 162 Dry heat, 146 Dynamic moisture permeable cell, 129 Dynamic sensations, 36 Dynamic transmissions, 136 Effective insulation, 99 Electrical capacitance technique, 118 Electrotactile stimulation, 48 Evaporation of sweat, 8, 51, 106, 155 Evaporative dish method, 124 Evaporative heat loss, 51, 88, 127, 147 Evaporative performance, 100 Evaporative transmissibility, 154 Extension, 68, Fabric clinginess, 57 Fabric extraction, 71 Fabric roughness, 73 Fabric scratchiness, 73 FAST, 67 Fechner’s law, 16 Fiber arrangement, 100 Fickian, diffusion, 120 Fit, 5, 14, 21 Flexural rigidity, 57, 74 Force feedback displays, 48 Forced convection, 122, 165 Formability, 68

174


Free nerve endings, 36 Garment dimension, 160 Garment patterns, 160 Garment pressure, 162, 169 Guarded hot plate, 95 Hair follicle nerve ends, 35 Hairiness, 102 Hand expressions, 15, 59 Heat balance equation, 87 Heat balancing, 50 Heat flux, 92 Heat of sorption, 120 Heat transfer, 138 Heaviness, 57 Holographic bench technique, 129 Hygral expansion, 70 Hygroscopicity, 141 Hyperthermia, 79 Hypothermia, 79, 138 Image analysis, wicking, 115 In plane wicking, 115 Insensible evaporation, 51 Insulation, 136 Insulation, 99 Intensity of stimulation, 47 Interval scale, 19 Inverted cup test, 126 Itch sensation, 37, 57 KES-F Thermo Labo, 96 KESF, 62 Kinesthetic pressure, 162 Knitted fabrics, 102 Krause’s end bulbs, 35 Latent heat, 50, 84, 123, 137 Loose-fit garment, 162 Low amplitude, 48 Low frequency, 48 Low stress mechanical properties, 58 Mass production, 170 Measurement of aesthetic, 24 Mechanical properties, 61 Mechanoreceptors, 34, 45, 54 Meissner’s corpuscles, 34 Merkel nerve ending, 35

Met, 99 Metabolic heat generation, 7, 84 Metabolic heat, 7, 48, 50, 83, 137 Metabolic rate, 86, 113, 137 Microclimate thickness, 145 Microclimate, 79, 103, 106, 121, 140, 145, 156, 165 Micro-pores, 144 Moisture build-up, 140 Moisture content, 57, 93, 106, 118 Moisture management, 118 Moisture sensation, 129 Moisture transmission, 79, 106 Moisture vapour permeability, 136 Moisture vapor transmission, 118, 124 Moisture vapour transmission cell, 128 Nerve endings, 33 Neurophysiological sensation, 47 Newton’s law of cooling, 89 Nociceptors, 33 Nominal scale, 17 Objective measurement of handle, 58 Ordinal scale, 17 Pacini’s corpuscles, 33 Packing coefficient, 110 Peak extraction force, 72 Permeability index, 100 Permeation efficiency factor, 154 Permetest, 128, 150 Perspiration, 118, 123, 138, 148 Photographic technique, 161, Physical stimulus, 16, 130 Physiological comfort, 6 Physiological factors, 13, Polar scales, 25 Prickle sensation, 37, 55 Prickliness measurement, 55 Psychological Perception, 14, 23 Psychological Scaling, 17 Psychophysical scaling, 165 Radiation, 85 Ratio scale, 18 Relaxation Shrinkage, 70 Roughness, 41, 55, 65, 73 Ruffini endings, 35

Index Selection of clothing, 1, 22, 145 Sensorial attributes, 15 Sensorial comfort, 4, 159 Skin layer, 33, 156 Skin sensors, 31 Skin temperature, 80 Skin, 31, 54 Somesthetic pressure, 162 Sorption-desorption, 121 Specialty yarns, 102 Stiffness, 68 Stimuli, 31 Subjective assessment, of handle, 59 Subjective clothing comfort, 20 Surface tension, 107 Sweat glands, 41, 51, 106 Sweat, 113 Sweating guarded hot plate, 127, 151 Sweating skin model, 152 Sweating, 7, 50, 82, 106, 138, 148 Tactile comfort, 54, 58 Tactile perception, 164 Tactile receptors, 40, 45 Tactile sensations, 21, 47, 54, 159, 162 Thermal comfort, 79, 86, 99, 139, 145, 156, 166 Thermal conductivity, 50, 94, 100, 139, 144 Thermal inertia, 91 Thermal insulation, 8, 79, 98, 107, 136, 148, 161 Thermal manikin, 97, 148 Thermal radiation, 48, 85 Thermal resistance, 90, 99, 150

175

Thermal sensation, 21, 41 Thermal transmission, 5, 83, 94, 154 Thermophysiological, 4, 25, 79, 106 Thermoreceptors, 31, 41, 91 Thickness, 67 Tight-fit garment, 162, 165 Tog, 99 Togmeter, 94 Tortuosity, 110 Transient heat flow, 91 Transient humidity, 140 Transverse wicking, 115 Two-point threshold method, 45 Upright cup method, 125 Van der waal’s forces, 121 Vasoconstriction, 49, 82 Vasodilatation, 49, 80 Ventilation, 156, 166 Vertical wicking, 115 Vibrotactile sensation, 47 Warmness, 57 Warmth thermoreceptor, 41, 91 Wear trial, 21, 148, 163 Weber’s Law, 16, 130 Wettability measurement Tensiometry, 112 Goniometry, 112 Wetting, 106, 112, 126 Wickability, 109 Wicking, 5, 107 Wind, windy, 136, 145, 156, 161 Yarn structure, 101

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