Fire Behavior of Upholstered Furniture and Mattresses

FIRE BEHAVIOR OF UPHOLSTERED FURNITURE AND MATTRESSES

by

John F. Krasny Fire Technology Consultant Kensington, MD 20895

William J. Parker Fire Technology Consultant Germantown, MD 20874

Vytenis Babrauskas Fire Science and Technology Inc. Issaquah, WA 98027

NOYES PUBLICATIONS Park Ridge, New Jersey, U.S.A. WILLIAM ANDREW PUBLISHING, LLC Norwich, New York, U.S.A.

Copyright © 2001 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 00-104716 ISBN: 0-8155-1457-3 Printed in the United States Published in the United States of America by Noyes Publications / William Andrew Publishing, LLC 13 Eaton Avenue Norwich, NY 13815 1-800-932-7045 www.williamandrew.com www.knovel.com 10 9 8 7 6 5 4 3 2 1

NOTICE To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information. This book is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. Final determination of the suitability of any information or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone intending to rely on any recommendation of materials or procedures mentioned in this publication should satisfy himself as to such suitability, and that he can meet all applicable safety and health standards.

Preface

This book is a collection of the up-to-date science and engineering knowledge in the field of furniture fire flammability. For continuity and perspective, citations to older work are still maintained, even in cases where newer research has brought forth improved methods or better knowledge. Thus, the advancement of the state of the art can be seen in these pages. In 1985, two of the present authors (Babrauskas and Krasny) published the first monograph devoted to upholstered furniture flammability. This was issued by National Bureau of Standards (now NIST, National Institute of Standards and Technology) as “Fire Behavior of Upholstered Furniture” (NBS Monograph 173). Many new concepts and experimental results have been published since that time. The most comprehensive recent research study in this area has been “Combustion Behavior of Upholstered Furniture” (CBUF) which was sponsored by the European Union. Two of the authors, Babrauskas and Parker, had the privilege of participating in CBUF. This project, as well as many others, resulted in major improvements in this field. Thus, it became opportune to revise the monograph. To be most useful to its intended user, this book was reorganized and structured more along the expected lines of enquiry from the user. This involved a major reexamination of the literature, especially coverage of new regulations and standard test methods. The review of regulations, however, is selective. Discussions are focused only on US, UK, and EU activities in this area. While numerous other countries have various regulations affecting

v

vi

Preface

aspect of furniture flammability, little if any technical work making reference to such regulations has ever been published in the English language. In this book, the term upholstered item will sometimes be used to include upholstered furniture as well as upholstered parts of bedding (solid core and innerspring mattresses and upholstered bed frames). In many cases, however, it is appropriate to consider that statements made about chairs or about upholstered furniture also apply to various other types of upholstered items. Bedding, such as blankets, sheets, pillows, etc., are treated separately. The book is arranged as follows: • Chapter 1 provides a brief overview of the structure and materials, fire safety design, fire statistics, and standards development. • Chapter 2 discusses some of the fundamentals of fire which affect the fire safety of upholstered furniture. These include smoldering and flaming ignition, flame spread, heat release, inter-item fire spread, room-fire interaction, flashover, smoke, and toxic gases. • Chapter 3 describes the pertinent test methods and regulations for smoldering and flaming ignition, flame spread, heat release rate (HRR), and smoke and toxic gas production for residential, public, and high risk occupancies. • Chapter 4 addresses smoldering and flaming ignition and includes the historical development and the details of the ignition tests. • Chapter 5 compares results obtained by different test methods, especially bench-scale and full-scale results, and furniture calorimeter and room results. • Chapter 6 covers fire safety design, considering the effects of upholstered item construction and materials, separately for smoldering (cigarette) and flaming ignition. Emphasis is on thermal behavior, flaming or smoldering; the relative rates of smoke and combustion products release, which are, in the first approximation, related to the HRR for flaming fires, are less extensively reviewed.

Preface

vii

• Chapter 7 briefly discusses room fire zone and field models as they pertain to furniture fires, furniture fire models, and correlation formulas, and a method for predicting the HRR of composites in the Cone calorimeter based on measurements of the individual components. • Chapter 8 discusses fire hazard analysis, and describes a method of predicting the available escape time based on the HRR of the burning furniture. • Chapter 9 offers brief conclusions about the current state of knowledge about furniture flammability. July, 2000 Issaquah, Washington

Vytenis Babrauskas

Contents

1 Introduction ............................................................................ 1 1.1.0 OVERVIEW.......................................................................... 1 1.2.0 ARRANGEMENT OF THIS BOOK .................................... 2 1.3.0 UPHOLSTERED FURNITURE STRUCTURE AND MATERIALS ........................................................................ 2 1.4.0 UPHOLSTERED FURNITURE AND MATTRESSES ....... 5 1.5.0 DESIGN AND FIRE SAFETY ............................................. 5 1.6.0 UPHOLSTERED ITEM FIRE STATISTICS ....................... 7 1.7.0 SUMMARY OF REGULATORY DEVELOPMENT ........ 10

2 Fundamentals ........................................................................ 18 2.1.0 PYROLYSIS AND COMBUSTION .................................. 18 2.2.0 SMOLDERING ................................................................... 21 2.2.1 Cellulosic Material ............................................... 21 2.2.2 Polyurethane Foam ............................................... 26 2.3.0 TRANSITION TO FLAMING ............................................ 27 2.4.0 FLAMING IGNITION ........................................................ 30

ix

x

Contents 2.5.0 FLAME SPREAD ............................................................... 34 2.6.0 HEAT RELEASE ................................................................ 39 2.6.1

Heat Release Rate ................................................. 39

2.6.2

Heat of Combustion .............................................. 42

2.6.3

Effect of The Ignition Source On The HRR Curve in Full-Scale Tests...................................... 51

2.7.0 PROPAGATING AND NON-PROPAGATING FIRES .... 52 2.8.0 INTER-ITEM SPREAD ...................................................... 56 2.9.0 INTERACTION WITH ENCLOSURE .............................. 62 2.10.0 FLASHOVER ...................................................................... 63 2.11.0 SMOKE AND TOXIC GASES ........................................... 66 2.11.1 General .................................................................. 66 2.11.2 Smoke ................................................................... 67 2.11.3 Toxic Gases .......................................................... 72

3

Test Methods, Standards and Regulations ......................... 83 3.1.0 CIGARETTE IGNITION .................................................... 83 3.1.1 Introduction ........................................................... 83 3.1.2 Upholstered Furniture ........................................... 87 3.1.3 Mattresses ............................................................. 96 3.1.4 Test Criteria For Cigarette Ignition Resistance .... 97 3.1.5 Critiques of Cigarette Ignition Standards ............. 99 3.2.0 FLAMING FIRE TESTS, STANDARDS, AND REGULATIONS ............................................................... 102 3.2.1 Introduction ......................................................... 102 3.2.2 Uses and Limitations of Flaming Fire Tests ....... 103 3.2.3 Description of Flaming Fire Tests ...................... 108 3.2.4 Development of Full-scale HRR Measurement Techniques .......................................................... 140 3.2.5 Flame Spread - Standard Tests ........................... 142 3.2.6 Transportation Seating Tests and Regulations ... 143 3.2.7 Miscellaneous Tests ............................................ 150

Contents

xi

3.3.0 SMOKE AND TOXIC GASES ......................................... 151 3.3.1 Smoke Tests ........................................................ 151 3.3.2 Toxicity Tests ..................................................... 156

4 Ignition Sources .................................................................. 163 4.1.0 MATCHES, SMALL GAS FLAMES, AND METHENAMINE PILLS .................................................. 168 4.2.0 WOOD CRIBS .................................................................. 171 4.3.0 NEWSPAPER SHEETS AND THEIR GAS BURNER REPLACEMENTS ............................................................ 175 4.4.0 WASTE PAPER BASKETS: REAL AND SIMULATED .. 178 4.5.0 RADIANT FLUX IGNITION SOURCES ........................ 179 4.6.0 OTHER IGNITION SOURCES AND LOCATIONS ....... 179 4.7.0 LARGE OPEN-FLAME OR RADIATION SOURCES ... 182

5 Effects of Test Apparatus and of Test Scale..................... 187 5.1.0 COMPARISON OF BENCH-SCALE RESULTS ............ 187 5.1.1 Comparison of Fabric and Fabric/Padding Composite Test Results ...................................... 188 5.2.0 COMPARISON OF BENCH AND FULL-SCALE RESULTS ......................................................................... 190 5.2.1 Flammability Results .......................................... 190 5.2.2 Comparison of Cone Calorimeter and Full-Scale Results ................................................................. 191 5.2.3 Comparison of Furniture Calorimeter and Room Results ................................................................. 200 5.2.4 Smoke Results .................................................... 204 5.2.5 Toxicity Test Results .......................................... 204

6 Upholstered Item Design Engineering .............................. 207 6.1.0 IGNITION RESISTANCE TO CIGARETTES ................ 207 6.1.1 Effect of Fabrics ................................................. 213 6.1.2 Effect of Padding Material ................................. 217

xii

Contents 6.1.3 Effect of Interliner (Barrier, Blocking) Materials .. 218 6.1.4 Effect of Welt Cords and Trim ........................... 218 6.1.5 Effect of Configuration ....................................... 219 6.1.6 Effect of Moisture ............................................... 219 6.1.7 Cigarette—Upholstered Item Interaction ........... 220 6.1.8 Low Ignition Propensity Cigarettes .................... 225 6.2.0 FLAMING FIRES ............................................................. 237 6.2.1 Ignitability........................................................... 237 6.2.2 Post-Ignition Behavior ........................................ 244 6.2.3 Flame Spread ...................................................... 299 6.3.0 SMOKE ............................................................................ 302 6.4.0 TOXIC PRODUCTS ......................................................... 307 6.5.0 FIRE INVESTIGATIONS ................................................ 316

7

Modeling .............................................................................. 320 7.1.0 INTRODUCTION TO MODELING ................................ 320 7.2.0 FURNITURE FIRE MODELS .......................................... 322 7.2.1 Physics-Based Models ........................................ 323 7.2.2 Combined Physics/Correlation Models .............. 328 7.2.3 Correlations-Based Models ................................ 331 7.3.0 A COMPONENT HRR MODEL FOR FURNITURE COMPOSITES .................................................................. 339 7.4.0 CFD ROOM FIRE MODELS ........................................... 342

8

Fire Hazard Analysis .......................................................... 346 8.1.0 SMOLDERING FIRES ..................................................... 346 8.2.0 FLAMING FIRES ............................................................. 347 8.2.1 Small Closed Rooms........................................... 347 8.2.2 Open (Ventilated) Rooms ................................... 348 8.3.0 THE ROLE OF HRR ......................................................... 351 8.4.0 THE ROLE OF OTHER FACTORS ................................. 352

Contents

xiii

8.5.0 RELATIONSHIP OF HRR AND AVAILABLE ESCAPE TIME ............................................................................ 354 8.6.0 HAZARD PREDICTIONS BASED ON MODELING ..... 354

9 Conclusions ......................................................................... 358 Exercises and Solutions ............................................................ 360 EXERCISES

............................................................................ 360

SOLUTIONS

............................................................................ 367

Abbreviations ............................................................................ 377 References .................................................................................. 379 Index .......................................................................................... 423

1 Introduction

This book is a comprehensive revision of a 1985 monograph[1] authored by Babrauskas and Krasny. The intervening years saw few advances in the basic science of furniture combustion, but much work was done in applied areas, both in empirical studies and in regulatory activities. Thus, this edition is organized differently from the preceding book, and is specifically intended to provide useful information to any individuals with a responsibility for the fire safety of furniture. Research from various parts of the world are encompassed in this book, but focus on regulation is mainly from the US perspective, with significant additional material on UK and EU activities, and more limited coverage of other parts of Europe. Other, briefer, overviews of the upholstered item fire situation in the UK and the US are found in Refs. 2–6. A comprehensive report on the results and analysis of an extensive European project on the post-ignition Combustion Behavior of Upholstered Furniture (CBUF)[7] references many important literature sources. This chapter contains an overview of the basics of upholstery structure and fire safety design. Following this, fire statistics are presented.

1

2

Fire Behavior of Upholstered Furniture and Mattresses

The chapter concludes with a brief history and status of US and EU regulations covering upholstered furniture and mattresses.

1.1.0

UPHOLSTERED FURNITURE STRUCTURE AND MATERIALS

Upholstered furniture has a complex structure, as shown in Fig. 1-1. One item can contain fifteen or more components. In the ignition process, whether it be from a cigarette (smoldering ignition) or small flame, the cover and interliner fabric, if any, and material immediately below them (one or several different padding materials) are important. As the fire progresses, other materials contribute, including the bulk of the padding as well as the frame, staples, and springs, which can affect the manner in which the burning item collapses. This in turn affects fire growth. Bedding (pillows, blankets, sheets, etc.), mattresses, and bed frames contain a different variety of materials and construction factors. A large variety of component materials are used in upholstered furniture and mattresses. Cover fabrics can be made from char-forming fibers, such as cellulosic, acrylic, wool, and silk fibers, or from thermoplastic fibers. Among the cellulosic fibers, cotton and rayon predominate but flax, hemp, jute, etc. are also used. Thermoplastic fibers include nylon, olefin (polypropylene or polyethylene), and polyester. Fabrics using blends of more than one fiber type have become very popular in the last decade. Fabrics often contain dyes and dye auxiliaries, print auxiliaries, and other finishes, e.g., stain and water repellents, and softeners. Raw cotton fabrics contain smolder promoting alkali metal ions, as do many of the fabric finishing agents. Many fabrics have latex back-coatings. Paddings today are predominantly polyurethane foams varying in density and additives. Polyurethane may be used as a thin layer combined with other paddings, or, more frequently, as the entire core. Cellulosic batting (mostly cotton but also containing hemp, jute, etc.), both untreated or flame retarded treated (FR), and cotton/man-made fiber blend batting are also used. Polyester batting is popular for special comfort and appearance effects.

Introduction

Figure 1-1. Upholstered furniture construction details.

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4

Fire Behavior of Upholstered Furniture and Mattresses

Interliners (also often called barrier materials, fire blockers, or blocking layers) are used between the cover fabric and padding to increase ignition resistance and improve burning behavior. For cigarette ignition resistance, thin layers of polyester batting are popular (paddings of 100% polyester batting are less widely used; they tend to have excessive loft and are mostly found in uses other than seats). Flame-resistant interliners are FR-treated cotton, aluminized materials, glass fabrics, layers of FR foams, aramid non-wovens, etc. In addition, furniture and mattresses often contain innersprings, frames (mostly wood or steel, but sometimes rigid polyurethane or highdensity polypropylene), springs, or straps to hold up cushions, bottom cover fabrics (usually nonwoven or cotton fabrics), nails, and staples.

1.2.0

UPHOLSTERED FURNITURE AND MATTRESSES

Many aspects of flammability are similar for upholstered furniture and mattresses. The similarities and differences pertinent to predicting the fire performance have been analyzed in Ref. 8. Upholstered items in residential or public occupancies, or air, maritime, and ground transportation are all subject to different functional requirements and regulations. Upholstered items include a large variety of constructions: three-ortwo seat sofas, chairs, mattresses, and some bedframes. The furniture geometry can have a large effect on progress of the fires. There are fully and partially upholstered types, recliners, and lightly upholstered office and stacking chairs. Chairs or sofas can have straight or curved sides, the seating area can be square or rounded (barrel chairs), there may or may not be upholstered armrests, etc. Loose or puckered cover fabrics have appeared in recent times, and their effect on flame and cigarette ignitability has not been established. There are also such features as open or padded seat sides, open spaces between seat and backrest, etc. Mattresses have predominantly flat, horizontal surfaces which are not as easily ignited from flame ignition sources as the vertical surfaces. Upholstered bed frames (divan bases) are more common in Europe than in the US. Both mattresses and upholstered furniture may have depressions at the welt cord and, due to tufting, these may affect cigarette ignition propensity.

Introduction

5

Cigarettes on mattresses may be covered inadvertently with sheets, blankets, and/or pillows. This increases the probability of smoldering ignition. These intermediary materials may also ignite more readily than mattresses from flames, and then expose the mattress to a much more severe fire than the original ignition source, e.g., a match. Fire development is also affected by the nature and materials of the bed frame. Consequently, it is more difficult to develop relevant ignition tests and standards for bedding than for upholstered furniture, which is not usually covered by extraneous items.

1.3.0

DESIGN AND FIRE SAFETY

While fire problems with upholstered furniture have been of concern for some decades, it has been mainly since the 1970s that quantitative data have been available for common materials used in upholstered items. These efforts have made it possible to treat the subject as a design or prediction problem. In a design problem, the designer is typically required to come up with materials and configurations suitable to meet a set objective, which may be resistance to ignition by cigarettes, resistance to small or large flames, or, smoke and toxic pyrolysis product release rates below some specified amount. Several means of solving such problems are: 1. Test items made from the same materials, in the same configuration, as the proposed line of furniture in a full scale facility, e.g., a room or furniture calorimeter; 2. Test large-scale mock-ups of the fabric, interliner, and padding; 3. Test bench-scale composites of the materials in item 1 and use the results in a full-scale model or correlation formula; 4. Test the fabrics and foams individually and use the results in a composite model or correlation formula to predict the results of tests of furniture composites in the Cone Calorimeter. However, the tests on the individual components require

6

Fire Behavior of Upholstered Furniture and Mattresses modified test procedures in the Cone Calorimeter which are not, at this writing, generally available in the testing laboratories; although the testing protocols are described in the CBUF report.[7]

In the last years, much progress has been made to eliminate the burden of full scale testing of every material/configuration combination a manufacturer may wish to produce, and to place greater reliance on strategy 3 and, possibly, strategy 4. At the end of this chapter, an overview is given of the CBUF project which is the latest of such efforts. Details of CBUF and other available literature on this subject are reviewed in Chs. 5, 6, and 8. Briefly, it is now possible, within certain limits, to estimate from benchscale tests of both mattresses and upholstered furniture fabric/padding composites whether the actual item will support a smoldering or a flaming ignition and whether the flaming ignition will lead to a fully involved fire. Such estimates can be used to decide whether certain regulatory pass/fail levels can be met. Progress has also been made in predicting the heat release rate (HRR) of propagating furniture and mattress fires.

1.4.0

UPHOLSTERED ITEM FIRE STATISTICS

During the 1991 to 1995 period, there was an average of 446,700 home fires, 3590 civilian deaths, 20,382 civilian injuries, and $4.5 billion property damage.[576]–[579] This is a decrease of almost 150,000 fires from the 1983–1987 period, and of a 19% decrease in civilian deaths. However, the number of deaths per 100,000 fires has not been decreasing. Furthermore, the number of civilian injuries and property damage increased during this period. Civilian deaths may be reduced by better medical treatment, and the increase in property damage can perhaps be explained by the 1991 Oakland, CA, firestorm. The table below shows the number of fires, deaths, injuries, and property damage due to upholstered item fires. As in earlier such compilations, these fires were by far the largest cause of fire deaths; ranked high in injuries and relatively low in property damage. While together they represented only about 10% of the fires, they caused about 35% of the deaths; this indicates that upholstered item fires are considerably more likely to cause death than other categories.

Introduction

7

Death, Injuries, and Damage Due to Fires in Which Upholstered Items Were The First Item to Ignite Number of Fires

Civilian Deaths

Civilian Injuries

Direct Property Damage, Million$

Upholstered Furniture 1985–1989

14,600

658

1810

237

1989–1993

16,000/3.4*

742/19

1967/9.4

239/5.5

1992–1996

13,900/3.2

653/18

1721/8.7

228/5.3

Mattresses and Bedding 1985–1989

39,000

774

3050

283/

1989–1993

31,200/6.7

627/16

3232/16

331/7.6

1992–1996

28,900/6.5

578/16

2997/15

320/7.4

*The number after the slash indicates the percent of the total fires, deaths, injuries, or property damage. Note: The next lower first item-to-ignite category was electrical insulation, 7.7% of fire deaths, followed by floor covering and cooking materials, 3.5% each.

Cigarettes or “discarded material,” presumably matches, account for 39% of the upholstered item fires and 52% of deaths dues to upholstered items. Incendiary fires were next in frequency. People falling asleep while smoking accounted for 13% of the fires. For mattresses and bedding, 56% of fires occurred on items with cotton fabrics, 24% in man-made fiber items, while the fabric was unclassified or classified “other known” in 20%. Children started 8700 (30%) fires, and smoking materials and matches or lighters, 5800 (20%). People falling asleep again were a major factor in these fires, with 1700 (6%) fires.

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Fire Behavior of Upholstered Furniture and Mattresses

The CPSC estimated that in 1996 there were 650 deaths due to upholstered furniture as first item to ignite, 1640 injuries and $250 million in direct damages and $3.7 billion in total societal cost.[580] There were 470 cigarette-initiated fire deaths (down from 1200 in 1981). Small open-flame ignitions accounted for an average of 90 deaths over the years 1990–1996, 440 injuries and $50 million damages. Two thirds of the deaths were children under five years of age. Children of that age also were the mostly responsible for these fires. An earlier analysis of US fire accident data by the National Fire Protection Association (NFPA) estimated that there were an average of 22,900 residential fires in which upholstered furniture was the first item to ignite, and 42,500 mattress and bedding fires during the years 1983 to 1987.[9] The upholstered furniture and mattress fires represented 11% of the residential fires, 40% of the deaths, 27% of the injuries, and 16% of the property damages. Upholstered item fires are the single largest cause of fire fatalities in the US; the next highest death figure is 7% for interior wall covering fires (as reported to be the first item to ignite but some walls might have been ignited by upholstered furniture). Another report reviews the available data in considerable detail.[10] It specifically discusses the increasing role of polyurethane foam in furniture fires, which may have increased the severity of furniture fires, even as their number is decreasing. In the US, 69% of all fires attended by the fire services are postflashover fires, with the majority of deaths occurring outside the room of fire origin.[11][12] This implies the need for studying both fire development in the room of origin and spread of hot smoke and toxic gases to the rest of the building. Computer fire simulation programs have been found very useful in this area. On the other hand, the majority of elderly fire victims in the UK die in the room of origin, as reported in Ref. 7. The conclusions of another NFPA investigation, this one specifically of US smoking-material initiated fires and using a variety of sources, were:[13] • In 1988, lighted tobacco products caused 230,500 fires, an estimated 1,660 deaths, 4,300 civilian injuries, and $440 million in damage. This represents about 27% of the total number of US civilian fire deaths, and is by far the leading

Introduction cause of fire deaths. However, smoking ranks only sixth among the causes of structural fires. Other major causes were heating equipment, matches and lighters, and electrical malfunction. • Upholstered furniture and mattresses and bedding were the first item to ignite in 24,000 smoking material initiated residential fires, and resulted in 1,250 deaths, 2,700 civilian injuries, and $200 million in property damage. Other major items ignited in smoking material residential fires were trash and clothing not on a person. Smoking-material initiated fires also were a major factor in nonresidential structure fires, with trash, discarded mattresses and bedding, and upholstered furniture leading the list of first ignited items. The mortality and morbidity in the latter fires were much lower than those in residential fires. • During the period of 1980 to 1988, the number of deaths per 100 smoking-related fires increased from 1.88 to 2.04 indicating an increase in the severity of such fires. This is in spite of the fact that there is a general reduction in fire deaths due to advances in the clinical treatment of burn injuries. Smoking-related fires were the second largest cause of civilian burn injuries. • More than 95% of the smoking-material caused fires were started by cigarettes. • Data for 1984 to 1988 shows that the risk of smoking material fire caused deaths increased with age. Death rate per million persons of all ages averaged 8.5 for men, and 4.2 for women. It was roughly twice that for the age

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Fire Behavior of Upholstered Furniture and Mattresses group 55 to 64, and three times that for those between 75 and 84. • Smoking-material fires of trash, grass, and brush were more frequent than those in residences, but caused fewer civilian deaths. However, such fires can destroy very large areas, and cause fire fighter injuries and deaths.

An ignition risk analysis for cigarette initiated upholstered furniture fires showed that the US incident rate from 1975 to 1982 was related to the annual cigarette consumption rate and the estimated average cigarette ignition resistance of the upholstered items in use.[14] The efficacy of upholstered furniture standards is illustrated by the California fire experience. In 1988, ten years after the cigarette and small flame ignition standards were first enforced, upholstered furniture fires had declined by 50%.[15] Part of this can be ascribed to the increased use of smoke detectors and lower percentage of smokers; on the other hand, the California population increased considerably during that period. California fire statistics for 1980 to 1984 show that upholstered items were the first to ignite in 35% of the hotel/motel and nursing home fires. Combined figures for the UK and the Netherlands showed similar trends.[16] Bedding and upholstered furniture accounted for approximately 10% of residential and 2% of public building fires. Smoking materials caused 32% of the residential fires (presumably mostly in upholstered items), electrical equipment 20%, and matches and lighters 10%. Figures for the UK alone show that upholstered furniture and bedding fires accounted for about 15% of the residential fires (total 63,000) but 50% of the deaths (total 710) and 30% of the injuries. About 30% of the deaths occurred in rooms other than that of the fire origin. Canada experiences about 4,000 upholstery fires a year, causing 100 deaths, 310 injuries, and a $28 million loss.[17]

1.5.0

SUMMARY OF REGULATORY DEVELOPMENT

This is a nontechnical overview of the voluntary and regulatory activities regarding upholstered item flammability. As the most significant recent development, the European Union (EU) (formerly called European Community, EC) completed the program leading to the CBUF report.[7] It was designed to provide a scientific base for potential regulations to control

Introduction

11

post-ignition fires of upholstered items. At the time of this writing, however, no regulatory activity has ensued. The technical details of tests and standards are given in Ch. 3. Recent reviews of US activities in this area can be found in Refs. 18–20. They cover the available tests, with emphasis on American Society for Testing and Materials (ASTM) standards. Damant has discussed the California standards, their effects, and changes made over the years on the basis of experience.[15][21] Because cigarette-initiated fires are much more frequent than flame ignition caused fires, particularly in residences, prevention of the former was assigned priority in the US in the 1970s. Voluntary standards for cigarette ignition exist in the US for residential and institutional upholstered furniture.[22][23] A Federal Standard applies to mattresses.[24] The upholstered furniture standards have almost identical ASTM and NFPA counterparts.[25]–[28] During 1994–1997, the US Consumer Products Safety Commission (CPSC) developed a small-flame ignition test in response to a petition from the National Association of the State Fire Marshals. Congressional action raised the spectre that meeting any new flammability requirements would require use of fire retardants and the latter might present toxic hazards. Despite decades of safe and effective use of fire retardants worldwide, CPSC was forced to pause the study and to commission a toxicity research project instead. The results are expected to become available towards the end of 2000. The draft CPSC test method itself is discussed in Ch. 3. California enforces its own regulations of cigarette and small flame resistance of fabrics and paddings, and of large flame resistance of institutional upholstered items.[29]–[31] Besides the long standing California Bureau of Home Furnishings and Thermal Insulation (BHFTI) Technical Bulletins TB 116 and TB 117 covering cigarette and small flame ignition requirements for residential furniture components,[29] standards were developed for institutional occupancies. TB 133 applies to furniture,[30] and TB 129 to mattresses.[31] They have been adopted as standards by ASTM.[32]–[33] TB 133 has also been adopted in a number of additional states and localities, and is being actively promoted for use all over the US. The International Association of Fire Fighters, an organization with about 200, 000 members, undertook a political effort to get TB 133 adopted in all 50 states.[34] Going to the individual states seemed indicated because of the anti-regulatory environment at the US Federal level, but presents difficulties because each legislature may use somewhat different wording, causing confusion for the furniture manufacturers. General adoption of TB 133 also was supported by

12

Fire Behavior of Upholstered Furniture and Mattresses

the American Furniture Manufacturers Association, BIFMA, and other industry organizations, and has made good progress. TB 133 applies to behavior after exposure to a substantial ignition source, about 18 kW, one of its (several) pass/fail requirements is that the HRR not exceed 80 kW. This requirement was based on a comprehensive series of full-scale room and furniture calorimeter tests on upholstered furniture conducted at the US National Institute for Standards and Technology (NIST) and BHFTI in which the original room temperature rise requirement was correlated with HRR.[35]–[36] At that peak in HRR, there is no possibility of flashover, and little possibility of the ignition of an adjoining or very close item. There are also prescribed pass/fail levels of smoke and CO. The NFPA Life Safety Code has provisions for upholstered furniture.[37] It requires cigarette resistance of components according to NFPA 260 for residential occupancies,[25] and of furniture mock-ups according to NFPA 261 for public occupancies.[26] For mattresses, the Federal Test is prescribed.[24] Exceptions are made for rooms with sprinklers installed. In addition, in the US, furniture in public occupancies is also frequently subject to state and local fire codes with widely differing requirements.[38] This variety of local codes can make compliance complicated for furniture manufacturers. With respect to flame ignitability, as well as behavior after ignition under flaming conditions, an important development of recent years is the use of HRR for characterization of materials, including upholstered items. The leading instrument for this is the Cone Calorimeter, which also permits measurement of smoke and toxic pyrolysis products.[39]–[41] A book covering the state-of-the-art use of HRR appeared in 1992,[42] and an annotated bibliography on the Cone Calorimeter publications through 1991 has been published.[43] One of the advantages of these measurements is that the results are in engineering units which can be used in computer programs for calculations of the fire hazard presented by various occupancies. The UK passed ignitability regulations in 1988 for all types of furniture and amended them in 1989.[44] An excellent guide to these regulations is Ref. 45, the testing is based on BS 5852.[46] A similar standard for mattresses is BS 6807.[47] Additional standards used for UK government procurement apply to various upholstered items.[48] In general, they are similar to those described in the UK Regulations but there are differences in specimen orientation, etc.

Introduction

13

Various provisions of the UK regulations had effective dates varying from only a few months after promulgation in 1988 to March 1993. Nevertheless, they were apparently accepted without major objections.[49] The development of combustion modified (CM) polyurethane, containing melamine or exfoliated graphite, made the regulation for polyurethane foam cushions technologically practicable for selected cover fabrics, without the use of interliners. However, the short lead times for enforcement (for some parts less than a year), the additional burden on the furniture industry of labeling and record keeping, the vagueness of the original regulations, and the more severe requirements for polyurethane foam than for latex foam were criticized by the British Furniture Industry Research Association. Brief histories of the circumstances leading to the British regulations and means to meet them are found in Refs. 50 and 51. Fire services all over the world are concerned about the rapidly developing fire in polyurethane foam containing furniture, compared to the older materials such as cellulosic battings and horse hair.[52] A survey of European fire brigades published in 1989 showed that 96% believed that fires became worse and produced more smoke and toxic gases than previously. Seventy seven percent ascribed this to the use of polyurethane foam, and 79% indicated that there was need for legislation to reduce the hazard. As discussed earlier, similar concerns were expressed by the US fire fighting community and led to its endorsement of California TB 133 for institutional furniture. Various aspects of the UK Regulations are discussed in Refs. 53 and 54. The regulations define occupancies according to the level of hazard presented by upholstered furniture and mattresses, and assign ignition sources used in BS 5852 (upholstered furniture) and BS 6807 (mattresses) accordingly. Resistance to ignition by cigarettes and a small gas flame simulating a match is required for all occupancies. Resistance to larger ignition sources is specified for High Risk Facilities, such as jails, prisons, detention centers, nursing care facilities, retirement homes, health care facilities, public auditoriums, condominiums, etc. The UK Regulations and related documents contain guidelines for the contents of homes, offices, work places, hospitals, residential care premises, places of entertainment, hotels, and boarding houses. A few years ago, a major effort was undertaken in Europe directed toward unification of flammability standards for the European Union (EU). Some of the preparatory activities were described in a series of papers presented at the Conference on Fire and Furnishing in Buildings and Transport, held in Luxembourg in November 1990[55] and subsequent

14

Fire Behavior of Upholstered Furniture and Mattresses

meetings. The Commission for European Standardization (CEN) formed Technical Committee TC 207 in 1989. Its activities as of 1992 were described by the Convener of its working group WG 6, Fire Test Methods, R. P. Marchant of the UK Furniture Industry Research Association (FIRA).[56] Meanwhile, the European Commission had announced a Draft Directive (subsequently withdrawn) relating to upholstered items, citing the following essential requirements: 1.

Ignitability—three levels of ignition resistance: cigarette resistance for all three levels; match equivalent flame resistance for residential furniture; resistance to an ignition source equivalent to a double sheet of newsprint for general public assembly occupancies; and resistance to five or six newsprint sheets for high risk areas such as locked wards of hospitals and prisons.

2.

Escape time in terms of smoke, toxicity, and heat released during the fire.

3.

Provisions to avoid adverse effects on the environment by FR treatments.

4.

Provisions for giving suitable information on the fire properties of furniture to the end user.

As a first step response, the present regulations, rules, and tests used by the EU members for residential, institutional, and transportation upholstered items were identified in 1990. The UK upholstered item fire regulations formed the basis for discussions in the organizations responding to CEN, ISO, and EU.[57]–[60] The ISO ignitability standard is patterned after BS 5862.[61] A study of the requirements and a proposal for further work was undertaken by the European Group of Official Laboratories for Fire-testing (EGOLF) and related to UK and ISO activities.[58] The Working Group evaluated ignition sources larger than cigarettes and matches, such as paper bags containing newspapers, gas flames, and wood cribs. Tests for mattresses based on the same concepts were proposed. Similarly, tests for post-ignition behavior, reaction-to-fire tests, using the full-scale NORDTEST 032 Furniture Calorimeter[62] and the Cone Calorimeter,[41] were considered. An ambitious inter-laboratory test evaluation, including six

Introduction

15

ignition sources (20 to 2500 kJ), six fabric/padding combinations, and 14 laboratories was planned in 1990. Use of full-scale room tests (ISO 9705)[63] to verify the results from the furniture calorimeter and methods of hazard assessment with use of computer codes were proposed. Measurement of heat release, smoke, and toxic product release rates on bench-scale (for example, with the Cone Calorimeter) would have to be related to full-scale experiments, and furniture geometry effects established. Fire spread to adjacent articles would have to be established by ignitability measurements at various levels of irradiance. Among the other items under discussion were the need to establish levels of protection needed for various occupancies, ranging from residential (for which the BS 5852[46] and ISO 8191[61] cigarette and match simulation flame ignitability tests seemed acceptable), to prisons and mental institutions, where escape is difficult or impossible and where arson is a distinct possibility. Individual governments would establish the levels of protection needed for various occupancies; interstate traffic of furniture would not be hampered by this because items would be labeled according to their behavior in various tests. The main research program, which actually materialized to increase the state of knowledge of fire testing upholstered items, was CBUF. The basic thinking leading up to the CBUF program is discussed in Refs. 7 and 64. The objective was the development of a new technology for assessment of the post-ignition burning behavior of upholstered furniture in support of the Second Essential Requirement included in the draft EU furniture directive. This requirement stated: “The atmosphere in the room in which the upholstered furniture or related article are on fire should despite the production of heat and smoke ... remain for a reasonable period of time after ignition such that it does not endanger the lives or physical well being of exposed persons. This will be achieved by controlling the rates of heat release, and of smoke and toxic gas production. This would allow time for the escape by alert and ablebodied persons.” The CBUF research program was authorized by the European Commission and part-funded by it. The rest of the funding came from industry, governments, and laboratories within the Member States of EU. The consortium conducting the research consisted of three organizations in

16

Fire Behavior of Upholstered Furniture and Mattresses

the UK, and one each in Belgium, Denmark, Finland, France, Germany, Italy, and Sweden. The technical coordinator was Björn Sundström of the Swedish National Testing and Research Institute in Borås. The major results of this research can be briefly summarized as follows:[65] 1. The HRR history of the full-scale furniture in the furniture calorimeter was identified as the principal measure of the hazard. From that, the height of the interface between the hot upper gas layer and the near ambient temperature lower layer, through which the room occupants would have to escape, could be calculated. The HRR combined with the yields of smoke and toxic gases could be used to predict the smoke obscuration and toxic gas concentrations in the room of fire origin and in the other rooms in the building. 2. A furniture fire model, a mattress fire model and a set of correlation formulas were developed to predict the HRR in the furniture calorimeter based on measurements on the furniture composites in the Cone Calorimeter. This would reduce the amount of full scale testing required. 3. A composite model was developed, as an option, to predict the HRR of furniture composites based on measurements on the individual components in the Cone Calorimeter. This would reduce the amount of bench scale testing required and make it practical to shift the responsibility for the testing from the furniture manufacturers to the material suppliers. 4. It was left to the regulator to specify the minimum time that must be allowed for escape from the room of fire origin and the minimum height of the hot gas interface during that period. In order to determine the acceptability of an upholstered furniture item, the actual escape time and minimum interface height can be predicted from its full scale HRR curve using existing room fire models.

Introduction

17

The full-scale fire test methods for the various furniture items varying in material assemblies and configuration were the ISO room/corner test[63] and the furniture calorimeter.[62] The fabrics and the foam were tested both as composites and as separate components in the Cone Calorimeter.[41] Detailed testing procedures for the room fire test and for the furniture and Cone Calorimeters were written based on an extensive investigation of the effects of the various test parameters. The ignition source for the full-scale tests was a gas burner with a nominal 30 kW HRR applied for 120 seconds, to assure ignition of most items. Commercial solid core and innerspring mattresses, innersprings, two and three seat sofas, and a large variety of upholstered chairs, as well as upholstered chairs varying systematically in fabric and padding and configuration were included. There were 71 room tests, 154 furniture calorimeter tests, and Cone Calorimeter tests of 1098 composites, along with 172 individual fabrics and foams tested. A number of preliminary experiments established the ignitability of the various items, and the effects of increased room size, variations in room ventilation, reproducibility and repeatability, and test procedure details. The report is an important source for information for the development of furniture fire models and furniture design engineering, as well as for the 1995 state of the art upholstered furniture fire experiments. The essential findings are summarized in the appropriate chapters of this book. Many countries currently have no encompassing upholstered item regulations but rely, to varying degrees, on local authorities and purchase specifications for the fire safety of upholstered items. Limited regulations apply to certain public occupancies in France, Germany, Italy, and Spain. The activities in the Nordic Countries, which proceed within the framework of EU, are described in Ref. 64. The regulation of transportation seating will also be unified throughout the EU. For aircraft, most countries comply with the US Federal Administration (FAA) standard[66] and the aircraft producers often have additional standards. For ships, compliance with the International Maritime Organization (IMO) rules is universally accepted. There appear to be few regulations for automobile upholstery except in the US,[67] however, this standard provides very little protection. On the other hand, bus upholstery is regulated in some countries, using paper or radiant ignition sources, or with the BS 5852 in the UK. Details are given in Ch. 3.

18

Fire Behavior of Upholstered Furniture and Mattresses

2 Fundamentals

This chapter briefly summarizes some of the fire science fundamentals that need to be considered in dealing with the fire behavior of upholstered furniture and provides references to research that has been carried out in these areas. The topics covered include pyrolysis, combustion, smoldering, transition to flaming, flaming ignition, flame spread, HRR, propagating and non-propagating fires, inter-item spread, interaction with the enclosure, flashover, smoke and toxic gases. For more detailed discussions of fire science fundamentals, the reader is referred to Drysdale’s book on Fire Dynamics[68] and the SFPE Handbook.[69] Because of the central role played by heat release rate, the book Heat Release in Fires[42] is also a useful reference. The report on the CBUF project deals with the application of many of these elements to a comprehensive research program on the fire behavior of upholstered furniture.[7]

2.1.0

PYROLYSIS AND COMBUSTION

Combustion of solid materials that exhibit charring can occur by smoldering, glowing, or flaming. They undergo thermal decomposition (pyrolysis) to produce volatiles and char at elevated temperatures. Here the term volatile refers to any substance that would be in the gaseous state at temperatures characteristic of a fire environment. The rate of production of volatiles, m· , (kg s-1) during the thermal decomposition of a solid element having a uniform absolute temperature, T (K) is usually expressed by the Arrhenius equation, 18

Fundamentals (Eq. 2-1)

19

m· = ( m-mf )n A exp(-E/RT)

where m is the remaining mass of the element at any time during the decomposition, mf is the final mass of its char, A is an effective frequency factor, E is an effective activation energy and R is the gas constant (8.3 × 10-3 kJ mole-1 K-1). The exponent, n, is the order of the reaction that is usually taken to be unity, which means that the rate of volatilization is directly proportional to the amount of mass left that can be volatilized. The constants A, E, and n have physical significance for gas phase reactions. In the case of the thermal decomposition of solids, they are only empirical constants which have been found to provide a good correlation of the mass loss rate with (1) the absolute temperature of the specimen and (2) the mass remaining to be lost before it reaches its final char state. To use Eq. 2-1 for computing the mass loss rate, one has to obtain the kinetic constants. These can be obtained by thermogravimetric analysis (TGA). Such an approach is usually reserved for research studies, since for product testing it is normally much easier to measure the mass loss rate directly (see Sec. 2.6.0). For cellulosic materials a rule of thumb is that a 10°C rise in temperature approximately doubles the rate of decomposition. If the volatiles form a combustible mixture with the surrounding air and come in contact with a flame, a spark, or a high temperature surface (in the neighborhood of 500°C) they will ignite to form a flame. Otherwise, the volatiles with boiling points above room temperature will condense into liquid droplets to form smoke. If the char structure is sufficiently porous and flaming does not occur, oxygen can diffuse into the pores and produce highly exothermic reactions with the char. If the rate of internal heat production is high enough and the heat losses are small enough, the temperature will be elevated sufficiently to sustain the reaction. In that case, a smoldering front will move through the material, eventually reducing it to ash. If the reaction rate eventually becomes high enough to produce a combustible mixture outside the surface and a surface temperature high enough to ignite it, there will be a transition to flaming. If there is a pilot such as a flame, spark, or high temperature surface nearby, the transition will occur earlier. If the porosity is too low to provide an oxygen supply sufficient to sustain the smoldering front and the rate of volatilization is too low to maintain a flame, oxidation of the charred surface can still occur. However, this requires a temperature around 600°C. The rate of heat loss from the surface is so high at this temperature that it can only be maintained by external radiant heat. At 600°C, the surface will appear to be red. The char

20

Fire Behavior of Upholstered Furniture and Mattresses

combustion will produce some CO which will undergo gas phase oxidation to CO2 in the neighborhood of the surface with the production of a faint blue glow. This process is referred to as glowing combustion. Sometimes glowing combustion is regarded as a form of smoldering. A combustible item, which is porous and allows air to permeate through it, can be subject to smoldering. Smoldering of an upholstered furniture item generally starts from a cigarette, or, less frequently, from sources such as heaters, etc. Flaming fires can be started with matches, lighters, or other, often much larger, flaming objects. The smoldering fire may turn into a flaming one later, or a flaming fire may convert to smoldering due to oxygen depletion or due to burning rates too small to support flaming. While the HRR of the smoldering fire is very small, its hazards can be significant. Even limited smoldering of upholstered items can cause casualties due to suffocation in the room of fire origin or even adjoining rooms in under-ventilated spaces. A given material may be capable of either smoldering, flaming, or both. Many materials used in upholstered furniture (for example, cellulosic and acrylic fabrics, polyurethane foam, and cellulosic batting) fall into the latter category. The rates of burning are very different, being on the order of 0.1 g s-1 for a smoldering chair and 100 g s-1 for a flaming one. The rate of glowing combustion is typically less than that of flaming but much greater than that of smoldering. The ignition scenarios are likewise different. Thermoplastic materials degrade at elevated temperature to produce lower molecular weight components which melt and evaporate. The liquid, which is not immediately converted to a vapor, can form pools or be absorbed temporarily by porous surfaces onto which they may fall. The rate of volatilization of the liquid pool is given by

(Eq. 2-2)

m ′′ =

′′ q net Lv

(kg m − 2s −1 )

where q·´´net is the net heat flux (kW m-2), which is the incident flux minus the heat losses, and Lv is the heat of vaporization (kJ kg-1). The mass loss rate from the solid surface can also be expressed by Eq. 2-2 if Lv is replaced by an effective heat of gasification hg which includes the heats of degradation and melting as well as the heat of vaporization. While Lv is a constant, hg can vary during the burning period when the liquid is not immediately converted to a vapor.

Fundamentals

21

Thermoplastic frames, padding, and fabrics initially consume energy during the decomposition and melting phase. After they form pool fires they typically have high HRRs because most of the heat feedback from the flames goes into the evaporation of the liquid pool. Radiation losses from the surface are small because the temperature of the boiling liquid is usually low compared to the surface temperature of charring solids. Also, the pool can spread to provide large burning areas. Melting foams can form large pool fires. Melting fabrics usually fall onto the padding to form puddles and evaporate from there to feed the flames. With many (but not all) upholstered items made from thermoplastic materials, a large pool tends to form on the floor under the object in burning. The HRR from this pool can be roughly similar to what is being released from the remaining components of the item.

2.2.0

SMOLDERING

This section is a brief review of the literature on smoldering of cellulosic materials and polyurethane foams. There is a considerable body of knowledge of the type of upholstered item constructions which ignite from cigarettes, as discussed in Ch. 6. A descriptive overview of the smoldering processes can be found in Ref. 70. Ohlemiller has provided the most recent and comprehensive review.[71] In the interest of safety, it is important to note that self-sustaining smoldering in cotton batting and polyurethane foams can occur well inside the slab, essentially undetectable from the surface, even if the fire has apparently been extinguished with, for example, water. When the smolder front reaches the outside of the slab, transition to flaming ignition is likely to occur. There is anecdotal evidence that smoldering mattresses have reignited after prolonged submersion in water. 2.2.1

Cellulosic Material

Smoldering ignition of upholstered items due to inadvertently dropped cigarettes is the major cause of residential fire deaths, as discussed in Ch. 1. Both the tobacco column and the paper covering a cigarette contain cellulose and are designed to smolder readily. Experience shows that smolder transfers easily from cigarettes to medium and heavy weight cellulosic and acrylic fabrics and from them to many commercial padding materials, especially cotton, cotton blend batting, and polyurethane foam.

22

Fire Behavior of Upholstered Furniture and Mattresses

Certain materials, such as medium to heavy weight thermoplastic fabrics and batting, wool fabrics, and halogen-containing materials (for example, vinyl-coated fabrics, vinyl-vinylidene back-coatings, or polyurethane with smolder resistant, SR, treatment) can interfere with this transfer. The literature on cigarette/upholstered substrate interaction is reviewed in Ref. 72. There is a plethora of analyses of cigarette (mostly tobacco column) smoldering behavior in air; however, it must be emphasized that cigarette behavior in air is not indicative of the behavior on a substrate. Figure 2-1 shows how the temperature/time relationship inside cigarettes and eventual ignition/non-ignition were influenced by the fabric on which the cigarette smoldered. A series of papers analyzes the smoldering behavior of cellulosic materials, for example, shredded paper insulation.[73]–[79]

Figure 2-1. Temperature versus time curves for cigarettes burning in air and on different fabrics.

The smoldering of cellulose is a two-step process. It is initiated by heating some local region up to its pyrolysis temperature. In the first step, char volatiles and gases are produced by thermal decomposition. The smoke observed during smoldering is due to the condensation of these volatiles. During the second step, the char is oxidized in a highly exothermic reaction (approximately 30 MJ kg-1) producing the heat required to

Fundamentals

23

bring the adjacent region up to its pyrolysis temperature. If the supply of oxygen is high enough and the heat losses are low enough, a smolder wave will propagate through the material, leaving only an ash deposit. If the oxygen supply is restricted, the first step is usually endothermic and the reaction rate is slow. If the supply of oxygen is adequate, oxidative pyrolysis occurs in the first step and the reaction will be mildly exothermic (less than 1.0 MJ kg-1). The products will still consist of char and volatiles. However, their rate of production will be faster and their detailed chemical composition will be different. Char will be formed in the first step whether the oxygen supply is adequate or restricted. However, the two chars are probably not identical in reactivity or in other properties. These chars are typically somewhat more resistant to oxidation than the initial fuel but ultimately can be completely consumed. The char oxidation wave can often be visually observed as a glow traveling over a previously charred area, e.g., paper or a log in the fireplace. In experiments involving shredded cellulose insulation on a heated plate, smoldering could be initiated at temperatures as low as 290°C.[76][77] Temperatures measured in free-burning tobacco columns ranged up to 900°C.[72][80]–[83] However, Salig found core temperatures in cigarettes at the beginning of smoldering on cotton print cloth/polyurethane foam and cotton duck/polyurethane foam composites of only about 600°C.[84] The heavier duck composite continued smoldering after the exposure to the cigarette was over, the print cloth composite self-extinguished as seen in Fig. 2-1. The smoldering rate increases with denser packing of the cellulose insulation, thicker insulation beds, increased oxygen supply, and favorable air current direction over the ranges tested.[76][77] However, the rate will decrease when the packing becomes so dense that the supply of oxygen becomes restricted. The role of smolder retardants like boric acid is to interfere with the oxidation process; it does not reduce temperatures in the smolder wave.[76][79] Much of the literature on burning cigarettes discusses the effects of tobacco type, packing density, cigarette paper porosity, etc., on the linear burning rate (the mass burning rate is less affected by these parameters); this is reviewed from the point of view of furniture item ignition in Ref. 72. As regulation of the ignition propensity of cigarettes appeared to become a possibility, a number of papers were published which touched on basic mechanisms of cigarette ignition of substrates but also covered items related to an appropriate test method; these are discussed in Sec. 6.1.7.

24

Fire Behavior of Upholstered Furniture and Mattresses

Two similar methods were chosen to investigate smoldering of fabrics. In both, cylindrical cartridge heaters were used to initiate smoldering because their heat flux could be controlled and they did not consume oxygen as cigarettes do.[573][574] In one, the heater tip was held on the top of the fabric specimens (five cotton ducks varying widely in weight and a 300 g m-2 cotton rayon, commercial upholstery fabrics) and an infrared imaging camera was placed underneath the fabric samples. No correction for emissivity was applied since the emissivity of cellulosic materials is >0.90, and that of char, 0.98. To study the effect of alkali metal ion concentrations, some duck samples were rinsed, and others treated in potassium acetate solutions. When the cartridge heater was placed on the fabric, a sharp rise in temperature and then a plateau was observed, followed by another steep rise to about 670°C when smoldering ignition occurred, followed by fairly stable temperatures. In the second method, the cartridge heater was placed flat on the fabrics which were supported by PU foam, and the ignition point was defined as the time when the specimen was visibly judged to have started glowing. The ignition times were shorter at higher heat fluxes, as expected, and there was a linear relationship between the heat flux and the inverse of the time to ignition. The ignition times increased with fabric weight and decreasing alkali metal and oxygen concentrations. Minimum heat fluxes for ignition were about 15 kW m-2, and a potassium ion level of about >1300 ppm was required to sustain smoldering after ignition took place. The authors found the ignition temperatures of the unwashed cotton ducks to be 384–424°C, for the washed ducks, up to 493°C. In a striped commercial upholstery fabric the ignition time varied for different colored stripes, from roughly 30 s to no ignition, because of differences in ion concentrations. This indicates that for appropriate testing of cigarette ignition resistance of fabrics, all major areas differing in color or construction should be tested separately. The results of this work were used to construct a model of fabric smoldering ignitions by cartridge heaters.[574] At about 400°C, the cellulose of the specimen is gradually converted to char, and ignition can be observed. The temperature then rapidly reaches a maximum as the char itself undergoes combustion. The mathematical model was developed from these observations based on the heat transfer mechanism involved, and agreed generally with the experimental results. The important parameters for fabric smoldering ignition are heating flux, heating area, fabric weight, and ion content; the latter may affect the reaction kinetics.

Fundamentals

25

Infrared imaging of the bottom surface of fabrics below a radiant heat source simulating smoldering cigarettes was used to measure dynamic surface temperature gradients.[575] The fabrics used were again the three heavy, dense, unfinished cotton duck fabrics and two commercial upholstery fabrics. They were tested without padding, so that the oxygen scavenging and heat sink effects normally encountered in upholstery were not present. The pyrolytic degradation and oxidation reactions are governed by the heat transfer from the ignition source and the kinetics of the oxidation in the initial stages of smoldering. The further growth of the smoldering zone depends on the oxygen supply and the heat losses, which, in turn, depend on fabric construction and permeability. In this experimental setup, two zones are clearly visible: red glow in a central charring zone and a discolored pyrolysis zone, with volatile gases emerging from the surface. All fabrics reached temperatures of 500°C, and the heavier fabrics generally maintained this temperature longer than the lighter ones. Smoldering was maintained above 450°C. Isothermal areas on the duck fabrics did not correlate to fabric weight or alkali metal ion contents. Based on peak temperatures and isothermal areas, washing to remove the smolder-promoting alkali metal ions did not affect smoldering propensity of the ducks. Measurements on the upholstery fabrics may have been affected by their loftier structures (which would affect the heat transfer through the fabric); their time/temperature curves are more irregular, and for the lighter fabric, the temperature after the peak descended more rapidly than for the other fabrics. The two upholstery fabrics also showed lower peak temperatures and smaller isothermal areas at 450°C after washing even though their original ion content was less than half of that of the ducks. This may indicate that there were also structural changes due to washing.

2.2.2

Polyurethane Foam

The smoldering of polyurethane foam is discussed in Refs. 84–88, and in the above-mentioned review.[72] Few polyurethane foams were found to smolder in contact with burning cigarettes unless a smoldering fabric cover was present.[84] Smolder temperatures are about 400°C and smolder front progress in those foams which smolder is about 0.1 mm s-1. About 5% of the mass of the combustion products consist of carbon monoxide (CO).

26

Fire Behavior of Upholstered Furniture and Mattresses

The smoldering process can be divided into two major competing phases: the formation of smoldering char and the formation of nonsmoldering tar.[75][85] The first phase of polyurethane foam pyrolysis, which involves 10–15% weight loss, is virtually the same in air as in an inert atmosphere. The product is colored but still has some of the resiliency of original foam. In the presence of air the initially degraded foam is cross-linked to form char with the release of water and heat. This black cellular char, which retains much of the foam structure, undergoes further oxidation in air and provides the heat required to drive the smolder wave. If the char oxidation is sufficiently fast then the rate of heat production may be adequate to replace the outside ignition source so that smoldering becomes self-sustaining. If not, smoldering may still proceed until it recedes so far from the external heat source (for example, smoldering fabric or cigarette) that its own heat generation can no longer overcome the heat losses; it will then extinguish. In the absence of air, or when the rate of char production is prohibitively slow, the degraded foam is converted to tar with the loss of the cellular structure essential for smolder. In the absence of air the tar is completely gasified leaving only a small residue (1–3%) at 500°C.[75][85] Several approaches to making SR (smolder resistant) foam have been suggested. One is the use of agents which would interfere with char formation during the degradation process. The second is promotion of tar formation by weakening the polyol chain and urethane links. This, however, may increase the probability for flaming combustion.[87] Others are inclusion of inert or hygroscopic materials in the foam; more recently, quite flame resistant foams are based on inclusion of melamine or certain forms of carbon, as discussed in Ch. 6. More specific modeling equations for the smoldering of polyurethane foam and reasonable experimental validation can be found in Ref. 87. The difficulties caused by the fact that smoldering is very incomplete combustion are discussed. Both conduction and radiation affect the smoldering rate in open structures, such as flexible polyurethane foams. Smolder intensity was found to be governed by oxygen supply, but smoldering can proceed at oxygen supply rates as low as 5% of the stoichiometric one. The threshold oxygen concentrations at which self-extinguishment, continued smoldering, or transition to flaming occur was established for three polyurethane foams.[88] This work was performed with an electrical heating coil rather than a cigarette ignition source. Such heating coils appear to give different results than cigarettes and seem to lead to faster transition to flaming than

Fundamentals

27

cigarette induced smoldering.[84] They obviously present a stationary ignition source, as compared to the moving smolder front of a cigarette.

2.3.0

TRANSITION TO FLAMING

Many, but by no means all, fires which start as smoldering fires eventually begin flaming. This transition is governed by a complex interaction of heat conduction, gas flows, and reaction chemistry and is not well understood.[75] For upholstered items, three main empirical observations can be made: 1. Oxygen availability and air currents play a major part in this transition. Typically, a smoldering furniture item may flame when a door is opened, assuring a new oxygen supply. Reports on shredded, tightly packed grass clippings are available;[80][81][89] they burst into flames most readily when the air movement exceeded 0.83 m s-1 (3 km hr -1). On the other hand, flaming may revert to smoldering if the oxygen in a room is depleted[90] or the mass loss rate in the form of gases and volatiles becomes too small to support flaming. However, the rate of mass loss may drop only slightly just after reversion to smoldering, while it always increases very significantly after a transition to flaming. The increase in the rate of mass loss rate during flaming is due to the flame heat transfer to the surface. When flaming stops, the oxygen which would have been consumed by the flame is now available to oxidize the high temperature char. Thus the high mass loss rate may continue for a while due to the char consumption contribution. In order to maintain the surface temperature high enough to sustain the char oxidation, it will generally require an external radiant flux which can include reinforcement from other parts of the burning item as discussed above. 2. When a transition to flaming occurs, HRR, smoke production, etc. were sometimes found to be essentially identical to a fire that would have been started at that instant by flaming means.[91] When this holds

28

Fire Behavior of Upholstered Furniture and Mattresses true, it can allow great simplification in practical analysis, since it states that the history of the fire does not have a “memory.” However, since changes in the chemical structure do occur during smoldering, there are undoubtedly conditions under which such simplification does not hold. 3. The transition to flaming cannot yet be predicted by knowing the materials of construction of the item.

The following studies are summarized in Table 2-1. The California Bureau of Home Furnishings (BHFTI) placed cigarettes on 15 commercial chairs.[92] Five chairs self-extinguished after prolonged periods; one smoldered until it was extinguished after 330 minutes; and nine eventually went into flaming. Because these were commercial furniture items varying greatly in shape as well as materials, little could be learned about construction factors which resulted in flaming except, perhaps, that the presence of thermoplastic fibers seemed to reduce the tendency for transition to flaming. Similar results regarding the transition to flaming were found in a few experiments with cigarette ignited chairs containing thermoplastic batting in the seats.[90] When polyester batting was the padding material, the probability of transition to flaming was decreased as compared to polyurethane and cotton batting. However, the rate of glowing (for those fabrics which did glow) seemed to be increased when polyester batting was substituted for polyurethane foam or cotton batting. The fastest transition from smoldering to flaming (22 minutes after placement of the cigarette) was in a chair in which a heavy cotton fabric covered the cotton batting; in chairs with lighter cellulosic fabrics and mostly polyurethane padding it was about one hour. A draft in the test room favored flaming. In tests performed in abandoned housing in the Indiana Dunes in the late 1970s, an assortment of commercially available new and secondhand furniture was ignited with a glowing heater element.[93][94] The average times of smoldering before flaming were 70 minutes for the chairs and sofas and 83 minutes for the mattress/box spring assembly in the original test series. Another test series was later conducted at NBS under somewhat different conditions on replicate chairs and resulted in flaming after an average of 44 minutes.[95] The chairs were covered with a cotton upholstery fabric, and padded with either cotton batting or polyurethane foam. None of the components were fire retarded. It is interesting to note that for this

Fundamentals

29

set of 12 identical chairs, the transition times varied from 29 to 63 minutes. This kind of variation emphasizes that all aspects of smoldering phenomena are highly statistically variable and that this variation must be considered before placing much reliance on mean values. In 1970, Southwest Research Institute reported cigarette ignition test results on innerspring mattresses using cellulosic ticking.[96] In some cases, where no smoldering ignition took place with one cigarette, two cigarettes were placed side by side; these results are thus pertinent for times from inception of smoldering to flaming but not for cigarette ignition resistance of the composites. These mattresses were tested without and with sheets or blankets containing various fibers; pillows were included in some test arrangements. A vinyl mattress cover underneath the sheet caused relatively slow transition to flaming. Six chairs with cover fabrics made from various fibers were also tested. A chair covered with polypropylene fabric flamed in 22 minutes (it may have been a rather thin fabric; medium or heavy weight thermoplastic fabrics generally have relatively good cigarette ignition resistance. However, this was one instance where two cigarettes side by side were used). Foam rubber (latex) seemed to transit into flaming relatively early. Table 2-1 shows that out of a total of 102 items subjected to smoldering ignition in laboratory tests, 32% burned up partially or completely without erupting in flaming; 64% did go to flaming, while the remainder were manually extinguished or were indeterminate. For the chairs which did not go to flaming, the time for the chair to be essentially consumed can be long; one study reported an experiment where smoldering persisted for over 6 hours. The mean smoldering-to-flaming transition observed in the laboratory tests was 88 min, the minimum 22 minutes, and the maximum 306 minutes. The conclusion can be drawn that transition times in the range 22 to 306 minutes are possible, but NOT that transitions outside of this range are impossible. The maximum and the minimum values found in a given sample of a population will depend not only on the traits of the population but on the sample size. Thus, if more than 102 fires of smoldering origin were examined, it is likely that values outside the given range would have been found. There are physical limits to this, however. Times in the range of seconds or days, for example, would be unlikely since smoldering is not established before some minutes have elapsed. Conversely, an item cannot transition to flaming if it has been smoldering long enough to be essentially consumed.

30

Fire Behavior of Upholstered Furniture and Mattresses

Table 2-1. Smoldering-To-Flaming Transition Times Reference Type of Item

Number of Items

Time to Flaming (min) Total Burned Went Other2 Avg. Range up or to went flaming out without flaming

[92]

Commercial chairs

15

5

9

1

142 60–306

[90]

NBS experimental chairs

6

3

3

0

48

[93][94]1

sofas and chairs

24

7

17

0

72

28–132

[93][94]1

mattresses and box springs

18

8

10

0

85

51–129

[95]1

chairs

22

10

12

0

44

29–63

[96]

solid foam and innerspring mattresses

11

0

8

3

140 97–233

[96]

chairs

Totals

22–65

6

0

6

0

97

22–152

102

33

65

4

88

22–30

1. Electric ignition source; the remaining test series used cigarette ignition 2. Manually extinguished, or status unclear

2.4.0

FLAMING IGNITION

Kanury provided a review of the ignition process of cellulosic materials[97] and a mathematical discussion of flaming ignition of solid fuels.[98] Also, Drysdale’s book on Fire Dynamics covers piloted ignition of solids.[68] A comprehensive treatment of piloted ignition is given by Janssens in his Ph.D. dissertation.[99] Flaming ignition occurs when (1) the flow of volatiles from a decomposing solid or a boiling liquid exceeds some critical value, depending on the material, its moisture content and orientation, the air flow conditions, ambient humidity and oxygen concentration and(2) the mixture of these volatiles with the surrounding air encounters a flame, a spark, or a high temperature surface (approximately 500°C or greater) which can serve as a pilot ignitor. The critical flow of volatiles required for flaming ignition is that which is needed to reach the lower flammability limit for the resultant fuel/air mixture.

Fundamentals

31

The ignition temperature is the lowest temperature at which piloted ignition of a material can occur. The minimum flux for ignition is the lowest incident flux for which this ignition temperature can be achieved. As the flux is increased above its minimum value, the time to ignition gets shorter, the depth of the thermal wave and the zone of pyrolysis at the time of ignition gets smaller. Therefore, the temperature in this zone must be increased in order to produce the required flow of volatiles to achieve ignition. Thus, the temperature at which ignition occurs is an increasing function of the incident flux. In order to avoid running tests at many different fluxes to find the smallest one for which the specimen will ignite, it can be determined graphically using a small number of different fluxes. Straight-line plots can usually be made by plotting the ignition flux on the x-axis versus some function of the ignition time (t -1, t -0.55, etc.) on the yaxis. The point at which the straight line intersects the x-axis is then defined as the critical flux for ignition. Further details are given in Ref. 99. Ignition by flame contact occurs if the incident flux is primarily due to the heat transfer from a flame contacting the surface. Radiant ignition occurs if the critical flux is primarily due to a radiation source not in contact with the surface. Radiant ignition is referred to as piloted or forced ignition if there is a flame, spark, or high temperature surface present. If there is no external pilot the surface needs to be heated to a sufficiently high temperature (for example, the generally accepted value is 400–500°C for cellulose) to act as its own pilot. In this case it is called auto-ignition or spontaneous ignition. The latter term is not preferred, since it may be confused with spontaneous combustion, below. Self-ignition or spontaneous combustion occurs under certain conditions of self-heating due to internal chemical reactions (bacteria can be involved). After long periods of time, the interior temperature can reach a sufficient level to produce the critical flow of volatiles required for ignition even when the ambient temperature is still relatively low. Some classic examples are oily rags and high piles of coal. A detailed discussion of spontaneous combustion is given in Ref. 100. Ignition by flame contact is always piloted. If the surface continues to flame after removal of the exposure flame, it is referred to as sustained ignition. If the flame on this surface is not maintained after removal of the ignition source, it is referred to as transient ignition. A uniform, large-area irradiance of about 20 kW m-2 suffices to ignite not only all common upholstered assemblies but even many fire retarded (FR) ones. While higher fluxes may be required to ignite over a small area, even these are normally achieved in typical fire scenarios. Yet,

32

Fire Behavior of Upholstered Furniture and Mattresses

in many practical cases, a small area ignition source may cause the material to be ignited locally but the fire will not spread; instead, it will die out once the source is removed. An extreme example of this is a welding torch. With many fabrics and foam compositions, sustained ignition by such torches is impossible in spite of their especially high fluxes; a hole is melted/burned through them but sustained ignition does not result. Sustained ignition is a necessary condition for flame spread. Ignition sources are discussed in Ch. 4. The most commonly used equation to predict the time to ignition is based on the solution of the classical heat conduction problem of an inert semi-infinite solid with constant properties exposed to a constant flux with no heat losses. The front surface temperature rise is given by Ref. 101

(Eq. 2-3)

T − To =

′′ t 2 q ext

π kρ C

where T is the surface temperature as a function of time, To is the ambient temperature, q·´´ext is the constant exposure flux, k is the thermal conductivity, ρ is the density of the solid, C is the specific heat of the solid, and t is time. The product kρ C is called thermal inertia. This equation can be inverted to yield the time to ignition as a function of the ignition temperature, the flux and the thermal inertia kρ C:

(Eq. 2-4)

(

ð kñ C Tig − To tig = ′′2 4qext

)

2

It must be recognized that in addition to the solid not being inert, both the thermal conductivity and the specific heat are increasing functions of the temperature for most materials. In the case of wood they are directly proportional to the absolute temperature. Also surface heat losses are too large to neglect at the pyrolysis temperature. Furthermore, Eq. 2-4 does not take into account the dependence of the ignition temperature on the flux. In spite of the shortcomings, it remains the classic analytical solution and has often been used for correlation of ignition time and flux data. However, improvements can be made with approximation methods as discussed by Janssens.[99] An analytical solution which takes linear heat losses from the surface into account does exist for the surface temperature. It is given by

Fundamentals

(Eq. 2-5)

T − To =

[

33

( )]

q ext ′′ 1− exp(ô ) erfc ô h

where

(Eq. 2-6)

τ=

h 2t kρC

Equation 2-5 reduces to Eq. 2-3 when the surface heat transfer coefficient h equals zero. The functions inside the brackets can each be replaced by a power series in order to provide for an approximate solution for the time to ignition. However, in order to account for the temperature variation in the thermal properties, the variation in ignition temperature with flux, and the heat of pyrolysis of the decomposing solid, it is necessary to use a computer model to predict the time to ignition. A computer model for piloted ignition of wood has been provided in Ref. 99. Another computer model has been constructed to calculate the HRR of wood.[102] The time to ignition is routinely obtained as a necessary by-product of this model. However, there is a scarcity of data on these properties at elevated temperatures to put into the models, particularly in the vicinity of the ignition temperature. Deterministic models for the HRR of upholstered furniture materials are not available at this time.

2.5.0

FLAME SPREAD

Typical upholstered item dimensions range from 0.5 to 2.0 m. Fires of this scale are large enough so that flame spread and heat release behaviors are predominantly by radiative, rather than convective, mechanisms. It can further be assumed that in most room fires, only natural convection and not forced convection flows need to be considered. Flame spread is usually distinguished as being one of two types: concurrent-flow (wind-aided) or opposed flow (against-the-wind). In natural convection situations, the wind is induced by the fire itself due to its buoyancy. Opposed-flow flame-spread can be along a horizontal surface or along a vertical surface in the lateral or downward direction. Horizontal and lateral flame-spreads are controlling factors in furniture fires; they have much lower rates than the upward or

34

Fire Behavior of Upholstered Furniture and Mattresses

wind aided spread. Wind-aided flame-spread is normally irrelevant to mattresses. On upholstered chairs, it will occur on vertical panels which are ignited at their bottom. The time for vertical flame-spread to occur is usually very fast and for furniture this component is sometimes approximated as being instantaneous. A theory was first presented by Rockett[103] for providing a calculational basis to determine opposed flow flame-spread when external radiation is the primary mechanism and convection is small enough that a simple treatment is adequate. The flame-spread process is assumed to be one-dimensional, i.e., spreading from a line source, not a point source. This theory has been applied, in a somewhat simplified way, to ASTM E 162, a test method for downward flame-spread on wall paneling materials.[104][105] It is sometimes used for other types of materials including upholstered items. An apparatus was constructed by Babrauskas and coworkers at NIST for making horizontal flame-spread measurements on fabric/padding composites in a thermal radiation field with the goal of obtaining data suitable for analysis with Rockett’s theory, but was used only for a few experiments.[106] However, the following observations were made: (1) at zero irradiance the spread rates ranged from 0 to 3.7 mm s-1 depending on the material combination; (2) at 2.5 kW m-2 irradiance the spread rates were typically about doubled, except for those cases which fell below 1.0 mm s-1 at zero flux. The flame-spread rate equations are usually based on the assumption that (1) there is a well defined preheating region which extends a distance beyond the area of the pyrolyzing surface along the direction of spread and (2) that the local heat flux from the flame to the surface in this region is constant. The flame-spread rate is given by

(Eq. 2-7)

v fs =

ä tig

where tig is time it takes to go from the surface temperature Ts ahead of the preheating region to the ignition temperature Tig. By substituting Eq. 2-4 into Eq. 2-7 Quintiere obtained the flame spread equation that was used in the development of the LIFT test method.[107][108]

(Eq. 2-8)

v fs (t ) =

( ) ð k ñ C (T − T ) 4ä q ′ft′

2

2

ig

s

Fundamentals

35

By letting (Eq. 2-9)

Φ=

( )

4 δ q ′ft′ π

2

Eq. 2-8 reduces to

(Eq. 2-10)

v fs (t ) =

(

Φ

k ρ C Tig − Ts

)

2

In principle, Eq. 2-10 could be applied for upward, lateral, downward and horizontal spread. The flame spread constant, Φ, would depend on the material, the scale of the problem, and the direction of flame spread. To a first approximation the ignition temperature, Tig, and the thermal inertia kρ C depend only on the material. However, k and C increase with the temperature which varies from the quasi-equilibrium temperature of surface ahead of the flame to the ignition temperature. Therefore, the effective thermal inertia should also depend to some extent on Ts. This has been ignored for practical reasons. The LIFT apparatus was designed to measure Φ, kρ C and Tig for use in the fire growth models to predict the lateral flame-spread using Eq. 2-10. A modified LIFT test has occasionally also been used in the horizontal orientation to obtainΦ for horizontal flamespread. The external flux plus the flux from the flame must be equal to or greater than the minimum flux for ignition in order to sustain the ignition and allow the flame to spread along the surface. Thus, the minimum flux for flame spread is given by (Eq. 2-11)

′′ = qigs ′′ − q ′ft′ q mfs

The minimum surface temperature for flame spread is the equilibrium temperature of the surface when it is exposed to this critical flux. Thus, Tmfs is defined by the following energy balance,

(Eq. 2-12)

(

) (

)

4 − T 4 + h T − T +  k ∂T  ′′ = εσ Tmfs q cfs o cfs o  ∂z  z =0

36

Fire Behavior of Upholstered Furniture and Mattresses

where z is the coordinate normal to the surface. The last term which is due to conduction losses to the interior can be considered to be small after long exposures. The total hemispherical emissivity, ε, for organic solids is usually close to unity. The Stefan-Boltzmann constant σ is 5.67 × 10-11 kW m-2 K-4. The convective heat transfer coefficient h will depend on the size and orientation of the surface. It can be determined experimentally from Eq. 2-12 by exposing an inert specimen of the same size and orientation to a known constant flux and measuring the surface temperature. Somewhat more empirical but closed-form expressions for mattress flame-spread were sought by Pagni et al.[109] They tested one half-size polyurethane foam mattress (0.89 m by 0.89 m) with a ticking and a cotton/ polyester sheet in two configurations intact, and with a 100-mm hole cut out of the ticking and sheet, surrounding the methenamine pill ignition point. For the specimens with the hole, they found three burning regimes: (1) for the first 30 s, flame-spread was traveling across the surface, up to the edge of the cut hole; (2) in the next 100 s, there was little horizontal flame-spread. Instead, the burning surface regressed downward until the bottom burned through; (3) in the final period, there was a steady, somewhat accelerating radial spread. However, this was not surface spread alone, but rather a progressively enlarging cylindrical consumed area. The interior of the cylindrical void was filled with a luminous turbulent flame. The uncutcover case was generally similar, but with less distinct regimes. (Similar observations also were made with specimens subjected to a linear ignition source on the apparatus described in Ref. 106.) Flame spread and mass loss rates, as well as flame base diameters for various times from ignition are listed in Table 2-2. An expression for flame height, measured above the specimen top, was obtained as:

(Eq. 2-13)

 m 2  = 0.36  5  D D 

h ft

14

where hfl is the height of the flame (m) and D is the flame base diameter (m). Other correlations have also been reported in the literature for similar, but not identical, polyurethane foam mattresses by Land[110] and Mizuno.[111] Land found a significant effect of foam moisture absorption on the flame spread. The higher moisture contents gave lower flame spread rates, smoke, and flame heights; the mass loss rates of samples conditioned to 92% RH were 50% or less than those conditioned to 35% RH. Other moisture effects are discussed in Ch. 6.

Fundamentals

37

Table 2-2. Flame Spread Rates, Flame Diameters, and Mass Loss Rates In Mattress Burns Time after Ignition (s)

Flame Spread Rate (mm/s)

Mass Loss Rate (g/s)

Flame Base Diameter (m)

0–30

1.82

≈0

3.84 × 10-3 t

0.028 e

0.024t

1.02 × 10-3 t + 0.061

30–130

0.51

130–220

-0.68 + 9.0 × 10-3 t

0.099 e 0.014t

220–360

1.69 - 1.58 × 10-3 t

0.099 e 0.014t -1.58 × 10-6 t2 + 3.38 × 10-3 t - 0.300

9.0 × 10-6 t2 - 1.36 × 103 t + 0.222

Mizuno[111] studied mattress foams, without ticking, in two sizes: 0.50 m by 0.50 m by 0.14 m thick, and 0.90 m by 0.90 m by 0.12 m thick. The foams were ignited in the center with a methenamine pill. The flame radius, r (m), was measured as: (Eq. 2-14) r = 2.3 × 10-3 t

0 < t < 30 s

r = 3.5 × l0-2 e0.026t for the smaller specimens 30 < t < 70 s r = 4.0 × 10-2 e0.021t for the larger specimens 30 < t < 130 s The mass loss rate (kg s-1) from t = 20 s to total surface involvement (t = 90 s for the smaller specimens) was evaluated as a function of the instantaneous radius: (Eq. 2-15)

m• = 0.0461 r 2.23

At the time of full surface involvement, 14% of the mass was lost. Some illustrations offlame spread over vertical polyurethane foam slabs are available.[112] In one configuration a single slab was ignited at the top with a point source. Melting, dripping, and cratering was seen. The basic burn pattern was V-shaped, with some additional burning at the top (Fig. 2-2). In the case of the corner-top ignition of two slabs in the same figure, the predominant flame pattern was straight down, due to radiation reinforcement from the second panel. The work of Mizuno[111] quantitatively illustrates the very difficult aspect of studies of furniture flame-spread. For reasons of tractability, theories of flame spread are invariably based on one of two simplifying

38

Fire Behavior of Upholstered Furniture and Mattresses

assumptions—the fuel is taken to be either thermally thick or thermally thin.[103][113] For the thick case, a negligible fraction of the mass is lost during flame spread; after initial flame involvement the burning surface regresses parallel to its original plane. For the thin case (for example, a burning card) the entire thickness is involved in the spreading flame. Actual measurements of the pyrolysis zone contours for a polyurethane foam (Fig. 2-3), however, show a behavior which does not conform to either of these limiting cases. For fabric/foam composites the reality is usually somewhat closer to the thermally thick case. It depends on the fabric which often burns by itself in the thermally thin case while the foam burns later in the thermally thick mode.

Figure 2-2. Downward burn pattern on a polyurethane foam slab for point ignition on a flat surface and on a corner configuration.

Figure 2-3. Pyrolysis zone contours for a horizontal polyurethane slab at different times.

Fundamentals

39

Intensive consideration was given by Friedman and others[114]–[115] in the 1970s and early 1980s to describing analytically a generalized fire growth process. In the conceptual model, the initial fire spread is considered as a growth in the fire area. This is reasonable for large surfaces, such as upholstered chairs, or for arrays of fuel items progressively becoming involved. In a process of this sort, it is often observed that the rate at which new material becomes involved in the fire is proportional to the amount already burning. That is (Eq. 2-16)

dAb /dt ~ Ab

This can be integrated to give the involved area, as an exponential function of time, as (Eq. 2-17)

Ab(t) = C1eC2t

where C1 and C2 are empirical constants. These constants were obtained for various individual items and collections of upholstered furniture and bedding.[116][117] The main application of such fire growth models, suggested by Friedman,[114] is for studying the time to such critical events as detection, alarm, and sprinklers. It is of importance in assessing fire hazard to determine the time from ignition to these critical events. The method works best when the objective is to determine fire growth from a non-zero start (for example, from detection) to a later event (for example, sprinkler activation). Exponential-growth schemes have not been found to be useful in assessing the hazard of furniture itself. This is because the constants needed for the exponential model could only be computed a posteriori— predictive methods were never found for them.

2.6.0

HEAT RELEASE

2.6.1

Heat Release Rate

Strictly speaking, the total heat release rate, (i.e. the HRR integrated over the area of the furniture item) in kW, should be used to express the HRR of a piece of furniture measured in the furniture calorimeter or in a room fire test and the HRR per unit area, in kW m-2, should be used to express the results of tests in the bench-scale HRR calorimeters such as the OSU,[118] Factory Mutual Research Corporation (FMRC),[119] and Cone Calorimeters.[41][120] However, it is common practice to refer to the results of both the bench-scale and the full-scale tests as simply “HRR,” even

40

Fire Behavior of Upholstered Furniture and Mattresses

though they have different units and cannot be compared directly. This causes no problem to those familiar with the field but could lead to some misunderstanding among those who are new to it. Nevertheless, the common practice is used in this book. It is essential, however, that the units are correct when expressing these results. The peak HRR per unit area is perhaps the most quoted parameter used to express the results of Cone Calorimeter tests in general, as described in Ch. 3. The 180 s average HRR per unit area refers to the average over the first 180 s after ignition. It is often used in expressing Cone Calorimeter results on furniture composites. These are 100 by 100 by 50 mm thick specimens containing the fabric, padding and interliner, if any. They are intended to represent small elements of the full-scale furniture. Averages over 60 s and 300 s intervals are also used. When the relationship between the 180 s average HRR (kW m-2) of the furniture composite in the Cone Calorimeter and the peak HRR (kW) of the burning furniture was investigated, it was found that the 180 s average HRR (kW m-2) provided a better correlation than the peak HRR (kW m-2) in the Cone Calorimeter.[121] This is because at the time of the peak full-scale HRR, not all portions of the upholstered item are at their peak burning rate. Some portions are not yet ignited, some are mid-way through burning, and others may already be burned out. Similarly, narrow peaks in the bench scale tests may not directly translate to the real-scale object, since no two portions of it are likely to undergo the same pyrolysis history at the same moment. The total heat released, in kJ or MJ, is obtained in the furniture calorimeter or room tests by adding up the products of the time between scans and the total HRR in each time interval over the duration of the test. This is also true of the total heat released (kJ m-2 or MJ m-2) in the bench-scale tests. Since the 1970s, HRR has come to be recognized as one of the most important fire properties of a material. The role of HRR in hazard prediction and optimization of its use is based on papers by Babrauskas and others.[7][122]–[126] A bibliography of HRR studies can be found in Ref. 43 and general and specific application discussions of it in Ref. 42. The usefulness of the HRR is exemplified by the following: • If the total HRR history in a room is combined with a detailed description of that room, the temperature of the hot upper layer and the elevation of its interface can be predicted as a function of time, as discussed in Ch. 1.[7] • If the smoke and toxic gas yields of the burning materials are also known, the smoke and toxic gas

Fundamentals

41

concentrations in the upper layer can be predicted from the HRR. • The rate at which smoke and toxic gases are transported to other parts of the building also depends on the total HRR and the smoke and gas yields of the burning materials. • The occurrence of flashover in the room depends on the HRR exceeding some critical level. • Flaming ignition of a material depends on exceeding some critical rate of pyrolysis which can be expressed in terms of a critical HRR. • The upward flame spread rate on a wall depends on the height of the flame which can be expressed as a function of the HRR. • Spread of flame from a burning chair to another chair some distance from it depends on the radiant flux at that distance which can be expressed in terms of the total HRR of the burning chair. Huggett has shown that the heat released during the combustion of polymers is directly proportional to the amount of oxygen consumed, to within about± 5%. The proportionality constant is 13.1 MJ per kg of oxygen consumed.[115] Note, by contrast, that the heat of combustion is given as the heat released in MJ per kg of fuel consumed and differs widely for different materials. Parker provided the basic formulas for the calculation of HRR from oxygen consumption measurements, opening a radical new approach to fire engineering.[127] A treatment which may be easier to use is presented in Ref. 128. These formulas provide the basis for the Cone Calorimeter, the furniture calorimeter, the room fire test and other applications of the oxygen consumption method. In the usual way of measuring the HRR by oxygen consumption, all of the combustion products along with the entrained air are collected by a canopy hood which is connected to an exhaust duct and fan. A small fraction of the exhaust gases are continuously drawn through a system of filters and gas analyzers to determine the concentrations of oxygen, carbon dioxide and carbon monoxide that are used along with volume flow in the duct as input to the formula for calculating the HRR. The technique is described in detail by Janssens and Parker.[128]

42

Fire Behavior of Upholstered Furniture and Mattresses

One important advantage of HRR measurements is that, unlike many other fire tests, the data are presented in engineering units which can be used for modeling, which is discussed in Ch. 7. The HRR of furniture items is affected by the material combination, the configuration of the item, its total mass and the ventilation conditions.[129] These can potentially be taken into account in a deterministic furniture fire model. Progress in this direction is discussed in Ch. 7. The first correlation formula was developed for residential furniture by Babrauskas and Krasny,[121][129] which takes the 180 s average HRR of the furniture composite in the Cone Calorimeter at an exposure flux of 25 kW m-2, the total mass of the furniture item and a style factor into account to predict its peak HRR in a full scale test in the furniture calorimeter. An improved correlation formula and more detailed furniture fire models for seating furniture and mattresses were developed on the CBUF program.[130] These are discussed in Ch. 7. 2.6.2

Heat of Combustion

The instantaneous effective heat of combustion is equal to the HRR divided by the mass loss rate and can vary considerably over the burning period for charring materials. During the flaming period the heat release is due to the volatile thermal decomposition products which typically have much lower heats of combustion than the virgin material. During the glowing combustion period which follows, it is due to the char which has a much higher heat of combustion than that of the virgin material. During a fire the water produced in the combustion process remains in the vapor phase. Thus, it is the net heat of combustion that is important in fires rather than the gross (upper) heat of combustion that is measured in an oxygen bomb calorimeter. The effective heat of combustion will, in general, tend to depend on burning conditions. In highly under-ventilated fires, the effective heat of combustion will be reduced. Its dependence on ventilation can be determined in a HRR calorimeter which can measure the HRR in reduced oxygen atmospheres. The average effective heats of combustion obtained in the CBUF room fire tests for a range of furniture in the European marketplace are shown in Table 2-3 while the average effective heat of combustion for specially designed chairs varying only in fabric and padding are shown in Table 2-4.[131] For the 13 commercial items which exhibited propagating fire behavior, the effective heat of combustion varied from 14 to 18 MJ kg-1. For the non-propagating ones it varied from 6 to 23 MJ kg-1. For

Fundamentals

43

the 14 fully upholstered items of marketplace furniture, the range was similar, from 15 to 20 MJ kg-1. With office chairs and mattresses the range was considerably greater. Table 2-5 shows the results obtained in the furniture calorimeter. The technical descriptions for these furniture items are given in Tables 2- 6 and 2-7. Table 2-3. CBUF Room Burn Results On Commercial Furniture Heat and Smoke Release Item

Peak HRR (kW)

Time to Peak HRR (s)

1:1a,b 1:2a 1:3a 1:4 1:5 1:6 1:7 1:8 1:9 1:10 1:11a 1:12 1:13 1:14 1:15 1:16 1:17 1:18 1:19 1:20 1:21a 1:22a 1:23 1:24 1:25 1:26 1:27a

1959 1714 2107 991 917 1696 664 1570 661 1027 1849 1181 662 614 1094 1035 933 44 1430 699 2122 1599 414 49 33 30 2363

152 608 232 424 480 337 656 168 1240 268 308 209 1313 1157 353 445 397 121 193 308 187 317 85 121 122 121 196

Total Heat Peak Heat of Smoke (mJ) Combustion Release (mJ/kg) Rate (m2/s)

Total Smoke (m2)

Smoke Yield (m2/kg)

Length of Test (s)

256.9 202.9 357.4 405.6 528.6 353.3 354.9 519.2 184.2 431.6 138.1 200.5 163.3 166.6 267.6 267.2 331.9 5.3 241.3 106.9 114.2 132.6 32.9 18.7 2.8 2.0 238.7

7334 2886 8119 1906 5032 2040 1466 3179 1532 3425 1058 1262 1460 1300 5459 5804 4554 146 3529 706 1966 10191 140 93 43 8 4690

429 235 463 74 147 95. 68 116 113 122 110 96 194 174 510 538 403 471 280 99 329 1528 104 119 286 83 339

215 616 270 1320 1800 713 1800 1276 1800 1296 311 1277 1800 1800 1217 1297 1173 1229 1205 1261 187 332 809 1161 1290 308 220

15.03 16.50 20.39 15.82 15.44 16.43 16.35 15.56 13.53 15.38 14.33 15.28 21.66 22.33 24.99 24.76 29.37 16.56 19.15 14.95 19.10 19.88 24.37 23.97 18.67 22.22 17.27

73 18 75 6 14 24 3 22 5 16 24 7 7 7 25 21 13 1 19 4 27 87 2