Update on Syntactic Foams

Update on Syntactic Foams

Bibin John C.P. Reghunadhan Nair

Smithers Rapra Update

Update on Syntactic Foams

Bibin John C.P. Reghunadhan Nair

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder.

A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-120-3 (Hardback) 978-1-84735-121-0 (ebook)

Typeset by Wordsworth Prepress, India Printed and bound by Lightning Source Inc.

C

ontents

Preface ........................................................................................vii 1

Introduction ......................................................................... 1 1.1

Syntactic Foams: Definition ........................................ 1

1.2

Matrices used in Syntactic Foams ............................... 2

1.3

Microspheres in Syntactic Foams ................................ 2

1.4

Structure of Syntactic Foams ...................................... 5

1.5

General Methods for Preparation of Syntactic Foams.......................................................... 7

1.6

Properties of Syntactic Foams ................................... 11 1.6.1

Comparison with Conventional Foams ........ 12

1.6.2

Property Tailoring ........................................ 12

1.6.3

Mechanical Properties .................................. 13

1.6.4

Moisture Absorption.................................... 14

1.6.5

Isotropic Nature........................................... 15

1.6.6

Dielectric Properties ..................................... 15

1.6.7 Thermal Properties ....................................16 1.7 Factors Affecting the Performance of Syntactic Foams........................................................ 16 1.7.1 Volume Fraction of Filler ............................. 16 1.7.2 Nature of Filler ............................................ 17 1.7.3 Nature of Polymer Matrix ........................... 17 1.7.4 Quality of the Filler-matrix Interface ........... 18 1.7.5 Method of Processing................................... 18 References .......................................................................... 18 iii

Update on Syntactic Foams 2

Types of Syntactic Foams ................................................... 23 2.1

Epoxy Syntactic Foams............................................. 23

2.2

Phenolic Syntactic Foams ......................................... 31

2.3

Cyanate Ester Syntactic Foams ................................. 39

2.4

Polyimide Syntactic Foams ....................................... 43

2.5

Polyurethane Syntactic Foams .................................. 51

2.6

Polyester Syntactic Foams ......................................... 53

2.7

Silicone Syntactic Foams ........................................... 54

2.8

Elastomeric Syntactic Foams .................................... 54

2.9

Nylon Syntactic Foams ............................................ 55

2.10

Polybutadiene Syntactic Foams ................................. 55

2.11

Polypropylene Syntactic Foams ................................ 56

2.12

Carbon–carbon Syntactic Foams ............................. 56

References ............................................................................ 58 3

Recent Developments in the Field of Syntactic Foams ........ 63 3.1

Fibre-reinforced Syntactic Foams ............................. 63

3.2

Nanoclay-incorporated Syntactic Foams ................. 76

3.3

Rubberised Syntactic Foams ..................................... 82

3.4

Functionally Graded Syntactic Foams (FGSF) ........... 83

3.5

Syntactic Foam Core Sandwich Composites ............. 86

3.6

Cement-based Syntactic Foams ................................. 89

References .......................................................................... 91 4

iv

Applications of Syntactic Foams ......................................... 95 4.1

Syntactic Foams in Buoyancy Applications ............... 95

4.2

Syntactic Foam as Thermal Insulation Material ........ 99

4.3

Syntactic Foams in the Aerospace Industry ............ 100

4.4

Syntactic Foams in Radomes ................................. 102

4.5

Syntactic Foams in the Sports Industry ................... 103

4.6

Syntactic Foams for Furniture Applications ............ 104

Contents 4.7

Syntactic Foams as Synthetic Marble ...................... 105

4.8

Syntactic Foam for Air-equivalent Solid Backing ... 105

4.9

Shape Memory Syntactic Foams ............................. 106

4.10

Syntactic Foam Plug Assist Materials ..................... 107

4.11

Expandable Graphic Art Printing Media using Syntactic Foam ...................................................... 108

4.12

Syntactic Foams in Underwater Sound Transducers ........................................................... 108

4.13

Syntactic Foams in the Airbus ................................ 109

4.14

Miscellaneous Applications .................................... 109

References .......................................................................... 111 Abbreviations ........................................................................... 117 Index ........................................................................................ 119

v

P

reface

Syntactic foams, which are composite materials obtained by dispersing hollow spheres in a matrix, have emerged as an attractive material for diverse applications. This field has evinced a lot of interest in high tech areas. Though several research papers have emerged in this field, there is practically no book or report that compiles the developments in this area. In this context, we thought it would be appropriate to write a book compiling the important works carried out in this field. The book is divided into four chapters. The first chapter gives an introduction to syntactic foams. The basics of syntactic foams, matrix systems used in syntactic foams, different types of microballoons, structure of syntactic foams, general methods of preparation and properties of syntactic foams are covered in this chapter. Chapter 2 deals with syntactic foams based on different types of resin systems such as cyanate, epoxies, polyimides and so on. Chapter 3 focusses on recent advances in the field of syntactic foams like fibrereinforced syntactic foams, nanoclay-reinforced syntactic foams, rubberised syntactic foams, functionally graded syntactic foams, syntactic foam core sandwich composites and cement-based syntactic foams. The applications of syntactic foams in different fields are detailed in Chapter 4. The applications of some commercial grades of syntactic foams are presented in this chapter to give the readers a better understanding of the importance of these materials. We hope that this book will be a useful guide and reference material to those working in this field. We thank iSmithers, UK, for publishing this book. We thank the Director of the Vikram Sarabhai Space Centre, Thiruvananthapuram for giving permission to publish this book. We also thank our family members for their constant support during the preparation of this book. vii

1

Introduction

1.1 Syntactic Foams: Definition Lightweight materials with high specific strength are essential in aerospace, marine and other structural applications. The commonest method to produce such a material is by introducing a lightweight material in a matrix. This concept forms the basis of syntactic foams: hollow microspheres are incorporated into a matrix [1–2]. As per the definition from the American Society for Testing and Materials (ASTM), syntactic foam is a ‘material consisting of hollow sphere fillers in a resin matrix’. The term ‘syntactic foam’ was introduced in the 1960s [3]. ‘Syntactic’ is derived from the Greek word ‘syntaktikos’, meaning ‘to arrange together’ [4]. The term ‘foam’ is used because of the cellular nature of the material. Syntactic foams are also known as ‘foam composites’ because the hollow microspheres can be viewed as reinforcements in a matrix [5]. This type of material was previously used in the marine industry, providing buoyancy for subsea apparatus such as submersible vehicles and oceanographic equipment. Nowadays, syntactic foams find many applications ranging from tea cups to the limits of space travel. In syntactic foams, the matrix is reinforced with hollow spherical particles which have a controlled systematic arrangement in the matrix. Incorporating hollow particles having a lower density compared with binder material allows for the manufacture of lightweight materials with the increase of filler content. Thus, syntactic foam with a filler density that is lower compared with the binder can be considered to be a special type of particulate-filled polymer composite [6–8]. Syntactic foams are categorised as physical foams because the matrix is not foamed chemically but instead the gas-containing

1

Update on Syntactic Foams particles are filled mechanically into the matrix. They have the real advantage of being fabricated over a large density range and possess useful properties that can be tailored for specific applications [9].

1.2 Matrices used in Syntactic Foams The matrices used in syntactic foam include polymers, metals or ceramics. In this review, we are confining the discussion to polymeric matrix-based syntactic foams. Thermoplastic and thermosetting polymers have been employed to process syntactic foams. The important thermosetting resins used are epoxies, phenolics, cyanate esters, bismaleimides, unsaturated polyesters, and polyurethanes. Examples of the thermoplastic resin matrices used include polyethylene, polypropylene, polystyrene, and Nylons. Syntactic foams based on various resin systems are detailed in Chapter 2. Syntactic foams are mainly prepared by using thermosetting matrices because of the favourable processing conditions, i.e., avoiding breakage by gently blending the hollow microspheres with thermoset precursors of very low viscosity. From the processing viewpoint, thermosetting syntactic foams have many advantages compared with thermoplastic ones. For example, thermosetting syntactic foams can be processed at much lower temperatures compared with thermoplastic syntactic foams, thereby reducing the material and energy costs for processing. Also, thermosetting resins have less solvent sensitivity and are not negatively affected by cleaning solutions [10]. Nevertheless, some attempts have been made to process syntactic foams with a thermoplastic matrix by using a solvent, or even by using a twin-screw extruder [11]. The important advantage associated with a thermoplastic matrix is the higher toughness compared with thermosets.

1.3 Microspheres in Syntactic Foams Hollow microspheres give the syntactic foam its low density, high specific strength, and low moisture absorption. Microspheres may comprise glass, polymer, carbon, ceramic, or even metal [12]. Other terminologies are used in the literature to describe microspheres

2

Introduction (e.g., microballoons, cenospheres). All these terms are intermittently used throughout this review to indicate hollow microspheres. The microspheres have a burst pressure sufficient to withstand the forces imposed upon them during the formulation, mixing and dispensing processes. The main processing advantage of microspheres is that the viscosity of the systems with spherical fillers is always less than that of systems with fillers of any other shape [13]. Properties such as high temperature resistance, good strength-to-weight ratios, clean surface chemistry, low thermal conductivity, low dielectric constant, and low dissipation factor make microballoons an important reinforcing material in these composites. Microspheres are characterised by their particle size, wall thickness and density. In general, microspheres used in syntactic foams have a diameter of 1–50 μm, wall thickness of 1–4 μm, bulk density of 70–500 kg/m3, and apparent density of 50–500 kg/m3. Hollow macrospheres (diameter, 1–100 mm) are also used as fillers in syntactic foams [13]. The wall thickness of a microballoon is related to a parameter termed the ‘radius ratio’, η which is defined as the ratio of the inner radius, ri, to the outer radius, ro, of the microballoons: η=

ri ro

where ri is the inner and ro is the outer radius of the microballoon, respectively. Increase in the radius ratio corresponds to a decrease in wall thickness, which leads to a decrease in true particle density of the microballoon. Therefore, microballoons having a higher radius ratio give rise to lower-density syntactic foams and vice versa [14]. The parameter η is related to the microballoon density (ρmb) and the material that the microballoon is made of, and is given by: ρmb η = 1– ρg





1 3

where ρg is the density of the microballoon material [15]. 3

Update on Syntactic Foams The most commonly used microspheres in syntactic foam are glass microspheres due to their high specific strength, regularity of the surface, good wetting characteristics, and low viscosity of the resin– microballoon mixture, as well as energy absorption properties, low cost and ease of fabrication [16–18]. Also, hollow glass particles have very low densities as compared with corresponding hollow metallic and ceramic particles [8]. In general, syntactic foams based on glass microspheres exhibit much better mechanical properties than those made with polymeric microspheres due to the substantial difference between the elasticities of glass and polymer. Glass microspheres can be made by heating tiny droplets of dissolved water glass in a process known as ‘ultrasonic spray pyrolysis’, and properties can be improved by using an acid treatment to remove some of the sodium [19]. Polymeric microballoons are commonly made up of epoxy resin, unsaturated polyester resin, silicone resin, phenolics, melamineformaldehyde, polyvinyl chloride, polypropylene, or polystyrene [12, 20]. As a general rule, these microspheres are produced by spraying low-viscosity solutions and melts [13]. Among the various polymeric microballoons, phenolic microballoons have been widely used for processing syntactic foams. The main advantage of phenolic microspheres over those of glass is their lower density. Another type of polymeric microballoons is based on polyvinylidene chloride (PVDC). These are known as ‘saran microspheres’, are mechanically tougher and have high resistance to organics. Ultra low-density resilient PVDC hollow microspheres can be premixed in polyester and stored for long periods. They also find application in urethane and epoxy foams [21]. Organic microspheres are also used in syntactic foams. They suffer from many limitations and their use is limited. Some organic microspheres can be converted into carbon microspheres. Usually, these spheres are derived from phenolic microspheres or carbon pitch spheres. Conversion of the organic spheres to carbon microspheres is usually accomplished by heating in an inert atmosphere to 800–1000 °C. This is sufficient to convert most of the organic material to carbon but, to achieve graphitisation, further heating to temperatures >2000 °C is

4

Introduction required. Carbon microspheres obtained from phenolic resins have densities of about 0.15 g/cm3, whereas those obtained from pitch sources have densities of 0.05–0.25 g/cm3 [20]. Materials made from carbon microspheres of low apparent density have a substantially lower thermal conductivity (0.049–0.064 W/(m.K) for densities of 200–300 kg/m3) than other syntactic foams [13]. Another type of microballoon known as shirasu microballoon is the yield of secondary popping expansion of volcanic ash sand particularly available in Kagoshima, Japan [22]. Fly ash in the form of hollow particles is also used in syntactic foams [20]. The Philadelphia Quartz Company (USA) produces ‘Q-Cel’ quartz microspheres which are mechanically very strong and very cheap (half the price of glass microspheres and one-third the price of phenolic microspheres). Metal, metal oxide and metal salt microspheres have also been used in syntactic foams [13].

1.4 Structure of Syntactic Foams Syntactic foams are usually tertiary systems because the matrix and gas-filled microspheres are usually composed of different materials. They are classified as two-phase systems and three-phase systems [16]. The close-packed arrangement of hollow microspheres in the matrix gives rise to two-phase syntactic foams. The two-phase structure is schematically shown in Figure 1.1(a) but this is only a hypothetical situation. During the processing of syntactic foams, air entrapment is possible, which leads to voids in the foam structure. In some other cases, a thin film of resin may surround the cluster of microballoons so that the resin cannot penetrate into the cluster, leaving an empty space in between the microspheres. If resin concentration is very low, the space in-between the microspheres is not wet, giving rise to voids. Sometimes, depending upon the application, voids are intentionally incorporated to obtain lower density. The existence of voids makes syntactic foams a three-phase system (unlike conventional polymer foams which are binary) [9, 23]. The three-phase structure is shown schematically in Figure 1.1(b). Thus, two-phase syntactic

5

Update on Syntactic Foams

Microballoons

(a)

Matrix resin

Void

Microballoons

(b)

Figure 1.1 Schematic representation of (a) two-phase and (b) three-phase structures of syntactic foam. Reproduced with permission from N. Gupta, C.S. Karthikeyan, S. Sankaran and Kishore, Materials Characterization, 1999, 43, 4, 273 ©1999, Elsevier [9] foams consist of hollow microspheres dispersed in a resin, whereas three-phase syntactic foams comprise hollow microspheres dispersed in a resin containing finely dispersed air bubbles [24, 25]. Two-phase syntactic foams offer good mechanical properties, but their electrical properties are limited by the lowest possible density for a given combination of matrix and filler. Moreover, three-phase syntactic foams

6

Introduction are limited as far with respect to mechanical properties. However, they possess low dielectric and loss coefficients compared with two-phase foams [23]. Two-phase syntactic foams are less moisture-absorbing due to their structure, whereas three-phase types absorb more moisture because of the presence of open-type voids [5]. The microstructure of syntactic foams can be observed using a scanning electron microscope (SEM) (Figure 1.2). The figure shows the SEM image of a glass-microballoon syntactic foam. Glass microballoons can be clearly seen as round particles embedded in the polymer matrix. Figure 1.2 also depicts matrix and voids, as marked.

1.5 General Methods for Preparation of Syntactic Foams The general methods for the preparation of conventional polymer foams require the use of at least two chemical constituents: one to decompose into a gas to form the bubbles, and one to form the

Microballoon

Void

Matrix

Figure 1.2 SEM image of a syntactic foam

7

Update on Syntactic Foams cell walls. Syntactic foams use ‘prefabricated’ microballoons or microspheres that are mechanically combined with a resin to form a composite material [4], i.e., syntactic foams can be processed by incorporating microballoons in a matrix. The commonest methods used to prepare syntactic foams differ from those used for preparation of other polymer composites. Hollow-glass, organic or carbon microspheres do not withstand high pressures. It is therefore practically impossible to extrude or injection-mould mixtures containing hollow microspheres. Various manufacturing techniques are available for processing syntactic foams. These vary from simple blending of the components to novel coating methods of the resin on the microsphere surface [20]. The microballoon concentration and matrix composition was found to be crucial for the ease of manufacturing syntactic foams [26]. The choice of process parameters (temperature, mixing time, addition sequence) is also a main challenge in the processing of syntactic foams [13]. The binder materials used in syntactic foams must have low viscosity, readily controlled gelation time, small exothermal effect during curing, low curing shrinkage, good adhesion and wettability to the filler as well as compatibility with modifiers and fillers (including diluents, plasticisers, dyes, flame retardants) [13]. A common method for the preparation of syntactic foams is by impregnation of microspheres in a resin solution. This method ensures uniform coating of each sphere by the resin. Homogeneous dispersion of the resin among the microspheres is therefore achieved. The main difficulty of this method lies in the need to entirely remove the solvent before the final curing. Several impregnation techniques are in common use. One method is to fill a mould with an appropriate amount of microspheres and then a pre-measured amount of resin solution is poured over it. The solution penetrates into the intersphere voids due to gravity, capillary forces and sometimes with the aid of vacuum or positive pressure. Final curing is after solvent removal. In another method, a measured amount of microspheres is introduced into a dilute resin solution. After removal of most of the solvent, the system has the consistency of wet sea-sand. This

8

Introduction ‘wet sand’ is then filled manually into a mould and cured. The ratio of polymer to microspheres in the dry powder is determined by solution concentration, time of contact, and temperature [20]. However, these methods have many drawbacks. These include potential environmental safety and health hazards of volatile solvents; formation of defects and structural non-uniformities when the solvent is being driven off by heat from the interior of the syntactic foam; difficulty in obtaining inter-batch reproducibility; and additional energy or cost associated with transporting the solvent-laden pasty mass to the moulding or curing equipment [18]. When the resin is available in a powdery form, a solid mixture of the hollow microspheres and the resin powder is prepared first. Then, a weighed quantity of the solid mixture is transferred to a mould of predetermined volume, pressed and cured [20]. In this method, the volume of microballoons used for each sample was constant, with the purpose of completely filling the mould volume with the closest packing possible [24]. An alternative approach to this method, when using thermoplastics, is to add microspheres to a melt of the thermoplastic in an extruder or a kneader. In this method, the microspheres are extremely lightweight and very fine in size, so many problems are encountered. The fine airborne dust characteristic of microspheres poses environmental problems. There is the possibility of breakage of the relatively fragile hollow microspheres during the dry mechanical/ frictional mixing and rough handling. Settling of the higher-density thermoplastic particles can cause non-uniformity in composition. Also, lower-density syntactic foams are difficult to prepare by this method. This is due to the inability to mix-in higher amounts, e.g., any amounts higher than approximately 15–20 wt% of microspheres in the resin matrix because of the very high volume-to-weight ratio of the microballoons compared with the resin [18]. A coating method has also been used for processing syntactic foams which consists of resin coating, vacuum filtration and polymer precipitation. In the coating step, a thin film of the polymeric solution is adsorbed onto the sphere surface. Polymer precipitation at this stage would lead to undesired agglomeration. The slurry is

9

Update on Syntactic Foams then vacuum-filtered and rinsed with liquids while on the filter. The purpose of this liquid rinsing step is to precipitate the polymer on the sphere surface and to simultaneously remove the solvent by leaching. Vacuum-drying of the coated spheres gives a moulding powder of discrete particles. By this method, uniform resin coating is achieved on the spheres, which subsequently leads to uniform resin distribution in moulded articles [27]. US Patent 4,595,623 [28] describes processing of syntactic foams from liquid thermosetting resins without using solvent. The use of liquid resin in polymeric foam materials has disadvantages. For example, liquid resin materials when used to form syntactic foams do not produce highly uniform distributions of the solid materials in the foam. The mixture is heated to allow the thermosetting resin to flow and wet the microballoons in the mixture. The mixture is then cured to set and crosslink the thermosetting resin to form the syntactic foam [10]. This method has been used for liquid polyesters and silicones. The development of extrudable epoxy syntactic foam systems has also been reported. Such systems offer a faster and more reliable way of making the models. Instead of laying on plaster by hand, resin is simply extruded onto a frame, allowed to harden, and machined to the correct shape by a numerically controlled machine tool. These types of syntactic foams find possible application in aircraft and other industries [29]. Easily removable, environmentally safe and low-density syntactic foams are prepared by mixing insoluble microballoons with a solution of water and/or alcohol-soluble polymer to produce a pourable slurry. Vacuum-filtering the slurry in varying degrees removes unwanted solvent and solute polymer. Finally, drying removes the residual solvent. The method is used when the solute polymers are non-toxic and soluble in environmentally safe solvents such as water or low-molecular-weight alcohols. The syntactic foams produced by this method are particularly useful in those applications where ease of removability is beneficial, and could find application

10

Introduction in: packaging of recoverable electronic components; drilling and mining applications; art works; the entertainment industry for special effects; manufacturing as temporary fixtures [30]. Spraying methods on a surface of a liquid resin stream and a microballoon stream meeting at some point while still in the air are also possible. This will produce discrete resin-coated particles which behave as a free-flowing material serving as a regular moulding powder [31]. Cornerstone Research Group, Incorporated have developed a unique process to produce strong (yet lightweight) syntactic foams. This process involves a low-stress resin removal system in which the excess resin is extracted from the syntactic material before the resin is cured. Along with the excess resin, microspheres that may have been broken during the mixing process are also removed. The resulting foam has low density and low void content, essential for high-integrity composites [32].

1.6 Properties of Syntactic Foams Syntactic foams possess many advantageous properties compared with conventional polymer foams and composites. The application of syntactic foams has centred around their ability to: reduce weight; increase stiffness; exhibit buoyancy; provide good nailability and screwability; reduce cost [33–34]. Syntactic foams are mechanically superior due to the rigidity of the microballoons. Hence, syntactic foams stand in-between conventional reinforced polymeric materials and foams with very high specific strengths [35–36]. Several other foamed materials offer better thermal insulation properties but none offer simultaneous high-strength properties [37]. They have excellent compressive and hydrostatic strength, good impact behaviour and damage tolerance compared with other closed-cell structured materials, which makes them very attractive for structural applications [33, 38–39]. They are found to be highly strain rate-sensitive, in contrast with metallic foams [14].

11

Update on Syntactic Foams

1.6.1 Comparison with Conventional Foams Blown and self-expanding foams develop a fairly random distribution of gas pockets of widely varying sizes and shapes. The porosity of syntactic foams can be much more closely controlled by careful selection and mixing of the microballoons with the matrix. The voids in syntactic foams are due to the cellular structure of the microsphere, so it is easier to predetermine and control the size and size distribution of the voids within syntactic foam [20, 31]. Ordinary foams are visibly porous, but syntactic foams have cells so small that the material looks like a homogeneous solid [4]. In conventional foams, cells are partially or fully connected whereas, in syntactic foams, the voids are enclosed within the walls of the microspheres and are therefore isolated from each other. The porosity of syntactic foams is typically at the microscopic level [40].

1.6.2 Property Tailoring The properties of syntactic foams can be tailored by changing a wide variety of parameters: matrix and microballoon materials, microballoon size, size distribution, wall thickness and volume fraction [25–26, 41–42]. The composition and density of syntactic foams can be predicted because these are fabricated by filling mechanically the gas-containing particles into the polymer matrix. Unlike most other foams, syntactic foam is a material whose density before curing is the same as that after curing [6]. The density of syntactic foam is proportional to the concentration of filler added. The limiting lower densities are determined by sphere density, the handling requirements of the uncured mix, and the property requirements of the final product. Sphere density also determines its efficiency as a bulking agent; the less dense the spheres, the lower the weight concentration required to achieve a specified composite density [34]. Syntactic foams can be designed and fabricated according to the physical and mechanical property requirements of the application. Depending upon the service conditions, the matrix resin can be

12

Introduction chosen from a wide range of thermosetting and thermoplastic resins. Similarly, microspheres of polymer, ceramic or metal can be chosen. Appropriate choice of fillers can produce composites with superior strength, damage tolerance, wear resistance and chemical resistance. The ability to cast a syntactic foam to any desired shape and the ease of machineability of the cured foam make syntactic foams very attractive structural materials [37].

1.6.3 Mechanical Properties Syntactic foams have excellent mechanical properties. They can absorb a considerable amount of energy under compressive loading [43–44]. The high specific compressive strength of syntactic foams derives from the resistance of microspheres to compressive loads [45–47]. The compressive properties of syntactic foams primarily depend on the properties of the microballoons, whereas the tensile properties depend on the matrix material used [26, 42]. Microspheres have an extremely strong structure and hence can withstand stresses very well. In the case of two-phase syntactic foams, properties are a function of the properties of the resin and the microballoon and their proportions. In the case of three-phase syntactic foams, the shape and content of voids play an important part. At lower resin concentration, failure is mainly that of structure collapse, whereas at higher resin concentration polymer properties become more important and more spheres break before the structure collapses [20]. A general stress–strain curve for the compression testing of syntactic foams is illustrated in Figure 1.3. A typical characteristic is a plateau region (marked as ‘densification region’ in the figure). This plateau region corresponds to energy absorption by the material in the process of compression. During the process of crushing, the hollow glass particles tend to fracture and expose the volume enclosed by them. This volume accommodates the material that is being compressed. Equilibrium between the new volume exposed and volume reduction due to compression is visible in the form of a plateau region in the stress-strain curve [37].

13

Update on Syntactic Foams

Densification region

40

Stress (MPa)

35 30 25 20 15

Elastic Deformation

10 5 0 0

0.05

0.1

0.15

Strain (mm/mm)

Figure 1.3 General stress-strain curve for the compression testing of a syntactic foam. Reproduced with permission from N. Gupta and E. Woldesenbet, Composite Structures, 2003, 61, 4, 311 ©2003, Elsevier [45]

1.6.4 Moisture Absorption The most significant property of syntactic foams is their low moisture absorption. Syntactic foams have completely closed cells and therefore absorb water to a lesser degree compared with foamed plastics in which the cells are open [12, 38, 45–46, 48]. The overall moisture absorption in syntactic foams is affected by many parameters. The most important ones are the nature of the resin–microsphere interface, and the concentration, size and strength of the microsphere. The chemical and physical properties of filler and binder are also decisive factors [49]. It is reported that for microballoon concentration 2.068 MPa). These syntactic foams can be exposed to water

51

Update on Syntactic Foams

Mechanical strength (MPa)

55 Tensile Strength Flexural Strength Compressive Strength

50 45 40 35 30 25 20 15 10 0

20

40

60

80

100

Weight percentage of K-25 in syntactic foams

Figure 2.17 Effect of microballoon (K-25) concentration on the strength of EPB syntactic foams containing K-25 and K-37. Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32]

at temperatures from 0 °C to 40 °C for up to 10 years without degradation. They exhibit a long pot-life and short demoulding times (