Effect of cell size on the quasi-static compressive

© Solmaz Zamanishourabi, 2021

Effect of cell size on the quasi-static compressive properties of silicone foams with spherical closed cells

Mémoire

Solmaz Zamanishourabi

Maîtrise en génie chimique - avec mémoire

Maître ès sciences (M. Sc.)

Québec, Canada

Effect of cell size on the quasi-static compressive properties of silicone foams with spherical closed

cells

Mémoire présenté à la Faculté des études supérieures de l'Université Laval dans le cadre du programme de maîtrise en génie chimique

Solmaz Zamanishourabi

Sous la direction de :

Denis Rodrigue, directeur de recherche

iii

Résumé

Dans ce travail, l'effet de la taille des cellules sur les propriétés de compression des mousses de caoutchouc

de silicone avec des cellules sphériques a été étudié expérimentalement. Les mousses ont été fabriquées

en utilisant une résine de silicone et des billes de polystyrène expansé (EPS) par une nouvelle technique.

Les billes ont été mélangées avec la résine réactive et le mélange a été laissé à température ambiante

jusqu'à ce que la résine soit durcie. Ensuite, le matériau solidifié a été chauffé pour rétracter les billes et

former la structure cellulaire. Trois mousses différentes avec des tailles de cellules différentes, allant

d'environ 1 mm à environ 2 mm, ont été fabriquées et testées sous compression quasi-statique. Pour chaque

échantillon, le module de compression par rapport au poids et la résistance à la compression par rapport

au poids ont été obtenus. Enfin, ces valeurs ont été comparées entre elles pour déterminer l'effet de la taille

des cellules sur les propriétés de compression. Les résultats montrent que l'augmentation de la taille des

cellules augmente la rigidité. Par exemple, l'augmentation de la taille des cellules de 1 mm à 1,5 mm à

densité constante (480 kg/m3) augmente le module de compression de 17%, tandis que la contrainte de

compression à 50% de déformation augmente de 14%. De tous les résultats obtenus, on peut conclure que

parmi les mousses de caoutchouc à cellules fermées ayant la même composition de matrice et des densités

similaires, mais des tailles d'alvéoles différentes, celle ayant la plus petite taille d'alvéole donne les

propriétés de compression les plus faibles par rapport au poids, tandis que celle ayant la plus grande taille

d'alvéole donne le rapport le plus élevé lorsqu'elles sont lentement comprimées.

iv

Abstract

In this work, the effect of cell size on the compressive properties of silicone rubber foams with spherical cells

was experimentally studied. The foams were made using a silicone resin and expanded polystyrene beads

(EPS) through a novel technique. The beads were mixed with the reactive resin and the mixture was left at

room temperature until the resin was cured. Then, the solidified material was heated up to shrink down the

beads to form the cellular structure. Three different foams with different cell sizes, ranging from about 1 mm

to about 2 mm, were made and tested under quasi-static compression. For each sample, the compressive

modulus to weight ratio and compressive strength to weight ratio were obtained. Finally, these values were

compared between them to determine the effect of cell size on the compressive properties. The results show

that increasing the cell size increases the stiffness. For example, increasing the cell size from 1 mm to 1.5

mm at constant density (480 kg/m3) increases the compressive modulus by 17%, while the compressive

stress at 50% strain increases by 14%. From all the results obtained, it can be concluded that among the

closed cell rubber foams having the same matrix composition and similar densities, but different cell sizes,

the one having the smallest cell size gives the lowest compressive properties to weight ratio, while the one

with the largest cell size gives the highest ratio when they are slowly compressed.

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Table of contents

Résumé ......................................................................................................................................... iii

Abstract ......................................................................................................................................... iv

List of Figures ............................................................................................................................... vii

List of Tables .................................................................................................................................. x

List of abbreviations ....................................................................................................................... xi

List of symbols .............................................................................................................................. xiii

Acknowledgements ...................................................................................................................... xv

Introduction ..................................................................................................................................... 1

POLYMER FOAMS ........................................................................................................................ 1

BLOWING AGENTS ....................................................................................................................... 3

OTHER COMMON POLYMER FOAMS .............................................................................................. 4

BIODEGRADABLE FOAMS .............................................................................................................. 4

SILICONE RUBBER FOAMS ........................................................................................................... 5

STATEMENT OF THE PROBLEM ..................................................................................................... 6

OBJECTIVES OF THE STUDY ......................................................................................................... 8

SCOPE OF THE STUDY ................................................................................................................. 8

SIGNIFICANCE OF THE STUDY ....................................................................................................... 8

Chapter 1 Literature Review .................................................................................................. 10

1.1 INTRODUCTION ................................................................................................................. 10

1.2 COMPRESSION BEHAVIOR OF POLYMER FOAMS ................................................................ 10

1.3 SILICONE RUBBER FOAMING CHEMISTRY .......................................................................... 13

1.4 RECENT SILICONE FOAM DEVELOPMENTS ......................................................................... 17

1.4.1 Silicone Foams Using Physical Foaming Agent ................................................... 17

1.4.1.1 Syntactic Silicone Foams ................................................................................ 18

1.4.1.2 Silicone Foams Using Physical Blowing Agent ............................................... 20

1.4.2 Silicone foams using chemical foaming agents .................................................... 21

1.5 SUMMARY OF LITERATURE REVIEW ................................................................................... 23

Chapter 2 Research Methodology ......................................................................................... 25

2.1 INTRODUCTION ................................................................................................................. 25

2.2 FABRICATION OF THE SILICONE FOAM SAMPLES ................................................................ 25

2.2.1 Materials ............................................................................................................... 25

2.2.2 Procedure ............................................................................................................. 26

vi

2.3 CHARACTERIZATION AND DATA ANALYSIS ......................................................................... 29

2.3.1 Foam Density ........................................................................................................ 29

2.3.2 Foam Morphology ................................................................................................. 29

2.3.3 Compression Properties ....................................................................................... 30

Chapter 3 Results and discussion ......................................................................................... 33

3.1 INTRODUCTION ................................................................................................................. 33

3.2 FOAM DENSITY MEASUREMENT ........................................................................................ 33

3.3 FOAM MORPHOLOGY ANALYSIS ........................................................................................ 35

3.4 COMPRESSION PROPERTIES ............................................................................................. 37

Conclusions and Recommendations ............................................................................................ 48

CONCLUSIONS ........................................................................................................................... 48

RECOMMENDATIONS FOR FUTURE WORKS ................................................................................. 50

References ................................................................................................................................... 51

vii

List of Figures

Figure I.1 Some examples of silicone foams application in the rail industry [18] .................... 6

Figure I.2 Schematic representation of three rubber foams with different cell sizes subjected to

quasi-static compressive loads. ............................................................................................. 7

Figure 1.1 Typical stress-strain curve for a polymer foam under compression [22]. .............. 11

Figure 1.2 Compressive stress-strain curves of a silicone rubber foam with different STG content

(0 wt% - 40 wt%) [16]. ......................................................................................... 12

Figure 1.3 Chemical structures of PDMS polymer (a) [28], and vinyl terminated PDMS polymer

(b) [29]. ................................................................................................................... 13

Figure 1.4 Silicone rubber products classification based on the processing technology and

curing mechanism [33]. ...................................................................................................... 14

Figure 1.5 Condensation curing mechanism for RTV1 or RTV2 systems (adapted from [25]) 15

Figure 1.6 Addition curing mechanism for RTV-2 systems, adapted from [25]. .................... 16

Figure 1.7 The foaming mechanism of a platinum catalysed silicone foam formulation [13]. . 17

Figure 1.8 SEM image of a syntactic rubber foam made from a polyurethane matrix and

polymeric microsphere [41]. ................................................................................................ 19

Figure 1.9 SEM images of the structure of a silicone foam produced by the hydrogen production

technology [56]. ................................................................................................... 22

Figure 2.1 The three-part mold used for foam sample fabrication. ...................................... 27

Figure 2.2 Schematic view of the foam sample fabrication process. ...................................... 28

Figure 2.3 Fabricated foam samples (S1, S2 and S3) with a thickness of 2 cm and a surface of 12

cm by 20 cm (before heating). ............................................................................................. 28

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Figure 2.4 A picture of the gas (nitrogen) pycnometer ULTRAPYC 1200e (Quantachrome

Instruments, USA). ............................................................................................................ 29

Figure 2.5 Pictures of the compression test rig (left) and testing machine (right). ................... 31

Figure 2.6 The specimens produced with a thickness of 2 cm and surface of 4 cm by 4 cm prepared

for compression tests. ........................................................................................................ 31

Figure 2.7 Illustration of compression modulus and compressive-deflection calculations from the

stress-strain curve. ............................................................................................................ 32

Figure 3.1 Averaged densities of the compression tests specimens ...................................... 34

Figure 3.2 The cellular structure of the foam samples at low (left) and high (right) magnifications

........................................................................................................................................ 36

Figure 3.3 Compressive stress-strain curve obtained for the unfoamed silicone rubber. .......... 37

Figure 3.4 Compressive stress-strain curves for sample S1. ................................................ 38

Figure 3.5 Compressive stress-strain curves for sample S2 ................................................. 38

Figure 3.6 Compressive stress-strain curves for sample S3 ................................................. 39

Figure 3.7 The averaged compressive moduli for the foam samples. .................................... 42

Figure 3.8 The averaged compression-deflection (compressive stress at 25% strain) for the foam

samples. ........................................................................................................................... 42

Figure 3.9 The averaged compressive stress at 50% strain for the foam samples. ................. 43

Figure 3.10 The compressive modulus to weight ratio for the foam samples. ......................... 45

Figure 3.11 The compression-deflection (compressive stress at 25% or 0.25 strain) to weight ratio

for the foam samples. ......................................................................................................... 46

Figure 3.12 The compressive stress at 50% strain (0.5 strain) to weight ratio for the foam samples.

........................................................................................................................................ 47

ix

Figure II.1 The effect of cell size on the compressive properties with respect to the sample density.

Sample S1 was made using 1 mm EPS beads, S2 using 1.5 mm and S3 using 2 mm beads. ... 48

x

List of Tables

Table 3.1: Physical properties of the fabricated foam samples. ............................................... 33

Table 3.2: Densities (kg/m3) of the compression tests specimens. .......................................... 34

Table 3.3: Physical and morphological parameters of the foams produced. ............................. 36

Table 3.4: Compressive properties of sample S1................................................................... 40

Table 3.5: Compressive properties of Sample S2. ................................................................. 40

Table 3.6: Compressive properties of Sample S3. ................................................................. 41

xi

List of abbreviations

AIBN - Azobisisobutyronitrile

Avg - Average

CBA - Chemical blowing agent

CERMA - Research center on advanced materials

CNT - Carbon nanotubes

CO2 - Carbon dioxide

EoF - End-of-life

EPS - Expanded polystyrene beads

EVA - Ethylene vinyl acetate

Gr - Graphene

H2 - Hydrogen

HTV - High temperature vulcanizing

LSR - Liquid silicone resin

N2 - Nitrogen

O-Si-O - Silicon-oxygen linkage

PBA - Physical blowing agent

PDMS - Polydimethylsiloxane

PE - Polyethylene

PS - Polystyrene

RTV - Room temperature vulcanizing

sCO2 - Supercritical carbon dioxide

xii

SD - Standard deviation

SEM - Scanning electron microscopy

Si - Silicon

SOVAR - Société de valorisation

STG - Shear stiffening gel

UV - Ultraviolet

PU - Polyurethane

xiii

List of symbols

ϑ𝑓 -

Poisson’s ratio of foam

ρ𝑓 -

bulk density of the foam

ρ𝑠 - density of the foam matrix material

𝐸𝑓 -

compressive modulus of the foam

𝐸𝑠 -

compressive modulus of the foam matrix material

𝑝0 -

pressure inside a cell

∅ - volume fraction of solid contained in the cell edges

xiv

To my kind family especially my beloved spouse

xv

Acknowledgements

I would first like to express my sincere gratitude to my supervisor Professor Dr. Denis Rodrigue without

whom this work would never have been done. I sincerely acknowledge his support and guidance throughout

my master program. I would also like to thank the Department of Chemical Engineering of Université Laval

for providing me with exceptional academic opportunities. Finally, I want to thank CERMA (research center

on advanced materials) for technical support and SOVAR (société de valorisation) for financial support.

1

Introduction

This thesis is composed of an introduction, conclusion and three chapters. The introduction presents a short

overview on polymer foams in general and silicone foams in particular. Then, the research problem and

objectives are presented and discussed with the scope of the study. Chapter 1 gives an overview of the

compression behavior of polymer foams and silicone rubber foaming chemistry. Then it discusses the recent

studies on silicone foam developments with a focus on their mechanical properties. In Chapter 2, the

methods, materials and equipment used to fabricate and characterize the silicone rubber foams are

addressed. Chapter 3 includes the results and findings from the characterization performed on the fabricated

silicone foams including densities, morphologies and compressive properties. It also discusses the effect of

cell size on the compressive properties of the foams. Finally, the last section gives the main conclusions of

the work and report a list of future works. In all these steps, I performed the experimental work (sample

preparation and characterization), as well as the analysis of the results and writing of the first draft of this

document.

Polymer Foams

A foam is a liquid or solid material containing a large number of small gas pockets [1]. Foam materials are

increasingly being used in various fields due to their positive role in material savings, as well as their

important characteristics such as large surface area and unique mechanical, thermal and electrical

properties [2]. Over the last decades, different types of foam materials have been produced. Among them,

polymer foams have attracted a great deal of attention from researchers and engineers due to their excellent

properties, such as low thermal conductivity, low moisture absorption, good chemical resistance and high

stiffness/strength to weight ratio [3]. In the modern world, polymer foams can be found almost everywhere.

They are used for disposable food packaging, furniture cushioning, energy absorption and insulation.

Polymer foams, which are lighter and less costly compared to other alternatives such as metallic foams [4],

found a broad range of applications in different industries such as automotive (seat cushion), aerospace

(vibration dampening), electronics (packaging), marine (insulation board), sport (helmet) and household

(furniture and bedding) [5]. However, there are some disadvantages associated with polymer foams like low

temperature resistance, low machinability and low biodegradation rate [6].

2

The first developments of polymer foams date back to the 1930s and 1940s with the invention of polystyrene

(PS) foam in 1931 now called expanded polystyrene (EPS). Polyurethane (PU) foams were introduced about

a decade later by Dr. Otto Bayer. They first found an application as a replacement for rubber and also impact

energy absorption to protect common materials of the time such as wood and various types of metals [1].

Then, in the 1940s, flexible polyurethane foam was developed for cushioning application in the furniture and

automotive industries.

Thanks to technological advancements in injection molding and extrusion over the next decades, polymer

foams became widely available and used in different industrial and residential sectors, while specific blowing

agents contributing to the ozone layer depletion became a concern highlighting the need for foaming agents

having less negative impact on the environment [6].

Polymer foams are made of thermoplastic or thermoset matrices [5]. This also includes elastomers/rubbers

(vulcanized or not). Thermoplastic foams are recyclable and can be reprocessed, while thermosetting foams

cannot be melted and reused leading to problems after their end-of-life (EoF) [5].

The production of polymer foams involves forming (nucleation), in a controlled manner, of a large number

of small gas pockets within a solid/liquid/melt matrix. The gaseous phase can be made using blowing agents

or gas containing particles (expandable beads). Extrusion, injection molding, compression molding and

casting are the most used techniques in the production of polymeric foams [5]. Nevertheless, other

techniques can be used today including rotomolding, calendaring, thermoforming, etc.

Polymer foams can be classified into two major groups based on the cell morphology. The first criterion is

the type of cell: open and closed cell foams [7]. In closed cell foams, the cells (gas pockets) are fully

surrounded by the matrix material, separating them from each other (cell walls). In contrast, the voids, in

open cell foams are not fully closed, but partially broken leading to some level of interconnection between

them [7,8]. Polymer foams can be also categorized as rigid, flexible or semi-rigid, based on their deformation

behavior [5]. The last criterion is the foam density. Depending on the density and industrial application,

foams are divided into low density, medium density and high density. For example, for polyurethane foams

used for mattress application, foams down to 30 kg/m3 are considered low density, while foams between 30

and 100 kg/m3 are called medium density foams. Foams above 100 kg/m3 are considered as high density

[9].

3

Blowing Agents

Along with the development of different polymer foams, different chemical (CBA) and physical (PBA) blowing

agents have been examined and developed to optimize the foam performances. A blowing agent is a material

generating the gas phase in the matrix to produce a cellular structure. In polymer foams, this can happen in

different ways: the blowing agent is a gas diffusing into the matrix under pressure at a specific temperature.

When the pressure is released, a thermodynamic instability occurs (supersaturation). The bubbles are

created and the then grow (expansion) under gas diffusion. Another possibility is to use a liquid blowing agent.

The liquid droplets dispersed in the matrix material turn into gas (phase change) or a chemical agent (solid)

thermally decomposes with or without additives (activator, nucleation agent) producing gas bubbles [5].

The selection of a blowing agent has a significant impact on the morphology, density and foaming process,

therefore having a direct effect on the performance of the finished foam product. For an insulation application,

in addition to the structural properties of the foam, thermal conductivity plays a very important role in the foam

performance and this must be considered when choosing the blowing agent, especially when the foam will

be used for long period of time [1].

However, for specific applications, such as load bearing and buoyancy, the composition of the blowing agent

does not play such a significant role on the final foam product. But, the composition/concentration of the

blowing affects the melt viscosity and changes the rheology of the liquid resin foam or the melt during the

manufacturing process, and hence the foam morphology, especially for high molecular weight polymers such

as polyesters and polyamides [2]. For thermosetting foams, like epoxy foams, in which the polymerization

and foaming take place in a single step, the choice of a suitable blowing agent is more complex. This is

because the blowing agent, during the foaming process, in addition to the viscosity of the liquid resin, has a

great influence on the curing reaction and the reactivity of the foam ingredients [5]. In this case, heat

management is important since phase change (PBA) and thermal decomposition (CBA) have related energy

involved.

4

Other Common Polymer Foams

Ethylene vinyl acetate (EVA) foam: This is a closed cell foam made form the copolymer of ethylene and vinyl

acetate (different ratios). It is often used as padding in different sports equipment, in exercise and floor mats,

in crafts and in protective products such as gloves and life jackets [2].

Polyethylene (PE) foam: This is a closed cell and durable foam made of different grade of polyethylene

which is one of the most common plastics in the world today. It is resilient and used for packaging different

fragile products due to its great vibration dampening capacity. It is also used as a thermal and acoustic

insulator in home appliance, automotive and aerospace industries [2].

Expanded polystyrene (EPS) foam: This is a very lightweight foam made from styrene monomer. It is an

excellent thermal insulator and impact energy absorber widely used for manufacturing lightweight protective

packaging and gears, food packaging, automobile parts and also for sporting goods like surfboards.

Expanded polystyrene was first developed in the 1950s and quickly found application in the construction

industry, then became popular in the packaging industries due to its very low weight, thermal conductivity

and acceptable mechanical properties [5].

Expanded polystyrene beads: EPS beads are mainly used to mold foams of various shapes for packaging

and thermal insulation applications. The bead molding process allows the mass production of foams with

complex geometries with good dimensional accuracy/stability in an economical way. Spherical polystyrene

beads are normally expanded up to about 40 times larger than their original size. To make the beads,

unexpanded polystyrene beads are heated using stream until the pre-added blowing agent evaporates. The

boiling foaming agent expand the beads leading to the generation of honeycomb cells and as a result the

bead expanding and touching each other [1].

Biodegradable Foams

Due to concerns about the environmental effects of petroleum-based foams and also their high price,

biodegradable foams have recently received a lot of attention. Biodegradable foams are mainly soluble in

water and play an important role in reducing CO2 emissions [6]. However, being sensitive to humidity has

5

limited their application. Another limiting factor for their wider application is their microcellular structure

making them difficult to manufacture in thin sheets. An example is starch foam which could be an alternative

to PS foams. Starch is extracted from materials containing polysaccharides like potato and corn. A common

method of making starch foam is extrusion. In this process, the molten starch is blown using a suitable

blowing agent, such as methanol, with the aid of suitable additives like talc as a nucleating agent [10]. During

the process, the methanol evaporates and is trapped in the molten starch forming the cells [1].

Silicone Rubber Foams

Silicone (Si) foams are flexible porous materials made of silicone rubber. They have characteristics of both

foam materials (low weight and large surface area) and silicone rubbers (high temperature resistance, high

weather and UV resistance, and high resilience) [11]. They have been around since the 1960s and have

found applications in different industries where a highly reliable elastic foam is required in terms of durability,

temperature resistance and resilience [12]. The main drawback of silicone foams is their higher cost

compared to other common polymer foams like polyurethane (PU) foams [13]. Relatively low tensile strength

and high density combined with difficult recyclability are other drawbacks associated with silicone foams

[12].

In the industry, cellular silicone materials are classified as silicone sponges and silicone foams, depending

on their fabrication methods [14]. Silicone foams are made through the curing of a liquid silicone resin (LSR)

system already expanded (physically or chemically) using a foaming agent [15]. On the other hand, silicone

sponges expand during heat curing and are usually made of gums like silicone systems containing a blowing

agent [16]. Silicone foams and sponges are mostly being used as sealing gaskets, as well as in different

enclosures (gaskets and sealings) like lighting fixtures and electronic cabinets, to avoid the penetration of

moisture, dust, light or air. They are also excellent candidates for highly resilient cushioning and for high

temperature resistance vibration insulation [14,17]. Some applications of silicone foams in the rail industry

(as an example) are shown in Figure I.1.

6

Figure I.1 Some examples of silicone foams application in the rail industry [18].

Statement of the Problem

The foaming method and process conditions highly affect the cell density, cell size, cell shape, cell

distribution quality, and consequently the mechanical properties [19]. This is especially true for blown

polymer foams [8]. For syntactic foams, which are expanded using hollow particles, in addition to the effects

of the cell geometry, their compressive properties highly depend on the properties of the embedded hollow

particles [20]. This is related to the fact that under compression, these hollow particles take a significant part

in the load bearing (mechanical contribution) and this highly affects the foam properties like the compression

modulus [20].

Recently, a new class of rubber foams was developed by the research group of Prof. Rodrigue at Université

Laval [11], which are made in a similar way as syntactic foams, but the solid particles (expandable beads)

added to control the foam structure have no effect on the foam’s compressive properties. The fabrication

method of these novel foams includes dispersing shrinkable lightweight particles (like expanded polystyrene

beads) as the foaming agent into a reactive liquid rubber (like silicone rubber). After solidifying the mixture,

7

the solid particles (bead skin) are shrunk leaving only voids in the solidified rubber resin. Although these

particles remain in the foam, they have no effect on the foam’s compressive modulus due to their small size.

The effects of cell size on the mechanical properties of these newly developed rubber foams have not been

studied yet. Therefore, this research project was conducted to partially fill this gap. In this research, the

effect of cell size on the compressive properties of silicone rubber foams with spherical closed cells was

studied within the scopes given in the next section (scope of the research). Actually, this research answered

the question illustrated in Figure I.2: among the three rubber foams having the same matrix composition but

different cell sizes, which one gives the lowest compressive properties to weight ratio and which one gives

the highest ratio, when they are slowly compressed?

Figure I.2 Schematic representation of three rubber foams with different cell sizes subjected to

quasi-static compressive loads.

8

Objectives of the Study

The main objective of this research is to determine the effect of cell size on the quasi-static compressive

properties of silicone foams with spherical closed cells. To achieve this objective, the following sub-objectives

are required:

i. To fabricate three different closed cell silicone foams with different cell sizes.

ii. To measure the quasi-static compressive properties of the foams.

iii. To compare the samples and analyse the results based on different cell size to

conclude on the quasi-static compressive properties of the foams.

Scope of the Study

This research experimentally investigates the effect of cell size on the compressive properties of rubber

foams with spherical closed cells. In this research, three different rubber foams with different cell sizes (1

mm, 1.5 mm and 2 mm in diameter) were fabricated using compression molding. The rubber foams were

made using a standard liquid silicone rubber resin system. Expanded polystyrene beads were used to form

the spherical closed cells within the silicone rubber matrix. The test specimens were prepared and tested in

a compression test rig with a quasi-static rate (0.015 s-1). The compressive properties including compressive

modulus and compressive strength were obtained from the stress-strain curve.

Significance of the Study

Foam morphology (cell geometry and density) is one of the main parameters significantly affecting the

compressive properties of the foam. Silicone foams are mostly used in application under compression,

therefore investigating their compressive properties is important. Since the preparation of silicone foams is

complex, the control of the cellular microstructure to achieve a uniform morphology is difficult [7]. But this

research uses a different method (EPS beads) giving a uniform structure allowing a more accurate control

9

of the final cellular structure. The results and finding of this research are useful for predicting the deformation

behavior of closed cell rubber foams subjected to compression loads.

10

Chapter 1 Literature Review

1.1 Introduction

This chapter gives an overview of the compression behavior of polymer foams with a focus on silicone rubber

foaming chemistry. Then, a discussion on recent studies related to silicone foam developments is presented

with specific investigations on their mechanical properties.

1.2 Compression Behavior of Polymer Foams

Foam materials have physical and mechanical properties which are measured by methods similar to those

used for unfoamed materials. But several parameters are affecting the mechanical behavior of a polymer

foam including cell density, cell size and cell morphology, as well as the composition of the matrix [21].

However, all foams, including polymer foams, show a strain-stress curve similar to the curve shown in Figure

1.1 under compression [22]. This curve can be divided into three different regions: elastic region (small

deformation), collapse plateau region (intermediate deformation) and densification region (high

deformation). Under compression, polymer foams, including silicone foams, first undergo elastic deformation

which is controlled by the cell wall bending or stretching. In open cell foams, the cell wall bends under

compressive load, while the cell wall stretches under the internal pressure of the contained gas in closed

cell foams [22]. After the elastic deformation, the foam continues to undergo a large compressive

deformation without significant stress increase in the collapse plateau region. In this step, flexible foams

deform as their cells collapse due to cell wall buckling under the compressive stress, while brittle foams

experience cell wall fracture and cell crushing leading to plastic deformation [23]. Once all the cells are

mostly collapsed, densification starts to occur. In this step, the opposing cell walls come into contact in every

cell and by imposing more deformation the stress substantially increases associated the matrix

compressibility, resulting in a sharp transition (higher slope) in the stress-strain curve [22].

11

Figure 1.1 Typical stress-strain curve for a polymer foam under compression [22].

Foams are able to absorb a high amount of mechanical energy when deformed under compression. This is

especially true for foams undergoing large plastic deformation and/or localized fracture. The amount of

energy absorbed is determined by the area under the compressive stress-strain curves minus the elastic

region. By increasing the brittleness and reducing the density of the foam, the energy absorption capacity

increases. Typically, a low-density foam under compression experiences a stress plateau representing its

high energy absorbing characteristics (region 2 in Figure 1.1). Therefore, materials with longer stress

plateaus have higher energy absorption capacity. However, closed cell silicone foams, because of the

rubber-like behavior of the matrix material and also because of the internal pressure of the contained gas,

mainly show elastic behavior and thus are not good energy absorber. In closed cell foams, the gas trapped

in the cells contributes to the overall stiffness of the foam, while in open cell foams the gas can easily escape

out of the foam once the foam is slowly compressed [7]. Recently, Zhao and Zhang [16] tried to improve the

energy absorption performance (safeguarding) of silicone foams using a shear stiffening gel (STG). As

shown in Figure 1.2, they found that the safeguarding performance of the foams improved with increasing

STG contents.

12

Figure 1.2 Compressive stress-strain curves of a silicone rubber foam with different STG content (0

wt.% - 40 wt.%) [16].

Silicone foams are mostly used in application under compression, so it is important to study their

compressive mechanical properties. Therefore, along with the development of silicone foams for different

applications, their compressive properties must also be studied. The compression behavior of a foam

depends on the microstructure of the foam and also the physical properties of the matrix polymer. It is also

well known that the properties of composite materials are highly dependent on the filler properties, as well

as matrix-filler interactions. During the past years, different attempts have been made to improve the

compressive properties of silicone composite foams using different fillers. For example, Verdejo et al. [3]

fabricated silicone composite foams using functionalized graphene (Gr) sheets and carbon nanotubes

(CNT). They found that the nanofillers negatively affected the foam morphology leading to larger cell size.

However, due to the strong interface of the nanofillers with the polymer matrix, the compressive modulus of

the foams increased. In another study, Bai et al. [24] used nanographite to improve the silicone foam cell

morphology. They observed that the heterogeneous nucleation effect of nanographite improved the

morphology of silicone foams by increasing the cell density and decreasing the cell size resulting in improved

compressive properties.

Li et al. [21], using scanning electron microscopy (SEM), observed that in silicone rubber foams having large

and small cells, the small cells mostly fail earlier than large cells during compression. In another research,

Tang et al. [7] fabricated silicone foams with different microstructures using supercritical carbon dioxide

13

(sCO2) and studied their compressive properties. They concluded that for closed cell foams under

compression, the stress increases with increasing strain due to an additional contribution of the gas

compression inside the cells.

1.3 Silicone Rubber Foaming Chemistry

Silicone rubber is a thermoset elastomer composed of a silicone polymer [25]. Silicone itself is a synthetic

polymer made up of silicon, oxygen, carbon and hydrogen atoms [12]. Silicon and oxygen atoms construct

the backbone of silicone, while organic side groups containing carbon and hydrogen (methyl, ethyl, etc.) are

attached to the silicon atom as shown in Figure 1.3. The silicon-oxygen linkage (O-Si-O) in silicone polymer

is called siloxane [13]. Therefore, silicone polymer is sometimes called polysiloxane [26].

Polydimethylsiloxane (PDMS), with the formulation shown in Figure 1.3a, is the most common member of

the polysiloxane family [27]. Depending on the application, the PDMS polymer chains are terminated with

functional groups like vinyl (Figure 1.3b) [28].

Figure 1.3 Chemical structures of PDMS polymer (a) [28], and vinyl terminated PDMS polymer (b)

[29].

Silicone rubbers are mostly made of PDMS polymers with high molecular weight through an exothermic

cross-linking reaction called curing or vulcanization [30]. There are three different types of curing for silicone

rubbers: addition curing (for vinyl terminated PDMS crosslinked with a curing agent containing Si-H groups),

condensation curing (for hydroxyl terminated PDMS) and free radical curing (for vinyl terminated PDMS

crosslinked to each other by ethylene bridges, using a peroxide crosslinking initiator) [31, 32]. The addition

curing and condensation curing occur in presence of metal catalysts, platinum and tin, respectively. But the

14

free radical cured rubbers do not require a catalyst and the reaction initiates using a peroxide compound

[13].

Based on the curing mechanism, the silicone rubbers can also be classified into two groups: high

temperature vulcanizing silicone rubbers (HTV) and room temperature vulcanizing (RTV) silicone rubber.

Each category is further classified based on the processing technology and curing mechanism, to subgroups

shown in Figure 1.4 [33].

Figure 1.4 Silicone rubber products classification based on the processing technology and curing

mechanism [33].

High temperature vulcanizing silicone rubbers or heat activated silicone rubbers are made of long chain

silicone polymers containing vinyl groups which are highly viscous and can be cured at temperatures above

room temperature, mostly at temperatures between 100 and 200°C. Based on the viscosity, they are

grouped as liquid silicone rubber and solid silicone rubber. The liquid silicone rubbers (LSR) are paste-like,

yet injectable, and are cured through platinum catalyzed addition curing. The solid silicone rubbers are gum-

like and are available in both peroxide cure and platinum catalyzed addition cure technologies. In the

industry, the term HTV usually refers to gum-like solid silicone rubbers since they were the first group of

HTV rubbers introduced [25].

Room temperature vulcanizing silicone rubber systems, which sufficiently cure at room temperature, are

available in one-part silicone (RTV-1) and two-part silicone (RTV-2) systems. RTV-1 system comprises a

one-component curable mixture and is mainly used for sealing application. It is ready to use, and once the

15

paste-like mixture is out of the package the curing process starts using the air moisture. The RTV-1 silicone

systems are condensation cure and mostly composed of hydroxyl terminated silicone polymer, reinforcing

filler, crosslinker and tin catalyst [25]. The condensation curing mechanism is shown in Figure 1.5. In this

curing reaction, the hydroxyl terminated silicone polymer reacts with a crosslinker with at least 3 organic

groups (carboxyl, alkoxy, oxime or amino groups) in the presence of moisture and tin catalyst.

Figure 1.5 Condensation curing mechanism for RTV-1 or RTV-2 systems (adapted from [25]).

A RTV-2 silicone system includes two liquid polymer mixtures, known as part A and part B. Each part

includes either a crosslinker or a catalyst. Once these two parts are mixed together (with a mixing ratio

recommended by the supplier), the curing process starts and the reactive mixture must be formed within a

certain period of time (pot life) before it gets gelled. The RTV-2 silicone systems are produced based on

either tin catalyzed condensation curing (Figure 1.5) or platinum catalyzed addition curing technology

(Figure 1.6). In this curing process, a PDMS polymer containing vinyl groups reacts with a PDMS polymer

with Si-H groups (crosslinker) in the presence of a platinum compound as a catalyst. This reaction does not

produce any by-product. The major use of RTV-2 silicone systems is for mold making applications [25].

16

Figure 1.6 Addition curing mechanism for RTV-2 systems, adapted from [25].

Silicone rubber foams are generally divided into two major categories based on the method used for their

expansion: blown silicone foams (using blowing agents) and syntactic foams (using hollow particles).

Blowing agents release gases in the matrix prior to curing resulting in a porous structure. They can be

classified into chemical or physical agents based on their mechanisms [1]. Carbon dioxide (CO2) and

nitrogen (N2) are good candidates as physical blowing agents due to their broad availability, as well as

limited health and safety hazards [34]. However, chemical blowing agents are still very common since the

direct use of CO2 and N2 is very challenging and requires optimized process conditions like pressure and

temperature. For silicone foams, hydrogen, which is produced during a chemical reaction of additives prior

to curing, has been the case since the 1950s [13].

Silicone foams based on their curing process are classified as RTV silicone foams and heat-activated foams.

Heat activated silicone foams are made of a mono-component gum-like mixture composed of HTV silicone

polymer, a peroxide compound, reinforcing filler, pigment and a heat activated chemical blowing agent which

releases gas within the matrix and creates the cellular structure. This one-component product at room

temperature remains stable, but the foaming and curing processes simultaneously take place and progress

until the final foam product is formed when sufficiently heated [25].

The RTV silicone foams are made of RTV liquid silicone polymers. Unlike the heat activated silicone foams,

they can be used for foam-in-place application and for foam filling of cavities with complex shape due to

their liquid natures. RTV silicone foam systems are mostly composed of a PDMS polymer, a metal catalyst

(a platinum complex or a tin salt), and a crosslinking agent such as silanes (compounds of silicon and other

17

atoms), or siloxanes containing a functional group of (hydroxyl, alkoxyl, acetoxyl or hydrogen atoms) [35].

RTV silicone foam systems are supplied as two-part products, with a specific mixing ratio. When these two

parts are mixed together, the cure reaction and gas generation processes simultaneously take place and

progress without requiring any heat application. In this case, a cured cellular rubber product is achieved

usually within a few minutes [25]. During the foam preparation, the silicone chains are crosslinked in the

presence of a platinum or tin catalyst [36, 37]. They react in parallel with a suitable additive like alcohol or

silanol to generate H2 [38]. These reactions can be accelerated by heat [39]. As an example, the foaming

mechanism of a platinum catalyzed silicone foam formulation is shown in Figure 1.7. In this formulation, the

silicone chains with hydrogen terminals are crosslinked with methyl terminated silicone chains, and react in

parallel with an alcohol to produce the hydrogen gas.

Figure 1.7 The foaming mechanism of a platinum catalysed silicone foam formulation [13].

1.4 Recent Silicone Foam Developments

1.4.1 Silicone Foams Using Physical Foaming Agent

During the past decades, along with rapid increase in silicone foams use, a great deal of attention has been

devoted to develop silicone foams using different methods with an extensive attention towards their

mechanical behavior. This section overviews the recent research works done on physically expanded

18

silicone foams including cellular silicone rubber materials fabricated using hollow particles (syntactic foams),

as well as foams blown using a physical blowing agent such as carbon dioxide.

1.4.1.1 Syntactic Silicone Foams

The idea behind the development of silicone based syntactic foams is to produce a cellular rubber structure

with a performance comparable to blown silicone foams, but at lower cost [40]. Syntactic silicone foams are

made through adding hollow microspheres (with a diameter in the range of 10-200 μm) in a silicone matrix

before curing [40-42]. As an example, a microstructure image of a syntactic rubber foam made from a

polyurethane matrix and polymeric microspheres, captured using scanning electron microscope (SEM), is

shown in Figure 1.8. Recently, different types of silicone polymers and hollow particles have been used to

fabricate syntactic silicone foams for different applications. For example, Nakanishi [40] fabricated a

syntactic silicone foam for an energy absorption application using an addition-cure room temperature

vulcanizing silicone rubber filled with hollow particles (microspheres). This cellular composite showed sound,

shock and vibration absorption capability.

19

Figure 1.8 SEM image of a syntactic rubber foam made from a polyurethane matrix and polymeric

microsphere [41].

In another study, Kenig et al. [42] fabricated a syntactic foam using hollow silica microspheres with an

average diameter of 100 microns and a powdered condensation curing silicone-based resin. They compared

the mechanical performance of this cellular composite with another composite made of the same matrix

material (with same volumetric resin content), but filled with solid glass beads. They found that the

compressive strength of the hollow sphere filled composites (syntactic foams) was higher than the one filled

with solid beads. Kessler et al. [43] studied the compressive properties of syntactic silicone foams filled with

polymeric calcium carbonate coated hollow spheres. They investigated the effects of microspheres

concentration on the compressive modulus and found that increasing the microspheres volume fraction from

10% to 30% decreased by 50% the compressive modulus due to a higher volume of compressible gas within

the foam.

Yazici et al. [37] fabricated syntactic silicone foams with different microsphere contents (10, 20 and 30%

wt.) using a liquid addition-cure silicone resin system and two different types of glass microspheres. They

tested the foams under quasi-static compression load and observed that by increasing the microspheres

content the plateau stress region of the stress-strain curve extended, thus the energy absorption capacity

20

of the foams improved. Schumann et al. [44] fabricated different types of syntactic silicone foams using two

different high temperature vulcanizing silicone rubber systems: a liquid silicone rubber system and a solid

silicone system. They filled the silicone rubber systems with ceramic and hollow glass microspheres of

different sizes and tested their tensile properties. They found that all the foams showed lower elongation at

break by increasing the microspheres content. However, the cell size did not significantly affect the tensile

properties. Comparing the ceramic microspheres to the glass ones, the former showed lower tensile

strength.

In another research, Alex et al. [45] fabricated syntactic foams using ceramic and glass microspheres in an

addition-curing room temperature vulcanizing liquid silicone resin system to investigate the physical

properties of the foams including tensile properties. They concluded that adding 5% wt. of microsphere

decreased the specific tensile strength, but increasing the microsphere content to 20% significantly

increased the specific strength due to the low density of the microspheres and also better interfacial bonding

with the matrix.

1.4.1.2 Silicone Foams Using Physical Blowing Agent

In recent years, many efforts have been made in developing foaming methods without causing significant

negative effects on the mechanical properties of the materials [46]. Some attempts have been made to foam

different silicone rubbers using a physical blowing agent. Recently, some literature reported on the

successful preparation of silicone foams using supercritical carbon dioxide (sCO2) as a physical blowing

agent. The sCO2 is a fluid state of CO2 where it is held above its critical pressure and critical temperature to

avoid phase change [47]. The solubility of sCO2 in polymers is higher than carbon dioxide in its gas state

[48]. However, foaming silicone rubber using sCO2 is very challenging, and in most silicone foams, fabricated

using this technology, some surface defects have been observed [49]. There are many parameters and

process conditions to be optimized and controlled in sCO2 foaming of silicone rubbers such as composition,

rheology and curing kinetics of the matrix, as well as temperature and pressure.

For instance, Song et al. [23] attempted to foam a pre-cured peroxide cure solid rubber resin system using

sCO2 foaming technology. They concluded that the sCO2 technology can be used to foam cross-linked

silicone rubbers. However, the pre-curing time is a key factor in the foaming process and affects the foam

morphology. Lee et al. [48] fabricated silicone rubber foams with a cell size of 12 microns using a pre-cured

21

high temperature vulcanizing liquid silicone resin system and sCO2. They investigated the effect of curing

degree prior to foaming on the final cell size of silicone rubber foams. They observed that the cell sizes

significantly decreased and the foam density increased by increasing the cure degree. Liao et al. [50]

fabricated silicone foams using a peroxide cure solid rubber resin system and sCO2. They found that the

composition and viscoelastic properties of the silicone rubber played a significant role in the sCO2 foaming

process. Yang et al. [51] fabricated different silicone foams using high temperature vulcanizing solid silicone

rubber and sCO2. They tried to improve the cell structure and decrease the cell size by optimizing the

process conditions. They achieved silicone foams with cell sizes smaller than 10 μm. A series of silicone

rubber foams were fabricated by Xiang et al. [8] using sCO2 to investigate the effect of silica as a reinforcing

agent on the cellular morphology and nucleation of silicone rubber composites. Their results showed that

silica can act as a nucleation agent in silicone rubber composite foams controlling the cell size and density.

1.4.2 Silicone foams using chemical foaming agents

Another method for the fabrication of silicone foams is to produce gas pockets within a silicone matrix using

a chemical blowing agent (CBA). This is usually done through blending a silicone resin rubber resin system

with a chemical blowing agent prior to curing. However, this method requires an optimized formulation to

obtain a silicone foam with an acceptable morphology. This is because during the foaming process, there is

another process taking place which is the crosslinking of the silicone polymer`s chains. These two processes

must be well balanced to obtain a foam with the desired density and good properties [13]. Chemical blowing

agents are usually directly added into the matrix. For silicone rubber foams, the chemical blowing agent has

to be dispersible in the selected silicone resin system prior to foaming, and must also be compatible with

the related catalyst. Otherwise, the CBA interferes with the crosslinking reaction and inhibits the curing

completion. Additionally, the CBA must be able to decompose close to an optimal temperature to produce

the gas [52].

During the past decades, different silicone foams have been successfully developed using CBA and

characterized for their properties. For example, Park [53] used AIBN (2,2-azobisisobutyronitrile) to foam a

peroxide cure high temperature vulcanizing solid silicone resin system. The AIBN blowing agent, when

sufficiently heated, decomposed and released nitrogen. The peroxide concentration in the silicone was

changed to investigate the effect of the curing level on the morphology and mechanical properties of the

foams. He observed that by increasing the peroxide concentration, the hardness, tensile strength and foam

22

density increased, but the cell size decreased. This is because an increase in the peroxide concentration

increases the crosslinking density and the silicone viscosity, therefore the cells were not able to grow. In

another study, Gao et al. [54] used (N,N-dinitrosopentametgylenetetramine) as the blowing agent to foam a

high temperature vulcanizing solid silicone resin system reinforced with nano-silica.

There is another chemical foaming method for silicone foam production, patented in 1958, which does not

require heating [36, 55]. This method uses the hydrogen gas generated through the reacting monomers in

a silicone-based mixture at room temperature during the foam preparation. The silicone foam resin systems

based on this technology are supplied in the form of two-part liquid polymers (parts A and B). When these

two parts are mixed, H2 bubbles are generated and grow in the matrix via diffusion. The structure of a silicone

foam produced using this technology is shown in Figure 1.9.

Figure 1.9 SEM images of the structure of a silicone foam produced by the hydrogen production

technology [56].

For such silicone foam, Jawhar et al. [56] studied the cell growth process during polymerization using optical

microscopy. After nucleation, the bubbles grow and reach their final sizes at the gel point of the silicone

matrix. Therefore, the crosslinking rate and gas generation reactions must be controlled and balanced to

achieve a foam with a desired morphology and density. This can be done using suitable inhibitors and also

23

by adjusting the concentration of catalysts, inhibitors and reinforcing/modifying fillers in the formulation [56].

Most of the recent silicone foam formulations have been based on the addition of fillers to improve or modify

the foam properties such as hardness, tensile strength, thermal/electrical conductivity and fire resistance

[57, 58]. But filler addition directly affects the rheology of the reactive mixture and thus the foam morphology

and properties. Fillers also prevent the proper development of the voids and negatively affect the shape of

the pores [56].

Verdejo et al. [57] fabricated room temperature vulcanized silicone foams reinforced with graphene sheets

and carbon nanotubes to investigate the fillers effect on the morphology and properties including mechanical

properties. They found that the foam morphologies were negatively affected by the fillers resulting in foams

with thicker cell walls associated to increased viscosity. Compared to a silicone foam made from the pristine

resin system, the compression modulus of the foams increased with filler addition. Very recently, Liu et al.

[58] added liquid fillers, including silicone oil, to the formulation of a room vulcanizing silicone foam system

to improve the electrical properties for sensor application. They found that adding liquid fillers decreased the

compression modulus of the foams.

1.5 Summary of Literature Review

In this chapter an overview of the compression behavior of polymer foams and silicone rubber foaming

chemistry was given. Recent studies on silicone foam developments were reported and discussed with a

focus on their mechanical properties.

Silicone foams have the combined properties of a foam and silicone which became a solution for tough

engineering challenges where stability against weathering, heat and UV is important. Several parameters,

including processing parameters and rheological properties of the formulation, affect the silicone foam

properties. As discussed, the method used for foaming has a direct effect on the cell shape and size which

will affect the mechanical properties of the foam including compression properties. Silicone foams are

produced through expanding a silicone resin system before/simultaneously with curing using a physical or

chemical process. Chemical foaming is the most common industrial method for silicone foam production. This

includes forming gas pockets within a silicone rubber resin system using gas produced through the

decomposition of a blowing agent or produced during a chemical reaction between the silicone monomers

and some additives. Silicone foaming based on the hydrogen production technology is limited to liquid rubber

24

silicone resin systems. Furthermore, there is some safety issues related to hydrogen in some environments

which is a concern. In physical foaming, a large number of gas pockets is introduced into a silicone rubber

matrix prior to curing. This can be done by the direct introduction of a gas as a blowing agent or by embedding

hollow particles.

Processing of silicone foams using a chemical or physical blowing agent is difficult as it is very sensitive to

the process conditions. This requires a balance between the cross-linking and the cell growth kinetics. It

becomes even more complex in the case of high filler content. On the other hand, processing syntactic foams

is very user friendly and does not require heat application. It also allows a better control of the cell size and

density, thus a direct control on the mechanical properties. However, the compressive properties of syntactic

foams highly depend on the properties of the embedded hollow particles as these particles take a significant

part in load bearing applications. Therefore, there is a need for an improved foaming method offering the

convenience and consistency of the syntactic foaming method without influencing the silicone matrix

composition.

Foam morphology (cell size, geometry and density) is one of the main parameters affecting the compressive

properties of the foams. Silicone foams are mostly used in application under compression, therefore

investigating their compressive properties is important, especially for material selection purposes. Although

several researches are available for silicone rubber foams, only a few studies focused on the relationship

between the cell geometry and compressive properties of silicone foams. Also, these studies were limited to

silicone foams with non-spherical cells. This limited amount of information motivated the present research

project by focusing on silicone foams with spherical cells.

25

Chapter 2 Research Methodology

2.1 Introduction

The main purpose of this research was to experimentally investigate the effect of cell size on the

compressive properties of rubber foams with spherical closed cells. This chapter addresses the methods,

materials and equipment which were used to achieve this goal. In this research, foam samples with different

cell sizes were fabricated and then tested for their compressive behavior. The results were then compared

to each other to find the effect of cell size on the compression properties

2.2 Fabrication of the Silicone Foam Samples

In this project, three rubber foams with different cell sizes were fabricated using compression molding. The

rubber foams were made using the materials and procedure reported below.

2.2.1 Materials

A pourable silicone rubber system, vulcanizing at room temperature and offering a reasonable pot life

(working time), was used as the matrix material. In our case, ELASTOSIL® M 4370 (from Wacker) was

selected. This silicone rubber includes two parts: part A and part B. At room temperature (23oC), part A has

a viscosity of 10000 mPa.s, a density of 1.5 g/cm3 and mainly contains polydimethylsiloxane (vinyl

terminated) and iron oxide. Part B has a viscosity of 350 mPa.s, a density of 0.97 g/cm3 and contains

polymethyl hydrogen siloxane and polydimethylsiloxane (vinyl terminated). This silicone resin system cures

through an addition-cure reaction using platinum catalyst which is already included into part A. The pot-life

of this silicone system is about 80 min at 23°C.

26

To form the spherical closed cells within the silicone rubber matrix, commercially available expandable

polystyrene (EPS) beads with a bulk density of about 20 kg/m3 and diameters ranging from 1 mm to 2 mm

were prepared and used. The beads were received from Squishy Deluxe company (China) in a pillow

package. The beads were then sieved and grouped in three categories based on their size: 1 mm, 1.5 mm

and 2 mm in diameter. Then, the EPS beads were washed using ethanol (95% purity, supplied by GreenField

Specialty Alcohols Inc.) to remove any possible contamination on their surfaces as this contamination could

negatively affect the catalyst performance, inhibit the curing reaction, as well as the interactions (adhesion)

with the matrix.

2.2.2 Procedure

A compression molding method was used to fabricate the silicone foam samples. A three-part mold having

a cavity with dimensions of 6 cm by 12 cm by 20 cm (shown in Figure 2.1) was used for this purpose. A

clearance of about 50 microns between the mold frame and the top part of the mold was included to allow

any exceeding silicone resin to get out of the mold, but not the EPS beads.

27

Figure 2.1 The three-part mold used for foam sample fabrication.

Foam samples with a thickness of 2 cm and a surface of 12 cm by 20 cm (volume of 480 cm3) were fabricated

through the following sequences.

Firstly, 2 cm of the cavity height was filled with a sufficient number of EPS beads. The next step was to add

the reactive silicone resin mixture with an amount of about 60% of the specimen volume into the mold. This

value was obtained from previous experiments. Therefore, about 290 cm3 of silicone resin was required

which was prepared as follow: Based on the mixing ratio recommended by the supplier (6:1 by volume),

about 270 cm3 of parts A with 45 cm3 of part B of the silicone resin system were mixed together in a

graduated plastic beaker using a dual hand mixer to prepare 315 cm3 of the reactive silicone resin. About

290 cm3 of the prepared resin was then poured in the mold and manually mixed with the beads using a stick.

Since the EPS beads are less dense than the silicone resin, they tend to move toward the surface and float

on top of the low viscosity resins, which was the case for the present research. To avoid this segregation

problem, subsequently, using the top part of the mold the height of the cavity was reduced to 2 cm. This

action applies a compression stress on the mixture and pushes the beads back into the liquid silicone while

pushing the extra amount of resin out of the mold through the mold clearance, resulting in a homogenous

beads distribution. By removing the exceeding resin, the compression stress in the mixture substantially

decreases and the flexible EPS beads recover their initial sizes and geometries. However, since the resin

is slightly viscous, the mixture remains under a low level of stress. Then, the mold was left at room

temperature for 24 h to cure the resin. Finally, the solidified material was demolded and put in an oven at

28

130oC for 1 h to shrink down the bead and to form the empty cells (voids). These conditions were also

obtained from previous experiments (preliminary tests). A schematic view of the fabrication steps is shown

in Figure 2.2.

Figure 2.2 Schematic view of the foam sample fabrication process.

The different foams were coded based on their cell size which is related to the EPS beads diameter used:

S1 (cell size = 1 mm), S2 (cell size = 1.5 mm) and S3 (cell size = 2 mm). A picture of these samples is given

in Figure 2.3. The base silicone resin is red, while the EPS beads are white.

Figure 2.3 Fabricated foam samples (S1, S2 and S3) with a thickness of 2 cm and a surface of

12 cm by 20 cm (before heating).

29

2.3 Characterization and Data Analysis

2.3.1 Foam Density

After foam samples preparation, their bulk density was measured. The samples were weighed using a digital

scale and the mass was divided by their volume. The foam volume for each sample was obtained by

multiplying its sides as measured by a digital vernier caliper. For comparison, the density of the unfoamed

matrix was also measured using a gas pycnometer as shown in Figure 2.4.

Figure 2.4 A picture of the gas (nitrogen) pycnometer ULTRAPYC 1200e (Quantachrome

Instruments, USA).

2.3.2 Foam Morphology

In the next step, the foam morphologies (cell size, cell density and cell deformation) were observed and

studied using an optical camera (Nikon COOLPIX P520). Although the compression load applied on the

EPS beads during the molding process was very low, the final structure of the foam samples was examined

30

using the above camera to see whether or not the geometries of the cells were changed during the process.

This observation also helped to see if the cells are interconnected or not.

For each foam, the cell density (number of cells in 1 cm3 of the foam) was determined as [59]:

Cell density = (n)3/2 * (Vf /Vm) (2.1)

where Vm and Vf denote the volume of the matrix material (unfoamed) and volume of the foam respectively,

while n denotes the number of cells in an area of 1 cm by 1 cm, which can be counted without using the

microscope.

By having the density of the unfoamed matrix from the pycnometer, the value of Vm was calculated as:

Vm = (mass of the foam before heating - mass of the contained EPS beads) / density of the matrix material

(2.2)

2.3.3 Compression Properties

In the next step, the foam samples were tested for their compression properties using a universal mechanical

tester Instron model 5565. The test rig and the testing machine are shown in Figure 2.5. A load cell with a

capacity of 5 kN was used to measure the applied load. For each sample, three specimens with an area of

4 cm by 4 cm were cut for the compression tests. The thickness of the specimen was about 2 cm. In total,

nine specimens were prepared as shown in Figure 2.6 and compressed with a quasi-static rate of 0.015 s-1

(1.8 mm/min) to obtain the compressive stress-strain curves.

31

Figure 2.5 Pictures of the compression test rig (left) and testing machine (right).

Figure 2.6 The specimens produced with a thickness of 2 cm and surface of 4 cm by 4 cm prepared for

compression tests.

32

Then, the compression modulus and compression strength at 0.25 strain (25% strain) and at 0.50 strain

(50% strain) were calculated for each foam from the stress-strain curve (Figure 2.7). In the industry, the

value of the compression strength at 0.25 strain is called the compression-deflection and represents a

measure of the compressive strength for silicone rubber foams [60]. The compressive modulus was obtained

by measuring the slope of the stress-strain curve in the elastic region (low deformation). The slope was

measured by dividing the stress variation by the strain variation between two points on the slope. This is

illustrated in Figure 2.7 using a typical stress-strain curve.

For each sample, three modulus values were obtained and used to calculate the averaged compression

modulus and determine their standard deviation as an approximation of the experimental error. Then, the

averaged values were divided by the foam density to obtain the compression modulus to weight ratio for

each foam. Finally, these values were compared to each other to determine the effect of cell size on the

compression modulus. The same approach was followed to determine the effect of cell size on the

compression-deflection values.

Figure 2.7 Illustration of compression modulus and compressive-deflection calculations from the stress-

strain curve.

33

Chapter 3 RESULTS AND DISCUSSION

3.1 Introduction

This chapter presents the results and discusses the findings from the characterization on the fabricated

silicone foams including densities, morphologies and compression properties. It also presents the

relationships between the cell sizes and the compressive properties of the foams with respect to their

respective densities.

3.2 Foam Density Measurement

From each sample produced (Table 3.1), three specimens were cut for compression tests. The bulk

densities of these nine specimens are listed in Table 3.2 and the averaged values are shown in Figure 3.1.

Using the gas pycnometer, the density of the matrix material (unfoamed) was 1445 kg/m3 which is in good

agreement with the value reported by the manufacturer (1.43 g/cm3) [61].

Table 3.1: Physical properties of the fabricated foam samples.

Sample Mass after heating (g) Width (cm) Length (cm) Thickness (cm)

S1 236 12 20 2.0

S2 228 12 20 2.0

S3 232 12 20 2.0

34

Table 3.2: Densities (kg/m3) of the compression tests specimens.

Sample S1 S2 S3

Specimen 1 488 472 483

Specimen 2 494 474 480

Specimen 3 492 480 486

Average 491 475 483

Standard deviation 3 4 3

Figure 3.1 Averaged densities of the compression tests specimens.

35

As it can be seen in Table 3.2, the standard deviations obtained for the specimens cut from samples S1, S2

and S3 are very small (less than 1%). This indicates that the samples S1, S2 and S3 are homogenous.

Table 3.2 also shows that within experimental uncertainty, there is no significant differences between the

sample densities.

3.3 Foam Morphology Analysis

Typical optical images of the samples cross-section are shown in Figure 3.2. It can be seen that the samples

are uniform. As expected, the low compressive stress applied on the EPS beads during the molding process

did not disturbed the cell shape (spherical) or their distribution (no agglomeration) leading to a homogeneous

structure. However, it can be seen that some cells are interconnected (not fully closed) in all samples.

Furthermore, the number of broken walls increases with increasing EPS beads size. Among all the samples,

sample S3 contains more open cells induced by broken cells walls. This can be attributed to the fact that for

some cells at the interface of EPS beads, where the beads coated with liquid resin come into contact, the

cell wall becomes very thin and even zero, resulting in interconnected cells. Sample S3 was made using

larger EPS beads and the interface between these larger beads is greater than that between smaller beads,

which facilitates these cell interconnections prior to curing.

36

Figure 3.2 The cellular structure of the foam samples at low (left) and high (right) magnifications.

From the cross-section images (Figure 3.2), the cellular morphology was analyzed and the results are

reported in Table 3.3.

Table 3.3: Physical and morphological parameters of the foams produced.

Sample Mass of the foam before heating (g)

Mass of the EPS bead added (g)

Vm

(cm3)

Vf

(cm3)

n

(-)

Cell density

(cells/cm3)

S1 238 11 157.1 480 111 3573

S2 230 10 152.2 480 71 1886

S3 234 9 155.7 480 49 1057

As seen in Table 3.3, samples S1, S2 and S3 contain similar volumes of silicone resin (Vm), but very different

numbers of cell (n). For example, in a volume of 1 cm3 of samples S1, S2 and S3, there are 111, 71 and 79

cells, respectively. Using Equation (2.1), the overall cell density decreases from 3573 to 1886 and 1057

37

cells/cm3 for samples S1, S2 and S3, respectively. This is attributed to a lower number of larger cells per

unit volume.

3.4 Compression Properties

As the first step, the unfoamed silicone rubber system (ELASTOSIL® M 4370) was tested under

compression. Five specimens were cut with areas of 47 mm by 47 mm and a thickness of 20 mm before

being subjected to compressive loads up to 3650 N (close to the load cell capacity of 5 kN) with a quasi-

static rate of 0.015 s-1 at room temperature. The obtained stress-strain curves are shown in Figure 3.3. This

allowed obtaining the compressive modulus of the silicone rubber which was measured at 3030 kPa.

Figure 3.3 Compressive stress-strain curve obtained for the unfoamed silicone rubber.

The foam samples were then tested for their compression properties as described in Chapter 3. A total of

nine specimens were prepared (three specimens from each sample as shown in Figure 2.6) and

compressed with a quasi-static rate of 0.015 s-1 at room temperature. The compression stress-strain curves

obtained are shown in Figures 3.4 to 3.6 for samples S1 to S3, respectively.

38

Figure 3.4 Compressive stress-strain curves for sample S1.


39


Comparing the compressive stress-strain curves from Figures 3.4-3.6 with the general curve for polymer

foams as shown in Figure 1.1, it is found that under quasi-static compression, the silicone foams (S1, S2

and S3) experienced a similar elastic and densification regions, but not an absolute plateau region. They

first undergo elastic deformations controlled by cell wall bending (in open cells) and cell wall stretching (in

closed cells) up to around 0.1 strain. In open cells, the cell wall bend under compressive load, while in closed

cells the cell wall stretches under the internal pressure of the gas enclosed [22]. Then, the foams

continuously undergo larger compressive deformations needing a lower level of increase in the stress, up

to around 0.2 strain. In this step, the foams deformed as their cells collapsed due to cell wall buckling under

the compressive stress [23]. Further increasing the deformation above 0.2 strain, the stress continuously

increased. In this step, the opposing cell walls came into contact with the shrunken EPS beads in open cells

first, then in closed cells containing a compressed gas, causing a sharp transition (higher slope) in the stress-

strain curve [24]. From the stress-strain curves for each sample, the values of compressive modulus (slope

of the stress-strain curve between 10% and 15% strain), compressive strength at 25% strain (compression-

deflection) and compressive strength at 50% strain were obtained as listed in Tables 3.4 to 3.6 and shown

in Figures 3.7 to 3.9.

40

Table 3.4: Compressive properties of sample S1.

Sample_Specimen Compressive modulus

(kPa) Compressive strength

at 25% (kPa) Compressive strength

at 50% (kPa)

S1_1 384 92 281

S1_2 390 98 303

S1_3 397 95 293

Avg 390 95 292

SD 6 3 11

Table 3.5: Compressive properties of Sample S2.




at 50% (kPa)

S2_1 436 101 323

S2_2 443 107 316

S2_3 449 112 326

Avg 442 107 322

SD 6 6 5

41

Table 3.6: Compressive properties of Sample S3.




at 50% (kPa)

S3_1 506 103 340

S3_2 505 106 336

S3_3 504 110 352

Avg 505 107 342

SD 1 4 8

The averaged compressive moduli of samples S1, S2 and S3 are compared in Figure 3.7. Among these

samples, sample S1, which was made using the smallest EPS beads (1 mm in diameter), gave the lowest

compressive modulus (390 kPa), while sample S3, made from the largest beads (2 mm in diameter), showed

the highest compressive modulus (505 kPa). Sample S2, made using 1.5 mm EPS beads, gave an

intermediate value (442 kPa). Therefore, it was found that by increasing the cell size the compressive

modulus increased. The moduli obtained for the foams are much lower than the modulus of the unfoamed

silicone rubber (3030 kPa) showing that the compressive modulus of the silicone rubber decreased by about

6-7 fold for the foams.

For the compression-deflection values (compressive stress at 25% or 0.25 strain), samples S3 and S2

showed similar compression-deflection level (107 kPa) as illustrated in Figure 3.8, while sample S1 gave

the lowest compression-deflection value (95 kPa), similar to what was observed for the compressive

modulus (Figure 3.7). These stress values (at 25% strain) are about 8 times less than for the unfoamed

silicone rubber from Figure 3.3 (850 kPa).

For the compressive stress at 50% strain, the averaged values obtained for samples S1, S2 and S3 are

compared in Figure 3.9. It can be seen that increasing the cell size leads to high compressive stress values.

42

At 50% strain, sample S1, having the smallest cell sizes, gave the lowest compressive stress (292 kPa),

while sample S3, made having the largest cell sizes, gave the highest compressive modulus (342 kPa).

Sample S2, made using the midsize EPS beads, showed an intermediate compressive modulus (322 kPa).

Figure 3.7 The averaged compressive moduli for the foam samples.

Figure 3.8 The averaged compression-deflection (compressive stress at 25% strain) for the foam samples.

43

Figure 3.9 The averaged compressive stress at 50% strain for the foam samples.

To conclude on the cell size effect on the compressive properties, the density of the foam must also be

considered. Therefore, the compressive properties, including compressive moduli and stresses, were

divided by the foam density for each sample. The values obtained are compared in Figures 3.10 to 3.12 for

a more complete analysis.

Figure 3.10 shows that increasing the cell size from 1 mm for S1 to 1.5 mm for S2, the compressive modulus

to weight ratio increases from 795 to 931 N.m/kg (17%). This indicates that silicone foams of similar density,

but with a larger cell size, may exhibit greater resistance to quasi-static compressive loads due to the higher

internal pressure of the contained gas, and may therefore be stiffer. As observed by Li et al. [21], in silicone

rubber foam having large and also small cells, the larger cells mostly fail after the small cells and thus

underwent larger stress levels before failure, and consequently the gas inside the cells experience higher

internal pressure. One reason why silicone foams of similar density, but with larger cell size, may exhibit

greater resistance to compressive loads could be theoretically explained using the Gibson-Ashby equation

given for closed cell foams [62]:

𝐸𝑓

𝐸𝑠 ≈ ∅2 [

𝜌𝑓

𝜌𝑠]

2+ (1 − ∅) [

𝜌𝑓

𝜌𝑠] +

𝑝0(1−2𝜗𝑓)

𝐸𝑠(1−𝜌𝑓

𝜌𝑠)

(3.1)

44

where, 𝐸𝑓, ρ𝑓, ϑ𝑓, 𝑝0, 𝐸𝑠 and ρ𝑠are respectively the foam modulus, foam density, Poisson's ratio of the

foam, pressure inside the cell, unfoamed matrix modulus and solid phase (matrix) density. Also, ∅ denotes

the volume fraction of solid contained in the cell edges, while the remaining fraction (1 - ∅) is in the cell

faces. At near atmospheric pressure, the third term of Equation (3.1) is much smaller than other terms rigid

materials (high Es) and thus it can be neglected [63]. But this is no longer the case for soft materials (low

Es), especially for closed cell foams with high internal cell pressure.

By substituting the values obtained from the experiments in Equation (3.1), the ∅ values for samples S1, S2

and S3 were calculated as 0.85, 0.70 and 0.61, respectively. This shows that by increasing the cell size from

1 mm for sample S1 to 1.5 mm for S2 and 2 mm for S3, the amount of silicone resin in the edges of the cell

decreases and therefore the amount of resin is more on the faces, resulting in a better distribution of the

resin improving the foam stiffness.

However, an increase in cell size may not always result in increased stiffness. This was found when

comparing the modulus to weight ratios obtained for S2 and S3. It was observed that a further cell size

increase from 1.5 mm for S2 to 2 mm for S3 led to an increase of only 12% (931 to 1046 N.m/kg) for the

modulus to weight ratio. This is probably related to the higher number of open cells with increasing cell size

leading to lower contribution of the cell walls.

In addition, the compressive modulus to weight ratios of the unfoamed sample and the foam sample with

the highest compressive modulus to weight ratio (S3) were compared with each other to study the

effectiveness of the present method for the fabrication of foams with high compressive moduli. It was found

that when the silicone rubber (matrix material) was expanded three times and its density was reduced from

1445 kg/m3 for the unfoamed state to 480 kg/m3 for the foam state (sample S3), its compressive modulus to

weight ratio was reduced by only two times, from 2097 N.m/kg to 1046 N.m/kg, as shown in Figure 3.10.

45

Figure 3.10 The compressive modulus to weight ratio for the foam samples.

A similar trend to Figure 3.8 is observed for the compression-deflection to weight ratio. As shown in Figure

3.11, increasing the cell size from 1 mm for S1 to 1.5 mm for S2 increased the compression-deflection to

weight ratio from 193 to 224 N.m/kg (14%) indicating that silicone foams of the same density, but with a

larger cell sizes, may exhibit higher compression-deflection due to the higher internal gas pressure of the

cell. However, similar to the modulus to weight ratio trend (Figure 3.10), increasing the cell size may not

always increase the compression-deflection level. As shown in Figure 3.11, further cell size increase from

1.5 mm for S2 to 2 mm for S3 slightly decreased the compression-deflection to weight ratio from 224 to 221

N.m/kg (1%), due to the increased number of open cells. However, the effect of open cells on the

compressive behavior of the foams at the beginning of the compression process is less pronounced than at

larger deformations. This is because the ratio of the closed cells to open cells at the beginning of the test is

greater than that at larger deformations. At a deformation of around 2.5% strain, where the modulus is

calculated, most of the closed cells were not broken and can resist/sustain the internal pressure of the

contained gas, while at 25%, where the compression-deflection is calculated, a high number of closed cells

were broken.

In addition, the compression-deflection to weight ratios of the unfoamed sample and the foam samples were

compared with each other to study the impact of the present foaming method on the compression-deflection

46

properties of the foams. It was found that when the silicone rubber (matrix material) was expanded three

times using the present method, its compression-deflection ratio to weight was also reduced by about three

times, as shown in Figure 3.11.

Figure 3.11 The compression-deflection (compressive stress at 25% or 0.25 strain) to weight ratio for the

foam samples.

Figure 3.12 shows that as 50% strain, unlike what was observed for the compression-deflection to weight

ratio (Figure 3.11), increasing the cell size resulted in high compressive to weight ratio. The compressive

stress to weight ratio increased from 595 N.m/kg for S1 to 677 N.m/kg for S2, and then to 709 N.m/kg for

S3. This is because at 50% strain, the presence of open cells is not as important as that at smaller strains,

where the compression-deflection is calculated. At higher strains, where a higher number of cells are broken,

the ratio of the closed cells to open cells becomes lower than that at smaller strains.

47

Figure 3.12 The compressive stress at 50% strain (0.5 strain) to weight ratio for the foam samples.

By increasing the cell size from 1 mm to 1.5 mm (about 50% increase) the level of compressive stress to

weight ratio at 50% strain increased by about 14%, while an increase from 1.5 mm to 2 mm (28% increase)

resulted in an increase of only 5%. The increased stress to weight ratio when the cell size increased from

1.5 mm for S2 to 2 mm for S3 is not as sharp as observed for the cells increasing from 1 mm for S1 to 1.5

mm for S2. This is because sample S3 has more open cells than samples S2. Since the increased

compressive properties in the closed silicone rubber foams is a result of higher internal pressure in the cells,

achieved by larger cell size, a foam with higher open cell content will show less compressive properties

improvement as the gas inside the cell can escape, loosing on the gas compressibility contribution.

48

Conclusions and Recommendations

Conclusions

In this research, the effect of cell size on the compressive properties of silicone foams with spherical closed

cells was investigated. Three different foams with different cell sizes ranging from 1 mm to 2 mm were made

and tested under a quasi-static compression rate at room temperature. For each foam sample, the

compressive modulus to weight ratio and compressive strength to weight ratio was obtained and these

values were compared. Based on the data obtained, Figure II.1 presents an overview of the results obtained

for the combined effect of cell size and foam density on the compressive properties.

Figure II.1 The effect of cell size on the compressive properties with respect to the sample density. Sample

S1 was made using 1 mm EPS beads, S2 using 1.5 mm and S3 using 2 mm beads.

49

From the results obtained, several conclusions can be made:

i) The method of using shrinkable beads to form the foam cells made it possible to produce homogeneous

foams with similar densities, but with different cell sizes.

ii) The cell size affects the quasi static compressive behavior of the silicone rubber foams in different ways.

iii) In closed cell silicone rubber foams, the compressive modulus increased by increasing the cell size for

the range of conditions investigated.

iv) Among the closed cell rubber foams having the same matrix composition and similar densities, but

different cell sizes, the one having the smallest cell size gave the lowest compressive properties to weight

ratio, while the one with the largest cell size gave the highest ratio when they are slowly compressed (0.015

s-1).

v) Increased compressive properties in the silicone rubber foams is a result of higher contribution of the

internal cell pressure, especially in larger cell sizes.

vi) For the closed cell silicone rubber foams with densities around 480 kg/m3, increasing the cell size from 1

mm to 1.5 mm increased the compressive modulus by 17%, as well as the compression-deflection and

compressive stress at 50% strain by 14%.

vii) For silicone rubber foams with similar densities, increasing the cell size may not always result in improved

compressive properties, depending on the ratio between closed cells and open cells as the internal gas

pressure is less important for the latter.

viii) For silicone rubber foams containing some open cells, the effect of open cells content on the foam

compressive behavior is different between the beginning of the compression test (small strains) compared

to later stages (larger deformations). This is because the ratio of closed cells to open cells changes and cell

walls contributions decrease with increasing deformation.

50

ix) For silicone rubber foams with similar densities, when increasing the cell size, a foam with higher open

cell content is producing leading to lower compressive properties due lower contribution of the cell walls and

the internal gas pressure compressibility.

Recommendations for Future Works

Based on the findings from this research, the following recommendations are made for further development:

i) To study the mechanical behavior of silicone rubber foams under other types of deformation:

tension, impact, bending, multi-axial, etc.

ii) To study the effects of different fillers/reinforcements on the mechanical behavior of composite

silicone rubber foams. This includes different types, sizes and geometry of particles.

iii) To study the effect of cell size and density on the thermal properties (heat conductivity) of silicone

rubber foams.

iv) To study the effect of other cell parameters (geometry, deformation, density) on the overall

properties of silicone rubber foams.

v) To investigate the possibility of using the EPS-based foaming technique to other types of rubbers

like polyurethane, polyisocyanurate, etc.

51

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Documents

Effect of cell size on the quasi-static compressive