Introduction -...

CHAPTER 1

Introduction

Chapter 1: Introduction

1.1 Introduction to polymer composites

The polymer composites are composed of polymer matrices and small amount of (a few wt %

of polymer matrices) of fillers. Type of the filler, polymer matrix and interface between filler

and polymer chains affect the properties of polymer composites. Load transfer is facilitated by

the interface between polymer and filler. The polymer matrix which is used in commercially

produced composites also called resin solution. Different polymers are available for synthesis

of polymer composite based on the starting raw ingredients. The most common polymers used

are known as polyethylene, polypropylene, vinyl ester polyester, epoxy, polyimide, phenolic,

polyether ether ketone (PEEK), and others. Nonreinforced polymers are not found to be

suitable as structural material due to low level of mechanical properties such as strength,

modulus, hardness and impact resistance. Size, shape, dispersion and orientation of the filler

affect the properties of the composites [1]. The fillers used for reinforcement are often fibers

or common ground minerals [2]. During the last years the properties and quality of the

composite materials are improved and made suitable for engineering application. Various

technical demands of the modern technology of such materials depend on their structure and

physical and mechanical behaviour. Knowledge of a number of physical parameters are

required to characterize such composite materials. The electrical and mechanical behaviour of

the two-phase polymer composite material depends not only on the type but also the on the

weight fraction of the filler and the matrix and their interaction. The performance of

composite can be improved by fillers in different ways [3]. For example the filler can improve

the tensile and tear strength without losing elasticity or extending the barrier effect to solvent,

gas and vapors. Moreover fillers can originate new functional properties in the polymer

composites such as flame retardancy or thermal and electrical conductivity which is not

possessed by the polymer matrix. The polymer composites are used widely in all industrial

sectors such as boat decking, transport, civil engineering, general engineering, aerospace,

sport, domestic consumer etc.

1.1.1 Types of polymer

Polymer composites are synthesised from a wide range of polymers such as thermoplastics,

thermosets and elastomers.

CHAPTER 1 INTRODUCTION

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Thermoplastic polymers: Thermoplastic polymers consist of linear or branched chain

molecules with strong intramolecular bonds but weak intermolecular bonds. They possess

semicrystalline or amorphous in structure which can be reshaped by application of heat and

pressure. Examples include polyethylene, polypropylene, nylons, polystyrene, polycarbonate,

polyacetals, polyether ether ketone, polyamide-imides, polyphenylene sulfide, polysulfone,

polyether imide, and so on.

Thermosetting polymers: The thermosetting polymer molecules are bonded by covalent

bonds having cross-linked or network structures. They can�t be reshaped once solidified by

cross-linking process as they do not soften and decomposes on application of heat. The most

Commonly used thermosetts are epoxides, polyesters, phenolics, ureas, melamine, silicone,

and polyimides.

Elastomers: An elastomer is a class of polymer characterized by viscoelasticity property

having low level of Young�s modulus and high yield strain in comparision to other

materials.Elastomers are often termed as rubber when referring to vulcanizates. The

monomers of the elastomers made of carbon, hydrogen, oxygen, and silicon. Elastomers are

amorphous polymers associated with considerable segmental motion they exist above their

glass transition temperature. Usually rubbers are used as adhesives, seals, and molded flexible

parts due to their softness and deformability at ambient temperatures. Natural rubber,

chloroprene rubber, butyl rubber, synthetic polyisoprene, polybutadiene, epichlorohydrin,

ethylene propylene rubber, silicone rubber, thermoplastic elastomers, fluoroelastomers,

polysulfide rubber are some of the examples of elastomers.

1.1.2 Types of filler

Fillers greatly improve the dimensional stability, tensile and compressive strength, impact

resistance, abrasion resistance and thermal stability of the composite when incorporated into

polymers. Extender fillers increase the bulk volume and reduce the price whereas reinforcing

fillers enhance the mechanical properties particularly tensile strength [4-7].The electrical and

thermal conductivity increase by adding conductive fillers to polymer matrix [8, 9, 10].

Wypych reported in a review [11] fillers used in thermoplastics and thermosets consisting

more than 70 types of particulates or flakes and more than 15 types of fibers of natural or

synthetic origin. The most commonly used particulate fillers are industrial minerals such as

calcium carbonate, talc, mica, feldspar, kaolin, wollastonite, and barite are some examples of


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particulate fillers which are commonly used. Different fillers with their primary function have

been provided in table 1.1.

Table1.1 Fillers and their function

Modification of mechanical property Glass fibers, mica, nanoclays, carbon

nanotubes, carbon /graphite fibers, aramid /

synthetic / natural fibers, talc, kaolin, CaCO3,

wood flour, glass spheres ,wollastonite.

Modification of electrical, magnetic

conductivity

Conductive, non-conductive ferromagnetic:

metals, carbon nanotubes and fibers, carbon

black, mica

Modification of surface property Antiblock, lubricating: silica, graphite,

CaCO3, MoS2, PTFE.

Enhancement of fire retardancy Hydrated fillers: Mg(OH)2, Al(OH)3

Enhancement of processability Thixotropic, anti-sag, thickeners and acid

scavengers: colloidal silica, hydrotalcite,

bentonite.

1.2 Background of carbon based polymer composites

Usually plastics are considered as insulating materials. It is possible to make them conductive

by addition of some carbon based fillers like carbon black, carbon fibre, graphite fibres,

graphene, carbon nanotubes, etc. Carbon black is one of the most frequently used filler in the

compounding of rubbers. Carbon black is proved to be an effective and low cost filler which

provides varying level of conductivity [12-15].Carbon black is used to reduce the cost of the

end product and modify the electrical and optical properties of the polymer matrix [16,17].

Filling carbon blacks in elastomer and plastics also reduces the cost of the end product and

modifies the mechanical, electrical and optical properties of the polymer matrix [16,17].The

resistivity of the plastic is lowered and antistatic, semiconductive or conductive properties are

imparted by adding carbon black as filler [9]. Loading level, particle size, aggregation, surface

chemistry and structure of carbon black are major five factors which affect the level of

conduction. Incorporation of carbon black into polymers yields positive temperature


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coefficient materials [18].Carbon black filled polymers are widely used in many applications

because of its low cost and good electrical conductivity [19, 20]. Carbon black which is a

form of amorphous carbon, has been used as filler in antistatic or electrical conductive

modification of polymer systems (Degussa 2008).Electrical conductivity can be introduced by

incorporation of small amount of carbon black to the polymer due to its high surface area to

volume ratio.The incorporation of carbon black to the insulating polymer matrix reduces the

electrical resistivity to several orders of magnitude [21]. The dielectric properties such as

dielectric loss tangent, dielectric permittivity are enhanced by the increase in concentration of

carbon black in the polymer matrix [22]. Polymer composites show a slow increase in

dielectric constant with increase in carbon black loading till roughly the percolation

concentration and then increases rapidly over the whole concentration ranges studied [23].

Both the physical and chemical interactions between elastomer and carbon black reinforce the

elastomer and improve other properties to a large extent [24, 25].The mechanical properties of

the composites are also strengthened by the interaction between the CB particles and the

polymer [26, 27]. In CB reinforced polymer composites the mechanical properties such as

modulus increases at low loading of carbon black whereas at higher loading the tensile

strength reduces [28, 29, 30].The degree of reinforcement is highly effected by the surface

area and the presence of functional groups present on the surface of carbon black [31, 32].

Litvinov and co-workers studied the adsorption of ethylene � propylene-diene rubber (EPDM)

upon the surface of carbon blacks using nuclear magnetic resonance (NMR) [33].They

observed that with increase in EPDM-carbon black interfacial area, the rubber molecule

physically adsorbed on the carbon black surface and the reinforcement enhances. Park and co-

workers studied the mechanical properties of carbon black/rubber composites and correlated

these to the surface energy of carbon blacks [34, 35].They reported that the specific surface

area affect the non-polar characteristics or the London dispersive component of the surface

energy of carbon blacks which leads to increase in vulcanization reactions and enhances the

mechanical properties composites [35].Not only the surface area but also the surface

chemistry of the carbon black affects the properties of carbon black filled elastomers

significantly. It is reported that the mechanical properties of composites can be altered by

modifying the carbon blacks dispersed in the rubbers. Leopoldes�s group reported that 100%

and 300% moduli were higher for oxidative gas treated carbon black reinforced natural rubber

[36]. Usually carbon black exists as the primary aggregation in rubber [37].When carbon

black is used as a filler it increases the thermal conductivity as well as the thermal degradation

temperature of the polymer composite than that of the pure polymer [38, 39].


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1.2.1 General characteristics of Ethylene octene copolymer

Ethylene Octene copolymer (EOC) is copolymer having trade name ENGAGE produced by

Dow Chemicals (USA) following a constrained geometry catalyst technology (CGCT).

Ethylene-Octene-copolymer (EOC) is a new copolymer consists of two monomers such as

ethylene and 1-octene.Table 1.2 provides the properties of ENGAGE 8150.

2 2 5H C CH (CH ) Me

2 2H C CH

Table 1.2 Properties of ENGAGE 8150

Physical form White Pellet

Molecular formula C10H20

Molecular composition 25% Octene

Molecular weight (g/mole) 140.266

Density (g/cc) 0.868

Melt flow index (g/min) (0C/Kg) 0.5/10 (190/2.16)

Mooney viscosity of ML1+4 (1210C) 35

The commoner is distributed homogeneously in the copolymer. The mechanical and thermal

properties of ethylene copolymers are affected by two factors such as decrease in crystallinity

by addition of the comonomer and increase in molecular segregation due to highly branched

chains [40]. Polyolefinic elastomer is available in commercial grade whose processability is

similar to plastic whereas flexibility and mechanical properties are as that of synthetic rubber.

Process technology is designed in such a way that it can be processed like thermoplastic and

compounded like elastomers [41]. It bridges between thermoplastics and elastomers. The

compression set, heat aging, and weather resistance properties are improved when cross-

linked by peroxide, silane, or irradiation. Moreover it possesses high peroxide cure and filler

loading capability. It is also applied to make outstanding electrical insulation. It has wide area


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of application such as cables, wires, impact modification, thermoplastic olefins (TPO),

general purpose thermoplastic elastomers etc. It enhances the physical and mechanical

properties when combined with fillers or crosslinked. It is widely used for general purpose as

it is flexible and light weight. It promotes filler acceptance for masterbatch application and

resiliency for foam applications. It is also used as soft and hard TPO skin applications such as

knee bolsters, door panels, instrument panels, pillar moldings and console moldings for its

excellent elasticity, toughness, low temperature ductility, impact resistance and long term

performance. It has very good processability since it is compatible with most olefins. It is

available in pellet form which leads to easy to handling, mixing, forming and processing as

shown in figure 1.1. It is also recyclable for in process scrap.

Figure 1.1 Pellets of EOC

The cold-temperature impact strength provided by ethylene octene copolymer is up to 20%

greater than equivalent loadings of EPDM. The crystallinity of EOC is between 10-15%

whereas that of EPDM is of 0-5% due to which the rigidity provided by EOC to a

thermoplastic olefin elastomer (TPO) is greater than EPDM. Moreover EOC possesses better

flexibility, clarity and crack resistance compared to ethylene vinyl acetate copolymer (EVA)

and also better weatherability than styrene-butadiene�styrene copolymer (SBS) [42]. The

conductivity properties of polyaniline, carbon black, and multiwall carbon nanotubes filled

EOC composite has been reported earlier [43-45]. Recently electrical and thermal

conductivity of composite based on ethylene-octene copolymer and expandable graphite has

been reported [46].

1.2.2 Conductive carbon black as filler in polymer composites

Carbon black (CB) is widely used as conductive filler in plastic to increase the electrical

conductivity [47]. Carbon blacks are mainly of five types such as furnace, thermal,


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impingement, acetylene and lampblack. Table 1.3 provides the range of particle sizes,

aggregate size and surface area of five types of carbon black.

Table 1.3 Particle size, aggregate size and surface area of carbon black produced by different

process [48, 49].

Carbon black Surface area (m2/g)Approximate

diameter of primary particle size (nm)

Diameter of aggregate (nm)

Oil-furnace 12�240 10�400 50-400

Thermal 6�15 120�500 400-600

Impingement (channel) 10�30 50-200

Lampblack 15�25 60�200 300-600

Acetylene black 15�70 30�50 350-400

Products with different physical and chemical properties are formed by the five different

processes [50-53]. Furnace and thermal blacks are the most commonly used CBs in rubber

and plastic industry.

Furnace process is widely used for production of carbon black, in which CB particles are

formed by thermal decomposition of oil. The aromatic oil molecules decompose by breaking

of C-H bonds. In the furnace method the aromatic oil is heated 200 to 2500C [54]. Firstly the

feed stock is preheated, and then the feedstock is placed in a hot-flame zone of the reactor

having burning natural gas inside it [47]. Oil is vaporized and thermally degraded to produce

CB particles inside the hot-flame zone. The reaction comes to an end after application of

water. Then the gas stream is released into the filter to separate carbon black. To increase the

bulk density of the carbon it is either mixed in pin machines with more water or tumbled in

horizontal drums, where it forms small pellets before being dried in a rotary kiln dryer

[47,51]. This process generates carbon blacks having size 10 to100nm and surface area from

25 to 1500m2/g. The thermal process produces the largest CB particle size [50].Lampblack

consists mid size particle with a high degree of aggregation and small surface area. Acetylene


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black is of high purity and high degree of aggregation and most crystalline form of the carbon

blacks. Channel black is characterized by small particle size and a high level of surface

oxidation [55, 56].

Since furnace black has large surface area and small particle diameter, it is suitable as filler to

impart electrical conductivity to polymer composites than the other four types of carbon

blacks. Table 1.4 provides the grades, production processes, particle size and surface area of

carbon black that are used in compounding of elastomers.

Table 1.4 Grades, production processes, Particle size and surface area of carbon black [50,

51]

Type DesignationAcronym

ASTM

Process and/or

feedstock

Average primaryParticle diameter

(nm)

Iodineabsorption

Numbera (g/kg)Superabrasionfurnace black

SAF N110

Oil furnace 17 145

Intermediate Superabrasion Furnace black

ISAF N220

Oil furnace 21 121

High-abrasionfurnace black

HAF N330

Oil furnace 31 82

Fast-extruding furnace black

FF N550

Oil furnace 53 43

General purpose furnace black

GPF N660

Oil furnace 63 36

Semi-reinforcing furnace black

SRF N762

Oil furnace 110 27

Medium thermal black

MTN990

Natural gas 320 9

Carbon black can be sold as pellets or as a powder. Particle size (surface area), surface

chemistry and structure of the CB are three properties which characterize it as conductive

filler [51-54]. A crystallographic structure is created from the layers produced by the reaction


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of carbon atoms and aromatic radicals. CB primary particles are formed from crystallites and

associated to form CB primary aggregates. Polley and Boonstra [57] reported that

conductivity is enhanced when the distance between the primary aggregates in the polymer

matrix approaches less than some critical distance. With increase in the surface area of the

carbon black the gap between the polymer and carbon aggregates decreases. The interparticle

distance ranges from 10 to 28 nm by adding carbon black of different size. The study says,

resistivity was exponentially related to the interparticle distance and does not depend on filler

size. CB aggregate size not only effect mechanical properties but also dynamic and

performance properties of the compounds [58]. Number of particles per aggregate, aggregate

size and shape are the three factors which determine the structure of carbon black. Low and

high structure carbon black contains few and many primary particles with numerous

branching and chaining respectively. A high structure carbon aggregates shows higher

conductivity in comparison to low structure carbon black at the same loading and the distance

of separation of aggregates is less in HS carbon black [59]. Moderate structure carbon black

possesses higher conductivity than high structure carbon black as the HS carbon black

increases the melt viscosity of the vulcanizate. The resistivity of a plastic matrix is

significantly affected by loading of carbon black. High resistivity is found when the adjacent

carbon black particles do not contact each other at low levels of loading. Increase in carbon

black loading results in formation of agglomerates of the carbon black particles. At a

particular carbon black loading conductivity increases sharply [60]. This critical concentration

of carbon black at which the conductivity increases rapidly is termed as the percolation

threshold [9, 60].

1.2.2.1Conductive carbon black (Ensaco 250G)

In the present study the carbon black used was Ensaco 250G of Timcal Singapore.

Carbochemical and petrochemical species are subjected to partial oxidation to produce 250G

carbon black. Table 1.5 gives the properties of carbon black (Ensaco 250G). These carbon

particles have mean diameter of 40 nm and round in shape. They have a branched structure

with aggregates of several hundreds of nanometer long formed by the association of CB

particles.


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Table 1.5 Properties of Carbon black (Ensaco 250G) [61]

Electrical resistivity 10 . Cm

Aggregate size 40 nm

OAN structure 190 ml/100g

Bulk density 180 Kg /m3

BET nitrogen surface area 65m2/g

Ash content 0.01 %

Moisture content 0.05 %

pH 10

The carbon black used in this work was designed for making the polymer composites and

their foam electrically and thermally conductive. It is characterized by high structure and low

surface area. The high structure provides required conductivity at low level of carbon black

loading and low surface area causes easy dispersion and processing. The unique combination

of high structure and low surface area also contributes to give outstanding dispersibility and

smooth surface finish [62]. The CB exhibits relatively moderate electrical conductivity and

some thermal conductivity [61]. This carbon black reduces the viscosity, cure inhibition and

vulcanization time due to high structure and low surface area. Ensaco 250G carbon blacks are

also used in non conducting applications where the low surface area and high structure of

those blacks provides very good mechanical performance at thin layer, low hysteresis with

relatively high hardness, good thermal aging and tear strength. Besides this low oxygen

content and high purity were other added advantages [61]. The structure of carbon black is

shown in figure 1.2.

Figure 1.2 SEM image of carbon black (Ensaco 250G)

It has wide area of applications due to low surface area which shows advantage on high

dispersion and processability. It is applied in making automotive fuel hoses, transmission


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belts, conveyor belts, thin membranes with high shock resistance, power cable accessories,

conductive profiles and seals, antistatic flooring, rollers, roller covering etc. It is suitable filler

for all rubber types including silicones and fluoroelastomers. Macutekevic J et al [63] studied

percolation threshold of carbon black (Ensaco 250G) reinforced epoxy resin. Addition of 10%

rubber microparticle to carbon black (Ensaco 250G) reinforced polycarbonate, decreases the

percolation threshold [64].

1.2.3 Processing methods of polymer composites

The main issue of fabricating polymer composite is establishing a good dispersion of fillers

throughout the polymer matrix. The unique properties of filler strongly affect the state of

dispersion throughout the matrix in the composites. Carbon black is well dispersed in the

polymer matrix by following the best processing method. Processing techniques should be

selected which provides good dispersion of filler so that desired properties can be achieved

for a specific study. Melt blending, solution blending and in situ polymerization are the

frequently used methods for processing of polymer composites.

1.2.3.1 Melt Blending

Melt blending has been widely used for processing of carbon black based conducting polymer

composites. The fillers are incorporated into the polymer and the mixture is heated to a

temperature higher than the softening point of the polymer. High temperature and shear force

are applied in melt blending method for well dispersion of the filler in the polymer matrix.

The commonly used compounding equipments are internal mixer, chaotic mixer, two roll

mills and extruder. The choice of the compounding equipment [65], processing technique [66-

68], process parameters during compounding, such as mixing time [69, 70] and temperature

[71], type and structure of the carbon black [70], and morphology of the polymer [70] have a

profound effect on the percolation threshold and electrical conductivity of the composite.

Melt processing is commercially much more attractive than a solvent method, as it is more

environmentally friendly and versatile. The drawbacks of this procedure are the lower degree

of dispersion sometimes causing formation of millimeter-scale inhomogeneities as compared

to solvent casting, and manipulation difficulties during processing due to the low bulk density

of the fillers [72].This lower degree of dispersion can have a detrimental influence on the

mechanical properties, conductivity and percolation threshold values. However, it has been


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reported by some studies that the percolation threshold decreases and conductivity increases

with increase in agglomeration of the filler in the polymer [73-76]. Thus melt processing

allows manufacturers many degrees of freedom with regard to the choice of the filler content

and selection of polymer [77].

1.2.3.2 Solution casting

Solution processing techniques have been widely used for the preparation of carbon black

[78-82] based electrically conducting polymer composites. Solution processing techniques, in

general, have three steps: dispersing the filler in a suitable solvent (usually by ultra-

sonication), mixing this suspension with the polymer in an appropriate solvent and drying the

composite solution by evaporation or distillation. An alternative method that is widely used is

the mixing of the filler suspension in a resin which is cured by the addition of a hardener [83].

The preparation of polymer composites may be simplified by developing the method further

[84, 85].

1.2.3.3 In-situ polymerization

In-situ polymerization is not a commonly used processing method. This is the only suitable

method of processing for thermoset polymers because of its inability to be remould once

cured. This method involves mixing of the fillers and monomers with or without the use of a

solvent, after that the polymerization reaction proceeds by adjusting parameters such as time

and temperature [72, 86]. Li et al [87] prepared carbon black reinforced amorphous

polystyrene (PS) composites by in-situ polymerization. In this method the monomers

penetrate into the filler agglomerates before polymerization as a result a conductive network

is developed in the polymer matrix. Generally the process is more difficult and complicated.

Molecular weight and molecular weight distribution of the polymer matrix are the two factors

which are considered prior to choice of polymerization process like anionic, radical, chain

transfer and ring opening polymerizations [88].


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1.2.4 Characteristic properties of CB/polymer composites

1.2.4.1 Cure characteristics

It is the curing behaviour or processing characteristics of the polymer composite mixes such

as scorch time, optimum cure time, maximum torque, minimum torque, cure rate index,

Mooney viscosity etc. The effects of carbon black-rubber interactions on rate of cure can be

evaluated by employing crosslinking which characterizes the critical parameters associated

with vulcanization process. It has earlier been reported that as the aggregate structure

distribution broadens, the trend in processing characteristic is towards slightly lower Mooney

Viscosity [89].The viscosity of a system with filler agglomerates is always found to be higher

than the well dispersed sample system for constant filler content. Carbon black shows higher

Mooney viscosity due to higher tendency for agglomeration. The cure kinetics of EPDM/

carbon black composites is affected by both surface area of carbon black and sulphur content

on the surface of carbon black. The physical crosslinking increases as the surface area of

carbon black increases so the increase in maximum torque, minimum torque are observed

[90].Carbon black affect on the rheometric characteristics of styrene butadiene rubber and

chloroprene rubber blends significantly [91]. The incorporation of carbon black decreases the

scorch time and increases the cure rate, and mechanical property. Many researchers tried to

co-relate cure kinetics and mechanical properties of a vulcanizate with the interactions

between elastomers and carbon blacks to explain the improved performance of rubber from

the microscopic point of view [92, 34].

1.2.4.2 Physico-mechanical properties

When a material is continuously stretched with increase in stress causes continuous

elongation and the mechanical properties are determined from the tensile stress strain curve.

Carbon black is generally added to the polymer as reinforcing filler. Surface characteristics,

structure and particle size of the fillers are the major factors which affect their reinforcing

effects in composites. The extent of the reinforcing effect is also enhanced by incorporating a

larger amount of carbon black in the polymer [93, 94]. It is reported that the hardness, tear

strength, tensile strength of carbon black filled NR/PP blends increase with increasing carbon

black incorporation. This observation is due to the carbon black (N330) reinforcing filler

which has good suface activity, chemical properties and non-uniform of porous surface which


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facilitates maximum interphase interaction between polymer chain and filler [95]. Composite

with conductive carbon black has the highest tensile strength, hardness, elongation at break,

tensile modulus at 300% and permanent set.

Drastic reduction is observed in the mechanical properties after 30 phr carbon black (N330)

loading except that the modulus at 300% and increased again at 60 phr filler loading which

indicates the improvement in the carbon black dispersion into the styrene butadiene rubber

matrix up to a certain amount at lower filler loading and further loading makes the dispersion

difficult as the specific surface area increases [96]. The carbon black having largest surface

area provides highest hardness, and 300% modulus to the polymer vulcanizate [90].The

physical and chemical interactions between the polymer matrix and carbon blacks provide

better reinforcement to the polymer and improves the physico-mechanical and electrical

properties greatly [97-99]. The electrical and mechanical properties of carbon black

(Conductex 975 ultra) filled ethylene octene copolymer studied and an interrelationship has

been established [100].

1.2.4.3 Sorption property

The transport behaviour of various organic solvents through different polymers and rubbers

plays a vital role as protective barriers in chemical and food industries, in membrane

separation processes, in solvent reservoirs and in the separation of organic-organic or

aqueous-organic mixtures by pervaporation. The sorption properties of polymers were found

to be affected by some factors such as the nature of the polymers, nature of the fillers, nature

of the penetrant, cross-link density, temperature etc. The solvent transport property of the

polymer composites such as diffusion, sorption and permeation are determined by the study of

sorption behaviour the polymer composites. Volume fraction of the filler, interaction of the

filler with the polymer matrix, shape and orientation of the filler particles etc determine the

degree of tortuosity of the path for the solvent. The increase in volume fraction of filler in the

polymer, increases the tortuosity of the path for the solvent molecules which affect the

sorption properties.

Porter [101] investigated the influence of addition of HAF black on degree of reduction of the

swelling of conventionally vulcanized natural rubber in n-decane. De Candia et al. [102]

studied the effect of rubber-filler interactions on transport property of elastomeric

networks filled with carbon black. Several researches have been carried out to study the


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influence of carbon black on the solvent transport and mechanical properties of carbon black

reinforced elastomers [103-106].

1.2.4.4 Dielectric properties

Dielectric properties of polymer composites such as dielectric permittivity, dielectric loss

tangent, ac conductivity, real and complex impedance and percolation threshold are studied

by using dielectric spectroscopy. It is based on the fact that an external field interacts with the

electric dipole moment of the sample. When the sample is subjected to an external electric

field the electron cloud moves away from the nuclei in the direction opposite to the applied

field. Thus the positive and negative charges are separated and the molecules get polarized

and act like an electric dipole. Various transitions are observed in the polymeric system due to

molecular relaxations which are associated with the complex motion of the dipolar chain

segments.Dielectric relaxation behaviour of composites is mainly affected by polymer-filler

interaction as well as size, shape, dispersion and distribution of filler particulates in the

polymer matrix. Dielectric materials have the ability to store electric charges. Polymer

structure can be studied easily by the dielectric constant and the loss factors values of the

material.

The definition for dielectric constant relates to the permittivity of the material. The complex

form of relative permittivity is expressed by equation [1.1]

''* j [1.1]

Dielectric constant ( )' is the real part and dielectric loss ( )" is the he imaginary part of

complex permittivity.

Structure of the polymer is one of the major factor due to which polymers possess lower value

of dielectric constant than other material. Dielectric materials have capacity to store electric

charge.

Dielectric constant is defined as the ratio of capacitance induced by two conducting plates

containing the insulating material to the capacitance of the same conducting plates with

vacuum within it. It is symbolized by ' and is expressed by the equation [1.2]


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0/' CC [1.2]

The dielectric constant value of the material depends on the polarizability of the dielectric

molecule. Equation [1.3] relates dielectric constant ' with the molecular polarizability t

0

1'C

N t [1.3]

Where N is the concentration of the molecules and C0 is the permittivity of the vacuum.

Thus dielectric constant increases with increase in molecular polarizability of the material.

The dielectric material is polarized when the atoms and molecules of the material are got

polarized under the application of an external field .The dielectric permittivity of materials is

achieved by the frequency dependent electronic, ionic, surface charge and dipolar

polarizations. But a dielectric medium can undergo more than one polarization.

When the molecules can�t undergo polarization in order to follow the rate of change of

oscillating applied electric field, it results in dielectric loss. The change in the applied electric

field changes the direction of polarization. A phase-lag results from the delay between the

change of field and change of polarization direction is known as the phase angle .The

dielectric loss tangent (tan ) is defined as the ratio between the dielectric loss to dielectric

constant i.e

'"tan

[1.4]

The incorporation of some conductive filler to the insulating materials like polymers increases

its conductivity to several orders of magnitude. The dielectric response of carbon black filled

polymer composites have been studied in past decades by several researchers [23, 107-108].

The detail about the internal structure and the role played by the polymer carbon black

interfaces are still to be studied. Carbon black or silver particles is incorporated to the matrix

resin to make dielectric radar absorbing material (RAM) which induces dielectric loss by

increasing the conductivity of the mixture. In high frequency bands the carbon black is

characterized by a good absorption performance [109].The dielectric properties such as

dielectric loss tangent, dielectric permittivity are enhanced by the increase in concentration of

carbon black in the polymer matrix [22].


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1.2.4.5 Thermal conductivity

Thermal conductivity of polymer composite is the quantity of heat transmitted through the

material in a given time, in a direction normal to the surface of unit area due to the

temperature gradient under the steady condition. It is the measure of ability of the substance

to transfer heat by conduction.

The heat dissipation from a heat source can be easily improved by thermally conductive

polymer composites. Thermally conductive polymer composites are used in heat sink

applications such as transformer housings, laptop cases and computers. The thermal

conductivities of the polymer composites lie between 1 to 30 W/mk for heat sink application.

This technology is also used as heat exchangers and radiators applications. Thermally

conductive polymer composites have other advantages such as light, good resistance to

corrosion, and processibility. Heat transfer through polymer composites takes place by three

mechanisms: radiation, convection, and conduction. Heat transfer in solid composites mainly

occur due to heat conduction mechanism which is focused in this work.

Heat conduction is calculated using Equation 1.5 [110, 111] thermal.

i

iji xTkh [1.5]

From this equation, it can be observed that heat transfer ( ih ) depends on the thermal

conductivity ( ijk ) and a temperature gradient. The thermal conductivity in Equation 1.5 is

assumed to be constant though the thermal conductivity of a material changes with

temperature. Thermal conductivity of the material as defined by Equation 1.5 is the sum of

heat transported by different methods from macroscopic point of view.

The transverse and axial thermal conductivity of a carbon black composite has already been

studied [112].The increase in thermal conductivity with the increase of carbon black volume

fraction is non-linear in nature which cannot be predicted by any theoretical model. The

thermal conductivity of 10 wt% CB loaded PDMS-CB composite increases an order of

magnitude compared to that of neat PDMS. To achieve same thermal conductivity PDMS

requires only 10% carbon black loading whereas epoxy resin requires 70% carbon black

loading. Also thermal decomposition of the PDMS composite is retarded due to the presence


18

of carbon black filler. Carbon black when loaded as filler in rubber, it provides better thermal

stability to the rubber composite [113]. Benli et al. [114] studied the influence of carbon black

on conductivity of polyurethane elastomer.

1.3 Background of cellular polymer and microcellular foam

Cellular materials widely occur in nature such as wood, cork, corals, sponge human bones etc.

These natural materials with cellular structure have been used for centuries [115]. The idea of

developing and manufacturing the broad range of cellular polymers arose when these natural

cellular structures exhibited excellent properties such as the high strength to weight ratio of

wood, cushioning properties of cork and straw and good insulating properties of cork and

balsa. The multifunctional cellular materials are widely used structurally for cushioning,

thermal insulation, energy absorption from impacts, sound absorption and vibration

dampening.

The apparent density of the cellular polymers decrease due to formation of huge numbers of

cells throughout the polymer matrix by foaming [116]. The cellular polymers consist of two

phase i.e solid and gas. The solid phase is made of either natural or synthetic polymer which

is continuous throughout the polymeric system. The gas exits inside the cells present in the

polymer. Microcellular foams are divided into two types open cell and closed cell depending

on the connectivity of the cell. A continous interconnectivity of the cells results in open cell

structure whereas discontinuous cells containing gas independent of other cells results in

closed cell structure [115,117-119]. The nomenclature and classification of cellular polymers

are done on the basis of properties of the base polymer, cellular structure, method of

manufacture and some combination of these factors [120].

Cellular rubbers are classified basing on the method of manufacturing by ASTM [121-

123].Generally the cellular rubbers are the cellular materials that has rubber matrix consisting

a mass of cells. Cellular rubbers are mainly classified as foam, sponge and expanded rubber.

The starting material for the foam rubber is liquid whereas for the open or closed cell sponge

and expanded rubber, starting material is solid. The expanded rubber is differentiated from the

sponge rubber based on the fact that expanded rubber has substantially closed cell structure

but sponge rubber consists open or inter communicating structure as a result of production

method for sponge.

Microcellular foams are the cellular structures found in polymer matrix or polymer

composites by adding blowing agent and sometimes other additives such as nucleating agent


19

to the polymer. Microcellular polymers are characterized by their cell size which is much

smaller than that observed in conventional foam. The cell size of conventional foam is 50 to

100 m whereas that of the microcellular polymers is 1-10 m [124-126]. Microcellular

polymeric foams cell densities more than 109 cells per unit volume. Nam Suh [127] was the

first who proposed to introduce small bubbles in solid polymers. The apparent density of the

foamed material decreases due to formation of huge number of cells throughout the polymer

matrix by foaming [116]. Polymeric microcellular foams are widely used for various

application due to their light weight possessing high strength to weight ratio and cost

effectiveness [128].Microcellular foams compared to conventional polymeric foams offer

increased toughness, higher impact strength, and longer fatigue life [129-132].Not only

mechanical property but also thermal, acoustical, or optical properties of the polymeric foams

can be improved by using appropriate additives, blowing agents, and processing conditions

which tailors the foam morphology. The different steps of foaming process such as the cell

nucleation, the cell growth, and the cell coarsening highly affect the cell morphology. Though

most of the properties of base polymers are retained by introducing foaming, the physical and

mechanical properties of the cellular material differs significantly than that of the base

polymer due to large volume expansion.Synthetic cellular polymers that are widely used

structurally for cushioning, in insulation, and in systems for absorbing energy from impacts.

Foams can be classified based on bulk foam densities into four categories: High density foams

have expansion ratio ( <4 fold), that of medium density foam lies between (4 to 10 fold) and

low density foams between (10-40 fold) and very low density foams between ( >40 fold).

Low-density foams are primarily used in cushion packaging whereas the high-density foams

are used for structural purposes such as wires and cables [133]. The disadvantage is the

relatively expensive production process. Microcellular structures have been developed in

amorphous [134,135] and semi-crystalline [134, 135] thermoplastics, as well as in elastomers

[135].

1.3.1 Microcellular elastomeric composite foam

Cellular polymers have been widely used for various applications for commercial purposes

since the 1940s because of their unique characteristics such as light weight, buoyancy,

cushioning performances, thermal and acoustic insulation, impact damping and cost

reduction. In literature most of the studies are based on thermoplastic polymer based

microcellular foams whereas less attention has been given to thermoplastic elastomer (TPEs)

[136-138]. Commercially available rubber based polymeric foams are produced from


20

synthetic polymers or rubbers such as ethylene vinyl acetate(EVA) copolymer, natural

rubber( NR), ethylene propylene diene (EPDM) terpolymer, acrylonitrile butadiene rubber

(NBR), polychloroprene rubber(CR) and acrylonitrile butadiene-polyvinyl chloride blend

(NBR/PVC)[139]. Polyolefin foams are used effectively in the area of packaging, automotive,

building and construction, medical, sports, marine and leisure markets due to their unique and

diversified properties like light weight, inertness, chemical resistance, buoyancy, good aging,

recyclability, cushioning performance, thermal and acoustic insulation [140]. Now a day low

and high density polyethylene, polypropylene, linear low-density polyethylene and

ethylene/vinyl acetate copolymers are used as base materials for commercial polyolefin

foams. Rigid fillers or short glass fibres are added to provide reinforcement to the polymeric

foams. The presence of reinforcing fillers such as carbon black, silica and silicate in the

polymeric foam affect the cell growth process and change the cell geometry and physical

properties the foam. Many rubber products use carbon black as a major component which

provides reinforcement and modifies the physical properties. Kim and co-workers [141] and

Le and Choi [142] studied the foaming characteristics and physical properties of natural

rubber foams filled with carbon black. Final foam density is affected by vulcanization

condition and compounding technique. Cell size, shape, density and modulus of the base

polymer characterize the foam [143].Good physical properties and optimal foam expansion

are obtained by the optimal crosslinking due to the loading of fillers. Recent studies have

demonstrated the feasibility of developing microcellular structure in graphene and wood fibre

polymer composites [144, 145].

1.3.2 Foaming of polymer composites

1.3.2.1 Fundamentals of processes of foaming

Mechanical, physical or chemical means are followed for foaming of polymeric materials.

The polymeric foam production system involves dispersion of a gas throughout the fluid

polymer phase and the resultant foam is stabilized. Generally expanded foam is obtained by

increasing the bubble size before stabilizing the system. Cellular polymers are produced by

the most common method in which the gas bubbles are entrapped in the polymer matrix by

mechanical whipping of gases into the polymer system (melt, solution or suspension) and

making it hard by heat or catalytic action or by both [146].These are also produced by

leaching out solid or liquid materials dispersed in the polymer matrix. The expansion process


21

is investigated by many researchers [147-152] and described in three steps such as formation

of small cells or discontinuities in the polymer matrix, cell growth to the desired volume and

stabilization of the cells by chemical or physical means. The gas dissolves in the polymer

phase under pressure and released out of the fluid polymer by reducing the pressure. Gases

may be produced by the decomposition of chemical blowing agent following a chemical

reaction.

Unlike the latex foam rubber, which is produced from latex, cellular elastomers are produced

by the expansion of the rubber stock. It is prepared by incorporating the vulcanizing agents

and a mainly nitrogen or carbon dioxide producing blowing agent, to the solid rubber. The

general procedure which is applied for production of cellular rubber is available in literature

[153-155].The uncured elastomer is compounded with the decomposable blowing agent,

vulcanizating agent and other additives at a temperature below the decomposition temperature

of the blowing agent. Increase in temperature reduces viscosity of the mix till the

vulcanization starts at a particular temperature and on further heating viscosity increases due

to formation of cross-links. Open cells are formed by introducing the blowing agent that

decomposes just before the achieving the minimum viscosity, as a result the cells expand

rapidly and membranes rupture before cross-linking stabilizes them. Closed cells are formed

by using the blowing agent that decomposes after the viscosity begins to rise; consequently

the cell membranes get sufficiently cured and do not rupture with cell expansion.

Several works on cell nucleation and growth in elastomer are available in literature [156-158].

Supersaturating the elastomer with an inert gas to a considerable extent, the gas may release

out of the solution internally as bubbles. The nature of the dissolved gas and the physical

properties of the polymer and the degree of supersaturation controls the quantity of bubbles

formed. Gent and Tompkins [156,157] have reported the formation and growth of gas bubbles

in cross-linked elastomers applying low degree of supersaturation. They noticed that bubble

formation is facilitated at critical conditions at most of the cases. They assumed that the gas

bubbles are originated from submicroscopic bubbles of air trapped during processing of

rubber or from the surfaces of badly wetted particles of dirt or dust. The nucleation and

growth of bubbles in several elastomers was studied by Stewart [158] using argon as the

dissolving gas at a variety of foaming condition. In this work the number of bubbles formed

under highly supersaturated condition can be calculated based on the combination of theory of

homogeneous bubble nucleation with an expression for the growth rate of bubbles in cross-


22

linked elastomers in an appropriate manner. Several authors have reported preparation of open

and closed cell cellular rubber from Styrene butadiene rubber (SBR), Natural rubber (NR),

Nitrile rubber (NBR), Polycloroprene (CR), Chlorosulphonated polyethelene (CSM),

Ethylene propylene terpolymers (EPDM), Polyacrylate, Butyl rubbers [153,159].

1.3.2.2 Microcellular foaming process

The microcellular foams are produced by batch and continuous foaming process. In the batch

foaming process blowing agent is incorporated to a polymer sample and placed in an

autoclave [160,161]. Pressure is dropped suddenly or steam heating is applied for production

of foam. Compression molding can also be applied in a batch foaming process, in case of

chemical blowing agent where as injection molding and extrusion are two typical methods

used for continuous foaming.

The mechanism for microcellular foaming consists of four basic steps [162,163]. Four basic

steps are: 1) polymer and blowing agent mixing 2) polymer/blowing agent solution formation

3) bubble nucleation; 4) cell growth and stabilization. In first step, the blowing agent is

injected into the polymer. Cells are formed and expanded due to the dissolved gas by reducing

the pressure or by steam heating. The solubility of the blowing agent inside the polymer

determines two parameters: i) the viscosity reduction of the polymer because of the

plasticizing effect and ii) the maximum expansion ratio of the foams that can be obtained

theoretically. The cell is nucleated, either by reducing the pressure fast or increasing the

temperature. Once nucleated, cells start to grow and stabilize. During this process, the

blowing agent diffuses from the polymer into the cells to support continuous expansion of the

bubble. Simultaneously, the blowing agent will also leak out of the foam, without expanding

the foam. In this case, a blowing agent with a lower diffusion coefficient facilitates the

production of foams with larger expansion ratio. When bubbles continue to grow, adjacent

cells squeeze the polymer walls between each other, and make the polymer wall thinner and

thinner. During this process, an extensional stress from expanding bubbles on the side is

applied on the polymer wall. The melt strength of the polymer or blowing agent is responsible

to maintain the foam structure by preventing the rupture of the bubble walls under stress and

adjacent bubble coalescence. An increase in temperature above the Tg can cause foaming.

However cell structure is again fixed by cooling.


23

Type of polymers, composition and process conditions of foaming affect the preparation

method of the microcellular polymer. Thermally induced phase separation techniques (TIPS)

are followed to produce spherical interconnected voids in thermoplastic resins with a narrow

range of pore and cell size distribution [164,165]. Microcells have both isolated and

communicating microspheres and their conglomerates which were reported by the

morphological studies of microcellular walls through SEM [119]. The microcellular foamed

plastics can be formed from both thermoplastics [166-167] and thermosets [168,169]

polymer. It has been reported by several authors that vulcanization temperature, time of

curing, blowing rate and external pressure are the important parameters controlling the

formation of microcellular structure [170-172].

Many researchers produced microcellular foams using amorphous and semi-crystalline

polymers in their solid state by the batching foaming process such as PS [173,174], PET

[175], PVC [176,177], POE [178] etc. Park and his co-workers have studied about the

development of the extrusion foaming processes using inert gases [179, 180]. Trexel has

developed the technology to produce microcellular foams in an injection molding machine

[181].

1.3.3 Blowing agents for foaming

Several compounds are used for foaming of polymer. These compounds liberate gaseous

products under heat and are called blowing agents. Blowing agent (BA) is defined as a

substance that produces a cellular structure in the polymer [182]. The blowing agents are

classified into two types: physical blowing agents (PBA) and chemical blowing agents (CBA)

[172].

1.3.3.1 Physical blowing agent in foaming

Physical blowing agents do not involve any chemical reactions for formation of gas during

foaming. Gas is formed by some physical processes such evaporation, desorption at elevated

temperatures or reduced pressures. Physical foaming agents include atmospheric gases or

volatile liquids that evaporate at certain conditions [183]. Nitrogen, CO2, argon, helium, and

short-chain (C2 to C4) aliphatic hydrocarbons and halogenated (C1 to C4) aliphatic

hydrocarbons are examples of permanent gas blowing agents [183]. Common liquid physical

blowing agents are low boiling liquids such as short-chain (C5 to C7) aliphatic hydrocarbons

and halogenated (C1 to C4) aliphatic hydrocarbons. Use of CFCs as a physical blowing agent


24

in industry declared illegal in the US since the year of 2010 according to the Montreal

Protocol due to its adverse effect on the ozone layer. CO2 and N2 gases are the two mostly

used blowing agent due to their low cost, relatively moderate critical temperature and

pressure, and environmentally friendly nature [184]. Inert gases have a relatively higher

diffusion coefficient compared with other blowing agents described previously.

1.3.3.2 Chemical Blowing Agent in Foaming

Chemical blowing agents are the compounds or mixture of compounds that releases gas

during foaming by chemical reaction associated with thermal decompositions [185,186]. A

chemical blowing agent should produce desired quantities of gas by heating to normal

vulcanizing temperatures (1200C-1700C).Besides evolution of gases it should be cheap, non-

toxic, odourless, non-discolouring, stable in storage and during processing, easy to disperse

and non reactive for the vulcanizing system. The cellular rubber manufactured using

chemical blowing agents are widely used for industrial, consumer and automotive application.

Blowing agents used are of two types based on its application i.e organic and inorganic

blowing agent. Ammonium carbonate, sodium bicarbonate etc. are examples of inorganic

blowing agents. The chemical blowing agent which evolves carbon dioxide are widely used

for producing open cell sponge. Closed cell products are manufactured using the organic

blowing agents, which produces mixture of gases predominantly nitrogen on decomposition.

Various types of organic blowing agents are used in practice which differ in their

decomposition temperature, physical appearance and chemical compositions.

Azodicarbonamide (ADC), Dinitrosopentamethylene tetramine (DNPT) and pp/-oxybis

benzene sulphonamide (OBSH) are some of the frequently used organic blowing agents.

There are two types of chemical blowing agents i.e exothermic and endothermic. Exothermic

blowing agents generate heat during the chemical reaction to produce gas, while endothermic

blowing agents require heat to react [184]. Exothermic CBAs generate heat in the

decomposition, and its major decomposition gas is N2 whereas endothermic CBAs absorb

heat in the decomposition, and its major decomposition gas is CO2. Polyurethane foams are

manufactured by using chemical foaming agents, where isocyanate group reacts with water to

produce amine and carbon dioxide (CO2) [187]. Use of chemical foaming agents is especially

beneficial for foaming of thermoset polymers such as polyurethane. It is because thermosets

cannot be processed after curing. Addition of chemicals in monomers enable foaming during


25

curing.The selection of the appropriate CBA for the plastic foaming should be done keeping

in view the decomposition temperature and processing temperature. It should be observed that

processing temperature of the polymer must not be close to decomposition temperature of the

CBA for smooth processing. If the decomposition temperature is too low, the polymer melt is

too rigid, which restricts foam expansion. If the CBA posses high decomposition temperature,

the polymer melt streangth must be more to maintain the bubble structure and prevent cell

coalescence. The residence of the CBA decomposition and the generated gases must be

compatible with the polymer and processing system [182].Another disadvantage is the

chemical residue. 4, 4�- oxybis benzene sulfonyl hydrazide (OBSH) and azodicarbonamide

(ADC) are the two frequently used exothermic blowing agents.

1.3.3.3 MICROFINE ADC-21 as blowing agent

MICROFINE ADC-21 is a specially modified form of Azodicarbonamide which is one of the

most widely used chemical blowing agent. The suitable decomposition temperature range is

155 0C - 175 0C and specially used to foam the rubber based compound [188]. It is applied to

get closed cell microcellular polymeric structure used in low density refrigeration, air

conditioning insulation and automotive door seals. It is also used for preparing precipitated

silica, CaCO3 and aluminium silicate filled ethylene-octene copolymer microcellular

composite to study the physical properties [189-191].The problem persists during

compounding when the MICROFINE ADC-21 remains insoluble in the oil or plasticizers. So

milling operation can be followed or it can be mixed with oil and plasticizers before

processing for good dispersion of the ADC-21in the polymer. It is preferable to produce light

colour product as the residue got after decomposition is whitish in colour. It burns when

exposed to flame. ADC-21 releases fumes like smoke during thermal decomposition. Its

decomposition residues possess low toxicity. Table 1.6 represents properties of MIKROFINE

ADC-21 blowing agent.


26

Table.1.6 Properties of MIKROFINE ADC-21 blowing agent [192]

Physical form Pale Yellow Powder

Chemical composition Specially modified Azodicarbonnamide

Gas composition Mostly N2 and CO2

Decomposition temperature in air(0C) 158 2

Solubility Insoluble in water

Moisture content (%) 0.2

Gas/gram 185 cc at STP

Azodicarbonamide (ADC) is one of the widely used exothermic CBA. It generates 65% N2,

32% CO2, CO and NH3.The residues include urea, biurea, cyanuric acid, urazole, and

cyamelide. The decomposition product consists of 32% gas, 27% sublimate and 41% solid

residue. The solid and sublimate residues contains 57% urazole, 38% cyanuric acid, 2%

cyamelide [193]. Its decomposition involves multiple steps of reactions, and here the first

major reaction is provided in equation

[1.6]

The thermal decomposition process of ADC foaming agent thermal is divided into three

phases, the first phase of the gas product were N2, CO, HNCO, solid residue biurea etc, the

second phase gas products were NH3, HNCO and the third phase gas products were NH3,

CO2, solid residue urazole etc[194].

1.3.4 Studies on characteristics properties of carbon black filled

microcellular ethylene-octene copolymer vulcanizates

1.3.4.1 Characterization of Cell Structure

The filler content in the microcellular polymer composite foams greatly affect the cell

morphology and foaming characteristics such as structure of cell, size of cell, cell density,


27

foam expansion ratio , relative density etc. It is reported that [195] carbon black filled

chlorinated polyethylene rubber microcellular foams have closed cell structure and increase

in carbon black content increases the cell density and decreases the foam expansion ratio and

average cell size. Density of carbon black reinforced NR foam increases with increase in

carbon black loading and foaming pressure but foaming efficiency decreases supported by the

decrease in expansion ratio [141]. The morphology of microcellular EPDM rubber

vulcanizates as a function of carbon black have already been studied [196, 197].

1.3.4.2 Rheometric characteristics

Both filler and blowing agent concentration affect the rheometric characteristics of polymer

composites such as scorch time, cure time, cure rate index, maximum and minimum torque

etc. Increase in concentration of carbon black in carbon black reinforced chlorinated

polyethylene rubber microcellular foams increases maximum torque, minimum torque, cure

rate index and decreases scorch time and optimum cure time filled chlorinated polyethylene

[195].The similar observation is reported in carbon black filled natural rubber foams[141].

1.3.4.3 Physical property

Spenadel [198], reported the physical properties of sponge rubber used in automobile for

sealing applications. The comparision between the physical properties of different sponge

rubbers and effect of fillers on foaming are reported in his study. Best overall properties are

observed in MT black filled ethylene propylene terpolymer (EPT) open cell sponge

compound compared to any other carbon black where as a closed cell network is formed by

reinforcing blacks (HAF, FEF, etc) which restrict blowing. The effect on morphology and

physical properties HAF black loading in microcellular EPDM rubber vulcanizates has been

studied by Guriya et. Al [199].

The physical properties of microcellular vulcanizates include tensile strength, elongation at

break, tear strength and modulos which are affected by reinforcing carbon black and foaming

pressure and temperature. It is reported [141] that increase in carbon black loading in natural

rubber foamed vulcanizate increases the tensile strength and tear strength whereas increase in

pressure has little effect. Moreover the hardness increases and elongation at break decreases

with increase in carbon black content. Carbon black reinforced chlorinated polyethylene

rubber foam shows increase in tensile strength, tear strength and decrease in elongation at


28

break with increase in carbon black loading [193].Thus carbon black increases the reinforcing

effect, crosslink density and viscosity in the foamed vulcanizate.

1.3.4.4 Dielectric property

The dielectric characteristics of microcellular vulcanizates such as dielectric permittivity,

dielectric loss tangent, ac conductivity, real and complex impedance and percolation threshold

are studied as function of frequency, blowing agent, filler and temperature by dielectric

spectroscopy. Closed cell microcellular EPDM has immense scope for application as

microwave dielectric materials for packaging material in radio and electronic engineering,

aviation and space applications.

Several researchers have studied the dielectric properties of microcellular vulcanizates at wide

range of frequency in recent years [200, 201]. It is also observed that increase in blowing

agent reduces the dielectric permittivity of Vulcan XC 72 carbon black reinforced EPDM

microcellular vulcanizates where as carbon black loading increases the dielectric loss tangent

making the polymer more nonlinear in nature [202].It is reported that [203] the dielectric

permittivity, loss tangent, ac conductivity increases with increase in Ensaco 350G carbon

black concentration in EPDM microcellular vulcanizates and the semicircular nature of the

Nyquist plot reduces .The percolation was found to be at 40 phr carbon black loading.With

increase in blowing agent loading the ac conductivity, dielectric loss tangent increases and the

semicircular nature reduces. In case of carbon fibre reinforced polypropylene composite foam

the dielectric permittivity increases significantly at lower frequency with increase in CF

content when the percolation is achieved.

1.3.4.5 Thermal conductivity

Foam is primarily applied for thermal insulation. Low-ambient temperature range is suitable

for the microcellular polymers to be used in thermal insulation. They are used for commercial,

industrial and domestic purpose due the thermal insulation property. Heat conduction occurs

in foams through the three methods such as conduction, convention and radiation

simultaneously. The heat convection does not occur when the cell size is smaller than 3mm

[204].


29

Cellular polymeric materials are the excellent thermal insulators but addition of carbon black

fillers make it able for thermal conduction. The studies on thermal conductivity of carbon

black reinforced polymeric foams are very few in literature. It is reported that a closed cell

rigid polymer foam containing at least about 2 % by weight of carbon black as filler , which

reduces the aged k-factor of the foam to below the aged k-factor of the corresponding unfilled

foam [205]. Also 2 wt % carbon black filled polyisocyanate and an isocyanate compound

has aged k-factor less than that of unfilled foam [206].

1.3.5 Ethylene octene copolymer based microcellular foamEthylene octene copolymer based microcellular foams have been researched less frequently

upto now [189-191, 207-209].The effect of precipitated silica,CaCO3, aluminium silicate and

blowing agent on the morphology and physical properties of the microcellular ethylene octene

copolymer foams has been studied earlier which reports that filler acts as a nucleating agent

and increases the cell density and blowing agent reduces the average cell size, relative

density, and physical properties such as tensile strength, elongation at break and

modulus[189-191].The effect of CaCO3, precipitated silica and aluminum silicate on the

deformation and energy absorption characteristics, rheological behaviour and thermal

conductivity of closed cell ethylene octene microcellular foam has also been studied [207-

209].

1.4 Research objective

The research has two main objectives and each having some specific objectives.

(I) The first objective is to fabricate carbon black (Ensaco 250G) reinforced EOC vulcanizates

and

(i)To study the enhancement of the cure characteristics and physico mechanical properties of

EOC/ CB polymer vulcanizates with variation of carbon black loading.

(ii) To study the sorption properties of EOC/CB polymer vulcanizates with variation of

carbon black loading and temperature by conducting sorption experiment for industrial

application.

(iii) To study the effect of conductive CB loading on dielectric properties of EOC/ CB

vulcanizates with variation of frequency and temperature by using dielectric relaxation

spectroscopy.

(II)The second objective is to fabricate carbon black filled microcellular EOC vulcanizates


30

(i) To study the cure characteristics and physico-mechanical behaviour of microcellular

ethylene octene copolymer/CB vulcanizates with variation of carbon black and blowing agent

loading.

(ii) To study the dielectric properties of microcellular ethylene octene copolymer/CB

vulcanizates with variation of carbon black and blowing agent loading over a wide range of

frequency and temperature by using dielectric relaxation spectroscopy.

(iii) To study the thermal conductivity of both unfoamed and microcellular ethylene-octene

copolymer foamed vulcanizates with variation of blowing agent, filler and temperature.

1.5 Organization of the thesis

This section provides an organization and overview of the present thesis.

Chapter 1 introduces the materials relevant to this work. It describes the background of

polymer composite, microcellular foaming of composite and the mechanism of cell nucleation

and growth in foam processing. It gives a literature review of foaming theory, describes and

examines the relevant existing studies. Chapter 2 describes the experimental methods

followed for the study of the mechanical, electrical, thermal and sorption property of EOC/

CB vulcanizates and their microcellular foam. Chapter 3 provides the effect of carbon black

loading on physico mechanical properties of EOC/ CB polymer vulcanizates. Chapter 4

provides the effect of carbon black loading and temperature on diffusion coefficient, sorption

coefficient, permeability coefficient, thermodynamic parameters related to sorption

phenomenon by EOC/CB polymer vulcanizates with three organic solvents. Chapter 5

presents the effect of conductive carbon black loading and temperature on dielectric

properties such as dielectric permittivity, tan , ac conductivity, and percolation threshold of

EOC / CB vulcanizates in the frequency range of 100 to 5MHz. Chapter 6 presents the

physico-mechanical behaviour of carbon black filled microcellular ethylene octene copolymer

vulcanizates with variation of filler and blowing agent loading. Chapter 7 describes the

variation of dielectric properties such as dielectric permittivity, ac conductivity, tan and

percolation threshold with carbon black and blowing agent loading in the EOC/CB

vulcanizates over a wide range of temperature by using dielectric relaxation spectroscopy.

Chapter 8 presents the thermal conductivity of both unfoamed and microcellular ethylene-

octene copolymer vulcanizates with variation of CB and blowing agent and temperature.

Chapter 9 represents conclusions of the present work as well as future scope.

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