Chapter I Introduction -...

Chapter I

Introduct ion

Chapter I - Introduction

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1.1 Introduction about Polymer and their classification:

Polymers are high molecular weight organic substances that are synthesized from

low molecular weight compounds by the process of polymerization, using addition

reaction or condensation reaction. In addition polymerization, the reaction is initiated by

a free radical which is usually formed due to the decomposition of a relatively unstable

component in the reacting species. In this reaction, repeating units add one at a time to

the radical chain, and reasonably high molecular weight polymers can be formed in a

short time by this polymerization. In condensation polymerization, the reaction takes

place between two polyfunctional molecules to produce one larger polyfunctional

molecule with the possible elimination of a small molecule such as water. Longer

reaction times are essential for forming high molecular weight polymers by this step

reaction. An elementary introduction to polymers is given here and more knowledge

about the physics, chemistry and engineering aspects of polymers are available in some

of the standard references [1-13]

on the subject.

Classification of polymers: Polymers formed through the polymerization processes

discussed above can be classified in a number of different ways based on certain chosen

characteristics for comparison.

Polymers are compounds formed by a more or less regular repetition of a large

number of same and different atomic groupings that are joined by chemical bonds into

long chains. e.g. A-A-AB-B-A-B-B

According to composition of polymers may be classified as

(a) Organic polymers e.g. Polyethylene

(b) Inorganic polymers e.g. Insoluble Sulphur

(c) Elemental organic – Polyurethanes, Polyesters & Polyamide

These polymers are further classified as :-

(i) Homopolymers containing one monomer in polymeric chain.

E.g. Homopolymers : -A-A-A-A-A Polyethylene, Polystyrene etc


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(ii) Copolymers containing two monomers

E.g : this type of polymers have pattern like : Styrene – Butadine rubber

In terms of configuration :-

(i) Linear

(ii) Branch

(iii) Cross linked/three dimensional polymers

In terms of configuration they are classified as :-

(i) Random polymer : -A-B-A-B-B-B-B-A-B-B-A-A-

(ii) Block Co-polymer : -AAA-BBB-AAA-BBB-

(iii) Graft Co-polymer : -

'Processing of polymer' is an important branch of polymer chemistry and

technology. Polymer processing is an engineering specialty which is used to convert

polymeric materials into useful products. In fact, polymer-processing technology makes

possible to convert refined and finer form of conventional materials.

A large number of synthetic polymers now exist which cover a wide range of

properties. These can be grouped into three major classes and they are briefed below.

(a) Plastics

(b) Fibres

(c) Elastomers

PLASTICS: Plastics are polymeric material in which a stress produces a

non-reversible strain. Plastic is defined as an organic polymer which is capable of

changing its shape on the application of a force and retaining this shape on removal of

this force. The plastic is a Greek word, which means a material that can be moulded and

formed into any shape of choice. Plastic is a synthetic polymer, which can be converted

into complex shapes by the application of heat and pressure. It is sub-divided into

Thermoset and Thermoplastics.


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The thermoset materials become permanently hard when they are heated above

the critical temperature and will not soften again on reheating. These are generally cross-

linked in this state. The thermoset plastics are more superior and have dimensional

stability characteristics as compared to the thermoplastics which have better impact

properties. The thermoset materials are changed irreversibly from fusible, soluble

products into highly intractable cross-linked resins which cannot be moulded by flow.

A thermoplastic polymer will soften when heated above the glass transition temperature,

Tg. It can then be shaped and on cooling will harden in this form. On heating it will

soften again and can be reshaped if required before hardening when the temperature

drops. This cycle can be carried out repeatedly. The changes that occur during this

process are physical rather than chemical and hence products formed from such polymers

can be re-melted and reprocessed. In actual sense, the plastic materials have properties in

between that of elastomers and fibre-forming polymers. They have good rigidity and

tensile strength. Plastics can either be fully amorphous or partially crystalline in nature.

A fully amorphous polymer will be a plastic material, when its glass transition

temperature (Tg) is above the 'use' temperature. Polystyrene with Tg= 100°C is a good

example. Amorphous plastics are brittle and exist in the rubbery state. Partially

crystalline polymers are tough plastics above Tg and below melting temperature, Tm.

Suitable example of this type of plastic is polyethylene. The drawback of fully

amorphous plastic is its brittleness. It can be removed by adding some elastomers during

the reaction. If a small percentage of butadiene rubber is dissolved in styrene monomer at

the time of polymerization, the polystyrene is formed. This product contains reduced

brittleness with improved strength and impact properties. Such polymer is known as 'high

impact polystyrene'. Cross-linking of plastic materials improves the structural rigidity,

resistance to swelling by solvents and also increases the upper range of the 'use' temperature.

Generally, the thermoplastic materials are uncross-linked and the thermoset plastics are

cross-linked systems. For example, phenol-formaldehyde, urea-formaldehyde and epoxy

systems involve cross-linking, while polyethylene and polystyrene are uncross-linked

systems. Thus, the cross-linking or plasticization is required when (i) the Tm is low

(ii) the material can be melted and moulded without decomposition (iii) the uncross-

linked material itself can show the properties of the end product. If the Tg is very high,


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e.g., as in PVC, which consists of too high Tm, 310°C. The processing of PVC is very

difficult, therefore, its flexibility and softness can be improved by adding plasticisers to it.

Consequently, the improved PVC is used to make the various products ranging from rigid

tubes to soft flexible toys.

FIBERS: When polymer is drawn into long filament-like materials, whose length is at

least 100 times its diameter, polymers are said to have been converted into 'fibres'. In

other words, a fibre is a polymer with a very high length to diameter ratio (at least 100: 1),

and most of the polymers, capable of being melted or dissolved, can be drawn into

filaments. The examples of natural fibres are cotton, flex, silk and wool, while nylon,

polyester, acrylics and polypropylene are manmade or synthetic fibres. Fibre-Forming

Polymers: Although there are various fibre-forming polymers, but only a limited number

have achieved great technological and commercial success. The polyamides are an

important group which contain natural proteins and synthetic nylons. The term nylon,

originally a trade name, which has now become a generic term for the synthetic

polyamides. Numerals in nylon-6, 6 is the number of carbon atoms present between

amide groups in the chain. Nylon-6, 10 is prepared from two monomers and has the

structure as:

[-NH)-(CH2)6-NHCO-(CH2) 8-CO-]n

Such structure consists of an alternative sequence of six and ten carbon atoms between

the nitrogen atoms. Nylon-6 is synthesized from one monomer and has the repeat formula

[- NH(CH2) 5 – CO -]n With regular sequence of six carbon atoms between the nitrogen

atoms. A nylon with two numbers is termed dyadic. It indicates that it possesses both,

dibasic acid and diamine moieties. Here, the first number represents the diamine and

second the diacid used in the synthesis. The monadic nylons contain one number,

indicating that synthesis involves only one type of monomer. Such terminology reveals

that a poly (a-amino acid) would be nylon-2. If a filament of fibre-polymer is tried to be

pull, it can be noticed that unlike a rubber band, this filament does not stretch. When the

force is released, the same filament neither get deformed, nor elongated or cracked. Thus

it is noticed that fibre-forming materials possess following characteristic properties:


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1. They exhibit high tensile strength and very high rigidity or stiffness.

2. Fibre-forming materials undergo irreversible deformation, i.e., after releasing the

force, the filament just retains its slightly elongated shape and does not go back to

its original position.

3. These materials have a very high crystalliniity. In other words, they are packed

very close to each other in a highly ordered manner and are held together by

strong inter chain cohesion forces which resist deformation and do not allow any

relative movement between the chains.

4. The molecules of these materials have a very high degree of polymerization.

5. The melting temperature (Tm) of these materials are much above the 'use

temperature.

6. Glass transition temperature (Tg) of fibre is also much higher than its 'use'

temperature, so that if some amorphous components are present in material, they

may not exhibit segmental mobility.

7. A good temperature range of Tg and Tm is between 200 - 300°C.

ELASTOMERS / Rubbers: Elastomers are popularly known as 'rubbers'. The first rubber

industry was found when the naturally occurring product 'latex' was isolated from a tree

Hevea Brasiliensis. It was first used by American Indians and was called caout chouc

from the Indian name. Later on, Priestley discovered its properties and found that this

material rubbed out pencil marks. After this discovery, the product is simply known as '

rubber'. From the 20th

century, scientists have been trying to synthesize the materials

whose properties could stimulate the natural rubber. This has led to the production of a

wide variety of synthetic elastomers. Now, a large number of synthetic elastomers are

available. But natural rubber still consists of the excellent balanced combination of

desirable qualities therefore; it is regarded as 'standard elastomer'. Synthetic Poly Isoprene

rubber still does not match with naturally occurring NR rubber. The balloons, tyres, belts,

hoses, gaskets, bushes, mounts shoe soles, surgical gloves are a few examples of articles

made of elastomers. The most important synthetic elastomer is styrene-butadiene (SBR),

which accounts for 35 % of the world market in elastomers. There are several other


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synthetic polymers now in use. These include polybutadienes (BR), styrene-butadiene,

(SBR), Acrylonitrile butadiene (nitrile rubber, a copolymer - NBR), Polyisoprene (IR),

Polychloroprene (CR) (Trade name neoprene), ethylene-propylene copolymer (EPR),

silicone rubber (Q), polyurethane(PU), polyfluorocarbon elastomer (FKM) [Trade name

Viton], isoprene-isobutylene copolymer (Butyl Rubber – IIR) etc.

A large number of polymers now available, which possess rubber-like behaviour

contains various significant characteristics as listed below:

(a) The materials are having glass transition temperature, well above room

temperature.

(b) They have the ability to stretch and retract.

(c) They possess high strength and modulus when stretched.

(d) The elastomers consist of low or negligible crystalline content.

(e) The molar mass of polymers is large enough for network formation.

(f) They must be rapidly cross-linked/ should have cross linking site.

For the elastomers, the most important factor is the Tg as it determines the range

of temperature where elastomeric behaviour is important and defines its lower limiting

temperature. Hence if the Tg of elastomer is well below the ambient temperature, it is a

good polymer for elastomeric application. The cross-linking process in elastomers is

known as vulcanization. After this process, the resultant polymer is a network of

interlinked molecules which is capable of maintaining an equilibrium tension i.e. stress

strain properties. The cross-linking is the most important feature which changes the

properties of an elastomer to a marked degree and extends the usefulness of the polymer

as a material.

Rubbers are classified based on property that they possess and they are commonly

called in industry as given below with examples:

Unsaturated rubbers:- NR, SBR, BR, IR etc.

Saturated Rubbers :- IIR, EPR, etc.

Heat Resistance Rubber:- Viton, FFKM, Fluorosilicone Rubber, etc.


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Oil Resistance Rubber :- NBR, CR, ACM etc.

Heat and Oil Resistance Rubber:- FKM,HNBR etc

Oil Additive Resistance Rubber:- CSM, AEM, ACM etc

Impermeable Rubber: IIR etc.

Rubbers are Classified based on structure as given below :

R group: Carbon main chain with unsaturated units e.g- NR, BR , SBR etc

--C—C=C—C--

M group: Carbon main chain with saturated units only e,g EPDM, ACM etc.

---C---C---C---C---C----

O group: Main chain with carbon and oxygen e.g CO

Q group: Siloxane main chain e.g MQ

T group: Main chain with carbon and sulphur e.g Polysulphide

U group: Main chain with carbon, nitrogen, and oxygen e.g AU /EU

The Table 1.1 shows some of the commonly used Rubbers with their

abbreviations, density, glass transition temperature and maximum service temperature

which can be used for selection of rubbers for a suitable application in a specified

conditions.


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Table 1.1: Various Rubbers and their characteristics

S.No. Common Names of

rubbers Abbreviations

Density,

g/cc Tg ° C

Maximum

Service

temp.° C

1 Ethylene-propylene

rubber

EPR 0.87 -55 149

2 Ethylene-propylene

Diene

EPDM 0.86 -55 125

3 Poly Butadiene BR 0.90 -85 70

4 Natural Rubber NR 0.92 -75 70

5 Synthetic Poly

isoprene

IR 0.92 -75 70

6 Isobutylene isoprene

rubber (Butyl

Rubber)

IIR 0.92 -65 100

7 Styrene butadiene

rubber

SBR 0.93 -55 70

8 Acrylonitrile

Butadiene Rubber

[Nitrile Rubber]

NBR 0.98 -40 100

9. Polychloroprene

Rubber

CR 1.23 -50 120

10. Polyacrylate Rubber ACM, AEM 1.18 -40 150

11. Silicone Rubber MQ 0.98 -110 225

12. Chlorosulphonated

Polyethylene

CSM 1.18 -50 125

13. Chlorinated

Polyethylene

CM 1.18 -40 150

14. Fluorocarbon

Rubber

FKM, FFKM 1.82 -30 250


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Based on heat and oil resistance properties, the elastomers are chosen out for their

application. Fig. 1.1 gives a general idea about the classification of elastomers based on

their heat and oil resistance properties:

Fig. 1.1: Chart on classification of elastomers based on their heat and oil resistance.

1.2 Rubber Compounding:

Rubber products are made out of rubber compounds which are made by mixing

selected rubber with different compounding ingredients and the same are added in

different proportion to have optimum set of properties. Ingredients used in compounding

to obtain the desired processing characteristics ultimate properties of the finished product

or cost control may be classified as follows:

Base Polymers-- (Natural or synthetic)

Curing agents (Sulfur, Organic Vulcanizing Agents] peroxides,

Secondary Accelerators

Accelerator activators and Retarders


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Stabilizer (antioxidants, anti-ozonants, protective wear)

Processing aids, plasticizers, softeners, tackifiers

Reinforcing fillers and resins inert fillers and diluents

Special purpose additives (Flame retardants, colour, blowing agents, deodorants etc.)

New special additives like nanofillers

The roles of each type of such compounding ingredients used in this research

work are explained below for more information. vulcanizing agents, Metallic oxides)

Compounding ingredients like carbon black, fumed silica, clay, calcium carbonate,

metal oxides etc are often used for compounding with thermoplastic polymers and

elastomers in order to improve their mechanical, thermal or other properties. Elastomeric

materials require reinforcement for application in their actual service condition, as they

lack in strength properties.

Mostly carbon black and high structure silica and sometimes few fibers have been

used as fillers & are found to improve properties of elastomeric composites5-6

.

Fillers: The name filler is given widely to almost all materials, which are added in

sufficiently large quantities to reduce the amount of rubber needed with or without the

improvement of rubber products. Filler is the ingredient used to reinforce physical

properties to impart certain processing characteristics or to reduce cost. A reinforcing

filler is a filler that improves the physical and failure properties (tensile strength, tear

strength and abrasion resistance) of the final vulcanizate. Reinforcing filler will enhance

the hardness, tensile strength, modulus, tear strength and abrasion resistance of a

compound. Selection of filler is the third most important task in compounding after the

elastomer and curing agent. Filler will influence physical and processing properties of a

compound. Degree of reinforcement of fillers will increase with the decrease in filler

particle size. The finer fillers require more energy for their dispersion into the elastomer

and therefore is more difficult to process. The particle size of filler plays a major role in

tensile strength of rubber compound and Mooney scorch.


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There are two types of fillers used in the rubber industry which classified as

1. Black filler

2. Non-black filler.

Example of black filler is carbon black while for non-black filler are silica, clay and

titanium dioxide. In tyre manufacturing, carbon black and silica are widely used. The

addition of active fillers to a rubber matrix leads to a considerably high reinforcement.

This greater reinforcement manifests itself in form of rise in the modulus leading among

all other things to a higher strain value, greater tensile strength and lower abrasion.

(i) Carbon Black –Carbon black is a colloidal form of element carbon consisting of 90 to

99 percent of carbon. The main non-carbon elements in carbon black are oxygen,

hydrogen and sulfur. Carbon black can be obtained commercially. Carbon black is

classified according to their particle size, surface activity, porosity and structure. These

parameters affect the properties of a rubber compound. For example, the smaller the

particle size, the greater is the increase in modulus, tensile property, abrasion resistance,

viscosity and electrical conductivity. It reinforces the rubber compound besides lowering

the compounding cost. Reinforcement of rubber with carbon black improves the rubber

properties due to the combination of physical and chemical interactions between carbon

black and rubber. Modulus is a primarily a function of carbon black structure and loading.

Compound containing high structure blacks have highest modulus. Mooney viscosity in

most elastomers is mainly dependent on carbon black structure. High structure black

contributes to the highest Mooney viscosity with particle size having a lesser effect. The

largest particle size black provides the greatest scorch resistance while the high structure

small particle size black usually reduces the scorch resistance.

Different types of Carbon black based on manufacturing are given below:

1. Lamp blacks

2. Channel blacks

3. Gas furnace.

4. Thermal.


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5. Acetylene.

6. Oil furnace.

Characteristics of Nature of Carbon black:

i) Carbon Blacks are essentially elemental carbon and are composed of

aggregated particles.

ii) Particles are partly graphitic in structure and colloidal in dimensions.

iii) The carbon atoms in the particle are in layer planes.

iv) The particle range is from 10 nm to 400 nm.

Carbon black is a material that has been known and produced since olden days but

only found its widespread manufacture and use in the last century when it was discovered

that when mixed into rubber it improves its mechanical properties. The increase in

strength of the rubber containing carbon black led to many practical applications of the

rubber throughout the world. The ability of the tyres to last longer is primarily due to the

strength imparted to rubber from carbon black. Carbon black refers to a group of

industrial product consisting of furnace black, channel black, thermal black and

lampblack. These are materials composed essentially of elemental carbon in form of

near-spherical particles of colloidal size combined mainly into particle aggregates

obtained by partial ignition or thermal decomposition of hydrocarbons.

Fig. 1.2 Nano structured Carbon Black – aggregates in rubber matrix

Furnace black is made in a furnace by partial ignition of hydrocarbons. Thermal

black and acetylene blacks are produced by thermal decomposition of natural gas and


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acetylene respectively. Channel black is manufactured by impingement of natural gas flames

on channel irons. Lampblack is made by burning hydrocarbons in open shallow pans. Only

the furnace and thermal decomposition process is significant and commercially important.

Carbon black classification: A classification system is used to classify rubber

grade carbon blacks by use of a four- character nomenclature system. The first character

in the nomenclature system for rubber-grade carbon blacks is the lettering, indicating the

effect of the carbon blacks on the cure rate of a typical rubber compound containing the

carbon black. The letter ‗„N”(Normal) is used to indicate a normal curing rate typical of

furnace black and ‗„S„„ (Slow) is used for channel black or for furnace black that have

been modified to effectively reduce the curing rate of rubber. The second character is a

digit to designate the average surface area of the carbon black as measured by nitrogen

surface area. The surface area range of the carbon black has been divided into ten

arbitrarily groups. The third and fourth character in this system are arbitrarily assigned

digits. It is to be noted that currently no ASTM grades are covered. Indeed, some main

carbon black manufacturers have been proposing grades on an experimental or commercial

basis. Carbon black is generally incorporated into rubber by shear experienced in an open

mill or banbury mixer. It is postulated that during the first stage of incorporation, carbon

black agglomerates becomes encapsulated by polymer. In the next stage of incorporation

the rubber is being forced through the channel between aggregates and agglomerates to

form a reinforced rubbery composite. The consequence of this incorporation of carbon

black into rubber is the creation of an interface whose total interfacial area and ability to

have interaction between the materials depends on the carbon black loading, structure,

specific surface area and dispersion obtained through the mixing process. The processing

properties of rubber are very important in industry. The incorporation of carbon black

into rubber greatly influences the properties of uncured rubber. Carbon black is known to

change the flow and viscosity of an uncured rubber compound significantly. The

hydrodynamic effect due to the presence of carbon black reduces the volume fraction of the

flow medium causing shear strain amplification when the compound is forced to flow, thus

increasing the viscosity. It is important that the asymmetric aggregate of carbon black

increases the flow resistance and it is known that as the structure is increased to even higher


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level then the viscosity increases. Filler-filler networking is also a factor that must be

overcome when the compound flows which will again increase viscosity.

The effects of different carbon black on rubber properties are dominated by the

carbon black its specific area and structure. In general, higher the surface area of carbon

black, impact higher levels of reinforcement with resulting higher hysteresis. Higher

structure generally gives improved extrusion behavior, higher compound modulus and

higher compound viscosity.

0.1% 100 %

Fig. 1.3 A qualitative interpretation of the Payne effect of filler.

The rubber properties are also influenced by the amount of carbon black in the

rubber compound. Some rubber properties like tensile strength, abrasion resistance are

increased with the increasing loading of carbon black to an optimum and then they

decrease.

Non-black fillers: This includes precipitated silica, clay and calcium carbonate. Among

the fillers, precipitated silica is widely used in rubber products especially in tyre. Silica is

used in the form of a synthetic, amorphous silicon dioxide (SiO2). Silica has hydroxyl

groups on its surface, it has resulted in a strong filler–filler interactions and adsorption of

polar materials by hydrogen bonds. Moreover, the surfaces of silica are polar and

hydrophilic; there is a strong tendency to adsorb moisture which adversely influences

cure and therefore, properties of vulcanized rubbers. Silica can be classified into two

types which based on the amount of water removed by ignition they are: i) hydrated and

ii) anhydrous type.


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Silica is added to rubber as reinforcing filler; where it improves the tensile

strength, hardness, tear strength and abrasion resistance. Fine particle silica blend with

carbon black is finding an increased use in commercial tyres. One reason is that silica

filled vulcanizate shows a low hysterisis in comparison with carbon black. Besides that,

some resulting trends exhibit a useful combination of resistance to abrasion, cut growth,

tearing, chipping, crack initiation and skidding. The fillers are primarily classified as

carbon black and light colour fillers. Chemical composition is primarily the basis for

classification of the light colour fillers. With each class of fillers, different degree of

activity is present. Basically, most carbon black, colloidal silica and most smallest

particle size silicates belong to the high and medium activity fillers, while chalk belongs

to inactive fillers.

In general, fillers with the smallest particle size or highest surface contribute to

the greatest degree of reinforcement in a rubber compound. The first fillers used in rubber

products were naturally occurring minerals that were readily available. They are zinc

oxide, clay, mica and asbestos that were added to natural rubber to reduce tack, increase

hardness and reduce the cost of the compound. Additional needs for reinforcing non-

black fillers generated the usage of calcium silicates, fumed silica, precipitated silica,

silicate and a variety of silane-modified products.

Non-reinforcing fillers such as china clay, calcium carbonate (whiting), barites

(barium sulphate), mica, titanium dioxide etc are added in polymers for cost reduction

purposes. However, presence of such non reinforcing fillers in the polymer matrix affects

the strength and key functional properties of the product. Filler size, shape, aspect ratio

and filler-matrix interactions are decisive factors in determining the effect of fillers on

properties of filled composites. In general, composite materials are formed when at least

two distinctly dissimilar materials are mixed to form a monolith9. The overall properties

of a composite material are determined not only by the basic components, but also by the

composite phase morphology and interfacial properties.

Silica classification: There are three categories of commercially available

synthetic silica and they are precipitated, fumed and surface treated silica. The

precipitated silica is by far the most common variety for general rubber usage and is


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commonly called „silica‟ as a generic term. Precipitated silica is made by acid

precipitation of sodium silicate. The sodium silicate solution is usually produced by

combining high purity sand and sodium carbonate in a high temperature furnace, then

dissolving the molten glass in water. Fumed silica is used primarily in silicone rubber and

compounds with special polymers or cure systems. Fumed silica is produced by high

temperature gaseous process and is more expensive. Lastly, the surface treated silicas are

specialty products used to advantage in certain function application.

At elevated temperatures such as those encountered during mixing, the silanol

groups on the surface of silicas, silicates, clays and talc may attach to a number of

chemical groups present in rubber compounds. Silanes are known to form strong

chemical bonds while others, such as water and glycols, form fairly weak adsorption

bonds via Van der Waals forces or hydrogen bonding .

Plasticizers: Commercial plasticizers or softeners are normally supplied in the form of

low or high viscosity liquids and more rarely as solid products. They are generally

incorporated for various aims :

as an extender to reduce the cost of the final product;

as a processing aid to improve the workability of the compounded rubber during

processing;

as a modifier of certain vulcanizate properties.

About 90% of all plasticizers used for commercial purposes are recovered from

petroleum. Generally, a large amount (above 20 phr) may act as an extender and a small

(2-5 phr) as a processing aid.

Vulcanizing Agents: Vulcanizing ingredients are those chemicals that form cross-links

between polymer chains when the compounded stock is heated to an appropriate

temperature. Elemental sulphur is most widely used as vulcanisating agent for crude

rubbers that contain double bonds in their chains. However, the reaction of rubber with

sulphur is slow even at high temperatures, in fact it usually requires several hours. In

order to increase the rate of vulcanization, it is necessary to add accelerators and

activators. By adding accelerators, the vulcanization time can be cut to minutes or


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seconds and in most cases the physical properties of the final product are also improved.

Activators are generally added in small amounts to increase the effectiveness of

accelerators. Zinc oxide and stearic acid are the most commonly used activators.

Saturated crude rubbers cannot be crosslinked by sulphur because of the absence of

double bonds in polymer chains. Consequently, they are usually vulcanized by organic

peroxides. Peroxides curing takes place via a free-radical mechanism and leads to carbon-

carbon cross-links. Other vulcanizing agents used for certain rubber kinds include metal

oxides, diamines, bisphenols and special resins.

Activators : Combination Zinc Oxide & Stearic acid play role of activation of the curing

/ vulcanisation reaction for sulfur –Accelerator curing.

Accelerators: Accelerators are organic substances, which enhances the rate of cure and

reduce cure time. A single/combination of accelerators is used. Different Curing systems

that are used for curing various types of Rubber are given in Table 1.2.

Table: 1.2 Various types of Rubbers and their curing system

S.No Type of Rubbers Curing system

1 Unsaturated Rubbers like

NR/IR, SBR, BR, NBR

Sulphur/Accelerator, Peroxide, Sulphur donor.

2 CR Rubber Metal Oxide / Accelerator

3 IIR (Butyl Rubber) Sulphur/Accelerator, Metal oxide, Phenolic Resin,

Para - quinine dioxime

4 Polyurethane -(AU) Rubber Self. i.e. two components mixed together

5 EPDM Rubber Sulphur/Accelerator, Peroxide

6 Silicone (PV MQ) Rubber Organic peroxide for HTV, Ethyl silicate,

Stannous Octate for RTV

7 Fluro elastomers Diak , Magnesium hydroxide


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Antidegradants – Antioxidants and Antiozonants:

Antidegradants: Elastomer-based products suffer irreversible changes in their

required design properties during service. In particular, a loss in mechanical properties

and alterations in surface aspect can occur. These changes are brought about by a number

of agents such as oxygen, ozone, heat, light, high energy radiation and high humidity

which is collectively referred to as ageing. In order to combat these changes additives

often collectively referred to as antidegradants or age resistors are employed. Typical

loading levels are of the order of 1-4 phr. The most common Antidegradants are the

antioxidants, which protect the elastomer against oxidation and the antiozonants that

retard or prevent the appearance of surface cracks caused by ozone. Age resistors may be

divided into two main groups: staining and non-staining. While the former are strong

protective agents however they discolor and stain to various degree, the latter are less

effective but can be used in white and colored rubber compounds. There are basically two

types of antioxidants: Phenolic & Amine.

Phenolic antioxidants are generally non-staining, non-discoloring and non-toxic

finding applications for colored products and food quality rubber items. The antioxidant

activity of phenolic antioxidants is less compared to amine type antioxidants. Amine

antioxidants are staining and dis-coloring, compounding and food quality products.

Special additives: There are several special additives are available for inducting special

properties in polymers. Examples of such special additives are listed below:

a. Antimony Trioxide is used as semi reinforcing flame resistant filler.

b. The use of chlorinated hydro carbon results in release of chlorine on decomposition

which reacts with antimony trioxide to form antimony tri-chloride that acts as a

flame suppressant.

c. Zinc Borate acts as a crust forming agent in rubber compound.

d. Tri-folyl Phosphate is used as non flammable plasticizer for processing benefits

along with flame retardency effect.

e. In certain rubber products like conveyor belt, textile rollers and spinning cots , the

build up of static electricity is undesirable. Antistatic agents are added to reduce


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the accumulation of dust or dirt on the surface of the elastomeric part during

service and also to minimize the possibility of sparking resulting from discharge

of accumulated static electricity. Usually antistatic properties are achieved by

compounding with conductive blocks, e.g. quaternary ammonium salts , ethylene

oxide , conductive fillers , Al / Co powder, etc..

f. Aluminium Silicate is added for obtaining high electrical resistivity.

g. Similarly for getting high magnetic properties one can use barium ferrite and for

inducing chemical resistance.

h. One can use barium sulphate which also reduces cost in tank rubber linings.

i. Similarly, for impermeability to radiation one can add lead & litharge and for heat

resistance in seals, gaskets magnesium oxide can be added.

j. For obtaining resistance to heat along with lower permeability to gases one can

add Mica powder and to reduce surface friction of products one can add

Molybdenum disulphide.

k. Sometimes, one needs to be very choosy about selecting ingredients specially

with specialty rubbers. For example, for heat resistance in silicone rubbers &

extreme whiteness one can add Titanium Dioxide.

l. Flame retardants are substances added to inhibit or to stop the rubber combustion.

In function of their nature, flame retardant systems can either act physically or

chemically14

. The most widely used fire retardant additive which acts physically

is the aluminium trihydroxide (ATH). Its endothermic decomposition occurs

between 180 and 200°C and leads to the release of water, which dilutes the radicals in

the flame and the production of alumina, which forms a protective layer:

2Al(OH)3 → Al2O3 + 3H2O (1050 kJ/kg)

m. Flame retardancy by chemical action can occur in either the gaseous or the

condensed phase15

.


Rajkumar. K 20

The combustion process can be retarded by physical action in different ways :

Cooling: endothermic decomposition of the flame retardant additive cools the

material.

Forming a protective layer: obstructing the flow of heat and oxygen to the polymer,

and of fuel to the vapour phase.

Dilution: release of water vapour or inert gases (CO2, NH3) may dilute the radicals in

the flame so it goes out.

Mixing methods: Rubber compounding is accomplished with two-roll mill (open mill)

or internal mixer. The two roll mill consists of two adjacent, hardened steel rolls rotate in

opposite directions at different speed in order to produce a friction between them

[Fig.1.4 ].

Fig. 1.4 Two roll mixing

The mixing process involves masticating or breaking down the crude rubbers into

an even and smooth band formed around the front roll. When the crude rubber becomes

soft and plastic, the other ingredients are added. The two roll mill mixing depends on

operator intervention. In fact, during the processing the powders drop into the mill tray

and the operators must collect them and to add back to the mix. In addition, in order to

obtain a uniform dispersion of the ingredients, cutting and folding are continuously

carried out. The behaviour of a rubber compound on a two-roll mill depends on its flow

characteristics and on the selected milling conditions. Good behaviour comprises

consistent banding round the working (front) roll during loading, an active rolling bank,

with little or no stagnation and a band that does not sag and remains in full contact with


Rajkumar. K 21

the mill roll16

. This mixing equipment is generally used for laboratory and low volume

production. However, long time is needed for filler incorporation, dust hazard is created

and poor ingredient dispersion is achieved, so it has been replaced with other devices.

And for this reason, nowadays it is mainly used as a second-stage mixing device for

adding vulcanizing agents and for completing the ingredient dispersion.

Fig. 1.5 Internal mixers – internal structure

The internal mixer (Fig. 1.5) is the mixing device mainly used in the rubber

industry. Its dominance can be assigned to several factors, among which is good filler

dispersion achieved and considerable reduction of the mixing time.

Vulcanization process: After the forming the green stock is converted into three-

dimensional elastic network. This is accomplished by the vulcanization process, usually

conducted under pressure at elevated temperature, in which the polymeric chains are

chemically linked together. The process of vulcanization is depicted graphically in Fig

1.6. In Fig 1.6 (a) it depicts the polymeric chains entangle among themselves and physical

cross linking takes place due to hydrogen bonding , Wander Waal force of attraction. Fig.1.6

(b) depicts the picture of polymer cross linked through hard segments chemically or

ionically.


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Fig. 1.6 Crosslinking of polymers – networking

The kinetics of vulcanization is studied using rheometers that measure the torque

variation as a function of time at a fixed temperature. The typical rheograph is explained

in Fig. 1.7.

Fig. 1.7 Typical graph of Rheological study of Rubber Compounds

Several important information about the rate and the extent of the compound

vulcanization can be derived from this curve. Initially, as the rubber compound is heated,

the torque decreases until it reaches a minimum value (ML), strictly correlated to a

decrease in the Mooney viscosity of the compound. Subsequently, the rubber begins to


Rajkumar. K 23

vulcanize and the torque rises and when it reaches a plateau a complete vulcanization and

the formation of a stable network are carried out. However, a chain scission may be

occurring and, if this phenomenon becomes dominant, the torque passes through

a maximum and finally decreases (reversion). On the other hand, some compounds show

a slowly increasing torque at long cure time (marching modulus) 17-18

. Besides the ML,

a vulcanization curve permits to determine the maximum torque achieved and the

optimum curing time (t90), that is the time require to reach 90% of full cure. Generally,

this is the state of cure at which the most physical properties achieve optimal results.

The curing ingredients are classified in two main groups: sulphur and non-sulphur

vulcanizing agents.

Sulphur based vulcanizing agents:

Fig.1.8 Sulphur vulcanization of Rubbers

To increase the rate and the effectiveness of the sulphur cross-linking,

accelerators are normally added. These materials, known also as sulphur donors, are used

to replace part of the elemental sulphur in order to produce vulcanizates with few sulphur

atoms per cross-link (mono- and di-sulphidic linkages). These are organic chemicals and

can be classified in five main groups: guanidines, thiazoles, dithiocarbamates, xanthates

and thiurams.

Non-sulphur vulcanizing agents: The majority of rubber compounds are vulcanized

using sulphur agents, however, there are some cases for which non - sulphur curing is

necessary or preferable. In the 1915 I. I. Ostromyslenski19

disclosed that peroxides could

be used as cross -linking agents for natural rubber. However, there was little interest in

peroxide cross -linking until the development of fully saturated ethylene -propylene

copolymers in the early 1970s.


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Peroxides decompose during vulcanization because of the increased temperature,

forming free radicals which lead to the formation of carbon -carbon cross-links.

Fig. 1.9 Peroxide vulcanization of Rubbers

Several organic peroxides are available as curing agents, the choice depends on

their stability, activity and cure temperature. Numerous advantages derive from the

peroxide vulcanization, such as good aging resistance and low compression set; but also

simple compound formulation and no discoloration of compounds. On the other hand,

peroxides initiated cure involves inferior mechanical properties, higher production cost,

greater care in storage and processing.

Some elastomers, such as polychloroprene rubber, are vulcanized with metal

oxides. The reaction mechanism occurs via allylic chlorines on the polymer chains.

Generally mixtures of ZnO and MgO are used. MgO is used to absorb the hydrochloric

acid liberated during mixing of CR rubber and ZnO reacts to CR rubber to get

crosslinked.

Application of Rubber Compounds: In most applications of rubber products there are

no alternative materials except other rubbers. Thank to their high deformability these

materials can retain a memory of their original unstressed state and can return to their

original dimensions when external forces are removed even after strains as high as

1000 %. This ability to recover their original dimensions leads to a broad range of

applications. In most tyre compounds, the rubber is the largest ingredient in the

formulation. It acts as a „binder‟ into which the other ingredients are dispersed.

Elastomers demonstrate a unique set of materials properties, including viscoelastic

characteristics and the ability to undergo extreme deformations. The modern pneumatic

tyre would be impossible without the use of elastomers. While specialty rubber types are


Rajkumar. K 25

usually employed in specific tyre components, a generic formulation for a SBR rubber

nanocomposite matrix might comprise

Carbon black and silica are the fillers mainly used in the tyre production. Silica

has greater reinforcing power, such as improving tear strength, abrasion resistance, age

resistance and adhesion properties, compared to carbon black. However, the

agglomeration nature of silica is generally believed to be responsible for considerable

rolling resistance in tyre applications18

.

Another field of rubber compounds application is the footwear industry14

. In some

ways, tyres and shoe outsoles have very similar requirements in terms of wear resistance

and traction, and compounds to meet these needs can be very similar. The differences are

in the durability and in the types and number of surfaces each comes in contact with. The

choice of polymers, fillers and additives for producing a shoe is related to the need

properties and requirements. For example, outsoles for running shoes need to provide

good durability, good traction, and high rebound. The polymers of choice for running

outsoles are natural rubber (NR) and polybutadiene (BR). Since one elastomer can rarely

meet all performance requirements, rubber blends are usually prepared to achieve the

desired balance of physical properties. In particular, very common are NR/BR blends

which can achieved rebound values greater than 90%; sometimes SBR is also added.

On the other hand, safety shoes provide protection against severe work conditions

and chemical contact14

. A rubber frequently used for manufacturing safety shoes is

polyacrylonitrile butadiene rubber. The advantage of this elastomer is the presence of two

components which provide different properties. In particular, acrylonitrile provides

resistance to oil and fuel, while the butadiene provides abrasion resistance and low

temperature flexibility.

Another interesting sector where natural and synthetic elastomeric materials have

a variety of applications is the construction industry14

. For example, in building

applications, rubber can help in the control or isolation of vibrations and noise generated

from the building itself. The use of rubber materials in buildings, both for construction

and decoration, continues to increase due to their advantages, such as sound proofing

effects, anti-quake performance, easy fabrication and installation, sealing properties14

.


Rajkumar. K 26

1.2 Nanocomposites

In today‟s world, Nanocomposites have drawn greater interest from both industry

and academia, because they often exhibit remarkable improvements in material properties

at very low clay loading (up to 10 weight %) when compared to pristine polymer or

conventional composites(20-24)

. Polymer nanocomposites are a special class of polymer

composites, a type of reinforced polymers having a two phase material with the

reinforcing phase having at least one dimension in the 10-9 m (nm) scale. It constitutes a

new class of material having nano-scale dispersion, typically 1-100 nm, of the filler phase

in a given matrix(25-26)

. The outstanding reinforcement of nanocomposites is primarily

attributed to the large interfacial area per unit volume or weight of the dispersed phase

(e.g., 750 m2/g). The nanofillers (especially nanoclay) have higher aspect ratio than

typical microscopic aggregates(27-29)

. Mineral clays which can be dispersed as silicate

nano layers of high aspect ratio are attractive for polymer reinforcements.

Polymer-clay nanocomposites have shown drastic enhancements in mechanical

properties (modulus and strength)(30-33)

, thermal properties (heat resistance and

flammability)(25,34)

, barrier properties(35-41)

, and biodegradability(42-43)

of pure polymer.

The colloidal state and the surface chemistry of the silicate layers in a polymer matrix

play important roles in the synthesis of polymer-clay nanocomposites. Electrostatic

forces maintain the clay layers together forming faceto- face stacks in agglomerate

tactoids, which complicate their dispersion in polymers(44)

. Also, the incompatibility

between the hydrophilic clay and the hydrophobic polymer hinders nanoscale dispersion

of the clay. In order to achieve a high degree of dispersion, the silicate layers can be

functionalized by adsorption of organic molecules such as surfactants to diminish surface

forces contributing to layer-stacking. Intercalation of organic molecules modifies the

hydrophilic surface into hydrophobic, raising the level of compatibility between the clay

and a polymer. Intercalation can be achieved in the clay inter layers by ion exchange of

cations loosely held by the negatively charged layers. Polar organic molecules replace

these cations rendering the clay organophilic. Thus, the greatest challenge in polymer

nanotechnology is to uniformly disperse these nanodimensioned fillers which offer

numerous advantages over conventional micron-sized fillers. The interest in the area of

polymer nanocomposites is many-fold(45-51)

. First, the present understanding of composite


Rajkumar. K 27

behavior does not fully satisfy or predict from a scientific standpoint why these systems

are superior in many properties. The second one, which is from an engineering standpoint,

that these materials can achieve significant enhancement in properties with such a small

amount of filler is an extremely attractive proposition. Also, there are huge numbers of

potential polymer-nanofiller combinations to explore. These factors have driven

extensive research work in this field for last couple of decades with expertise from

variety of scientific disciplines(45-59)

. Particles have long been added to polymers to

improve their physical properties, such as strength, toughness, thermal behavior, etc(45-65)

1.3. Nanofillers: Definition and General Behavior of Nano-sized Structures

Because of their small size, structures with nanoscale dimensions have relatively

huge surface areas per unit weight, and these surface areas often dominate the behavior of

these materials. Thus, the chemistry of the surfaces of these materials has taken on a

special significance. Some important nanostructures include carbon nanotubes,

montmorillonite type clays, and biomolecules such as proteins and DNA. Frequently,

these nanomaterials self-assemble into highly ordered layers or structures arising from

hydrogen bonding, dipolar forces, hydrophilic or hydrophobic interactions, etc. For

maximum reinforcement, however, proper dispersal of these nanostructures has become a

major research effort.

Nanoparticles have high free energy because of their large surface area. In order

to minimize it, they agglomerate to form bigger particles. They are stabilized by various

procedures. Reviews are available on synthesis of these particles (66-68)

. They have been

synthesized by methods such as arrested precipitation in aqueous as well as nonaqueous

solutions(69)

, reverse micelle (70)

, using organo- metalic precursors (71)

and by chemical

capping (72-74)

.

In this thesis, four different nano particulate fillers have been used as nanofillers

for preparation polymer nanocomposites .

1. Nano graphite

2. Nanoclay

3. Nanosilica

4. Nano TiO2


Rajkumar. K 28

1.4. Synthesis methods

Several methods can be used to produce nanocomposites from polymers using

nanofillers(27-33)

. However, in general, polymer nanocomposites are achieved by either

direct polymer from solution or melt or intercalation of monomers followed by in-situ

polymerization (Figure 1.5).

1.4.1. In-Situ polymerization

In-situ polymerization covers any process where nanocomposite is made by

performing some sort of polymerization reaction in the presence of nanofiller. There are

many variations on this technique, all generated from the need to disperse the silicate

layers. The simplest technique involves mixing a monomer with nanofiller and

polymerizing it. The growing polymer chains can push the nanofillers leading to an

uniform dispersion (Figure 1.6). Instead of a linear polymerization, in-situ crosslinking

can be used. In this case, the nanofiller is swollen in one of the reactants, or in the

mixture, and the reaction takes place to form a cross-linked network(75,56)

1.4.2. Melt mixing

In this technique, using an extruder or heated internal mixing chamber, polymer

and nanofillers are physically mixed at high shear rate, and a nanocomposite is obtained

(Fig.1.7). In this case, no impurities or residues are introduced into the sample, as

observed in the case of in-situ or solvent casting methods. But there is a problem with

dispersion of nanofiller. In addition to these concerns, new problems are created when

heat is used to enhance the polymer mobility. In order to achieve good mixing, a long

mixing time is preferable, but this must be balanced against the normally undesirable thermal

degradation of polymer and nanofiller. Finding an optimum can be difficult, especially for

polymers that thermally degrade readily (e.g., some biodegradable polyesters), or for those

systems where the processing temperature must be high in order to achieve polymer flow

and effective mixing (many polyamides and polycarbonates fall into this category). Also,

the organic Compatibilizer present in silicates may degrade at high temperature.


Rajkumar. K 29

1.4.3. Solution mixing:

Solvent casting is one of the simplest techniques by which nanocomposites are

produced(48)

. A polymer dissolved in an appropriate solvent and nanofiller, are combined

and thoroughly mixed, and the solvent is then allowed to evaporate, leaving the

nanocomposite behind, typically as a thin film. The solvent imparts enhanced mobility to

polymer chains so that they can easily intercalate between the silicate layers. A solvent

should be chosen that completely dissolves the polymer and fully disperses the nanofiller.

This describes the ideal case; there are, however, a number of complications. In practice,

the solvent described may not exist. Settling of the nanofiller out of the solvent is a

significant problem, if the solution concentration is not properly chosen. Care should be

taken, especially when casting the mixture, since evaporation must be performed very

slowly so as not to produce bubbles in the sample. But, it is a useful technique as it can

be easily performed with a wide variety of polymers and nanofiller, and is particularly

useful with systems that are thermally unstable or where melt-mixing is otherwise very

difficult or impossible.

1.5. Polymer-Nanocomposites- Recent Advancement:

Polymer nanocomposites are materials in which nanoscopic inorganic particles,

typically 10-100 Å in at least one dimension, are dispersed in an organic polymer matrix

in order to dramatically improve the performance properties of the polymer. Systems in

which the inorganic particles are the individual layers of a lamellar compound – most

typically a smectite clay or nanocomposites of a polymer (such as nylon) embedded

among layers of silicates – exhibit dramatically altered physical properties relative to the

pristine polymer as given in Fig. 1.10.

Clay MMT structure Polymer Intercalated structure Polymer Exfoliated structure

Fig. 1.10 Structures of Clay and its interaction with polymer


Rajkumar. K 30

For instance, the layer orientation in polymer-silicate nanocomposites exhibit

stiffness, strength and dimensional stability in two dimensions (rather than one). Due to

nanometer length scale which minimizes scattering of light, nanocomposites are usually

transparent.

Polymer nanocomposites represent a new alternative to conventionally filled

polymers. Because of their nanometer sizes, filler dispersion nanocomposites exhibit

markedly improved properties when compared to the pure polymers or their traditional

composites. These include increased modulus and strength, outstanding barrier properties,

improved solvent and heat resistance and decreased flammability.

Layered silicate/polymer nanocomposites exhibit superior mechanical

characteristics (e.g. 40 % increase of room temperature tensile strength), heat resistance

(e.g. 100 % increase in the heat distortion temperature) and chemical resistance (e.g. ~10

fold decrease in O2 and H2O permeability) compared to the neat or traditionally filled

resins. These property improvements result from only a 0.1-10 vol. % addition of the

dispersed nanophase. Polyimide-clay hybrids represent another example of polymer

nanocomposites. These nanocomposites have been prepared by intercalation of the

organoclay with a polyamic acid. The clay polyimide hybrid composite films exhibit

greatly improved CO2 barrier properties at low clay content; less than 8.0 vol. % clay

results in almost a ten-fold decrease in permeability. Adding nanoscale ceramic powders

to commercial products can produce another class of polymer nanocomposites. The

addition of reinforcing agents is widely used in the production of commodities

(packaging films and tyres). It is expected that the reduction of the added particle size

down to nanometric scale could enhance the performance of these materials, even though

not to the extent as layer addition. These new materials are aimed at being a substitute for

more expensive technical parts (gear systems in wood drilling machines, wear resistance

materials) and in the production of barrier plastic film for food industry.

Besides structural applications, polymer nanoparticle compounds have very

interesting functional applications. For instance, Fe2O3 / polymer nanocomposites are

used as advanced toner materials for high quality colour copiers and printers and as

contrast agents in NMR analysis, memory devices. The key to forming such novel


Rajkumar. K 31

materials is understanding and manipulating the guest-host chemistry occurring between

the polymer and the layered compounds or the nanoparticles, in order to obtain a

homogenous dispersion and a good contact between polymer and added particle surfaces.

There have been major advances in solid state and materials chemistry in the last two

decades and the subject is growing rapidly. The coatings of magnetic particles are of

special interest because of their important applications viz. technological energy

transformation, magnetic recording, magnetic fluids and magnetic refrigeration system.

Polymer materials have been filled with several inorganic compounds in order to increase

properties like heat resistance, mechanical strength and impact resistance and to decrease

other properties like electrical conductivity, dielectric constant thereby increasing the

permeability for gases like oxygen and water vapor.

In recent years considerable efforts have been devoted to the development of

methods for the preparation of composite particles consisting of polymer cores covered

with shells of different chemical composition. In several of these powders, particles

covered with magnetic materials have been used as beads for gas separation, or as

pigments, catalysts, coatings, flocculents, toners, raw materials recovery, drug delivery

and anticorrosion protection.

Polymer composites containing ferrites are increasingly replacing conventional

ceramic magnetic materials because of their mouldability and reduction in cost. They are

also potential materials for microwave absorbers, sensors and other aerospace

applications. These flexible magnets or rubber ferrite composites are possible by the

incorporation of magnetic powders in various elastomer matrices. This modifies the

physical properties of the polymer matrix considerably. Solvent casting method is one of

the easiest methods for the preparation of polymer nanocomposites. It needs simple

equipment and is less time consuming. The fine dispersion of the magnetite inside the

polymer matrix makes it a magnetic polymer.

1.5.1. Rubber Nanocomposites

Nano reinforcement of rubbers has a long and solid background since a plethora

of compounding recipes, which contain particles of nanodimension range, like carbon

black and silica grades, have been developed by both academia and industry14

. However,


Rajkumar. K 32

recently several other kinds of nanofillers have received attention for reinforcement

characteristics in rubbers and, amongst all, the nanoclays have been more widely

investigated probably because they are easily available in nature and cheaper. The

ongoing R&D interest is mostly due to the remarkable properties improvement which is

observed when nanoclays are added to a rubber matrix. This enhancement depends on the

nanometric-scale dispersion that the nanoclays can achieve in the compound; contrary to

the conventional fillers, such as carbon black and silica, which carry out a micrometric

scale dispersion.

The service performance of rubber products can be improved by the addition of

fine particle size carbon blacks or silica. The most important effects are improvements in

wear resistance of tire treads and in sidewall resistance to tearing and fatigue cracking.

This reinforcement varies with the particle size, surface nature, state of agglomeration

and amount of the reinforcing agent and the nature of the elastomer. Carbon blacks normally

are effective only with hydrocarbon rubbers. It seems likely that the reinforcement

phenomenon relies on the physical adsorption of polymer chains on the solid surface and

the ability of the elastomer molecules to slip over the filler surface without actual

desorption or creation of voids.

Polymer Nanocomposites[76]

exhibit improved physical properties when compared

with conventional silica filled rubber compounds due to a platelet-type dispersion of the

clay within the rubber matrix. These enhanced physical properties [77-83]

are only obtained

when nano-sized clays are uniformly dispersed throughout the rubber matrix, without

appreciable agglomeration.

Dispersement can be achieved through solution or melt intercalation. Depending

on the degree of dispersement, the polymer-clay nanocomposites can be classified as

either intercalated or exfoliated nanocomposites.

In intercalated nanocomposites, the clay particles are dispersed in an ordered

lamellar structure with large gallery height as a result of the insertion of polymer chains

into the gallery. In exfoliated nanocomposites, each silicate layer is delaminated and

dispersed in a continuous polymer.


Rajkumar. K 33

Fig. 1.11 Chemical Structure of MMT Clay

Fig. 1.12 Scheme of different types of composites arising from the interaction of

nanoclay and polymers

In recent years rubber-clay nanocomposites have attention in both academic and

industrial researchers due to their phenomenal improved performance compared to


Rajkumar. K 34

matrices containing micron-sized particulate fillers like carbon black or non black fillers.

In particular, mechanical properties enhancement has been observed with less

concentration. Thermal stability and fire retardancy through char formation are other

interesting and widely searched properties displayed by rubber nanocomposites.

1.6. Scope and objectives

1.6.1 Scope of Research

Synthetic Rubbers like Acrylonitrile-butadiene rubber (NBR), Hydrogenated

nitrile butadiene rubber (HNBR) and Ethylene propylene diene polymethylene rubber

(EPDM) rubbers are widely used in automotive and industrial applications. NBR and

HNBR are of polar in nature and containing oil resistance properties and EPDM is of

non-polar in nature having heat and weather resistance. With acrylonitrile content the

rubber shows higher strength, greater resistance to swelling by hydrocarbon oils, and

lower permeability to gases. At the same time, however, the rubber becomes less flexible

at lower temperatures, owing to the higher glass transition temperature of poly

acrylonitrile (i.e., the temperature below which the molecules are locked into a rigid,

glassy state). Nitrile rubber is mostly used where high oil resistance is required, as in

automotive seals, gaskets, or other items subject to contact with hot oils. HNBR is

classified as thermally improved version of NBR and has combination of oil as well as

heat resistance properties. As nanofillers found to improve various properties of polymers,

an attempt has been made to prepare polymer nanocomposites based on these three

rubbers [NBR, HNBR and EPDM] and study the improvement of respective properties.

1.6.2 Objectives

1. To prepare and characterize NBR rubber based nanocomposites using

various nanofillers like nanosilica, nanoclay, nanographite and nano TiO2

and study their effects on physico-mechanical, thermal and barrier

properties.

2. To prepare and study the effect of nano silica and nano clay on physico-

mechanical, thermal stability, degradation and solvent resistant properties

of HNBR rubber based nanocomposites.

http://www.britannica.com/EBchecked/topic/511800/rubber


Rajkumar. K 35

3. To prepare and study the effect of nano silica and nano clay on physico-

mechanical, thermal stability and electrical resistivity of EPDM rubber

based nanocomposites.

4. To compare the effect of liquid ingredients like liquid NBR, DOP as a

media for nano-fillers dispersion in NBR and HNBR rubbers and to

characterize the dispersion using optical, mechanical, and thermal

properties of polymer nanocomposites.

5. To study the thermal ageing resistance of polymer nanocomposites and

their importance due to incorporation of nano-fillers as thermal resistance

properties


Rajkumar. K 36

1.7 References

1. Wagner, A.H., Kalyon, D.M., Yazici, R. and Fiske, TJ. (1997) Extensional flow

of engineering plastics with glass fibers, Paper 8-D, presented at the 13th

international meeting of the Polymer Processing Society (June 10-13).

2. Chapman, P.M. and Lee, T.S. Effect of talc filler on the melt rheology of

polypropylene, SPE /., 26, 37-40 (1970).

3. Mills, NJ. The rheology of filled polymers, /. Appl Polym. ScL, 15, 2791-805

(1971).

4. Nazem, F. and Hill, C.T. Elongational and shear viscosities of beadfilled

thermoplastic, Trans Soc. RheoL, 18, 87-101 (1974).

5. Han, C.D. Rheological properties of calcium carbonate-filled polypropylene melts.,

Appl. Polym. Sa'., 18, 821-9 (1974).

6. White, J.L. and Crowder, J.W. The influence of carbon black on the extrusion

characteristics and rheological properties of elastomers: polybutadiene and

butadiene-styrene copolymer, /. Appl. Polym. ScL, 18,1013-38 (1974).

7. Minagawa, N. and White, J.L. (1976) The influence of titanium dioxide on the

rheological extrusion properties of polymer melts, /. Appl. Polym. Sd., 20, 501-23.

8. Faulkner, D.L. and Schmidt, L.R. Glass bead-filled polypropylene Part I:

Rheological and mechanical properties, Polym. Engg ScL, 17, 657-64 (1977).

9. Boira, M.S. and Chaffey, C.E. Effects of coupling agents on the mechanical and

rheological properties of mica-reinforced polypropylene, Polym. Engg ScL, 17,

715-18 (1977).

10. Bigg, D.M. Rheology and wire coating of high atomic number metal low

density polyethylene composites, Polym. Engg Sd., 17, 745-50 (1977).

11. Kataoka, T., Kitano, T., Sasahara, M. and Nishijima, K. Viscosity of particle

filled polymer melts, Rheol. Ada, 17,149-55(1978).

12. Kataoka, T., Kitano, T. and Nishimura, T. Utility of parallel-plate plastometer

for rheological study of filled polymer melts, Rheol Ada, 17, 626-31 (1978).


Rajkumar. K 37

13. Copeland, J.R. and Rush, O.W. Wollastonite: short fiber filler/ reinforcement,

Plastic Compounding, 1, 26-36 (Nov./Dec.) (1978).

14. T. Sabu and S. Ranimol, Rubber Nanocomposites: Preparation, Properties and

Applications, John Wiley & Sons (2010).

15. F. Laoutid, L. Bonnaud, M. Alexandre, J.M. Lopez-Cuesta and Ph. Dubois, Mater.

Sci. Eng., R, 63, 100 (2009).

16. T.R. Hull and B. K. Kandola, Fire Retardancy of Polymers: New Strategies and

Mechanisms, Royal Society of Chemistry (2009).

17. J.R. White and S.K. De, Rubber Technologist’s Handbook, Rapra Technology

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18. B. Rodgers, Rubber Compounding: Chemistry and Applications, Marcel Dekker

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19. I. I. Ostromysklenki, J. Russ. Phys. Chem. Soc., 47, 1467 (1915)

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Elsevier; NY, Vol 9, (1979).

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theory and practice‟ Hanser Verlag,( 2008).

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