INTRODUCTION TO ELASTOMERS

Submitted By: - Acharya Raghav

Roll Number - 155501

Department of Metallurgical and Materials Engineering

NIT Warangal

Polymer

Definition: A group of engineered materials characterized by large

molecules that are built up by the joining of smaller molecules.

They are natural or synthetics resins.

Properties of Plastics

Light weight

Good resistance to corrosion

Easy of fabrication into complex shapes

Low electrical and thermal conductivity

Good surface finish

Good optical properties

Good resistance to shock and vibration.

Classification of Engineering Polymers:-

Classification based on their industrial Usage :-

a) Plastics

b) Elastomers

Classification based on their temperature dependence :

a) Thermoplasts

b) Thermosets

The word plastic comes from the Greek word Plastikos, meaning “able to be

shaped and molded”. Plastics can be broadly classified into two major groups

on the basis of their chemical structure i.e. thermoplastics and thermosetting

plastics.

Thermoplastics:-

The material that softens when heated above the melting temperature and

becomes hard after cooling is called thermoplastics. Thermoplastics can be

reversibly melted by heating and solidified by cooling in limited number of cycles

without affecting the mechanical properties. On increasing the number of

recycling of thermoplastics may result in color degradation, thereby affecting

their appearance and properties. In the molten state, they are liquids, and in the

mushy state they are glassy or partially crystalline. The molecules are joined end-

to-end into a series of long chains, each chain being independent of the other.

Above the melting temperature, all crystalline structure disappears and the long

chain becomes randomly scattered.

The molecular structure of thermoplastic has an influence on the chemical

resistance and resistance against environmental effects like UV radiation. The

properties may also vary from optical transparency to opaque, depending on the

molecular structure. The important properties of the thermoplastics are high

strength and toughness, better hardness, chemical resistance, durability, self

lubrication, transparency and water proofing.

With the application of heat, thermoplastic softens and it can be molded into

desired shapes. Some thermoplastics can be joined with the application of heat

and pressure. There are several techniques available for the joining of

thermoplastics such as mechanical fastening, fusion bonding, hot gas welding,

solvent bonding, ultrasonic welding, induction welding, and dielectric welding.

Applications

Thermoplastics can be used to manufacture the dashboards and car trims, toys,

phones, handles, electrical products, bearings, gears, rope, hinges and catches,

glass frames, cables, hoses, sheet, and windows, etc.

Few Examples and their applications :-

Acrylonitrile-butadiene-styrene (ABS):

Characteristics: Outstanding strength and toughness, resistance to heat

distortion; good electrical

properties; flammable and soluble in some organic solvents.

Application: Refrigerator lining, lawn and garden equipment, toys,

highway safety devices.

Acrylics (poly-methyl-methacrylate) PMMA

Characteristics: Outstanding light transmission and resistance to

weathering; only fair mechanical properties.

Application: Lenses, transparent aircraft enclosures, drafting equipment,

outdoor signs.

Fluorocarbons (PTFE or TFE,Teflon)

Characteristics: Chemically inert in almost all environments, excellent

electrical properties; low coefficient of friction; may be used to 260ooC; relatively

weak and poor cold-flow properties.

Application: Anticorrosive seals, chemical pipes and valves, bearings, anti

adhesive coatings, high temperature electronic parts.

Polyamides (nylons)

Characteristics: Good mechanical strength, abrasion resistance, and

toughness; low coefficient of friction; absorbs water and some other liquids.

Application: Bearings, gears, cams, bushings, handles, and jacketing for

wires and cables

Thermosets :-

The property of material becoming permanently hard and rigid after cooling when

heated above the melting temperature is called thermosets. The solidification

process of plastics is known as curing. The transformation from the liquid state

to the solid state is irreversible process, further heating of thermosets result only

in the chemical decomposition. It means that the thermosets can’t be recycled.

During curing, the small molecules are chemically linked together to form

complex inter-connected network structures (figure 2). This cross-linking

prevents the slippage of individual chains. Therefore, the mechanical properties

(tensile strength, compressive strength, and hardness) are not temperature

dependent, as compared to thermoplastics. Hence, thermosets are generally

stronger than the thermoplastics.

The joining of thermosets by thermal processes like ultrasonic welding, laser

welding, and gas welding is not possible, but mechanical fastening and adhesive

bonding may be used for low strength applications.

Applications:-

Thermosets are commonly used for high temperature applications. Some of the

common products are electrical equipments, motor brush holders, printed circuit

boards, circuit breakers, encapsulation, kitchen utensils, handles and knobs, and

spectacle lenses.

Examples:-

Epoxies

Characteristics: Excellent combination of mechanical properties and

corrosion resistance; dimensionally stable; good adhesion; relatively

inexpensive; good electrical properties.

Application: Electrical moldings, sinks, adhesives, protective coatings,

used with fiberglass laminates.

Phenolics

Characteristics: Excellent thermal stability to over 150o C; may be

compounded with a large number of resins, fillers, etc.; inexpensive.

Application: Motor housing, telephones, auto distributors, electrical

fixtures.

Mechanical Properties of Plastics

The mechanical properties of the plastics are given below:

(a) Hardness: Plastics are not very hard, their hardness being comparable to

that of brass and aluminum. Generally, thermosets are harder than

thermoplastics. The temperature of the material substantially affects its

properties. The hardness of commonly used plastics is in the range of 5- 50 BHN.

(b) Stress-Strain Behavior: When the plastic is subjected to uni-axial load, it

deforms permanently and ultimately fails as shown in figure 10. Tensile

strengths of plastics may be in the range of 10 to 100 MPa. The modulus of

elasticity for plastic is in the range of 10 MPa to 4000 MPa. Tensile strength

decreases with increasing temperature. Some of the plastics are brittle in

nature. The mechanical properties of the plastics depend upon the strain rate,

temperature, and environmental conditions.

(c) Fatigue Behavior: Fatigue failure of thermosets is brittle in nature, but in

case of thermoplastic failure occurs due to initiation of crack propagation. The

flexural fatigue strength of plastics may be in the range of 105 - 107 numbers of

cycle to failure at room temperature (20˚C).

INTRODUCTION TO ELASTOMERS :-

Etymology – Elastomers came from the word Elastic + mer

Elastic come from the Latin word Elasticus which means cord, tape,

or fabric, woven with strips of rubber, which returns to its original

length or shape after being stretched.

In 17th century it originally describes a gas in the sense ‘expanding

spontaneously to fill the available space

An elastomer is a material that can exhibit a rapid and large reversible strain in

response to a stress. An elastomer is distinguished from a material that exhibits

an elastic response that is characteristic of many materials. An elastic response is

where the strain is proportional to stress according to Hooke’s Law, though the

strain may only be a small amount, such as 0.001 for a silicate glass. An elastomer

can exhibit a large strain of for example 5–10 and to be able to do this an

elastomer must be a polymer.

Elastic strain may be due to chemical bond stretching, bond angle deformation

or crystal structure deformation. In an elastomer under strain bond are not

elongated and bond angles not deformed. An unstrained elastomer will exist in a

random coil structure. As strain is increased the molecules will uncoil to the

limiting linear structure. Therefore, to be an elastomer a substance essentially

must consist of macromolecules. Large strain required very long molecules so

that uncoiling can be considerable. Formation of an unstrained random coil means

that the elastomer must be non-crystalline since any regular

crystal structures will be unable to contribute to elastomeric properties.

The large reversible strain must be rapid which means the restraining

intermolecular forces must be minimal. Elastomers will have minimal hydrogen

bonding or polar functional groups that contribute to intermolecular forces. Steric

hindrance to uncoiling should be minimal so that elastomers are unlikely to have

bulky pendant groups or rigid intra-chain groups. This is why most common

elastomers consist of simple hydrocarbon high molar mass macromolecules. An

elastomer will therefore be a polymer stripped of all molecular complexity.

An elastomer is defined by mechanical response not by chemical structure.

Elastomers comprise a diverse range of chemical structures although they are

characterized as having weak intermolecular forces. An elastomer will undergo

an immediate, linear and reversible response to high strain to an applied force.

This response has a mechanical analogy with a spring according to Hooke's Law.

Non-linear, time dependent mechanical response is distinguished as

viscoelasticity according to the parallel spring and dashpot model. Time

dependent irreversible response is a viscous response according to a dashpot

model. An ideal elastomer will only exhibit an elastic response. Real elastomers

exhibit a predominantly elastic response, however they also exhibit viscoelastic

and elastic responses especially at higher strains.

The chemical structure and molecular architecture of elastomers is tightly related

to elastomeric mechanical response. High strain requires a polymer with high

molar mass preferred. Many materials can exhibit an elastic response, that is

immediate, reversible and linear strain with stress, however only a polymer can

exhibit additionally high strain. High strain is due to uncoiling of random

molecular coils into more linear conformations. The limit to elastic response is

when molecules are in fully extended conformations. This mechanism is due to

uncoiling of chain segments. Molecules do not move relative to each other, there

are reversible random coiling not translational motions.

Reversibility and immediate response is obtained with macromolecules that have

flexible chains with weak intermolecular forces. Rigid groups such a benzene,

bulky side-chains such as isopropyl, polar groups such as ester and hydrogen

bonding groups such as hydroxy are not desirable if a polymer is to be an

elastomer. This description supposes elastomeric properties at ambient

temperatures, since at elevated temperatures above the glass transition

temperature many polymers become elastomers.

At high extensions and when under strain for longer times viscous flow occurs,

known as creep when over longer times. Chemical cross-linking prevents viscous

flow, the movement of molecules relative to each other. Elastomers are cross-

linked after moulding or shaping to fix molecules into their relative positions.

Once cross-linked the unstrained shape of an elastomer cannot be altered and the

elastomer cannot be reprocessed or recycled. The

permanence brought about by cross-linking and the need to perform a cross-

linking reaction

on elastomers are disadvantages for their applications.

This figure shows the viscous deformation that can’t be recovered after the load

the removed. But this is very less when used in practical cases of elastomers.

Limiting the Expansivity of Elastomers:-

Cross linking of polymers using Silicon.

In 1839, American Chemist Charles Goodyear discovered that rubber can be

made stronger by heating it with sulphur.

Cross Linking changes the properties of the Polymers to a very great extent. Small

amount of cross-linking leave the elastomer soft and flexible, as in rubber band.

Additional cross linking restricts some of the uncoiling and the material become

harder, like the rubber used in bowling balls.

Thermodynamics of Elastomers:-

In most solids, the atoms or molecules are held in place by strong intermolecular

potentials. These determine the equilibrium state of the solid and, because each

atom or molecule sits in a deep potential well, it can take a great deal of energy

to deform or expand a normal solid. The thermodynamics of most solids are

determined by the variation of internal energy with shape and size and this also

determines the thermal Expansivity of the material. When heated most solids

expand and this is because, with higher thermal energy, each atom or molecule

can oscillate further from its equilibrium position leading to an effective increase

in volume.

In ideal rubber on the other hand, each polymer chain is free to rotate about its

bonds and each chain can therefore coil or uncoil without changing the internal

energy, U. The internal energy of rubber is therefore independent of the shape at

constant volume and, like an ideal gas, rubber obeys Joule’s law: rubber can

stretch and un-stretch with no change in U. However, we know that when rubber

is stretched we feel a restoring force, so what is the origin of this force? The

answer is that the force is entropic in nature. When rubber is stretched the polymer

chains uncoil and begin to align into a more ordered state with

correspondingly lower entropy. The force we feel is a direct result of the second

law of thermodynamics: the rubber is trying to pull back into a more disordered

state where its entropy is a maximum. Again, this behaviour is analogous (but

opposite in sign) to the ideal gas.

Theory: Thermodynamics of ideal rubber

The first law of thermodynamics is simply a restatement of the conservation of

energy. It can be written as

Fdl = dU − TdS, (1)

where F is the force acting over distance dl,

dU is the change in internal energy and

dS is the change inentropy S at constant temperature, T. We will always work in

quasistatic equilibrium, so the load F on the rubber is always equal to the

restoring force F generated by the rubber. Then,

It is clear that there are two contributions to the load needed to stretch a substance,

a contribution related to the change in internal energy with respect to length and

a contribution from the change in entropy with respect to length. In most solids,

the first term is large and the second negligible. In rubber (and the ideal gas) the

first term is negligible and it is the entropic term that is important.

The entropy is essentially a measure of the disorder of the system. For rubber, the

entropy is large when

the polymer chains are coiled and tangled, and small when the rubber is stretched

so that the polymer

chains uncoil and align. It is possible to obtain a good physical understanding of

the entropy of rubber with a simple 1-dimensional model of a single rubber

molecule

The entropy is related to the number of ways, W, in which each possible length

of the rubber molecule can be achieved. Each time the chains rearrange through

random thermal motion each particular arrangement of links will arise with equal

probability but some particular lengths of chain can be obtained in many more

ways than others and so the random thermal motion will likely push the molecule

into these most probable lengths. The Boltzmann relation (which is inscribed on

Ludwig Boltzmann’s tombstone) relates the entropy

to W,

S = k ln W

where k is the Boltzmann constant.

Specific Elastomers and their Applications:-

Aliphatic and Aromatic Hydrocarbon Elastomers

Natural rubber (NR) is an elastomer with a basic monomer of cis-1,4-isoprene. It

is made by processing the sap of the rubber tree (i.e., Hevea brasiliensis) with

steam, and compounding it with vulcanizing agents, antioxidants, and fillers.

Natural rubber is widely used for applications requiring abrasion or wear

resistance electric resistance and damping or shock absorbing properties such as

large truck tyres, off-the-road giant tyres and aircraft tyres. It is chemically

resistant to acids, alkalis and alcohol. However, it does not do well with oxidizing

chemicals, atmospheric oxygen, ozone, oils, petroleum, benzene, and ketones.

Halogen and Nitrile Substituted Elastomers

Polychloroprene (CR) was one of the first commercially successful synthetic

rubbers with an annual consumption of about 3,00,000 tons worldwide (excluding

former Soviet Union and PR of China). It is a chlorinated rubber material, which

was developed in 1932 by Carothers, Collins, and co-workers using emulsion

polymerization techniques.

Originally developed as an oil-resistant substitute for natural rubber, CR has a

good resistance towards various organic chemicals including mineral oils,

gasoline, and some aromatic or halogenated solvents. It also has good aging

resistance, high ozone and weather resistance. In contrast to the majority of other

rubber types, CR shows a surprisingly higher level of resistance to

microorganisms, such as fungi and bacteria. Moreover, it has low flammability

and outstanding resistance to damage caused by flexing and twisting, an elevated

toughness

Future Trends

A trend with new polymers is toward specialty applications where advanced and

unique properties are required. The requirements are met through improved

control of molecular structure, copolymerization and formulation of existing

polymer types. Elastomers follow this trend with new polymerization techniques

and catalysts to control tacticity, comonomer composition, molar mass and molar

mass distribution. Formulation innovation has come from polymer blends that are

compatible, though not miscible when averging of properties would occur.

Recent publication frequencies show that nano composites are the elastomers

with most rapid development. Elastomers nano composites have always been

significant in that carbon blacks and silicas used in the traditional rubber industry

are nano particulate materials. Enhanced elastomeric properties and energy

damping are prime areas of new developments. In addition, resistant elastomers

are in demand for chemical resistance, thermal resistance, radiation resistance,

wear/abrasion resistance and weathering resistance.

A second trend is towards biomaterials, or materials derived from renewable

resources. The source of monomers for elastomer synthesis has been mentioned

in this review. Natural rubber has always been a biomaterial and it would be

rational to continue to innovate with its use and enhancement through

compounding.

Natural rubber crop design is another area with potential. The rubber derived from

plantations in different locations and hence climates or microclimates, as well as

weather trends and seasonal variations for collection cause variation in the rubber.

New plant breeds or genetic modification is likely to yield rubber with enhanced

properties and of greater initial purity. These plantation improvement processes

have been underway throughout the history of natural rubber, however

biotechnology has recently advanced rapidly.

Conclusion

Elastomers are unencumbered polymers preferably of high molar mass so that

random coils will form and the coils can be extended towards an entropically

unstable linear conformation. Elastomeric polymers have contributed

significantly to understanding of the behaviour of macromolecules. Structural

refinments are required in practice, firstly some threshold of crosslinking prevent

flow thus restricting molecules to reversible uncoiling.

References :-

1. General Purpose Elastomers – Structure Chemistry Physics &

Performance, Robert Shanks & Ink Kong

2. Elastomers By – Robert Shanks & Ink Kong, Applied Science RMIT

University

3. NPTEL II – Thermoplastics & Thermosets

4. Polyurethane Elastomer, 2nd Edition New York 1992

5. http://prb.abs.org Issue of 2011

6. L. Miller, P Strehlow, Rubber and Rubber Ballons Paradigms of

Thermodynamics, Springer 2009