Download - Introduction to Elastomers
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