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Material Science

POLYMERS

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POLYMERS

Polymers are materials whose molecules are built up from a series of smaller units, and are based on the carbon atom.

Three types of polymers: thermoplastics, thermosets Elastomers (rubber).

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Thermoplastics

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Thermoplastics Thermoplastics soften when heated and

become hard again when it is cooled They are generally flexible and relatively

soft. Material can be recycled Formed into the shape of final component

by the use of moulds The raw materials are usually in powder

or granular form

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Thermoplastics Commonly used thermoplastics:

Polyethylene Polypropylene PVC Polystyrene PTFE Nylon Acrylics ABS

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Polyethylene Thermoplastics

Commonly called polythene High resistance to corrosion caused by

solvent or chemicals Unaffected by food Tough and flexible Cheap to produce Common uses:

Packing for food and sweet Industrial and household container Toys, cold water pipe and electrical insulation

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Polypropylene Thermoplastics

More rigidity, strength and heat-resistance than polyethylene

Better resistance to corrosion caused by solvent or chemicals than polyethylene

Can be sterilized by steam Withstand repeated bending without cracking Widely used in:

Packaging Electrical insulation Automobile parts Case and containers Safety helmets

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PVC Thermoplastics

Can be produced in either a rigid form or a flexible form

It is hard when solid and possesses both strength and toughness

Have exceptional resistance to acid and solvents

Excellent insulation properties Common uses are:

Helmets Bottles, handbags Protective gloves and clothing Garden hose, floor tiles, luggage Electrical insulation, interiors of refrigerators

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Polystyrene Thermoplastics

Can be produced in clear form with hard, glossy surface Strong, flexible and light in weight but has low impact

strength Can be easily moulded in shape High electrical resistance and good resistance to foods

and household acids Can be produced in form of foams Common uses are:

Toys and refrigerator parts Packaging for foodstuffs and fragile parts Trays, pens and hull for some sailing boat Thermal and sound insulation in house and factories

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PTFE (Teflon) Thermoplastics

Unaffected by concentrated sulphuric acid which is capable of dissolving some metals

Can be used at temperature ranging from -200 to 300 0C

Most materials do not adhere to PTFE Very low coefficient of friction One of the most costly thermoplastics It is produced in a fashion similar to metals in

powder metallurgy It is widely used for bearings, fuel hoses, gaskets,

valve seat, pump parts and as a non-stick coating for pans and similar appliances

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Nylon Thermoplastics

Nylon is very strong, tough and light in weight Good resistance to abrasion and low

coefficient of friction It is flexible ,has high impact strength and can

be used without lubrication Operation temperature about 150 0C Uses of nylon include small gear, bearing,

combs, ropes, fishing lines, tennis strings, casting and machine parts for domestic appliances, helmets, tubing, rollers pulleys and etc.

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AcrylicsThermoplastics

Clear glass-like material, tough and lighter than glass, commonly called perspex

Hard and rigid with high impact and tensile strength

Its surface is easily scratched Easily moulded into shape Perspex is used for aircraft glazing,

roofing panels, lenses, drawing instruments, telephone, baths, sinks, display signs, clock faces, handles and reflectors

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ABS Thermoplastics

Have exceptional resistance to impact with good tensile strength

Resist to most acids, to many alkalis and to some solvents

Operation temperature of about 80 0C Easily processed by a number of methods

commonly used for thermoplastics ABS is used for pipes, helmets, tool handles,

toys, wheels, refrigerator parts, pump components, luggage, battery cases, widely used in the automobile industry for parts of bodywork ( may be chromium plated)

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Thermosets

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Thermosets When thermoplastics are heated above

their glass transition temperatures they can be reshaped

Thermosetting plastics undergo chemical change (curing) during moulding and can never again be softened by heating

Thermosets are stronger, more rigid and more brittle than thermoplastics

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Thermosets Common used thermosets:

Phenolics (bakelite) Urea formaldehyde Melamine formaldehyde Polyurethane Epoxy resins Polyester resins

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Phenolics (bakelite) Thermosets

Hard, strong and rigid and able to operate at about 200 0C

High electrical resistance, being used for electrical fittings

High chemical resistance and dimensional stability

Bakelite is used for electrical fittings, switchgear, handles, motor car parts, and parts for domestic appliances

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Urea formaldehyde Thermosets

The basic resin is hard, brittle, rigid and scratch resistant

Like phenol formaldehyde it is never used alone but additives

It is resistant to most solvents and household detergents

It has good electrical insulation properties and being virtually colourless, it can be coloured by pigments to suit any decorative requirements

For this reason it is widely used for domestic electrical equipment (plugs, sockets and switches) and most domestic appliances, toys and buttons

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Melamine formaldehyde Thermosets

This material is similar to urea formaldehyde, but is more resistant to heat and is less water absorbent.

This not only improves its electrical properties but makes it suitable for tableware.

This material also used for decorative panels (formica), electrical fittings, trays, light fixtures and handles

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Polyurethanes Thermosets

One type is used in the manufacture of bottles and films

Many types are used as foams The foams range from a hard and rigid material

to a soft and flexible material The rigid foams are generally used for heat

insulation and for strengthening hollow structure The soft and flexible foams used for upholstery

and for item such as artificial sponges They also used in adhesives (usually glass to

metal) and base of paints

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Epoxy resins Thermosets

The resins are usually combined with a hardener The resulting properties of the material are:

High mechanical strength and toughness Excellent adhesion to many other material High resistance to chemicals and moisture

Epoxy resins are used for bonding glass fibre fillers. They are resistant to water and most reagents and have excellent electrical insulation properties.

Epoxy are also widely used as casting material for small components and material for sealing electrical equipment such as transformer and chokes

‘Araldite’ is a common epoxy resin glue

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Polyester resins Thermosets

These materials have good heat resistance and excellent electrical insulation properties

They have good dimensional stability and are unaffected by water and most organic solvents

This makes them suitable for mouldings for high voltage insulators in television sets and for mouldings for the electrical equipment of road vehicles and aircraft

Alkyd resins are also used as the basis for the paint systems used on cars and domestic appliances

Their low water absorption and resistance to weathering make them an excellent binder for use with glass fibre reinforcement for mouldings ranging from domestic baths, chairs, to boat hulls

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Elastomer

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Elastomer Elastomers (or rubbers) are polymers with

considerable extensions which are reversible Example: natural rubber and synthetic rubber Synthetic rubbers are now used much more extensively

and they are based on other polymers and produced from the distillation of oil.

Many components do not require great strength but they do require softness, flexibility and reversible elongation. Thus elastomers are ideal for such applications as resilient floor coverings, weather stripping, footwear, vehicle tyre, joint sealants, and anti-vibration mountings.

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Application of Elastomer I

Vibration insulation and isolation Shock absorbers Anti-vibration machine and engine

mountings Sound insulation

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Application of Elastomer II

Distortional systems Correctives for misalignment: such as

flexible couplings Changing shapes: such as belts, flexible

hose, covered rollers, and tyres Seals and gaskets Rubber hydraulics.

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Application of Elastomer III

Protective systems protection against abrasion protection against corrosion electrical insulation protective clothing: gloves, aprons,

boots.

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Elastomer Commonly used elastomer:

Acrylic rubbers Butyl rubbers Nitrite rubbers Polychloroprene rubbers Polyisoprene (natural rubber) Polysulphide rubbers Rubber hydrochloride Silicone rubbers

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Acrylic rubbersElastomer

These are derived from the same family of polymeric materials as ‘Perspex’

This group of rubbers has excellent resistance to oils, oxygen, and ultraviolet radiation, and they are used as the basis for the paints developed for motor vehicles.

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Butyl rubbersElastomer

This rubber is impervious to gases and is used as a vapour barrier and for hose linings

It is highly resistant to outdoor weathering and ultraviolet radiation and is used for construction industry sealant.

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Nitrite rubbersElastomer

This has excellent resistance to oils and solvents and can be readily bonded to metals

It is used for petrol and fuel oil hoses, hose linings, and aircraft fuel tank linings. It is also resistant to refrigerant gases.

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Polychloroprene rubbersElastomer

It has good resistance to oxidation, aging, and weathering

It is resistant to oils, solvents, abrasion and high temperatures

Because of its chlorine content it is fire resistant

It is used as a flexible electrical insulator and for gaskets, hoses, engine mounts, sealants, rubber cements and protective clothing.

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Polyisoprene (natural rubber) Elastomer

This is derived from the sap of a tree It has a relatively high tensile strength Unfortunately solvents, petrol, mineral

oils, and ozone readily attack it It degrades rapidly in the presence of

strong sunlight  Modified by additives to give it increased

strength and wear resistant properties Natural rubber is used for vehicle tyres

as it has excellent antiskid properties

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Polysulphide rubbersElastomer

Although this rubber has low mechanical strength, its resistance to solvents and its impermeability to gases are excellent and its weathering characteristics are outstanding

It also has good bonding properties and is widely used in the construction industries as a sealant. They are also used as fuels.

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Rubber hydrochloride Elastomer

This material is better known as ‘Pliofilm’ and is used to form a transparent film for the vacuum packaging of foods

Its unusually high tensile strength and tear resistance easily identifies it.

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Silicone rubbersElastomer

Although silicone rubber has a relatively low tensile strength, it has an exceptionally wide working temperature range of -80°C to +235°C

It is also used in space vehicles and artificial satellites.