1 material science polymers. 2 polymers are materials whose molecules are built up from a series of...
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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 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 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 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.