Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
Amorphous
Polystyrene (PS) is a highly transparent, brittle material of good thermal
and dimensional stability, and is relatively inexpensive. It is widely used for
cheap moulded objects such as appliance housings and toys; when foamed
with CO 2 the Styrofoam product is used for packaging and wall tiles.
Acrylics. Polymethylmethacrylate (PMMA, ‘Perspex’) has outstanding light
transmission characteristics and resistance to weathering, although its
mechanical properties are only fair. In the form of transparent sheet it is
used for aircraft windows and laminated windscreens; it can also be used
for lenses and outdoor signs.
Polycarbonates show excellent dimensional stability, with good impact
resistance and ductility. They are employed as a base for photographic
film and also for lenses. Their toughness makes them appropriate for the
manufacture of safety helmets.
Polyvinylchloride (PVC) is good for low-cost general application. It is
widely used for floor coverings. Though normally rigid, it may be made
flexible by the use of plasticizers to make (e.g.) artificial leather, garden
hoses and clothing.
5.1.2 Thermosets
Epoxies possess an excellent combination of mechanical properties and
corrosion resistance. They are familiar as an adhesive (‘Araldite’) and as
the matrix of fibre-glass laminates. They are also used for electrical
mouldings and for protective coatings.
Phenolics (phenolformaldehyde) are inexpensive and show excellent thermal
stability to over 150°C, but are rather brittle ‘Bakelite’ is used for electrical
fixtures. Phenolics may be combined with a large number of resins and
fillers to make ‘Tufnol’ and ‘Formica’, for example.
Unsaturated polyester resins (UPRs) are thermoset materials that are often
fibre reinforced as fibreglass or laminates, and are significantly cheaper
than epoxy. Propylene glycol is the preferred glycol for manufacturing
UPR for many thermoset composite applications: the liquid resins are
stable for long periods, but can be triggered to cure by a peroxide catalyst.
They are used for fibreglass boat hulls, automobile body components
such as bumper beams, body panels and catalytic converter heat shields.
5.1.3 Elastomers
Elastomers are all amorphous, except at high strains, which tend to order
the molecules of the material (see Section 5.4.2).
Polyisoprene, natural rubber (NR) has good abrasion resistance, but low
resistance to heat, ozone and oil.