Building Design and Construction Handbook - Merritt - Ventech!

4.92 SECTION FOUR
Nylon. Molded nylon is used in increasing quantities for impact and high resistance
to abrasion. It is employed in small gears, cams, and other machine parts,
because even when unlubricated they are highly resistant to wear. Its chemical
resistance, except to phenols and mineral acids, is excellent. Extruded nylon is
coated onto electric wire, cable, and rope for abrasion resistance. Applications like
hammerheads indicate its impact resistance.
Polystyrene. This is one of the lightest of the presently available commercial
plastics. It is relatively inexpensive, easily molded, has good dimensional stability,
and good stability at low temperatures; it is brilliantly clear when transparent and
has an infinite range of colors. Water absorption is negligible even after long immersion.
Electrical characteristics are excellent. It is resistant to most corrosive
chemicals, such as acids, and to a variety of organic solvents, although it is attacked
by others. Polystyrenes as a class are considerably more brittle and less extensible
than many other thermoplastic materials, but these properties are markedly improved
in copolymers. Under some conditions, they have a tendency to develop
fine cracks, known as craze marks, on exposure, particularly outdoors. This is true
of many other thermoplastics, especially when highly stressed. It is widely used in
synthetic rubbers.
4.73 ELASTOMERS, OR SYNTHETIC RUBBERS
Rubber for construction purposes is both natural and synthetic. Natural rubber, often
called crude rubber in its unvulcanized form, is composed of large complex molecules
of isoprene. Synthetic rubbers, also known as elastomers, are generally
rubber-like only in their high elasticity. The principal synthetic rubbers are the
following:
GR-S is the one most nearly like crude rubber and is the product of styrene and
butadiene copolymerization. It is the most widely used of the synthetic rubbers. It
is not oil-resistant but is widely used for tires and similar applications.
Nitril is a copolymer of acrylonitrile and butadiene. Its excellent resistance to
oils and solvents makes it useful for fuel and solvent hoses, hydraulic-equipment
parts, and similar applications.
Butyl is made by the copolymerization of isobutylene with a small proportion
of isoprene or butadiene. It has the lowest gas permeability of all the rubbers and
consequently is widely used for making inner tubes for tires and other applications
in which gases must be held with a minimum of diffusion. It is used for gaskets
in buildings.
Neoprene is made by the polymerization of chloroprene. It has very good mechanical
properties and is particularly resistant to sunlight, heat, aging, and oil; it
is therefore used for making machine belts, gaskets, oil hose, insulation on wire
cable, and other applications to be used for outdoor exposure, such as roofing, and
gaskets for building and glazing.
Sulfide rubbers—the polysulfides of high molecular weight—have rubbery
properties, and articles made from them, such as hose and tank linings and glazing
compounds, exhibit good resistance to solvents, oils, ozone, low temperature, and
outdoor exposure.
Silicone rubber, which also is discussed in Art. 4.71, when made in rubbery
consistency forms a material exhibiting exceptional inertness and temperature re-