Materials:
Plastics
Introduction
Humans have taken advantage of
the versatility of polymers for centuries in the
form of oils, tars, resins, and gums. However,
it was not until the industrial revolution that
the modern polymer industry began to develop.
In the late 1830s, Charles Goodyear succeeded
in producing a useful form of natural rubber through
a process known as "vulcanization." Some 40 years
later, Celluloid (a hard plastic formed from nitrocellulose)
was successfully commercialized. Despite these
advances, progress in polymer science was slow
until the 1930s, when materials such as vinyl,
neoprene, polystyrene, and nylon were developed.
The introduction of these revolutionary materials
began an explosion in polymer research that is
still going on today.
Some degree of compromise is
almost always necessary in designing plastic parts.
Arriving at the best compromise usually requires
satisfying the mechanical, thermal, and electrical
requirements of the part, utilizing the most economical
resin or compound that will perform satisfactorily
and be attractive, and choosing a manufacturing
process compatible with the part design and material
choice.
Probably no plastic will provide
100% of the requirements for the desired performance,
appearance, processibility, and price. Selecting
the best qualified material is not based simply
on comparing numbers on published data sheets;
such values can be grossly misleading. For example,
choosing the most economical material for a part
by comparing the cost per pound of various plastics
is a mistake. Some plastics weigh twice as much
per cubic inch as others and so would require
twice as much to fill a given cavity and cost
twice as much to ship.
Polymers have a wide range of mechanical
properties. Network polymers are often quite strong
and stiff (high yield strength and modulus of elasticity),
although they have poor ductility. Linear polymers
have much lower strength but quite high ductility,
and elastomers have very large values of ductility
and a variable modulus of elasticity. Polymers are
generally classified according to their structure,
properties and use as:
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