| |
|
|
| |
|
The
World of Plastics |
|
|
Plastic is the general term for a wide
range of synthetic or semi-synthetic polymerization products.
They are composed of organic condensation or addition
polymers and may contain other substances to improve performance
or economics. There are few natural polymers generally
considered to be "plastics". Plastics can be
formed into objects or films or fibers. Their name is
derived from the fact that many are malleable, having
the property of plasticity.
Plastic can be classified in many ways, but most commonly
by their polymer backbone (polyvinyl chloride, polyethylene,
polymethyl methacrylate and other acrylics, silicones,
polyurethanes, etc.). Other classifications include thermoplastic,
thermoset, elastomer, engineering plastic, addition or
condensation or polyaddition (depending on polymerization
method used), and glass transition temperature or Tg.
|
|
Some plastics are partially crystalline and partially
amorphous in molecular structure, giving them both a melting
point (the temperature at which the attractive intermolecular
forces are overcome) and one or more glass transitions (temperatures
above which the extent of localized molecular is substantially
increased). So-called semi-crystalline plastics include polyethylene,
polypropylene, poly(vinyl chloride), polyamides (nylons), polyesters
and some polyurethanes. Many plastics are completely amorphous,
such as polystyrene and its copolymers, poly (methyl methacrylate),
and all thermosets
|
High
Density Polyethylene |
|
| |
Pellets
|
Plastics are polymers: long chains of atoms
bonded to one another. Common thermoplastics range from
20,000 to 500,000 in molecular weight, while thermosets
are assumed to have infinite molecular weight. These chains
are made up of many repeating molecular units, known as
"repeat units", derived from "monomers";
each polymer chain will have several 1000's of repeat
units.
|
The vast majority of plastics are composed
of polymers of carbon and hydrogen alone or with oxygen,
nitrogen, chlorine or sulfur in the backbone. Some of
commercial interest are silicon based. The backbone is
that part of the chain on the main "path" linking
a large number of repeat units together. To vary the properties
of plastics, both the repeat unit with different molecular
groups "hanging" or "pendant" from
the backbone, (usually they are "hung" as part
of the monomers before linking monomers together to form
the polymer chain). This customization by repeat unit's
molecular structure has allowed plastics to become such
an indispensable part of twenty first-century life by
fine tuning the properties of the polymer.
The most commonly used thermoplastic materials are polystyrene
(low cost, lacking the strength and longevity of other
materials), ABS or acrylonitrile butadiene styrene (a
co-polymer or mixture of compounds used for everything
from Lego parts to electronics housings), nylon (chemically
resistant, heat resistant, tough and flexible - used for
combs), polypropylene (tough and flexible - used for containers),
polyethylene, and polyvinyl chloride or PVC (more common
in extrusions as used for pipes, window frames, or as
the insulation on wiring where it is rendered flexible
by the inclusion of a high proportion of plasticiser).
|
|
Much of the information on this page
was sourced from Wikipedia.org
|
|
|
|
|
|
|
|
|
|
