Plastic is the general common term for a wide range of
synthetic or semisynthetic organic amorphous
solid materials suitable for the manufacture of
industrial products. Plastics are typically polymers of high
molecular
weight, and may contain other substances to improve
performance and/or reduce costs.
The word derives from the Greek
????????? (plastikos), "fit for molding", from
??????? (plastos) "molded".[1] [2] It
refers to their malleability, or plasticity during manufacture, that allows
them to be cast, pressed, or extruded into an enormous variety of
shapes—such as films, fibers,
plates, tubes, bottles, boxes, and much more.
The common word "plastic" should not be confused with the
technical adjective "plastic", which is applied to any material
which undergoes a permanent change of shape (a "plastic
deformation") when strained beyond a certain point. Aluminum, for
instance, is "plastic" in this sense, but not "a plastic" in the
common sense; while some plastics, in their finished forms, will
break before deforming — and therefore are not "plastic" in the
technical sense.
There are two types of plastics, thermoplastic and thermoset.
Thermoplastics, if exposed to heat, will melt in two to seven
minutes. Thermosets will keep their shape until they are charred and
burnt. Some examples of thermoplastics are grocery bags, piano keys
and some automobile parts. Examples of thermosets are kid's dinner
sets and circuit boards
Plastics can be classified by their chemical structure, namely
the molecular
units that make up the polymer's backbone and side
chains. Some important groups in these classifications are the acrylics, polyesters, silicones, polyurethanes, and halogenated
plastics. Plastics can also be classified by the chemical
process used in their synthesis, e.g. as condensation, polyaddition, cross-linking, etc.[3]
Other classifications are based on qualities that are relevant
for manufacturing or product design. Examples of such classes are
the thermoplastic and thermoset, elastomer, structural, biodegradable, electrically
conductive, etc. Plastics can also be ranked by various
physical properties, such as density, tensile strength, glass
transition temperature, resistance to various chemical
products, etc.
Due to their relatively low cost, ease of manufacture,
versatility, and imperviousness to water, plastics are used in an
enormous and expanding range of products, from paper clips to
spaceships. They have already displaced many traditional
materials—such as wood, stone, horn and bone, leather, paper, metal,
glass and ceramic—in most
of their former uses.
The use of plastics is constrained chiefly by their organic
chemistry, which seriously limits their hardness, density, and their
ability to resist heat, organic solvents, oxidation, and ionizing radiation. In particular, most
plastics will melt or decompose when
heated to a few hundred celsius.[citation
needed] While plastics can be made
electrically conductive to some extent, they are still no match for
metals like copper or aluminum.[citation
needed] Plastics are still too expensive
to replace wood, concrete and ceramic in bulky items like
ordinary buildings, bridges, dams, pavement, railroad
ties
etc