PLASTIC PRODUCTS - Plastics and Environmental Sustainability - page 119

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PLASTIC PRODUCTS
There are over 80,000 different plastic compounds or mixed plastic
formulationsavailablecommercially.Thesearebasedonabout20chemically
distinct classes of polymers , 1 such as polyethylenes (PE), polystyrenes (PS),
and polypropylenes (PP). Most of the common plastics produced in high
volume, called commodity plastics, are based on only about 5-6 of such
classes. A single class of polymer such as PP, however, is available in a
wide range of different grades with identical repeat-unit chemistry but with
widely different molecular weights, chain branching, degrees of crystallinity,
stereoregularity, and specific gravity.
The average molecular weight of the polymer is the single most important
metric in specifying plastics for a given application. For instance, PE with
an average molecular weight 2 of Mn ~ 1000 g/mol with an average of only
40 repeat units per chain molecule is a soft waxy material. But
ultrahigh-molecular-weight polyethylenes (UHMWPE) with Mn ~ 2-5
million g/mol are very strong materials 3 used as gel-spun fibers in
bulletproof vests and helmets. Yet, both are polyethylenes with the same
repeat units. PE with the same Mn but with different molecular weight
distributionsorpolydispersityindices(PI)canhaveverydifferentproperties.
In highly oriented fibers of PE, the tensile strength doubled (Smith et al.,
1982) when the PI was reduced from 8 to 1.1 at the constant Mw ~ 10 5
g/mol. The same grade of the same variety of plastic resin is manufactured
by a host of different companies worldwide. There will be subtle changes in
properties in resins with similar specifications but manufactured by different
companies.
Plastic resins are rarely used in virgin form but are intimately mixed with
several chemicals (called additives) before they can be formed into useful
plastic products. Processing plastics requires them to be subjected to harsh
mechanical and thermal treatment in processing machinery. Some of the
additives incorporated reduce the amount of thermal degradation or
breakdown of the resin during processing. Other additives ensure that the
resin is imparted with specific properties such as high strength or long-term
durability required in the intended product. Popular formulations or