Some Industrially Important Polymers - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 109

CHAPTER 8
Some Industrially Important Polymers
8.1 POLYETHYLENE
Polyethylene is the largest volume polymer produced and its precursor
ethylene is the largest volume organic chemical produced. The reason
that polyethylene is so widely used is that it has an attractive balance of
properties at an economical price. Polyethylene can be fabricated by a variety
of techniques such as injection molding, blow molding and rotomolding. It
has excellent chemical resistance and strength. The end use of a plastic is an
article - a bag, bottle, film, bumper, computer housing, medical implant, and
so forth. The polymer to make that article is chosen based upon its physical
properties and ability to be formed into the article. If these criteria are met
by several polymers, then cost usually determines which polymer is used.
Polyethylene is the lowest cost polymer for a variety of reasons. The raw
material, ethylene, is low cost. Because polyethylene is made in such high
volumes, the manufacturing cost is low. Because the density is relatively low,
it takes less weight to make an article than if a higher density material were
used. To emphasize this last point, consider an article that could be made
from either polyethylene having a density of 0.92 g/cm 3 or polycarbonate
having a density of 1.20 g/cm 3 . These polymers are purchased by weight, so
many cents or dollars per pound. Even if the purchase price were the same
(and it is not - polyethylene is much lower), the material cost to make the
polyethylene article is about 23% lower.
Polyethylene was originally made by a high pressure, high temperature
radical process. Reactor pressures may be as high as 40,000 psig and
temperatures as high as 300 ∘ C. Because of the indiscriminant nature of
radical reactions done at these high temperatures the polymer is not linear.
It has oligomeric branches as shown below. Note that the polymer has many
more repeat units and this is a sketch of only a section of the molecule.