Some Industrially Important Polymers - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 111

The result is a linear polymer with branches that are four carbons in length.
The branches decrease the ability of the polyolefin chains to pack tightly and
decrease the density. The greater the number of branches, the lower is the den-
sity. Depending on the process and the catalyst system, the comonomer can
incorporate somewhat uniformly. However, in other instances the comonomer
incorporation is less uniform and can be more prevalent in lower molecular
weight chains. In the extreme, this can result in a lower melting waxy material
which can give process problems. The choice of comonomer, the amount of
comonomer, and the uniformity of incorporation all influence properties and
differentiate various grades of polyethylene.
HDPE has small levels of or no comonomer and a density of
0.940 - 0.970 g/cm 3 . Linear low density polyethylene (LLDPE) has a
greater amount of comonomer. The grade cutoffs can vary from com-
pany to company, but generally linear polyethylene with a density from
0.890 - 0.920 g/cm 3 is considered LLDPE and 0.920 - 0.940 g/cm 3 is medium
density polyethylene (MDPE). It may seem surprising that such distinctions
are made among seemingly small changes in density, but these changes are
significant. In the industry, densities are routinely measured to four significant
figures. As a general trend, as density increases, crystallinity increases. Mod-
ulus (stiffness) and heat softening points also increase with increasing density.
Permeation decreases with increasing density. Low permeation is usually
desirable so a polyethylene pipe for natural gas would have a lower rate of
natural gas permeation if it were made from HDPE than if from LLDPE.
Generally, low temperature toughness decreases with increasing density. A
polyethylene tub on a cold winter day in Buffalo left outside and then dropped
is more likely to break if it is made from HDPE than if made with LLDPE.
Another differentiating property of polyethylene is molecular weight.
The molecular weight is influenced by the choice of catalyst system and the
polymerization conditions. As the polymerization proceeds, the active site
can decompose in a chain termination reaction. The molecular weight is
determined by the relative reaction rates of the propagation and termination
reactions. Many catalyst systems make very high molecular weights, and in
these circumstances a chain termination agent, usually hydrogen, is added
to control molecular weight. The polydispersity can be influenced by the
nature of the catalyst system. A single-site catalyst will give a narrower
polydispersity versus a Ziegler - Natta system. Combinations of catalysts can