Environmental Engineering Reference
In-Depth Information
11.3
Groups and Composition of Plastics
In contrast to pure substances, plastics are in general compositions of different
compounds. They are prepared by the reaction of monomers to form long polymer
chains. In a single plastic material, polymer chains with various chain lengths are
present, and strictly speaking each chain length represents one pure substance with
its own physical and chemical properties. Therefore, the properties of the bulk ma-
terial are derived from the combination of all the chains present. If a polymer made
from the same monomer differs in the composition of the chains, it may also differ
in its properties. In general, the molecular weight of a polymer chain is a multiple of
the monomer molecular weight. The number of monomers connected in one chain
is called the degree of polymerization. Since various chain length are present, the
polymer is commonly characterized by its molar mass distribution. Several defini-
tions are used, but most important are the number average
M
n
and the weight aver-
age
M
w
of the molar weight. The number average tells us at what molar mass the
highest number of chains can be found. The weight average tells at what molar mass
the chain length fraction with the highest mass can be found. Since longer chains
contribute more to
M
w
than to
M
n
due to their higher molar mass,
M
w
is always high-
er or equal to
M
n
. The quotient of
M
w
and
M
n
is called the polydispersity index PDI:
M
M
w
PDI
=
n
For a pure substance PDI is 1. For most polymer formulations, the PDI is much
higher. Some PE or PS polymer might reach PDIs of 6 and higher.
11.3.1
Polymer Groups
Polymers are also defined by their method of synthesis. Most common are polymers
obtained by a polymer chain reaction. Monomers with double bonds are linked to-
gether forming long chains. The chain reaction is started either by an initiator or a
catalyst. The initiator is only able to start a limited number of chain reactions and is
consumed during the process. A catalyst is able to start chain reactions several times
so that only a small amount of catalyst is required. The reaction proceeds until the
chain reaction is terminated. Once terminated, a second activation of the chain is
in general not possible. Typical examples are PE, PP, PS, and PVC (Fig.
11.2
). The
polymerization of more than one monomer at the time is also commonly conducted,
resulting in copolymers. Examples for copolymers are acrylnitrile-butadiene-sty-
rene copolymer (ABS) and ethylene-propylene-diene monomer rubber (EPDM).
Another possibility is the combination of two different polymer chains in a graft
polymer. One example is high-impact PS (HIPS) in which at first styrene is polym-
erized and after styrene is consumed, small amounts of butadiene are added forming
polybutadiene at the end of the PS chains.
