Page 17 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

INTRODUCTION TO POLYMERS AND PLASTICS 1.3

FIGURE 1.1 Polymerization.

weight species are polymerized, the resulting polymer is termed a copolymer—for exam-
ple, ethylene vinylacetate. This is depicted in Fig. 1.2. Plastics can also be classified as ei-
ther thermoplastics or thermosets. A thermoplastic material is a high molecular weight
polymer that is not crosslinked. It can exist in either a linear or branched structure. Upon
heating, thermoplastics soften and melt, allowing them to be shaped using plastics pro-
cessing equipment. A thermoset has all of the chains tied together with covalent bonds in a
three-dimensional network (crosslinked). Thermoset materials will not flow once
crosslinked, but a thermoplastic material can be reprocessed simply by heating it to the ap-
propriate temperature. The different types of structures are shown in Fig. 1.3. The proper-
ties of different polymers can vary widely; for example, the modulus can vary from 1 MN/
2
2
m to 50 GN/m . For a given polymer, it is also possible to vary the properties simply by
varying the microstructure of the material.

FIGURE 1.2 Copolymer structure.

FIGURE 1.3 Linear, branched, and cross-linked polymer struc-
tures.

There are two primary polymerization approaches: step-reaction polymerization and
1
chain-reaction polymerization. In step-reaction (also referred to as condensation poly-
merization), reaction occurs between two polyfunctional monomers, often liberating a
small molecule such as water. As the reaction proceeds higher molecular weight species
are produced as longer and longer groups react together. For example, two monomers can
react to form a dimer then react with another monomer to form a trimer. The reaction can
be described as n-mer + m-mer → (n + m)mer, where n and m refer to the number of
monomer units for each reactant. Molecular weight of the polymer builds up gradually
with time, and high conversions are usually required to produce high molecular weight

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