Page 131 - Handbook of Plastics Technologies

Source: Handbook of Plastics Technologies

CHAPTER 3

THERMOSETS

Rudolph D. Deanin
University of Massachusetts
Lowell, Massachusetts

Plastics are organic polymers that can be poured or squeezed into the shape we want and
then solidified into a finished product. Thermoplastics are linear polymer molecules that
soften or melt when heated and solidify again when cooled. This is a reversible physical
process that can be repeated many times. Thus, it is a simple low-cost process that ac-
counts for 85 percent of the plastics industry.
Thermosetting plastics are low-molecular-weight monomers and oligomers with multi-
ple reactive functional groups, which can be poured, melted, or squeezed into the shape
we want and then solidified again by chemical reactions forming multiple primary cova-
lent bonds that cross-link them into three-dimensional molecules of almost infinite molec-
ular weight. These are irreversible chemical processes that cannot be repeated. They
account for 15 percent of the plastics industry, they include a great variety of chemical re-
actions and conversion processes, and they go into a very broad range of final products.
Thus, there is a great difference between thermoplastics and thermosets, both in terms
of materials chemistry and applications, and in terms of the mechanical processes used to
produce finished products.

3.1 MATERIALS AND APPLICATIONS

The major thermosetting plastics, in order of decreasing market volume, are polyure-
thanes, phenol-formaldehyde, urea-formaldehyde, and polyesters. More specialized ther-
mosets include melamine-formaldehyde, furans, “vinyl esters,” allyls, epoxy resins,
silicones, and polyimides. While they may sometimes compete with each other and with
thermoplastics, for the most part, each of them has unique properties and fills unique mar-
kets and applications.

3.1.1 Polyurethanes
With a U.S. market of 6 billion pounds per year, polyurethanes are the leading family of
thermosetting plastics. Of the 100 or so families of commercial plastics, they are the most

3.1
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