Page 61 - Handbook of Plastics Technologies

Source: Handbook of Plastics Technologies

CHAPTER 2

THERMOPLASTICS

Anne-Marie Baker, Joey L. Mead
University of Massachusetts
Lowell, Massachusetts

2.1 INTRODUCTION

Plastic materials encompass a broad range of materials. The effect of structure on the re-
sulting properties was discussed more fully in Chap. 1. Here, we describe the details of the
wide variety of plastic materials available for use. For a comprehensive listing of proper-
ties, the reader should refer to Chap. 1.

2.2 POLYMER CATEGORIES

2.2.1 Acetal (POM)
Acetal polymers are formed from the polymerization of formaldehyde. They are also
given the name polyoxymethylenes (POMs). Polymers prepared from formaldehyde were
studied by Staudinger in the 1920s, but thermally stable materials were not introduced un-
1
til the 1950s, when DuPont developed Delrin. Hompolymers are prepared from very pure
formaldehyde by anionic polymerization as shown in Fig. 2.1. Amines and the soluble
2
salts of alkali metals catalyze the reaction. The polymer formed is insoluble and is re-
moved as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzip-
ping with the release of formaldehyde. The thermal stability of the polymer is increased by
esterification of the hydroxyl ends with acetic anhydride. An alternative method to im-
prove the thermal stability is copolymerization with a second monomer, such as ethylene
3
oxide. The copolymer is prepared by cationic methods developed by Celanese and mar-

FIGURE 2.1 Polymerization of formaldehyde to polyoxymethylene.

2.1
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