12 Polymer Categories 121 Acetal POM

Acetal polymers are formed from the polymerization of formaldehyde. They are also known by the name polyoxymethylenes (POM). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s when DuPont developed Delrin.1 Homopolymers are prepared from very pure formaldehyde by anionic polymerization, as shown in Fig. 1.4. Amines and the soluble salts of alkali metals catalyze the reaction.2 The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldhyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stability is copoly-

Atactic Syndiotactic

Isotactic Syndiotactic

Figure 1.3 Isotactic, syndiotactic, and atactic polymer chains.

Isotactic Syndiotactic

Figure 1.3 Isotactic, syndiotactic, and atactic polymer chains.

Atactic merization with a second monomer such as ethylene oxide. The copolymer is prepared by cationic methods.3 This was developed by Celanese and marketed under the tradename Celcon. Hostaform is another copolymer marketed by Hoescht. The presence of the second monomer reduces the tendency for the polymer to degrade by unzipping.4

There are four processes for the thermal degradation of acetal resins. The first is thermal or base-catalyzed depolymerization from the chain, resulting in the release of formaldehyde. End capping the polymer chain will reduce this tendency. The second is oxidative attack at random positions, again leading to depolymerization. The use of antioxidants will reduce this degradation mechanism. Copolymerization is also helpful. The third mechanism is cleavage of the acetal linkage by acids. It is, therefore, important not to process acetals in equipment used for polyvinyl chloride (PVC), unless it has been cleaned, due to the possible presence of traces of HCl. The fourth degradation mechanism is thermal depolymerization at temperatures

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