Polyoxymethylene

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It has been suggested that Delrin be merged into this article or section. (Discuss)

Polyoxymethylene (POM), also known as acetal resin, polytrioxane, polyformaldehyde, and paraformaldehyde, is an engineering plastic used to make gears, bushings and other mechanical parts (cf. nylon, teflon, UHMWPE). The most important polyacetal resin, it is a thermoplastic with good physical and processing properties. It is also sold under the trade names Delrin, Kepital, Celcon, "Hostaform" and Ultraform, the last three being copolymers. Its chemical formula is -(-O-CH₂-)_n-.

Long molecules of this compound are also found in formalin preserving solutions. Since formaldehyde polymerizes in water, formalin contains very little formaldehyde in the form of H₂CO monomer; most of it forms short chains of polyformaldehyde. A few percent methanol is often added to limit the extent of polymerization.

[edit] Synthesis

To make polyoxymethylene homopolymer, anhydrous formaldehyde must be generated. The principal method is by reaction of the aqueous formaldehyde with an alcohol to create a hemiformal, dehydration of the hemiformal/water mixture (either by extraction or vacuum distillation) and release of the formaldehyde by heating the hemiformal. The formaldehyde is then polymerized by anionic catalysis and the resulting polymer stabilized by reaction with acetic anhydride.

To make polyoxymethylene copolymer, formaldehyde is generally converted to trioxane. This is done by acid catalysis (either sulfuric acid or acidic ion exchange resins) and concurrent removal of the trioxane by distillation or extraction. The trioxane is then dried to remove all water and other active hydrogen containing impurities.

The co-monomer is typically dioxolane but ethylene oxide can also be used. Dioxolane is formed by reaction of ethylene glycol with a formaldehyde source (trioxane or concentrated aqueous formaldehyde) over an acid catalyst. Other diols can also be used.

Trioxane and Dioxolane are polymerized using an acid catalyst, often boron trifluoride etherate. The polymerization can take place in a non-polar solvent (in which case the polymer forms as a slurry) or in the melt (e.g. an extruder). After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by heat.

Stable polymer is melt compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers.