polyoxymethylene

Polyoxymethylene (POM), also known as polyformaldehyde or acetal resin, is a crystalline engineering thermoplastic valued for high stiffness, good strength, low friction, and excellent dimensional stability. It resists many fuels and oils and maintains shape under load, making it a preferred material for precision mechanical parts.

Chemically, POM consists of repeating (-CH2-O-) units. It is available mainly as two families: homopolymers (POM-H) and copolymers (POM-C). POM-H is derived from formaldehyde and offers very high rigidity but is more sensitive to hydrolysis and thermal degradation. POM-C incorporates comonomers such as 1,3-dioxolane to improve hydrolytic stability and processability, yielding greater service temperature and moisture resistance.

Manufacture involves polymerization of formaldehyde under acidic or catalytic conditions, followed by stabilization with acetyl end groups to reduce chain scission. Commercial POM is often sold as acetylated polyformaldehyde (acetal). Copolymers are synthesized by co-polymerizing formaldehyde with the chosen comonomer, which enhances resistance to water and heat during processing and use.

Key properties include high modulus and creep resistance, low coefficient of friction, and low to moderate water absorption. POM displays good chemical resistance to fuels and many solvents, but is susceptible to hydrolysis under hot water or steam and shows limited resistance to strong bases and acids. Continuous service temperatures are typically around 80–100°C, with melting points in the range of roughly 165–180°C depending on grade.

Processing methods include injection molding, extrusion, and blow molding. Common applications are gears, bushings, bearings, valve components, fittings, pump parts, and electrical connectors. POM is not biodegradable and is mechanically recyclable; however, high-temperature decomposition can release formaldehyde, so processing and disposal require appropriate control.