Polyoxymethylene (POM)

Polyoxymethylene (POM, polyacetal) Polyoxymethylene (polymethylene oxide) is a synthetic polymer, the product of formaldehyde polymerization. Designation: POM. Other designations: POM C (copolymer), POM H (homopolymer), POM-HI (impact-resistant grades containing non-thermoplastic rubber), ACETAL, acetals, etc. General group name: polyacetals. When choosing a particular polyacetal grade, it is important to understand their similarities and differences. POM copolymer and homopolymer are similar in most of their properties: hardness, toughness, low coefficient of friction, good wear resistance, excellent fatigue endurance. Both types of material are opaque, have low UV resistance, are prone to thermal decomposition, are readily attacked by acids and alkalis, and have poor fire performance. However, there are also some differences between these types of POM. Homopolymers generally have a higher degree of crystallinity. Consequently, they offer the following short-term mechanical properties: higher stiffness, tensile strength, impact resistance and initial creep resistance. Compared with homopolymers, copolymers have better oxidation resistance and show better resistance to creep and creep rupture over longer time scales. There are also polyacetals with fillers. Glass-fiber-filled copolymers have better mechanical properties than the corresponding homopolymers. This is because a slightly different chemical structure provides better bonding between the polymer and the glass fiber.

Comparing copolymers and homopolymers: copolymers, having lower crystallinity, generally offer better dimensional stability, a lower coefficient of friction and greater wear resistance. Although homopolymers have lower moisture absorption, the copolymer is less prone to hydrolysis in hot water.

Copolymers also have better resistance to alkaline media. Owing to their higher crystallinity, homopolymers have a higher heat deflection temperature, whereas copolymers allow higher continuous-use temperatures thanks to better long-term stability. The properties of POM C copolymers and POM H homopolymers are set out in more detail below. POM C polyacetals POM copolymer has a linear structure with a high degree of crystallinity, which provides a wide range of characteristics, namely: excellent wear resistance, long-term fatigue endurance, impact strength and creep resistance, as well as excellent resistance to moisture, solvents and strong alkalis. Its chemical structure provides higher resistance to thermal and oxidative degradation compared with acetal homopolymers. Properties: — Very strong at sub-zero temperatures (-40°C) — Very hard and stiff — Easy to color — Heat deflection temperature up to 100 °C — Very good sliding and wear characteristics — Excellent chemical resistance to fuels, solvents, strong alkalis. — Excellent resilience — Low moisture absorption — Resistance to stress cracking Existing POM C grade groups base grades high-flow extrusion reinforced with glass fiber and glass beads with improved tribological characteristics (low coefficient of friction and high wear resistance) high impact strength medical grades low odor emission with increased resistance to aggressive cleaning agents or chlorinated water resistant to hot diesel fuel UV-stabilized electrically conductive with MetaLX ™ metallic effect Applications: Injection molding, extrusion, centrifugal molding, physical foaming. POM H polyacetals (Delrin) Has a higher melting temperature and higher strength, with fast molding cycles thanks to a very high degree of crystallinity, but is sensitive to hydrolysis (exposure to moisture when heated). Special properties: — Higher stiffness compared with POM copolymer — Excellent resilience characteristics — Very good electrical insulation properties — Absorbs virtually no moisture — Higher resistance to chemicals (pH 4 - 9) — May exhibit crystalline shrinkage during molding, which must be taken into account when designing molds Possible uses of POM H Sliding elements, gears, control cams, snap-fit joints, machine and mechanism components.