Review of POM

Polyoxymethylene (POM) is one of the important engineering plastics. It has high mechanical strength, fatigue resistance, creep resistance, wear resistance, and chemical resistance. Therefore, it has an irreplaceable position in engineering materials. POM has been widely used in aviation, automotive, precision instruments, electrical and electronic, machinery, construction and other industries.

POM resin is made of methanol as the raw material, and is made of formaldehyde. There are two kinds of homopolymers and copolymers, but their synthetic routes and synthetic methods are different. Generally, homopolymerized POM is directly polymerized by high-purity gas formaldehyde, while copolymerized POM is polymerized by copolymerization of paraformaldehyde with ethylene oxide, dioxolane and other comonomers.

At present, most of industrial formaldehyde is produced by methanol oxidation, using methanol as the raw material, by mixing methanol and air, and contacting with a heated catalyst. Since the upper limit of the explosive limit of the mixed gas of a drunk and air is 37%, and the lower limit is 7%, according to the difference of methanol concentration in the mixed gas, the methanol oxidation method can be divided into the methanol excess method and the air excess method.

Homopolymerized POM is generally made by directly polymerizing high-purity formaldehyde, and its synthesis process can be divided into three stages, namely: formaldehyde refining, polymerization of homopolymerized POM, and stable treatment of homopolymerized POM.

The terminal hydroxyl group formed at the end of the homopolymerized POM macromolecule is easily decomposed due to thermal instability. Therefore, the industry generally adopts the method of esterification to thermally stabilize the homopolymerized POM. The esterification agent used in the heat treatment of homopolymerized POM is generally acid anhydrides, such as acetic anhydride.

Copolymerized POM is high-purity formaldehyde or paraformaldehyde and a cyclic ether or cyclic methylal containing at least one C-C one unit, such as ethylene oxide, 1,3-dioxolane, 1, 4-Butanediol diformal is copolymerized, and the current industrial method mostly uses paraformaldehyde as the monomer. The synthesis process of copolymerized POM can also be divided into three stages, namely synthesis and purification of paraformaldehyde, polymerization of copolymerized POM, and stabilization of copolymerized POM.

Because homopolymeric POM is composed of the same kind of monomers, the molecular chain has high structural symmetry and strong order, which is conducive to the formation of relatively complete crystals with high crystallinity and a melting temperature of 175 ° C. The copolymer molecular chain has poor structural symmetry, low crystallinity and melting temperature of 165 ℃. The advantages of homopolymerized POM are good mechanical properties, with high tensile strength, bending strength, fatigue strength, impact strength, rigidity, surface hardness and heat deformation temperature, etc. The disadvantages are poor heat resistance and narrow molding temperature range, etc. . The heat resistance of copolymerized POM is better than that of homopolymerized POM, and the molding temperature range is wider. The crystallinity, melting temperature and mechanical properties of copolymerized POM are generally lower than those of homopolymerized POM.