Introduction To POM

Around 1955, DuPont obtained homopolymer of formaldehyde by polymerization of formaldehyde. Polyoxymethylene is easy to crystallize, with crystallinity above 70%. The melting temperature of homo formaldehyde is about 180 ℃.
It is another engineering plastic with excellent comprehensive properties after polyamide. It has high mechanical properties, such as strength, modulus, wear resistance, toughness, fatigue resistance and creep resistance. It also has excellent electrical insulation, solvent resistance and processability. It is one of the five general engineering plastics.
The acetal polymer, namely polyoxymethylene (POM), is formed by the polymerization of formaldehyde, which is also often called polyoxymethylene (POM). The preparation of polymers from formaldehyde has been studied as early as the 1920s, but thermal stable materials have not been prepared until DuPont developed Delrin (Dailin) in 1950. Homopolymer is prepared by anionic polymerization of very pure formaldehyde. The polymer formed is insoluble. With the progress of polymerization, precipitation continues. As the acetal resin selected by formaldehyde is pulled apart, thermal degradation occurs. The thermal stability of the polymer can be improved by esterification of hydroxy end groups with acetic anhydride. Another way to improve the thermal stability is to copolymerize with the second monomer, such as ethylene oxide, whose polymer is prepared by cationic polymerization.
There are four mechanisms of acetal resin thermal degradation. The first is thermal or alkali catalyzed chain depolymerization; The result is that formaldehyde is released, and the end group cutting of polymer can reduce this tendency; The second is that oxygen attacks the irregular position of polymer, which also leads to depolymerization. The use of antioxidants can reduce the occurrence of this degradation mechanism, and copolymerization can also help reduce this tendency; The third mechanism is that the acetal resin chain is broken by acid. The fourth degradation is thermal depolymerization when the temperature gradient exceeds 270 ℃, which is very important. It warns the operator to keep the processing temperature below 270 ℃ to avoid polymer degradation.
Acetal resin is highly crystalline, with typical crystallinity of 75% and melting point of 180 ℃. Compared with polyethylene (PE), the molecular chains are stacked more closely due to the shorter C-O bond, resulting in higher melting point of the polymer. High crystallinity endows acetal polymer with good solvent resistance. Polymers are mainly linear. Their molecular weights range from 20000 to 110000.
Acetal resin is a strong and hard thermoplastic with good fatigue and thermal stability. It has low friction coefficient and good heat resistance. It is considered that acetal resin is similar to nylon, but its fatigue resistance, creep resistance, hardness and water resistance are better than nylon. However, the creep resistance of acetal resin is not as good as that of polycarbonate. As previously mentioned, acetal resin has excellent solvent resistance, and no organic solvent that can dissolve acetal resin below 70 ℃ has been found; But it can swell in some solvents. Acetal resin is sensitive to acid, alkali and oxidant. Although the C-O bond is polar, it has been balanced, and its polarity is much smaller than the carbonyl group in nylon, resulting in a relatively low moisture absorption of acetal resin. A small amount of moisture adsorbed may cause swelling and size change, but will not cause polymer hydrolysis and degradation. The effect of moisture is much smaller than that of nylon polymers. Ultraviolet light can cause polymer degradation, which can be reduced by adding carbon black. Copolymers usually have properties similar to homopolymers, but the mechanical properties of homopolymers are slightly higher than copolymers. Their melting points are also higher, but their thermal stability and alkali resistance are worse than copolymers. Both homopolymer and copolymer are filler materials (glass fiber, fluoropolymer, aromatic polyamide fiber and other fillers), which are made of toughened materials and ultraviolet light stabilized materials. Acetal resin and polyurethane elastomer are blended to improve their toughness, and these materials can be bought in the market.
Acetal resins for injection molding, injection molding and extrusion molding are available. It is important not to overheat or seriously overpressure caused by formaldehyde during processing. The polymer shall be cleaned before shutdown to avoid overheating during startup. Acetal resin shall be stored in a dry place. The apparent viscosity of acetal resin is less dependent on shear stress and temperature than that of polyolefin, but its melt has low elasticity and low strength. Low melt strength is a problem in blow molding. Copolymers with branched chain structure are more suitable for blow molding. The crystallization speed is very fast, and the shrinkage after molding can be completed within 48 hours after molding. It is difficult to make transparent films due to rapid crystallization.
The market demand of acetal resin in the United States and Canada in 1997 was 368 million pounds. Applications of acetal resin include gears, rollers, pipe components, pump parts, fan blades, aerosol containers made of blown film, molded chain wheels and chains, which are often used to directly replace metals. Acetal resin is mainly used for injection molding, followed by extrusion of plates and bars. The low friction coefficient of acetal resin makes it possible to make good bearings.