Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. Short-chained POM (chain length between 8 and 100 repeating units) is also better known as paraformaldehyde (PFA). As with many other synthetic polymers, polyoxymethylenes are produced by different chemical firms with slightly different formulas and sold as Delrin, Kocetal, Ultraform, Celcon, Ramtal, Duracon, Kepital, Polypenco, Tenac and Hostaform. POM is characterized by its high strength, hardness and rigidity to −40 °C (−40 °F). POM is intrinsically opaque white because of its high crystalline composition but can be produced in a variety of colors. POM has a density of 1.410–1.420 g/cm3. POM's electrical resistivity is 14×1015 Ω⋅cm making it a dielectric with a 19.5MV/m breakdown voltage. Typical applications for injection-molded POM include high-performance engineering components such as small gear wheels, eyeglass frames, ball bearings, ski bindings, fasteners, gun parts, knife handles, and lock systems. The material is widely used in the automotive and consumer electronics industry.
Development Polyoxymethylene was discovered by Hermann Staudinger, a German chemist who received the 1953 Nobel Prize in Chemistry. He had studied the polymerization and structure of POM in the 1920s while researching macromolecules, which he characterized as polymers. Due to problems with thermostability, POM was not commercialized at that time. Circa 1952, research chemists at DuPont synthesized a version of POM, and in 1956 the company filed for patent protection of the homopolymer, forgetting to mention in the patent the term copolymer, opening thus the road to competitors. DuPont credits R. N. MacDonald as the inventor of high-molecular-weight POM. Patents by MacDonald and coworkers describe the preparation of high-molecular-weight hemiacetal-terminated (~O−CH2OH) POM, but these lack sufficient thermal stability to be commercially viable. The inventor of a heat-stable (and therefore useful) POM homopolymer was Stephen Dal Nogare, who discovered that reacting the hemiacetal ends with acetic anhydride converts the readily depolymerizable hemiacetal into a thermally stable, melt-processable plastic. In 1960, DuPont completed construction of a plant to produce its own version of acetal resin, named Delrin, at Parkersburg, United States. Also in 1960, Celanese completed its own research. Shortly thereafter, in a limited partnership with the Frankfurt firm Hoechst AG, a factory was built in Kelsterbach, Hessen; from there, Celcon was produced starting in 1962, with Hostaform joining it a year later. Both remain in production under the auspices of Celanese and are sold as parts of a product group now called Hostaform/Celcon POM.
Production
Different manufacturing processes are used to produce the homopolymer and copolymer versions of POM.
Homopolymer 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. Due to the manufacturing process, large-diameter cross-sections may have pronounced centerline porosity. Typical examples of polyoxymethylene homopolymers are Delrin variants.
Copolymer The polyoxymethylene copolymer replaces about 1–1.5% of the −CH2O− groups with −CH2CH2O−. To make polyoxymethylene copolymer, formaldehyde is generally converted to trioxane (specifically 1,3,5-trioxane, also known as trioxin). This is done by acid catalysis (either sulfuric acid or acidic ion-exchange resins) followed by purification of the trioxane by distillation and/or extraction to remove water and other active hydrogen-containing impurities. Typical copolymers are Hostaform from Celanese and Ultraform from BASF. The co-monomer is typically dioxolane, but ethylene oxide can also be used. Dioxolane is formed by reaction of ethylene glycol with 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, BF3OEt2. The polymerization can take place in a non-polar solvent (in which case the polymer forms as a slurry) or in neat trioxane (e.g. in an extruder). After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by melt or solution hydrolysis to remove unstable end groups. Stable polymer is melt-compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers.
Fabrication POM is supplied in a granulated form and can be formed into the desired shape by applying heat and pressure. The two most common forming methods employed are injection molding and extrusion. Rotational molding and blow molding are also possible. Typical applications for injection-molded POM include high-performance engineering components (e.g. gear wheels, ski bindings, yoyos, fasteners, lock systems). The material is widely used in the automotive and consumer electronics industry. There are special grades that offer higher mechanical toughness, stiffness or low-friction/wear properties. POM is commonly extruded as continuous lengths of round or rectangular section. These sections can be cut to length and sold as bar or sheet stock for machining.
Typical mechanical properties POM is a hard plastic that cannot be glued, but can be joined to POM by melting. Melted POM does not adhere to steel tools used to shape it.
POM is a relatively strong plastic, nearly as strong as epoxy, or aluminum, but a bit more flexible:
POM is wear-resistant:
Availability and price POM materials can have trademarked producer-specific names, for example "Delrin". Prices for large quantities, in October 2023, in US$/kg:
US: 3.26, Europe 2.81, China 2.58, SEA 2.30, Middle East 1.68 . Prices and availability retail / small wholesale:
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