Polyacetal (Polyaldehyde)


Polyoxymethylene (POM), sometimes called polyformaldehyde, is the most important polyacetal. It is a highly crystalline polymer that is known for its high flexural and tensile strength, stiffness, hardness, and excellent chemical resistance. It also has a low coefficient of friction, and good heat and fatigue resistance, but insufficient flame resistance.

Due to the low ceiling temperature, polyaldehydes are generally unstable at ambient and elevated temperatures and easily depolymerize. For this reason, most polyaldehydes, like poly(acetaldehyde) and poly(n-butyraldehyde), have found little or no commercial use. Polyformaldehyde is an exception; its ceiling temperature is noticeably higher than those of all other polyacetals.
The temperature at which depolymerization occurs can be increased by converting the less stable hydroxyl end groups into more stable ester groups, for example by reaction with anhydrides. This reaction is referred to as end-blocking or end-capping. The stability of polyacetals can be also improved by copolymerization with other monomers. The main method is ring-opening copolymerization of trioxane (the cyclic trimer of formaldehyde) with a small amount of cyclic ether (ethylene oxide, 1,3-dioxolane).

Another drawback of polyacetals is their poor UV stability; that is, prolonged UV radiation causes degradation, leading to color change, enbrittlement, and loss of strength. To improve the UV stability, hindered-amine light stabilizers and UV adsorbers are often added to the blend.



Commercial grades of POM are available under the trade names Delrin®, Duracon®, Celcon® (copolymer) and Ultraform® (copolymer). Rubber-toughened grades of polyacetal are also available for more demanding applications, where higher impact and flexural strength are required.



POM is a high volume engineering plastic. The annual worldwide production lies in the range of half a milliom tons. It is known for its high dimensional stability, hardness and creep reisistance. These qualities allow POM to be used as a replacement for metal parts. Along with its copolymer resins, it is widely used as an engineering plastic across almost every industry. This includes mechanical, automotive, plumbing, hardware, and machinery parts as well as electronic and electrical components. Examples are pump and filter housings, shower heads, machinery parts such as gears, bearings, rollers, and conveyor chains, airflow valve fittings and valves, parts in the transportation industry such as fuel pump housings, cooling-fan parts, fuel caps, door handles, steering column - gear shift assemblies, and household appliances such as food mixer parts, pump and water sprinkler parts.



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