Polyacetal (Polyaldehyde)
Properties
Polyoxymethylene (POM), sometimes called polyformaldehyde or
acetal, is the most important polyacetal. It is a highly
crystalline thermoplastic that is known for its high flexural and tensile strength, stiffness, hardness, and
low creep under stress.
It also has a low coefficient of friction, excellent chemical resistance and outstanding fatigue properties, but
only moderate heat stability and 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 or without a small amount of cyclic ether (typically ethylene oxide or
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. Some pigments such as carbon black also provide some UV protection.
POM and its copolymers are often an excellent choice for applications that require low friction, tight tolerances, and high impact and creep resistance. It can be processed by all common methods used for thermoplastics. It can be economically injection molded to even extremely intricate parts or it can be extruded to rods, pipes, profiles and sheets which are often further machined with cutting tools to produce (high precision) finished parts.
COMMERCIAL POM's
Commercial grades of POM are available under the trade names Delrin®, Duracon®, Celcon® (copolymer) and Ultraform® (copolymer). Rubber-toughened and fiberglass-reinforced grades of polyacetal are also available. These grades are used for more demanding applications where higher impact and flexural strength and/or increased stiffness and hardness are required.
APPLICATIONS
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 resistance. 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. Some examples are machinery parts such as gears, bearings, rollers, conveyor chains, pump and filter housings, airflow valve fittings and valves; parts in the transportation industry such as fuel pump and filter housings, cooling-fan parts, fuel caps, door handles, steering column - gear shift assemblies, cranks of sliding sunroofs, windshield wiper clips and windshield washer nozzles; household appliances and tools such as food mixer parts, water sprinkler nozzles, shower heads, furniture joints and drawer runners, parts in espresso and coffee-making machines, knife and tool handles.