The production of stable thermoplastics from formaldehyde was discovered around 60 years ago, using two independent approaches. To prevent unzipping of the polymer chains at high temperature, DuPont used an end cap mechanism. Celanese opted for incorporation of a comonomer which also stopped the unzipping. Today you still have a choice between the homopolymer grades and the copolymer grades of polyacetal (POM) but are they totally interchangeable?
For most of the attributes of polyacetal, they are similar —: hard, tough, low coefficient of friction, good wear resistance, excellent fatigue endurance. They are both opaque, with poor uv resistance, prone to thermal degradation, easily attacked by acids and alkali and have poor fire performance.
However there are subtle differences. Homopolymers tend to have higher crystallinity levels. Hence they have better short term mechanical properties — stiffness, tensile strength, impact resistance and initial creep resistance. Copolymer grades have better oxidation resistance and exhibit better resistance to creep and creep rupture at longer time scales. Surprisingly, glass fibre filled copolymer grades have better mechanical properties than the corresponding homopolymer. This is because the slightly different chemical structure gives better coupling to the glass fibre.
Having lower crystallinity, copolymers tend to have better dimensional stability and win out on lower friction and less wear. Although homopolymer grades have a lower moisture uptake, the copolymer is less susceptible to hydrolysis in hot water. Likewise copolymers have better resistance to alkali materials. Because of the higher crystallinity, homopolymer has a higher heat distortion temperature but copolymer grades have higher continuous use temperatures because of better long term stability.
For processors, the copolymer grades are attractive because of the lower process temperature associated with lower crystallinity and also a wider process window.
Copolymer: Celcon, Hostaform, Kepital, Lucel, Schulaform, Tecaform, Ultraform
This Polyacetal article was written by Dr.Charlie Geddes for Hardie Polymers
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