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. A typical example is DuPont’s Delrin.
Copolymer
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 purificaiton of the trioxane by distillation and/or extraction to remove water and other active hydrogen containing impurities. Typical copolymers are Hostaform from Ticona 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, BF3 OEt2. 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 in order to remove the unstable end groups.
Stable polymer is melt compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers.
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