Phase II: Production of POM Sheet and POM Rod

Different manufacturing processes are used to produce the homopolymer and copolymer versions of POM sheet and POM rod.

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 purification 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.

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, fasteners, lock systems) and 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.

Machining

When supplied as extruded bar or sheet, POM may be machined using traditional methods such as turning, milling, drilling etc. These techniques are best employed where production economics do not merit the expense of melt processing. The material is free-cutting, but does require sharp tools with a high clearance angle. The use of soluble cutting lubricant is not necessary, but is recommended.

Because the material lacks the rigidity of most metals, care should be taken to use light clamping forces and sufficient support for the work piece.

Machined POM can be dimensionally unstable, especially with parts that have large variations in wall thicknesses. It is recommended that such features be ‘designed-out’ e.g. by adding fillets or strengthening ribs. Annealing of pre-machined parts before final finishing is an alternative. A rule-of-thumb is that in general, small components machined in POM suffer from less warping.

Bonding

POM is typically very difficult to bond. Special processes and treatments have been developed to improve bonding. Typically these processes involve surface etching, flame treatment or mechanical abrasion.

Typical etching processes involve chromic acid at elevated temperatures. DuPont has a patented process for treating acetal homopolymer called satinizing which creates anchor points on the surface, giving an adhesive something to grab. There are also processes involving oxygen plasma and corona discharge.

Once the surface is prepared, a number of adhesives can be used for bonding. These include epoxies, polyurethanes, and cyanoacrylates. Epoxies have shown 150-500 psi shear strength on mechanically abraded surfaces and 500-1000 psi on chemically treated surfaces. Cyanoacrylates are useful for bonding to metal, leather, rubber and other plastics.

Solvent welding is typically unsuccessful on acetal polymers, due to the excellent solvent resistance of acetal.

Thermal welding through various methods has been used successfully on both homopolymer and copolymer.