Homopolymer[ edit ] To make polyoxymethylene homopolymer , anhydrous formaldehyde must be generated. The formaldehyde is then polymerized by anionic catalysis , and the resulting polymer stabilized by reaction with acetic anhydride. Due to the manufacturing process, large-diameter cross-sections may have pronounced centerline porosity. Copolymer[ edit ] The polyoxymethylene copolymer replaces about 1—1. The co-monomer is typically dioxolane , but ethylene oxide can also be used.
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Homopolymer[ edit ] To make polyoxymethylene homopolymer , anhydrous formaldehyde must be generated. The formaldehyde is then polymerized by anionic catalysis , and the resulting polymer stabilized by reaction with acetic anhydride.
Due to the manufacturing process, large-diameter cross-sections may have pronounced centerline porosity. Copolymer[ edit ] The polyoxymethylene copolymer replaces about 1—1.
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 , BF3OEt2. The polymerization can take place in a non-polar solvent in which case the polymer forms as a slurry or in neat trioxane e. After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by melt or solution hydrolysis to remove unstable end groups.
Stable polymer is melt-compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers. Fabrication[ edit ] 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. The material is widely used in the automotive and consumer electronics industry. 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[ edit ] 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. POM sheets can be cut cleanly and accurately using an infrared laser, such as in a CO2 laser cutter. 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.
As can be the case with many polymers, 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. 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[ edit ] POM is typically very difficult to bond, with the copolymer typically responding worse to conventional adhesives than the homopolymer. Typical etching processes involve chromic acid at elevated temperatures.
DuPont uses a patented process for treating acetal homopolymer called satinizing that creates a surface roughness sufficient for micromechanical interlocking.
There are also processes involving oxygen plasma and corona discharge. These include epoxies , polyurethanes , and cyanoacrylates. Cyanoacrylates are useful for bonding to metal, leather, rubber, cotton, and other plastics. Solvent welding is typically unsuccessful on acetal polymers, due to the excellent solvent resistance of acetal. Electrical engineering: insulators , bobbins , connectors , parts for electronic devices such as televisions , telephones , etc.
Model: model railway parts, such as trucks bogies and hand rails handle bars. Medical: insulin pen, metered dose inhalers MDI. Food industry: Food and Drug Administration has approved some grades of POM for milk pumps, coffee spigots, filter housings and food conveyors. Sports: paintball accessories. It is often used for machined parts of paintball markers that do not require the strength of aluminium, such as handles and reciprocating bolts.
POM is also used in airsoft guns to reduce piston noise. Longboarding : puck material for slide gloves help the rider touch the road and lean on their hand to slow down, stop, or perform tricks. Music: picks , Irish flutes, bagpipes , practice chanters , harpsichord plectra, instrument mouthpieces, tips of some drum sticks.
Horology : watch bracelets e. IWC Porsche Design POM homopolymer is also susceptible to alkaline attack and is more susceptible to degradation in hot water.
Defective mouldings are most sensitive to cracking, but normal mouldings can succumb if the water is hot. Both POM homopolymer and copolymer are stabilized to mitigate these types of degradation. In chemistry applications, although the polymer is often suitable for the majority of glassware work, it can succumb to catastrophic failure. An example of this would be using the polymer clips on hot areas of the glassware such as a flask-to-column, column-to-head or head-to-condenser joint during distillation.
As the polymer is sensitive to both chlorine and acid hydrolysis, it may perform very poorly when exposed to the reactive gases, particularly hydrogen chloride.
Failures in this latter instance can occur with seemingly unimportant exposures from well sealed joints and do so without warning and rapidly the component will split or fall apart. This can be a significant health hazard, as the glass may open or smash. Here, PTFE or a high-grade stainless steel may be a more appropriate choice. In addition, POM can have undesirable characteristics when burned. The flame is not self-extinguishing, shows little to no smoke, and the blue flame can be almost invisible in ambient light.
Burning also releases formaldehyde gas, which irritates nose, throat, and eye tissues.
The general macromolecular structure of these two types of POM resin is expressed as following: About the properties of POM resin Since Polyacetal POM resin is an engineering plastic material with very high crystallinity, it has superior abrasion resistance, creep resistance a property of bearing very slow deformation due to long-term loading , fatigue resistance, and chemical resistance resistant to almost all chemicals except for strong acid. However, it also has limitations in performance such as difficulties in adding flame retardancy because of no aromatic structure such as a benzene ring in the macromolecule and high content of oxygen in the chemical composition , as well as difficulty in post-molding processing of the surface. POM resin having characteristics as described above is used for automotive parts abrasion-resistant sliding parts, fuel-related parts, and supporting structure, etc. As a chemical property unique to Polyacetal POM resin, if the pyrolytic reaction of the repetitive oxymethylene groups in the polymeric chain starts from the chain end, it progresses in a chain reaction character, resulting in molecular weight decrease. In the case of copolymer type POM resin, it has improved thermal stability compared to homopolymer type POM resin because oxyethylene groups, co-monomer units, stop the pyrolytic chain reaction.
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