| Polyoxymethylene | |
|---|---|
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Other names
Poly(oxymethylene) glycol; Polymethylene glycol |
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| Identifiers | |
| CAS number | 9002-81-7 |
| Properties | |
| Molecular formula | (CH2O)n |
| Molar mass | Variable |
| Appearance | Colorless solid |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Polyoxymethylene (POM), also known as acetal,[1] polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability.
It was discovered by Hermann Staudinger, a German chemist who received the 1953 Nobel Prize in Chemistry. He studied the polymerization and structure of POM in the 1920s to research the theory of macromolecules, which he characterized as polymers. Due to initial problems with thermal stability, POM was not commercialized.
First synthesized by DuPont research chemists around 1952, the company filed for patent protection of the homopolymer in 1956 and completed construction of a plant to produce Delrin at Parkersburg, West Virginia, in 1960. Celanese completed its study on a copolymer in 1960, producing Celcon in 1962 and Hostaform in 1963 in Kelsterbach, Germany, under a limited partnership with Ticona.
Contents |
POM is characterized by its high strength, hardness and rigidity to -40°C. POM is intrinsically opaque white, due to its high crystalline composition, but it is available in all colors. POM has a density of ρ = 1.410 - 1.420 g / cm3.[2]
POM homopolymer is a semi-crystalline polymer (75-85% crystalline) with a melting point of 175° Celsius. The POM copolymer has a slightly lower melting point of 162 – 173° Celsius.
POM is a tough material with a very low coefficient of friction. However, it is susceptible to polymer degradation catalyzed by acids, which is why both polymer types are stabilized. Both homopolymer and copolymer have chain end groups (introduced via end capping) which resist depolymerization. With the copolymer, the second unit normally is a C2 (ethylene glycol) or C4 (1,4-butanediol) unit, which is introduced via its cyclic acetal (which can be made from the diol and formaldehyde) or cyclic ether (e.g. ethylene oxide). These units resist chain cleavage, because the O-linkage is now no longer an acetal group, but an ether linkage, which is stable to hydrolysis. POM is sensitive to oxidation, and an anti-oxidant is normally added to molding grades of the material.
POM advantages:
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. 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.
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.
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 are ‘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.
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.[3][4]
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.
Acetal resins are sensitive to acid hydrolysis and oxidation by agents such as mineral acids and chlorine. POM homopolymer is also susceptible to alkaline attack and is more susceptible to degradation in hot water. Both POM homopolymer and copolymer are stabilized to mitigate these types of degradation. Thus low levels of chlorine in potable water supplies (1–3 ppm) can be sufficient enough to cause stress corrosion cracking to develop, a problem which has been experienced in both the USA and Europe in domestic and commercial water supply systems. Defective mouldings are most sensitive to cracking, but normal mouldings will succumb if the water is hot.
Widespread failure of acetal mouldings in potable and hot water supplies resulted in one of the largest class actions in the USA when acetal plumbing fittings cracked and caused flooding of homes, a problem exacerbated by similar problems with polybutylene pipework. The acetal fittings tended to fail first, followed by the pipework.
In chemistry applications, whilst 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.
Leading POM producers (approximately 800 kt market):
Other significant producers (less than 3%):
Significant coupounders (less than 1%):