 | |
 | |
| Names | |
|---|---|
| Other names Poly(oxymethylene) glycol; polymethylene glycol | |
| Identifiers | |
| ChemSpider |
|
| UNII | |
| Properties | |
| (CH2O)n | |
| Molar mass | Variable |
| Appearance | White solid (but can be dyed) |
| Density | 1.41–1.42 g/cm3[1] |
| Melting point | 165 °C (329 °F)[2] |
| Electrical resistivity | 14×1015 Ω⋅cm[2] |
| −9.36×10−6 (SI, at 22 °C)[3] | |
| Thermochemistry | |
| 1500 J/kg·K[2] | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Polyoxymethylene (POM), also known asacetal,[4]polyacetal, andpolyformaldehyde, is an engineeringthermoplastic used in precision parts requiring highstiffness, lowfriction, and excellent dimensional stability. Short-chained POM (chain length between 8 and 100 repeating units) is also better known asparaformaldehyde (PFA). As with many other syntheticpolymers, polyoxymethylenes are produced by different chemical firms with slightly different formulas and sold as Delrin, Kocetal, Ultraform, Celcon, Ramtal, Duracon, Kepital, Polypenco, Tenac and Hostaform.
POM is characterized by its high strength, hardness and rigidity to −40 °C. POM is intrinsically opaque white because of its high crystalline composition but can be produced in a variety of colors.[1] POM has a density of 1.410–1.420 g/cm3.[5]
Typical applications forinjection-molded POM include high-performance engineering components such as small gear wheels,eyeglass frames,ball bearings,ski bindings, fasteners,gun parts, knife handles, and lock systems. The material is widely used in the automotive andconsumer electronics industry. POM's electrical resistivity is 14×1015 Ω⋅cm making it a dielectric with a 19.5MV/m breakdown voltage.[2][6]
Polyoxymethylene was discovered byHermann Staudinger, a German chemist who received the 1953Nobel Prize in Chemistry.[7] He had studied thepolymerization and structure of POM in the 1920s while researchingmacromolecules, which he characterized as polymers. Due to problems withthermostability, POM was not commercialized at that time.[8]
Circa 1952, research chemists atDuPont synthesized a version of POM,[9] and in 1956 the company filed for patent protection of thehomopolymer,[10] forgetting to mention in the patent the termcopolymer, opening thus the road to competitors. DuPont credits R. N. MacDonald as the inventor of high-molecular-weight POM.[11] Patents by MacDonald and coworkers describe the preparation of high-molecular-weighthemiacetal-terminated (~O−CH2OH) POM,[12] but these lack sufficient thermal stability to be commercially viable. The inventor of a heat-stable (and therefore useful) POM homopolymer was Stephen Dal Nogare,[13] who discovered that reacting the hemiacetal ends withacetic anhydride converts the readily depolymerizable hemiacetal into a thermally stable, melt-processable plastic.
In 1960, DuPont completed construction of a plant to produce its own version of acetal resin, namedDelrin, atParkersburg, United States.[14] Also in 1960,Celanese completed its own research. Shortly thereafter, in a limited partnership with theFrankfurt firmHoechst AG, a factory was built inKelsterbach,Hessen; from there,Celcon was produced starting in 1962,[15] withHostaform joining it a year later. Both remain in production under the auspices ofCelanese and are sold as parts of a product group now called 'Hostaform/Celcon POM.
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Different manufacturing processes are used to produce the homopolymer and copolymer versions of POM.
To make polyoxymethylenehomopolymer,anhydrousformaldehyde must be generated. The principal method is by reaction of the aqueous formaldehyde with an alcohol to create ahemiformal, dehydration of the hemiformal/water mixture (either byextraction orvacuum distillation) and release of the formaldehyde by heating the hemiformal. The formaldehyde is then polymerized byanionic catalysis, and the resulting polymer stabilized by reaction withacetic anhydride. Due to the manufacturing process, large-diameter cross-sections may have pronounced centerline porosity.[16] A typical example is DuPont's Delrin.
The polyoxymethylenecopolymer replaces about 1–1.5% of the −CH2O− groups with −CH2CH2O−.[17]
To make polyoxymethylenecopolymer, formaldehyde is generally converted totrioxane (specifically1,3,5-trioxane, also known as trioxin).[18] This is done byacid catalysis (eithersulfuric acid or acidicion-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 fromCelanese and Ultraform fromBASF.
Theco-monomer is typicallydioxolane, butethylene oxide can also be used. Dioxolane is formed by reaction ofethylene glycol with aqueous formaldehyde over an acid catalyst. Other diols can also be used.
Trioxane and dioxolane are polymerized using an acid catalyst, oftenboron trifluoride etherate, BF3OEt2. The polymerization can take place in anon-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 to remove 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.[19] The two most common forming methods employed areinjection molding andextrusion.Rotational molding andblow molding are also possible.[citation needed]
Typical applications for injection-molded POM include high-performance engineering components (e.g. gear wheels, ski bindings,yoyos, fasteners, lock systems). 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.
POM is a hard plastic, that cannot be glued, but can be joined to POM by melting. Melted POM does not adhere to steel tools used to shape it.[20][21]
| Density | 1.41 | kg/dm3 |
| Melting point | 165 | °C |
| Specific thermal capacity | 1500 | J/kg/K |
| Specific thermal conductivy | 0.31 to 0.37 | W/m/K |
| Coefficient of thermal expansion | 120[21] | ppm/K |
POM is a relatively strong plastic, nearly as strong as epoxy, or aluminum, but a bit more flexible:
| Property | value | units |
|---|---|---|
| Tensile yield stress | 62 | MPa |
| Tensile modulus | 2700 | MPa |
| Elongation at yield | 2.5 | % |
| Tensile breaking stress | 67 | MPa |
| Elongation at break | 35 | % |
| Impact strength | 80 | kJ/m2 |
POM is wear-resistant:
| Property | conditions | value | units |
|---|---|---|---|
| Coefficient of friction against steel | 0.3 m/s, 0.49 MPa | 0.31 | |
| Coefficient of friction against steel | 0.3 m/s, 0.98 MPa | 0.37 | |
| Specific wear against steel | 0.49 MPa | 0.65 | mm3/N/km |
| Specific wear against steel | 0.98 MPa | 0.30 | mm3/N/km |
| Coefficient of friction against POM | 0.15 m/s, 0.06 MPa | 0.37 |
POM materials can have trademarked producer-specific names, for example "Delrin".
Prices for large quantities, in October 2023, in US$/kg:[22]
Prices and availability retail / small wholesale :
Retail price November 2023 in the Netherlands : from 19 to 27 euro/dm3
POM is a strong and hard plastic, about as strong as plastics can be, and therefore competes with e.g.epoxy resins andpolycarbonates.
The price of POM is about the same as that of epoxy.
There are two main differences between POM and epoxy resins:
while POM can be cast when melted and adheres to practically nothing.
Epoxy resins are often used withglass fiber reinforcement, but for POM that is not an option because it does not adhere to the glass fibres.
Epoxy resins needs time to cure, while POM has fully matured as soon as it has cooled down.
POM has very little shrinkage: from 165 °C to 20 °C it shrinks by just 0.17%.
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 CO2laser 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.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, with the copolymer typically responding worse to conventional adhesives than the homopolymer.[24] Special processes and treatments have been developed to improve bonding. Typically these processes involve surface etching,flame treatment, using a specific primer/adhesive system, or mechanical abrasion.
Typical etching processes involvechromic 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.[25][26] In order to get a high bond strength without specialized tools, treatments, or roughening, one can use Loctite 401 prism adhesive combined with Loctite 770 prism primer to get bond strengths of ~1700psi.[24]
Once the surface is prepared, a number of adhesives can be used for bonding. These includeepoxies,polyurethanes, andcyanoacrylates. Epoxies have shown 150–1,050 psi (1,000–7,200 kPa)[24] shear strength. 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.[citation needed]
Thermal welding through various methods has been used successfully on both homopolymer and copolymer.[27]
Acetal resins are sensitive toacidhydrolysis andoxidation by agents such asmineral acid andchlorine.[36] POM homopolymer is also susceptible to alkaline attack and is more susceptible to degradation in hot water. Thus low levels of chlorine in potable water supplies (1–3 ppm) can be sufficient to causeenvironmental stress cracking, a problem experienced in both the US and Europe in domestic and commercial water supply systems. 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, particularlyhydrogen chloride (HCl). 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 releasesformaldehyde gas, which irritates nose, throat, and eye tissues.
