Polyoxymethylene

Polyoxymethylene (POM), in the USA also commonly known under DuPont's brand name Delrin, is an engineering plastic, a polymer with the chemical formula -(-O-CH₂-)_n-. It is often marketed and used as a metal substitute, Delrin is a lightweight, low-friction, and wear-resistant thermoplastic with good physical and processing properties and capable of operating in temperatures in excess of 90 degrees Celsius (approx 200 degrees Fahrenheit). According to the material safety data sheet from DuPont, the material has a slight odor of formaldehyde.^[1]

It is also known as polyacetal, acetal resin, polytrioxane, polyformaldehyde, and paraformaldehyde; the latter term is usually restricted to the short-chained polymer. The plastic is sold under the trade names Kepital, Celcon, Hostaform and Ultraform^[2], the last three being copolymers.

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Occurrence

Since formaldehyde polymerizes in water, formalin-preserving solutions actually contain very little formaldehyde in the form of H₂CO monomer; most of it forms short chains of polyformaldehyde. A few percent methanol is often added to limit the extent of polymerization.

Synthesis

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.

To make polyoxymethylene copolymer, formaldehyde is generally converted to trioxane. This is done by acid catalysis (either sulfuric acid or acidic ion exchange resins) and concurrent removal of the trioxane by distillation or extraction. The trioxane is then dried to remove all water and other active hydrogen containing impurities.

The co-monomer is typically dioxolane but ethylene oxide can also be used. Dioxolane is formed by reaction of ethylene glycol with a formaldehyde source (trioxane or concentrated 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 the melt (e.g. an extruder). After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by heat.

Stable polymer is melt compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers.

Applications

The Food and Drug Administration has approved Delrin for use in the food industry. Delrin was used by Mattel from 1968 to 1972 to produce the low-friction wheel bearings found on redline Hot Wheels. Delrin is made from acetal homopolymer resin, and, when extruded into large basic shapes (i.e., sheets, rods, and tubes), tends to be subject to porosity problems. These porosity problems can make the product less reliable in certain applications. Acetal copolymer (Acetron GP) is often used as a replacement for Delrin (acetal homopolymer) when porosity risk is a factor in material selection.

It is also used in yo-yos such as the Silk by Alchemy yoyos, Lyn Fury by YoYoJam, and the Milk by Born Crucial. Delrin's lubricity leads to a yo-yo with fantastic grinding ability.

It is also used extensively in paintball markers, where it is used to make bolts, pump handles and many other parts. Its low cost, adequate strength, light weight, and self-lubricating properties make it ideal for markers. The other material commonly-used for markers, Nylatron, while slightly lighter, is more prone to swelling. A swelled part can make the mechanism impossible to operate and sometimes even damages the marker. Nylon-based products such as Nylatron have better wear-resistance than Delrin, but do not have good moisture-resistance, and are therefore not suitable to high-humidity or underwater applications.

Delrin's resistance to liquids and low coefficient of friction have also made it useful as a bearing-replacement in casters and wheels, to be selected in cases where corrosive environments make a traditional roller or ball bearing impractical.

Delrin has also recently found use in the manufacturing of Irish flutes (traditionally made of wood), tin whistles (traditionally made of metal), and bagpipes (traditionally made of wood). Delrin flutes sound similar or identical to wooden flutes, but have none of the shrinkage or cracking issues usually associated with wooden instruments in hot, cold, or dry environments. Builders such as Des Seery, Michael Cronnolly of M&E Flutes, Tony Dixon, and Rob Forbes all build Delrin flutes.

Delrin has become an increasingly popular material in the fabrication of guitar picks. It has excellent durability; it is much more resistant to wear than nylon at the point of string contact, especially on round-wound strings. Delrin does not crack or break like celluloid or polyvinyl chloride (PVC), provides a solid, slip-free grip, and develops a "memory" over time; that is, it gradually develops a slight curvature that conforms to the user's grip.

Delrin is also much treasured by many makers and players of harpsichords, being used for the plectra, which pluck the strings when a key is pressed. Delrin has been found to provide a tonal quality very similar to that of the bird-quill used in the heyday of the instrument, while being far more durable and consistent.

Delrin is used to make the combination wheels in most high-security safe locks. Using delrin in the wheels makes the lock resistant to decoding by X-raying because of delrin's low density. However, neutron bombardment techniques may still be used to decode delrin combination wheels. Delrin also increases the lock's resistance to wear.

It is also used to make frame sliders and knee pucks for motorcycle riders/racers, as well as for slide gloves used when longboarding.

Delrin is gaining popularity for the manufacture of diving equipment, due to its light weight and its low porosity to gas under pressure. Some other plastics can absorb gas under pressure, and swell or deform.

Delrin has been used increasingly to create body-piercing jewelry, especially tunnels or plugs for use in stretched holes. While viewed by many as relatively safe for daily wear, the safety of this practice is still hotly debated within the piercing community.

Delrin is also being commonly used as a substitute for horn in the koiguchi on modern-made Japanese swords.

Two companies that made watthour meters also used Delrin components in their cyclometer-type registers for a while. However, Delrin has an unfortunate tendency to deteriorate when exposed to UV (from sunlight) for long periods of time - leading to performance issues in the register and a costly situation for the manufacturers.

Two percent to four percent solutions of polyformaldehyde in phosphate buffered saline are used as a fixative. It is commonly used in in situ hybridization including the TUNEL staining. In the TUNEL assay polyformaldehyde fixation increase the accessibility of the intracellular component therefore increasing the sensitivity of the staining.

Fabrication

Supplied in granulated form, Delrin 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 and blow molding are also possible.

Typical applications for injection-molded Delrin 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

Delrin is commonly extruded as continuous lengths of round or rectangular section. These sections can be sawn off to length and sold as bar or sheet stock for machining. More information about possible considerations can be found in the design guide available from DuPont.^[3]

Machining

When supplied as extruded bar or sheet, Delrin 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 workpiece.

Machined Delrin 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 Delrin suffer from less warping.

Bonding

Acetal polymers are typically very difficult to bond. Special processes and treatments have been developed to improve bonding of acetal. 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^[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.

History

Delrin was first synthesized by DuPont's research chemists around 1952. The company filed for patent protection of the material in 1956 and completed construction of a plant to produce the material at Parkersburg, West Virginia in 1960.

References

1. ^ Dupont MSDS for Delrin
2. ^ BASF Ultraform product information
3. ^ Delrin Design Guide
4. ^ Snogren, R. C. (1974). Handbook of Surface Preparation. New York: Palmerton Publishing Co..