Plastic Materials Close Up: Polyoxymethylene (POM), Acetal, Delrin®, and Celcon®

Polyoxymethylene (POM)

POM is a thermoplastic material also known as acetal. It is a molecule containing the functional group of carbon bonded to two –OR groups. POM was first discovered by German chemist Hermann Staudinger who won the 1953 Nobel Prize in Chemistry. He had studied it in the 1920s but found it to be thermally unstable. DuPont synthesized a version and filed for patent protection of the homopolymer crediting R.N. MacDonald as the inventor.

POM

This material had the same problem as Staudinger’s POM, in that it was also thermally unstable and therefore not useful commercially. A heat-stable POM homopolymer was finally discovered by Dal Nagore. He realized that by reacting the hemiacetal ends with acetic anhydride he could readily depolymerize hemiacetal into a thermally stable and melt processable thermoplastic. In 1960, Du Pont built a manufacturing plant to produce Delrin®, its version of the POM homopolymer, while in the same year Celanese completed its study of the copolymer and in 1962 started production of Celcon®. Other manufacturers followed with their own versions of these materials.

Polyoxymethylene demonstrates the following properties:

• High stiffness
• Low coefficient of friction
• Hardness
• High abrasion resistance
• High heat resistance
• Low water absorption
• Good electrical and dielectric properties
• Excellent dimensional stability
• More creep resistance than nylon
• Low smoke emissions
• High gloss surface
• Highly crystalline

On the down side:

• Flammable liquid with a dangerous fire hazard (not available in a flame retardant grade)
• Poor resistance to acids/alkalies
• High shrinkage rate
• Limited operating temperature range

Different Processes Used to Make Homopolmer and Copolymer POM

It is important to note that different manufacturing processes are used to produce the homopolymer and the copolymer versions of POM. To make the homoploymer, first anhydrous formaldehyde must be made. The formaldehyde is then polymerized by anionic catalysis and the resulting polymer is stabilized by reaction to acetic anhydride. Water produced must be removed from the reaction. The homopolymer has excellent creep resistance. Delrin® by Du Pont is typically made this way. However. the copolymer of POM requires that the formaldehyde is converted to trioxane, which can be done by acid catalysis followed by purification of the trioxane by distillation or extraction to remove water and other active impurities containing hydrogen. Celanese produced the copolymer named Celcon and others other companies produce it under their own brand name.

How Pom Can be Made into Shapes

POM is supplied in a granulated form and, with heat and pressure, can be molded into a desired shape. POM can be injection molded as well as rotational and blow molded. It can also be extruded into lengths of rectangular or round bars which can be used for machining. POM is very, very difficult to bond. Solvent welding of acetal polymers is usually unsuccessful due to the excellent solvent resistance of this material. Thermal welding however has been used successfully on both the homoploymer and copolymer.

Engineering Material

POM is an engineering material used in parts that require precision. Applications for POM include high performance components such as gear and fasteners (screws, nuts, washers, spacers, etc.). It is also used in other applications such as ski bindings, yoyos, electronic cigarettes, watch bracelets, zippers, insulin pens, and metered dose inhalers. It is used in musical instruments for picks and instrument mouthpieces, in the food industry in applications like milk pumps and coffee spigots, and also as a solvent for perfumes and as a synthetic flavoring ingredient.

For guidance on material selection for your application, contact us at Craftech.

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