Acetal Polyoxymethylene Homopolymer - POM

It is made in both homopolymer and copolymer forms, which are not very different from each other. Both are tough, hard, stiff polymers with good appearance (can give high surface gloss) and excellent abrasion resistance.

Usually highly crystalline and opaque, their natural colour is white but they are typically used coloured. They have poor resistance to UV and generally to alkali and acids.

When compared to the copolymer, the homopolymer has slightly better mechanical properties but somewhat worse thermo-oxidative stability. The copolymer alone is suitable for continuous use with hot water.

Polymer Type

Thermoplastic

Advantages

Advantages of Acetal Polyoxymethylene Homopolymer :

  • Hard, tough and stiff.
  • Excellent creep resistance and solvent resistance, only dissolved by hot solvents, eg. Chlorophenols and Benzyl alcohol above 70 °C.
  • Good hydrolytic stability.
  • Higher tensile strength, flexural strength, fatigue resistance and hardness than acetal copolymer.
  • Good resistance to stress cracking.
  • Low coefficient of friction.

Disadvantages

High mould shrinkage (appr.2%), post-moulding shrinkage (appr. 0.1%) takes about 48 hrs.

Other disadvantages of Acetal Polyoxymethylene Homopolymer include:

  • Attacked by acids and bases, very rapid attack by nitric acid.
  • Very poor resistance to UV radiation.
  • Black components are necessary for outdoor applications.
  • Copolymers have better creep resistance thermal stability and processability.
  • Unsuitable for continuous use in hot water or steam.
  • Poor fire resistance (UL) fire retardants cannot be incorporated into polymer.
  • Halogens can cause explosive decomposition during processing.

Applications

POM is used in many applications where good abrasion resistance and low coefficient of friction is required such as bearings, gears, conveyor belt links, snap fittings, and many other engineering components.

Other applications include cams, plumbing components such as valve stems, pump impellors, carburettor bodies, plumbing fittings, appliance casings, and jug kettles.

Typical Properties

Property Value
Density (g/cm3) 1.42
Surface Hardness RM94
Tensile Strength (MPa) 69
Flexural Modulus (GPa) 2.83
Notched Izod (kJ/m) 0.075
Linear Expansion (/°C x 10-5) 12.2
Elongation at Break (%) 40
Strain at Yield (%) 12
Max. Operating Temp. (°C) 85
Water Absorption (%) 0.25
Oxygen Index (%) 15
Flammability UL94 HB
Volume Resistivity (log ohm.cm) 15
Dielectric Strength (MV/m) 19.7
Dissipation Factor 1 kHz 0.003
Dielectric Constant 1 kHz 3.7
HDT @ 0.45 MPa (°C) 172
HDT @ 1.80 MPa (°C) 136
Material. Drying hrs @ (°C) 2 @ 90
Melting Temp. Range (°C) 200 - 220
Mould Shrinkage (%) 2
Mould Temp. Range (°C) 80 - 100

Source : Abstracted from Plascams

For more information on Plascams please visit The Rubber and Plastics Research Association

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