Polymer Data File:

Polyoxymethylene
- POM (Acetal)

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Introduction

Advantages and Limitations

Typical Applications

Processing

Physical and Mechanical Properties

Finishing

Thermal, Electrical and Optical Properties

Health and Safety

Chemical Resistance Properties

Other Information


Introduction

POM is a highly crystalline linear thermoplastic which has predictable mechanical, chemical and other properties over a wide time and temperature range. As for PP the material is available in homopolymer and copolymer grades.

POM is rigid, translucent and tough with good spring like qualities, it has good wear and electrical properties and it is generally resistant to creep and most organic solvents. The high heat distortion temperature makes it suitable for many applications at elevated temperatures and it can also be used at temperatures down to -40oC.

POM does show some notch sensitivity.

The higher cost of POM (in comparison to the commodity plastics) reduces the application range but it is also one of the cheaper 'engineering' thermoplastics.

Typical Applications

Mechanical engineering: Clock and watch parts, rollers, bearings, gearwheels, housing parts.

Household goods: Control dials, pump parts, valves, gears.

Office equipment: General business machine parts, spring and screen parts, coils, rewind rollers for radio and video cassettes, precision parts for measurement and control technology.

Transportation: Snap fittings and fixing parts for interior linings, functional parts in the heating, ventilation and coolant sector, door handles, styling strips, seat belt components.

Plumbing and installation sector: Small pressure vessels, sound-damping mountings for WCs and bathrooms, pump and filter housings, fittings, hinges.

Consumer sport and leisure sector: Aerosol valves and heads, disposable cigarette lighters, ski bindings, toy parts.

Physical and Mechanical Properties

General

POM is not hygroscopic and water absorption is very low, dimensional stability is good with changes in humidity.

POM has a relatively high density of 1.41 and POM parts are heavier than equivalent parts in other plastics.

POM has a relatively high shrinkage of 2%, due to the crystallinity effects and this must be taken into account in tool design.

Mechanical properties

POM has an ideal combination of strength, stiffness and toughness. The stiffness and strength, particularly in the temperature range of 50 to 120oC, are greater than those of most other thermoplastics. At room temperature, POM has a distinct yield point at approximately 8 to 10% elongation. Below the yield point it has good elastic recovery, even when stressed repeatedly and this gives good spring capacity and suitability for snap fastenings.

POM also has high creep strength and a low creep tendency. This combination of properties in conjunction with good wear resistance and a low dynamic friction coefficient make it suitable for demanding industrial applications.

The strength, E modulus and deflection temperature under load can be increased by glass fibre reinforcement.

POM has a low co-efficient of friction (0.1 - 0.3) against itself or other materials, but this can be improved by blending with molybdenum disulphide or PTFE.

Property

Approximate Value

POM

POM - Glass filled

Tensile strength

55 - 80 MN/m2

80 - 105 MN/m2

Tensile Modulus

3-4 GN/m2

>4 GN/m2

Elongation at Break

10 - 50 %

<10 %

Flexural Strength

100 - 150 MN/m2

100 - 150 MN/m2

Notched Impact Strength

10 - 20 kJ/m2

3 - 10 kJ/m2

Specific Heat

1.25 - 1.70 kJ/kgoC

1.25 - 1.70 kJ/kg/oC

Glass Transition Temperature

-76 oC

 

Heat Deflection Temperature

100 - 150 oC

150 - 200 oC

Coefficient of Thermal Expansion

5 - 10 x 10-5 / oC

5 - 10 x 10-5 / oC

Long Term Service Temperature

90 oC

100 - 150 oC

Specific Gravity

1.41

>1.6

Mould Shrinkage

0.01 - 0.025 m/m

0.005 - 0.025 m/m

Water Absorption

0.1 - 0.5 % (50% rh)

0.1 - 0.5 % (50% rh)

Transparency

Opaque

Opaque

Thermal, Electrical and Optical Properties

Thermal properties

POM has a narrow melting point range (164 to 168oC for the copolymer). Parts can withstand thermal stresses up to temperatures approaching this melting range for very short periods without distortion.

The co polymer glass transition temperature is -60oC. At temperatures just over 100oC, molecular movement takes place in the crystalline region and deflections under load begin to become significant. POM copolymer is more thermally stable than the POM homopolymer.

Permissible service temperatures under hot conditions are 140oC (for under short terms) and 80 to 100oC under constant stress. At 120oC the material will start to become brittle after three months and then rapidly decrease in strength.

Fire behaviour

POM is rated as UL 94-HB and the material is a slow clean burning material with little or no smoke generation. The homopolymer has a high self-ignition temperature of 376oC and is classified as slow-burning.

Electrical properties

POM has a high electrical insulating capacity, high electrical strength and good dielectric behaviour. The low moisture absorption of the material makes it highly suitable for electrical applications.

Optical properties

POM is opaque and uncoloured moulded parts are translucent to white. All muted colours can be produced and a good surface gloss can be achieved from a polished tool and correct moulding conditions.

Chemical Resistance Properties

POM has excellent chemical resistance to organic compounds with the exception of halogenated hydrocarbons where the resistance is lower. Chemicals to which POM is resistant include alcohols, aldehydes, esters, ethers, glycols, hydrocarbons (petrol, engine oil), agricultural chemicals, weak acids and alkalis.

Weathering

POM is susceptible to UV radiation and if exposed to sunlight for long periods, the parts lose their surface gloss and become brittle. UV stabilisers can be added to double the service life, but marked improvements can be achieved by adding 2 to 3% of a special type of carbon black.

Stress cracking

POM has good stress crack resistance in the air, but stress cracking is a concern in some gases and liquids

POM Chemical Resistance Chart

  Advantages and Limitations

 

Advantages

Limitations

1. Excellent combination of toughness, rigidity, fatigue strength, and creep resistance (suitable for clips and springs).

1. Attacked by strong mineral acids.

2. Good moisture and chemical resistance; resistant to nearly all organic solvents and alkali solvents.

2. Homopolymers are not stable in alkalis.

3. Good dimensional stability over a wide temperature range (-40oC to 165oC).

3. Adversely affected by prolonged exposure to UV in standard in pigmented form.

4. Maximum service temperature 90 to 120oC, depending on grade, stress, and time.

4. Molecular structure is such that non-flammable forms are not practicable.

5. High resistance to thermal and oxidative degradation.

5. Above about 115oC significant changes in physical properties occur, particularly weight loss and change in metal-flow index.

6. Very good resistance to stress relaxation.

6. Cannot be used for applications involving high electric stress or power frequencies at temperatures above 70oC.

7. Good friction and wear properties.

7. Materials in moulded form normally contain traces of free formaldehyde (4 ppm.) which could be absorbed by foodstuffs.

8. Low gas and vapour permeability.

8. Strains in well-fabricated components for long-term applications should not exceed 2 to 5 per cent.

9. Processable by conventional techniques or injection moulding with excellent dimensional accuracy and post-moulding stability.

9. Fillers can reduce strength and elongation.

10. Range of grades available with or without fillers, additives etc. for specific applications.

10. Joining to self or other materials is difficult.

Processing

Injection moulding

Pre-drying is not necessary but can be carried out at 110oC for 2 hours if the material has become moist or if the uniformity of the material needs to be improved.

Injection pressures of 1200 to 1500 bar are used depending on the viscosity of the melt, the flow to wall thickness ratio and the type of sprue. Processing temperature is 180 to 220oC and up to 230oC in the event of deep flow and thin walls. The best processing temperature is around 205oC. Thermal damage may occur above 230oC unless the residence time in the cylinder is kept short.

At a mould temperature of 120oC the mouldings are tougher and have greater rigidity and the general range is 50 to 120oC. The material is partially crystalline thermoplastics and the mechanical properties are determined by the degree of crystallisation which increases with the mould temperature. Mould tempering is important in the production of high surface quality parts with low distortion and care needs to be taken in this area. Mouldings produced at 90oC have less post-shrinkage than mouldings produced at lower temperatures. Mould shrinkage is about 2% but is greatly dependent on the processing conditions. The glass fibre reinforced materials types have lower shrinkage but distortion can occur if the shrinkage is not uniform. Moulded parts with a high accuracy in gauge or parts used at high temperatures can be tempered for 24 hours at 110 to 140oC in order to allow for post-shrinkage.

As with all crystalline polymers the follow-up pressure can be influential on the shrinkage of the part and a longer hold time is preferred to reduce shrinkage.

High thermal stress during processing will result in the formation of formaldehyde.

Extrusion

Extrusion mainly to produce semi-finished articles (round and flat rods, hollow rods, slabs) and can be carried out easily on single and multi screw extruders. The extrusion temperatures are between 180 and 220oC.

High molecular weight types are the most suitable; highly viscous types with a good melt strength are best for extrusion blow moulding.

Short or long compression screws should have an L/D ratio of 15 to 25.

Processing Method

Applicable

Injection Moulding

Yes.

Extrusion

Yes

Extrusion Blow Moulding

Yes

Injection Blow Moulding

Yes

Rotational Moulding

Yes

Thermoforming

Yes

Compression Moulding

Yes

Casting

No

Bending and joining

Yes (but sometimes difficult)

  Finishing

Machining

POM is easily machined. Use a high cutting speed with a slow feed. Cutting coolants are not required. It is important to use sharp tools so as to avoid excessive heat development.

Surface treatment

The smooth hard surface and the good chemical resistance of POM requires pretreatment to improve the adhesion of coatings. This pretreatment can be by mechanical roughening, chemical etching or by applying and baking lacquers. There are special printing inks and embossing foils for printing and hot stamping; these give sufficient bond and wipe strength without any pretreatment of the surfaces.

Welding

POM can be welded by the hot gas, heated tool,  ultrasonic, friction (spin) and vibration welding methods. High frequency welding is not suitable.

Bonding

Bonding is difficult because of the good chemical resistance and bond strengths will always be low. Gas-tight, air-tight and moisture-tight joins can be obtained with impact, reactive and polyisocyanate adhesives, but they have only low mechanical strength.

Health and Safety

Health and Safety

POM is suitable for food contact applications and for use with potable water.

Other Information

Identification

  • Sample will be difficult to ignite but will burn with a yellow flame and smell strongly of formaldehyde.

 

Last edited:11/03/10

Tangram Technology Ltd. 2000

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