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Acetal Delrin Copolymer Homopolymer - What are the differences?

Acetal Delrin Copolymer Homopolymer What are the differences?

By Kevin Chen - ENERGETIC Plastics
 
Acetal (Polyoxymethylene or polyacetal) is a highly crystalline polymer. POM is most commonly known for its high stiffness, mechanical strength, abrasion resistance and good resistance to chemicals and solvents. It also displays good low temperature impact resistance, high dimensional stability, favourable frictional properties and low water absorption. However, polyacetal has poor resistance to acids and alkalis, and that it burns easily. It also has a restricted processing temperature range and poor UV resistance.
 
There are mainly two types of acetal: acetal homopolymer (POM-H, commonly called Delrin) and acetal copolymer (POM-C). They are polymerized via two different methods. One is anionic polymerisation of formaldehyde which produces homopolymers that crystallize very well and the end product have high stiffness and strength. The other method is cationic polymerisation of trioxane. The addition of small amounts of comonomers lowers the crystallinity but increases toughness. The stiffness and strength are however somewhat lower than for POM-H.
 

The table below presents a comparison of the performance properties of polyacetal homopolymers, copolymers and 25% glass fibre-reinforced copolymers.
 


POM-C accounts for around 75% of the global POM sales. It is mainly because they are easier to process and cost less. POM-H, on the other hand, have better physical and mechanical properties than copolymers but slightly higher in price.
 
To sum up:
 
POM-C Homopolymer:
 
•      Slightly lower cost
•      Free of centreline porosity
•      Good dimensional stability up to a maximum continuous temperature of 80°C
•      Better dimensional stability due to lower level of crystallinity.
•      Better resistance to hot water and chemicals
•      Lower coefficient of friction and better impact and wear properties, especially in wet or moist environments.
 
POM-H Delrin
 
•      Offers slightly higher mechanical properties
•      May contain a low-density centre (centre line porosity)
•      Slightly less chemical resistance
•      Greater stiffness, with higher flexural modulus at room and elevated temperature applications.
•      Slightly higher tensile and impact strength at and below room temperatures.
•      Slightly harder and has a lower coefficient of friction.

 
Applications
 
POM-H are best suited to applications where good abrasion resistance and a low coefficient of friction is needed. Applications include bearings, gears, conveyer belt links and safety systems like seat belts.
 
POM-C are best suited to applications that require low coefficient of friction such as electric kettles and water jugs, components with snap fits, chemical pumps, bathroom scales, telephone keypads and housing for domestic appliances.
 
Polyacetals can be CNC machined, extruded and injection moulded and. Most of the acetal parts are injection moulded. Modifiers can also be added such as impact modifier for high toughness applications and glass fibres for high strength.