Acetal is an engineering thermoplastic. It is also known as polyoxymethlene or POM. POM is used in precision parts that require high stiffness, low friction and dimensional stability. From -40 degrees C to 80 degrees C POM is known for its high strength, hardness and rigidity. Natural POM is an opaque white material. However, it can easily be dyed to almost any color. It has a high crystalline composition. It is available as a homopolymer and a copolymer. Different chemical companies sell POM under different names including Delrin®by DuPont and Celcon® by Celanese.
Acetal Copolymer versus Acetal Homopolymer
There are two types of POM, acetal copolymer and acetal homopolymer These are very similar however they are not exactly the same material. The difference is in the structure of each. Acetal homopolymers have chains made of one monomer while acetal copolymers have chains made of two different monomers. For many applications either will perform well as most properties are within 10% of each other. However, there are important differences.
Characteristics of POM (Acetal)
Both homopolymer and copolymer POM have the following characteristics:
- Creep resistant
- Excellent mechanical stability
- Good electrical insulating and dielectric properties
- Strong, rigid and tough
- Excellent moisture resistance, low water absorption
- Low coefficient of friction, high lubricity
- Chemically resistant. Mostly unaffected by solvents and fuels.
- Low thermal expansion
- High stiffness and hardness therefore a good alternative to metals
- FDA approved
- Easily machined and molded
- Easy to color
- Not paintable
- Flammable and heat sensitive
- Does not adhere well to other objects due to high lubricity
- Does not perform well in Chlorine thus not for swimming pool use.
Acetal copolymer is a semi-crystalline thermoplastic. Importantly, the copolymer has the advantage of reduced centerline porosity. In addition, the copolymer has better chemical properties. It exhibits good pH resistance from pH 4-9. It is hydrolysis resistant up to 60 degrees Celsius. The continuous service temperature in air is 90 degrees Celsius. And the acetal copolymer exhibits higher out-gassing. It is available in natural and black. In addition, the copolymer is a little cheaper.
Acetal homopolymer is a crystalline thermoplastic. It is the ideal material to replace metal parts. The down side of the homopolymer is that it exhibits centerline porosity. Generally, the homopolymer has better mechanical properties:
- 15% higher stiffness and strength.
- 10% better creep resistance
- Higher Rockwell hardness rating
- Natural grade is FDA, NSF and USDA compliant
- Higher tensile strength
- Significant impact resistance
Why Use Delrin® (Aetal Homopolymer) to Replace Metals
Delrin® exhibits high tensile strength, low friction, and high wear resistance. In addition, it has low creep, is resistant to warping and is overall tough and durable. Delrin® has low moisture absorption and is chemically resistant to hydrocarbons, solvents and neutral chemicals.
While acetal copolymers exhibit some of these characteristics it is the unique combination of properies that make the acetal homopolymer the right plastic to replace metals in some applications.
This is not to say that Delrin® is as strong as metals. It is not. However, Delrin keeps on performing in applications such as pump and valve components where parts are exposed to moist or wet environments. Metals may fail while Delrin® keeps on performing.
Typical applications for POM include high performance parts such as gears, safety restraints, bearings, bushings rollers, fittings and electrical insulator parts. POM is used to make many fasteners such as screws, hex nuts, washers. hex bolts and many other parts. Also, it is used to make consumer products such as eyeglass frames, ski bindings, guns and knife handles. Additionally, Delrin® is used widely in the consumer electronics industry. And it is preferred by the automotive industry for use on car interiors because of its impact strength.
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