Everything You Should Know About Acetal
Acetal materials are known to the commercial world for more than 50 years now. The use of this material takes us back to 1920s where it was widely used in the laboratory. They are also known as Polyoxymethylene or POM and is a thermoplastic. They are of two types Copolymer or POM-C and Homopolymer of POM-H, the latter is also known as Delrin.
Acetals are highly crystalline, and therefore, they must be heated above their melting point for processing. It is a semi-crystalline material. It provides high strength and stiffness along with enhanced dimensional stability and ease of machining. Acetal can absorb a minimal amount of moisture. Because of this feature, it results in inconsistency of its physical properties in all types of the environment.
The low moisture absorption property allows excellent dimensional stability for close-tolerance machined parts. In high moisture conditions or submerged applications, the bearings made of acetal can outperform nylon. This is the best choice for close tolerance mechanical parts and electrical insulators.
Types of acetal and their benefits
Majorly two types of acetal resins are available in the market today:
Both of these have their own set of advantages and disadvantages.
Homopolymer has the following benefits:
- Homopolymers possess a greater degree of regularity in their structure, and thus they are said to be more crystalline than their copolymer counterparts.
- It mostly used in high load mechanical application, vivo medical devices, automotive applications, plumbing, and irrigation applications.
- They are stiffer than
- They have higher impact strengths at room temperatures and low temperatures.
- The products made from homopolymer show better short-term mechanical properties than copolymers, but they tend to degrade faster over time, and this could lead to performance issues fairly quickly.
- It has the ability to resist scale build-up.
- It has Good Creep Resistance, Rigidity and Toughness along with Surface Hardness and Lubricity
The copolymer has the following advantages over Homopolymer:
- It is mostly used in high-performance engineering components, the consumer electronics industry, and the automotive industry.
- They have much more improved dimensional stability because of a low level of crystallinity.
- They are chemical resistance with high pH (basic) solutions.
- They are creep resistance.
- They have fatigue endurance.
- They are inherently more stable and resistant to thermal degradation.
- They are Easier to process
- They have Higher UV / Color stability
- They can wear solvent resistance over the long term under very demanding service conditions.
The major difference between homopolymer and copolymer acetal is because of the phenomenon called “centerline porosity”. It is most prominent in extruded parts, particularly in the thick slab. It is a whiter shaded region around the centre portion of a rod. It extends down the entire length of the rod. In slab, it appears as a line along the centre of each cut edge. Sometimes slab may appear to be laminated or glued together. When it comes to porosity traditionally, copolymers perform better than homopolymer.
Excessive centerline porosity is unwanted because of the following reasons:
- Inconsistent colour appearance in finished parts.
- Compromises the structural integrity
- Creates routes for leakage of gas and liquids.
- Provides areas where bacteria can be grown in food processing applications.
TEA is a diagnostic test; it helps in determining whether an acetal part was moulded from a homopolymer or a copolymer. In the tests, if a sample of a homopolymer is placed in TEA at an elevated temperature, it dissolves fairly rapidly whereas the copolymer does not.
Both homopolymer and copolymer have their own sets of benefits and advantages that make it useful for different industries and also enhances its areas of application.