Engineering thermoplastics are valued for their superior mechanical strength, thermal stability, and reliability, making them suitable for demanding applications. Within this group is Polyoxymethylene, commonly known as Acetal or POM, a high-performance plastic developed from formaldehyde-based chemistry. Acetal is frequently used to replace metals in many precision components due to its excellent balance of properties. Acetal Copolymer (POM-C) represents a specific and highly utilized variant of this material, engineered for enhanced stability and robust performance in varied environments.
The Chemistry: Defining Acetal Copolymer
Polyoxymethylene is fundamentally a chain of repeating formaldehyde units, but the structure varies significantly between its two main forms: homopolymer and copolymer. A homopolymer consists of polymer chains made up of only one type of monomer, which in the case of Acetal Homopolymer (POM-H) is polyoxymethylene. The Acetal Copolymer (POM-C), however, incorporates a second type of monomer unit, typically oxyethylene or dioxolane, into the polyoxymethylene chain.
The comonomer units are randomly distributed throughout the primary chain, disrupting the continuous POM structure. This strategic structural alteration prevents depolymerization, or “unzipping,” which is the thermal degradation of the polymer chain ends. Without the stabilizing comonomer, a POM chain is more susceptible to breaking down into formaldehyde gas when exposed to heat or certain chemicals. The incorporation of oxyethylene groups effectively acts as a block, granting the copolymer significantly better resistance to thermal and chemical attack, specifically against hot water and alkaline solutions.
Essential Physical and Mechanical Properties
Acetal Copolymer is valued by engineers for its high intrinsic strength and rigidity, often exhibiting a tensile strength ranging from 6,400 to 10,600 psi. Its semi-crystalline nature contributes to this hardness and provides excellent dimensional stability across a range of temperatures. This stability is further supported by the material’s very low rate of water absorption, which prevents the swelling and warping that can compromise the fit of precision parts.
The material is particularly well-suited for moving parts because of its naturally low coefficient of friction and high resistance to wear and abrasion. Acetal’s inherent lubricity makes it appear almost self-lubricating, allowing components to slide against each other with minimal energy loss. POM-C also offers reliable electrical insulating properties, making it useful for components within electronic assemblies. The copolymer exhibits good chemical resistance, remaining largely unaffected by organic solvents, fuels, and many inorganic compounds.
Common Industrial Applications
In the automotive sector, POM-C is frequently used for fuel system components, safety belt hardware, and internal gear wheels because of its resistance to petroleum products and its low-friction properties. Its dimensional stability and durability are utilized in various industrial machinery parts, such as precision rollers, conveyor components, and bushings.
Consumer goods also heavily rely on POM-C for small, highly functional parts that must withstand repeated use. Examples include durable zippers, snap fittings, aerosol valves, and lock system mechanisms where reliability and a smooth operation are expected. The material’s compliance with certain food and medical regulations, coupled with its resistance to sterilization processes, also makes it suitable for components in medical devices and food processing equipment.
Key Differences from Acetal Homopolymer
The Acetal Homopolymer (POM-H) generally offers approximately 10% to 15% higher tensile strength and stiffness at room temperature, but this comes at the cost of stability in harsh conditions. POM-H’s continuous polymer chain is more vulnerable to depolymerization when exposed to hot water, steam, or high-pH alkaline chemicals. Consequently, POM-C is overwhelmingly chosen in applications involving wet environments because of its superior hydrolysis resistance and greater ability to withstand basic solutions. The copolymer also exhibits better dimensional consistency in extruded forms, as it is far less susceptible to developing internal voids known as “centerline porosity,” which is common in larger POM-H cross-sections. Additionally, the copolymer has a lower melting temperature, which allows for easier processing during manufacturing and results in less outgassing compared to the homopolymer.