Overview of POM
Polyoxymethylene (POM), also known as acetal or polyacetal, is a high-performance engineering thermoplastic. It is characterized by its high strength, hardness, rigidity, and excellent dimensional stability, making it a common replacement for metals in precision parts . POM is available in two main types: homopolymer and copolymer, each with slightly different performance characteristics .
Key Properties
The specific properties of POM can vary slightly depending on the grade and whether it is a homopolymer or copolymer. The following tables summarize typical values gathered from the search results.
1. Mechanical Properties
| Property | Typical Value | Notes / Test Method |
|---|---|---|
| Tensile Strength | 54 - 70 MPa | High strength and stiffness are key characteristics . |
| Tensile Modulus | 2.2 - 3.1 GPa | Indicates rigidity. |
| Flexural Modulus | 2200 MPa | |
| Elongation at Break | 35 - 75% | |
| Izod Notched Impact Strength | 43 - 130 J/m | Varies significantly based on grade and formulation . |
| Hardness (Rockwell) | M81 - M94 | |
| Coefficient of Friction | 0.2 - 0.35 | Low friction, good tribological properties . |
2. Physical Properties
| Property | Typical Value |
|---|---|
| Density | 1.41 - 1.56 g/cm³ |
| Water Absorption (24 hrs) | 0.15 - 0.25% |
| Water Absorption (Equilibrium) | 0.6 - 0.9% |
3. Thermal Properties
| Property | Typical Value |
|---|---|
| Melting Point | 165 - 183 °C |
| Heat Deflection Temp (HDT) @ 1.8MPa | 110 - 155 °C |
| Upper Working Temperature | 80 - 120 °C (long-term) |
| Coef. of Thermal Expansion | 120 - 122 x 10⁻⁶ K⁻¹ |
4. Electrical Properties
| Property | Typical Value |
|---|---|
| Dielectric Strength | 18 - 20 kV/mm |
| Volume Resistivity | 10¹⁵ Ω·cm |
| Surface Resistivity | 2x10¹⁵ - 10¹⁵ Ω/sq |
| Dielectric Constant | ~3.7 - 3.8 @ 1MHz |
5. Chemical Resistance & Other Properties
POM exhibits good resistance to many chemicals, solvents, and fuels at room temperature. However, its resistance to concentrated acids, dilute acids, and alkalis is generally poor . It has poor resistance to UV radiation and is not suitable for long-term outdoor use without modification . Its flammability rating is typically UL94 HB .
Homopolymer vs. Copolymer
It is important to distinguish between the two main types of POM, as their properties differ:
POM Homopolymer (e.g., Delrin®): Generally offers higher short-term mechanical strength, stiffness, and wear resistance. However, it is less thermally stable and more prone to degradation, which can lead to depolymerization .
POM Copolymer (e.g., Celcon®, Hostaform®): Features improved thermal and chemical stability due to the random distribution of oxyethylene groups in its polymer chain. While it may have slightly lower crystallinity and short-term mechanical strength compared to the homopolymer, it offers better long-term mechanical and thermal properties .
