Acetal Copolymer Properties — POM-C Mechanical & Thermal Datasheet
Acetal copolymer (POM-C) delivers a property profile nearly identical to acetal homopolymer (POM-H/Delrin) at room temperature, with important differences emerging in hot-water resistance, large-cross-section uniformity, and hydrolytic stability. This datasheet compiles mechanical, thermal, electrical, chemical resistance, and tribological properties for standard natural-grade acetal copolymer (Celcon M90 / Hostaform C 9021 equivalent), with comparison notes to POM-H throughout.
At a glance:
- Tensile strength: 9,500–10,000 psi — about 5% lower than POM-H (Delrin 500)
- Fatigue endurance (10⁷ cycles): ~4,000–4,500 psi vs. ~5,000 psi for Delrin
- Continuous service temperature: 185°F (85°C); melting point 329°F (165°C)
- Hot-water resistance: substantially better than POM-H above 60°C
- No centerline porosity in large cross-sections: key advantage in rod >3" dia
- Density: 1.41 g/cc; CTE: ~5.4–6.5 × 10⁻⁵ in/in/°F
- UL 94 HB flame rating
Mechanical Properties
Tensile and Compressive Data (73°F / 23°C)
Impact and Hardness
Fatigue Properties
Acetal copolymer's fatigue endurance limit — approximately 4,000–4,500 psi at 10⁷ cycles — is about 10–20% below acetal homopolymer (Delrin, ~5,000 psi). This difference is relevant in cyclic-stress applications such as gears, snap-fits, and cam followers. For most general-purpose machined parts under static or low-cycle loading, the difference is insignificant.
Fatigue strength is sensitive to stress concentrations (notches, corners, thread roots). A generous fillet radius at stress risers is even more important in POM-C than in metals because plastic fatigue crack propagation can be rapid once initiated.
Creep and Stress Relaxation
Acetal copolymer creep behavior closely parallels POM-H. At 23°C and 5,000 psi stress, typical creep strain over 1,000 hours is 1.5–2.5%. At 75°C and 3,000 psi, expect 3–6% creep strain over 1,000 hours. Design bolted or clamped POM-C joints with 20–35% relaxation allowance over the service life.
Comparison: POM-C vs. POM-H (Delrin)
Thermal Properties
POM-C's heat deflection temperature at 264 psi (230°F / 110°C) is about 25°F lower than POM-H (255°F / 124°C). In applications where parts approach the HDT — sustained elevated-temperature bolted joints, proximity to heat sources — this distinction can drive grade selection. Verify with application-specific testing rather than relying on tabulated HDT alone.
Electrical Properties
Acetal copolymer is an electrical insulator in standard grades. Antistatic and conductive grades are available in molding compounds from Celcon and Hostaform but are not commonly stocked in rod and sheet form. For ESD-critical applications in stock-shape format, evaluate alternative materials.
Chemical Resistance
The comonomer in POM-C provides meaningfully better chemical resistance in two categories compared to POM-H: hot water and dilute alkalis. In other categories the materials are essentially equivalent.
| Reagent / Environment | Resistance | Notes |
|---|---|---|
| Water (ambient) | Excellent | 0.20–0.22% absorption in 24 hr |
| Hot water (60–90°C) | Good — Better than POM-H | POM-C heat-stabilized grades rated to ~90°C continuous |
| Steam (>100°C) | Moderate | Expect some degradation; not a steam-service material |
| Aliphatic hydrocarbons | Excellent | Fuels, oils, mineral spirits |
| Aromatic hydrocarbons | Good | Some swell at prolonged exposure |
| Alcohols | Excellent | IPA, ethanol, methanol |
| Ketones (acetone, MEK) | Good | Minor surface effect at extended exposure |
| Dilute acids (pH > 4) | Good | |
| Strong mineral acids (pH < 2) | Poor | Attacks chain |
| Dilute alkalis (NaOH <10%) | Good–Better than POM-H | Comonomer backbone is more alkali-stable |
| Strong oxidizing agents | Poor | Bleach, peroxides |
| Gasoline / diesel fuel | Excellent | Common fuel-contact application |
| Hydraulic fluids | Excellent | Mineral and synthetic-base hydraulic fluids |
Tribological Properties (Wear and Friction)
POM-C's tribological performance is essentially equal to POM-H in dry-sliding applications. The marginally higher wear factor (K) compared to POM-H is within experimental variability between test lots. For high-duty sliding applications, lubricated or PTFE-filled grades are available in both POM-H and POM-C formulations.
Physical and Other Properties
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