ACETAL RESINS
ACETAL RESINS. Highly crystalline resins that have the repeating group (OCH2)x. The resins are polyformaldehyde. The natural acetal resin is translucent white and can be readily colored. There are two basic types: a homopolymer, such a Du Pont’s Delrin, and a copolymer, such as Hoechst Celanese’s Celcon. In general, the homopolymers are harder and more rigid and have higher tensile, flexural, and fatigue strengths, but lower elongation. The copolymers are more stable in long-term high-temperature service and have better resistance to hot water. Glass-filled acetals provide greater strength and stiffness, and tetrafluoroethylene (TFE)-filled acetals provide low friction and high wear resistance. The 500 Series of Glidestar, from EM Corp., consists of acetal-based self-lubricating (oil or TFE) compounds.
Acetals are among the strongest and stiffest of the thermoplastics. Their tensile strength ranges from 8,000 to about 13,000 lb/in2 (55 to 89 MPa), the tensile modulus of elasticity is about 500,000 lb/in2 (3,445 MPa), and fatigue strength at room temperature is about 5,000 lb/in2 (34 MPa). Their excellent creep resistance and low moisture absorption (less than 0.4%) give them excellent dimensional stability. They are useful for continuous service up to about 220°F (104°C). Acetals’ low friction and high abrasion resistance, though not as good as nylon’s, rates them high among thermoplastics. Their impact resistance is good and remains almost constant over a wide temperature range. Acetals are attacked by some acids and bases, but have excellent resistance to all common solvents. They are processed mainly by molding or extruding. Some parts are also made by blow and rotational molding. Typical parts and products made of acetal include pump impellers, conveyor links, drive sprockets, automobile instrument clusters, spinning reel housings, gear valve components, bearings, and other machine parts. Acetal homopolymers are used for mechanical and electrical parts. They have a specific gravity of 1.425, a tensile strength of 10,000 lb/in2 (69 MPa), 15% elongation, a dielectric strength of 500 V/mil (19.6 X 106 V/m), and Rockwell hardness M94. They retain their mechanical strength close to the melting point of 347°F (175°C). Acetal copolymers are thermoplastic linear acetal resins produced from trioxane, which is a cyclic form of formaldehyde. The specific gravity is 1.410, flexural strength 12,000 lb/in2 (83 MPa), Rockwell hardness M76, and dielectric strength 1,200 V/mil (47 X 106 V/m). It comes in translucent white pellets for molding.
Tenac SH, a homopolymer from Asahi Chemical, uses a Nylon 3 heat stabilizer to enhance processibility but retains the thermal stability and mechanical properties of the standard homopolymer.

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