ACETAL RESINS.
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Written by tom
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Friday, 29 February 2008 |
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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 (Delrin) and a copolymer (Celcon). In general, the homopolymers are harder, more rigid, and have higher tensile flexural and fatigue strength, but lower elongation. The copolymers are more stable in long-term high-temperature service and have better resistance to hot water. Special types of acetals are glass filled, providing higher strengths and stiffness, and tetrafluoroethylene (TFE) filled, providing exceptional frictional and wear properties.
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. Delrin, of Du Pont, is used for mechanical and electrical parts. It has a specific gravity of 1.425, a tensile strength of 10,000 6 lb/in2 (68 MPa) with elongation of 15%, dielectric strength of 500 volts per mil (19.6 X 106 volts per meter), and Rockwell hardness M94. It retains its mechanical strength close to the melting point of 347°F (175°C). Celcon, of the Celanese Corp., is a thermoplastic linear acetal resin produced from trioxane, which is a cyclic form of formaldehyde. The specific ~gravity is 1.410, flexural strength 12,000 lb/in2 (82 MPa), Rockwell hardness M76, and dielectric strength 1,200 volts per mil (47 X 106 volts per meter). It comes in translucent white pellets for molding.
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Last Updated ( Saturday, 01 March 2008 )
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