Acetal resins are highly crystalline plastics based on formaldehyde polymerization technology. These engineering resins are strong, rigid, and have good moisture, heat, and solvent resistance. Acetals produced in the U.S. include homopolymers by Du Pont Co. (Delrin) and copolymers by Celanese Engineering Plastics Div., Hoechst Celanese Corp. (Celcon) and copolymers by BASF Corporation Plastic Materials (Ultraform).
Melting points of the homopolymers are higher, and they are harder, have higher resistance to fatigue, are more rigid, and have higher tensile and flexural strength with generally lower elongation. Some high-molecular-weight homopolymer grades are extremely tough and have higher elongation than the copolymers. Homopolymer grades are available that are modified for improved hydrolysis resistance to 180 °F, similar to copolymer materials.
The copolymers remain stable in long-term, high-temperature service and offer exceptional resistance to the effects of immersion in water at high temperatures. Neither type resists strong acids, and the copolymer is virtually unaffected by strong bases. Both types are available in a wide range of melt-flow grades.
The copolymers process easier and faster than the conventional homopolymer grades. However, with the introduction in 1987 of Delrin II homopolymer resins, Du Pont claims to have eliminated that difference. The improve moldability is attributed to the use of a new thermal stabilizer, which gives Delrin II the thermal stability of copolymers while retaining the higher properties of the homopolymer.
Both the homopolymers and copolymers are available in several unmodified and glass-fiber-reinforced injection-molding grades. Both are available in PTFE or silicone-filled grades, and the homopolymer is available in chemically lubricated low-friction formulations. A blow-molding grade (actually, a terpolymer) of Celcon is also produced, and special, high-productivity molding grades of Delrin are available. Several grades of both types comply with Food and Drug Administration regulations for repeated contact with food at temperatures to 250 °F. The National Sanitation Foundation has approved several homopolymer and copolymer resins for use in potable water to 180 °F.
The acetals are also available in extruded rod and slab form for machined parts. These materials are readily machined with standard brass-cutting tools. Property data listed in the table apply to the general-purpose injection-molding and extrusion grade of Delrin 500 and to Celcon M90.
The homopolymers are available in several viscosity ranges that meet a variety of processing and end-use needs. The higher viscosity materials are generally used for extrusions and for molded parts requiring maximum toughness; the lower viscosity grades are used for injection molding. Elastomer-modified grades offer greatly improved toughness.
Properties: Acetal homopolymer resins have high tensile strength, stiffness, resilience, fatigue endurance, and moderate toughness under repeated impact. Delrin 100, a high-impact grade, is the toughest acetal. Delrin 100 ST is a super tough grade, delivering up to seven times greater toughness than unmodified acetal in Izod impact tests and up to 30 times greater toughness as measured by Gardner impact tests.
Homopolymer acetals have high resistance to organic solvents, excellent dimensional stability, a low coefficient of friction, and outstanding abrasion resistance among thermoplastics. The general-purpose resins can be used over a wide range of environmental conditions; special, UV-stabilized grades are recommended for applications requiring long-term exposure to weathering. However, prolonged exposure to strong acids and bases outside the range of pH 4 to 9 is not recommended.
Acetal homopolymer has the highest fatigue endurance of any unfilled commercial thermoplastic. Under completely reversed tensile and compressive stress, and with 100% relative humidity (at 73 °F), fatigue endurance limit is 4,500 psi at 10 (to the sixth power) cycles. Resistance to creep is excellent. Moisture, lubricants, and solvents including gasoline and gasohol have little effect on this property, which is important in parts incorporating self-threading screws or interference fits.
The low friction and good wear resistance of acetals against metals makes these resins suitable for use in cams and gears having internal bearings. The coefficient of friction (nonlubricated) on steel, in a rotating thrust washer test, is 0.1 to 0.3, depending on pressure; little variation occurs from 73 to 250 °F. For even lower friction and wear, PTFE-fiber-filled and chemically lubricated formulations are available.
Properties of low moisture absorption, excellent creep resistance, and high deflection temperature suit acetal homopolymer for close-tolerance, high-performance parts. Long-term behavior in various environments can be predicted from design handbook data.
Applications: Automotive applications of acetal homopolymer resins include fuel-system and seat-belt components, steering columns, window-support brackets, and handles. Typical plumbing applications that have replaced brass or zinc components are shower heads, ballcocks, faucet cartridges, and various fittings. Consumer items include quality toys, garden sprayers, stereo cassette parts, butane lighter bodies, zippers, and telephone components. Industrial applications of acetal homopolymer include couplings, pump impellers, conveyor plates, gears, sprockets, and springs.
The copolymers have an excellent balance of properties and processing characteristics. Melt temperature can range from 360 to 450 °F with little effect on part strength. The resin is available in natural (translucent white) and in a wide range of colors. UV-resistant grades (also available in colors), glass-reinforced grades, low-wear grades, and impact-modified grades are standard. Also available are electroplatable and dimensionally stable, low-warpage grades.
Properties: Acetal copolymers have high tensile and flexural strength, fatigue resistance, and hardness. Lubricity is excellent. They retain much of their toughness through a broad temperature range and are among the most creep resistant of the crystalline thermoplastics. Moisture absorption is low, permitting molded parts to serve reliably in environments involving humidity changes.
Good electrical properties, combined with high mechanical strength and a UL electrical rating of 100 °C, qualify these materials for electrical applications requiring long-term stability.
A new acetal copolymer resin, Ultraform S 1320X-003, is available from BASF Corporation Plastic Materials. It has a combination of high mechanical and heat deflection properties (close to those of acetal homopolymers) and the good thermal stability and processing properties of acetal copolymer resins. The tensile strength, modulus of elasticity, impact strength, heat-deflection temperature, and surface hardness are about 10% higher than those of general-purpose acetal copolymers.
Impact-modified grades of acetal copolymers have notched Izod impact strengths up to nearly 3 ft-lb/in. at room temperature. The impact-modified acetal copolymer resins have a good balance of toughness and rigidity, with modulus of elasticity as high as 305,000 psi. This compares to 435,000 psi for general-purpose acetal copolymer resin with 1.3 ft-lb/in. notched Izod.
Acetal copolymers have excellent resistance to chemicals and solvents. For example, specimens immersed for 12 months at room temperature in various inorganic solutions were unaffected except by strong mineral acids -- sulfuric, nitric, and hydrochloric. Continuous contact is not recommended with strong oxidizing agents such as aqueous solutions containing high concentrations of hypochlorite ions. Solutions of 10% ammonium hydroxide and 10% sodium chloride discolor samples in prolonged immersion, but physical and mechanical properties are not significantly changed. Most organic reagents tested have no effect, nor do mineral oil, motor oil, or brake fluids. Resistance to strong alkalies is exceptionally good; specimens immersed in boiling 50% sodium hydroxide solution and other strong bases for many months show no property changes.
Strength of acetal copolymer is only slightly reduced after aging for one year in air at 240 °F. Impact strength holds constant for the first six months, and falls off about one-third during the next six-month period. Aging in air at 180 °F for two years has little or no effect on properties, and immersion for one year in 180 °F water leaves most properties virtually unchanged. Samples tested in boiling water retain nearly original tensile strength after nine months.
Applications: Industrial and automotive applications of Celcon acetal copolymer include gears, cams, bushings, clips, lugs, door handles, window cranks, housings, and seat-belt components. Plumbing products such as valves, valve stems, pumps, faucets, and impellers utilize the lubricity and corrosion and hot-water resistance of the copolymer. Mechanical components that require dimensional stability, such as watch gears, conveyor links, aerosols, and mechanical pen and pencil parts, are other uses. Applications for the FDA-approved grades include milk pumps, coffee spigots, filter housings, and food conveyors. Parts that require greater load-bearing stability at elevated temperatures, such as cams, gears, TV tuner arms, and automotive underhood components are molded from glass-fiber-reinforced grades.