ACETAL

 

Acetal resins are engineering thermoplastics based on formaldehyde polymerization technology.  These highly crystalline resins are strong, rigid and have a good moisture, heat and solvent resistance.  Acetals produced in the U.S. include homopolymers (Delrin, by DuPoint) and copolymers (Celcon, by Celanese).  A copolymer is formed by the polymerization of two monomers.  Because of the chemical make-up differences, do not substitute one acetal for the other without discussing it with the customer.

 

Both the homopolymers and copolymers are available in several unreinforced and glass-fiber-reinforced injection molding grades.  Both are available in PTFE-filled or silicone grades and the homopolymer is available in chemically lubricated low-friction formulations for self-lubricating applications.  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 have been cleared by the FDA for repeated food contact at temperature usage of 250 degrees F.  The acetals are also available in extruded rod and slab for machined parts.  These materials are readily machined with standard brass cutting tools.

 

Acetal homopolymers are available in several viscosity ranges to 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.

 

Properties

                                                                                                Delrin®                                 Celcon®

                                                                                                (homopolymer)                      (copolymer)

 

Tensile Strength, psi                                                           10,000                                     8,800

Flexural Modulus, psi`                                                        410,000                                   375,000

Impact Strength ft-lbs/in. of notch                                    1.4-2.3                                     1.3-1.5

Heat Resistance in air                                                          160⁰F                                                          200⁰F

Heat Resistance in H₂0                                                        180⁰F                                      212⁰F

Dielectric Strength                                                               500V/mil                                 500V/mil

Rockwell Hardness                                                              94 M                                       78-80 M

 

 

Acetal homopolymer resins have high tensile strength, stiffness, resilience, fatigue endurance and moderate toughness under repeated impact.  Homopolymer acetals have high resistance to organic solvents, excellent dimensional stability, a low co-efficient of friction and outstanding abrasion resistance among thermoplastics.  Resistance of acetal 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.

 

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.

 

The copolymers have an excellent balance of properties and processing characteristics.  Melt temperature can range from 360 to 450 degrees F with little effect on part strength.  The resin is available in natural (translucent white) and in a wide range of colours.

 

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.  A low moisture-absorption characteristic permits molded parts to serve reliably in environments involving humidity variations.  Good electrical properties qualify these materials for electrical applications requiring long term stability.

 

Industrial and automotive applications 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 acetal 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 FDA-approved grades include milk pumps, coffee spigots, filter housings and food conveyors.