Acetal plastics

Acetals are independent structural units or a part of certain biological and commercial polymers, and 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 were specially developed to compete with zinc and aluminum castings. The natural acetal resin is translucent white and can be readily colored with a high sparkle and brilliance. There are two basic types — homopoly-mer (Delrin) and copolymer (Celcon). In general, the homopolymers are harder, more rigid, have higher tensile flexural and fatigue strength, but lower elongation; however, they have higher melting points. 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 82°C, 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, but the copoly-mers process more easily and faster than the conventional homopolymer grades.

Both the homopolymers and copolymers are available in several unmodified and glass-fiber-reinforced injection-molding grades. Both are available in polytetrafluoroethylene (PTFE) or silicone-filled grades, and the homopolymer is available in chemically lubricated low-friction formulations.


The acetals are also available in extruded rod and slab form for machined parts. Property data listed in Table A.2 apply to the general-purpose injection-molding and extrusion grade of Delrin 500 and to Celcon M90.

Acetals are among the strongest and stiffest of the thermoplastics. Their tensile strength ranges from 54.4 to 92.5 MPa, tensile modulus is about 3400 MPa, and fatigue strength at room temperature is about 34 MPa. Acetals are also among the best in creep resistance. This combined with low moisture absorption (less than 0.4%) gives them excellent dimensional stability. They are useful for continuous service up to about 104°C.

TABLE A.2

Properties of Acetals

ASTM or UL Test

Property

Copolymer

Homopolymer

Physical

D792

Specific gravity

1.41

1.42

D792

tmp61-2

19.7

19.5

D570

Water absorption, 24 h, 1/8-in. thk (%)

0.22

0.25

Mechanical

D638

*Tensile strength (psi)

tmp61-3

8,800

10,000

tmp61-4

5,000

6,900

D638

*Elongation (%)

60

40

D638

tmp61-5

4.1

5.2

D790

Flexural strength (psi)

13,000

14,100

D790

tmp61-6

At 73 °F

3.75

4.10

At 160°F

1.80

2.30

D256

Impact strength, Izod (ft-lb/in.)

Notched

1.3

1.4

Unnotched

20

24

D671

tmp61-7

3,300

4,300

D785

Hardness, Rockwell M

80

94

Thermal

C177

Thermal conductivity

(10-4 cal-cm/s-cm2-°C)

5.5

8.9

tmp61-8

1.6

2.6

D696

Coefficient of thermal expansion

tmp61-9

8.5

10.0

D648

Deflection temp (°F)

At 264 psi

230

277

At 66 psi

316

342

UL94

Flammability rating

HB

HB

Electrical

D149

Dielectric strength

Short time (V/mil)

5 mils

2,100

3,000

20 mils

2,000

90 mils

500

500

D150

Dielectric constant

At 1 kHz

3.7

3.7

At 1 MHz

3.7

3.7

D150

Dissipation factor

At 1 kHz

0.001

0.001

At 1 MHz

0.006

0.005

D257

Volume resistivity (ohm-cm)

tmp61-10

1014

1015

D495

Arc resistance (s) 120 mils

240 (burns)

220 (burns, no tracking)

Frictional

Coefficient of friction

Self

0.35

0.3

Against steel

0.15

0.15

* At 0.2 in./min loading rate.

Injection-molding powders and extrusion powders are the most frequently used forms of the material. Sheets, rods, tubes, and pipe are also available. Colorability is excellent.

The range of desirable design properties and processing techniques provides outstanding design freedom in the areas (1) style (color, shape, surface texture and decoration), (2) weight reduction, (3) assembly techniques, and (4) one-piece multifunctional parts (e.g., combined gear, cam, bearing, and shaft).

Acetal Homopolymers

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. Some tough grades can deliver up to 7 times greater toughness than unmodified acetal in Izod impact tests and up to 30 times greater toughness as measured by Gardner impact tests (Table A.2).

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 30.9 MPa at 106 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 make these resins suitable for use in cams and gears having internal bearings. The coefficient of friction (nonlubri-cated) on steel, in a rotating thrust washer test, is 0.1 to 0.3, depending on pressure; little variation occurs from 22.8 to 121 °C. 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.

Applications

Automotive applications of acetal homopoly-mer 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 showerheads, ball cocks, 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.

Acetal Copolymers

The copolymers have an excellent balance of properties and processing characteristics. Melt temperature can range from 182 to 232°C with little effect on part strength. 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 an Underwriters’ Laboratories (UL) electrical rating of 100°C, qualify these materials for electrical applications requiring long-term stability.

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 1 year in air at 116°C. Impact strength holds constant for the first 6 months, and falls off about one-third during the next 6-month period. Aging in air at 82°C for 2 years has little or no effect on properties, and immersion for 1 year in 82°C water leaves most properties virtually unchanged. Samples tested in boiling water retain nearly original tensile strength after 9 months.

The creep-modulus curve of acetal copoly-mer under load shows a linear decrease on a log-log scale, typical of many plastics. Acetal springs lose over 50% of spring force after 1000 h and 60% in 10,000 h. The same spring loses 66% of its force after 100,000 h (about 11 years) under load.

Plastic springs are best used in applications where they generate a force at a specified deflection for limited time but otherwise remain relaxed. Ideally, springs should undergo occasional deflections where they have time to recover, at less than 50% design strain. Recovery time should be at least equal to time under load.

Applications

Industrial and automotive applications of 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, television tuner arms, and automotive under-hood components, are molded from glass-fiber-reinforced grades.

More costly acetal copolymer has excellent load-bearing characteristics for long-lasting plastic springs. To boost resin performance, engineers use fillers, reinforcing fibers, and additives. Although there are automotive uses for large fiber-reinforced composite leaf springs, unfilled resins are the better candidates for small springs. Glass fibers increase stiffness and strength, but they also limit deflection. And impact modifiers reduce modulus and make plastics more flexible but decrease creep resistance.

Acetal Resins Processing Acetals

Acetal resin can be molded in standard injection molding equipment at conventional production rates. The processing temperature is around 204°C. Satisfactory performance has been demonstrated in full-automatic injection machines using multicavity molds. Successful commercial moldings point up the ability of the material to be molded to form large-area parts with thin sections, heavy parts with thick sections, parts requiring glossy surfaces or different surface textures, parts requiring close tolerances, parts with undercuts for snap fits, parts requiring metal inserts, and parts requiring no flash. It can also be extruded as rod, tubing, sheeting, jacketing, wire coating, or shapes on standard commercial equipment. Extrusion temperatures are in the range of 199 to 204°C.

Generally the same equipment and techniques for blow molding other thermoplastics work with acetal resin. Both thin-walled and thick-walled containers (aerosol type) can be produced in many shapes and surface textures.

Various sheet-forming techniques including vacuum, pressure, and matched-mold have been successfully used with acetal resins.

Fabrication

Acetal resin is easy to machine (equal to or better than free-cutting brass) on standard production machine shop equipment. It can be sawed, drilled, turned, milled, shaped, reamed, threaded and tapped, blanked and punched, filed, sanded, and polished.

The material is easy to join and offers wide latitude in the choice of fast, economical methods of assembly. Integral bonds of acetal-to-acetal can be formed by welding with a heated metal surface, hot gas, hot wire, or spin-welding techniques. High-strength joints result from standard mechanical joining methods such as snap fits, interference or press fits, rivets, nailing, heading, threads, or self-tapping screws. Where low joint strengths are acceptable, several commercial adhesives can be used for bonding acetal to itself and other substrates.

Acetal resin can be painted successfully with certain commercial paints and lacquers, using ordinary spraying equipment and a special surface treatment or followed by a baked top coat. Successful first-surface metallizing has been accomplished with conventional equipment and standard techniques for application of such coatings. Direct printing, process printing, and roll-leaf stamping (hot stamping) can be used for printing on acetal resin. Baking at elevated temperatures is required for good adhesion of the ink in direct and screen-process printing. In hot stamping, the heated die provides the elevated temperature. Printing produced by these processes resists abrasion and lifting by cellophane adhesive tape.

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