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Thermoplastic Composites

Nov. 15, 2002
No longer is product design constrained to the property limits and performance characteristics of unmodified grades of resins.

No longer is product design constrained to the property limits and performance characteristics of unmodified grades of resins. Thermoplastics that are reinforced with high-strength, high-modulus fibers provide dramatic increases in strength and stiffness, toughness, and dimensional stability. The performance gain of these composites usually more than compensates for their higher cost. Processing usually involves the same methods used for unreinforced resins.

Glass, mineral fibers: Glass fibers used in reinforced compounds are high-strength, textile-type fibers, coated with a binder and coupling agent to improve compatibility with the resin and a lubricant to minimize abrasion between filaments. Glass-reinforced thermoplastics are usually supplied as ready-to-mold compounds. Molded products may contain as little as 5% and as much as 60% glass by weight. Pultruded shapes (usually using a polyester matrix) sometimes have higher glass contents. Most molding compounds, for best cost/performance ratios, contain 20 to 40% glass.

Practically all thermoplastic resins are available in glass-reinforced compounds. Those used in largest volumes are nylon, polypropylene, polystyrene, ABS, and SAN, probably because most experience with reinforced thermoplastics has been based on these resins. The higher performance resins -- PES, PEI, PPS, PEEK, and PEK, for example -- are also available in glass-fiber-reinforced composites, and some with carbon or aramid fibers as well.

Glass-fiber reinforcement improves most mechanical properties of plastics by a factor of two or more. Tensile strength of nylon, for example, can be increased from about 10,000 psi to over 30,000 psi, and deflection temperature to almost 500°F, from 170°F. A 40% glass-fortified acetal has a flexural modulus of 1.8 10 (to the 6th power) psi (up from about 0.4 10 (to the 6th power), a tensile strength of 21,500 psi (up from 8,800), and a deflection temperature of 335°F (up from 230 °F). Reinforced polyester has double the tensile and impact strength and four times the flexural modulus of the unreinforced resin.

Also improved in reinforced compounds are tensile modulus, dimensional stability, hydrolytic stability, and fatigue endurance. Deformation under load of these stiffer materials is reduced significantly; deformation tests must be conducted at 4,000 instead of 2,000 psi stress (used for unreinforced materials) to produce usable results.

Fiber reinforcement of a resin always changes its impact behavior and notch sensitivity. The change may be in either direction, depending on the specific resin involved. But even when the change is an improvement, these properties may not be high enough for certain demanding applications. This need has led to the development of impact-modified compounds -- specifically, nylon 6 and 6/6 alloys, a nylon 6/6 copolymer and a polypropylene copolymer -- with up to 50% improvement over reinforced unmodified compounds. While the impact properties of a glass-reinforced compound are not always superior to those of the unreinforced compound, the reinforced modified compounds are always superior to the reinforced unmodified grades.

Molded glass-reinforced and mineral-reinforced plastics are used in a broad range of structural and mechanical parts. For example, glass-reinforced nylon, because of its strength and stiffness, is used in gears and automotive under-the-hood components, while mineral-reinforced nylon is used in housings and body parts because it is tougher and has low warpage characteristics. Polypropylene applications include automotive air-cleaner housings and dishwasher tubs and inner doors. Polycarbonate is used in housings for water meters and power tools. Polyester applications include motor components -- brush holders and fans -- high-voltage enclosures, TV tuner gears, electrical connectors, and automobile exterior panels. Camper tops, pallets, and hand luggage are typical applications of reinforced HD polyethylene.

The newest glass-reinforced compounds are the long-fiber materials. These compounds, available principally in a nylon 6/6 base resin, are fabricated by pultrusion. The injection-moldable pellets thus contain fully wetted fibers equal in length to the pellet -- typically 0.400 in. This compares to 0.030 to 0.060-in. fiber lengths in conventional, short-fiber products. In fiber loads of 50% by weight, mechanical properties are improved dramatically over those of the short-fiber compounds. Long-fiber-reinforced compounds are available in the U.S. from ICI Advanced Materials, Polymer Composites Inc., and Dexter Composites.

Continuous-fiber glass-reinforced polypropylene is available in sheet form, for stamping or hot-flow forming of large, thin-wall parts such as automotive front-end retainer panels, oilpans, fender liners, upper grille panels, and station-wagon load floors, and for lawn-mower shrouds, luggage, and housings and guards for farm equipment and snowmobiles. The product is marketed by Azdel Inc. The company also markets a glass-reinforced PET polyester sheet product and plans to add sheet materials based on other resin matrices.

Another new composite form is a sheet material in various thermoplastic matrix resins developed by Du Pont. Reinforcement is unidirectional but discontinuous glass, carbon, or aramid fibers.

Carbon fibers: Carbon-fiber-reinforced compounds are available in a number of thermoplastics, including nylon 6/6, polysulfone, polyester, polyphenylene sulfide, polyetherimide, polyetheretherketone, and ETFE and PFA fluorocarbons.

The carbon-fiber-reinforced materials, at two to four times the cost of comparable glass-reinforced thermoplastics, offer the ultimate in tensile strength (to 35,000 psi), stiffness, and other mechanical properties. Compared to the glass-reinforced materials these compounds (10 to 40% carbon) have a lower coefficient of expansion and mold shrinkage, and improved resistance to creep and wear. Strength-to-weight ratios are also higher.

Carbon-fiber reinforcement also makes plastic compounds partially conductive. In compounds containing small amounts of carbon, this characteristic is useful for applications where static charges cannot be tolerated. Compounds containing higher percentages of carbon can be used for applications such as business-machine housings to shield the equipment from electromagnetic interference (EMI). Attenuation of electromagnetic radiation in carbon-fiber-reinforced nylon, for example, has been reported to be 36 to 40 dB in the frequency range from 50 kHz to 1 GHz.

Commercially available structural-carbon fibers are derived either from polyacrylonitrile (PAN) fibers or a special petroleum pitch. PAN-derived fibers have been available for several years and, for several of the lower modulus varieties, large databases have been developed through their use in aerospace programs. These fibers are generally selected for their high strength and efficient property translation into the composite.

The pitch-based fibers are newer and, while they are not as strong as the low-modulus PAN fibers, the ease with which they are processed into high-modulus components makes them attractive for stiffness-critical and thermally sensitive applications.

Pultrusion technology first provided long-glass fiber-reinforced composites with high-performance capabilities. Following close on the heels of this development are new long-carbon fiber-reinforced composites with even higher properties. Specifically, flexural and tensile moduli are the highest ever measured in discontinuous fiber-reinforced thermoplastic composites.

High stiffness-to-weight ratios and greater wear resistance allow these composites to compete against many metals. In addition, long-carbon composites are excellent candidates for applications requiring electrostatic dissipation and EMI shielding. Tests on some of the new composites have produced surprisingly high (60 dB and above) electromagnetic radiation attenuation values.

Aramid fibers: Aramid fibers, with greater specific strengths than steel or aluminum, should be an ideal reinforcement for thermoplastic resins. However, chopped aramid fibers do not adhere as well as the conventional glass or carbon-fiber reinforcements. Proprietary sizing systems aid in wetting of the fiber, but extensive fiber damage results in properties for the composite that are less than spectacular.

On the plus side, aramid-fiber-reinforced composites have low warpage, excellent wear and abrasion resistance, low coefficient of friction, and low thermal expansion. In addition, the mechanical properties of the composites are relatively uniform in all directions.

Aramid-fiber-reinforced composites have been applied in applications such as chain snubbers and guides, where low wear and mating-surface wear are important. They have also been evaluated successfully in gears, bearings, compressor vanes, and pump impellers.

In the past, two drawbacks have made aramid-reinforced thermoplastic composites impractical. First was the difficulty of getting the chopped fiber dispersed in the resin because the high surface energy of the lightweight fibers inhibited wetting at the fiber/resin interface. This and the unusually low bulk density of commercially available aramid fiber did not allow processing in conventional compounding equipment. A second problem was the fiber damage that occurred, both during mixing and injection molding. Damage is usually severe because of the low compressive strength of aramid fiber.

The first of these problems has now been minimized, and fibers can be incorporated uniformly into a resin matrix. Consequences of the fiber damage show up, of course, as reduced properties which are similar to, or lower than, those of their glass-reinforced counterparts.

There is a silver lining, however, to the dark cloud of disappointing mechanical properties. The aramid-reinforced composites have been found to have low warpage, good wear and abrasion resistance, low thermal expansion, and -- most important of all -- uniform mechanical and thermal properties in all directions. Conventional fiber-reinforced composites have lower shrinkage and higher strength and modulus in the flow direction. Not so with the aramid-reinforced materials. Properties, including mold shrinkage and thermal expansion, are nearly isotropic, regardless of flow pattern.

Internal lubrication: The first thermoplastics that were recognized for their inherent lubricity were nylon, acetal, and polytetrafluoroethylene (PTFE). These materials perform well, but for the more critical uses, their coefficients of friction may be too high, or wear may be too rapid.

The next generation of self-lubricated thermoplastics was formulated of various base resins that contained molybdenum disulfide, graphite, or PTFE particles to improve both lubrication and wear characteristics. Although wear resistance was indeed upgraded considerably, mechanical strength and dimensional stability of these compounds are often insufficient.

To minimize these deficiencies, reinforcing fibers of glass or carbon are added. The resulting composites are several times stronger than the unreinforced materials, and they are extremely stable in a wide range of service environments. But these materials too have a shortcoming: In service, a period of time is required for the internal lubricant to become exposed and to be burnished over the wear surfaces. During this run-in period, as a bearing or wear member is put into service, the unlubricated surfaces are in contact, and damage may occur.

Two approaches to eliminating these problems use silicone fluids to provide the lubrication function. Both of these, Migralube and Rimplast, are proprietary formulations. Thermoplastic composites can also be internally lubricated with a variety of systems to improve wear resistance. PTFE and silicone, separately or in combination, provide the best improvements in wear characteristics. Graphite powder and molybdenum disulfide are also used, primarily in nylons. The PTFE lubricants are specially modified to enhance their lubricious nature in the compound. The optimum level of lubricating filler varies depending on filler type and resin, but typical ranges are:

  • PTFE 15-20%
  • Silicone 1-5%
  • PTFE/silicone 15-20%
  • Graphite 10%
  • MoS2 2-5%

Addition of these lubricants further improves wear characteristics of good bearing materials such as nylon and acetal. The lubricants also allow the use of poor wearing, but close-tolerance materials, such as polycarbonate, in gear or bearing applications. Lubricants can be used by themselves or in conjunction with glass or carbon-fiber reinforcements.

Polytetrafluoroethylene and silicone fluids; glass, aramid, and carbon fibers; and graphite powder are the primary reinforcements and lubricants used in internally lubricated composites. The composites are based on engineering resins for injection-molded wear and structural parts.

Polytetrafluoroethylene (PTFE) lubricants dispersed into a thermoplastic base resin greatly improve surface-wear characteristics. Molecular weight and particle size of the PTFE lubricant are designed to provide optimum improvements in wear, friction, and PV values for selected resin systems. PTFE has the lowest coefficient of friction (0.02) of any known internal lubricant. Its static coefficient of friction is lower than its dynamic coefficient, which accounts for the slip/stick properties associated with PTFE/metal sliding action. During the initial break-in period, the PTFE particles embedded in the thermoplastic matrix shear to form a high-lubricity film over the mating surface. The PTFE cushions asperities from shock and minimizes fatigue failure.

Silicone fluids are chosen for their ability to perform as boundary lubricants and for partial compatibility with a particular base resin. The silicone is sufficiently compatible with the base resin to form an alloy, yet incompatible enough to cause migration to the surface of the compound. The silicone moves to the surface of a molded or extruded part by two mechanisms: diffusion by random molecular movement, and exclusion from the matrix (migration) because of its limited compatibility. The result of the migratory action is a continuous generation of a silicone film, which serves as a boundary, or mixed-film lubricant.

Glass fibers improve both short-term and long-term mechanical properties of a resin. The fibers also improve creep resistance, thermal conductivity, and heat-deflection temperature as well as the tribological properties of the base resin. The degree of improvement depends on the efficiency of the sizing system that bonds the resin to the fibers. Glass beads and unsized milled-glass fibers, on the other hand, increase the wear factor of the mating surface and the coefficient of friction.

Glass fibers are frequently used in combination with silicone and PTFE lubricants which offset the negative wear effects that the glass fibers have on surface characteristics. The use of silicone only, in conjunction with glass fibers, is not recommended, however. PTFE provides far more protection to the mating surface and should be used (with or without silicone) if the wear rate of the mating surface is important.

Carbon fibers added to thermoplastic resins provide the highest strength, modulus, heat-deflection temperature, creep, and fatigue-endurance values commercially available in composites. These property improvements, coupled with greatly increased thermal conductivity and low friction coefficients, make carbon fibers ideal for wear and frictional applications where the higher cost can be tolerated. In applications where the abrasive nature of glass fibers wears the mating surface, the softer carbon fibers can be substituted to reduce the wear rate. Carbon fibers can also be used in conjunction with internal lubricants to further improve surface characteristics of most thermoplastic resin system.

Another useful property of carbon-fiber-reinforced thermoplastics is their low volume and surface resistivities. Most resin systems reinforced with 15% or more carbon fibers can effectively dissipate static charge, which is a problem common to gears, slides, and bearings used in business-machine, textile, electrical, and conveying equipment.

Aramid fibers are stronger on a weight basis than steel or aluminum, but they are not as easy to work with as are glass and carbon. The high surface energy of the lightweight fibers makes wetting of the fibers and dispersion in resins difficult. Also, the fibers, having low compressive strength, are easily damaged in mixing and molding operations, which reduces composite properties. The first problem has been minimized somewhat with proprietary sizing/coupling systems, but fiber damage is still being worked on. Consequently, strength improvement in aramid-reinforced composites is not a reason to use these materials. But there are advantages offered by aramid/resin composites: Warpage and thermal expansion are low, abrasion resistance is high, and mechanical and thermal properties in molded parts are nearly uniform in all directions.

Graphite powders are low-friction, high-temperature solids traditionally used to lubricate moving metal parts where boundary lubrication is required. Their ability to lubricate depends on their structure, purity, and particle size. Properly selected graphite powders can be extrusion compounded with a variety of thermoplastics to provide coefficients of friction and wear factors between those of the base resin and the PTFE/silicone-lubricated versions. An important use for graphite-lubricated thermoplastics is in components that operate in aqueous environments.

Reinforced and/or internally lubricated compounds are used in a variety of applications. Polycarbonate-based composites are used for gear and bearing surfaces where accuracy is important and chemical resistance is not a severe problem, such as in cameras and office equipment. Acetal and PPS-based compounds have found wide acceptance in hostile environments such as gasoline metering and pumping devices. Nylon composites are often chosen for certain harsh environments because of their excellent chemical resistance.

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