Machine Design

Thermoset Composites

Thermoset matrix systems dominate the composites industry because of their reactive nature and ease of impregnation. They begin in a monomeric or oligomeric state, characterized by very low viscosity. This allows ready impregnation of fibers, complex shapes, and a means of achieving cross-linked networks in the cured part. The early high-performance thermoset-matrix materials were called advanced composites, differentiating them from the glass/polyester composites that were emerging commercially in the 1950s. The "advanced" term has come to denote, to most engineers, a resin-matrix material reinforced with high-strength, high-modulus fibers of glass, carbon, aramid, or even boron, and usually laid up in layers to form an engineered component. More specifically, the term has come to apply principally to epoxy-resin-matrix materials reinforced with oriented, continuous fibers of carbon or of a combination of carbon and glass fibers, laid up in multilayer fashion to form extremely rigid, strong structures.

Resin systems: More than 95% of thermoset composite parts are based on polyester and epoxy resins; of the two, polyester systems predominate in volume by far. Other thermoset resins used in reinforced form are phenolics, silicones, and polyimides.

Polyesters can be molded by any process used for thermosetting resins. They can be cured at room temperature and atmospheric pressure, or at temperatures to 350°F and under higher pressure. These resins offer a balance of low cost and ease of handling, along with good mechanical, electrical, and chemical properties, and dimensional stability.

Polyesters can be compounded to be flexible and resilient, or hard and brittle, and to resist chemicals and weather. Halogenated (chlorinated or brominated) compounds are available for increased fire retardance. Low-profile (smoother surface) polyester compounds are made by adding thermoplastic resins to the compound.

Polyesters are also available in ready-to-mold resin/reinforcement forms -- bulk-molding compound (BMC), and sheet-molding compound (SMC). Bulk-molding compound is a premixed material containing resin, filler, glass fibers, and various additives. It is supplied in a doughlike, bulk form and as extruded rope.

Sheet-molding compound consists of resin, glass-fiber reinforcement, filler, and additives, processed in a continuous sheet form. Three types of SMC compounds are designated by Owens-Corning Fiberglas Corp., as random (SMC-R), directional (SMC-D), and continuous fiber (SMC-C). SMC-R, the oldest and most versatile form, incorporates short-glass fibers (usually about 1 in. long) in a random fashion. Complex parts with bosses and ribs are easily molded from SMC-R because it flows readily in a mold. SMC-C contains continuous glass fibers oriented in one direction, and SMC-D, long fibers (8 to 12 in. long), also oriented in one direction.

Moldings using SMC-C and SMC-D have significantly higher unidirectional strength but are limited to relatively simple shapes because the long-glass fiber cannot stretch to conform to a shape. These two types of SMC are usually, but not always, used in combination with SMC-R. Various combinations are available that contain a total of as much as 65% glass by weight. These materials are used for structural, load-bearing components.

High-glass-content sheet-molding compounds are also produced by PPG Industries, designated as XMC. These compounds contain up to 80% glass (or glass/carbon mixtures) as continuous fibers in an X pattern.

Epoxies are low-molecular-weight, syruplike liquids that are cured with hardeners to crosslinked thermoset structures that are hard and tough. Because the hardeners or curing agents become part of the finished structure, they are chosen to provide desired properties in the molded part. (This is in contrast to polyester formulations wherein the function of the catalyst is primarily to initiate cure.) Epoxies can also be formulated for room-temperature curing, but heat-curing produces higher properties.

Epoxies have outstanding adhesive properties and are widely used in laminated structures. The cured resins have better resistance than polyesters to solvents and alkalies, but less resistance to acids. Electrical properties, thermal stability (to 550°F in some formulations), and wear resistance are excellent.

Phenolics, the oldest of the thermoset plastics, have excellent insulating properties and resistance to moisture. Chemical resistance is good, except to strong acids and alkalies.

Reinforced phenolics are processed principally by high-pressure methods -- compression molding and continuous laminating -- because volatiles are condensed during the molding process. Recently developed injection-moldable grades, however, have made the processing of phenolics competitive with thermoplastic molding in some applications

Silicones have outstanding thermal stability, even in the range of 500 to 700°F. Water absorption is low, and dielectric properties are excellent. Chemical resistance (except to strong alkalies) is very good.

Reinforcements: Glass is the most widely used reinforcing material in thermoset composites. Glass fiber, with a tensile strength of 500,000 psi (virgin fiber at 70°F), accounts for almost 90% of the reinforcement in thermosetting resins. Other reinforcements used are carbon, graphite, boron, and aramid (Kevlar) for high-performance requirements; glass spheres and flakes; and fibers of cotton, jute, and synthetic materials.

Glass fibers are available in several forms: roving (continuous strand), chopped strand, woven fabrics, continuous-strand mat, chopped-strand mat, and milled fibers (hammermilled through screens with openings ranging from 1/32 to … in.). The longer fibers provide the greater strength; continuous fibers are the strongest.

All forms of glass fibers are produced in the standard E-glass reinforcement type. Some of the higher strength forms are also available in S glass, which has a tensile strength about one-third higher than that of E-glass. Cost of S-glass is considerably higher. S-2 Glass, a product of Owens-Corning, is a variant of S-glass, having the same batch composition but without the rigid, military quality-control specifications. Properties are similar to those of S-glass; cost is between that of E and S-glass.

Carbon fibers in composites can be long and continuous, or short and fragmented, and they can be directionally or randomly oriented. In general, short fibers cost the least, and fabrication costs are lowest; but, as with glass, properties of resulting composites are lower than those obtainable with longer or continuous fibers.

Milled fibers are the shortest carbon fibers used for reinforcement. They range in length from 30 to 3,000 microns, averaging approximately 300 microns. Mean L/D ratio is 30. Short chopped fibers (about … in. long), with an L/D ratio of about 800, increase strength and modulus of a composite more than milled fibers do. Cost of a molding compound reinforced with … -in. fibers is about twice that of one containing milled carbon fibers.

Long chopped fibers (to 2 in.) are often added to a thermosetting glass/polyester sheet-molding compound to increase the stiffness of compression-molded parts. Continuous carbon fibers provide the ultimate in performance and/or weight reduction. Continuous fibers are available in a number of forms including yarns or tows containing 400 to 160,000 individual filaments; unidirectional, impregnated tapes up to 60 in. wide; multiple layers of tape with individual layers, or plies, at selected fiber orientation; and fabrics of many weights and weaves.

The outstanding design properties of carbon fiber/resin matrix composites are their high strength-to-weight and stiffness-to-weight ratios. With proper selection and placement of fibers, the composites can be stronger and stiffer than equivalent thickness steel parts and weigh 40 to 70% less. Fatigue resistance of continuous-fiber composites is excellent, and chemical resistance is better than that of glass-reinforced systems, particularly in alkaline environments. Like most rigid materials, however, carbon-fiber composites are relatively brittle. The composites have no yield behavior, and resistance to impact is low.

Thermal characteristics of carbon fibers are different from those of almost all other materials. Linear expansion coefficients range from slightly negative for 30 million-psi modulus fibers to approximately -1.3 10 (to the 6th power) in./in.- °F for the ultrahigh-modulus fibers. This property makes possible the design of structures with zero or very low linear and planar thermal expansion -- a valuable characteristic for components in precision instruments such as telescopes and for the alignment requirements of aerospace antennas and similar critical parts. Transverse coefficients of expansion are quite different -- typically 15 10 (to the 6th power) in./in.- °F.

The thermal conductivity of ultrahigh-modulus pitch-based carbon fibers exceeds that of copper. When density differences are considered, the specific conductivities can be as much as eight times that of copper.

Applications: Glass-reinforced polyester composites are used for automobile body panels, seats and panels for transit cars, boat hulls, bathroom shower-tub structures, chairs, architectural panels, agricultural seed and fertilizer hoppers, tanks, and housings for a variety of consumer and industrial products. Glass/epoxy applications include filament-wound pipe and tanks, and circuit boards.

Phenolic-matrix composites are used in printed-circuit boards, gears, and in insulators and other components for electrical equipment and appliances. Melamine applications include circuit boards, thermal and electrical insulation, and components requiring good chemical resistance. Silicones are chosen for service as electrical and thermal insulation, such as in circuit boards requiring maximum heat resistance.

The aircraft industry was quick to see the advantages of carbon/epoxy composites, which offered light weight, high strength and modulus, and -- most important -- excellent fatigue performance. Most engineering and manufacturing experience with these composites has been gained in developing military-aircraft components. While the manufacture of composite components is still highly labor intensive and expensive, the technology has advanced in recent years to improve production quality and speed with the use of systems such as automated tape laying.

Nevertheless, the high cost of carbon fibers and of processing the composites has limited most uses of carbon/epoxy composites to aerospace components -- fuselage panels for military aircraft and cargo doors for the space shuttle, for example -- and high-priced sports equipment -- tennis-racquet frames, golf-club shafts, skis, and archery bows.

TAGS: Materials
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