Machine Design


Polyester resins, in thermoset, glass fiber-reinforced formulations, have been used for a variety of industrial, marine, and consumer products for at least 40 years. These are the materials of "fiberglass" boats, fishing rods, and automobile body panels. More recently, thermoplastic grades of polyesters have become available for use in high -performance, injection-molded parts.

Thermoset polyesters: Polyester and alkyd molding compounds are usually based on unsaturated polyester resin systems. The term alkyd refers to those using lower amounts of monomer; when the monomer level is higher, the compound is called a polyester. At room temperature, the solutions - mixtures of resin and a liquid monomer (usually styrene) - are stable. Any of a variety of peroxide catalysts can initiate crosslinking (curing) at room temperature or at higher temperatures.

Unlike most other plastics that are based principally on a single ingredient, polyester formulations usually contain substantial amounts of several materials. Special advantages and limitations apply to each type of polyester compound, and to the various processing methods.

Fabrication of parts from thermoset polyester resins - usually glass-fiber-reinforced compounds - is more varied than with any other type of plastic. Methods for producing polyester/glass-fiber (FRP) parts include hand lay-up and spray-up for small to moderate quantities of large parts; com- pression molding from sheet-molding compound or from glass-fiber preforms for high-volume production of moderate-sized,intricate parts; cold-press molding for smaller components; pultrusion for constant-section shapes; and resin-injection molding, also called resin-transfer molding,a low-pressure method for intermediate production quantities.

Polyesters are also available as casting resins, both in water-extended formulations for low-cost castings, and in compounds filled with ground wood or pecan-shell flour for furniture components.

Low-profile molding resins are mixtures of polyester resins, thermoplastic polymers, and glass-fiber reinforcement. These are used to mold parts with smooth surfaces that can be painted without the need for prior sanding.

Bulk-molding compounds (BMC) are mixtures of polyester resin, short-glass fibers (1/8 to '/, in. long), filler, catalyst, and other additives for specific properties. BMC is supplied in bulk form or as extruded rope for ease of handling.

Sheet-molding compounds (SMCs) consist of polyester resin, long-glass fibers (to 2 in.), a catalyst, and other additives. They are supplied in rolls, sandwiched between polyethylene carrier films. Uniformity of SMC materials is closely controlled, making these materials especially suited for automated production.

Structural SMCs contain up to 65% glass in continuous, as well as random- fiber orientation. This compares with ~O to 35% glass, in random orientation only, of conventional SMC. For more details, see the chapter on thermoset composites.

Polyester resins are also available for use as coatings for curing by ultraviolet radiation. These 100%-solids materials cure in a matter of seconds and give off no solvents. Although some styrene may be lost upon exposure to UV radiation, the amount is small.

Properties: Properties of thermoset polyesters are so dependent on type, compounding, and processing method that a complete listing covering all combinations would be almost impossible. However, typical strength ranges obtainable in parts fabricated from various forms of polyester/glass compounds and processed by several methods are listed in the table.

Outdoor weatherability of polyester resins specifically compounded for such service is good. Thermal, electrical, and environmental properties are generally good and can be individually improved by formulation.

Thermoplastic polyesters: Known chemically as polybutylene tereplithalate (PBT) and polyethylene tereplithalate (PET), the thermoplastic-poly ester molding compounds are crystalline, high-molecularweight polymers. They have an excellent balance of properties and processing characteristics and, because they crystallize rapidly and flow readily, mold cycles are short.

In addition to several unreinforced molding resins, the polyesters are available in glass- reinforced grades. Unreinforced and glass-filled grades are available with UL flammability ratings of 94 HB and 5V.

Properties; Thermoplastic polymers have excellent resistance to a broad range of chemicals at room temperature including aliphatic hydrocarbons, gasoline, carbon tetrachloride, perchloroethylene, oils, fats, alcohols, glycols, esters, ethers, and dilute acids and bases. They are attacked by strong acids and bases.

High creep resistance and low-moisture absorption give the polyesters excellent dimensional stability. Equilibrium water absorption, after prolonged immersion at 73° F, ranges from 0.25 to 0.50% and, at 150°F, is 0.52 to 0.60%. Black-pigmented grades are recommended for maximum strength retention in outdoor uses.

Toughness is another strong point of the unreinforced, high- mo lecu lar-weight thermoplastic polymers. Unnotched specimens are rated "no-break" both at room temperature and at -40° F. In falling-dart tests, the unreinforced grades withstand more than 40 ft-lb. A toughened grade of 30% glass-reinforced resin has an unnotched impact strength of 24 ft-lb/in. - about 40% greater than that of conventional 30% glass-reinforced polyesters. Although impact strength, is exceptional, special care must be observed in designing parts beca polyesters are notch sensitive.

Stiffness and creep resistance can be increased by the addition of high glass loadings to thermoplastic polymers. Both 45 and 55% glassre inf orced versions are available having flexural moduli of 2,000,000 and 2,600,000 psi. These materials also have higher strength properties than conventional 30% glass-reinforced polyesters.

Warpage, which is caused by anisotropic shrinkage, has long been a problem with glass-reinforced thermoplastic polyester. Several options are available, however, to reduce warpage. For applications requiring maximum toughness, an amorphous resin, such as polycarbonate, can be blended with the polyester. Warpage is also reduced considerably with the addition of glass and mica reinforcement.

Glass-reinforced polyesters have outstanding electrical properties, which are retained after prolonged exposure to a wide range of temperature and humidity conditions. For continuous exposure, however, maximum recommended water temperature is 125°F.

Polyester film is tough, durable, and dimensionally stable. It can be metallized, embossed, slit, die cut, and laminated. It is available in crystal clear, white, opaque, or translucent forms, and its surface can be glassy smooth, matte, antistatic, adhesion promoting, or heat sealable.

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