Machinery's Handbook, 31st Edition
Copolymers, BLENDS, AND ADDITIVES 557 acetals, polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET). The crystallites are linked by amorphous zones; the resulting microstructures make these materials stronger and stiffer than similar materials without crystallinity. They generally possess higher end-use chemical and temperature resistance, and melt at higher tempera- tures, with more sharply defined melting ranges than amorphous resins. Liquid-Crystalline Polymers (LCP): These contain crystal-like structures even in the melted resin. They have highly ordered, rod-like microstructures with good mechanical properties and high melting temperatures. They have the lowest shrinkage and warpage of the three types of thermoplastics. Copolymers, Blends, Additives, and Reinforcements.— Plastics can be manipulated to modify mechanical, chemical, electrical, and cost characteristics. Copolymers: Combining two or more different polymers can create a new material with properties that are quite different from those of the individual polymers ( homopolymers ) from which they are made. The copolymers are alloyed: mechanically blended from the two or more different polymers, often with special additives to make them compatible. These “alloys” are compounded so as to retain the most desirable characteristics of each constituent, especially impact strength and flame resistance. The properties usually are between those of the constituent homopolymer materials. Copolymers such as SAN and ABS have long been commercially available. While tens of thousands of grades have been developed, less common blends and alloys are not always readily available in sheets or in standard industry stock shapes supplied for machining. Additives and Fillers: Plastics compounds with desired sets of properties are produced from “neat” resins by adding many types of other chemicals and materials. Antioxidants, lubricants, pigments and dyes, fire retardants, and ultraviolet protectants in small percent ages usually do not significantly alter mechanical and physical properties. Larger quantities of particulate fillers, such as wood flour or fibers, silica powder, glass or phenolic micro balloons, and carbon and metal powders, are often used to tune density, thermal and electri cal conductivity, mechanical properties, and unit cost. The use of such additives makes available many grades of a given plastic type from each commercial plastics manufacturer. Such additives can increase value in application but also may increase difficulty in recycling. Plasticizers: Plasticizers soften and toughen the base resins with which they are com pounded. While they are used in several resin families to some extent, polyvinyl chlo - ride resins (PVC) and related plastics use plasticizers and many other additives in high percentages. These modified resins produce a myriad of grades of PVC from the familiar rigid pipe material (the dominant consumer of PVC) to wire coatings, flooring, and elas - tomerics used in footwear. Fibers: Plastics, which have much lower moduli and strengths than glass and metals, can be made stiffer and stronger by incorporating glass, graphite or other fibers. Short chopped fibers in fractions up to about 40 percent, randomly oriented in molding com - pounds, provide substantial increases in stiffness and strength. But more dramatic im - provements are obtained by incorporating longer fibers in carefully chosen directions, as in fishing rods and golf club shafts, where content of glass or carbon fiber may be more than 50 percent by weight. Historically, sturdy composites have been and still are made by compression molding multilayer canvas, paper, or cotton-felt sheets impregnated with phenolic or other thermosetting resins. (See also Plastics Gears on page 606.) A rela- tively recent reinforcement material is oriented aramid (aromatic polyamide, Kevlar), whose strength and modulus rival those of metals. Its most familiar use has been in “bul - let-proof” vests for law-enforcement and military personnel. Cellular Plastics.— Many rigid and flexible thermoplastic and thermosetting resin fam ilies are available as foamed plastics in the form of sheets, boards, bars, and cylinders, as well as molded shapes. The internal structure of plastic foam consists of tiny gas-filled cells bounded by shared membranes of the resin. In closed cell foams, each cell is a tiny polyhedral cavity adjacent to, but not opening into, other cells. Foamed polystyrene hot liquid beverage cups are an example of closed
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