Machinery's Handbook, 31st Edition
556 PLASTICS MATERIALS Plastics Materials.— Plastics encompass a wide range of materials that consist of long chains of carbon atoms with attached atoms of other elements, mainly hydrogen, and small, repeating chemical functional groups that influence the plastics’ properties. Some other elements found in plastics are oxygen, nitrogen, chlorine, sulfur, and fluorine. A special class, the silicones , have chains of alternating silicon and oxygen atoms with small carbon-based side groups. These repeating chain segments are called mers , giving the term polymers for these materials. The pure ( neat) materials are also known as resins . Commercial plastics are resins containing additives in amounts ranging from trace quan - tities to 50 percent or more. In any sample of resin, molecular chain lengths will generally vary widely, but the average typically exceed several thousand mer units if the resin is to possess good mechanical properties. There are two main classes of resins: thermoplastics (which includes elastomers ) and thermosets . Thermoplastics: Plastics that soften when heated are called thermoplastics. These plas tics can be repeatedly melted by heating and then solidified by cooling. This reversible change may actually be repeated several times until a final product is made. For example, freshly polymerized polypropylene is a fine powder that is converted into pelletized feedstock by extrusion. The extruder compacts and melts the powder and pumps it through a strand die, where the emerging strands are water-chilled and sliced into 1 ⁄ 4 -inch (10 mm) BB-sized pellets. A sheet maker may then reprocess the pellets in an extruder to produce long rolls of thin sheet or shorter slabs of thicker sheet. These can be used by a thermoformer, which again heat-softens the sheet and pulls it down over a chilled mold to make packages from the thin sheet or wastebaskets from the thicker ones. Some examples of commodity thermoplastic families are polystyrenes (PS), olefins including polyethylenes (PE) and polypropylenes (PP), acrylics (PMMA), cellulosics (e.g., cellulose acetate butyrate, CAB), and polyvinyls. Engineering thermoplastics, such as the nylon family, acetal resins, polycarbonate, and others listed in the preceding table, generally have superior strength, toughness, and resistance to heat and chemicals. Although the heating/cooling cycle can be repeated, each recycling tends to reduce mechanical properties and appearance. Thermosets: Thermosetting plastics, such as amino, epoxy, phenolic, and unsaturated polyesters, are supplied to the final processor as partially polymerized mixtures of chemically active, long-chain molecules. When they are heated during molding, the mixture components react to form chemical bonds, called cross-links , between the original chains, forming huge three-dimensional molecules. Such products generally have improved mechanical properties and heat resistance. However, if heated above their molding temperatures, they do not melt but instead decompose. They cannot be reprocessed, so opportunities for salvaging scrap for recycling are limited. Many rubbers that are processed by vulcanizing, such as butyl, latex, neoprene, nitrile, polyurethane, and silicone, also are classified as thermosets. Elastomers and Rubbers: Raw natural or synthetic rubber, initially thermoplastic, is mixed with finely divided carbon and sulfur. During molding, these materials undergo cross-linking by sulfur, a process called vulcanizing . Elastomers typically have low elastic moduli (stiffness) but high elasticity (stretch). Many, if stretched to twice their length (or much more) and then released, will snap back resiliently to their original length and shape. Thermoplastic elastomers are often used in place of rubber, and also may be used as addi tives in other thermoplastic resins for improving impact strength of rigid thermoplastics. Thermoplastic Structures.— Thermoplastics can be classified by their structures into three categories: Amorphous: These randomly structured materials are sometimes called noncrystalline . Such thermoplastics include acrylates, polycarbonate (PC), polystyrene, acrylonitrile- butadiene-styrene (ABS), styrene acrylonitrile (SAN), and polyvinylchloride (PVC) and its acetate copolymers. Semi-Crystalline: Half or even more of the chains in crystalline thermoplastics are or - ganized into tiny regular regions called crystallites . Representative members are nylons,
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