Plastics

  A major group of materials that are primarily non cyristalline hydrocarbon substances composed of large molecular chains whose major element is carbon. The three terms—plastics, polymers, and synthetic resins (or resins)—are sometimes used interchangeably to identify these materials. However, the term plastics has now come to be the commonly used designation.

The first commercial plastic, celluloid, was developed in 1868 to replace ivory for billiard balls. Phenollic plastics, developed by Baekeland and named Bakelite after him, were introduced around the turn of the century. A plastic material, as defined by the Society of the Plastics Industry, is “Any one of a large and varied group of materialsconsisting wholly on in part of combinations of carbon with oxygen, hydrogen, nitrogen, and other organic an inorganic elements which, while solid in the finished state, at some stage in its manufacture is made liquid, and thus capable of being formed into various shapes, most usually through the application, ether singly or together, of heat and pressure.”

There are two basic types of plastic basedon intermolecular bonding. Thermoplastics, because of little or no cross-bonding between molecules, soften when heated and harden when cooled, no mather how often the process is repeated. Thermosets, on the other hand, have strong intermolercular bonding. Therefore, once the plastic is set into permanent shape under heat and pressure, reheating will not soften it.

Within these major classes, plastics are commonly classified on the basis of base monomers. There are over two dozen such monomer families or categories: rigid, flexible, and elastic. Another method of classification is by the “level” of performance or the general area of application, using such categories as engineering, general-purpose, and specialty plastics, or the two broad categories of engineering and commodity plastics.

Some of the major characteristics of plastics that distinguish them from other materials, particularly metals, are : 1) they are essentially noncyristalline in structure; 2) they are conductors of electricity and are relatively low in heat conductance; 3) they are, with some important exceptation, resistant to chemical and corrosive environments; 4) they have relatively low softening temperatures; 5) they are readily formed into complex shapes; and 6) they exhibit viscoelastic behavior—that is, after an applied load is removed, plastics tend to continue to exhibit strain or deformation with time.

Polymers can be built of one, two, or even three different monomers, and are temed homopolymers, copolymers, and terpolymers, respectively. Their geometrical form can be linear or branched. Linear or unbranched polymers are composed of monomers linked end-to-end to from a molecular chain that is like a simple string of beads or a peace of spaghetti. Branched polymers have side chains of molecules attached to the main linear polymer. These branches can be composed either of the basic linear monomer or a different one. If the side molecules are arranged randomly, the polymer is atactic; if they branch out on one side of the linear chain in the same plane, the polymer is isotactic; and if they alternate from one side to the other, the polymer is syndiotactic.

Few plastic in use are totally composed of polymer resins. Nearly all contain one or more additive materials to modify or control properties, or to reduce costs. Fillers are probably the most common of the additives. They are usually used to either provide bulk or modify certain properties. Generally, they are inert and thus do not react chemically with the resins during processing. The fillers are often cheap and serve to reduce cost by increasing bulk. For example,  wood flour, a commond low-cost filler, sometimes makes up 50% of a plastic compound. Other typical fillers are chopped fabrics, asbestos, talc, gypsum, and milled glass. Besides lowering costs, fillers can improve properties. For example, asbestos increases heat resistance, and cotton fibers improve toughness.

Plasticizers are added to plastics compounds either to improve flow during processing by reducing the glass transition temperature or to improve properties such as flexibility. Plasticizer are usually liquids that have high boiling points, such as certain phthalates. Substances which are used as plasticizers. Stabilizer are added to plastics to help prevent break down or deterioretion during molding or when the polymer is exposed to sunligh, heat, oxygen, ozone, or combination of these. Thus there is a wide range of compounds, each designate for a specific function. Stabilizer can be metal compounds, based on tin, lead, cadmium, barium, and others. And phenols and amines are added antioxidants that protect the plastic by diverting the oxidation reactions to themselves.

Catalysts, by controlling the rate and extent of the polymerization process in the resins, allow the curing cycle to be tailored to the processing requrements of the application. Catalysts also affect the shelf life of the plastics. Colorants, added to plastics for decorative purposes, come in a wide variety of pigments and dyestuffs. The traditonal colorants are metal-base pigments such as cadmium, lead, and selenium. More recently, liquid colorants, composed of dispersions of pigments in a liquid, have been developed. Fire retardants  are added to plastics products that must meet fire-retardants requirements, because polymer resins are generally flammable, except for such notable exceptations as polyvinyl chloride. In general, the function of fire retardants is limited to the spread of fire. They do not normally increase heat resistance or prevent the plastic from charing or melting. Some fire-retardants additives include compounds containing chlorine or bromine, phospate-ester compounds, antimony thrioxide, alumina trihydrate, and zinc borate.

Reinforcements obtained with plastics are not normally considered additives. They are used in plastics primarily to improve mechanical properties, particularly strength. Although asbestos and some other materials are used, glass fibers are the predominant reinforcement for plastics.

Plastics are produced in a variety of different forms. Most common are plastic moldings, which range in size from less than one inch to several feet (two centimeters to several meters). Thermoplastics, such as polyvinylchloride and polyethylene, are widely used in form of plastic film and plastic sheeting. The term film, is used for thicknesses up to and including 10 mils (0,25 cm), while sheeting refers thickness over that.

Both thermosetting anf thermoplastic materials are used as plastic coatings on metal, wood, paper, fabric, leather, glass, concrete, ceramics, or other plastics. There are many coating process, including knife or spread coating, spraying, roller coating, dipping, brushing, calendring, and the fluidized-bed process. Thermosetting plastics are used in high-pressure laminates to hold together the reinforcing materials may be cloth, paper, wood, or glass fibers. The end product may be sheets, or decorative sheets as in counter tops, rods, tubes, or formed shapes.

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