8. POLYMERS ANDTHEIR ADDITIVESThe term macromolecule, or polymer, isapplied to substances of high molecularweight that are composed of a large number (usually at least 100) of units of low molecular weight joined by covalent bonds. If the low molecular weight units making up the macromolecule are bonded end-to-end in a long chain and no covalent chemical bonds exist between the chains, the macromolecules are called linear polymers. Such polymers, unless of extremely high molecular weight (1,000,000), can usually be dissolved and, when heated, they soften or melt so that they can be extruded into fibers or moulded into desired shapes. These polymers are said to be thermoplastic. On the other hand, if the polymer chains are linked together at numerous points, the polymer is one large three-dimensional molecule, infusible and insoluble. Such polymers are called cross-links. Certain linear polymers, referred to as thermosetting, contain groups which, when heated, react to give cross-linked polymers.1The process by which small molecules undergo multiple combination to form macromolecules is polymerization. Small molecules from which amacromolecule or polymer can be made are calledmonomers. Two types of polymerization arerecognized: (1)condensation polymerization and (2) addition polymerization.A polymer-forming reaction involving elimination of a small molecule such as water or alcohol between monomer units is described as condensation polymerization.2 In addition polymerization, unsaturated or cyclic molecules add to each other without elimination of any portion of the monomer molecule. The empirical formula of the polymer is then, of course, the same as that of the monomer.Reactions capable of forming macromolecules by either addition or condensation polymerization must be functionally capable of proceeding indefinitely. Whenever two monomer molecules react, the product must contain a functional group capable of reacting with another molecule of monomer. In condensation polymerization, each monomer unit must have at least two functional groups. In addition polymerization, the monomer need have only one functional group - the presence of two or more functional groups usually leads to the production of cross-linked addition polymers.The modulus and other physical properties of polymer can be improved by the addition of fillers and other additives, such as plasticizers, antioxidants, heat stabilizers, ultraviolet stabilizers, and flame retardants. Additives are essential functional ingredients of polymers, and whenever possible, each should be used in optimum amounts for the attainment of high-quality products.Plasticizer: According to the ASTM-D-883 definition, a plasticizer is a material incorporated in a plastic to increase its workability and flexibility or distensibility. The addition of a plasticizer may lower the melt viscosity, elastic modulus, and glass transition temperature (Tg) of a plastic.Antioxidants: Polymers such as polypropylene (PP) are not usable outdoors without appropriate stabilizers, because of the presence of readily removable hydrogen atoms on the tertiary carbon atoms. PP and many other polymers (RH) are attacked during processing or outdoor use in the absence of stabilizers because of a chain degradation reaction. The rate ofchain-reaction degradation is retarded by the presence of smallamounts of antioxidants.Heat Stabilizers: In addition to the free-radical chaindegradation described for polyolefins, another type of degradation (dehydrohalogenation) also occurs with chlorine-containing polymers such as PVC. When heated, PVC may lose hydrogen chloride and form a chromophoric conjugated polyene structure. This type of degradation can be decreased by the presence of lead, barium and cadmium salts which act as scavengers for hydrogen chloride and therefore can be used as heat stabilizers.Ultraviolet Stabilizers: While much of the sun’s high-energy radiation is absorbed by the atmosphere, some radiation in the 280 to 400 nm(ultraviolet) range reaches the earth’s surface. Since the energy of this radiation is 100 to 72 kcal, it is sufficiently strong to cleave covalent bonds and cause yellowing and embrittlement of organic polymers. Since the effect of ultraviolet radiation on synthetic polymers is similar to its effect on the human skin, it is not surprising that ultraviolet stabilizers such as phenyl salicylate have been used for many years in suntanning lotions.Flame Retardants: Since some polymers are used as shelter and clothing and in household furnishing, it is essential that they have good flame resistance. Combustion is a chain reaction that may be initiated and propagated by free radicals like the hydroxyl free radical. Hydroxyl radicals may be produced by the reaction of oxygen with macroalkyl radicals. Halogen radicals produced by the reaction of hydroxyl radicals with halides, such as HX, may serve as terminators for the chain reaction.Other polymer additives are colorants, antistatic agents, foaming agents, preservatives, and processing aid or lubricants.译文:８聚合物及其添加剂大分子或聚合物这个名称，被用来表示一些高分子量的物质。