IX. PLASTICS 

 1 . General . 



a. Chemical and Manufacturing Properties . Chemically the term "plastics" 

 is applied to a large group of synthetic materials that are processed by 

 molding or forming into a final shape. Plastics are composed of chainlike 

 molecules of high molecular weight, called polymers, that have usually been 

 built up from simpler monomers. 



All plastics share many common properties and, in general, have four 

 things in common. First, at some stage in their production they are soft and 

 pliable and can be formed, by the application of heat, pressure or both, into 

 definite desired shapes. Second, plastics are organic materials; i.e., they 

 are based on a carbon structure. This distinguishes them from such materials 

 as metals, ceramics, and concrete. Third, plastics are synthetic materials 

 and are products of chemical processes that alter the characteristics of the 

 raw materials from which they derive. Fourth, plastics are high polymers; 

 they consist of monomer atoms joined together into molecular aggregations. 



Different monomers are used to manufacture each different type or family 

 of plastics. Each plastic has a particular combination of properties, pro- 

 cessing requirements, and economics that make it ideally suited for certain 

 applications, yet unsuitable for many others. 



b. Thermoplastics and Thermosets. Plastics in general may be classified 

 into distinct groups. These are thermoplastics and thermosetting plastics 



or thermosets. Thermoplastics soften repeatedly when heated and harden 

 when cooled. At high temperatures they may melt and at low temperatures 

 become brittle. The process of heating and softening and cooling and 

 hardening may be repeated indefinitely for plastics such as polyethylene, 

 polyvinyl chloride, acrylics, nylon or polystyrene. Thermosetting plastics 

 go through a soft plastic stage only once. When hardened, an irreversible 

 change takes place and they cannot be softened again by reheating. Some 

 thermosetting plastics are polyesters, epoxies, phenol- formaldehydes, 

 melamine-formaldehydes and silicones. 



c. Copolymers and Composites . Plastics can be combined like metal 

 alloys to attain the best qualities for a particular end use by selectively 

 drawing from the best attributes of the blended components of the polymers. 

 The process is referred to as copolymerization and the products are called 

 copolymers. Plastics used for structures, including those used in the 

 coastal zone, are most commonly composites or copolymers rather than pure 

 forms. Reinforced plastics are one category of composites in which the 

 plastic is strengthened and stiffened by combining it with high strength 

 fibers such as glass. Sandwich- type plastics contain a variety of strong, 

 thin facings and lightweight cores. There are also the polymer concretes, 

 which contain plastic matrix in place of or in addition to inorganic 

 cement. 



d. Structural Properties from Additives . Because all plastics are 

 synthetic, during their production various things can be done to alter 

 their characteristics by the introduction of additives. These include 



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