Thermoplastics and Thermosets

        All plastics, whether made by addition or condensation polymerization, can be divided into two groups, called thermoplastics and thermosets. Thermoplastics are solids at room temperature that can be melted or softened by heating, placed into a mold or other shaping device, and then cooled to give the desired shape. The thermoplastics can be reshaped at any time by reheating the part. A kitchen analogy to thermoplastics would be candy caramels (Photo 2.la), which soften and melt when heated. When melted, they can be put into molds and cooled, thus taking the shape of the mold. If desired, they can be reheated and reshaped many times. For instance, the caramels could be reheated to a melt and then an apple could be dipped in the caramel to coat the apple (Photo 2.lb). The coating stays on the apple because the apple cools the caramel coating.

        Thermosets can be either liquids or solids at room temperature. When placed into a mold and then heated, they initially drop in viscosity (liquefy) but then cure (harden) upon further heating, thus giving the desired shape and solid properties. After curing, if thermosets are reheated, they will soften somewhat but do not melt and, therefore, cannot be reshaped. A kitchen analogy to thermosets would be a cake. The batter is poured into the cake pan (mold) and then placed in the oven and heated. Because the liquid cake batter contains polymeric materials that will crosslink when heated, the cake will eventually harden (cure). When the liquid has all solidified, it is removed and cooled, but if the cake is reheated, it will not remelt but will degrade (burn). The behavior of the cake is typical of thermosets.

Figure 2.26 Cross-linking of polymer chains.

        Two such very different behaviors would be expected to arise from a basic difference between thermoplastics and thermosets. In addition to the normal covalent bonds that join the atoms together in the polymer chain, the thermoset plastics also have covalent bonds that join the chains one to another. These bonds between chains have been given a special name, crosslinks, illustrated in Figure 2.26.

Photo 2.1 (a) Caramels (analogous to thermoplastics). (b) Caramel apples (reshaped thermoplastics). (c) Cake (thermosets). 

        When the appropriate sites for reactions exist, crosslinks are normally formed by heating the polymer materials, a process called curing. The heating gives sufficient energy to excite the molecules and cause them to move close enough together that attractions between the bonding sites along the polymer chains can occur, causing the bonds to form. Hence, thermoset materials become firm, cured, or "set" with thermal energy. The crosslinks can either be formed as the polymers themselves are being formed or can be formed between specific sites along existing polymer chains that can join together in normal covalent bonds. Thermoplastic materials, which are characterized by not being crosslinked, will soften and become more "plastic" or pliable or melt with thermal energy. Details of the nature of thermoplastics (commodity and engineering) and thermosets are given in later chapters.

        Some polymers, such as many rubbers and other elastomers, have properties that are intermediate between thermoplastics and thermosets. This occurs because these intermediate materials have only a few crosslinks. Therefore, some oi the polymers may not be tied into others with cross-links and even if tied into others, there are so few crosslinks that the mass of molecules will, when heated, greatly soften and some parts may even melt. This situation means that the elastomers can be thought of as partly thermoplastic and partly thermoset.

        Because of this intermediate nature of elastomers, their compounding (mixing-in additives) and molding are also somewhat different from that of either thermoplastics or thermosets. Since elastomers do not fully melt when heated, compounding is done on the semisolid elastomers, usually by mixing the additives and the polymer together in a powerful mixer (somewhat like a bread dough mixer) or by using a roller mill. After mixing, the elastomer is molded under heated conditions, as is typical of thermosets. More discussion about these materials will be given in Chapter 10.