The initial application of heat changes the structure

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Unformatted text preview: The elements that are most likely to form two covalent bonds are those in Group VIB of the periodic table. The two most common elements in this column are S and O. Therefore, the answer to this problem is oxygen. The crosslinks in rubbers usually involve a short chain of S or O atoms rather than a single S or O atom. The level of crosslinking in an automobile tire is not great, and there are large numbers of unsaturated double bonds remaining. Therefore, when atmospheric sulfur or oxygen attacks the tire in use, the sidewalls become brittle and crack. ....................................................................................................................................... As mentioned above, the key difference between thermoplastic (TP) and thermoset (TS) structures is that TSs generally have a 3-D primary bond network while TPs have weak secondary bonds between the covalently bonded polymer chains. This difference has a profound influence on the properties of polymers. The change in structure of a TS polymer as a result of an increase in temperature can be compared to that of a raw egg during cooking. In either case, the raw material— monomer, or uncooked egg— can be formed easily into almost any desired shape by “pouring” the raw material into a suitably shaped mold. The initial application of heat changes the structure by supplying the energy necessary to form a three-dimensional bond network. Upon cooling from the “fabrication” temperature, the material retains the shape of the mold. It is not possible, however, to repeat this process. Upon reheating, both the egg and the TS polymer will degrade — char or burn — rather than revert to their original fluidlike structure. Thus, TS polymers can be formed initially into almost any desired shape, but they cannot be re-formed at a later time. In contrast, the behavior of a typical TP polymer is much closer to that of candle wax. Because of the comparative ease with which the secondary bonds between polymer chains can be temporarily suspended with heat, TPs...
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