Additives enable microwave curing of most elastomers Nonpolar rubbers cured continuously using diethylene glycol, triethanolamine Continuous microwave curing applied to almost any elastomer brings the rubber industry's long-time objective of continuous processing a step closer to reality. Dr. Jakob Ippen, of Farbenfabriken Bayer's technical service department, Leverkusen, West Germany, told the 98th meeting of the ACS Division of Rubber Chemistry in Chicago, 111., that continuous microwave curing is possible with almost any elastomer if suitable compounding ingredients are chosen, and notably if diethylene glycol and triethanolamine are used. In most methods of vulcanization in which rubber goods are subjected to high temperatures, the heat is supplied by air, steam, or a liquid. Because of low thermal conductivity of rubber mixtures, Dr. Ippen explains, articles with a relatively large cross section are slow to cure and do not acquire an optimum degree of cross-linking over their entire cross sections. In microwave curing, he says, the rubber material to be cured is passed through an electromagnetic alternating field, and this field's energy is absorbed and converted to heat throughout the object. UHF fields. Rubber mixtures respond differently to ultrahigh-fre-
quency fields. For example, polar elastomers—such as nitrile rubber (a copolymer of butadiene and acrylonitrile) and neoprene rubber (polychloroprene)—become hot very quickly when placed in a microwave oven, which Dr. Ippen uses to test dielectric heating of elastomers. Polar elastomers typically reach a temperature of 200° C. within half a minute. On the other hand, nonpolar elastomers, including natural rubber, styrene-butadiene rubber (SBR), and butyl rubber (a copolymer of isobutylene and isoprene), hardly become hot at all. The highest temperature reached in 3.5 minutes, for example, is only 60° C. for any of the nonpolar elastomers tested. Dr. Ippen points out that his laboratory experience indicates that an elastomer stock is suitable for microwave curing only if it reaches a temperature of 150° C. or more in 180 seconds or less in the microwave oven test apparatus. Addition of carbon black to nonpolar elastomers enhances their ability to heat up in a UHF field, Dr. Ippen says. Thus it's not too difficult, he says, to formulate mixes for microwave curing based on polar polymers or—if suitable carbon blacks are added—on
Add fives fit natural rubber for microwave curina Temperature reached in UHF field (" C.)
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Source: Farbenfabriken Bayei
34 C&EN NOV. 2, 1970
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nonpolar polymers. No light-colored fillers, he finds, are alone capable of producing sufficient heat to permit continuous microwave curing. It would appear then that the limits of microwave curing are reached with a stock without carbon black based on a nonpolar polymer. An initial remedy to surpass this limit is to add a polar polymer, which functions as an energy absorber in the resulting blend. This procedure, however, is unsatisfactory, Dr. Ippen points out, because the addition of another polymer changes the physical data and also introduces an added cost. Cumulative effect. In looking for an alternative, Dr. Ippen and his coworkers find that some ordinary rubber compounding ingredients are capable of absorbing electromagnetic energy and producing heat—and in cumulative amounts. Investigating this approach, Dr. Ippen uses a red laboratory tubing stock based on natural rubber and made in a Bayer pilot plant. Originally the tubing was vulcanized in a liquid curing device; its energy absorption in a UHF field was not sufficient for use with the microwave method. By systematically replacing some of the compounding ingredients of the rubber stock with substances which have perceptible, though slight, absorption, Dr. Ippen developed a stock which can be microwave-cured easily under industrial conditions. Large increase. Alone, natural rubber for the tubing stock shows little heat development in the microwave oven, but with several compounding ingredients added, such as silica, stearic acid, antioxidants, accelerators, and sulfur, the mixture does surpass Dr. Ippen's critical time-temperature limit for suitability for microwave curing. However, there is a particularly large additional increase in temperature when diethylene glycol and triethanolamine are added to disperse the filler. Both substances are polar, but the observed increase is much greater than would be expected, he says. To bring about sufficient heat development for microwave curing of light-colored nonpolar stocks, it usually suffices to add an amount of diethylene glycol and triethanolamine in equal amounts to represent 10% of the amount of filler used. Dr. Ippen says that in addition to natural rubber many other nonpolar elastomers—including SBR, ethvlenepropylene-diene terpolymer (EPDM), and even butyl rubber—can be continuously microwave-cured, despite the absence of carbon black, when diethylene glycol and triethanolamine are used. However, the Bayer chemist admits that so far he has found no suitable formulation for silicone rubber.
