Jani 



THE INDIA RUBBER WORLD 



The Vulcanization of Rubber at Constant Temperature and 

 by a Series of Increasing Temperatures/ 



By G. D. Kratz and Arthur H. Flower 



SLIGHTLY CONDENSED, this paper is given as prepared by 

 the authors. 



When vulcanization of rubber is effected by heating for a 

 period of time at a definite and constant temperature, the rate of 

 combination of the sulphur with the rubber decreases with the time. 

 In this particular instance we have endeavored to maintain a con- 

 stant rate of combination of the sulphur and rubber by a variation 

 in the temperature. Our efforts have been confined primarily to 

 devising a method for calculating a series of temperatures by the 

 use of which the rate of vulcanization might be accurately con- 

 trolled and to make a comparison of the physical characteristics 

 of a rubber mixture vulcanized to the same point by both 

 methods. 



The vulcanization of rubber at constant temperature was re- 

 garded by Weber" as consisting in a chemical reaction between 

 the rubber and sulphur. Later, Skellon' also recorded results 

 which tend to show that the combination of sulphur with rubber 

 is strictly a chemical reaction, which is first preceded by the 

 melting of sulphur and its solution in the rubber. Likewise he 

 maintains that the rate of combination for unit time and constant 

 temperature decreases with the decrease in the active mass of the 

 sulphur present. Ostwald,* on the contrary, has regarded thf 

 vulcanization phenomena as due to an adsorption of the sulphur 

 by the rubber, the rate of which, when expressed graphically, fol- 

 lows the typical adsorption isotherm. Spence" and his co- 

 workers, however, have demonstrated that Weber's vulcanization 

 curves, on which Ostwald based his calculations, are subject to 

 correction. They" have also shown almost conclusively that the 

 vulcanization phenomenon is the resultant both of an adsorption 

 and a chemical interaction of the sulphur with the rubber, so 

 that the views of others are probably not entirely free from need 

 of modification. 



From the results obtained by Spence it is quite obvious that 

 when vulcanization is effected at constant temperature, the major 

 portion of the sulphur combined with the rubber during the early 

 stages of the reaction. Lowering of the initial temperature and 

 subsequently increasing it at regular intervals tends to make the 

 reaction proceed more uniformly. 



In fact, for many years it has been common technical practice 

 to employ this method, popularly known as a "rising cure," 

 based mainly upon the fact that it affords a means whereby 

 the low heat conductivity of the rubber may be minimized rather 

 than for the above reason. It is well established that in the case 

 of bulky articles, unless the vulcanizing temperature is exceed- 

 ingly low, or unless it is initially low and gradually increased as 

 the reaction proceeds, the outside surface may be over-vulcanized 

 before the heat has thoroughly penetrated to the interior of the 

 mass. 



There is a distinct and readily measurable relationship be- 

 tween the time required for vulcanization and the temperature at 

 which it is effected, although there is hardly sufficient evidence 

 to warrant its expression as a law. Based on our previous 

 observations we have been able to calculate with accuracy the 

 relative rates of vulcanization for various temperatures and to 



•Presented before the Rubber Section at the S6th meeting of th( 

 American Chemical Society, Cleveland, September tO to 13, 1918. 



'"Chemistry of India Rubber," 1906 edition, pp. 85-88. 



•"India Rubber Journal," 46 (1913), 723; "Rubber Industry," 1914. 



<"Kolloid-Zeitschrift." 6 (1910), 136. 



'"Kolloid-Zeitschrift," 11 (1912), 28; "Chemiker-Zeitung," 36 (1912) 

 1162; "Kolloid-Zeitschrift," 11 (1912), 274. 



'Ibid., 8 (1911), 304; 11 (1912), 28; 13 (1913). 265. 



apply these different temperatures so that the vulcanization-time 

 curve did actually take the form of a straight line. The satis- 

 factory results obtained were remarkable in that the degree of 

 vulcanization was measured not by chemical but by physical 

 means, which is shown later to be a dangerous procedure.' 



In the application of the above, however, although the accur- 

 acy of our calculations and the control obtained over the rate of 

 combination of the sulphur witli the rubber exceeded our ex- 

 pectations, the differences noted in the physical characteristics 

 of a mixture vulcanized by the two methods were not widely 

 different until a sulphur coefficient of 3.9 was obtained. At this 

 point, vulcanization at constant temperature resulted in a pro- 

 duct which was noticeably inferior to the same mixture when 

 vulcanized by a series of increasing temperatures. 



This was not entirely unexpected, as, in the case of a mixture 

 containing 5 per cent of sulphur, vulcanized to the point of 

 "technical cure"' with a sulphur coefficient of 1.69, the vulcaniza- 

 tion-time curve at constant temperature so closely approxi- 

 mated a straight line that, for all practical purposes, a unit 

 amount of sulphur may be said to have combined in unit time 

 by eitlier method. In fact, the same statement may be made 

 with reservation even up to a vulcanization coefficient of 2.9, 

 although at this point both the tensile strength and elongation, 

 particularly the latter, of the mixture vulcanized at constant 

 temperature were found to be slightly inferior to those obtained 

 when vulcanization was effected by a series of increasing tem- 

 peratures. 



Thus, our results would tend to show that the values for the 

 sulphur coefficient, as previously given by others, are in all cases 

 high. Even the figures 2.8 to 3.0, recommended by Spence, ap- 

 pear to be excessive, while the values established by Eaton and 

 Day are entirely out of question. Furthermore, it seems evi- 

 dent, that, as has previously been stated by De Vries, changes in 

 the rubber-sulphur mixture which determine the physical prop- 

 erties of the mixture, proceed independently of those which de- 

 termine the vulcanization coefficient. Or in other words, the 

 past history of the sample must be known if it is to be judged 

 solely on the basis of its sulphur content. 



Likewise, whatever figure may be decided upon as the cor- 

 rect vulcanization coefficient for Hevea rubber, it is essential in 

 order to obtain maximum physical results by vulcanization at 

 constant temperature, that restrictions be placed upon the mini- 

 mum amount of sulphur and catalyst allowable in the original 

 mixture. For best results at constant temperature there should 

 be present in the mixture such quantities of both sulphur and 

 catalyst that the active mass of the sulphur is not decreased to 

 an extent that will slow up the rate of reaction before the de- 

 sired sulphur content is attained. If this is not taken into con- 

 sideration the continued heating necessary to effect the com- 



•This work is now being repeated, the rate of vulcanization being 

 by both chemical and physical means. 



'The term "technical cure" is used to itidicate that decree of vul- 

 canization at wliuh arc- t<pi:ii<l n.iMiidciu maximum tensile strength and 

 maximum eIon,;:itiMTi 'I~' [ Lilly dependent upon the vulcani- 



zation coefficient, iid- ' 'innship does appear to exist be- 



tween them. I i ■ III I in contradistinction to "optimum 



cure." as it hi. - i it what is generally known as 



'" * ' ■ ' purposes an over-cure 



'- correct vulcanization 

 :bouId not be in excess 

 ith other work we have 

 .lin rubbers other than 

 ■ients for these rubbers 

 by Spence and Eaton, they 



and that the ].r : 



coefficient are t -i i 



of 2.8 per cent 



also determin'-il 



Hnea. These .. -nil- -!i.. ■ ili i' .'li 



are not nccessnrily ...nstants, as suciiestcd by S. _ __ _ ^__ 



should lot be in excess of the following figures: Plantation Hevea, 2.8', 

 Fine Itsrd Par.i. 3.2; Red Kas^-ii, 3.4: and (>ara (Minihot). 4.4. Samples 

 of these rubbers with high coefficients were found to age quickly with rapid 



