May 1, 1921 



THE INDIA RUBBER WORLD 



579 



What the Rubber Chemists Are Doing 



THE RELATIVE ACTIVITY OF VARIOUS ALLOTROPIC FORMS OF 

 SULPHUR TOWARDS CAOUTCHOUC 



By D. F. Twies and F. Thomas 



THE INVESTIGATIONS ill receiit years of the allotropic forms 

 of sulphur capable of existence within the range of tempera- 

 ture used in ordinary vulcanization, naturally prompt en- 

 quiries as to a possible difference in the activity of these various 

 modifications of the vulcanizing agent. The fonns most likely 

 to be involved are (a) S^, the modification represented by or- 

 dinary crystalline sulphur, (b) S/^, the insoluble modification con- 

 stituting part of genuine "flowers of sulphur" and corresponding 

 with the viscous constituent of molten sulphur, and (c) Sf, a 

 more brightly colored and more soluble variety, present in a small 

 proportion with much S^ in the equilibrium mixture yielded by 

 ordinary sulphur at temperatures a little above its melting point ; 

 the proportion of S'^ increases markedly if the temperature is 

 raised to 140 degrees or higher. 



Tests made with mixtures of rubber with soluble sulphur (S?*.) 

 and insoluble sulphur (S") respectively, under ordinary technical 

 conditions, indicate that the vulcanizing effect of these two varie- 

 ties is practically the same; this result, however, is probably to 

 be attributed to the rapid change of Sa at such temperatures into 

 the equilibrium mixture consisting mainly of S^ with some S"'. 



Between 128 degrees and 168 degrees C. the temperature co- 

 efficient of the chemical reaction between rubber and sulphur, i.e. 

 the proportion by which the rate is increased on raising the tem- 

 perature 10 degrees C, is surprisingly constant, and in view of 

 the alteration of the composition of the equilibrium mixture with 

 rise of temperature, the comment has been made that this is in- 

 dicative of comparable vulcanizing capacity on the part of the 

 modifications present. 



Although the difference observable between the forms of sul- 

 phur naturally existent at ordinary vulcanizing temperatures is 

 thus shown to be relatively slight, it must be remembered that 

 under such conditions the sulphur will undergo fusion and then 

 rapidly yield the equilibrium mixture. If the temperature could 

 be maintained much lower, the rate of attainment of equilibrium 

 could be reduced and the increased persistence of each form would 

 afford greater opportunity for observing any difference in vul- 

 canizing capacity. The use of a suitable vulcanization catalyst, 

 e. g., aldehyde-ammonia, enables experiments to be made at a 

 sufficiently low temperature. It is a little unfortunate that the 

 powerful organic catalysts generally should be basic substances 

 such as also tend to catalyze the mutual interconversion of the 

 sulphur allotropes, but as is demonstrated by the results now sub- 

 mitted, the disturbance from this direction is not sufficient to 

 mask completely the sought effect. 



In the first set of experiments the varieties of sulphur used 

 were (o) a high grade finely powdered sulphur soluble in carbon 

 bisulphide and consisting entirely of S^^, and (b) an extracted 

 sublimed flowers of sulphur 93 per cent insoluble in carbon bi- 

 sulphide and consisting mainly of Sm. Mixtures were made of 

 each of these with selected pale crepe rubber of known rate of 

 vulcanization ; the composition in each case was rubber 90 parts, 

 sulphur 10 parts, and aldehyde-ammonia 1 part. In order to 

 ensure as closely comparable composition and conditions as pos- 

 sible, the sulphur in both cases was first mixed with an equal 

 weight of the rubber and the composition of each stock checked 

 before introducing the correct amount of each into the final 

 mixtures. Similarly the aldehyde-ammonia was first mixed with 

 nine times its weight of the rubber so as to increase the accuracy 



with which equal proportions of this "stock" could be introduced 

 into the two batches of equally "worked" rubber. Vulcanization 

 was effected, with the two mixtures simultaneously, at 98 degrees, 

 108 degrees, and 118 degrees C. 



It will be noted that under the conditions of these experiments 

 the vulcanizing effect of S", particularly at the lowest tempera- 

 ture, is definitely inferior to that of S^. 



Taking an extensibility of 700 per cent as a standard for com- 

 parison, the difference in the rate of vulcanization of the two 

 mixtures is clearly seen from the respective periods required, and 

 the more rapid conversion of S'' into S^ with rise of temperature 

 is reflected in the corresponding decrease in the temperature co- 

 efficient." 



Rate of vulcanization 



Sn (93%) 



SX 



Degrees Time 



C required 



98 32 hours 



108 550 mins. 



118 185 mins. 



Temp. 

 coetTt. 



3.5 

 3.0 



Time 



required 



23 hours 



440 mins, 



140 mins. 



Relative 



Temp. effect. 



coefft. Sa:S\. 



— 1:1.4 



3.1 1:1.3 



3.1 1:1.3 



1 .TournrtI of the Society of Chemical Industry, Febru.iry 28, 1921. 



An experiment with similar mixings of the same rubber and 

 two forms of sulphur in the same proportions, but without the 

 catalyst and at 148 degrees, confirmed the earlier observations 

 as to the closely comparable effectiveness of the S'' and 5?^ at 

 this temperature, the relative effectiveness calculated in the same 

 way as above being 1 :1.06. 



These results as a whole demonstrate that at the ordinary vul- 

 canizing temperatures, e. g., 148 degrees, Sa changes so rapidly 

 into S'l that no appreciable difference can be detected in the rate 

 of vulcanization, the effect in each case being that actually pro- 

 duced by S^. At lower temperatures it is possible to detect a 

 distinctly feebler effect in the mixture containing S/*. The pos- 

 sibility that this difference may be characteristic not of S» but 

 of St, whfirh is formed together with S^ from the less stable 

 S** at such temperatures, is hardly in accord with the almost 

 identical effect observed at 148 degrees C. (see above) and with 

 the results given below. In all likelihood therefore the difference 

 arises from the Sa itself and is probably not explained completely 

 by the lower solubility of this form of sulphur. 



From the constancy of the temperature co-efficient of the re- 

 action between ordinary sulphur and rubber at 138 degrees — 168 

 degrees C, over which range S^ gives rise to an increasing pro- 

 portion of St, it would appear that Sir and SX are of almost 

 equal activity in this direction. 



There appeared to be some hope of obtaining more evidence of 

 the activity of S/* relative to S^ by making vulcanization experi- 

 ments with rubber at higher temperatures, at which sulphur is 

 known to undergo a marked change in molecular complexity 

 probably corresponding with a rapidly increasing proportion of S" 

 in the equilibrium mixture. Unfortunately with ordinary rubber 

 the chemical change accompanying vulcanization would then be 

 too rapid for convenient examination, and recourse was there- 

 fore had to the use of synthetic rubber which, as is well known, 

 is relatively sluggish in vulcanization; the sample used was of 

 "methyl-rubber," i. e., polymerized dimethylbutadiene, which was 

 free from artificial catalysts and "clasticators." On account of 

 the hardness of the vulcanized products obtained with such syn- 

 thetic rubber, no physical tests were possible and the rate of re- 



=In these experiments vulcanization was effected uninterruptedly for the 

 full periods, the temperature of the oil bath being maintained constant for 

 the whole of the time. The greater value of the temperature coefficient ob- 

 rained previously with SX for the interval 98 degrees-108 degrees C. is to be 

 ascribed to the fact that the earlier experiment at 98 degrees C. was made 

 in two stages, with the result that the vulcanization would be retarded 

 somewhat on account of the partial separation of sulphur from solution in 

 the rubber halfway through the experiment. 



