22 



THE INDIA RUBBER WORLD 



[October 1, 1919. 



What the Rubber Chemists Are Doing. 



THE NATURE OF VULCANIZATION. points to very incomplete vulcanization. This docs not happen 



/>'.v Dr. H. P. Stevens' with moderately under-vulcanized rubber. Harries and Fon- 



P-'^RT I. robert e-xtracted a large quantity (500 grms.) with acetone for a 



THE COMBINATION OF RUBBER WITH SULPHUR. long period (60 days)." At the end, the rubber contained only 



To THEORIES have been put forward to explain the change 0.29 per cent of sulphur. It was assumed that, given sufficient 



which takes place in the physical and chemical properties time, this residue would eventually have been removed and 



of rubber when vulcanized. According to one theory, therefore the sample may be said to contain no combined sulphur, 



vulcanization is primarily an adsorption process; according to Owing to the large amount of rubber taken for extraction and 



the second, a chemical reaction is involved, the sulphur enter- the defects of the extraction apparatus, it appeared probable that 



ing into a chemical combination with the rubber hydrocar- this result would be more quickly obtained by the extraction of 



bon. It would, however, appear that those who uphold the small samples, rolled very thin, using a form of apparatus in 



physical theory admit the possibility of a subsequent chemical which extraction takes place at the boiling point of acetone 



combination of the rubber and sulphur taking place, while the (Analyst, 1913, 38, 143). 



supporters of the chemical theory admit that adsorption of the I determined therefore to vulcanize a mixture of Para' rub- 

 sulphur may precede chemical combination. It remains to be ber and 10 per cent of sulphur under the same conditions as em- 

 shown whether or no the technical efifect of vulcanization can ployed by Harries, that is for 30 minutes at 145 degrees C, but in 

 be produced without any chemical combination between sulphur the form of a thin sheet ^—1 mm. thick so to obtain uniform 

 and rubber hydrocarbon taking place. vulcanization. At the same time a second experiment was made 



The systematic researches of Spence and his collaborators with the same mixture of rubber and sulphur, vulcanizing for 19 



("Kolloid Zeitsehrift," 1911, 8, 304; 9, 300; 1912, 10, 300; 11, 28, minutes only. It is well known that Para rubber from different 



274; 1913, 265), which in the main have been corroborated by sources varies in rate of vulcanization. The experiments were 



later work, clearly show that sulphur enters into chemical com- therefore extended to include a sample of fast vulcanizing planta- 



bination with rubber during the ordinary vulcanizing process as, tion Para rubber. Approximately one-quarter of each sample 



for instance, when a mixture of rubber and sulphur is heated for was removed after 1, 2, 4, and 9 weeks, extraction (1 week= 



an hour or two at temperatures such as 130 degrees— 150 degrees approximately 50 hours), and the sulphur estimated with the 



C. The velocity of the reaction is proportional to the time of following results : 



heating at a constant temperature ; the temperature coefficient p^^^ -Rubber A Vulcanized in Ste.^m at 145 Degrees C. 



normally lies between 2 and 3 {Ibid., 11, 32), but is considerably ^^^.^^ „f Combined sulphur per cent, 



influenced by catalysts (the so-called accelerators). extraction. (i)30min. (2) 19 min. 



More recently the subject has been studied by Harries and 2 weeks . .......... ...... ...... ...... 1.54 0.96 



Fonrobert (Berlin, 1916, 49, 1196, 1390); these authors, without ^ weeks i;4? aw 



controverting the work of Spence, claim that vulcanization is es- n ,-, ,, 



... u • I J u ■- 1 • J uu v Para Rubber B. 

 sentially a physical process and that vulcanized rubber can be 



prepared which is free from combined sulphur. This conclusion (3) a"d (4) were vulcanized to correspond with (1) and (2> 



is based on one vulcanizing experiment only, carried out on a above. They were extracted for periods of 2 weeks and 9 weeks, 



rather large scale. The acetone-extracted vulcanizate was prac- fxtr'ac'tion *'nT M^'^''' ^"'"'''m'^fQ '^^•"*' 



tically free from sulphur. In the course of my own work on 2 weeks' l.U ' 0.67 



vulcanization I have made numerous sulphur estimations in 9 weeks 1.08 0.70 



acetone-extracted vulcanizates but have never obtained a figure Fast Vulcanizing Para Rubber C. 



comparable to that of Harries. It was therefore thought advis- This sample was treated similarly to B. 



able to carry out a control of Harries' vulcanization experiment .Analyses were made in duplicate. 



before proceeding with further research. Period of Combined sulphur per cent. 



There are several points in regard to Harries' experimental extraction. (5) 30 mm. (6) 19 mm. 



procedure which are open to criticism. The rubber was mixed 2 weeks f 2.69 1.40 



with 10 per cent of sulphur and disks vulcanized for a short time 12.39 i'47 



(^-hour) at a fairly high temperature (145 degrees C). The 9 weeks (2:33 1:46 



thickness of the disks is not. given but, as these were used for j,,, vulcanised specimens were subjected to physical tests, 



tensile tests with a Schopper machine, they were probably 5-6 Ri^g.^h^ped test pieces gave the following results- 



mm. thick so as to give a ring of standard size. It is doubtful .^ime of Breaking Final length 



whether sufficient time was given for the heat to penetrate Rubber vulcanizing strain, grms. (original 



evenly through so thick a layer of rubber, and it is probable that ^"""nl^A (minutes). per |q-^nim. lengih=l). 



the outer layers were more fully vulcanized than the inner parts a] 'b , 30 750 il'l 



of the disks. It is stated that the conditions of vulcanization were (4i ....[.[.[...'..'.'.'.'...'.'. 19 290 12!2- 

 similar to those technically used in the manufacture of inner (|) !\:[[[[.'.\'.'.',\'.'.'..::\\ 19 "90 n'.l 

 tubes, but these latter usually contain an appreciable proportion With regard to these fjgures, the breaking strain of (5) is 

 of antimony sulphide in addition to the sulphur which facilitates only a little below that normally given for a fully vulcanized 

 the conduction of heat through the mass, also of "accelerators" Para rubber. This sample contains about 2.4 per cent of com- 

 which are capable of reducing the time required for vulcanization bined sulphur whereas a fully vulcanized rubber usually contains 

 from one-half to one-third of that otherwise required. It is 2.8—3 per cent calculated on the rubber (coefficient of vulcaniza- 

 stated that the physical qualities of the vulcanized rubber were tion). The breaking strains of (1) and (3) are not much over 

 satisfactory, but the tensile figures are not given and the fact that one-half of what would be obtained if fullv vulcanized. The 

 the rubber became soft and sank together during extraction . Presumably the extraction was not carried on during the night as the 

 ■ apparatus was leaky. Sixty days mav therefore be taken 



iDr. H. P. .Stevens in "The Journal of the Society of Chemical In- about 500 hours, 



dustry," July 15. 1919. a That is, derived from Hevea brasiliensis. 



