XOVEMBER 1. 1919] 



THE INDIA RUBBER WORLD 



85 



What the Rubber Chemists Are Doing. 



The Nature of Vulcanization.' 



By Dr. H. P. Stevens? 

 Part II. {Continued.) 



THE ACTION OF SOLVENTS ON VULCANIZED RUBBER. 



PORTIONS of the four vulcanized samples were kept for 3 

 niunlhs at winler temperatures and other portions for the 

 same period in an incubator at 30 degrees C. On examining 

 the specimens for solubility in benzene, those specimens preserved 

 at winter temperatures were somewhat less soluble, the specimen 

 (2) now behaving similarly to specimen (3) when originally 

 tested. Those specimens kept at 30 degrees C. were more 

 altered, being still less soluble; thus (1) could no longer be 

 dissolved in benzene on standing overnight and shaking. It was 

 found, however, that all specimens dissolved to a large extent in 

 benzene when left long enough immersed in the solvent. 



It is generally agreed that it is exceedingly difficult to remove 

 the last traces of free sulphur from an unvulcanized mixture of 

 rubber and sulphur. Thus the following figures were obtained 

 on the analysis of an unvulcanized mixture of 90 parts of rubber 

 and 10 parts of sulphur before and after extraction for one 

 week : — before extraction, 9.84 per cent sulphur. After extraction, 

 0.13 and 0.12 per cent sulphur. 



The unvulcanized mixture dissolved more readily in benzene 

 than sample (1) above. A diflFerence could be detected in the 

 physical properties so that it may be said that sample (1) 

 showed the effect of vulcanization to a slight degree. 



Solvent Action of Benzene on Fully Vulcanized Rub- 

 ber. Although benzene has but little solvent action on fully 

 vulcanized rubber in the cold, I find that, by prolonged extrac- 

 tion of the acetone-extracted rubber in a Soxhlet at the boiling 

 point, a considerable proportion may be dissolved. The ex- 

 tracted rubber tends to solidify on the sides of the flask in 

 which the solvent is boiled, but part remains in solution. On 

 evaporation of the benzene, the rubber remaining no longer 

 dissolves appreciably when fresh benzene is added and set aside 

 in the cold or the liquid boiled.' In this respect the solution 

 of vulcanized rubber obtained resembles that produced by the 

 action of ultra-violet rays on a solution of raw rubber and 

 sulphur in benzene (Helbronner and Bernstein, "Coj)j/'(w/?(?niM," 

 1914, 158, 1343). The recovered rubber is of inferior physical 

 properties. The following experiments were made to ascertain 

 whether it was possible to separate vulcanized rubber into 

 parts containing different proportions of "combined" sulphur. 



Two samples of vulcanized rubber, A and B, were extracted 

 with acetone at the boiling point for one week and the combined 

 sulphur estimated. Parts of the extracted rubbers were dried, 

 weighed, and similarly exhaustively extracted with benzene at 

 the boiling point for one week. The extracted rubber was dried 

 and weighed, and the sulphur estimated in the residue. 



Benzene extract: (A) 30.8 per cent; (B) 13.7 per cent. Sulphur 

 content of residue before benzene extraction: (A) 3.80, 3.80 per 

 cent;* (B) 8.64, 8.77 per cent Sulphur content of residue after 

 benzene extraction: (A) 3.85, 3.86 per cent; (B) 8.35 per cent. 



It will be seen that, although an appreciable proportion of 

 the vulcanized rubber was dissolved by the benzene, the residue 

 of A contains the same percentage of sulphur as the original 

 acetone-extracted rubber, and the residue of B almost as much. 



•Continued from The 



»Dr. H. P. Stevens 

 dustry," July 15. 1«19. 



.'The rubber sometimes dissolves when covered with benzene and set 

 aside for two or three months. 



'Here and subsequently duplicate figures give the results of duplicate 

 uialyscs. 



The benzene extractions were now repeated with portions of 

 the acetone-extracted rubbers, A and B, which had been put 

 aside for 2^3 weeks in the dark. The benzene extracts were 

 {.\) 53.3 per cent; (B) 53.1 per cent. 



That is to say, the benzene-soluble had increased to over 

 50 per cent in the course of aging. I have already drawn 

 attention (8th International Congress of Applied Chemistry, 1910, 

 25, 585; "Kolloid Zeilschrift," 1914, 14, %) to the rapid deteri- 

 oration that rubber undergoes after it has been extracted with 

 acetone, particularly when vulcanized. Such "perished" or 

 "resinified" rubber is readily soluble in benzene. In the above 

 cases the extract had the physical properties of a rubber rather 

 than of a resin, although it underwent a further change on 

 keeping, becoming brittle and "resinous." It was also found that, 

 on keeping the above-mentioned acetone-extracted rubber for 

 S — 6 weeks, it dissolved completely in benzene with the exception 

 of a small amount of a flocculent precipitate, probably due to the 

 protein constituent of the original rubber. The benzene solution 

 when evaporated gave a varnish-like film, moderately hard but 

 inelastic. Portions of the extracted rubber and the extracts of 

 A and B were analyzed. 



The extracted rubber contained (A) 3.57, 3.59 per cent; 

 (B) 8.57, 8.38 per cent sulphur; and the benzene extracts (A) 

 3.34, 3.36 per cent; (B) 8.09, 8.22 per cent sulphur. In both cases 

 the extract contained slightly less sulphur than the extracted 

 rubber. There was also an apparent loss of sulphur during the 

 aging of the acetone-extracted rubber. Some weeks later a 

 portion of the acetone-extracted rubber. A, then aged and 

 perished, was found to contain 3.32 and 3.33 per cent sulphur. 

 These figures compare with 3.80 per cent originally present in 

 the acetone-extracted rubber, A. 



As the foregoing results show that the solubility in benzene 

 of an acetone-extracted vulcanized rubber depends on the period 

 that has elapsed since the rubber was subjected to extraction 

 with acetone, it was desirable to ascertain in what degree a 

 sample freshly extracted with acetone dissolved in benzene. 



A freshly vulcanized sample of rubber, C, was extracted with 

 acetone for one week and immediately extracted with benzene 

 for a like period. It yielded 10.9 per cent of benzene extract. 

 The original acetone-extracted rubber contained 3.80, 3.70 per 

 cent of sulphur, and the benzene extract 3.68 per cent. 



The residue was then immediately reextracted with acetone 

 and gave a small extract amounting to 1.7 per cent containing 

 3.77, 3.82 per cent sulphur. It appeared that the process of benzene 

 extraction, or the removal of the benzene preparatory to extrac- 

 tion with acetone, had produced some degradation of the rubber, 

 as it then yielded 1.7 per cent to acetone although previously 

 exhaustively extracted with acetone. There was no appreciable 

 variation in the sulphur content. 



A portion of the original rubber, C, was also extracted with 

 benzene without previous acetone extraction. The extract con- 

 sisted almost entirely of sulphur. The residue contained 3.90, 

 3.97 per cent sulphur. 



These figures are a trifle higher than for the acetone-extracted 

 rubber. The fact that very little rubber was extracted by the 

 benzene seems to indicate that vulcanized rubber with this 

 proportion of "combined" sulphur is almost insoluble in benzene 

 and only dissolves when degraded or oxidized, and that such 

 degradation takes place rapidly when exposed to air after ace- 

 tone extraction or during the acetone-extraction process itself: 



