218 



THE INDIA RUBBER WORLD 



[January 1, 1920. 



What the Rubber Chemists Are Doing. 



AGING EXPERIMENTS ON VULCANIZED PLANTATION 

 PARA RUBBER. 



CAPTAIN B. J. Eaton, F. 1. C, anu 1". W. F. Day of the E-x- 

 perimental Vulcanizing and Chemical Laboratory, .Agricul- 

 tural Department, Federated Malay States, have reported 

 their investigations on the aging changes which take place in vul- 

 canized Para rubber, of which the following is a condensed ac- 

 count : 



Preliminary investigations were made as a guide to a systematic 

 examination of aging phenomena by e.xperimenting on samples 

 of rubber which had been considerably overvulcanized, since the 

 changes would be emphasized in such samples and would be ac- 

 celerated. 



These samples consisted of "slab" rubber (coagulum matured 

 for at least six days before machining to crepe and crepe rubber. 

 The samples were prepared under comparable conditions and mix- 

 ings containing ten per cent of sulphur and 90 per cent of rubber 

 were prepared and cured at 140 degrees C. for varying periods of 

 excessive cure. It was discovered, however, that no useful deduc- 

 tions could be drawn from the figures obtained owing to the re- 

 markable increase in weight which occurred in the crumbled sam- 

 ples after storage, this is, on aging. These large increases in 

 weight were not followed up in these preliminary experiments. 

 The results indicated that much overcured samples would give 

 even greater increases over similar periods. 



As it w^as practically certain that the increases in weight were 

 due to oxidation, the presence of substances soluble in water was 

 tested by extraction of the samples in boiling water and the per- 

 centage of extract (by difference), the percentage of sulphur in 

 the form of sulphate, and the total acidity of the extract of sul- 

 phuric acid were determined in terms of sulphur. The extrac- 

 tion with water was followed by extraction with acetone in order 

 to obtain the free sulphur content and the additional matter ex- 

 tractable by acetone. .\ remarkable percentage of matter ex- 

 tractable by water was formed during the aging. The amount of 

 sulphur present as sulphates in the aqueous extract was also 

 marked. The acetone extract contained substances other than the 

 sulphur and the resin which would be present in the original raw 

 rubber (which ordinarily amounts to 2.3 per cent). 



No further deductions can be drawn from the results since 

 the increases in weight of the samples had not been determined. 

 Some of these increases are nearly 40 per cent, which would bring 

 the total sulphur content of the samples, calculated on original 

 weight, up to about nine per cent, which still indicates an actual 

 loss of sulphur on aging. 



SCHEICE OF EXPEKIMENT. 



As a result of the preliminary experiments mentioned above it 

 was decided to trace the changes, due to aging, in the combined 

 sulphur content, and the aqueous and acetone extracts, after mak- 

 ing allowances for the oxidation or other changes in weight. For 

 this purpose, two sets of about ten grams of crumbed rubbers 

 were stored in specimen tubes. The first set was used to provide 

 samples for analysis at different periods, while the second set was 

 weighed at intervals but not otherwise disturbed. In addition, two 

 grams of each crumbed rubber were left on watch-glasses and 

 also weighed at intervals, as it was thought that aging (as indi- 

 cated by increase in weight) would proceed more rapidly in the 

 case of crumbs exposed to the air and under different conditions 

 from those packed in closed tubes. 



Further, eight-gram disks about five millimeters thick and 44 

 millimeters in diameter were also exposed in small watch-glasses. 

 These were used in order to retain any sulphur exuding on the 



'"Journal of the Society of Chemical Industry," Septemtwr 15, 1919. 

 page 339T. 



surface due to the well known "blooming" of vulcanized rubber. 

 INCREASE IN WEIGHT ON BTOBAGE. 



During the first ten days a slow increase in weight took place 

 in all the samples, after which period the factors favoring a rapid 

 increase are: (a) overcuring of the rubber; (b) freedom from 

 complete exposure to the air. The most rapid increase was shown 

 by the most overcured sample exposed on a watch-glass. 



A similar relationship holds good between the tube stored and 

 watch-glass stored samples for each pair of samples. In every 

 instance the increase in weight of the tube stored samples over- 

 look those of the watch-glass stored samples after periods ranging 

 from one to two and one-half months, the period increasing ac- 

 cording as the overcuring of the sample is less. Comparing the 

 cures obtained by plotting the data from these samples shows that 

 excessive overcuring appears to be the principal factor affecting 

 the type of curve In the case of watch-glass stored samples the 

 tendency to keep below the tube stored samples for an indefinite 

 period, that is, to gain weight more lowly, was very marked. 



It seems clear that the explanation lies in the fact that while 

 increases in weight are proceeding uniformly in both sets of sam- 

 ples, a loss of weight is proceeding at the same time in the freely 

 exposed samples, which leads to an apparent retarded increase. 



Results from four samples investigated indicated that over- 

 curing beyond a certain point ceases to increase the capacity of a 

 rubber to gain weight during storage, and warrants the assump- 

 tion that excessive overcuring does actually decrease the capacity 

 of aging as measured by weight increase. 



COMBINED SULPHTJH CONTENT. 



Only the much overcured crumbled samples showed combined 

 sulphur changes on aging. Normally cured samples did 

 not show much variation in this respect, although some changes 

 could be traced. These changes were, however, small and 

 irregular. 



It was noticed generally that the color changes of the samples 

 of stored crumbs which took place were greatest in those por- 

 tions in contact with the sides of the tubes, that is to say, the por- 

 tions exposed to light. The central portion or core of the tube- 

 stored crumbs appeared to be unaffected in the undercured sam- 

 ples. The overcured samples, however, broke down to a powder. 

 Probably, therefore, if given sufficient time, the full aging effect 

 of 40 per cent increase in weight would develop in all the 

 samples. 



From a study of the changes in sulphur content and weight 

 it is evident that very complex changes are proceeding in which 

 the loss of sulphur compounds at one time exceeds, proportion- 

 ately, the loss of other constituents. It is also clear that at a 

 later stage the conditions are reversed, leading to an increase in 

 sulphur content, and throughout the whole period of change a 

 steady gain in weight, due presumably to oxidation, is taking 

 place. 



AaiTEOtJS AND ACETONE EXTRACTS. 



Some idea of the changes in the proportions of sulphur soluble 

 in water and in acetone can be obtained by extraction in boiling 

 water and acetone. Close analysis of the results is impossible, 

 owing to the weight increases, and it is not possible to eliminate 

 the effect of these changes. The results show that complex 

 changes are occurring which it is impossible to examine at this 

 stage of the work. 



CONCLUSIONS. 



It is not yet possible for the authors to draw very definite con- 

 clusions, since the changes taking place in vulcanized rubber on 

 aging are obviously complex. They have placed their results 

 on record because the literature on this subject does not appear 



