Iay 1, 1920.] 



THE INDIA RUBBER WORLD 



495 



What the Rubber Chemists Are Doing. 



EXTRACTION OF RUBBER GOODS.' 



IN RUBBER ANALYSIS, where rubber is determined indirectly, as 

 the difference between 100 per cent and the sum of the 

 organic extracts, ash, and total sulphur, it is important tliat the 

 extracts represent as much as possible of the organic materials 

 which were added to tlie rubber batch and which are not de- 

 termined separately. Those who have tried to extract glue from 

 a vulcanized compound containing it, by means of boiling water, 

 know how tenaciously rubber clings to certain admixed materials. 

 In this work it was our intention to ascertain as far as possible 

 how much, if any, soluble organic material is retained in a com- 

 pound after it has been extracted with acetone for eight hours 

 and chloroform for four hours in the usual way. 



The results obtained by successive extractions of cheap rubber 

 mixtures containing mineral rubber and pine tar demonstrated 

 that in the case of cheap compounds, the figure for rubber ob- 

 tained by the "difference" method cannot be reliable at all, be- 

 cause of the retention of the organic material unextracted. As 

 long as the ashing method is used it is clear that an accurate 

 figure for rubber cannot be expected for this reason. At best it 

 can be but an approximation. 



Finding that such mixtures as acetone, carbon bisulphide, and 

 acetone-chloroform, were more effective than the simple solvents 

 in washing residues on Gooch pads, it occurred to us that if a 

 constant boiling mixture of these solvents could be obtained, this 

 mixture might find application in the extraction of rubber goods 

 in the Soxhlet. 



Ryland" found that a mixture of 55 per cent carbon bisulphide 

 and 45 per cent acetone by volume has a constant boiling point 

 of 39.25 degrees C. Likewise a mixture of 68 per cent chloroform 

 and 32 per cent acetone by volume boils constantly at 64.7 de- 

 grees C. Accordingly, it was decided to extract samples of 

 cheap rubber with acetone for eight hours, and then with such 

 a constant boiling mixture, and compare the w-eight of extract 

 so obtained with that obtained by means of pure carbon bisul- 

 phide, or pure chloroform. 



From the results obtained, it is plain that chloroform is slightly 

 better than carbon bisulphide as a solvent for extracting rubber, 

 while the constant boiling mixture, whether it be 55 per cent 

 carbon bisulphide, 45 per cent acetone or 68 per cent chloro- 

 form, 32 per cent acetone, is a much more efficient solvent than 

 either carbon bisulphide or chloroform. 



Further, one extraction with a constant boiling mixture may 

 take the place of the two extractions, namely, acetone for eight 

 hours, followed by chloroform for four hours, according to 

 common practice in the rubber laboratory to-day. To ascertain 

 whether such a substitution would be possible, the following 

 mixture was made by distilling a mixture of 55 per cent by 

 volume of carbon bisulphide and 45 per cent by volume of ace- 

 tone over copper at a temperature of 39.25 degrees C. The 

 solvent therefore contained no sulphides or free sulphur so that 

 sulphur determined in the extracts could not be attributed to 

 the solvent. Likewise, the mixture of 68 per cent by volume of 

 chloroform and 32 per cent by volume of acetone was freshly 

 distilled, and contained no sulphur. 



A study of the results with these mixed solvents reveals that: 



(a) A small amount of sulphur, usually less than 0.1 per cent 

 is extracted by the chloroform. 



(b) About the same amount of sulphur is extracted by the 

 mixture of 55 per cent carbon bisulphide and 45 per rent acetone 

 when it is used in place of chloroform. 



•Abstract of paper by S. W. Knslem. assistant chemist, and B. L. Honyo, 

 laboratorv assistant. Bureau of Standards. Washington. D. C, read before 

 the Rubber Division of the American Chemical Society, at Philadelphia, 

 Bennsylvania. September 2-6. 1919. 



'Ryland. "Tournal of the American Chemical Society." 1899. volume 22, 

 paRc 3R4. See also Young. "Fractional Distillation," pages 68 and 69. 



(c) Either mixture extracts from the original sample, in eight 

 hours, as much or more material than the sum of the extracts 

 obtained by the eight-hour acetone extraction and the four-hour 

 chloroform extraction. 



(d) Either mixture extracts from the original sample, in eight 

 hours, as much or more mateiial than the sum of the extracts 

 obtained by an eight-hour extraction with acetone and a four- 

 hour extraction with a mixture of 55 per cent carbon bisulphide 

 and 45 per cent acetone. 



(e) The free sulphur present in the extracts obtained by the 

 use of the mixtures is usually from 0.1 per cent to 0.3 per cent 

 higher than the amount present in the acetone extract. 



(f) There seems to be no advantage in extracting 12 hours 

 or longer with the mixed solvents, since the figures are at the 



maximum in eight hours. 



(g) The mixture of carbon bisulphide and acetone does not 

 decompose during the extraction to liberate sulphur or sulphur 

 compounds and in this way affect the free sulphur determination. 



(h) The mixture of 68 per cent chloroform and 32 per cent 

 acetone produces heavier extracts than that of 55 per cent carbon 

 bisulphide and 45 per cent acetone. 



Determination of the eft'ect of the mixed solvents was made 

 on high-grade stocks for the purpose of ascertaining what vary- 

 ing solvent effect they would have on vulcanized rubber. 



Four compounds containing (.A.) 70 per cent, (B) 85 per cent, 

 (C) 95 per cent and (D) 45 per cent of rubber, respectively, 

 were extracted eight hours with acetone and four hours with 

 chloroform and then one was subjected to four hours' additional 

 extraction with chloroform, another to four hours with 55 per 

 cent carbon bisulphide, 45 per cent acetone, and a third to four 

 hours with 68 per cent chloroform and 32 per cent acetone. 



All three solvents dissolve vulcanized rubber to an extent 

 which seems to depend upon how heavily it is compounded. It 

 is interesting to note that the mixture of 68 per cent chloroform 

 —32 per cent acetone exhibits this ability to dissolve rubber to 

 a marked extent. The rubber dissolved by it in the above is 

 nearly twice that dissolved by pure chloroform. On the other 

 hand, the mixture of 55 per cent carbon bisulphide — 45 per cent 

 acetone hardly exhibits this ability at all, so that the rubber dis- 

 solved by it is inappreciable as compared with that dissolved by 

 chloroform or the mixture of chloroform and acetone. Obvi- 

 ously, this is a point in favor of the use of the mixture of 55 

 per cent carbon bisulphide — 45 per cent acetone. 



These results obtained by the carbon bisulphide-acetone mix- 

 lure on these high-grade compounds are in every case lower than 

 the sum of the acetone and chloroform extracts. The figures for 

 free sulphur are also in accordance with previous statements 

 given here. It is evident, therefore, that it is highly advan- 

 tageous to use the mixture of carbon bisulphide and acetone to 

 take the place of acetone and chloroform, because of this less 

 active solution effect upon rubber as shown. 



Before recommending the adoption of this mixed solvent, a 

 few words must be .said about deterioration of the solvent, and 

 blank extracts that will form in it. It was found that the freshly 

 distilled mixture would give no blank, and at the end of two 

 weeks would still give no blank. However, at the end of a 

 month's storage 75 cc. of the mixture would give a residue 

 which, although it contained no sulphur, yet weighed up to 

 0.006-gram. Therefore, it is advised that where the mixture is 

 being used in large quantities for routine analysis, it be redis- 

 tilled over copper every week. Where it is used only occasion- 

 ally, it should be mixed up fresh and distilled before using. 



