112 



THE INDIA RUBBER WORLD 



[December 1, 1913. 



What the Rubber Chemists aie Doing. 



[Extracts from Recent Articles on the "Chemistry of Rubber,' 



IN the "Zcitschrift fuer angcwandtc Clicniic." X'olume 26, page 

 494, Ernest Deussen communicates a new method for "The 

 Quantitative Determination of Sulphur in Rubber." The 

 principle of the method is based on the fact that on heating so- 

 dium sulphate with sodium carbonate and filter paper in the 

 presence of reducing gases, there is formed sodium sulphide. 

 In the present application of this principle the rubber is treated 

 with concentrated nitric acid, as a result of which a yellow ni- 

 trosit is formed which is soluble in a sodium carbonate solution. 

 The sulphur has been for the most part oxidized to sulphuric 

 acid. The mineral matter has been converted either to nitrates 

 or sulphates, or in some cases has remained unaltered. 



After the excess of nitric acid has been lioiled off, sodium 

 carbonate is added, which dissolves the nitrosit and neutralizes 

 the sulphuric acid, witli the formation of sodium sulphate. Those 

 minerals which have been rendered soluble by the nitric acid 

 treatment are precipitated as carbonates. The solution, there- 

 fore, contains only the nitrosit, sodium sulphate, and excess of 

 sodium carbonate. This solution is filtered, evaporated to dry- 

 ness, and the residue reduced with filter paper. The sodium 

 sulphide formed is determined colorimetrically in the usual way. 

 As a standard for this colorimetric determination there is used 

 a sodium sulphate solution of known strength which has been 

 evaporated and reduced under the same conditions as the sample 

 to be analyzed. The reduction of the sodium sulphate to sulphide 

 is not quite complete, there being formed small quantities of 

 sodium sulphite and sodium thiosulphate. By using a sodium 

 sulphate solution of known strength and treating it in the exact 

 manner as the sample to be analyzed, the error incurred, owing 

 to the incomplete reduction of the sulphate, is overcome. 



The' nitric acid used in the above determination must be dis- 

 tilled over barium hydroxide and only the middle fraction ot 

 this distillation accepted. 



In the "India Rubber Journal" of October 18. 1913. 

 H. Skellon, B. Sc, contributes a paper on "The Role of Pol;, - 

 prene Sulphide in Vulcanization." By polyprene sulphide the 

 author understands the final interaction product of the rubber 

 hydrocarbon and sulphur. Regarding the rubber hydrocarbon 

 as (CioH,e)n, polyprene sulphide would have the formula 

 (Cn,H,eS.)n. In the theoretical discussions of vulcanization this 

 possible influence of polyprene sulphide has been disregarded. 

 It would, of course, be most marked in the case of hard rubber 

 where considerable polyprene sulphide is present. The author 

 expresses the opinion, which is supported by experimental data, 

 that sulphur is soluble in polyprene sulphide, and, furthermore, 

 that this solubility is greater than the corresponding solubility of 

 sulphur in rubber. Considering the reaction : Rubber -j- sulphur 

 = polyprene sulphide, it is found that the initial reaction ve- 

 locity is greater than the reaction velocity toward the end of the 

 reaction. (This is ascribed to the solubility of sulphur in poly- 

 prene sulphide, which latter is formed in the reaction.) As 

 a result, the concentration of the unchanged rubber 

 and sulphur is altered, which would not be the case if the 

 sulphur were not soluble in the polyprene sulphide. The fact of 

 sulphur being more soluble in polyprene sulphide than in rubber 

 also explains why hard rubber articles do not generally bloom, 

 altho they contain a very large quantity of free sulphur. 

 Blooming in soft ri.bber goods may be looked upon as the crys- 

 tallization of sulphur from a supersaturated solution. Since 

 sulphur is more soluble in polyprene sulphide and since hard 

 rubber contains a much larger amount of polyprene sulphide 

 than does soft rubber, a free sulphur concentration which would 

 produce a supersaturated .solution in the case of soft rubber 

 eoods, produces a solution which is not supersaturated in the 



which have appeared in some of the Foreign Publications.] 



case of hard rubber. Accordingly, blooming docs not take place, 

 even tho a large amount of free sulphur be present. 



It is probable that when sulphur dissolves in polyprene sul- 

 phide, polysulphidcs result, as is the case when sulphur is dis- 

 solved in inorganic sulphides. It is therefore very probable that 

 polysulphidcs of polyprene sulphide exist in vulcanized rubber 

 goods. 



In the "Gummi Zeitung," Volume 27, page 1906, Felix Jacob- 

 solm contributes a paper on "The Determination of the Mineral 

 Matter in Rubber Compounds." It was recently suggested in 

 Germany to use a solution method as the standard method for 

 the determination of mineral matter in vulcanized rubber goods. 

 Kerosene (B. P. 230 to 260 deg. C.) was suggested as a suitable 

 solvent, tho in some cases camphor oil or paraffin oil would 

 be found to be more effective. The author found on working 

 with paraffin oil that results too high in mineral matter were very 

 frequently obtained. This erroneous value, he suggests, is prob- 

 ably due to the carbonization of the rubber which takes place in 

 the process of solution owing to the high boiling point of the 

 paraffin oil. The carbonized matter being insoluble would be 

 weighed up together with the minerals, and accordingly a value 

 too high would be found for the latter. The same compound 

 which gave too high a value with paraffin oil was also analyzed, 

 using camphor oil as a solvent. In this case values much too 

 low in mineral matter were obtained. The same held true with 

 a mixture of '/a of paraffin oil and J4 of camphor oil. Satisfac- 

 tory results were obtained, however, by using a mixture of 2/5 

 paraffin oil and 3,''5 camphor oil. It should be noted that the 

 above considerations all refer to the same compound which con- 

 tained 33 per cent, rubber. For a rubber compound containing 

 50 per cent, rubber, it was found that equal portions of paraffin 

 oil and camphor oil give the most satisfactory results. 



In the "Gummi Zeitung," Volume 28, page 7, H. Loewen con- 

 tributes an article under the same title as Jacobsohn's article. 

 This investigator finds in the majority of cases that the utiliza- 

 tion of paraffin oil as a solvent gives satisfactory results. How- 

 ever, in one particular case excessively high values were ob- 

 tained for the mineral matter as determined by this method. It 

 was observed during the heating of the rubber with the oil that 

 after .some of the rubber had gone into solution a peculiar floccu- 

 lent substance separated out from the solution and subsequently 

 settled together with the mineral matter. This substance was 

 evidently rubber which had once been in solution and for some 

 reason had subsequently become insoluble. In order to investi- 

 gate this matter more closely, experiments were carried out with 

 unvulcanized rubber. A qualitative experiment was carried out 

 by heating 0.1 grams plantation crepe with 2^ c. c. paraffin oil 

 at 170 deg. C, till the rubber was completely dissolved. The 

 solution was thereupon heated for several hours at 230 deg. C, 

 altho by this treatment the solution was not markedly af- 

 fected. On increasing the temperature, but keeping it below the 

 boiling point of paraffin oil, brown flakes immediately separated 

 out of the solution. The heating was therefore discontinued, 

 and these flakes were found to be soluble in benzol, but on con- 

 tinuing the heat treatment they no longer were benzol soluble. 

 By using a mixture of equal parts of paraffin oil and kerosene 

 (B. P. 160 to 260 deg. C), the separation of these brownish 

 flakes was not obtained. 



Jacobsohn found that paraffin oil gives too high a value and 

 camphor oil too low a value, while certain mixtures of these two 

 oils give satisfactory results. In view of the above observations, 

 Loewen suggests that these apparently accurate results are 

 merely due to the counter balancing of the positive and negative 

 errors. 



