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THE INDIA RUBBER WORLD 



[August 1, 1914. 



WHAT THE RUBBER CHEMISTS ARE DOING. 



[Extracts from recent articles on the Clicniistry of Rubber ivhich 



liai'e appeared in some of the foreig,n publications.] 



THE ACTION OF COPPER AND OTHER METALS ON RUBBER. 



IN the "Gummi Zeitung," vol. 28, page 1280, F. Frank and E. 

 Marckwald publish a;i investigation on "The Action of 

 Copper and Other Metals on Rubber." The authors exam- 

 ined a number of decomposed rubber-insulated cables. In 

 some cases the decomposition was such that the insulation 

 was soft and sticky while in other cases hardening in the 

 insulation had resulted. It was observed that the tin plating 

 of the copper wire had been corroded in every case where the 

 decomposition of the insulation had resulted. This corrosion 

 of the tin plating resulted in the copper being attacked, and 

 it was possible to identify copper sulphide wherever such cor- 

 rosion of the tin had taken place. It was found on analysis 

 that the insulations of the cables which showed no tendency 

 to decompose had a vulcanization coefficient of 4.5. The tin 

 plating on the copper wire had not I)een aflected and only 

 very small specks of copper sulphide could be observed on 

 the wire. Careful examinations revealed no trace of copper 

 in the insulation. 



In the case of the decomposed cables, the analysis showed 

 a much lower coefficient of vulcanization, and also a much 

 lower total sulphur content, the coefficient being in the neigh- 

 borhood of 1 as opposed to 4.5 in the case of the satisfactory 

 insulations. As only traces of free sulphur were present in 

 the decomposed insulations, it was evident that an error had 

 been made in the weighing out of the sulphur, with the result 

 that under-vulcanization had resulted. It was easily possible 

 to identify copper in the decomposed insulations. The rub- 

 ber substance of the decomposed insulations was completely 

 soluble in acetone, only the inorganic minerals remaining 

 behind after extraction with this solvent. This acetone ex- 

 tract on purification with petroleum ether yielded a hard 

 brittle product, free from nitrogen, and containing 2.7 per 

 cent, mineral matter. The analysis of this material, after 

 allowing for the mineral matter, gave the following values: 



Carbon 73.21 per cent. 



Hydrogen 9.19 per cent. 



Sulphur 1.43 per cent. 



Oxygen (by difference) 16.17 per cent. 



The analysis shows the material to be a highly oxidized prod- 

 uct. In the presence of the copper, therefore, the under- 

 vulcanization has brought about the high state of oxidation 

 of the rubber. 



As a result of their investigation the authors arrive at tlie 

 following conclusions: 



(It The composition of the rubber insulation must be such 

 that a quick and reliable combination of the sulphur and 

 rubber results. 



(2) The vulcanization should be a quick one. a high tem- 

 perature being resorted to if necessary. By the latter means 

 it is always possible to cure in a very short period of time. 



(3) The tin plating on the copper wire must be uniform. 



(4) Under-vulcanization. even with a subsequent after vul- 

 canization, is not permissible. 



THE COAGULATION OF HE\EA LATEX AND ITS BEARING ON 

 THE STRENGTH OF RUBBER. 

 In the "Journal of the Society of Chemical Industry," page 

 289, Newton W. Barritt contributes an article on "the Coagu- 

 lation of the Latex of Hcvca hrasilicnsis and its Bearing on 

 the Strength of Rubber." The author carried out a large 

 number of test tube experiments on the coagulation of fresh 

 Hevea latex in solutions of varying concentrations of acid 

 and salt. The acid concentrations varied from five-hundredth 

 normal to twice normal, while the salt concentrations varied 

 ^rom one-hundredth normal to twice normal. Standard solu- 



tions of acid and salt were run into the test tubes to make the 

 required strengths and Yz cc of fresh latex was then added to 

 each tube. The liquid was thoroughly stirred to effect com- 

 plete decomposition of the latex, and allowed to stand for 

 24 hours to insure equilibrium. The tubes in which coagula- 

 tion was just short of completeness were then noted and the 

 acid and salt concentrations plotted on squared paper with 

 the concentrations of acid as abscissae and concentrations of 

 salt as ordinates. All points within the curves show concen- 

 trations of acid and salt at which coagulation does not take 

 place, while all points above the curve show concentrations of 

 acid and salt at which coagulation is complete and no 

 caoutchouc globules are to be found in the serum. 



It was found tliat the coagula formed by high concentration 

 of salt at points distant from the curve were tougher and 

 stronger in te.xture than those found at low concentration of 

 salt, which latter were soft and spongy. It was also found 

 that the water-absorbing capacity of the coagulum varies in- 

 \ersely as the concentration of salt in the serum. The in- 

 hibiting effect of acids on coagulation increases with increase 

 of concentration of acid up to a certain maximum. Beyond 

 that, in the case of hydrochloric and sulphuric acids, the in- 

 hibiting effect begins to diminish with increasing concentra- 

 tion of the acid, until at a concentration of N/2, coagulation 

 takes place without the addition of salt. With acetic, lactic, 

 and phosphoric, acids further increase in concentration has no 

 effect, and these acids are unable to produce coagulation at 

 any concentration in the absence of salt. The comparison of 

 the coagulating effect of various salts shows that the valency 

 of the radicals does not appear to exert much influence. .\c- 

 cording to Seeligmann. Hcvea latex from Brazil'- contains 55 

 per cent, water and 9 per cent, mineral salts. The author sug- 

 gests that it is very probable that this high salt concentration 

 is a factor determining the superior quality of Brazilian rub- 

 ber, and that the dilution of the latex by the direct addition 

 of water and acid solution, as practiced by the eastern plan- 

 tations, is largely responsible for the inferiority of plantation 

 rubber. This theory would also account for the variation 

 which exists among plantation rubbers, even from the same 

 estate, since it has been shown that the yield and composition 

 of the latex vary considerably with respect to percentages of 

 caoutchouc, protein, and ash, according to length of tapping, 

 age of trees, and rainfall. In order to obtain uniform qual- 

 ity it would appear necessary to insure uniform compositions 

 of the late.x, especially regarding its ash and protein contents 

 and concentrations. 



The author has experiments in progress which are intended 

 to throw light on this point but postpones discussion of the 

 results until vulcanization tests of the samples have been 

 obtained. 



THE VISCOSITY OF RUBBER SOLUTIONS. 



In the "Journal of the Society of Chemical Industry," vol. 

 33, page 446, R. Gaunt publishes an investigation on "The 

 Viscosity of Rubber Solutions." In making up the rubber 

 solutions for the viscosity e.xperiments, it was observed that 

 greater quantities of rubber yield a rather more than propor- 

 tionate amount of dissolved matter to the same quantity of 

 solvent. It would therefore appear that the dissolved rubber 

 causes a partial solution of the insoluble constituent. For the 

 subsequent investigations the author used samples of (a) Pale 

 Fine Plantation Crepe. Hcvca: (b) Fine Hard Para; (c) Cas- 

 tilliia: (d) Funtumia Sheet; (e) Ceara Biscuits. 



Of these rubbers Fine Hard Para is the most insoluble, 

 especially in the unwashed condition. By passing the Fine 

 Hard Para through a washer for 45 minutes in a stream of 

 cold water, the solubility is considerably increased. Heating 

 with solvents also increases the solubility. The viscosity of 

 rubber solutions varies greatly with diflferent solvents. Ben- 

 zol and chloroform produce solutions whose viscosity is 



