536 



THE INDIA RUBBER WORLD 



[July .. 1915. 



What the Rubber Chemists Are Doing. 



A is for the Determination 



Ri bber. Under this titU' in " I - 

 nologic Papers of the Bureau of Standards. No. 45," J. B. 

 Tuttle and A. Isaacs review and compare the various method! < I 

 the dtermination of total sulphur as given by well-known rubber 

 chemists, discussing the results and outlining the method ado 

 by the Bureau of Standards. Eight methods were compared. Of 

 these two were by direct solution, three were by direct fusum. two 

 by solution and subsequent fusion and one special method by ace- 

 tone extraction. The tabulation of the results obtained by these 

 methods shows remarkably uniform determinations by the 

 method of Waters and Tuttle and justify its selection as the 

 preferable methoi 



The treatment in detail was as follows; The rubber (0.5 gram) 

 was treated in a porcelain crucible with 25 c.c. of concentrated 

 nitric acid saturated with bromine, the vessel covered with a 

 watch glass and allowed to stand one hour. It was heated on a 

 steam bath for one hour and then the cover was removed and 

 the solution evaporated to dryness. The residue was fused with 

 5 grams of fusion mixture (sodium carbonate and potassium 

 nitrate), extracted with hot water, filtered, the filtrate acidified 

 with hydrochloric acid and the sulphur precipitated as usual. 



The authors state as the summary of their investigation the 

 following : 



It is shown that the methods which have been proposed for the 

 determination of the total sulphur, other than that present as 

 insoluble metallic sulphates, are not satisfactory. 



It is shown that loss of sulphur is likely to occur in the direct 

 fusion methods, and this loss is apt to increase with increasing 

 free-sulphur content. The method of Waters and Tuttle is 

 recommended for the determination of total sulphur. This 

 method is accurate and comparatively rapid and has given satis- 

 factory results in the hands of a number of analysts over a 

 rather extended period of time. 



A new suggestion is offered, namely, to determine separately 

 the free sulphur and the sulphur remaining after the acetone ex- 

 traction, reporting the sum of the two quantities as the total 

 sulphur. This procedure eliminates the troublesome effect of 

 the free sulphur upon the determination of the total sulphur. 



Analysis \nh Valuation of Rubber. Philip Schidrowitz, 

 Ph. D. in the "Analyst." May. 1915, publishes a comprehensive 

 paper, with copious references to original sources, under the title 

 of "Recent Advances in the Analysis and Evaluation of Rubber 

 and Rubber Goods." The leading features of the paper are as 

 follows : 



I. CRUDE RUBBER. 



The examination may involve: 



(a) Chemical analysis, with a view to determining the quantity 

 of pure rubber and of various impurities, and, to a certain extent. 

 in some instances, the nature of the latter. 



(b) Physical or mechanical tests, carried out either on the crude 

 material or on the latter . the vulcanizing process, with 

 a view to determining the physical and mechanical qualities of the 

 rubb< nee. 



Chemical analysis has hitherto been subordinate in the com- 

 mercial evaluation of rubber, partly on account of lack of exact 

 knowledge regarding the nature of the secondary products 

 i resins, nitrogenous substances, etc.). and partly owing to the 

 absence of specific information on the influence exercised by 

 them on the vulcanization process on the one hand, and on the 

 more imp tributes (strength, elasticity, etc.) on the 



other hand. 



If the difficulties associated with the chemical i fl of 



the nature and influence of the "impurities" necessarily make 



progress in this direction slow, it is not surprising that work hav- 

 ing as its object the identification and evaluation by chemical 

 means of different rubber substances or caoutchoucs is still in a 

 more or less embryonic state. 



Recent work by Caspar] suggests the possibility of discriminat- 

 ing, up to a point, by physico-chemical methods, between caout- 

 choucs of different commercial quality. According to I aspari, 

 rubber is of a composite character and consists of ii i "soluble" 

 rubber, which is a weak but elastic colloid, soluble in light petro- 

 leum, and (2) of "insoluble'' or "pectous" rubber, which is an 

 elastic colloid of considerable mechanical strength. The latter, 

 in some respects resembling a slightly vulcanized material, pre- 

 serves its structure on contact with solvent- It is. however, 

 gradually dissolved by benzene and carbon tetrachloride, but 

 whereas the viscosities of the soluble in Brazilian and plantation 

 I'ara. respectively, arc verj -miliar, the "pectous" in the latter is 

 far more readily attacked by benzene or carbon tetrachloride than 

 the "pectous" of the former. According to Caspari, Brazilian 

 fine contains 35 to 50 per cent, of "pectous," whereas plantation 

 rubber examined by him showed no more than 10 to 25 per cent. 

 Caspari believes that "nerve" or strength is mainly due to the 

 "pectous" variety. The work of Caspari will require confirma- 

 tion and amplification before it is applied to rubber evaluation. 

 It suggests a new field of research indicating the possibility of 

 estimating the quality by a direct physico-chemical method. 



Secondary Products — Rubber Resins. The outstanding fea- 

 ture of the work of Heinrichsen and Marcusson is that all 

 resins excepting that from Para (Hevea) are optically active. 

 In certain cases, therefore, the absence of optical activity in 

 the extracted resin may be taken as evidence that no rubber 

 other than Hevea is present. Para resin contains 15 per cent, 

 and other resins up to 100 per cent, of unsaponi liable matter. 

 The optical activity appears to be mainly due . to the latter. 

 Iodine values varying from 30.6 for Jelutong resin to 118 

 for Para resin were found. So far as the investigation has 

 been carried it appears that the resins from vulcanized rubber 

 exhibit the same characteristics as those from the crude material. 

 D. Bloom, as the result of the examination of 150 samples of 

 resin from different species, concluded that the "acid value" of 

 the resin from the same species is constant. 



The effect of rubber resin on vulcanizing capacity is a matter 

 of controversy. Litharge has been shown to be practically in- 

 operative as a catalyst in the absence of rubber resins. Where 

 litharge or other catalyst was not employed it has been found 

 that the rubber resins do not exercise any marked effect on the 

 curing capacity. 



Mechanical Impurities. Beadle and Stevens give the follow- 

 ing method (for these materials only). They "depolymerize" the 

 rubber by heating with a solvent of high boiling point, thinning 

 still further with a solvent of low viscosity, filtering and weigh- 

 ing. 



Insoluble Matter — Xitrogenous Substances This item does 

 not apply to accidental mechanical impurities, but to natural and 

 normal substances always present to some extent in crude rub- 

 ber. While there is no proof that normal "insoluble" is essen- 

 tially a nitrogen product (a part doubtless consisting of oxidation 

 products) it is fairly certain that it normally contains a high 

 proportion of nitrogen. 



Methods of Separation and Estimation. In W. Schmitz's 

 method 2.5 grams rubber are treated with 50 c.c. of pentachlor- 

 ethane for 5 to 7 hours at 85 to 90 degs. C. with the formation 

 of very fluid solution readily filterable, particularly if somewhat 

 diluted with chloroform. The residue can be further purified by 

 dissolving in 5 per cent, solution of sodium hydroxide and repre- 

 cipitatintr with hydrochloric acid. 



