Si 111 \IUKR 1, 1915.] 



THE INDIA RUBBER WORLD 



647 



of the rubber itself. \t present there i- no possibility oi serious 

 competition of artificial with plantation rubber as regards price. 

 \s regards practical utility synthetic rubbers seem to lack the 

 durability of natural rubber because the latter, by its vegetable 

 origin, contains a series of associated substances, resins, albu- 

 men, etc., which undoubtedly have an influence on it-, durability, 

 for it is well known that deresinated rubber is much more easilj 

 attacked by tlu- oxygen of tlie air than rubber containing resin. 

 Possiblj these associated substances acl as protective colloids 

 which reduce the vulnerability of the pure substance. 



A further reason why synthetic rubbers are inferior to natural 

 rubber in mechanical properties is that the former are not uni- 

 form substances but mixtures. According to recent invi 

 tions of Steimmig, in the oxygen splitting of synthetic rubbers 

 there appears in addition to Coulinie acid and Coulinie aldehyde, 

 which, according to Harries, correspond to natural rubber, resin- 

 ous acid and acetonyl-acetone. 



The two last mentioned substances indicate that in the polymei 

 ization of isoprene, in addition to the 1.5-dimethylcyclooctanes, a 

 smaller amount (20 per cent.) of the 1.6 compound must have 

 been formed by abnormal condensation, which, upon being split 

 by means of ozone, furnishes the two components mentioned. 

 I hi latter have never been found in natural rubber. Until pos- 

 sible to arrange the conditions of polymerization so that the 

 synthetic rubbers will constitute uniform compounds, it is not 

 to be expected that synthetic rubbers will equal natural rubber in 

 its useful properties. 



SULPHIDE AND SULPHATE SULPHUR AND THE ACTION OF 

 SOLVENTS ON VULCANIZED RUBBER. 



The presence of metallic sulphides and sulphates in technical 



rubber articles complicates the estimation of the "combiner!" 

 sulphur or "coefficient of vulcanization." Ordinarily the free 

 sulphur and that present as substitute are extracted with acetone 

 and alcoholic potash respectively. In the absence of sulphides 

 and sulphates an estimation of sulphur in the residue gives the 

 percentage in combination with the rubber. In the presence of 

 sulphides and sulphates it is usual to heat a portion of the 

 residue with high boiling point solvents to destroy the rubber 

 and render it soluble. The sulphur is then estimated in the 

 washed mineral residue. The sulphur is also estimated in an- 

 other portion of the extracted rubber, and the sulphur combined 

 with the rubber estimated by difference. 



The method is unsatisfactory and has two disadvantages. 

 I ii-i. many vulcanized rubbers are decomposed with difficulty. 

 'I- hey carbonize and cake even at carefully regulated tempera- 

 tures Consequently the residue, after washing with benzene 

 contains undissolved organic matter which protects the de- 

 composition of the mineral sulphides. Second, the method as- 

 sumes that the vulcanized rubber does not react with basic 

 substances, such as litharge or magnesia, present in the mixing 

 during heating, with formation of metallic sulphides, although 

 vulcanizing temperatures are employed. 



The method described below is applicable to those sulphides 

 decomposable by heating with acids. It is, therefore, suitable 

 for the estimation of the sulphides of zinc and lead. The metal- 

 lic sulphides in either vulcanized or unvulcaui/ed rubbers are 

 so protected by the rubber surrounding the mineral particles 

 that the surface only is attacked by prolonged boiling with strong 

 hydrocholoric acid solution. If the vulcanized rubber be first 

 swollen in a suitable solvent in which the aqueous acid is partly- 

 soluble, the metallic sulphides of lead and zinc are easily and 

 completely decomposed. Ordinary methylated ether has been 

 found the most suitable solvent. If preferred, benzene or one of 

 the chlorinated hydrocarbons, such as dichlorethylene, can be 

 employed. Liberated hydrogen sulphide is estimated and calcu- 

 lated to percentage of sulphide sulphur. 



Estimation of hydrogen sulphide by oxidation to sulphuric 

 acid does not prove satisfactory. Best results are obtained by 



precipitation in lead acetate solution. The absorption is very 

 complete in the first bottle. The freshly precipitated and washed 

 sulphide is decomposed by shaking with iodine solution. 

 ESTIMATION OF SULPHIDE SULPHUR. 

 To determine the sulphide sulphur, 20 c.c. of concentrated 

 hydrochloric acid and 30 c.c. of ether are placed in a Voigt's 

 flask (a flask having a ground in stopper carrying an outlet tube 

 and a side inlet tube which passes through the side of the llask 

 and reaches nearly to the bottom). The air is expelled from the 

 flask by a current of carbon dioxide, The flask is then con- 

 neeted with an n apparatus containing lead acetate solu- 



tion and a weiglu-.l quantity of rubber is introduced. The rub- 

 < I swells gradually and after about 1? minutes the ether, to- 

 gether with evolved hydrogen sulphide, is driven over into the 

 rptio'n apparatus by gentle heat. The decomposition is com- 

 pleted by boiling the mixture a few minutes. Traces of hydro- 

 gen sulphide are removed by a current of carbon dioxide and 

 the lead sulphide is collected, washed and titrated iodometrically. 

 ESTIMATION 01 SI I I'll Ml SI LPHUR. 

 The residue in the Voigt flask, containing the sulphates is ex- 

 tracted repeatedly with hydrochloric acid and the sulphates deter- 

 mined as barium sulphate. 



ACTION OF SOLVENTS ON VULCANIZED RUBBER. 

 Ether, in presence of hydrochloric acid, gradually dissolves 

 vulcanized rubber at the ordinary temperature, and the dis- 

 solved rubber contains about 1.5 per cent, sulphur. A mixture 

 of benzene and hydrochloric acid also dissolves vulcanized rub- 

 ber. Chlorohydrocarbons act similarly to the mixture of solvents 

 and hydrocholoric acids, but are no more rapid than a mixture 

 of benzene and acids. 



REAGENT FOR RUBBER ANALYSIS. 



Douglas P. Twiss, in analytic work on rubber, finds that a 

 mixture of equal parts by volume of concentrated hydrochloric 

 acid and ether acts readily on rubber mixings at ordinary tem- 

 perature, the penetration of the acid being greatly facilitated 

 by the swelling action of the ether. The use of a similar mixture 

 has been proposed by H. P. Stevens as the basis of his method 

 for the estimation of free sulphur in rubber mixings. 



Another application for this reagent is the neutralization of 

 accelerators, such as litharge, before attempting the removal 

 of free sulphur from rapid curing mixings. Where necessary to 

 examine the content of combined sulphur in a partially cured 

 rubber mix which contains much mineral accelerator and free 

 sulphur, the conversion of the accelerator into an inert sub- 

 stance before the acetone extraction has the- advantage of re- 

 moving the likelihood of vulcanization during extraction. 



To effect such purpose the procedure used is to treat 1 to 2 

 grams of the rubber with the acid-ether reagent until this reagent 

 penetrates throughout iln mass The- pt gress of the act 

 in presence of litharge, is easily followed by the change in 

 color. The change is usually completed in a day. The rubber 

 mass can then be removed or the ether evaporated. The mass is 

 next washed in running water and dried. It is then ready for 

 acetone extraction and the combined sulphur estimated in the 

 d rubber. If mineral sulphates are absent, the sulphur 

 in the extracted rubber may be considered as organically com- 

 bined sulphur. It is safer to begin with two samples and to 

 estimate the total free and combined sulphur in one and free 

 sulphur in the other. The above process appears to be desirable 

 where there are large quantities of free sulphur and accelerator. 

 In the- opposite instance the method of Stevens is pronounced 

 perfectly satisfactory. 



PATENTED TREATMENT OF RUBBER. 



Improved Composition for RrnnER Thread. — British patent No. 

 14355 (1914), W. P. Bradley. Addition of 2 to 7 per cent, each 

 of lampblack and cercsin wax to the quantity of rubber used. 



