November 1. 1918.] 



THE INDIA RUBBER WORLD 



79 



These facts having been stated and the action of the resins 

 and the proteins, in the vulcanization, having been established, 

 let us examine successively the reactions which are produced 

 •when we cause sulphur, at the temperature of vulcanization (135 

 degress C), to react on these two substances. 



The resins of caoutchouc are present as ethers of cholesterol, 

 acetates of alpha and beta amyrine, acetate of lupeol, that is 

 to say as ethers resulting from the action of a fat acid, acetic 

 acid, on a secondary alcohol, the cholesterol. Under the in- 

 fluence of heat these ethers tend to decompose and the molecule 

 of cholesterol, very complex, tends to split, giving rise to less 

 complex products. This change takes place, as we can verify 

 in the heating of the resins of jelutong, with an abundant lib- 

 eration of hydrogen and acetylene. 



At the temperature of vulcanization, this decomposition is 

 only partial, but the quantity of hydrogen produced is sufficient, 

 in the presence of sulphur, at 135 degrees C, to form consid- 

 erable sulphydric acid. This reaction was noticed by one of 

 the discoverers of vulcanization, Hancock, and confirmed by 

 Payen, who mentioned it carefully in his studies on caoutchouc. 



If, therefore, we study the action of resins on sulphur in 

 glass, at the temperatures of vulcanization, we find that the 

 result is the formation of sulphydric acid in abundant quantity. 

 We shall see later what part this sulphydric acid can play in 

 vulcanization. 



The proteins of caoutchouc, isolated for the first time by 

 Spence, appear as conjugated proteins, muscines or glucopro- 

 teins, which have been studied by Schmiedberg. They are 

 formed by the union of a carbohydrate with an amine after the 

 fashion of the amino acids, and by hydrolysis, under the action 

 of the enzymes which always accompany it, they can be hydro- 

 lized, producing glucosamines : 



H H OH 



I I I 

 CH, OH — C — C — C — CHO — CH — NH. 



I I I 

 OH OH H 



If we prepare the protein of caoutchouc by the method of 

 Spence and Kratz by dissolving crude gum in benzine in the 

 presence of trichloracetic acid, and then submit the product 

 thus obtained to enzymic action, either with the enzymes of 

 caoutchouc, or with the amylase of Effront, we obtain a sub- 

 stance which behaves, in vulcanization, as the fermented proteins 

 of Eaton and Grantham do and which, when analyzed, presents 

 all the characteristics of glucosamine. 



It combines with the isocyanate of phenyl, giving a substance 

 which melts at 211 degrees C. With the phenylhydrazine, it gives 

 a glucosazone, and with hydroxylamine a glucosamine oxime 

 melting at 122 degrees C. Bromine water oxidizes it into gluco- 

 samic acid, and nitric acid into isosaccharic acid. 



If we take this substance, which is only the product of the 

 transformation of the protein under enzymic action in the 

 caoutchouc, and heat it with sulphur, to the temperature of 

 vulcanization, 135 degrees C, we shall find that it gives rise to : 



1. Sulphydric acid; 



2. Sulphocyanic acid. 



The equation can be written as follows : 

 CH,0H[CH0H]3 CH(NH,) CHO + 2S^CNHS + H,S. 



Sulphydric acid is easily identified by acetate of lead and 

 nitroprussiate of sodium, sulphocyanhydric acid by perchloride 

 of iron and the protosalts of copper, which give an insoluble 

 precipitate of sulphocyanate of copper. 



Thus the action of resins and proteins on sulphur, at the 

 temperature of vulcanization, forms sulphydric acid and sulpho- 

 cyanic acid. 



The formation of this last substance need not astonish us, for 

 it always is formed when sulphur, nitrogenous products and 



derivatives of coal are heated together, as in the destructive 

 distillation of coal which contains nitrogen, sulphur and carbon 

 It IS produced also to a considerable extent in the wash waters 

 and in the materials used in purifying illuminating gas, not un- 

 like sulphydric acid. 



How can these two substances, which are surely formed by 

 the action of sulphur at 135 degrees C. on a caoutchouc con- 

 taining a normal quantity of resins and of proteins, produce 

 rapid vulcanization of the gum, increase its resistance to rup- 

 ture and its polymerization? 

 Let us examine successively the cases of these two acids. 

 Harries has recently shown, bringing the theories of Weber 

 and of Oswald into agreement, that vulcanization takes place 

 m two phases: the first one, a phase of adsorption, in which 

 the sulphur is extractible by acetone, but in which the caout- 

 chouc passes from the metastable form to the stable form 

 characterized by the insolubility of its chlorhydrate in chloro- 

 form ; then, a second phase, the chemical one, in which the ad- 

 sorbed sulphur combines with the gum, giving a sulphide of 

 polyprene and becoming unextractible by acetone. 



Remembering the principles of colloidal chemistry, sulphur to 

 be adsorbed by a colloid, such as caoutchouc, must itself be 

 in a colloidal state, S=, but it is introduced into the mixture to 

 be vulcanized m a polymeric state, S^; therefore, our first task 

 must be to break up its aggregate and bring it to a colloidal 

 state. 



Under the influence of heat, this transformation takes place 

 but slowly and in proportion to the means to be acted on That 

 IS why we can reduce the time necessary for vulcanization by 

 increasing the quantity of sulphur used. 



Sulphydric acid, produced by the reactions of the resins and 

 the proteins on the sulphur enables us to obtain colloidal sulphur 

 much more easily. 



If we heat sulphur in the presence of air, or of a substance 

 containing oxygen, such as an oxidized caoutchouc or a metallic 

 ox.de, we produce sulphurous acid and the sulphide correspond- 

 mg to the oxide used; it is the reaction which is constantly 

 used in vulcanization with litharge or with oxide of magnesia, 

 both of which are excellent accelerators. 

 The reactions are produced under the following equations 

 I6PbO 4- 3S» = 16PbS + 8S0,. 

 16MgO + 3Ss = 16MgS + 8S0,. 

 The reaction is produced more or less rapidly according to 

 whether the thermic equation, corresponding to the chemical 

 equation, is positive or negative. 



With litharge the equation is exothermic and produces -f202 

 calories, the reaction is very rapid and the acceleration is highly 

 accentuated. 



With magnesia, the equation is endothermic and requires the 

 addition of 328 calories, but this fault is compensated for by the 

 considerable quantity of occluded air which light magnesia con- 

 tains. It is to this occlusion that this substance owes its ac- 

 celerating power. 



With oxide of zinc the reaction is clearly endothermic, and 

 a^ this substance contains little or no occluded air, it remains 

 a simple charge and has no accelerating action. 



The same is the case with most of the metallic oxides whose 

 thermic equation is negative, in the case of formation of sul- 

 phurous acid. 



In presence of sulphydric acid, tlie sulphurous acid thus pro- 

 duced reacts in the production of colloidal sulphur and of water. 

 SO, -f 2H,S = 2H=0 + 3S. 

 It is the classical reaction which is applied constantly in the 

 purification of illuminating gas. 



The formation of colloidal sulphur adsorbable by caoutchouc 

 is thus explained, its rapidity of formation being the function 

 of the rapidity of formation of sulphurous acid. 

 On the other hand, we must not forget that sulphydric acid 



