78 



THE INDIA RUBBER WORLD 



[November 1, 1918. 



Applications of Catalysis to Vulcanization. 



By AndrS Dubosc. 



CAOUTCHOUCS contain besides pure gum a certain number 

 of impurities of which the characteristics are: 



1. Saponifiable or unsaponifiable resins with or without 

 rotatory power. 



2. Nitrogenous substances in which, besides the proteins iso- 

 lated by Spence, we find enzymes the o.xidizing action of which 

 on the gum, in the presence of air, causes tackiness and destruc- 

 tion. 



These substances, for a long time, have been considered in- 

 jurious and attempts have been made, by washing and other- 

 wise to eliminate them as completely as possible. 



The production of chemically pure synthetic rubbers from 

 isoprene revealed the impossibility of properly vulcanizing them 

 and gave rise to the thought that the proteins and the resins 

 which were lacking were essential in effecting vulcanization. 



The same negative results were obtained with plantation gums 

 from which, either during coagulation or by intensive washing, 

 all foreign matters had been removed. 



It therefore seemed quite important to allow a certain quan- 

 tity of resins and proteins to remain in the gum so as to free 

 the gum from destructive enzymes. 



Following Weber's researches the importance of the proteins 

 in rubber was suspected but their action was considered more 

 physical than chemical, something like the action that Selig- 

 mann attributed to the elastic matter of caoutchouc. 



On the basis of microscopic analysis, in which the proteins, 

 dyed black by silver salts, are easily visible. Biffin maintained 

 that they play an important part in coagulation, forming a 

 network surrounding globules of pure gum. 



Franck, after numerous experiments on Hcvea latex, shared 

 this opinion. 



Other scientists, however, have shown that the protems are 

 not indispensable in obtaining a good coagulum from the latex, 

 long and Tromp de Haas, for instance, easily obtained the 

 coagulation of the latex of CastilUoa after removing the pro- 

 teins by alcohol. . 



Spence and Crossly obtained perfect coagulation with latex 

 considerably diluted with water, in which the proteins could not 

 coagulate at the same time as the caoutchouc. 



Victor Henry Zimmermann, after microscopic 'n- 



cluded that the coagulation of the gum can take place when the 

 proteins are not present. 



Barrett declared that in the coagulation with acetic acid, as 

 practiced in plantations, there are two phases, one rich and 

 the other poor in proteins, and that in both phases coagulation 

 goes on equally well. 



All these studies dealt only with the physical part played by 



proteins and neglected their chemical effect, which is much more 



important. . v t i 



The first observations along these lines were made by Lock 



and Bamber. 



These scientists, studying the physical constants of purified 

 plantation gums after vulcanization, found that the resistance 

 to breaking of these caoutchoucs was in inverse ratio to their 

 pure gum contents. 



The importance of resins and proteins during the vulcaniza- 

 tion was then indicated for the first time. 



The part they play has been still more clearly described by 

 Beadle and Stevens, and Lothar Weber, at the Congress of Ap- 

 plied Chemistry at New York, in 1912. 

 From their studies it results: 

 1. That a caoutchouc, completely free from resin after pro- 



longed extraction by alcohol, does not vulcanize and that a 

 minimum of three per cent of resin is necessary for vulcaniza- 

 tion to take place. 



2. That a caoutchouc, after losing its proteins, vulcanizes 

 very poorly and very slowly ; that its breaking strength, com- 

 pared with an ordinary piece of caoutchouc, is reduced from 

 five to one ; and that the coefficient of vulcanization drops from 

 3.62 to 1.15. 



Beadle and Stevens also established other facts of importance, 

 viz. : that if one augments artificially the protein contents or 

 those of nitrogenous matter in a mass of caoutchouc, vulcaniza- 

 tion takes place more rapidly, the physical constants, partic- 

 ularly the breaking strength, are increased, and the coefficient 

 of vulcanization becomes higher. 



These valuable observations showed conclusively the chemical 

 importance of resins and proteins in vulcanization, as acceler- 

 ators and vitalizers. Tlieir action is probably catalytic, for, 

 although they clearly stated their results, neither Beadle and 

 Stevens, nor Lothar Weber furnished any explanations. 



Later work, done by Eaton and Grantham and published by 

 them last July, has not only confirmed, but has shown the in- 

 creasing importance of the part played by the proteins and by 

 the resins in the vulcanizing process. 



Eaton and Grantham have shown that, under the action of 

 anaerobic bacteria, the proteins of caoutchouc are transformed 

 into a substance which they have not isolated, but which, dur- 

 ing vulcanization, diminishes the curing time, acting as an ac- 

 celerator, and improves the physical constants, acting as a vital- 

 izer. In the presence of antiseptics such as formaline, the 

 bacterial transformation does not take place and acceleration 

 does not occur, although there is a slight increase of the physical 

 constants, which we shall explain later on. The same negative 

 result is found if the caoutchouc is sterilized by steam or by 

 freezing, which is easily explained by the fact that the microbic 

 action on the protein is prevented. When acting similarly on 

 other proteins, such as casein or the peptones, and the proteins 

 precipitated from the serum, results are obtained which, though 

 not identical, resemble acceleration and vitalization. 



But it must be remarked that the results obtained are not 

 dependent on the amount of nitrogen in the caoutchouc, and 

 that a sample containing niuch nitrogen often behaves much 

 worse than another containing considerably less. This is easily 

 explained if we note that the nitrogen can react only under 

 certain conditions determined by the microbic action. 



This can all be understood very clearly by referring to the 

 writings of Effront on the action of the proteins and of the 

 diaminated acids under the influence of bacteria or rather of 

 enzymes, which writings were probably unknown to Eaton and 

 Grantham. 



The principle announced by Effront is that if one causes the 

 reaction, under proper conditions, of an enzyme or a diastase 

 on a protein, it is divided into fat acids on the one hand and 

 into amines or ammonium compounds on the other. Remem- 

 bering the studies of Spence, we know that the protein of caout- 

 chouc exists as a conjugated protein, probably a glycoprotein, 

 which by hydrolysis, changes into glycosamine. It is almost 

 certain that it is this substance which originates in the reactions 

 pointtd out by Eaton and Grantham, and that it reacts as an 

 accelerator and a vitalizer. 



These reactions are therefore due to an amine or to an amin- 

 ated body, a valuable observation which allows us to understand 

 the chemistrv of the reactions to which we have referred. 



