174 



THE INDIA RUBBER WORLD 



December 1, WM 



THEORY OF ACCELERATION IN VULCANIZATION 



THH well known Freiicli rubber chemist, Andre Dubosc, re- 

 cently published' his very illuminating theory on acceleration 

 of vulcanization of rubber in the case of such organic accelerators 

 as piperidinc, para-nitroso-dimcthyl aniline, hexamethylene 

 tetramirte and thio-carbanilide. Incidentally he regards Spence's 

 discovery and introduction of organic vulcanization accelerators 

 as marking as important an epoch in the development of the rub- 

 ber industry as the discovery of vulcanization by Goodyear. 



The author's theory, for which he does not claim perfection, 

 clearly accounts for the hitherto unexplained singular and ener- 

 getic action of the accelerators mentioned. The leading features 

 of the author's paper are given in the following abstract : 



REACTION OF AMINES WITH SVLPHUR 

 If a known accelerator, hexamethylene tetramine for example, 

 is mixed with sulphur, placed in a sealed tube, and heated for 

 a few minutes at 135 to 145 degrees C, it is found that besides 

 sulphide of carbon or hydrosulphuric acid, sulfocyanic acid is 

 generated. All organic accelerators derived from amines inevit- 

 ably give this same reaction. 



PRODUCTION OF HEXAVALENT SULPHUR 



At the vulcanization temperature, sulfocyanic acid separates, 

 yielding hexavalent sulphur and cyanhydric acid. The latter 

 body, kept at vulcanization temperature, combines in the presence 

 of ordinary divalent sulphur, producing unstable sulfocyanic acid, 

 which by dissociation again furnishes hexavalent sulphur. If the 

 temperature is kept constant, the reaction continues while free- 

 divalent sulphur remains. 



This is a catalysis where the cyanhydric acid acts as catalyzer. 

 Its practical effect is to transform ordiiiary divalent sulphur into 

 hexavalent sulphur, the action of which during vulcanization 

 is entirely different. This variation of valence in an elementary 

 body is common. 



ACTION OF HEXAVALENT SULPHUR 



Hexavalent sulphur, freed at the time of dissociation ot sul- 

 focyanic acid, is susceptible in vulcanization of saturating 

 three free double bonds belonging to rubber with which it is 

 in contact. These double Ixmds may belong either to two dif- 

 ferent molecules of rubber or to three. A single molecule of rub- 

 ber reacting with ordinary, divalent sulphur will saturate only 

 one double bond. 



SPEED OF VULCANIZATION 



If vulcanization is considered as the saturation, by sulphur, of 

 a certain number of free double bonds in the rubber molecule 

 and three such bonds, instead of one, are saturated in a given 

 time, the speed of the reaction will evidently be tripled. 



It is found, experimentally, with most of the accelerators con- 

 taining an active amino group, that the lime needed for vul- 

 canization is reduced in the proportion of three to one. This 

 is the case with piperidinc and thio-carbanilide. If the accelerator 

 contains several active amino groups the reduction of time for 

 vulcanization may be considerably further reduced. This is the 

 case with hexamethylene tetramine or furfuramide. 



The hypothesis explains the modification of the speed ot vul- 

 canization by the liberation of hexavalent sulphur by dissociation, 

 resulting from the products of decomposition of the initial ac- 

 celerator and the sulphur of an intermediary body (sulpho- 

 cyanic acid). This is a case of catalysis by stages: first, there 

 is decomposition of the initial accelerator producing a catalyzer, 

 cyanhydric acid ; second, the sulphur of the mixing or the hydro- 

 sulphuric acid combines, generating sulfocyanic acid, which, 

 third, dissociates furnishing hexavalent sulphur and regenerating 

 the catalyzer, cyanhydric acid. Thio-sulfocarbanic acid, which, 

 according to Kratz' and Bedford,' is present in all reactions due 

 to accelerators, also leads to the formation of sulfocyanic acid 

 and the liberation of hexavalent sulphur under the influence of 



heat, with saluratu n of three double bonds of the rubber in 

 reaction, and regeneration of catalytic cyanhydric acid. In fact, 

 this latter would seem to be the true accelerator and to cor- 

 respond with Spence's "active principle." 



If it be admitted, as Kratz has observed, that the cyclic nucleus 

 of accelerators is broken during vulcanization, it easily explains 

 the formation of sulfocyanic acid. This hypothesis also ex- 

 plains why, in rubbers vulcanized with the aid of accelerators, 

 there is never found a trace of these bodies, either in the 

 aqueous, or in the acetone extract. They are not true accelerators 

 but under suitable conditions of temperature they are capable of 

 generating, by decomposition, a catalyzer which will react on 

 the sulphur. This catalyzer, cyanhydric acid, which remains 

 in the mass after vulcanization, in the form of sulfocyanic acid, 

 or rather, of sulphocyahatcs, insoluble in consequence of combi- 

 nations with the charges, can be determined by Chelle's method 

 lor cyanhydric acid, which is used in toxicological analysis. 



INCREASE IN TENSILE STRENGTH 



Rubber vulcanized in the presence of accelerators shows marked 

 increase of breaking strength over the same rubber vulcanized 

 in the ordinary manner. This change seems to indicate that rub- 

 ber vulcanized in the presence of accelerators is more highly 

 polymerized than that treated without accelerators. These facts 

 of practical experience have, up to the present time, remained un- 

 explained. Our theory affords a means of explanation. In fact, 

 if we admit the formation of hexavalent sulphur during vul- 

 canization, it is easy to see the cause of the change of polymeriza- 

 tion which occurs. 



Reverting to Weber's theory of vulcanization', we know that 

 tlie different molecules of rubber united to each other, form 

 physical aggregates, a kind of open chain preserving at each 

 end a double bond which will saturate an atom of divalent sul- 

 phur in ordinary soft vulcanization. In this case polymerization 

 is limited to a single aggregate containing a smaller or larger 

 number of molecules. 



Considering the action of accelerators, let u? admit that they 

 generate hexavalent sulphur. .\s already explained, this body 

 may react on free valences of two or three molecules of rubber. 

 In explanation of this, our theory provides that on the same atom 

 of hexavalent sulphur polymerization will take place as follows : 



(1) In the case of an aggregate of rirbber molecules, the end 

 molecules of which have a double bond, these will be broken and 

 give a molecule of rubber of which the four valences will be 

 saturated. The aggregate will have its polymerization increased 

 by one molecule and its resistance to break will be modified, in a 

 slight degree only. 



(2) In the case of vulcanization by hexavalent sulphui, satu- 

 ration of the terminal free valences of three physical aggregates 

 of rubber will take place. Polymerization will therefore be 

 three times as great as that produced by ordinary vulcanizations, 

 because it acts on three aggregates instead of one. Resistance to 

 break, dependent on polymerization, will therefore be very notably 

 iiicrca.sed and theoretically ought to be tripled, and this has been 

 demonstrated experimentally. 



THE CASE OF NITROSO ACCELERATORS 



At lirst sight the theory seems to apply only to accelerators of 

 the amino (NH.) or imino (NH) groups and to leave out the 

 nitroso compounds discovered by Peachey. but such is not the 

 case. The nitroso bodies decompose during vulcanization and 

 generate cyanic acid. The latter, under the influence of sulphur, 

 yield sulphurous anhydride and sulfocyanic acid. The sulfocyanic 



M.c Caoutchouc et la flulta Pcrch.i. Scptimhtr 15. 1920. 



""The Effect of Certain Accelerators Upon the Properties of Vulcanized 

 lUibher," The Inbia Rubber World, June 1, 1919, page 485; November 

 1, 1920. page 95. 



'"Reactions of Accelerators During Vulcanization," The Isdh Rubber 



World, January 1, 1920. page 206. 

 *"The Clicmistry of India Rubber,' 



1906 edition. 



