June 1, 1921 



THE INDIA RUBBER WORLD 



665 



In the course of the experimental work described in this paper 

 we have made a comparison of the sulphur coefficients of a type 

 mixture vulcanized with the assistance of a number of accel- 

 erators closely related to aniline and for which the dissociation 

 constants are known. We have also employed the hydrochlorides 

 of two of these substances, relatively weak and strong bases, in 

 order to observe the effect of the acid portion during the vulcan- 

 ization. The results obtained and the conclusions drawn led us 

 to employ the sulphides of ammonia as accelerators and vulcan- 

 izing agents. 



Briefly summarizing these results, it was found that with the 

 substances tested there was apparently no direct relationship 

 between their dissociation constants and their excess sulphur co- 

 efficients or physical properties after vulcanization. In a closely 

 related series, such as aniline and its methyl derivatives, the 

 substance with the largest dissociation constant was found to 

 be the most active. However, the relative activities of the mem- 

 bers of this series were not proportional to their dissociation con- 

 stants. Generally speaking, the activity of all of the substances 

 could be traced to the amino group, and depended to a large ex- 

 tent upon whether or not substitution had taken place in this group. 

 In this respect, they should probably be regarded as substituted 

 ammonias, rather than as the more complex derivatives of other 

 substances. 



One effect of the basicity of two of the substances, inethylaniline 

 and /i-toluidine, was determined with the hydrochlorides of these 

 two substances. Our results showed that with substances of 

 this type, the first effect of the base is to neutralize the retarding 

 action of the acid formed in the decomposition of the salt during 

 vulcanization. We had previously suggested this in a foot-note 

 in a former paper." We also found that when the acid liberated 

 in the decomposition of such a salt is neutralized by other sub- 

 stances in the mixture, the activity of the hydrochloride is very 

 close to that of the free base. These results are of particular 

 interest, as Van Heurn" has shown that, whereas ammonium car- 

 bonate is moderately active as an accelerator in a mixture of 

 rubber and sulphur, ammonium chloride is inert. The former 

 salt decomposes into ammonia and a weak acid, the latter into 



and those obtained years ago by Gerard' with potassium tri- and 

 pentasulphides, is taken up in greater detail in the experimental 

 part of this paper. It is equally evident, however, that if this 

 explanation is advanced in the case of ammonium polysulphide, 

 vulcanization with ammonium hydrosulphide requires that this 

 substance decompose not into ammonia and hydrogen sulphide 

 only, but with the subsequent formation of a polysulphide which 

 liberates sulphur iri the active form.' 



It has been shown by Bedford and Scott" that many of the 

 more complex substances which accelerate the vulcanization of 

 rubber react with sulphur, with the liberation of HjS and the 

 formation of thiourea derivatives. In view of our results with 

 the ammonium sulphides, the action of such thiourea derivatives 

 would depend upon their ability to enter into a subsequent reac- 

 tion with the 1I;S formed, or the sulphur present in the mixture, 

 with the formation of a polysulphide. Further, although the 

 formation of a polysulphide in this manner would, to a certain 

 extent, be dependent upon the basicity of the substance originally 

 added as the accelerator, it is obvious that the dissociation con- 

 stant of the reaction product would be a better indication of its 

 activity than the dissociation constant of the original substance. 

 In a previous paper" we have suggested that the activity of cer- 

 tain nitrogenous substances may be interpreted on the basis of a 

 change in valency of the nitrogen, with the nitrogen functioning 

 as a sulphur carrier. This suggestion was made to assist in cor-. 

 relating the nitrogen content with the activity of the substances 

 employed, although, as pointed out in the above paper, results 

 obtained by others already indicated that the sulphur is not nec- 

 essarily attached to the nitrogen. While our present results show 

 that vulcanization may be effected by polysulphide formation, 

 they do not exclude the possibility of the active nitrogen group 

 acting as a catalyst. 



The experimental results are shown in the following table : 

 Table I 



First latex pale crepe 



Sulphur 



Accelerator 



Vulcanized for 90 min. 



lOO 

 8.1 



Substance l-'ormula 



Control 



Aniline CH^NH^ 



Methvlaniline C.H^NH.CH^ 



Dimcihvlaniline r,Hr,NfCH3). 



(.-Toluidine CH.vC„H,.NHj 



m-Plienvlcncdiamine NHj CeHj.NH^d • 



^i-Phenylenediamine NH-.CflH,.NH,f 1 : 



^Benzidine NH;.C,H,.C,H.NH, 



Phenylhvdrazine C«H.N H.NH- 



Hydrazobenzene" CrtNH.NH.CeHj 



O.Ol G.Mol. 



0.93 



1.07 



1.21 



1.07 . 



1.08 



l.OS 



1.84 



1.08 



1.84 



Determined 

 M. P. or B. P. 



of 

 Accelerator^ 



at 148° C. 



Physical Properties 



183.1 

 192.0 

 192.5 

 45.0 

 62.6 

 140.6 

 126.2 

 24O.0 

 126.0 



FHsscciation 



Constant K 



at 15° to 18° C. 



3.. SO x'l0-i» 

 2.55 y lO-w 

 2.42 y in-" 

 1.60 X 10-» 

 1.35 X 10-i» 

 2.4S X 10-"'' 

 7.40 X 10-" = 

 1.60 X 10" 



Excess 



Sulphur 



Coefficient 



(2.58!) 



3.400 



0.612 



0.250 



2.987 



2.986 



5.248 



3.056 



0.751 



0.777 



Tensile Strength 



Lbs. Per Sq. In. 



at Break 



1.229 



2,005 



1,665 



1,938 



2,476 



1,933 

 193 



1,464 



1,052 



2,165 



Final 



Length 



at Break 



1,090 



910 



1.050 



1,060 



920 



830 



430 



810 



1.080 



1.140 



•All m. p. are below, and b. p. above the temperature of vulcanization. 'Figure applies lo second "K." 

 ammonia and a strong acid, according to the following reactions : 

 {NH,),CO, >-2NH, -f H,0 + CO, 



'Does not have basic properties. 



CONCLU.SIONS 



NH.Cl- 



■^NH, -f HCl 



Our final experiments, wherein we found that in a closed system 

 rubber is vulcanized by heating with ammonium polysulphide or 

 ammonium hydrosulphide, were carried on in order to obtain a 

 reaction mixture of undoubted basic character, which at the same 

 time would include H,S as one of the decomposition products. 

 The function of H,S in connection with the vulcanization of 

 rubber has long been made a subject of controversy. In the 

 present instance it may be regarded as a very weak acid. 



Our results with ammonium polysulphide may be explained as 

 due to the decomposition of this substance into ammonia, hydro- 

 gen sulphide, and sulphur, the latter substance being liberated in 

 an active (nascent) form which readily combines with the rubber. 

 The analogy between our results with ammonium polysulphide, 



•Chemical & Metallurtdcal Engineering, 20, 1919. 420. 



^Communications of the Netherland Government for Advising the Rubber 

 Trade and the Rubber Industry. Part 6. 202. 



1. The activity of synthetic nitrogenous organic substances as 

 accelerators is not proportional to the dissociation constants of 

 the original substances and, with the exception of members of 

 a closely related series, no definite relationship exists between 



H. 



'R. Hoffer, "Treatise on Caoutchouc and Gutta-Percha" 

 C. Baird & C'.., London. 1883. 



(trans. Brannt), 



*As an aqnenus solution of NH^HS was employed, the action of this 

 substance may aNo he explained by its dissociation products. It would 

 dissociate with NIT,* as the cation and IIS- the anion. As the IIS- ion 

 ■Iself is weakly acid, there would probably be many H* and HS" iona 

 and but few S" " ions in the aqueous solution. The II* and S — ions 

 in turn react to form H^S. On the other hand. (NH,)iS dissociates with 

 Nir**, the cation, antl .S- -. the anion. The latter, in the presence of 

 water, dissociates with the formation ff OH- and HS- ions. Thiw, 

 NILHS dissnciatcs with the formation of a greater number of H* ions than 

 in the case of (NIl4)jS, and ccnsenuently with a greater reformation of 

 II;S. This may arcrunt for the difrercnce in the relative activities of the 

 two substances. The same may be true in the absence of water, as most 

 organic accelerators are apparently soluble in rubber, the high dielectric 

 constant of which indicates that this substance itself may be a good dis- 

 sociating medium. 



"Journal of Industrial and Engineering Chemistry. 12. 1920, 31. 



'".Tournal of Industrial and Engineering Chemistry. 12, 1920, 317. 



