August 1, 1920.] 



THE INDIA RUBBER WORLD 



731 



better class examples. As the rate of vulcanization of rubber- 

 sulphur mixings containing less than, roughly, 10 per cent of free 

 sulphur is directly proportional to the percentage of free sulphur, 

 the effect of such additional sniphur-containing ingredients on the 

 rate of vulcanization is obvious, and today is commonly realized. 

 For experimental work on the relative rates of vulcanization of 

 different rubbers or on the effect of various catalysts, the selec- 

 tion of a mixture of rubber and sulphur containing as much as 

 10 per cent of the latter is advisable, not only because this quan- 

 tity is more than sufficient to permit the progress of the change to 

 well beyond the characteristic maximum strength, but also be- 

 cause it is sufficiently high to reduce in extent any disturbance 

 ansing from slight inaccuracies in the proportion of sulphur 

 present. The possibility must always be borne in mind, however, 

 that the activity of an accelerator may possibly be influenced by 

 the proportion of free sulphur simultaneously present. 



USE OF A CATALYST. 



.iMmost all basic substances can act as catalysts to the vul- 

 canization process, for example, magnesium oxide, lead oxide, 

 lime, sodium hydroxide, potassium hydroxide, and also substances 

 such as sodium amide, potassium glyceroxide and sodium 

 pheno.xide, which, by interaction with the traces of water in- 

 variably present in rubber, are able to give rise to alkalis or 

 bases ; tj-pical examples of organic accelerators are piperdine, 

 quinine (the crude alkaloid mixture known as "quinoidine" is 

 commonly used) aniline, naphthylamine, /)-phenylenediamine, 

 hexamethylenetetramine. anhydroformaldehydeaniline, benzylidi- 

 neaniline, aldehyde-ammonia, and simple carbon bisulphide or 

 carbon dioxide derivatives of the amines, such as dimethylam- 

 monium dimethylcarbamate, the corresponding dimethyldithio- 

 carbamate and thiocarbanilide. Compounds containing a nitroso- 

 group substituted into an aromatic cyclic nucleus are also effective 

 catalysts, the best known example being /)-nitrosodimethylaniline. 

 Clearly there is a wide range of possible accelerators of these 

 various types.^ In this connection there must also be mentioned 

 the possibility of forming catalysts in rubber during its produc- 

 tion. If the wet rubber clot, freshly coagulated from the latex, is 

 kept for several days before being rolled and washed, partial 

 decomposition of the nitrogenous constituents of the retained 

 serum sets in with the formation of organic bases. These are 

 not eliminated by the subsequent rolling and washing, so that the 

 resulting rubber exhibits exceptionally rapid vulcanization. 



As is illustrated clearly by our results in Fig. 1, the curve rep- 

 resenting the rate of combination of rubber and sulphur does not 

 follow the course expected from a simple chenlical reaction, but, 

 with less than 10 per cent, of free sulphur, is approximately rec- 

 tilinear until the almost complete exhaustion of the sulphur." 

 This is probably to be explained by the occurrence of auto- 

 catalysis ; with mixtures of rubber and sulphur containing more 

 than 10 per cent of the latter, the progress of the fixation of 

 sulphur follows the sinuous S course, which is commonly re- 

 garded as characteristic of an autocatalytic process.' In the 

 presence of an artificial catalyst, therefore, the compensation 

 relation between the effect of the disappearance of sulphur and 

 the extent of the increasing catalytic effect may be disturbed, so 

 that the fixation of sulphur no longer follows a rectilinear 

 course.' 



Although the results as to the rate of vulcanization of a simple 

 sulphur mixing, as decided by these three methods, are com- 

 parable, in the presence of an extraneous catalyst, this is not 

 necessarily so. The chemical action of sulphur on the rubber 

 induces the physical alterations which constitute the advantage to 

 be gained by vulcanization, but the chemical and physical proc- 

 esses are not necessarily strictly proportionate, and some "ac- 

 celerators" influence one more than the other. In the presence of 

 certain accelerators the physical or mechanical alteration is dis- 

 proportionately rapid, and the tensile strength attains its maxi- 



mum" at a coefficient of vulcanization (combined sulphur )< 100 

 -^ rubber) well below the normal value of 5 (see table below). 



Other accelerators, on the other hand, reduce the sharpness of 

 the optimum, so that the peak of the curve is less pronounced. 

 In yet other cases the catalyst may give rise to a vulcanized 

 rubber with an abnormal extensibility relative to its coefficient 

 of vulcanization. Most of them, but not all, by reducing the 

 time of heating necessary, cause the production of a rubber with 

 a higher tensile strength than would be obtained by more tardy 

 \ulcanization at the same temperature without the catalyst, and 

 in this direction reduction of the time of vulcanization by using an 

 increased percentage of sulphur can have a similar effect." 



The effectiveness of one of the above-named organic catalysts, 

 namely, aldehyde-ammonia, is demonstrated with a mixture of 

 pale crepe and sulphur (90: 10), at various temperatures from 

 148 degrees C. (51 pounds' steam pressure) downwards, in an oil 

 bath vulcanizer. Even at a concentration of % per cent the 

 effect is clearly observable while, with 1 per cent, vulcanization 

 occurs so readily as to be possible in a reasonable period at 108 

 degrees C. (or less than 5 pounds' steam pressure) ; the progress 

 of vulcanization has been recorded at 98 degrees C. It will be seen 

 that the temperature coefficient calculated by the ratio of the 

 speed of reaction at intervals of 10 degrees C. from 108 degrees 

 to 148 degrees is practically the same as for the reaction in the 

 absence of an artificial catalyst; the average value for the ac- 

 celerated mixings being 2.4. This observation militates against 

 the belief of some investigators in this field that vulcanization 

 catalysts are not themselves able to expedite vulcanization, but 

 that during the early stages of the process they combine with 

 sulphur, giving rise to substances which possess the desired ac- 

 tivity. This view may be correct in certain cases, but evidently 

 cannot be accepted generally for all vulcanization catalysts. The 

 average values of the coefficient with the various proportions of 

 accelerator are given in the table : 



Method of tes 

 (a) Combina 



AVERAGE TEMPERATtlRE COEFFICIENT. 



1% H% !4% H% 



(118°- UlS"- (.11!!"- (128"- 



14S° C.) 148° C.) 148° C.) 168° C.) 



2.5 



2.5 



It is a striking fact that although the individual values of the 

 temperature coefficient between 108 degrees and 148 degrees C. 

 without exception oscillate closely about the mean value of 2.4, 

 the interval 98 degrees — 108 degrees C. shows a much greater 

 value exceeding 5.0. This is doubtless due to the melting of the 

 sulphur between these two latter temperatures, the normal melting 

 point being lowered under the obtaining conditions. This ob- 

 servation supplies a confirmation of the argument as to the com- 

 parable effectiveness of the various allotropic forms. 



The fact observed above, that the temperature coefficient pos- 



817; 1912, 81, 785. 



'Skellon, Rubber Industry, 1914, 172; van Iterson, "Communications of 

 ilie Netherland Government Institute for Advising the Rubber Trade and 

 the Rubber Industry," 1916, 7, 247. 



^van Iterson, he. cit. 



'This does not refer to the so-called "technical optimum" of vulcaniza- 

 tion for which the corresponding coefficient of vu!caniz.alion in the pres- 

 ence of a catalyst may fall as low as 2 or 1. (Kratz and Flower, "Journal 

 of Industrial and Engineering Chemistry," 1919. 11, 30.) The technical 

 optimum cure is probably capable of less definite measurement than the 

 maximum tensile strength in the type of mixing used above. 



"van Rossem, loc. cit., p. 210. 



