December 1, 1919.] 



THE INDIA RUBBER WORLD 



137 



The Effect of Organic Acceleratorson the Vulcanization Coefficient! 



By D. F. Cranor. 



A brief discussion of the already published work on the relationship of the mechanical properties of VMlcanized rubber to 

 the chemical composition, and a description of experiments with certain powerful accelerators which demonstrates tlutt under 

 certain conditions good mechanical properties can be obtained in rubber with a very low degree of chcntical combination of 

 sulphur. 



DURING THE PAST FEW YEARS Considerable material has been 

 published regarding the relationship of the mechanical 

 properties to the chemical composition of vulcanized rub- 

 ber. It has been demonstrated that if vulcanization is carried 

 to a point where maximum physical properties are obtained in 

 the freshly cured goods, such goods are overvulcanized and sub- 

 ject to rapid deterioration; also, that the proper degree of 

 vulcanization for soft rubber can be accomplished with less 

 chemical combination of sulphur in the case of quick curing 

 stocks, than with compounds which require prolonged heating 

 to effect vulcanization. It is apparently quite generally believed 

 that the coefficient of vulcanization for a properly cured pure 

 gum mixing of good quality plantation Hcvea rubber lies in the 

 neighborhood of three, although it has been impossible to de- 

 termine a coefficient which definitely represents the so-called 

 optimum cure. 



The fact that almost all practical rubber formulas to-day con- 

 tain some one of the many organic accelerators makes further 

 research necessary if scientific methods are to be applied to the 

 estimation of the degree of cure of the mixings containing 

 these substances. I have, therefore, undertaken extended work 

 along these lines with view to making my testing methods more 

 accurate and satisfactory. It has been the thought of the writer 

 that results of greatest significance would be such as would indi- 

 cate the amount of sulphur combination required to effect 

 proper cure in a given sample of crude rubber, as compared with 

 the combination necessary when the same rubber has added to 

 it organic accelerators of various degrees of efficiency. Accord- 

 ingly I have determined the physical properties and vulcanization 

 coefficients of various samples of crude rubbers, when cured with 

 and without accelerators, and have chosen certain series cover- 

 ing some of the results obtained in connection with powerful 

 accelerators. 



The determinations here described were made upon a mixing 

 of standard ribbed smoked sheet and a sample of inferior brown 

 plantation crepe. In both cases 100 parts of rubber were mixed 

 with 6 pans of sulphur and 1 of zinc oxide. The latter was 

 included in order to have the compound workable to best 

 advantage with accelerators which exert a maximum effect only 

 when used in the presence of zinc oxide. The sulphur was set 

 at 6 per cent on the weight of rubber, it being desired to use 

 enough to get a stock capable of being overcured before show- 

 ing the phenomena of reversion, which has been mentioned by 

 Schidrowitz, and at the same time one in which the amount of 

 sulphur is sufficiently low to obviate acceleration due to an 

 excess, thus allowing the effect of the catalysts to be most 

 clearly demonstrated. The percentage of zinc oxide is held at a 

 minimum, it being used only to activate the accelerators added 

 to these mixings. Even such an amount exerts a slight retard- 

 ing action on the combination of sulphur, this having been 

 proved by Van Heurn (See The India Rubber World, page 251, 

 February 1, 1919), and also by personal experience in our lab- 

 oratory. This substance is also a factor in fixing the physical 

 properties of the product, but it is necessary to include it in 

 the basic formula in order to preserve the similiarity of these 

 mixtures with the treated compounds. 



It will be noted by referring to Table I that the smoked sheet 

 was vulcanized through a wide range of time changes at a vul- 

 canizing temperature of 292 degrees F., or approximately forty- 

 five pounds' steam pressure. All test specimens were cut by a 

 die from sheets approximately 0.100-inch thick, which were vul- 

 canized in an ordinary platen press, the temperature of which 

 was controlled by a reducing valve. The tensile strength, ultimate 

 elongation and loads required to produce definite elongation 

 were determined by a Scott vertical combination rubber-and- 

 fabric testing machine, the pulling jaw of which travels at a 

 uniform rate of 20 inches per minute. The tensile product was 

 calculated according to the method of Stevens. "Set" was 

 determined in the case of the results recorded in Table I by 

 stretching the specimen from one to eight inches and holding 

 at such elongation for a period of ten minutes. The measure- 

 ments were made ten minutes after releasing the samples from 

 the jaws of the stretching device. 



In the following tables the samples used for the "set" test 

 were also stretched to eight inches, except in series where the 

 figures for ultimate elongation are less than one to nine, in 

 which case the samples for "set" were stretched from one to 

 seven inches. As a matter of fact, practically no value is to be 

 attached to the "set" results, as in many cases the figures are iden- 

 tical for samples having otherwise widely vary characteristics. 

 They are inserted only to make the results complete, since the 

 test is one which seems to be much used in general testing 

 practice. The coefficient of vulcanization has in every instance 

 been calculated from a direct determination of combined sulphur, 

 made upon a sample extracted in acetone for a period of 24 

 hours at the boiling temperature of the solvent. The method 

 used is the nitrate acid-bromine oxidation followed by fusion with 

 potassium nitrate in presence of sodium carbonale. Two deter- 

 minations were made upon each sample, and in almost every 

 case the results check to wilhin 0.10 per cent. 



Chemical Society 



220 

 235 



It is seen with the smoked sheet that the series commences 

 with a "heat" quite insufficient for proper vulcanization, and 

 continues through 15 minutes' time changes to cover the entire 

 range, including under, good and overvulcanization, and finally 

 in the case of the last two members of the series, reversion and 

 degradation due to overheating after the free sulphur content 

 has been reduced to less than 1 per cent. This is especially 



