November 1, 1920 



THE INDIA RUBBER WORLD 



95 



should be educated to know thoroughly the dangers which a safe- 

 guard covers, that they inay furnish the precautions which the 

 safeguard cannot. It may be possible to safeguard a machine 

 so thoroughly that it would be impossible for a workman to in- 

 jure himself dehberately, but with the result that the workman 

 would eventually dismiss from his mind any thought of danger. 



It would be folly to place such a man on another machine which 

 cannot be so well guarded. Thorough education in safety is, 

 after all, the most important requirement, and it not only pro- 

 vides protection in his mechanical duties, but it expands his men- 

 tal capacity and alertness to provide protection for himself and 

 his fellow-men at all times, everywhere. 



The Effect of Certain Accelerators Upon the Properties of Vulcanized 



Rubber— ir 



By G. D. KraU and A. H. Flower' 



IN A RECENT p.^PER,' H. P. Stcvens has given new figures, and 

 from them made a number of deductions in regard to certain 

 discrepancies befween results obtained by the present authors* 

 and earlier results obtained by him.' We do not entirely agree that 

 these latest deductions will suffice for the complete coordination 

 of his former results with ours. This view is confirmed by the 

 repetition and amplification of our former e.Kperiments, including 

 work with extra light instead of heavy calcined magnesia. 



This work was carried out with a sample of the rubber pre- 

 viously employed and also with another rubber of similar physical 

 appearance. Entirely different results were obtained with the two 

 rubbers. In neither instance, however, was extra light magnesia 

 found to develop greater activity than Accelerator A, and, in one 

 case, it was markedly inferior to the latter. In both cases where 

 Accelerator A was employed, the load required to effect a given 

 extension led to erroneous conclusions, if used as a criterion of 

 the rate of cure. 



As these results were obtained with accelerators of definite com- 

 position and purity, the differences may be attributed to variations 

 existing in the rubbers themselves, and most probably in the na- 

 ture, amount, or condition of the extraneous materials present. 

 As a considerable portion of this extraneous matter was extract- 

 able with acetone, an investigation was made of the relative effect 

 of the two accelerators upon the two rubbers after extraction. 

 Since the nature of the substances removed by the extraction' was 

 not studied, no attempt can be made to correlate the effect of the 

 extra light magnesia with any definite one of the extraneous sub- 

 stances originally present in the rubber. Certain facts, however, 

 have been well enough established to deserve brief consideration. 



It was noted by Spence' that the nitrogen in rubber was not 

 entirely of protein origin, and that nitrogenous bodies of well- 

 defined alkaloidal character could be detected in the acetone ex- 

 tract of Para rubber. This was subsequently confirmed by Spence 

 and Kratz' for plantation crepe (Hez'ea), although a difference 

 in the character of the protein material in the two rubbers was 

 found. Further, certain of their results indicated that in planta- 

 tation Hezva the non-protein nitrogenous substance was not easily 

 extractable with acetone. Dekkar' also noted the presence of 

 nitrogen in the acetone extract, and gave figures for nitrogen 

 distribution in the extracted rubber and its acetone extract which 

 closely confirmed those originally obtained by Spence. Prior to 

 Dekkar's observations, Beadle and Stevens" noted that the rate 

 of vulcanization of certain rubbers decreased if the rubbers were 

 previously extracted with acetone. After vulcanization the phy- 

 sical properties of the acetone-extracted samples were so greatly 

 impaired, due either to the loss of the resin or the physical effect 

 of the solvent upon the rubber, that the decrease in the rate of 

 cure was considered of secondary importance. 



It would therefore appear that the removal of the acetone- 

 soluble nitrogenous constituent is responsible for the decrease 

 in the rate of cure of the rubbers, rather than either of the 

 causes originally assigned by Beadle and Stevens." This is also 

 in accordance with the later results of Eaton, Grantham and Day," 

 and of Stevens," wherein the accelerating substance of plantation 



Hcvca rubber was found to be an organic base or mixture of 

 bases, probably formed by the degradation of the protein portion 

 of the nitrogenous material originally present in the rubber." 



The possibility that magnesia may hasten this degradation, with 

 the formation of an accelerator similar to that produced liy the 

 biological decomposition of the proteins, has already been pointed 

 out by Eaton" in commenting upon the patent of Esch." 



In view of the well-known action of many synthetic organic 

 accelerators in the presence of certain mineral oxides, such as 

 that obtained by Cranor" with zins oxide, we are led to the con- 

 clusion that the effect of small amounts of magnesia in accelerat- 

 ing the vulcanization of rubber is of a secondary or contributory, 

 rather than a primary nature, and consists largely in effecting a 

 response from the natural accelerator in the rubber. This finds 

 further confirmation in the observation of Stevens in his previous 

 paper, wherein he pointed out that the accelerating effect of 

 extra-light magnesia decreases when a sulphur coefficient of 

 2.0 to 2.5 has been attained. At this point, increased amounts of 

 magnesia would have no effect, as they would be in excess of the 

 amount required by the natural accelerator, which is present in 

 the same definite and limited amount in all of the mixtures." 



The same would not be true for Accelerator A, which is 

 regarded as a primary accelerator and is present in the various 

 mixtures in increasing amounts up to 1 per cent of the rubber. 



In conclusion, we wish to draw a distinction between the terms 

 "effect of accelerators" and "action of accelerators." This paper 

 deals primarily with the effect produced by certain accelerators 

 upon the sulphur coefficient and the physical properties of the 



[ tli« 

 439. 



' Published by courtesy of the American Chemical Society. Paper read 

 before the Rubber Division of the Society at St. Louis, Missouri, April 1216 

 1920: 



- The Falls Rubber Co., Cuyahoga Falls, Ohio. 



» The India-Rubber Journal, 58 (1919). 52?. 



* Oieinical and Metallurgical Engineering, 20 (1919). 417. 



^Journal of the Society of Chemical Industry, Z7 (1918), 156t. 



° .\ qualitative determination showed llie presence of nitrogen 

 extracts of both rubbers. 



^ Herbert Wright, "Hcvea Brasiliensis, or Para Rubber," 1912, 

 London. 



s KolloidZeitschrift. 14 (1914), 268. 



8 Communications of the Netberland Government Institute for Advising 

 the Rubber Trade nnd the Rubber Industry, Part II, p. 55. 



'"International Congress of Applied Chemistry, 25 (1912). 581. 



'1 In a previous paper [The Journal of Engineering and Industrial Chem- 

 istry, 12 (1920), 317], we liave mentioned that results obtained with certain 

 syntlietic organic substances indicate, in some cases, that the accelerator 

 may be clnsely l)ound to tlie rubber. Should this also be found true in the 

 case of the natural accelerator, the removal of this substance by extraction 

 would markedly impair the pliysical properties of the .sample after vulcaniza- 

 tion, as well as slow down the rate of cure. (Compare with footnote 25.) 



" "Vari.ibility in Plantation Rubber," Journal of the Society of Chemical 

 Industry, 35 (1916). 715. 



"Journal of the Society of Chemical Industry, 36 (1917), 365. 



'* The protein portion of this nitrogenous material which is in.soIublc in 

 rcctone and benzene has been shown to act as an accelerator (Beadle and 

 Stevens. Kolloid-Zeitschrift. 11 (1912), 61; 12 (1913). 46: 14 (1914). 91). 

 It has the further advantage of being present in relatively large amount as 

 compared with the acetone-soluble constituent. As it does not, however, 

 respond to magnesia to the same extent as the latter substance, and, as 

 certain results (not included in this paper) indicate that the extraction with 

 acetone docs not cause a marked degradation of this protein material into 

 the soluble varietv, we have not made reference to it. 



"'Agricultural ilullclin Federated Malay States, 5 (1915), 38. 



"German Patent 273,482 (November 22, 1912). 



"The India Rubber World. 61 (1919), 137. 



*^ It is interesting to note that Dekkar's figures for the nitrogen in the 

 2cetone extract of TIevea crene 'when calculated on a protein basis) are 

 very close to the amounts of the accelerators cmploj'ed in these experiments. 



