November 1, 1920 



THE INDIA RUBBER WORLD 



97 



the results of physical tests are taken as measure of the rate or 

 state of cure of accelerated mixtures, such measurements must 

 be made at or near the point of break of the respective mixtures. 

 The effect of such small amounts of accelerators on the elonga- 

 •tion of vulcanized mixtures has already been commented upon 

 in our previous article. In contradistinction to Stevens' view, 

 we do not regard the composition of a mixture as fixed when 

 different accelerators are used, even if they are employed in 

 amount less than 1 per cent on the rubber. 



Table II — Tests on Rubber No. 444 



Load in Hectograms 



In his second communication,^ Stevens has drawn attention to 

 the relationship between the coefficient of vulcanization and the 

 load at a given extension. Our present results, for Samples 408 

 and 444, expressed in the same manner, are shown graphically 

 in Fig. 2. It is at once evident in both cases that they differ 

 markedly from the results obtained by Stevens. In the case of 

 the mixtures which contained magnesia, the curves for both 

 rubbers are practically straight lines up to coefficients of about 

 2.0 to 2.5, and, as Stevens has already noted, the excess load 

 required to effect a given extension affords a fair measure of 

 the rate of cure. In the case of Accelerator A, however, these 

 curves are not straight lines, which shows that the load required 

 to effect a given extension is not a measu;-e of the rate of cure, 

 as indicated by sulphur coefficients. Evidently, 'both rubbers vul- 

 canize at almost the same rate when this accelerator is employed. 



Both Stevens' and our own results are subject to the same' 

 interpretation. Our Accelerator A has been shown to decrease 

 the load required to effect a given extension, namely, has increased 

 the elongation as compared with extra lig'ht magnesia. It is quite 

 possible, however, that Stevens' Accelerator I induced a greater 

 resistance to extension under a given load than our Accelerator 

 A. Furthermore, it is not entirely out of the question to select 

 an organic accelerator which would actually increase the resistance 

 to extension to more than that obtained with a similar quantity 

 of extra light magnesia. 



Experiment III. A comparison was made of the results ob- 

 tained with the two rubbers before and after extraction with 

 acetone. 



A 10-gram sample of the rubber was sheeted thin and extracted 

 with acetone in a Soxhlet apparatus for 36 hours. At the end 

 of this time, the sample was dried in z'acuo to constant weight 

 and resheeted. The mixture was made by carefully sieving the 

 required amount of sulphur and accelerator into the sheeted 

 rubber and then rolling into a cylinder. The sample thus pre- 

 pared was squeezed between the rolls of the mill to press the 

 ingredients into the rubber without loss of either sulphur or 

 accelerator. Subsequently, it was thoroughly mixed by resheet- 

 ing and rerolling until a homogeneous mix was obtained. After 

 the samples had been allowed to age for 24 hours they were 

 vulcanized in a button mold in the platen press and the combined 

 sulphur estimated in the usual manner. Owing to the small size 

 of the samples,^ no physical tests were made. 



The results of this experiment are shown in tabular form 

 in Table III. It is seen that, despite the small size of the samples 



employed, the results obtained with the unextracted rubbers are 

 in good accord with those previously found for similar quantities 

 of the accelerator (Tables I and II). In the case of the extracted 

 rubbers, however, this was not true. 



Table III 



Un- 

 extracted 

 Rubber 



-Sample 408- 



Extracted 



Rubber 



(Ext. 36 Hrs.) 



Acetone 

 Ext. = 2.62 



Un- 

 extracted 

 Rubber 



-Sample 444- 



Extracted 



Rubber 



(Ext. 36 Hrs.) 



Acetone 



Ext. = 2.99 



, A 



Sulphur Kxcess Sulphur Excess Sulphur Excess Sulphur Excess 



Co- Co- C(- Co- Co- Co- Co- Co- 



efficient efficient efficient efficient efficient efficient efficient efficient 



0.831 



1.009 



1.000 



Control . , . 0.580 

 Extra light 



magnesia, 



0.5 percent 1.874 1.294 1.290 0.459 3.132 2.123 1.343 0.343 

 Accelerator 



A, 0.5 per 

 cent 2.925 2.345 3.204 2 373 2.938 1.929 3.424 2.-f24 



After extraction with acetone and vulcanization with the assis- 

 tance of Accelerator A, Samples 408 and 444 were both found 

 to have approximately (slightly higher) the same sulphur co- 

 efficients as were obtained with the unextracted rubbers, which 

 have already been shown to be almost equal to each other. The 

 extracted samples which were vulcanized with the assistance of 

 extra light magnesia, however, gave entirely different results. 

 .'\lthough the unextracted samples had sulphur coefficients of 

 1.874 and 3.132, respectively, the same mixtures, when prepared 

 with acetone-extracted rubbers, had approximately the same 

 sulphur coefficient, 1.3. 



It Would appear that Sample 444 differed from Sample 408 

 quite markedly in the nature or condition of its acetonc-extract- 

 able components. Although we recognize that the extraction 

 with acetone may not be without effect upon the rubber or 

 upon the extraneous substances left in the rubber, it would appear 

 that, if' the acetone-soluble substances are removed, not only is 

 the response of the two rubbers to the accelerating influence of 

 extra light magnesia decreased in both instances, but also the 

 excess sulphur coefficients obtained are small and almost equal. 

 The excess coefficients obtained were, indeed, so little above 

 their controls that it would appear that a more complete re- 

 moval of these extraneous substances would prove extra light 

 magnesia to be almost inactive as an accelerator. As Accel- 

 erator .\ functions equally well with both rubbers, either before 

 or after extraction, when judged on the basis of the sulphur 

 coefficients obtained, the results obtained with it require no 

 further comment. 



Final emphasis is placed upon the fact that all results were 

 obtained with mixtures of rubber, sulphur, and accelerator only, 

 and that the amounts of accelerators employed were small in 

 all instances. 



CONCLUSIONS 



In view of the above experimental results, we are warranted in 

 drawing the following conclusions: 



I — The activity of small amounts of magnesia as an accelerator 

 is largely of a secondary or contributory character, and acts in 

 conjunction with, or obtains a response from, certain extraneous 

 substances (probably nitrogenous) present in the rubber. 



II — The activity of small amounts of magnesia is limited by 

 the amount and nature of these extraneous substances originally 

 present in the rubber. 



" Loc. cit. 



^ It was obvious that the physical properties of the control mixture and 

 the mixture which contained magnesia were very inferior to similar mixtures 

 of unextracted rubber. This was not true, however, for the mixture which 

 c<'ntaincd Accelerator A: the nhysical properties of this mixture were good 

 and not greatly below a similar mixture prepared from unextracted rubber. 



