June 1, 1919.] 



THE INDIA RUBBER WORLD 



485 



What the Rubber Chemists Are Doing. 



EFFECT OF CERTAIN ACCELERATORS UPON THE PROPERTIES 

 OF VULCANIZED RUBBER. 



AT the meeting of the New Jersey Chemical Society in New- 

 ark, January 13, 1919, a paper on the "Et=fect of Certain 

 Accelerators Upon the Properties of Vulcanized Rubber," 

 by George D. Kratz and A. H. Flower, was read by the former. 

 The main features of this paper and the authors' conclusions 

 are given in the following extended quotations : 



INFLUENCE OF ACCELERATORS. 



The results recorded were obtained in the course of several 

 investigations to determine the relative activity of certain inor- 

 ganic and organic accelerators, and the permissibility in the use 

 of the sulphur coefficient in evaluating samples of vulcanized 

 rubber known to contain accelerators. This has consisted of: 

 (a) a comparison of the relative effects of heavy calcmed mag- 

 nesia and an (unidentified) organic accelerator ni a mixture 

 containing only rubber and sulphur, and (fc) a comparison of 

 the effect of larger amounts of heavy calcined magnesia, light 

 magnesia and lime in a mixture which contained an excess of 

 zinc oxide. ... 



Irrespective of the effect of accelerators upon the physical 

 properties of the vulcanized mixture, possibly the advantage most 

 often claimed for the organic variety in preference to inorganic 

 substances is that in accelerating vulcanization, the former are 

 much more active than the latter and can be used in small 

 amounts to replace much larger quantities of litharge, magnesia 

 or lime. 



These investigations have not included a comparison of the 

 relative effects of magnesia and p-nitroso-dimethyl-aniline. The 

 organic accelerator which we employed in place of the latter sub- 

 stance was found to be far more active, when used in srnall 

 amount, than were similar quantities of magnesia. Likewise, 

 the physical properties of the mixtures which contained the or- 

 ganic accelerator were shown to be superior to those obtained 

 with similar amounts of magnesia. 



When the load required to effect a given extension is taken 

 as a measure of the physical properties, we have found a marked 

 decrease in the extension of the vulcanized mixture to be true 

 for small amounts of magnesia; certain organic accelerators, 

 however, were found to increase the extension. Under these cir- 

 cumstances, entirely erroneous results were obtained when two 

 scries of mixtures, one of which contained small amounts of 

 magnesia and the other similar amounts of a strong accelerator, 

 were tested by this method. Comparisons made on the basis 

 of the loads required to effect a given extension are less reliable 

 than those obtained by comparing the tensile strength and per- 

 centage elongation at break. In this instance the sulphur coeffi- 

 cient may be said to afford a fair index of the state of cure as 

 measured by the physical properties of mixtures known to con- 

 tain small amounts of either inorganic or organic accelerators. 



This statement, however, was not found to be of general ap- 

 plication, and certainly is not true for mixtures which contain 

 larger amounts of inorganic accelerators in the presence of an 

 excess of zinc oxide. We find that large amounts of zinc oxide 

 effect a slight and limited increase in the rate of vulcanization 

 (sulphur coefficient). 



Contrary to what might be expected, we have found that mix- 

 tures which contained fairly large quantities of both magnesia 

 and zinc oxide, when vulcanized to maximum physical properties, 

 had lower sulphur coefficients than a control mixture which was 

 vulcanized to the same degree without the assistance of an ac- 

 celerator. The substitution of lime for magnesia in such a mix- 

 ture, however, was found to produce quite a different effect. 

 With lime, after an initial decrease in the sulphur coefficient, 

 the latter value was found to respond to a further increase in 

 the amount of the accelerator. This difference in the action 

 of the two substances would indicate that their function in the 

 mixture is not identical. 



Our results with heavy mineralized mixtures have shown that 

 such mixtures are not_ only subject to misinterpretation, but the 

 constituents of the mixtures tend to obscure or mask the in- 

 dividual properties of the rubber used. 



INDICATION or SULPHUR COEFFICIENT. 



Fundamentally, the purpose of any ordinary vulcanization is to 

 obtain the maximum physical properties which the mixture will 

 retain unimpaired over the longest period of time under the con- 

 ditions to which it will be subjected. This physical condition 



may be largely independent of the sulphur coefficient, particu- 

 larly if the mixture has been vulcanized with the assistance of 

 an accelerator. Generally speaking, we have found that the 

 sulphur coefficient does not afford a reliable indication of the 

 physical properties of a vulcanized rubber mixture or that it can 

 be taken as a measure of the state of cure except possibly in 

 the case of mixtures which consist of rubber and sulphur only. 

 We agree with Stevens, however, that for Hevea rubber prac- 

 tically all mixtures with a sulphur coefficient in excess of 3.2 

 will be subject to rapid deterioration. 



In a former paper' we have given the limits for the vulcaniza- 

 tion coefficient of Hcvea rubber at 1.7 to 2.8. Under standardized 

 conditions, the higher figure has been found to be consistently 

 approximated by mixtures vulcanized without the aid of an 

 accelerator, or with the assistance of one which is only mildly 

 active. The lower figure has been found to apply for mixtures 

 vulcanized with the assistance of even small amounts of powerful 

 organic accelerators_ or larger amounts of magnesia. The 

 anomaly found to exist between the action of magnesia and lime, 

 however, indicates that a certain amount of reservation should 

 be made in interpreting the coefficients of mixtures which con- 

 tain either of these substances. 



In view of our results as a whole, while we agree with 

 Stevens that the sulphur coefficient is most important as an 

 indication of the ultimate stability of the product, in general 

 practice it should be considered as an indication only. Even 

 in the case of mixtures composed solely of rubber and sul- 

 phur, we have found it dangerous to evaluate between them on 

 the strength of their sulphur coefficients alone, unless the past 

 histories of the samples in question are definitely known. As 

 stated by Stevens, the true value of a product, as expressed by 

 Its state of cure, is obtained from physical and chemical tests 

 only when they have been made after a definite period of aging 

 conducted under carefully standardized conditions. 



EXPERIMENTAL RESULTS. 



per Cent 



Sq. Mm. Elongation 



(at Break), at Break. 



636 



871> 900» 



1153= 975» 



1170" 1025" 



1250= 1037' 



1223 1087 



In the case of mixtures vulcanized with the assistance of 

 "Accelerator A," it is evident from the figures obtained for the 

 percentage elongation at break that this property increased 

 proportionately with the tensile strength ; with magnesia, on the 

 contrary, tensile strength was increased at the expense of 

 elongation. 



This difference in the effect produced in the percentage elonga- 

 tion by small amounts of accelerators prohibits the use of the 

 load required to effect a given extension as a measure of the 

 physical properties of the two series of mixtures. As the per- 

 centage elongation of a mixture is increased by the action of an 

 organic accelerator or otherwise, unless this is accompanied by 

 a corresponding and uniform increase in the tensile strength, a 

 given extension will be effected by a lesser load than would nor- 

 mally be required. This was roughly found to be true with 

 "Accelerator A." On the other hand, when the tensile strength 

 is increased at the expense of elongation, as was found to be the 

 case with magnesia, it will require an excessive load to effect 

 the same extension. This decrease in the percentage elongation of 

 mixtures which contained small amounts of magnesia was so 

 marked that, with one per cent of this substance, an extension 



^ "Journal of Industrial and Engint 



'These samples were pinched thr 



nachine before the point of rupture, i 



he results for tensile strength and e 



iring Chemistry," 11, 1919, page 30. 

 iugh by the clamps of the testing 

 r break, was reached. ConsequcntlYr 



