May 



1917.] 



THE INDIA RUBBER WORLD 



451 



What the Rubber Chemists Are Doing. 



COEFFICIENT OF VULCANIZATION AND THE STATE OF CURE. 



DR. Henry P. Stevens discusses in ;'The India Rubber Jour- 

 nal" (February 10, 1917) the paper of Dr. O. de Vries on 

 the above topic, The India Rubber World (]VI{irch 1, 1917). 

 Doctor Stevens remarks that if physical tests on vulcanized 

 rubber are to be of practical value in order that a decision may 

 be made regarding the manufacturing value of any particular 

 specimen, these tests must be carried out either on a rubber 

 cured suitably to manufacturing conditions, or if cured beyond 

 this stage (over-cured) there must be available some method 

 by which the tests on the over-cured rubber may be corre- 

 lated to tests on correctly cured rubber. No such method of 

 calculation is at present available, as the necessary relation- 

 ship has not been worked out between stress-strain curves 

 and coefficient of vulcanization. A correctly cured rubber is 

 one fully cured from the manufacturer's standpoint, not be- 

 yond that, point at which aging is satisfactory. Otherwise 

 the specimen is over-cured. 



Rubber is a colloid, and shows the phenomena of hysteresis. 

 Its physical properties at any moment depend partly on its 

 previous history. It is therefore obvious that no conclusions 

 should be drawn from any stress-strain curve without taking 

 into consideration the previous history of the specimen. 

 Dr. Stevens agrees with the stat;ement of Dr. de Vries that 

 "the percentage of combined sulphur is quite independent of 

 the state of cure as expressed by the position of the stress- 

 strain curve," but adds that the "state of cure" cannot be ade- 

 quately expressed under present conditions by the stress- 

 strain curve. It may, however, be possible when the "standard 

 curve" or other methods are correlated to manufacturing con- 

 ditions and the tests carried out on rigidly standardized line.;. 



Vulcanization is essentially a chemical process; so also is 

 the subsequent decomposition of rubber which has been over- 

 cured, and hence the proportional relationship of rubber and 

 sulphur in combination is the best guide for a stable product, 

 the first essential in the manufacture of rubber goods. 



Messrs. Schidrowitz and Goldsborough have evidently cor- 

 related their stress-strain curve method to manufacturing 

 conditions, as shown by their remarks on the subject in "The 

 India Rubber Journal" (March 3, 1917). 



Discussing the case, in which the "coefficient" (according 

 to Stevens) may be wrong, but the mechanical properties 

 correct, Schidrowitz and Goldsborough note that the question 

 at issue is whether state of cure or correct cure is to be 

 judged by the chemical or the mechanical properties of the 

 vulcanized article — by sulphur combined with the rubber or 

 by the stress-strain curve. Their answer is that ultimately 

 the attributes or quality of vulcanized rubber must be judged 

 by the physical or mechanical properties. 



Certain low-curing rubbers require, in order to acquire the 

 necessary mechanical properties, a protracted cure, and in the 

 course of such cure will combine with more than three per 

 cent of sulphur, and in general slow curing rubbers deteriorate 

 more rapidly than rapid curing goods, and will not age well 

 because of the excess of combined sulphur, and possibly also 

 by reason of the long heating necessary. 



The physical and mechanical effects which vulcanization 

 has upon rubber are shown in the clearest manner by the 

 stress-strain curve method, and there is no other known 

 method whereby the mechanical aspect of vulcanization can 

 be systematically and accurately followed and measured. 



Schidrowitz and Goldsborough remark in conclusion: 



In view of the apparent lack of comprehension concerning 

 stress-strain curves, we take this opportunity of briefly re-stating 

 some of the more important pointsi 



1. The "type" of the curve is independent of the state of cure. 

 It therefore connotes inherent properties. 



2. As the "type" varies for different rubbers, its determination 

 affords a valuable method of comparison in regard to important 

 mechanical properties. 



3. We obtain a graphic representation of tlie progress of vul- 

 canization. 



4. We are able to cure to a definite mechanical condition, and 

 to estimate the rate of cure necessary to attain that condition. 



5. Having settled the position of the curve for a given rubber 

 mi.xing we are able to control the vulcanization of the factory 

 product. 



Much else may be done with and deducted from the stress- 

 strain curve method. In more propitious times the authors pro- 

 pose to recur to the subject at length. 



While agreeing that stress-strain curves are of great value, 

 intelligently applied. Dr. Stevens holds that the coefficient of 

 vulcanization is the safest guide as to the state of cure. 



VULCANIZATION BY AGENTS OTHER THAN SULPHUR. 



Reviewing the recently published work of Ostromyslenski, 

 Dr. H. P. Stevens presents some of his results in the "Journal 

 o"f the Society of Chemical Industry" (February IS, 1917). 

 V■ULCANI^ATION WITH NITROBENZENE. 



Ostromyslenski's results (published in The India Rubber 

 World, November 1, 1916) have not been duplicated and his 

 claim cannot at present be accepted. 



The effect of vulcanization has been obtained with mixes 

 of rubber with di- or trinitrobenzene in the presence of bases 

 such as litharge or magnesia without the least difficulty. 

 Trinitrobenzene is far more efficient than dinitrobenzene. Mono- 

 nitrobenzene, so far, has not given satisfactory results. 

 Figures are given obtained with 100 parts pale sheet rubber, 

 eight parts litharge and varying proportions of );i-dinitro- 

 benzene and sym-trinitrobenzene. The specimens were cured 

 in a steam vulcanizer at 135 degrees C. for times varying 

 from 5 to 120 minutes. 



The results of the physical tests show that the products 

 so far obtained possess relatively poor physical qualities. 

 The best results were obtained with the larger proportion 

 (four parts) of trinitrobenzene, but even in this case the 

 strength as measured by the tensile product is not much more 

 than half that obtained with a properly vulcanized rubber and 

 sulphur compound. 



The color of rubber vulcanized with nitrobenzenes is 

 brown to black. Viewed by transmitted light in a thin 

 stretched sheet it is yellow-brown and translucent or almost 

 transparent.- Litharge can be replaced by magnesia and prob- 

 ably by other basic oxides with similar results. 



VULCANIZ.\TION WITH BENZ0\T:. PEROXIDE. 



The discovery of peroxides as vulcanizing agents to re- 

 place sulphur was announced by Ostromyslenski. Of the 

 peroxides employed, most of the experiments were made 

 with benzoyl peroxide. 



As is well known, benzoyl peroxide, like other peroxides, is 

 very explosive, and consequently cannot be handled in the 

 dry state. For these experiments it was used in the form of a 

 paste with about its own weight of water. In this connection 

 it presents no difficulty when added to the rubber on the mix- 

 ing rolls, and the small amount of water added is lost by the 

 time the mi.xing is complete. 



In the first series of experiments three mixes were made 

 of the same plantation crepe rubber with the following per- 

 centages of benzoyl peroxide (reckoned dry): A, 0.33 per 

 cent; B, 1.33 per cent; C, 6.67 per cent. 



Mixings A and B gave very weak rubbers. Nevertheless 

 the influence of even 0.33 per cent of benzoyl peroxide is 

 clearly marked. Mix C gave the best results, the vulcanized 



