336 



THE INDIA RUBBER WORLD 



February 1, 1921 



What the Rubber Chemists Are Doing 



THE ACTION Of LIGHT ON RUBBER' 



IX this paper the author, B. D. Porritt, cites many observa- 

 tions and investigations of the prejudicial effects of light on 

 rubber, technically known as "perishing," begnniing with the 

 observations of Thomas Hancock, nearly 100 years ago, who 

 found that by blackening the surface of the rubber, deterioration 

 could be prevented. Similarly the deleterious effect on the rub- 

 ber proofing of certain dressings present in the dyed fabrics, pre- 

 sumably metallic mordants, was remarked by him, as also was 

 the fact that the non-caoutchouc constituents of the rubber exer- 

 cised a marked protective effect. 



DETERIORATION OF BALLOON FABRICS 



Many eminent chemists have studied at many different angles 

 the subject of perishing of rubber, one of the practical aspects 

 being the deterioration of balloon fabrics. 



Two types of protection have been found to give satisfactory 

 results. One involves the employment of small proportions of 

 litharge and other ingredients in the proofing mixture and prob- 

 ably depends mainly on the formation in the rubber of lead sul- 

 phide in a colloidal form. The other, which has been termed the 

 ■"molecular protection process," involves the selective absorption 

 of the ultra-violet light by a suitable colored organic compound 

 actually dissolved in the rubber. 



By the use of either of these two methods, especially if com- 

 bined with some of the earlier protective processes, it is probable 

 that at present the determining factor for the life of a balloon 

 envelope in the tropics is the durability of the textile rather than 

 of the rubber. 



MASTICATION AND SUSCEPTIBILITY OF RUBBER TO LIGHT 



With a view to determining whether the changes produced by 

 the process of mastication have any effect on the susceptibility 

 of raw rubber to light, a range of samples was obtained through- 

 out the operation, and from these solutions were prepared and 

 exposed, the viscosities being determined at intervals. The re- 

 sults would suggest that the physical changes produced in rubber 

 by mastication are not necessarily the same as the initial effects 

 arising from the exposure to light and air, and that the stability 

 to light may increase rather than diminish with the severity of 

 the treatment, though this is at variance with the results of 

 Weber, who found that overmilled rubber was prone to oxi- 

 •dation. 



SOLARIZATION AND USE OF ULTRAVIOLET RAYS FOR VULCANIZING 

 SOLUTIONS 



It is desirable to draw attention to the fact that the action of 

 light and air on rubber is not always associated with ill effects. 

 In the early days of the industry, exposure to sunlight, a process 

 tenned "solarization," was frequently employed to produce a 

 type of surface vulcanization. 



A comparatively recent development of this old process in- 

 volves the use of light sources rich in ultra-violet^ for the pro- 

 duction of "solutions" of vulcanized rubber. Under the influence 

 of a quartz mercury vapor lamp, a solution of rubber and sul- 

 phur undoubtedly sets to a stable "gel," accompanied by the 

 combination of a small proportion of the sulphur. 



Under ordinary daylight conditions it is found, however, that 

 the addition of sulphur to a rubber solution, if anything, ac- 

 <elerates the rate of viscosity diminution, and it was consequently 

 thought of interest to see whether the removal of oxygen would 



' Joint mectinR of Far.iday Sociely and the Physical Society of London, 

 t920. Published in The India-Rubber Journal, December 4, 1920, page 21. 



- Helbronner and Bernstein, I.e Caoutcliouc et la Gutta-Perclia, 1915, 

 12. 8720. 



enable the sulphur to respond to the more feeble incident radia- 

 tion. An experiment in this direction did not, however, produce 

 tlie anticipated result, those solutions to which crystalline sulphur 

 had been added showed no apparent change after several months' 

 exposure, while the blank sample containing rubber only, after 

 a few days suddenly set to a "gel" without any obvious prelimi- 

 nary increase in viscosity. This gel on exposure was found to 

 be rapidly transformed to a liquid. 



PROJECTED NEW VISCOMETER 



It is hoped shortly to design a viscometer which will enable 

 these peculiar changes to be studied more thoroughly, using 

 specially purified materials in view of the marked effects pro- 

 duced by traces of acid, and to verify Van Rossem's statement 

 that, even in the absence of air, the action of light results in a 

 reduction of the viscosity of rubber solutions. 



While it would appear likely that the presence of air is not 

 favorable to the light vulcanization effect, it must be noted that 

 in the nascent condition oxygen is itself capable of producing a 

 similar result, since organic peroxides which have recently been 

 put forward as vulcanizing agents' have been proved in a measure 

 to function as such. 



AN IMPORTANT EFFECT 



It may be of interest to draw attention to the important, but 

 generally unrecognized effect which is produced in the vulcaniza- 

 tion of rubber by the agency of sulphur and heat when this process 

 is carried out in the presence of air. Goodyear's discovery of 

 this remarkable transformation was due to the accidental heating 

 of a piece of rubber containing sulphur and white lead. Han- 

 cock, when independently endeavoring to obtain the same result, 

 secured no success by heating samples containing rubber and 

 sulphur alone and ultimately discovered that to effect the change 

 desired it was necessary to immerse the rubber in molten sulphur. 



Technical experience has confirmed these early observations 

 and proved that with sulphur only the absence of air is necessary 

 to insure vulcanization, while to secure satisfactory results by 

 the "dry heat" process, the use of a positive catalyst such as 

 litharge, in addition to sulphur, is indispensable. 



OXIDATION IN ABSENCE OF LIGHT 



Comparatively little is know'n regarding the mechanism of the 

 changes which take place during the perishing of vulcanized rub- 

 ber, beyond the fact that the final products resemble those of raw 

 rubber in properties and composition ; that oxidation takes place 

 in the absence of light if the material has been overvulcanized 

 and is probably promoted by the presence of various catalysts. 



Recent work, however, has shown that marked alterations take 

 place on aging in the tensile properties of overvulcanized rubber 

 prior to oxidation .setting in,* and it is possible that in this pre- 

 liminary physical change oxygen is again functioning as a catalyst 

 — a suggestion which is now under investigation. 



WAYS IN WHICH LIGHT AND OXYGEN OPERATE TO PRODUCE 

 CHANGES 



1. I'nder ordinary conditions the action of light and oxygen 

 would appear to take place in two stages, the first being an altera- 

 tion in the state of molecular aggregation during which oxygen 

 •functions mainly as a catalyst and the second a series of chemical 



reactions in which the active participation of oxygen is promoted 

 by the formation of an autocatalyst, probably a peroxide. 



2. The development of the former alteration, which constitutes 

 "tackiness," is probably further promoted or retarded by other 



^ I. I. Ostromyslenski. Journal of the Russian Physico-Cheniical Society, 

 191.^. 47, 1453-61. ?nd The India Rubber World. November 1. I9I6, 65. 

 * .Annual Report on the Progress of Applied Chemistry, 1919. page 338. 



