196 



THE INDIA RUBBER WORLD 



[Janv. 



What the Rubber Chemists Are Doing. 



INVESTIGATIONS OF THE VULCANIZATION PROCESS.' 



DR. .\. W. \"AN ROSSEM, Director of the Institute of the 

 International Association for Rubber Cultivation in the 

 Netherland Indies, has compiled the Institute studies of 

 the influence of various factors on the vulcanization process. 

 The studies here reported were restricted to the treatment of 

 hot vulcanization only. Following brief introductory remarks 

 on the well-known objects and results of the vulcanization 

 process, the influence of various important factors are considered. 

 The results obtained are given below in condensed form. 



Influence of the Method, of Plasticizing and Washing on 

 THE Vulcanization Coefficient. 

 There is no doubt that very intense plasticizing can be carried 

 out without having any influence on the velocity of vulcanizing, 

 as appears from results expressed in terms of the vulcanization 

 coefficient of a mixture of 92"/^ parts Hevea crepe and 7^^ parts 

 sulphur, cured according to the standard method. (The India 

 Rubber World, September 1, 1918, page 723.) 



Influence of Plasticizjng on Relative Viscosity and Vei.ocity of 

 Vulcanization. 



Relative Viscosity 

 5% Solution in 



^xr^^ c.{ Benzine of Plas- Vulcanization 



Plasticizing ticized Mixture. Coefficient. 



Normal '" fH 



10 times extra '-| 1,8 



30 times extra . 990 



60 times extra " 2.28 



The marked decrease of viscosity shown in the table is notable. 

 This decrease takes place at a slower rate when the manipulation 

 is carried out without rest. In this case the period of rest was 

 short and had little influence. If the rubber is allowed to cool, 

 as over night, it is much more affected when milled again. Prob- 

 ably on much longer plasticizing a greater decline of viscosity 

 will take place. 



Whereas, Weber', and Spence and Ward' find that the velocity 

 of vulcanization is independent of the duration of plasticizing, 

 Axelrod' finds that strongly plasticized rubber takes up more 

 sulphur than the normally treated. No indication of this fact 

 with very strongly plasticized rubbers was found at the Institute. 

 There is no doubt that on the washing rolls rubber is de- 

 polymerized in a way corresponding to that on the mixing rolls 

 but also there may be removed by washing substances which 

 affect the velocity of vulcanization. This has been proved with 

 certainty by the Institute for Para rubbers and also for special 

 rubbers prepared according to evaporation processes. 



Time of Cure and the Vulcanization Coefficient. 



It appears that the quantity of combined sulphur increases 



about in proportion to the duration of the vulcanizing time. 



Temperature and the Vulcanizing Coefficient. 



From the tabulated results of three series of experiments on 



the same rubber and sulphur mixing it is deduced that the 



temperature coefiicient for an interval of nine degrees C. is 



greater than two. With ten degrees difference of temperature 



the resuhs diverge more, and with six degrees difference, have 



a greater tendency to similarity. Consequently it may be safely 



assumed that for temperatures not varying much from the 



technical ones, van't Hoff's rule will prevail, namely, that the 



temperature coefficient is between two and three. Whether this 



iCoomunication^ of the Netherland Government Institute for advising 

 the Rubber Trade and the Rubber Industry— Part VI, 

 '"The Chemistry of India Rubber," 1902, page 16, 

 •"Kolloid-Zeifschrift," 11, 1912, page 27;-. 

 •"Gummi-Zeitung," 24, 1909. page 352. 



holds for temperatures below 100 and above 160 degrees C. re- 

 mains to be investigated. 



Added Sulphur and the Vulcanization Coefficient. 

 This matter was studied on mixes of rubber and sulphur rang- 

 ing from 2^ to 20 per cent of sulphur, vulcanized at SO pound* 

 of steam for V/2 hours. The results show that there is a direct 

 relation between the coefficient of vulcanization and the sulphur 

 present. 



Viscosity of Raw Rubber and the Coefficient of Vulcaniza- 

 tion. 

 There exists for first latex rubbers a close correlation between 

 the viscosity of the raw product and the velocity of vulcanization. 

 This correlation is striking because the rubber has been strongly 

 depolymerized before devulcanization, even with the normal 

 way of working. All first latex rubbers will probably be depoly- 

 merized on prolonged plasticizing to an identical degree of vis- 

 cosity. In practice a highly viscous rubber will be less depoly- 

 merized before vulcanization than a less viscous one. The above- 

 mentioned correlation, however, does not become less striking 

 in consequence, for it has been seen above that the velocity 

 of vulcanization is independent of the amount of plasticizing. 

 It is very probable, in view of experimental results, that rubber 

 low in viscosity vulcanizes much more slowly than highly 

 viscous rubber. 



In order to throw light on the influences which dominate the 

 process of transforming pure raw rubber into the cured product 

 the following points were investigated: (1) Is increase of 

 breaking load attributable to less depolymerization of the rubber 

 before vulcanization? (2) Is increase of breaking load due to 

 the specific property of building up more quickly while being 

 vulcanized in the case of a rubber of higher viscosity number? 

 Experimentally it was found that a test piece made from a 

 brand of rubber whose solution shows a high viscosity number, 

 becomes stronger than one made from a species which gives a less 

 viscous solution, because the inherent properties of the first cause 

 it to vulcanize with greater rapidity. Under normal circum- 

 stances such a test piece becomes still stronger, because the 

 highly viscous rubber is far less depolymerized during the same 

 amount of work exerted upon it by the mixing rolls than a 

 rubber which is less viscous. 



Influence of the Percentage of Resin, Ash, Nitrogen and 

 THE Degree of Acidity on the Vulcanization Coefficient. 

 Opinions are divided as to whether the presence of resin is an 

 advantage to vulcanization of rubber or not. The work of the 

 Institute, by Van Heurn, indicates, without restriction, that with 

 standard vulcanization the influence of resin on the mechanical 

 properties of rubber is to lower the breaking strength. 



Later, on elaborating numerous data, Van Rossem found the 

 peculiarity of first latex rubbers, that there exists a correlation 

 between the percentage of resin and the velocity of vulcaniza- 

 tion and that with the increase of the resin percentage, velocity 

 increases on the average. Though the correlation coefficient has 

 a comparatively low value, still there is a distinctly positive 

 correlation between the percentage of resin and the vulcaniza- 

 tion coefficient. That this correlation exists is especially re- 

 markable because from Van Heurn's tests just mentioned, it 

 appeared that the addition of rubber resins causes the breaking 

 strength to decline and elongation at break to increase. 



The way the resins can affect the mechanical properties of 

 rubber was revealed by a study of elongation diagrams and led 

 to the following conclusions : 



1. By extracting the resin a change in mechanical propertie* 



