264 



THE INDIA RUBBER WORLD 



[February 1, 1917. 



What the Rubber Chemists Are Doing. 



VULCANIZATION TESTS. 



EATON and Grantham's recent paper on "Variability of Planta- 

 tion Rubber in Technical Mixings," abstracted in The In- 

 dia Rubber World last inonth, is discussed by Doctor H. P. 

 Stevens in the "Journal of the Society of Chemical Industry" 

 (November 30, 1916). 



Doctor Stevens states that he is in general agreement with 

 Eaton and Grantham as to the effect of introducing mineral mat- 

 ter or reducing the proportion of sulphur. The result is that the 

 introduction of a few per cent of litharge almost obliterates the 

 difference in the rate of cure between the fast and slow curing 

 rubbers. The importance of litharge as an ingredient of tech- 

 nical mixings cannot be over-rated; possibly more than half the 

 rubber goods manufactured contain litharge, and that is why 

 some manufacturers use a mixing containing litharge for testing 

 purposes. 



Eaton and Grantham attribute the effect of litharge entirely to 

 its action as an accelerator, but if this explanation were sufficient, 

 magnesia should produce a similar effect. A better explanation 

 is found in Doctor Stevens' research on the function of litharge 

 in vulcanization [Journal of the Society of Chemical Industry, 

 May 31, 1915, and The India Rubber World, August 1, 1915]. 

 The conclusions there reached were; (a) That the addition of 

 litharge in moderate quantities increases the coefficient of vul- 

 canization ; (b) that the maximum coefficient of vulcanization is 

 obtained where there is just sufficient sulphur to cure the rubber 

 fully and to convert all the litharge to lead sulphide and sulphate; 

 (c) that increasing proportions of litharge cause a progressive 

 reduction in the coefficient of vulcanization, a larger percentage 

 of lead sulphide and sulphate being formed; (d) that the per- 

 centage of free sulphur drops suddenly at the point where the 

 rubber is fully cured; (e) that even with large proportions of 

 litharge a little free sulphur always remains. 



If a simple relationship between the "optimum" cure and the 

 correct cure be required merely a^ a working hypothesis. Doctor 

 Stevens suggests that a geometrical relationship be adopted and 

 to calculate the correct cure of the second sample by multiplying 

 by the correct cure of the first and dividing by the "optimum" 

 cure of the first. 



IMPERIAL INSTITUTE VULCANIZATION TESTS. 



Further investigations have been made at the Imperial In- 

 stitute, London, on samples prepared in Ceylon by L. E. Camp- 

 bell to determine the influence of various factors on the vulcaniz- 

 ing and mechanical properties of rubber. The subject is treated 

 in full in the Department of Agriculture Bulletin, Ceylon, No. 24. 



The following summary of these investigations is from "The 

 India Rubber Journal" (December 2, 1916) and deals with the 

 following : 



(1) The effect of the form of the rubber, sheet, crepe and 

 block ; (2) the effect of drying in air at ordinary temperature, in 

 hot air and in a vacuum drier; (3) the effect of over-working the 

 freshly coagulated rubber. Incidentally, comparison of rubber 

 coagulated with acetic and hydrofluoric acid showed the former 

 usually had the shorter time of vulcanization. 



SUMMARY OF RESULTS. 



The results recorded in this summary confirm those previously 

 obtained in showing that plantation Para rubber is quite satis- 

 factory in mechanical properties, the average tensile strength be- 

 ing fully equal to that of specimens of the best hard Para and 

 the average elongation at the breaking point, only very slightly 

 lower. 



The chief variation is in the time required for correct vulcani- 

 zation. There is no doubt from the results now available that 



the conversion of the freshly coagulated rubber into crepe length- 

 ens the time of vulcanization, as compared with that of the cor- 

 responding sheet. In eleven comparative sets of specimens dealt 

 with in this summary, the thin crepe rubber had a distinctly 

 longer time of vulcanization than the sheet, the figures ranging 

 from 105 to 130 minutes for the crepe and from 60 to 75 minutes 

 for the sheet. It is noteworthy, too, that this lengthening of the 

 time of vulcanization is brought about by passing the rubber 

 through the rollers only five or seven times, and that additional 

 treatment in the machine, up to 70 times through the rollers, has 

 little further effect on the time of vulcanization. Thus the times 

 of vulcanization of crepe rubber passed through the rollers 7, 35, 

 and 70 times were 113, 115 and 130 minutes, and the times for 

 rubber treated 5, 25 and 50 times were 105, 115 and 115 minutes, 

 respectively. The time for the control sheet was 75 minutes in 

 each case. 



Although the conversion of freshly coagulated rubber into 

 crepe has this marked effect on the time of vulcanization, the 

 tensile strength is again shown to be but little affected, the dif- 

 ference in the breaking load of the sheet and crepe being small 

 In the specimens previously dealt with the advantage in average 

 tensile strength was invariably in favor of the sheet, but the 

 crepe may have the higher value, as is the case in four of the 

 six sets of specimens. 



The "over-working" of the freshly coagulated rubber in the 

 washing machine had little effect on the tensile strength, or on 

 the time of vulcanization. Rubber passed through the rollers 50 

 or 70 times differed only slightly in either of these respects from 

 rubber treated five or seven times. The common opinion that the 

 mechanical properties may be easily impaii'ed by "over-working," 

 does not receive support from the results of these experiments. 



The conversion of thin crepe into thick crepe, by rolling sev- 

 eral pieces together, did not produce any difference in the time 

 of vulcanization, and the differences in tensile strength were not 

 very marked or constant. The block rubber, made by compress- 

 ing thin crepe, had the same time of vulcanization as the latter, 

 but in five out of six sets of specimens its tensile strength was 

 a little lower. 



The different methods of drying employed: (1) Air drying 

 at the ordinary temperature; (2) drying in hot air; and (3) in 

 vacuo, had very little effect on the time of vulcanization or tensile 

 strength of the rubber. 



EELATIONSHIP OF MECHANICAL To'cHEMICAL PEOPEHTIES. 



Doctor D. Spence in "The India Rubber Journal" writes as fol- 

 lows on the relationship of mechanical to chemical properties of 

 vulcanized rubber: 



From experiments made there is no question that the com- 

 bined sulphur at "optimum" cure in the case of Hevea plantation 

 rubber is a remarkably constant quantity, equal on the average 

 to approximately 2.8-3 per cent. Where more than this amount 

 of combined sulphur has been found, either the method of vul- 

 canization is at fault or the means of determining the "optimum" 

 cure are inaccurate. In this connection it is necessary to point 

 out that in the case of very soft, low-grade rubbers it is difficult 

 to judge of the "optimum" cure, and there is always the tendency 

 to increase the cure to beyond the "optimum" point in the hope 

 of thereby improving the physical or tensile properties of the 

 product. In the case of any good grade of Hevea plantation rub- 

 ber there is no such difficulty, however, and where more than 2.8- 

 3 per cent of combined sulphur is reported in this case, either 

 the sample is over-cured, or what amounts to the same thing, 

 vulcanization has not been properly carried out. With proper 

 methods of vulcanization, and with the requisite experience in 

 the judging of the proper cure, the combined sulphur at "op- 



