THE INDIA RUBBER WORLD 



[AicrsT 1, 1920. 



What the Rubber Chemists Are Doing. 



THE ACCELERATION OF VULCANIZATION.' 



THRKE METHOD.-; arc availaljlc for speeding up the vulcaniza- 

 tion process: (1) raising the temperature, (2) increasing 

 the proportion of sulphur relative to rubber, (3) intro- 

 ducing an accelerator. 



EFFECT OF TEMPERATURE. 



The eflfect of alteration of temperature is similar to that for 

 other chemical reactions, the temperature coethcient being be- 

 tween 2 and 3 (for 10 degrees C). The suggestion has been 

 made that some accelerators, lead o.xide in particular, are not 

 genuine catalysts, but that they merely react with part of the 

 sulphur with evolution of heat, thereby raising the temperature 

 of the reacting mass above that of the surrounding heating 

 medium." The insufficiency of this explanation is evident from 

 the fact that such an effect should be almost negligible at the 

 surface of the rubber in contact with the molds, while in the 

 interior it would be marked ; thick slabs also would vulcanize 

 much more rapidly than thin sheets ; both these consequences of 

 the theory are contrary to experience. It is quite possible, how- 

 ever, that many vulcanization accelerators do exert a slight 

 thermal effect in addition to their purely catalytic influence. 



The curves given in Fig. 1 represent results of some of our 

 experiments as to the rate of vulcanization at temperatures rang- 

 ing from 138 degrees C. (35 pounds' steam pressure) to 168 de- 

 grees C. (95 pounds) for a mi.xture of pale crepe rubber (90) and 

 sulphur (10). Pale crepe rubber was chosen as showing greater 

 uniformity in rate of vulcanization than other forms of rubber, 

 and was taken as far as possible from one case. For the intro- 

 duction of the sulphur a stock mixing of sulphur with approxi- 



degrce of accuracy attainable with careful working. For the 

 purpose of comparison between different samples an elongation 

 of 600 per cent (including the original length) at a load of 0.5-kilo 

 ■per square mm. has been arbitrarily assumed as a standard 

 throughout this paper; (c) by the time required to produce maxi- 

 mum tensile strength. 



The last method, although of less importance than might be 

 expected in technical practice, is of considerable value in experi- 

 mental work as supplying a convenient and rapid method for 

 comparing rates of vulcanization, for example, of different rub- 

 bers or at different temperatures, the maximum tensile strength, 

 determined within three days of vulcanization, being observed 

 with a product containing approximately 5 per cent of combined 

 sulphur calculated on the rubber. The actual value of the break- 

 ing strength of a rubber test piece is always more or less for- 

 tuitous ; however, as vulcanization beyond the condition necessary 

 for the attainment of the maximum strength causes a very rapid 

 weakening, the position of the maximum is relatively easily de- 

 termined. The peaked curves in Fig. 1 indicate the position of the 

 maximum rather than the actual magnitude of the values. 



It will be observed that the temperature coefficient manifests 

 no tendency to any regular increase or decrease with rise of 

 temperature, the mean value calculated (by all three methods of 

 comparison) from the figures represented in the curves for the 

 rubber-sulphur mixture at 128 degrees to 168 degrees C. ap- 

 proximating to 2.31 This appears to indicate that the allotropic 

 forms ordinarily present in molten sulphur in relative pro- 

 portions dependent on the temperature, must possess equal or 

 at least comparable vulcanizing activity.' 



Vulcaniiation at Different Temperatures 



Effect of 0%tol%Acceleratoratl48°C 



Average effectiveness of 

 Accelerator at various 





?0 40 60 go 100 120 140 160 180 200 m 240 260 

 Time of Cu re (mins) -» 



10 20 30 40 50 60 

 Timeof Cure(r 



70 80 90 100 



0,25 0.5 0.75 1.0 1.25 

 Percentage of Accelerator— 



mately an equal weight of rubber was used. The composition of 

 this mixture was checked by analysis, and the correct proportion 

 weighed out for mixing with the remainder of the rubber. In 

 this way the almost inevitable loss of sulphur dust during the 

 customary method of mixing was avoided. 



The progress of vulcanization may be followed in at least three 

 different ways: (a) by the combination of sulphur; (b) by the 

 gradual decrease in the extensibility of the rubber at a definite 

 load (or increase of the load necessary to produce a definite 

 elongation) ; within the range of the experiments in this paper, 

 this shows a roughly rectilinear relationship whh the period of 

 vulcanization. Unfortunately, this method is influenced to a 

 greater extent than either (a) or (c) by the mechanical treat- 

 ment of the rubber during mastication and the subsequent mixing 

 operation, increased working or heating having a tendency to 

 exaggerate the extensibility figure. The approximation of the 

 curve to a straight line and the close proximity of extensibility 

 values for duplicate samples are evidence of the considerable 



Fig. 



Fig. 3. 



The extent to which the vulcanization period in technical work 

 can be reduced by raising the temperature is limited by various 

 considerations, not the least of which is the poor thermal con- 

 ductivity of rubber and the consequent danger of unequal heating, 

 involving local irregularities in the degree of vulcanization. 



INCREASE OF PROPORTION OF SULPHUR. 



Many old compounding ingredients and specifics of the rubber 

 trade, often bearing fanciful names, contained — undeclared — con- 

 siderable proportions of free sulphur; mixtures of sulphur with 

 various waxes and also with antimony sulphide may be quoted as 



of Chemical 



iBy D. !••. Twiss and S. A. Brazier, "Tournal of the 

 Industry," May 15, 1920, page 125-132T. 



=SeidI, -GnmnuZci>ung:> 1911, 25, 710, 748. 



^On account of tlie inconveniently rapid progress of vulcanization 

 168 degrees C. and its slowness at 128 degrees C. the measurements 

 138 degrees C. to 158 degrees C. are possibly to be preferred; a me 

 value of 2.6 is then obtained for the temperature coefficient. 



'See Twiss, Annual Report of the Society of Chemical Industry, 1919, 

 page 327. 



