720 



THE INDIA RUBBER WORLD 



lAutusT I. 1920. 



storage st rjuiu teinpcraturc. The effect of zinc oxide in activat- 

 ing organic accelerators is not in the least understood. Twiss, 

 in a recent paper, gave figures for some other organic accelerators 

 which are similarly activated by a small quantity of zinc oxide; 

 in fact without zinc oxide many of them would be of little use. 

 We have considered the accelerating effect — that is to say, the 

 catalytic action — of these substances in promoting the union of 

 caoutchouc and sulphur, but it is also of interest to compare the 

 physical properties of rubber vulcanized with and without an 

 accelerator. For this purpose we make use of the coefficient or 

 percentage of combined sulphur calculated on the raw rubber as 

 the basis on which to compare the physical properties; in other 

 words, we determine the breaking strain and elongation of the 

 specimens vulcanized to give the same coefficient. Gottlob noticed 

 some time ago that there was considerable danger of overcuring — 

 that is to say, formation of an unstable vulcanizate — if rubber 

 containing an organic accelerator was fully vulcanized. In a se- 

 ries of cures it was found that without the accelerator the break- 

 ing strain of the overcured specimens showed a gradual reduc- 

 tion with the excess curing, while in the presence of an accelerator 

 the decrease in tensile strength was very sudden. He did not, 

 however, publish any aging tests in confirmation. The writer 

 and others Iiavc shown that a pure rubber mix is best cured to 

 give a ctefficient of appro.ximately 3, if tensile figures are con- 

 sidered in conjunction with aging tests. When, however, accel- 

 erators are added, it appears from Cranor's figures that the 

 coefficient should be much lower if satisfactory aging results are 

 to be obtained. When using powerful catalysts with a small 

 proportion of ;:inc oxide it is probable that the rubber is suf- 

 ficiently vulcanized when the Coefficient amounts to one unit. 

 The higher tensile figures obtainable by the use of organic cata- 

 lysts appear, therefore, to some extent illusory. Some recent 

 figures of Seidl are worth quoting in this connection. He made 

 up four "mixings" and vulcanized for varying periods at 138 de- 

 grees C. The results in the accompanying table show the physi- 

 cal properties and time of cure for six fixed percentages of com- 

 bined sulphur ("Gummi-Zcituiig," June 18, 1920, pages 797-8). 

 The physical tests were made on rings of 4 square millimeters 

 cross-sectional area, and I have recalculated the breaking; strain to 

 grams per square millimeter cross-sectional area. 



the same breaking strain as that without accelerator having a co- 

 efficient ol 2.47. Consequently to obtain a breaking strain of 590 

 grams per square millimeter a rubber and sulphur mixing must 

 be vulcanized to double the coefficient which would be necessary 

 if a suitable accelerator were added. To yield a breaking strain 

 of 1,160 to 1,170 the use of a mix without accelerator will neces- 

 sitate vulcanizing to give a 50 per cent higher coefficient. For 

 a breaking strani of 1,540 to 1,550 the coefficient must be raised 30 

 per cent. With breaking strains over 2,000 the coefficient does 

 not require to be raised and is practically the same for both 

 mixes. We therefore have a progressive relationship. The 

 difference in the coefficient required to produce the same break- 

 ing strain being less and less the higher the coefficient. With 1 

 per cent of the accelerator used by Seidl, the rubber would ap- 

 pear to be fully cured round about a coefficient of 2 to 2.5 against 

 the figure of 1 suggested by Cranor for the very efficient accel- 

 erator which has been used in his experiments. 



A comparison of columns 3 and 4 is also, of great interest. 

 The compounds used differ only in the percentage of sulphur. 

 It has been stated that approximate proportional vulcanization 

 exists between the coefficients and added quantities of sulphur 

 in ordinary rubber sulphur mixings. The figures show that a 

 similar condition holds for accelerator compounded mixings. 

 The time required to produce the same sulphur coefficient is ap- 

 proximately two-thirds, in the case of the 15 per cent sulphur 

 mix, of what it is in the 10 per cent sulphur mix. It is said 

 that rubber can dissolve only a limited amount of sulphur — 

 about 10 per cent and, therefore, larger quantities merely act as 

 a diluent and tend to hinder rather than promote the reaction. 

 As caoutchouc sulphide is formed, larger quantities of sulphur 

 can be dissolved, as caoutchouc sulphide is a better solvent for 

 sulphur than raw rubber. (Compare work of Skellon, also 

 "Communications of the Netherland Government Institute for 

 .\dvising the Rubber Trade and the Rubber Industry," 1916, page 

 239 ef seq.) Consequently, as vulcanization proceeds, a larger 

 proportion of sulphur is dissolved (if available) and an increase 

 in rate of cure results. One would not expect such an increase 

 until an appreciable proportion of sulphur had combined with 

 the rubber. If we compare the third and fourth columns it will 



.PHYSic.\r. Properties 



Time of Cure for Six Fixed Percentages of Combixed Sulphur. 



To study this table wc may concentrate on the first and third 

 series of figures. The compounds differ only in that (3) contains 

 1 per cent of accelerator. This reduces the time of cure to about 

 one-seventh or less. It will be noted that the sulphur is present 

 in considerable excess even when the maximum amount (4.57 

 per cent) is combined. If the breaking strains are compared 

 it will be seen that the addition of the accelerator produces a 

 very considerable increase of breaking strain at the lower cures, 

 that is, up to about 3 per cent of combined sulphur. For 3.9 and 

 4.5 per cent of combined sulphur the breaking strains are prac- 

 tically the same for both mixings. At this stage both are over- 

 cured. The difference in breaking strain is therefore confined to 

 normal cures. The breaking strain of the accelerator compounded 

 rubber with a coefficient .82 is approximately equal to that without 

 accelerator having a coefficient of 1.63. Similarly the accelerator 

 compounded rubber with a coefficient of 1.63 gives approximately 



be seen that a considerable increase in rate of cure results from 

 increasing the proportion of sulphur from 10 to 15 per cent. 

 The time of cure is reduced from T/z to 5 minutes and propor- 

 tionately. At the same time the breaking strain is more than 

 doubled. In fact with 15 per cent of sulphur the rubber appears 

 fully cured with a coefficient but little exceeding one unit. If this 

 is so, the coefficient corresponding to the correct cure will depend 

 not only on the nature and proportion of accelerator but also 

 on the proportion of sulphur added. It is not possible to speak 

 positively until aging experiments have been carried out. In this 

 connection I would much prefer tests made at about normal tem- 

 perature, say not above 30 degrees C, rather than so-called ac- 

 celerating aging tests at higher temperatures. Temperature is 

 not the only factor in aging and it is doubtful whether it is safe 

 to assume that the accelerated aging tests will give results com- 

 parable with those carried out at room temperatures. 



