Decfmber 1. 1919] 



THE INDIA RUBBER WORLD 



HOURS AFTER 



In this series, the sample cured five minutes (coefficient 2.68) 

 closely approaches the 2.85 coefificicnt of the correctly cured 

 standard smoked sheet. Comparing the physical characteristics 

 of these two, we find a difference of 1,554 pounds tensile 

 strength in favor of the accelerated stock, a loss of 1.2 inches 

 in elongation at the breaking point, a tensile product higher by 

 1,021, and an even greater resistance to stretching than in the 

 case of the rubber containing hexamethylene tetramine. This 

 sample is undoubtedly much overvulcanized. 



Figure 1 shows the stress-strain curves of the three samples, 

 all taken from the same lot of rubber and all cured to vulcani- 

 zation coefficients of 2.68 to 2.85 by means of vulcanizing periods, 

 ranging from five minutes to two hours and ten minutes. It 

 afTords a graphic illustration of the variation in physical prop- 

 erties referred to essentially the same vulcanization coefficient. 



Figure 2, on the other hand, illustrates the range of the vul- 

 canization coefficients for the same samples, referred to the load 

 required to produce a uniform elongation of 700 per cent. By 

 reference to these curves, we find that about 4.9 per cent of 

 combined sulphur is necessary to produce a sample of un- 

 treated rubber requiring a load of 1,000 pounds to stretch it 

 through 700 per cent. In the case of the same rubber containing 

 an addition of ]/i of one per cent of "hexa," the same effect was 

 produced when about 1.8 per cent of sulphur has been combined. 

 With the introduction of the dimethylamine addition product, the 

 corresponding coefficient drops to 0.9 per cent. Figure 2 also 

 includes a curve representing the compound reported in Table 

 VI. which consists of the basic formula and 1 per cent of the 

 dimethylammonium-dimethyl-dithiocarbamate. 



TABLE VI. 



CoMPOl-\D .\. UinuED SMOKED SHEET, 6 PER CENT SULPHUR .AND 1 PER 



These samples were not heated at all, being mi.xed, calendered 

 and stored in the dark between sheets or holland cloth. The 

 tests shown were made one month and two months after prepara- 

 tion, and certainly show that spontaneous vulcanization has taken 

 place to a very considerable extent. With this mixing we find, 

 by extending the curve toward the zero point, that this unhealed 

 material would require the load of 1,000 pounds to stretch it to 

 700 per cent when only 0.7 per cent of sulphur has entered into 

 combination with the rubber. The general tendency is for the 

 curves to flatten toward the load axis as the period of heating 

 is reduced, and this gives some indication of the extent to 

 which rubber is depolymerized by heat and shows that ex- 

 tremely low sulphur combination is sufficient to vulcanize when 

 the period of heating is reduced to a minimum or entirely 



omitted. The turn of the curve representing the untreated rub- 

 ber in the neighborhood of the 5 per cent combined sulphur line 

 again illustrated the reversion which takes place when heating 

 is continued after the bulk of the sulphur has entered into com- 

 bination. The turn at the 3 per cent point, in the case of the 

 "hexa" sample is indicative of the great care necessary when 



Fig. 2. Range of Vulcaniz.\tion Coefficient. 



using this substance, since if the sulphur in the compound is 

 held down to a figure which is too low, results equally as dis- 

 astrous as those coming from overcure will inevitably follow the 

 overheating of such compounds. 



Work along very similar lines was also carried out with an 

 inferior type of brown plantation crepe, the results of which are 

 indicated in Tables VII and VIII: 



TABLE VII. 



TUTED FOR STANARD RIBBED SMOKED SHEET. SAMPLES TESTED 24 HOURS AFTER 



In this case, the figures reported are limited to the tests made 

 on the straight rubber and the same rubber to which was added 

 M of 1 per cent of the dimethylamine-carbon bisulphide addition 

 product. Preliminary tests on this rubber treated with hexame- 

 thylene tetramine showed that satisfactory vulcanization could 

 only be obtained with the addition of higher proportions of 

 zinc oxide. Figure 3 shows the complete stress-strain curves 



