138 



THE INDIA RUBBER WORLD 



[December 1, 1919. 



apparenl in the 235-mimnc cure whiili ha> a slrcii.mh of 1^0 

 pouiuls and an elongation of only X>0 jicr cciii or 1 to -I'/i 

 inches. If superior physical characteristics were to be taken as 

 the vulcanization criteria, the 190-minute cure which shows the 

 greatest tensile product, or the 20S-minute which shows the 

 j,reatest resistance to stretching, would be selected. However, the 

 foriner has a vulcanization coefficient of 4.76, and the latter of 

 5.07, and the author is confident that any rubber man would 

 pronounce these pieces to be overvulcanized after making the 

 most superficial examination. Unfortunately, insufficient time 

 has elapsed since the original preparation of the samples to 

 publish the results of natural weathering tests at this time. But 

 accelerated aging test, if applied to an entire series such as 

 this, will show those members which are decidedly over or 

 undercurcd, and the results of physical and chemical tests made 

 upon specimen.^ so treated are. therefore, given in Table II : 



TABLE II. 



(leyree deteriorated quite rapidly. This same figure has been 

 indicated as the proper coefficient for a mixing of the nature, 

 in question by Spence, Kratz, Van Heurn and others. 



Following the preparation and testing of the series above 

 mentioned, the accelerators hexamethylcne tetramine and 

 dimethylammonium-dimelhyl-dithiocarbamate (the . addition 

 product of dimethylaminc and carbon bisulphide) were separately 

 added to the original mixing, namely: smoked sheet 100 parts, 

 sulphur 6 parts, zinc oxide 1 part, in the proportion of Vz of 1 

 per cent to the rubber contained in the batch. The former was 

 selected as a moderately active accelerator and the other as an 

 example of a group which probably comprises the most active 

 accelerators known and, therefore, one of the most valual)le for 

 use in illustrating the extreme possible effects of accelerators 

 on the mechanical and chemical properties of the resulting vul- 

 canizates. This substance has been mentioned in the literature 

 of rubber chemistry and should be a familiar one (see "Chemi- 

 cal .\bstracts," January 10, 1918). The results in Table III 

 .show the effect of hexametbylenc tclraminc on the mixing. 



TABLE ni. 



1673 

 1943 

 2283 

 1807 



It is seen by the accelerated age test, that the sample cured 

 130 minutes, having original vulcanization coefficient of 2.85 

 (Table I), is the best of the series. It will be noted that this 

 cure yields sample having a maximum strength, tensile product 



It is apparent from these figures that the compound contain- 

 ing this substance not only cures in about one-third the time 

 required for vulcanization of the untreated rubber in the same 

 formula, but the relationship of mechanical properties to chem- 

 ical composition is altogether altered. If we refer the physical 

 properties to the vulcanization coefficient as standard, by com- 

 paring the SO-minute cure with coefficient of 2.85 with" the 130- 

 minute "correct" cure of the untreated smoked sheet with 

 coefficient of 2.85, it is seen that the "hexa" stock has 1,044 pounds 

 per square inch greater tensile strength, one-half inch less 

 stretch, a tensile product higher by 824, and that more than 

 twice the load is required to stretch this stock to a given elonga- 

 tion. Making allowance for the toughening effect of the accel- 

 erator, the "hexa" compound is somewhat overvulcanized at 

 the 50-minute cure. 



In Table IV are indicated llie results obtained by using the 

 dimelhylamine addition product at a vulcanizing temperature of 

 292 degrees F. 



Fig. 1. Stress-Strain Curves. 



and the greatest resistance to stretching after the accelerated 

 aging treatment. The selection of this as the best cured 

 sample is well in accordance with the conclusions of Stevens in 

 a paper read before the Society of Chemical Industry. In this 

 it was shown that first-grade plantation rubbers vulcanized to a 

 si'lphur combination of about three stood up well during an 

 aging period of 120 weeks, whereas those cured to a greater 



TABLE IV. 



ADDITION Vi 



DIMETHYLAMMO- 



URS AFTER 



1680 

 2340 

 2418 

 2691 

 2698 



655 



1365 



1675 

 2207 

 2398 

 2508 



In this instance it is seen that samples having satisfactory 

 physical properties were obtained in vulcanizing periods of 

 three, four and five minutes with the abnormally low coefficients 

 of 1.09, 1.21 and 1.45, respectively. The same mixture was also- 

 vulcanized at 295 degrees F., the results being given in Table V: 



