194 



THE INDIA RUBBER WORLD 



U' 



bination of the latter fractions of tlie sulphur undoubtedly im- 

 pairs the quality of the final product. 



Under certain conditions we have found it possible to obtain 

 a uniform rale of vulcanization for a rubber-sulphur mixture 

 by employing a previously calculated series of increasing tem- 

 peratures such tliat the vulcanization-time curve is reduced to a 

 straight line. 



By so doing we have shown that at comparatively high sul- 

 phur coefficients a better product is obtained by vulcanization 

 with a series of increasing temperatures than with a constant 

 temperature. We have also found the vulcanization coefficients 

 previously recommended by others to be excessive, and that, 

 unless the history of the vulcanization phenomenon is fully 

 known, it is unsafe to judge samples solely on the basis of their 

 sulphur content. 



The mixture used by the authors in their experiments con- 

 sisted of the following proportions by weight: 



First latex plantation crepe 100 



Zinc oxide 100 



l^lS'yn,ine:;:::;:;;;::;::;;:;::::;;::;;::::::v. hz 



The rubber used was the best qualily typical of its variety 

 and was subjected to the minimum amount of milling necessary 

 to work in all of the ingredients. The basic amine (catalyst) 

 was ground to 200-mesh and was worked into the rubber be- 

 fore the pigment and sulphur were added. 



An investigation was made of the rate of vulcanization of 

 this mixture at a constant temperature of 298 degrees F. Our 

 results, which confirm those previously obtained by others, are 

 tabulated in Table I and are expressed graphically in Figure 1. 

 By this table and figure it is also shown that although 60 per 

 cent of the sulphur present in the mixture combined with the 

 rubber during the first two hours, less than 80 per cent had en- 

 tered into combination at the end of 4 hours. 



T.\BLE 1— Vulcanization at a Constant Temperature of 298 Degrees F. 



Time of Combined Tensile Permanent 



Vulcanization. Sulphur. Strength. Elongation. Set. 



Min. Per cent. Lbs. per Sq. In. Per cent. Per cent. 



10 0.404 



15 0.578 



20 0.663 ... 



25 0.840 .... ^... ■■■■ 



30 1.080 1527 /23 14.32 



40 1.330 1533 710 14 06 



50 1.490 1930 707 15.23 



60 1.690 2277 697 15.62 



70 1.875 2015 685 17.97 



80 2.170 2102 690 20.30 



90 2.485 2055 680 21.09 



120 2.945 2156 678 24.22 



150 3.410 2060 685 25.78 



180 3.600 1725 677 23.44 



210 3.780 155S 680 23.44 



240 3.920 1435 652 18.75 ^ 



It is seen in Table I that the "technical cure" for this mix- 

 ture is obtained in about 60 minutes at 298 degrees F. with a 



vulcanization coefficient of 1.69. Further, while the general 



Figure 1.— Vulcanization-time Curve at Constant Tem- 

 peratures. 



shape of the curve in Figure 1 was found to be convex to the 

 .r-axis, that portion up to and including this 60-minute point very 

 closely approximates the straight line OA drawn through this 

 point. Thus in the case of this particular mixture "technical 



l-'iGURE la — Temperature-Timb 



Curve for Vulcanization 



to Technical Cure. 



treating each temperature in the 



vulcanization" is attained before the active mass of the sulphur 

 present is decreased to an extent which would produce a marked 

 decrease in the rate of vulcanization. 



It has been shown above that at a coefficient of 1.69 our 

 method was not applicable for the differentiation of the small 

 variations in the quality of 

 the mixture, due to the 

 method of vulcanization _^^ 

 employed. For this reason 

 a comparison was made of j^^ 

 the physical characteristics 

 of the mixture when vulcan- ,3<; 

 ized to a coefficient of 3.9 at 

 constant temperature and hysoo 

 a series of increasing tem- 

 peratures. '"■' 



The temperatures and 

 times to be employed to ef-'"* 

 feet vulcanization by the 

 latter method may be read- 

 ily obtained by applying ^^^ 

 the data given in Table I 

 and Figures 1 and \a. ,^^ 

 First, the number of differ- 

 ent temperatures to be^.^j 

 used should be previously 

 decided upon. Second, the 

 number of minutes required 

 to effect a "technical cure" 

 at each of these tempera- 

 tures may be obtained 

 from Figure la. Th 

 series individually, let 



t = time in minutes required to effect a "technical cure" at 

 that temperature. (From Figure la.) 



t' = time in minutes required to effect a "technical cure" at 298 

 degrees F. (From Table I.) 



c =^ time in minutes required to attain the desired vulcaniza- 

 tion coefficient at 298 degrees F. (From Table I.) 



n ^= number of temperatures in the series. 



/ X — 

 Then, = T, where T equals the number of minutes re- 

 quired for vulcanization at a given temperature when employed 

 in the previously established series. 



It is quite obvious that, if temperatures are chosen at random 

 from the figure, the sum of the times for the complete series of 

 temperatures may not be the same as the time required to effect 

 the desired degree of vulcanization at a constant temperature of 

 298 degrees F. On the other hand, it is easily possible to make 

 a selection such that the total time of vulcanization is the same 

 by either method. In order that our results might be strictly 

 comparable, we chose the following series of four temperatures, 

 the sum of the times of which was exactly equal to 240 minutes, 

 the time required to obtain a coefficient of 3.9 at 298 degrees F. 



285.5 degrees F. for 107 minutes 



298.0 degrees F. for 60 minutes 



302.5 degrees F. for 43 minutes 



307.0 degrees F. for 30 minutes 



Total 240 minutes 



Employing the foregoing series of temperatures and times, 

 the mixture was vulcanized in a button mold for a total time of 

 240 minutes, samples being removed for combined sulphur esti- 

 mation at hourly intervals and at each change in the vulcanizing 

 temperature. These results are shown in the first four columns 

 of Table II and expressed graphically by the solid line in Figure 

 2. It is readily observed that the results obtained coincide al- 



