INDUSTRIAL A N D ENGINEERING CHEflISTRY
512
Vol. 15, No. 5
Effect of Variations in t h e Sulfur and Hexamethylenetetramine Content on Properties of Compounded Rubber' By Harlan A. Depew T H ENEWJERSEYZINC Co., PALMERTON, PA.
FEW YEARS ago most tire treads were compounded with not less than 6 parts of sulfur to 100 of rubber. At the present time probably the majority of the tires contain less than 6 per cent calculated on the rubber. One well-known tire on the market has 3 parts of sulfur added to 100 parts of rubber. Although there is a great deal of technical knowledge regarding the effect of reducing the percentage of sulfur in rubber compounds, an examination of the literature shows that very little has been published. Recent interest in this subject has led to the presentation of-this report from filed data, which were collected in the early summer of 1920, together with recently determined tensile tests on the balance of the original slabs showing the effect of 25 mo. storage,
A
rearranged to compare the compounds under maximal area conditions. The interval between curves is a trifle large. For instance, the optimum cure of Compound A, is probably about 40 instead of 30 min. An examination of Compounds A, B, C, and D shows that the tensile strength, elongation, and maximal area increase with a decrease of sulfur content. The shapes of the stressstrain curves, as shown by the load at 300 per cent and 450 per cent elongation, are not appreciably affected by the decrease in sulfur content. I n a general way there is also an indication that the physical properties are higher with increase of accelerator content. Compound H, with the highest accelerator and lowest sulfur content, has the highest physical properties.
TABLEI-COMPOUNDSINVESTIGATED A B C 920 920 920 92 55 46 15 15 15 560 560 560 34 34 34 10 6 5 1.6 1.6 1.6
First latex pale crepe.. ............................................ Sulfur ........................................................... Hexa .............................................................
................................................ ................................................... ........................................ ........................................
XX red zinc oxide Carbon black., Per cent sulfur o n rubber.. Per cent "hexa" on rubber.
The compound chosen consisted of 100 volumes of rubber, 10 volumes of XX red zinc oxide, 2 volumes of carbon black to color the compound black, and the curing agents. It was desirable t o have a black compound in order to be able to observe the bloom more easily. The sulfur content was varied from 10 to 4 per cent by weight and the accelerator content from l / Z to 2.2 per cent by weight. The tensile strength and elongation were determined on an autographic Olsen navy testing machine. The load and area figures were taken directly from the curve sheets. The samples were allowed to stand for 2 wks. and then the degree of bloom was observed. A heavy bloom was chosen as 100 and no bloom as 0. The intermediate values are attempts t o evaluate the relative degree of bloom. Free sulfurs were determined only on the 75-min. cure. Where it is given for other cures it is estimated on the basis that the curve for rubber and sulfur combination is a straight line until most of the sulfur is combined. The results show that the stocks became nonblooming with free-sulfur contents (calculated to 100 of rubber) of 1.1, 1.4, 1.0, 0.8, 1.2, which average 1.1. A heavy bloom results with 2.0 per cent free sulfur. Therefore, to produce a nonblooming stock, the total sulfur added calculated on the rubber should not exceed the desired vulcanization coefficient plus one. For example, a nonblooming stock with a vulcanization coefficient of 2.2 should not contain over 3.2 per cent of total sulfur (This does not take into consideration such substances as shoddy, which inhibit blooming to a considerable degree.) Considering the maximal area under the stress-strain curve to be a criterion of optimum cure, the data have been 1 Presented
before the Division of Rubber Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922.
D
E
F
920 37 15
920 55 6
920
560
660
34 4
84 6
1.6
0.5
55 10 560
G 920 46 20 560
34
34
6
5 2.2
1.1
H 920 37 20 560 34 4 2.2
It has been noted by Cranor2 and others that the vulmnization coefficient a t the optimum cure depends considerably upon the accelerator used. The results in Table I11 indicate that the vulcanization coefficient a t the optimum cure is also influenced to a considerable extent by the sulfur content. With 6 per cent of sulfur the vulcanization coefficient is about 2.6 to 2.9, but on reducing the sulfur content to 4 per cent the vulcanization coefficient is 2.0 to 2.2 a t the optimum cure.
Area FIG.1-RELATION BETWEEN THE AREAUNDER THE STRESS-STRAIN CURVE AFTER 25 MONTHS'STORAGE AND THE INITIAL VULCANIZATION COEFFICIENT. COMPARISONS MADEAT THE OPTIXUMCURE
The results of the tests on the samples which had been stored check the statement by Geer3 and Evans that over* Zndza Rubber J . , 58 (1919),1201. 8
India Rubber Wovld, 64 (1921f,887.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
May, 1922 Time of Cure a t Sample
A
B
C
D
E
F
G
H
141 C. Mxn.
30 45 60 75 90 105 120 30 45 60 75 90 105 120 30 45 60 75 90 105 120 30 45 60 75 90 105 120 30 45 60 75 90 105 120 30 45 60 75 90 105 120 30 45 60 75 90 105 120 30 45 60 75 90 105 120
Tensile Strength Lbs./Sq. % ElonIn. gation
3132 3037 1813 896 807 546 538 2845 3110 3297 3103 2592 1618 1288 2892 3302 3483 3519 3310 2935 3023 2518 2757 3387 3323 3270 2843 2912 1947 2310 2735 2868 2937 2337 2988 2200 2915 3228 3200 1877 1488 1233 2832 3257 3300 3057 2815 1825 1822 2680 3182 3493 3232 3117 2757 2493
686 629 488 315 261 160 154 714 698 665 627 573 478 374 733 689 673 656 619 598 593 717 688 696 665 646 617 585 690 699 697 677 660 620 659 681 702 691 651 532 465 413 701 673 635 613 586 492 486 708 696 678 653 618 588 569
-
nounrl
A B C D
6.5 5.9 3.4 1.2
%:z. $80
540 728 795
0.8
0.4 0.3 6.1 6.1 6.5 6.0 5.4 3.4 2.2 6.1 6.6 6.8 6.4 6.2 6.5 5.4 5.4 5.8 7.3 7.2 6.6 5.9 5.2 3.8 5.0 5.6 5.8 6.1 6.0 6.2 4.0 6.0 7.0 6.5 3.6 2.7 2.2 6.2 6.9 6.3 6.2 5.7 3.5 3.5 5.8 6.3 7.6 6.9 7.3 6.5 5.3
... 307 373 484 658 680 722 793 255 360 890 508 603 650 832 262 340 437 513 538 553 590 216 280 323 356 438 493 528 235 344 455 535 578 662 725 362 455 545 625 700 705 710 294 373 468 548 585 620 623
940 1185 1455
...
... ... ...
100 100 100 100 100 100 70
760 820 1070 1505 1405 1490
80
...
0 0
600 945 1015 1190 1350 1350 1310 620 855 994 1100 1230 1475 1275
80 60 40 20
600
100 100 100 100 100 100 100 100 100 100 100 80 40 0 60 50 30 10
865 802 920 1030 1110 1100 565 795 980 1150 1178 1155 1425 820 1065 1220 1350 1425 1360 1468 705 895 1100 1240 1240 1260 1385
80 70 60 10
0 0 0
60 40 30 10 0 0 0
0
0 0 50 40 10 0
0 0 0
E
F G H
Time of Tensile LoAD-LBs./SQ. I N . Maximal Cure, Min. Strength % Elon300% 450% Area 40 Lbs. Lbs./Sa. In. gation Elong. Elong. ~~~
6.5 6.5 6.8 7.3 6.1(?) 7.0 6.9 7.6
313i 3297 3483 3387 2937 3228 3257 3493
--
TABLE 11-RESULTS OF THE EXAMINATION OF THE VULCANIZED RUBBER % Free AFTER25 MONTHS’ STORAGE Area unSulfur Estimated Tensile Loaddera Calcd. to Free Sul- Strength Lbs./Sq. Stress- Load-Lbs./Sq. In. 100 of Vulc. fur t o 100 Lbs./Sq. % ’ ElonIn. 300 450% $7 Free Strain Rubber Coef. Rubber In. gation Arean % Elong. Elong. Bloom &fur Curve
... ...
... ...
... ... ... ... ...
4.97
... 2.82
... 1.48 ... ... ...
... ...
... 1.12 ... ... ...
... ... ...
0.87
... ...
... ... ... ...
1.97
... ... ... ... ... ... 1.70 ... ... ... ... ... ...
0.88
... ... ...
... ...
... 0.71
... ... ...
...
... ... ... ... ...
2:bb
... ...
... ... ... ... 1.92 ... ...
...
... ... ... 1.48 ... ... ... ... ...
... 3.38 ... ...
...
... ... 2:ii ... ... ... ... ... ...
1.51
... ... ...
... ...
... 1.21 ... ...
...
... ...
... 5.0 ... ... ...
... ... ... 3.5 ... ... ...
... ... ... 3.1 ... ... ... ... ...
... 2.5
... ... ...
... ... .*.
2.6
... ... ...
... ... ...
3.1
... ... ... ... ... ...
3.5
... ... ... ... ...
2:i
... ...
... ...
TABLE 111-PROPERTIESAT COMPARABLE CURES Com-
513
-
686 665 673 696 660 691 673 678
’
380 484 390 437 438 455 455 468
940 1070 1015 994 1030 980 1065 1100
cure is the most important factor in the rapid aging of rubber. The tests also show that compounds with low sulfur contents are less affected by overcure than those with higher sulfur contents. It might be more accurate to say that low sulfur compounds overcure less easily. At the optimum cure neither the amount of hexamethylenetetramine nor the amount of sulfur, within the limits of this investigation, greatly affect the aging deterioration. However, there is a rough parallelism between aging, deterioration, and vulcanization coefficients, and it has been noted that low sulfur compounds reach their optimum cure a t comparatively low vulcanization coefficients, which would indicate a superiority in aging qualities of low sulfur stocks The stress-strain curves are Rtiffened in nearly all cases on aging, as shown by the lead at 300 per cent elongation. The extent of the stiffening is approximately the same for all the compounds.
... ... ... ... ...
2.0
... ... ... ...
2875 2530 650 700 595 555 370 2865 2755 2625
625 518 165 166 120 78 65 665 630 575
6.1 2.3 0.4 0.5 0.2 0.2 0.2 6.1 5.6 3.9
375 440 530 3120 3000 2945 1555 845 720 545 2615 2910 2895 1555 1065 680 580 2200 2675 2710 1800 1820 593 465 2750 2990 2620 1500 610 565 450 2740 2675 2120 820 745 480 460 2500 2570 2450 1940 1015 630 545
125 116 125 695 625 575 400 250 190 135 680 655 585 425 330 194 165 705 675 625 496 485 225 160 715 660 590 450 195 170 125 695 640 505 245 215 120 1.15 710 615 555 490 325 185 145
0.3 0.3 0.4 6.2 6.0 5.8 2.6 1.0 0.6 0.3 6.2 6.3 5.4
...
... ... ... ... ... ... 1.4 ... ...
1.1
... ... ... ...
1.0
... ... ... ...
... ... ... ... 1.8
... ... ... ... ... ...
1.0
... ... ... ... ...
0.8
...
... ...
... 1.2 ... ... ...
...
520 865
... ... ... ...
420 535 665
... ... ... ...
...
...
... ...
2.7
1.5 0.6 0.4 4.1 5.2 5.8 3.3 3.4 0.5 0.4 6.3 6.3 5.1 1.2 0.6 0.2 0.2 5.6 6.1 3.9 0.8 0.7 0.2 0.2 6.3 5.3 5.1 3.8 1.9 0.6 0.4
... ... ... ...
... ... ...
... ... ... ... 390 495 715 798 845
--
-
AFTER 25 Mo. STORAGE Tensile Load-Lbs.1 Estimated S .In. Vulcaniza- % ’ Sulfur yo “Hexa” Strength % Elontion Coef. on Rubber on Rubber Lbs./Sq. In. gation Area 3009 Elong.
2.0 2.8 2.5 2.0 3.1 2.5 2.1 2.2
10 6 5
1.6 1.6 1.6 1.6 0.5 1.1 2.2 2.2
~
2450
6.1 3.9 5.8 5.4 3.4 5.1 6.1 5.1
520
Fellowship for S t u d y of Soap as a Detergent The Palmolive Company, of Milwaukee, announces the establishment of a fellowship for the study of the fundamental principles connected with the detergent action of soap. The fellowship carries an annual stipend of $2000. The fellow will have the privilege of pursuing his studies at any institution in the country which is properly equipped for this purpose. The candidate must possess the equivalent of a master’s degree. The fellowship will be awarded by a committee consisting of the following: W. D. BANCROBT, Professor of Chemistry, Cornel1 University E. C. FRANKLIN, President, AMERICAN CHEMICAL SOCIETY H. N. HOLMES,Chairman, Colloid Division, National Research Council VICTORLENHER, Professor of Chemistry, University of Wisconsin J. C. SELLMER, representing Palmolive Company
Application should be made to Victor Lenher, Chairman, Palmolive Fellowship Committee, P. 0. Box 281, Milwaukee, Wis.
