April, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
(22) Pease and Durgan, Ibid.,52, 1262 (1930). (23) Podbielniak, Oil Gas J., 27, 38 (Jan. 17, 1929); and 29, 235 SOct. 2, 1930); IKD. EKG.CHEM.,Anal. Ed., 3, 177 (1931). (24) Rice, J. Am. Chenc. SOC.,53, 1959 (1931).
449
(25) Schneider and Frolich, Ibid.,23, 1403 (1931). (26) Shepherd and Porter, IND. EX. CHEM..,15, 1143 (1923)
RECEIVED September
22, 1932.
Low Sulfur Compounding A. A. SOMERVILLE AND W. F. RUSSELL, R. T. Yanderbilt Company, Inc., New York, N. Y. Since the beginning of the rubber industry, sul- and gas black stocks compounded with 3 per cent f u r dosages used for soft rubber have declined f r o m sulfur on the one hand, and 0.3 and 1 per cent (gas 20 to 2 or 3 per cent customary today. T h e de- black stock) on the other. cline is still under way, and present sulfur ratios Advantages of very low sulfur compounds are are high in comparison with what is actually re- less tendency to scorch, better physical properties, quired to produce satisfactory commercial vulcaniza- better aging, higher resilience and lower power tion-fractions of 1 per cent. losses, less discoloration. Vulcanization with extremely g r e a t e r resistance to hot sollow sulfur ratios, the use of $ vents, g r e a t e r a b r a s i o n re$le sistance, and less flex-cracking s e l e n i u m a n d tellurium as 2I on aging: disadvantages are auxiliary vulcanizing mater i d s , and the proper type of accelerahigher set at low femperatures 2a tion required to “cure in” all and reduced capacity io the sulfur and produce satisfaca d h e r e to m e t a l s . I t seems tory commercial r e s u l t s , are 2 p r o b a b l e Ihat, in t h e future, c o m p o u n d i n g will be based discussed. Comparative data, YEARS A D ‘Overing many physical FIGURE1. ConrMERCIALSULFUR largely on the use of very lous bufes, are shown f o r pure gum RarIos, P w , PRESEZT, ASD FUTLRE sulfur ratios.
S
IKCE Goodyear in 1839 discovered the change produced in rubber by heating it with sulfur, no better commercial vulcanizing agent than sulfur has been discovered, and the vast industry of today is based on this fundamental discovery. Other vulcanizing materials have been proposed but have not been commercially successful. Sulfur, in some form, remains the chief commercial vulcanizing agent for rubber either in the dry or met form.
HISTORICAL The proportion of sulfur (to rubber) used for technical vulcanization, however, has not remained constant. It was quickly found that sulfur, in addition t o the bencficial changes it produced, could also damage the rubber. The gains in elasticity, strength, and resistance t o freezing were not permanent but were rapidly lost as the vulcanized rubber changed on aging t o a brittle, resinous product. The proportions of sulfur necessary t o produce the most lasting improvements in the rubber became a question of prime imDortance to the developing i n d u s f6 3000 try. 210.9 T h e original v) \ patent awarded to i U f’ ’ 2000 G o o d y e a r ( 5 ) in 140.66 u) 1844 specified 20 e 0 per cent of sulfur on : 1000 70.3 the r u b b e r a s t h e e v) amount “to which w it is d e s i r a b l e to 2 0 0 approximate.’’ As 2- 30 60 90 I20 I50 100 the new i n d u s t r y P TIMEOFCURE:MIN.AT 220-F. ( l o r - c j FIGURE 2. RATEOF SCORCHING OF began t o grow, exPUREGUMSTOCKS perience showed
that the sulfur figure could be shaded a bit, and it probably soon dropped to about 15 per cent in highly efficient plant-. After the use of the roller mixing mill became more general, there followed a long period of empirical development during which almost every available material was tried in rubber to modify its properties and cheapen its cost. Lime and magnesia, as \yell as the lead compounds known t o Goodyear, were found to hasten the reaction with sulfur; and the use of these inorganic “catalysts” became universal. On the other hand, the world-wide search for new and cheaper sources of rubber popularized the African and other low-grade rubbers, the slow curing properties of which tended to offqet the accelerating effects of the inorganic accelerators. Thus there was probably little change in commercial sulfur ratios until about 1900. From 1900 to 1910 there appears to have been a gradual drop to about 10 per cent, and this was the amount generally used a t that time for testing the vulcanizing quality of raw rubber. At the time of the discovery of organic accelerators in 1906, sulfur ratios seem to have been within the range of 8 t o 12 per cent for p r o d u c t s such as tires and t u b e s in which sulfur bloom was not 2 considered objecf tionable. B o o t and shoe compounds, on the other hand, appear 2 t o have always 0 HRS 2 HRS 48HRS h a d considerably TlMEOF HEATINQ I N VACUO AT 10°C lower and FIGURE 3. PL.4STICITY CHANGES ON i n t h i s r e s p ect LOVGHEATING OF PUREGUMSTOCKS
$
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLEI. RATEOF SCORCHINQ OF PRESS-CURED PUREGUM STOCKS
450
C O M P O U NAD
PRESE C U R EAT 220” F. Min. 30 45 60 90 120 180
Stress a t 500% Tensile strength Lb./sq. in. Kg./sq. cm. Lb./sq. in. Kg./sq. cm. Slight cure Slight cure 95 6.7 695 49 185 13 1600 113 320 22 2330 164 395 28 3100 218 530 37 3250 229
have shown a much smaller change in the last twenty years. After 1910 the use of organic accelerators gradually became widespread, and by 1920 few plants were operating without them. Their use brought about a steady decline in sulfur ratios because lower sulfur plus the accelerators gave better physical quality than higher sulfur without them. Unless the sulfur ratio was reduced as well as the time of cure, the accelerators had too much tendency to cause overcuring and premature aging. By 1920 sulfur ratios in tires and tubes had declined to 4-6 per cent. The next decade saw greatly increased activity in the a p p l i c a t i o n of accelerators; new and 281 2 more active products, including the 210 0 I so-called ultra and U 5: semi-ultra s u l f u r 140 0 a b e a r i n g accelera: tors, were i n t r o 7030 L duced and rapidly became p o p u l a r . B y 1930 s u l f u r O 0 TIMEOFCURE’MINUTESAT P74.F (I34.C) ratios in the tire inFIGURE4. TENSILEPROPERTIES OF dustry had dropped PRESS-CURED PUREGUMSTOCKS t o a b o u t 3 p e r cent, and probably throughout the whole industry the average amount of sulfur used for vulcanization was not much above this figure. Since 1930 the tendency has been to still lower sulfur ratios. Figure 1 shows in graphical perspective (which must not be interpreted too literally) the downward trend of sulfur ratios since the beginning of the industry.
Ultimate elongation
% Slight cure 980 800 830 830 790
. -
Vol. 25, No. 4
-
C O M P O U NBD
Stress a t 500% Tensile strength Lb./sq. in. Ko./sq. cm. Lb./sq. in. Kg./sq. em. Uncured Uncured Uncured Uncured Slight cure Slight cure .175 .. .. 280 20 12 1210 85 405 28 2780 196
Ultimate elongation
% Uncured Uncured Slight cure 950 900 800
per cent when commercial compounds with considerably less than 1 per cent of sulfur are now coming into use. Therefore the term “low sulfur compounding” will be reserved here for the latest developments in the art, which deal with fractions of 1 per cent of sulfur on the rubber as compared with 2 or 3 per cent commonly employed. L O W SULFUR COMPOUNDIXG
Bruni (3) in 1921 showed that rubber could be vulcanized with thiuram disulfides in the absence of added sulfur; since then it has been generally known that satisfactory commercial vulcanization can be accomplished with 3 to 4 per cent of its weight of tetramethylthiuram disulfide and no other vulcanizing agent. This accelerator is believed to have one atom of sulfur available for vulcanization, so that a vulcanizate obtained from the proportions mentioned cannot have more than about 0.5 per cent of combined sulfur. Bongs a n d Blake ( 2 ) have stated that approximately 0.5 2100 . per c e n t of com3 bined s u l f u r is all 140 that is needed for L the f o r m a t i o n of D pure soft rubber and 70 3 have suggested for this the f o r m u l a 0 7 14 PI 0 5 IO 0 (CsH&aSs. Rubber DAYS O F AGING HRS O F A Q I N G vulcanized with 3 FIGURE 6 . AGINGOF PUREGUM STOCKS to 4 per c e n t of tetramethylthiuram disulfide is somewhat softer than ordinary sulfur-vulcanized rubber, but it far surpasses the latter in stability-i. e., resistance to age and heat deterioration. WHAT IS LOW SULFUR? As an outcome of work done by others with tetramethylThe proportion of sulfur commonly used today is around 2 or 3 per cent on the rubber; this is frequently called (‘low thiuram disulfide as a vulcanizing agent in the production sulfur,” which it is when compared with what was customary Of heat-resisting stocks and more recently as a result of a t e n y e a r s a g o . study of the theory proposed by Boggs and Blake (2), this a There is also a tend- laboratory began several years ago the intensive study of ency in the indus- very low sulfur ratios in conjunction with suitable com2812; try to r e g a r d this binations of accelerators and vulcanizing agents. The idea 4 amount of sulfur as of trying to duplicate rubber cured with thiuram disulfide 21o0,Y about t h e lowest by rubber cured with sulfur to a coefficient of 0.5 had not limit for s a t i s f a c - been thought of commercially, owing to technical difficulties 5: 1 4 0 e E t o r y c o m m e r c i a l of acceleration and control of the amount of sulfur combining. vulcanization. The first attempts made in this laboratory to produce 7o The most recent commercial compounds with added sulfur approximately ” developments shorn equivalent to that called for by Boggs’ coefficient of vulsI that not o n l y can canization-i. e., 0.5 per cent on the rubber-were carried 0 complete \,ulcaniza- on in 1930. They were based on the principle of using the TIMEOFCURE MINUTES AT 214.F (134.C) FIGURE 5 . TENSILEPROPERTIES OF t i o n be a c c o m - exact amount of sulfur and “driving it home” with suitable PUREGUM STOCKS CUREDIN DRY p l i s h e d w i t h far high-powered accelerators. The experiments proved that HEAT lower s u l f u r per- the kind and amount of accelerators employed were of first centages than those importance for successful vulcanization by this method. now in use, but also that the products possess special charac- These have to be able to pass the small amount of sulfur teristics and qualities which it is impossible to obtain with on to the rubber and not exhaust it by reaction with themthe higher sulfur ratios. The latest sulfur ratios are prac- selves as many other accelerators appear to do. The sulfur-bearing accelerators, such as mercaptobenzotically as far below those currently used as the latter are below those of twenty years ago. Hence it is confusing thiazole, benzothiazyl disulfide. tetramethylthiuram disulfide, to use the term “low sulfur” for compounds having 2 or 3 and combinations of these, were found, in reasonable amount,
4
(I
April, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
to produce satisfactory vulcanization with 0.5 per cent, and even less, of sulfur. The vulcanizates differed outstandingly from those customarily obtained with 2 or 3 per cent of added sulfur in their enhanced resistance t o aging in the oxygen bomb. The resistglooo ance to aging diminished -m as the sulfur ratio was in:800 creased. h I When sulfur-bearing accelerators are thus used in 2 600 b. p r o p e r combinations, the W resistance of the cured com5pound to aging is improved V as the sulfur is lowered and z w 32 0 0 -\$[, the accelerator increased; ' ' -I when the sulfur a p p r o x i g
~--LJ----L~A'
451
gives the compound a flatter curing range, and contributes materially to the physical quality. Tellurium ( 4 ) was subsequently found to act in these low sulfur compounds similarly t o selenium except that it is less reactive. Because of this and of its higher atomic weight, more of it may be used (up t o about 1 per cent). Tellurium is particularly advantageous in white or delicately colored articles because it has practically no discoloring effect when the rubber is exposed to light. Like selenium, tellurium tends to prevent reversion of low sulfur compounds. Without sulfur and accelerator, however, it has little if any v u l c a n i z i n g Iaction. w 80
ACCELERATIOX a The modern s u l f u r - b e a r i n g accelerators are most suitable for u s e i n t h e lorn s u l f u r field of 4f 4 0 compounding. Of t h e s e , m e r captobenzothiazole, together with a smaller amount of tetramethylthiuram disulfide, has given the best results. About 1 per cent of O30 40 50 60 70 80 90 the former and from 0.25 to 0.5 T'MEoFCURE: 'INUTESAT 274.F (134OC) per cent of the latter generally F~~~~~8. poWERL~~~ produce excellent r e s u l t s . The A N D RESILIENCE OF PURE GUMSTOCKS t h i u r a m disulfide, if present in small a m o u n t s (up to 0.25 per cent), functions chiefly as an activator for the primary accelerator; when used in larger amounts, its vulcanizing action undoubtedly also plays a part in the reaction. Benzothiazyl disulfide produces results similar t o those of mercaptobenzothiazole, but with a slower curing rate. The amount of acceleration indicated is usually enough for practically any type of compound with a sulfur ratio as low as 0.3 per cent and selenium ratio up to 0.5 per cent. These proportions may be varied within narrow limits to suit different purposes. Gas black, however, has a retarding action so that with high gas black compounds it is necessary to raise the percentage of sulfur to 1 or 1.2 per cent; this amount is still low in comparison with the ratios commonly used for gas black stocks (3 to 4 per cent). In the following series of tests a comprehensive c o m p a r i s o n is made between two typical sulfur stocks containing 3 per cent sulfur and the s a m e s t o c k s compounded on the new lorn sulfur basis. The first two compounds compared are not loaded except CURE MINUTES AT 2 7 4 O F for 10 per cent of zinc oxide, FIGURE 9. ENERGY Loss and may be designated as pure ON I~~~~~OF pURE G~~ STOCKS gum; the second pair are typical gas black stocks. IN A R B I T R A R Y UNITS
SELESIUVAND TELLURIUM These elements resemble sulfur in being able to combine with rubber under certain conditions, but more slowly than sulfur. They differ from sulfur in their inability to form hard rubber. According to Blake ( I ) , the maximum coefficient of vulcanization obtainable with selenium is about 5.1 (corresponding to a sulfur coefficient of 2.04). According to Boggs and Blake (a),all combined sulfur in excess of 0.5 per cent is detrimental to soft rubber, causing it to age more rapidly. It seemed reasonable, therefore, to try using very small quantities of selenium together with extremely low sulfur for the purpose of improving the tensile properties of the low sulfur compounds without affecting their high resistance to artificial aging. Selenium, furthermore, had previously been found useful for improving the tensile quality of rubber cured with thiuram disulfide without greatly affecting its age and heat resistance. After considerable experimenting, compounds containing small amounts of sulfur and selenium in combined proportions equivalent to approximately 0.5 per cent of sulfur were developed and studied. These compounds showed high physical quality as well as remarkable resistance to aging in both the oxygen and air bombs. It has been found impossible to duplicate their age resistance by using higher amounts of sulfur even with the best aging accelerators. The stability of these compounds, however, is not as great as that of rubber cured with 3 or 4 per cent of tetramethylthiuram disulfide alone. Indeed, it has not yet been found possible to equal this in any compound vulcanized with elementary sulfur. The use of selenium, and later tellurium, as a secondary vulcanizing agent was thus found to be essential to the quality of the low sulfur compounds. It tends to prevent the softening (reversion) of the rubber on prolonged heating,
PUREGulLr STOCKS The formulas for the two pure gum compounds are as follo~vs: Rubber Stearic acid Agerite powder (phenyl-@-naphthylamine) Zinc oxide Captax (mercaptobeneothiaeole) T u a d s (tetramethylthiuram disulfide) Vandex (selenium) Sulfur
-4
B
100
100
1 1
10
1
...
...
3
1 1 10 1 0.5 0.5
0.3
Since it is generally agreed that a good antioxidant is necessary to produce a stock having superior aging qualities, phenyl-@-naphthylamine was used in these compounds. RATEOF SCORCHISG. Figure 2 and Table I show the
INDUSTRIAL AND ENGINEERING CHEMISTRY
452
Vol. 25, No. 4
show the comparative resistance of the two compounds to oxygen-bomb and air-bomb aging. Stock B has far better aging resistance in the oxygen and air bombs than stock A, although A is regarded as a good aging compound in commercial practice.
results of curing the two compounds in a press a t a steam pressure of 2.5 pounds-i. e., a temperature of 220" F. (104" C.), The results in Figure 3 were obtained by heating the uncured stocks in a vacuum drier a t 158" F. (70" C.). and determining the decrease of plasticity with time by means of a Williams plastometer. Both methods prove the low sulfur stock to be markedly less scorchy than the other. This is an advantage from a factory point of view since it means easier mixing and better processing.
RESISTANCE TO LIGHT DISCOLORATION
AND CRACKING.
White stocks were exposed to an arc light in the Fade-0-Meter for 12 hours. Compounds G and H correspond to A and B, respectively; they are colored white with 25 per cent of titanium dioxide and contain no antioxidant. Compound CHANGES ON LONGHEATING OF PTJRE H1 is the same as H except that it has 0.5 per cent of telTABLE 11. PLASTICITY GUMSTOCKS lurium instead of selenium, as follows: TIMEOF HEATINQ Hours 0 2 4s
-
C O M P O U RA -D RecovS t a t e of ery cure
Plasticity
1/100 mm.
1/100 mm.
387 618 847
26 331 233
Uncured Slight cure Cured
-COMPOUND PlasRecovticity ery 1/100 mm.
1/100 mm.
347 533 726
18 284 340
B-
S t a t e of cure
Rubber Stearic acid Zinc oxide Titanium dioxide Captax (mercaptobenzothiazole) Tuads (tetramethylthiuram disulfide) Vandex (selenium) Telloy (tellurium) Sulfur
Uncured Uncured Cured
TENSILE PROPERTIES AND RATEOF CURE. Figures 4 and 5 show the tensile properties and curing rates of compounds A and B as obtained from press cures a t 30 pounds of steam pressure [274" F. (134' C.)] and from dry heat cures a t the same temperature. Results are also given in Tables I11 and IV. RESISTANCE TO AGING. Figure 6 and Tables V and VI
CURINQ
COMPOUND A
Stress at 500% Lb./sq. in. K Q . / E em. ~.
PRESSCURE Min. 10 20 30 45 60 90
155 325 390 440 475 415
11 23 27 31 33 29
3 5 10 15 20 30
175 345 390 455 480 475
12 24 27 32 34 33
H 100 1 10 25 1 0.5 0.5
... ... ...
... 0.3
3
H1 100 1 10
25 1 0.5
...
0.5 0.3
The high sulfur stock G developed a brown color over the exposed area; stocks H and H 1 showed much fainter discoloration, that of H1 being scarcely noticeable. On prolonged exposure to sunlight under tension, the surface cracks on stock G were larger and deeper than those on H and H1.
OF PTJRE GUMSTOCKS AT Two TABLE111. TENSILEPROPERTIES
-
G 100 1 10 25 1
Ultimate Tensile strength elongation Stress a t 500% Lb./sq. in. Ke./sq. cm. % Lb./sq. in. Kg./sq. em. TEMPERATURE, 274' F. (134' C.) 151 925 110 8 2150 425 30 3180 224 865 590 41 3480 245 855 690 49 3570 251 845 755 53 3510 247 835. 665 46 3160 222 765 TEMPERATURE, 298' P. (148' C.) 9 129 950 125 1830 32 188 850 450 2670 44 208 835 630 2960 54 236 840 765 3360 238 820 800 56 3390 53 243 830 755 3450
TEMPERATTJRES COMPOUND B Tensile strength Lb./sq. in. Kg./sq. cm.
Ultimate elongation
%
1205 3360 3560 3650 3580 3240
85 236 250 257 252 228
900 820 750 720 700 730
675 3290 3600 3600 3620 3440
47 231 253 253 255 242
850 800 750 710 700 700
PROPERTIES OF PCREGUMSTOCKS CUREDIN DRYHEAT TABLEIV. TENSILE
- -
COMPOECND A DRY-HEATCERW' Stress at 500% Tensile strength Min. Lb./so. . _ in. Ka./sa. em. Lb./sa. in. Ka./so. cm. 23 2270 160 10 325 190 31 2700 20 440 195 32 2780 30 450 190 33 2700 45 470 170 33 2420 60 470 138 37 1960 90 450 5 30 pounds air pressure; 45-minute rise t o 274' F. (134' C.).
Ultimate elongation
% 900 780 790 800 710 670
-
COMPOUND B-
Stress a t 500% Lb./sq. in. Kg./sq. cm. 125 9 575 40 575 40 650 37 690 49 700 49
Tensile strength Lb./sq. in. Kg./sq. cm. 1300 91 2 16 3070 3150 22 1 3000 211 190 2700 2230 157
Ultimate elongation
% 990 730 730 740 650 640
IN OXYGENBOMB TABLEV. AGING OF PUREGUMSTOCKS [At 300-pound pressure a n d 158' F. (70' C . )I
COMPOEND A
c
PRESS CERE AT 274' F. Min.
Stress a t 500% Lb./sq. in. Kg./sq. cm.
Tensile strength Lb./sq. in. Kg./sq. em.
Ultimate elongation
%
. -
COMPOUND B
Stress at 500% Lb./sq. in. Kg./sq. cm.
Tensile strength Lb./sq. in. Kg./sq. cm.
Ultimate elongation
%
BEFORE AGINQ
30 45 60 90
390 440 475 415
27 31 33 29
3480 3570 3510 3 160
245 251 247 222
30 45 60 90
920 955 950 835
65 67 67 59
3180 3070 2470 2110
224 216 174 148
30 46 60
975 990 875 735
69 70 62 52
2600 2340 1700 1160
183 165 120 82
30
945 1000
66
70
2060 1430 280 225
145 101 20 16
855 845 835 765
590 690 755 655
41 49 53 46
3560 3650 3580 3240
250 257 252 228
750 720 700 730
980 1120 1155 1135
69 79 81 80
3160 3310 3220 3190
222 233 226 224
6.50
1280 1250 1170 1050
90 88 82 74
3390 3360 2760 2730
238 236 194 192
620 620 610 6 10
1110 1160 1030 965
78 81 72
2700 2860 2780 2400
190 201 195 169
610 610 615 620
A G E D 7 DAYS
690 680 650 650
640 630 640
A G E D 14 D A Y S
90
630 630 600 580 A G E D 2 1 DAYS
45
60 90
... ...
.. ..
6 10 555 4 10 3 00
68
I N D U S T R I A L A N D E N G I N E E R I N G C H E RI I S T R Y
April, 1933
TABLEVI. AQINGO F PGRE Guhr PRESS CURE AT
274' F.
Min.
S T O C K S IN
453
AIR BOMB
[ A t 80-pound pressure and 260' F. (127O C . ) ] c COMPOUND A-, COMPOUSD B Ultimate Stress a t 500% Tensile strength elongation Stress a t 500% Tensile strength Lb./sq. i n . Kg./sq. em. L b . / s q . in. Kg./sq. cm. % Lb./sq. in. Kg./sq. cm. Lb./sq. in. K g . / s q . cm.
Ultimate elongation
BEFORE AG1h-Q
30 45 60 90
390 440 4i5 415
27 31 33 29
3480 3570 3510 3160
245 251 247 222
30 45 60 90
710 705 730 635
50 50 51 45
2230 2190 2060 1840
157 154 145 129
... ...
.. ..
775
54
675 885 885 1275
47 62 62 90
180 180 180 180
13 13 13 13
855 845 835 765
590 690 755 655
41 49 53 46
3560 3650 3580 3240
250 257 252 228
750 720 7 00 730
785 765 735 765
55 54 52 54
2480 2480 2670 3100
174 174 188 218
640 650 650 685
705 625 675 630
50 44 47 44
1940 1920 1950 2250
137 136 138 158
620 630 650 660
530 500 450 440
37 35 32 31
1150 1440 1310 1290
81 101 92 91
610 640 640 650
.4GED 3 H O U R S
30 45 60 90
...
..
680 680 665 665 A G E D 5 HOURS
260 430 460 580 h G E D 10 H O U R S
30 45 60 90
SWELLING IS HOT SOLVESTS.Figure 7 and Table S'II indicate slightly greater resistance to swelling for the B stock than for A in lard and fuel oil a t 158" F. (70" (2.). b u t m u c h I5 greater resistance in fuel Ioil a t 212" F. (100" C.). 10 Since specifications for z W oil-resisting compounds
260 240 230 230
TABLEIx. ESERGYLOSS PRESSC U R E &T
274' F
Min. 30 45 60 90
O S IMPACT OF
PURE GUM STOCKS
ENERGY Loss
Compound A
Compound B
%
%
22,25
19 17.75 16.25 17.5
22 22.5 2
EFFECT OF SULFUR RATIOSo x SET. Figure 10 shows the set obtained on a low sulfur compound by stretching it arbitrarily to 300 per cent elongation at room temperature 8 5 L are becoming more and and also a t 32" F. 0 more rigid as r e g a r d s (0°C.). The effect 05 I 15 2 25 3 both time and temperaP E R C E N T SULFUR of gradually increasFIGUREIO. EFFECTOF SULFUR t u r e of immersion, the ing the sulfur is also RATIOSON SET OF PUREGUM lorn, s u l f u r m e t h o d of 9 shown, the tests beSTOCKS c om p o u n d i n g o f f e r s nlo92 ing made on t h e d is t i n c t a d v a n t a g e s S o p t i m u m c u r e in for this class of goods. each case. At the f r e e z i n g temperaTABLEVII. P W E L L I S G O F PURE GUMSTCICKS IX ture there is a disHOT SOLVENTS tinct increase in the (Percentage volume increase after 7 days' immersion) P R E S 6 C U R E -COMPOUXD %. --cOMPOrlvD Bset with sulfur perAT 274' F. Larda Fuel oila Fuel oilb Lard" F u r l oila Fuel oilb FIGURE 11. TENSILE PROPERTIES OF centages below one Mm. GASB L ~ C STOCKS K 30 162 395 This is undoubtedly 945 ?30 435 140 45 145 394 940 123 604 377 a disadvantage in 60 147 387 835 121 290 368 90 157 401 890 120 302 367 extremely low sulfur compounds, which as yet has not been 0 At 158' F. (70' C.). overcome. b At 212' F. (100' C.). Since there is no known laboratory test for measuring the effect of low temperatures that gives results which can be POWER LOSS- 4 3 RESILIENCE. ~ Figure 8 and Table present the results of resilience tests made with compounds correlated with service tests, it is difficult to determine how A and B on the Dunlop Pendulum Hysteresis machine. serious a drawback the defect Stock B shows greater resilience and lower power loss than may be. Low s u l f u r comA over the range of cures tested. Similar results expressed pounds with 1 per cent of sulf ;do00 281 P as percentage energy loss, obtained by means of Lupke's fur are being regularly used z Impact Resiliometer, are given in Figure 9 and Table IX, commercially a t temperatures lower than 32" F. (0" C.) z3Ooo 2109; the B stock again showing the smaller power loss. m C and from commercial experi: I40 6 f TABLE VIII. POWER Loss, RESILIESCE, AND INDENTATION OF ence there appears t o be no 0 m PUREGUMSTOCKS reason t o fear bad results. $ low 70 3 -COJ~POUND A -COMPOUND B It is frequently necessary to ,. PREBSCURE Power ResiliIndentaPower ResiliIndentaalter the composition of varioo 0 ~ ~ 2 7 F. 4 ' loss ence tion loss ence tion 7 14 21 Arbzfrary Arbztraty ous manufactured articles to DAY3 OF AGING Min. unzts % untfs % s u i t climatic d i f f e r e n c e s . FIGURE12. ACINGOF G~~ 30 35.9 77.2 17.80 35.8 77.2 18.19 45 25.8 81.5 16.42 21.9 80.8 16.24 Rubber being no exception to BLACKSTOCKS IN OXYGEN 60 24.5 82.0 15.90 19.5 85.0 15.36 BOMB 90 25.3 81.2 16.04 19.6 this r u l e , i t would seem 85.0 15.45 reasonable that, when excesThe superior mechanical properties of stock B indicate sively low temperatures are t o be combated, the sulfur should that such compounds have distinct advantages for tire car- be set a t a somewhat higher level than for ordinary purposes. casses and other articles subjected to continuous flexing On the other hand, at extremely lorn temperatures such as in service. Lower power losses indicate reduced generation -20" F. (-29" C.), even compounds vulcanized with 3 per of heat. cent of sulfur stiffen and show an abnormally high set.
t40e5
-
p
I N D U S T R I A L A N D E N G I N E E R I N G C H E IM I S T R Y
454
Vol. 25, No. 4
TABLEX. TEXSILEPROPERTIES OF GAS BLACK STOCKS COMPOCND
PRESSCURE AT
274' Min. 10 20 30 45 60 90
F.
Stresa a t 300% L b . / s q . in. K g . / s q . c m . 320 22 53 755 70 1000 1235 87 1360 95 1545 109
c
Ultimate elongation
% 610 600 605 600 595 575
Stress a t 300% Lb./sq. in. Kg./sp. cm. 535 38 1205 85 1520 107 1760 123 1780 125 1920 135
-
D
COhlPOCXD
Tensile strength Lb./sq. in. KQ./sq. cm. 101 1430 2920 205 264 3760 297 4220 305 4340 299 4260
Tensile strength Lb./sq. in. KQ./sQ.. cm. 2380 1i 7 4250 299 4575 322 4460 314 4440 312 4400 309
Ultimate elongation % 610 625 585 545 530 505
IN OXYGES BOMB TABLEXI. AGING OF G.4s BLACKSTOCKS
[At 300-pound pressure a n d 158' F. (70' C.)] COMPOUND
PRESSCCRE AT
274' F. Min. 20 30 45 60 90 20 30 45 60 90
Stress a t 300% Lb./sq. in. K g . / s q . c m .
... 1000 1235 1360 1545
...
1125 1150 1270 1380
..
70 87 95 109
c
COMPOUSD D Ultimate elongation
Tensile strength Lb./sq. in. K y . / s q . e m .
Stress a t 300'% Lb./sq. in. Kg./sq. cm.
%
Tensile strength L b . / s q . in. KQ./sq. c m .
Ultimate elongation
%
BEFORE AGIKQ
3760 4220 4340 4260
...
...
236 298 304 300
605 600 595 575
1205 1520 1760 1780
85 107 123 125
4250 4575 4460 4440
299 322 314 312
625 585 545 530
1060 1260 1575 1670
75 89 111 117
2660 2700 2550 2350
187 190 179 165
550 530 465 400
765 1050 1270 1340
54 74 89 95
1630 1830 1680 1620
115 129 118 114
530 490 370 370
575 835 1000
io
40 59
1040 1210 1060 1005
73 85 75 71
455 425 320 285
..
..
..
...
...
AQED 7 DAYS
...
.. 79 81 89 97
li60 2070 1950 1425
89 146 137 100
360 500 450 310
..
..
...
..
...
A G E D 14 DAYS
20 30 45 60 90 20 30 45 60 90
...
..
690 875 955
49 62 67
...
...
... ... ... ...
.. ..
.. .. .. ..
...
...
49 I D
690 1060 1000 670
300 360 310 200
70 47
... 21
i05 600 610 575
42 43 40
GAS BLACKSTOCKS Experiments were carried out on two gas black stocks, one compounded according to present day usage, the second similar to the first except that it is compounded on a low sulfur basis: C Rubber Plastogen Stearic acid Agerite powder (phenyl-@-naphthylamine) Zinc oxide Gas black Captax (rnercaptobenzothiazole) Tuads (tetramethylthiuram disulfide) Vandex (selenium) Sulfur
100 5 4 1
10 45 1
...
... 3
..
..
..
...
...
A G E D 21 D A Y S
D 100
5 4 1 10 45 0.5 0.25 0.5 1
...
110 280 250 170
..
.. ..
...
..
..
...
Gas black compounds are notoriously poor agers, and stock C represents what may he considered to be good by the usual method of compounding. Compound D, however, shows a substantial improvement in age resistance. TABLEXII. ABRASIONLoss PRESS CCRE AT 274' F.
OF
Gas BLACKSTOCKS VOLUMELOSSOF D
VOLCMELoss OF C KELLY MACHINE
.%fa 71. 20 30 45 60 90
Cc.
x
Comparable unzts
100
GO
Cc. X 100 420 250 20.5 210
10 ' 6.55 5.09 4.05
380 295 235
...
Cornparable units 7.25 4.31 3.54 3.62
...
GRASlELLI YACHISE
TENSILE PROPERTIES AND RATE OF CURE. The tensile strength and modulus of the two stocks cured in a press a t 30 pounds of steam are compared in Figure 11 and Table X. The low sulfur stock (D) cures faster than the stock with higher sulfur (C) and develops a somewhat higher tensile strength and a considerably higher stress a t 300 per cent elongation. W
Min. 20 30 45 60 90
Mzn. 305 40 55 70
255 235 200 215
10 9.22 7.85 8.43
...
Cc./h. p . hour 225 205 185 165
...
Comparable units 8.82 8.05 7.25 6.47
...
CC. 10.37 9.87 10.02 10.52
Comparable units
...
10
9.52 9.66 10.14
.
Cc 8.93 9.1 9.43 10.0
...
Comparable units 8.61 8.77 9.09 9.64
...
U. S . R U B B E R C O M P A N Y M A C H I K E
$ 6 W
A 8 K U
$ 4
2
Comparable units
...
GOODYEAR M A C H I N E
100
-I
C c . / h . p . hour
0 20 4 0 60 00
PO 40 60 80 T I M E OF CURE
PO 4 0 60 80 100 PO 40 00 80 MINUTES A T 2 7 4 . F . (I34.C)
20 4 0 60 60
FIGURE13. ABRASIONLoss OF GAS BLACKSTOCKSBY DIFFERER'T METHODS RESISTANCE TO AGING. Figure 12 and Table XI show the comparative resistance of the two stocks t o aging in the oxygen bomb a t 300 pounds pressure and 158' F. (70" C.).
Min. 20 30 45 60 90
Cc .
...
5.29 4.77 4.82 4.87
Comparable units
...
10 9.02 9.11 9.21
cc. 4.87 4.61 4.80 4.67
...
Comparable units 9.21 8.71 9.08 8.83
...
X E W JERSEY ZINC C O M P A N Y M A C H I K E
Xzn. 20 30 45
CC .
Comparable units
3.20 2.57 2.37 2.36
10 8.03 7.41 7.36
...
...
Cc. 2.67 2.20 1.95 2.01
Comparable units 8.34
6.87 6.09 6.28 60 ... ... 90 a Includes 10 minutes of extra cure t o compensate for extra thickness of test pieces.
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I ST R Y
April, 1933
ABRaSION Loss. The relative losses on abrasion of compounds C and D were studied orer a range of cures, five abrasion methods being used. These results are compared in Figure 13 and Table X I I . The results by the different methods were all reduced to a comparable scale by arbitrarily f i x i n g a t 10 t h e abrasiori loss of the 30-minute c u r e of s t o c k C for e a c h method *andrecalcul a t i n g each set of values on this basis. Low sulfur stock D s h o w s t h e loJTer a b r a s i o n l o s s by ' TIMEOFCURE MINUTES AT 2 7 4 ° F (134.C) each method. FIGURE14. ABR.4SION LOSS ON h B R A S I O N LOSS AGING G4s B L ~ CSTOCKS K OX AGISG. Figure 14 and Table XI11 show the relative abrasion losses of compounds C and D after aging 7 days in the oxygen bomb a t 158" F. (70" C.) and 300 pounds pressure, and 2 hours in the air bomb a t 260" F. (127" C.) and 80 pounds pressure. I n each case, low sulfur compound D has the lower abrasion loss after aging.
455
Unaged, the low sulfur compound D shows slightly more tendency to crack than does the high sulfur stock C. After aging in the oxygen and air bombs, however, this condition is reversed and D shows much less cracking than C.
COSCLCSIOK The advantages of low sulfur compounding are believed to be of value to the industry and to constitute a long step in advance in the field of rubber technology, the method having been recently a d a p t e d with success t o a number of commercial products. This relaa tively new method has obstacles to overcome; the most difficult of t h e s e a r e the high set (at 2: low temperatures) of fully cured AT 2 7 4 O F . (134aC.) rubber containing less than one 1s. ENERGY LOSS per cent of combined sulfur, and FIGURE the apparently greater difficulty ON IhiPAcT OF GAS BLACK of making it adhere to metals STOCKS than when higher sulfur dosages are used. Much more research will be necessary to iron out these problems and to explain why 0.5 per cent of combined sulfur added by means of tetramethylthiuram disulfide gives a better aging product than does the same amount added as elementary sulfur. TABLEXIII. ABRASIOS Loss (KELLYMACHISE)OF GAS As for the future one can only guess a t what sulfur ratios BLACKSTOCKS AFTER AGING will be used ten years from now. The indications are that L'OL. L O S S AFTER VOX.. L O S S AFTER the decline of the past mill continue and that the ratios of PRESSCCRE VOL. Loss B E F O R E 7 DAYSIN 2 HOURS IN A T 274' F. O X Y Q E N BOMB AIR BOMB the future will probably be from 0.5 to about 1.5 per cent c D C D c D unless they disappear altogether, which could happen through Min. Cc.Xl00 C c . X l O 0 C c . X l O 0 C c . X l 0 0 Cc.XlO0 C c . X l 0 0 com ple t e displace20 ... 420 980 ... 505 30 580 250 ii6o 690 700 420 IO ment of s u l f u r by 45 380 205 920 560 555 385 60 295 210 SO0 560 445 380 c o m b i n a t i o n s of 90 235 ... 575 ... 395 ... e selenium or t e l l u rium with o r g a n i c ENERGY Loss o s IMPACT. Hysteresis, expressed as per0 6 disulfides or through z centage energy loss, of compounds C and D are given in U d i s c o v e r y of still Figure 15 and Table XIV. These values were obtained by 2 4 m o r e efficient vulmeans of Lupke's Impact Resiliometer and show compound B canizing agents. D to be superior to C in mechanical properties. Y = There will probably J a l w a y s be uses for TABLEXIV. ENERGY Loss ON IMPACT OF Gas BLACK STOCKS 0 0 special compounds FIGURE16. FLEX-CRACKING OF GAS with h i g h e r sulfur PRESS CURE E N E R Q Y Loss AT 274' F. Compound C Compound D BLACKSTOCKS for cements or adMin. % % hesive purposes, for 20 42.25 ... 45.5 30 39.75 which reason the line of maxima in Figure 1 has been specu37 41.5 45 40.5 60 38 latively extended to 4 per cent for 1943. 90 ... 43.25
i:
':
ACKNOWLEDGMEXT FLEX-CRBCKIKG. Flexing tests were made on a De hIattia The writers are indebted to M. J. De France, Goodyear fatigue machine. Uniform flexing strips 25 mm. wide X Tire and Rubber Company, for abrasion figures obtained 6 mm. thick, cured with a half-round groove (3 mm. radius) on the Goodyear abrasion machine; to G. S. Haslam, New across the middle of the strip to act as the flexing focus, were Jersey Zinc Company, for abrasion figures obtained on the given one hundred thousand 180" flexing cycles a t 82" F. New Jersey Zinc Company and U. S. Rubber Company (28" C.). The extent of cracking was estimated visually, abrasion machines; to H. E. Elden, Dunlop Tire and Rubber and the results appear in Figure 16 and Table XV, the Corporation, for resilience values obtained on the Dunlop average condition of the four cures flexed being plotted in Pendulum Hysteresis machine and to Paul Liipke, Jr., Essex each case. 'Rubber Company, for the use of his Impact Resiliometer. TABLEXI'.
FLEX-CRACKISG~ OF Gas BLACK STOCKS
PRESS CURE BEFORE AGIXQ A T 2 i 4 O F. C D
AGED7 DAYSI X OXYGENBoMBb C D
AQED
&IR
.Win.
a b c
Visual estimate 10 = specimen cracked completely through, At 300 pounds bressure and 158' F. ( i o " C.). At 80 pounde pressure and 260' F. (127' C . ) ,
2
HR. I N
BorBcD
LITERATURE CITED (1) Blake, J. T.,
ISD. EKG.C H m f . , 22, 746 (1930).
(2) Roggs, C. R., and Blake, J. T., Ibid., 22, 749 (1930). (3) Bruni. G . , and Romani, E . , Giorn. chin,. i n d . applicaia, 3, 351 (1921); I n d i a Rubber ,J., 62, 63 (1921). (4) Edland, L. A , , U. S.Patent l,S75,997 (Sept. 1, 1932). ( 5 ) Goodyear, Charles, U. 9. Patent 3,633 (June 15, 1844). RECEIVED February 18, 1933. Presented before the Division of Rubber Chemistry at the 85th Meeting of t h e American Chemical Society, Washington, D. C.,March 26 t o 31, 1933.
