qR-s
C O M P A R I S O N W I T H N A T U R A L RUBBER
TENSILESTRENGTH AT BREAK. In general, GR-S vulcanizates retain a much greater percentage of their original tensile strength than do natural rubber vulcanizates. CHAXGESIN MODVLUS. Most natural rubber vulcanizates tend to revert and show lower modulus upon aging. This is particularly true with the more severe tests and in the latter stages of the tests. (Some modern rubber vulcanizates show stiffening in the early stages.) GR-S vulcanizates almost always show a pronounced stiffening A. M. NEAL AND P. OTTENHOFF upon aging; this effect appears to increase E. 1. d u Pont d e Nemours & Company, Inc., Wilmington, Del. with rising temperature. ELONGATION 4~ BREAX. Both natural rubber and GR-S vulcanisates tend to lose elongation on aging. The magnitude of the Data are presented o n the effect of various typesof accelerated aging o n a simple GR-S change is much greater with GR-S vulcanvulcanizate as well as a typical smoked sheet vulcanizate-namely, the oxygen presiaates. sure test, air oven test, and air pressure heat test. The importance in GR-S vulcanizates TEAR RESIST~XCE. Natural rubber vulof evaluating the effect of accelerated aging on properties other than tensile strength is canizates lose tear resistance upon accelerated stressed, particularly o n modulus, elongation at break, tear resistance, hardness, and aging, regardless of the type of test. GR-S dynamic properties. The effect of small amounts of copper and manganese on the vulcanizates sometimes show an increase in age resistance of GR-S vulcanizates is determined b y the oxygen pressure test; the tear resistance upon aging, which is particucatalytic effect of these metals is much less severe o n the oxidation of GR-S than of natural rubber. The effect is shown of simultaneously increasing the accelerator and larly apparent in the oxygen pressure test. decreasing the sulfur on the resistance of GR-S vulcanizates to each type of aging. Even when GR-S loses tear resistance, the The approximate severity of the accelerated aging tests on GR-S vulcanizates with percentage change is much less than with respect to changes in tensile strength, elongation at break, and modulus i s presented. natural rubber, probably due in part to the low original tear resistance of the GR-S vulcanizates. CHAXGE IN HARDNESS.GR-S vulcanixates increase in hardNE of the early deficiencies of natural rubber vulcanizates ness during all types of accelerated aging. Natural rubber vulwas their susceptibility t o deterioration through oxidation. canizates generally show a slight increase in hardness during the I t is probably fair to state that the utility of natural early stages of the test and then the hardness decreases. rubber, for most of its widespread modern usages, depended in CHANGES IN DYNAMIC PROPERTIES. The effect of acclerated large degree on the development of antioxidants and accelerators aging on the dynamic properties of GR-S vulcanizates appears to which produced age-resistant vulcanizates. Part of the research differ considerably from the corresponding effects in a natural of the past twenty years has been directed toward the development rubber vulcanizate. K i t h natural rubber stocks the change in of so-called accelerated aging tests for evaluating the probable heat build-up values (Goodrich flexometer) can be fairly acculife of rubber vulcanizates under a variety of service conditions. rately estimated from a consideration of the corresponding changes The most important of these are the oxygen pressure test in in modulus. With GR-S stocks, on the other hand, no correlawhich aging is carried out at 70' C. in oxygen a t 300 pounds per tion appears to exist. The oxygen pressure test is particularly square inch; the Geer oven test in which the aging is carried out severe in destroying the resilience of GR-S stock although there a t 70" C. in air a t atmospheric pressure; the oven test in which is an increase in modulus during the aging period. The air oven the aging is carried out in air at atmospheric pressure and a t and air pressure heat tests do not effect this property very temperatures higher than 70" C., usually either 121' or 150"; severely. and the air pressure heat test in which the aging is carried out at STOCKS STUDIED.A simple GR-S compound was chosen rather 127' C. in air at 80 pounds per square inch. than a stock developed for any special usage. On the other Since the synthetic rubber being produced in greatest volume hand, the rubber stocks selected for comparison were practical is GR-S. the discussion in this paper is limited to the resistance types designed for passenger and truck tire tubes. Although the of GR-S vulcanizates t o various types of aging. GR-S contains quantitative results on GR-S are restricted to a single accelerator an efficient antioxidant added during the course of manufacture; blend of. mercaptobenzothiazole and diphenylguanidine, the therefore GR-S vulcanizates, regardless of whether they contain same type of results was observed for all of the accelerator comfurther antioxidants added during the compounding stage, should binations considered. Since one of the best ways of increasing be compared with natural rubber vulcanizates containing effective the resistance of natural rubber vulcanixates t o accelerated aging antioxidants. has been to reduce the sulfur and increase the accelerator content, I n evaluating the resistance of natural rubber vulcanizates this method was also applied to GR-S vulcanizates. The comte aging by the accelerated tests, greatest emphasis has, in pounds and original physical properties of their vulcanizates are general, been placed upon their retention of tensile strength. given in Table I. The existing literature is unanimous in stating that the age reThe results obtained on aging these vulcanizates in the oxygen sistance of GR-S vulcanizates is vastly superior to that of natural pressure test under the standard conditions of 70" C. and oxygen rubber. Koch ( 2 ) and Stocklin (4, 6) measured the resistance at 300 pounds pressure are listed in Table I1 and plotted in of GR-S (Buna S) vulcanizates to aging in air, oxygen, and steam Figure 1. at various temperatures. These articles give few data 'and are OXYGEN PRESSURE TESTS of little value in critically examining the resistance to aging of GR-S vulcanizates. Some articles ( 5 , 7 )are now appearing which These data show the excellent resistance of GIZ-S vulcanizates deal largely with acceleration and compounding of GR-S and as judged by retention of tensile stength; the GR-S vulcanizate give some aging data. is more resistant to aging in this respect than the natural rubber
RESISTANCE TO VARl TYPE§ OF AGING
0
352
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1944
353
OXYGEN PRESSURE AGING OF GR-S
I
-
-
3cn 80-
U
o
30-
A 0.1% M n
\0 RUBBER 812.0SULFUR -
29
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90
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I
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90
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L ~ G R - aS1.0 SULFUR\DRUBBER R 2.0 SULFUR 1 x G R - S & 1.5 SULFUR-
70
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I
2
3
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R
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a 2.0
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&RUBBER K
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vulcanizate. In contrast t o the rubber, decreasing the sulfur in the GR-8 vulcanirates while simultaneously increasing the accelerator results in little change in retention of tensile strength; what differences do occur favor the higher sulfur content. In regard to changes in modulus which take place during the test, the marked stiffening of GRS vulcanizate is striking. Since such stiiening is generally considered undesirable, the GR-S vulcanizate is much less resistant to aging than natural rubber from this standpoint. Simultaneously decreasing the sulfur and increasing the accelerator content of the GR-S stock markedly reduce the magnitude of the change in modulus on aging. As judged by the effect on elongation at break, there is no striking difference between natural rubber and GR-Svulcanizates
354
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 36, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1944
TABLEI. RUBBERAND GR-S STOCKS USEDIN AGINGTESTS ComDound Smoked sheets GR-S Zinc oxide Fine particle whiting MPC black FT black Stearic acid Palm oil Pine tar Neoeone A Akroflex C Retarder W Zenite A
2-MT
A 1100.0
... _ 5_ . 0
16.0
15.0
... 1.5
.1.0 .. 1.5
...
...
1.75
Thionex MBT-DPG blend Sulfur Optimum cure at 274O F., min.
... ... ..
2.0 7'
B 100.0
O
D
E
... 1oo:o 1oo:o 1oo:o 5.0 5.0 . . . .50.0 . . . 50.0 .5.0. 6 5b .: 0O 40.0 .1 ..0 . . 1.0 . . . .1.0. 2.0
. . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .I.0. . . .. ... ... ... ... ... ... ... ... u.l .1 .. 2 . . 1.6 . . . .3 ..0 ...
1.0 1.25 0.75 0.5
1.5 60
1.0 45
Properties of Unaged Vulcaniaates at Optimum Cure Stress. Ib./sq. in. At 1007 ... .. 225 225 At 30003 375 1450 275 1550 1275 3325 At 500'f 1100 3400 4250 Tensile 1 . / s q . in. 3700 4200 3600 Elonpahon at break, % 760 800 525 520 Hardness (Shore A) 62 60 Winkelmann tear, lb./in. width 80' F. 430 360 240 245 70" C. 265 225 Heat build-up,b ' C. 55 ... 54
1425 3300 3300 500 59
... ...
1.0 7"
... ...
2.0 60
200
295 235 53
At 298' F,. In Goodrich flexometer: speed 1800 cycles per min I/a-in. stroke, 153.3 Ib./sq. in. load on sample, reading after 20-min. operaijon. b
355
The usual laboratory procedure is to introduce known amounts of copper in a soluble form, generally as the stearate. The GR-5 stocks used and the results obtained on aging these stocks in the oxygen pressure test are listed in Table I11 and plotted in Figure 2. Both copper and manganese exert a catalytic effect upon the deterioration of GR-S vulcanisates although the effect is considerably less than that on natural rubber. The GR-S vulcanisates can apparently tolerate a concentration of copper or manganese twenty to forty times as great as can natural rubber. As would be expected, the metals act as true catalysts; i.e., they speed up the normal rate of deterioration and do not change their direction, GR-S vulcanizates containing copper show a marked stiffening, decrease in elongation a t break, and resulting embrittlement rather than reversion and resinification usually associated with metal contamination in natural rubber. Copper inhibitor X-872-A is effective in overcoming the catalytic action of copper on G R S vulcanizates just as it is with natural rubber; 2% of this inhibitor appears to be sufficient to overcome the catalytic effect of 0.1% of copper (percentages based on elastomer content). HEAT AGING
Although the limited scope of high-temperature aging tests is set forth in the A.S.T.M. specification (I),thwe has been a grow-
TABLE 11. AGINGOF RUBBERAND GR-S VULCANIZATES IN OXYGEN BOMB in the oxygen pressure test. The GR-S stock Compound A B C D E C D E having normal sulfur content (2 %) is somewhat Elastomer Rubber Rubber GR-S GR-S GR-S GR-S QR-S GR-S R S on elastomer 2.0 1.0 2.0 1.5 1.0 2.0 1.5 1.0 inferior to the rubber stock, but the GR-S stock having lower sulfur (1%) is superior to the rubber Aging period weeks 5 weeks1925 1575 Stress at 300% 525 2475 2000 1550 2250 550 stock. Tensile strength 2200 1850 2800 2500 2200 2425 3325 2375 Elon ation at break 680 360 400 600 350 400 440 320 The effect of the oxygen pressure test on the Hardrness 415 75 400 74 74 73 72 77 ... tear resistance of GR-S vulcanisates is surprising, Winkelmann tear, 8O0 F. 370 380 345 395 215 ... 240 295 70' 260 250 220 2fO c. ... ... In contrast to natural rubber, the GR-S stocks b c Heat Build-up 80 93 kested show an increased resistance to tear after . a Pellets blew out in 17 min. b Pellets blew out in 10 minutes. 6 Pellets blew out in 9 minutes. aging. Although such an increase may not be found for all GR-5 stocks, in general this test does not affect the tear resistance of GR-S stocks TARLB 111. INFLUENCE OF COPPER AND MANGANESE ON to the extent that it does rubber stocks, The original tear resistOXYGENBOMBAGINGAT 70' C. AND 300 POUNDS OXYGEN ance of natural rubber stocks is much greater than that of PRESSURE comparable GR-S stocks. Although the increase in tear resistCompound F G H I J K ance on aging of GR-S stocks indicates resistance to this type OR-s 100.0 100.0 100.0 100.0 100.0 100.0 MPC black 50.0 50.0 50.0 50.0 50.0 -50.0 of deterioration, it is minimized by the low original values. Zino oxide 5.0 5.0 5.0 5.0 5.0 5.0 No conclusions can be drawn from the data on the effect of Stearic aoid 1.0 1.0 1.0 1.0 1.0 1.0 Cu stearate ... 0.10 1.0 1.0 ... varying the accelerator-sulfur ratio. Mn stearate . . . . . . 0.20 . . . . . . 2.0 Cu inhibitor The hardness (Shore type A) of the G R S stock increases more X-872-A ... ... .2 ..0 . . .2 ..0 22 .. 00 2 . 0 than for the natural rubber stock on this type of aging but the Sulfur 2.0 2.0 Thionex 0.5 0.5 0.5 0.5 0.5 0.5 extent of hardening is not great. PR-S ... 0.01 ... 0 1 0.1 ... Oxygen pressure aging rapidly destroys the dynamic properties ... 0.01 . . . . . 0.1 of GR-S vulcanizates as judged by the Goodrich flexometer; Cu inhibitor ... ... ,.. . . . . . 2.0 ... GR-S stocks are much less resistant to aging in this respect Unagedo than natural rubber vulcanizates. I n view of the poor original Stress at 200% 650 700 700 850 625 750 resilience of GR-S vulcanizates, this type of failure warrants Tensile strength 2450 2300 2625 2500 2500 2575 430 420 420 280 470 Elongation at break 415 considerable attention. Reducing the sulfur and increasing the Hardneea 68 68 68 69 67 69 accelerator content in the GR-S stocks appear to accelerate this After 2-Week Aginga type of deterioration.
-
. I .
e
... ...
...
...
8 f&p"
s
CATALYTIC EFFECT O F METALS
Although the amount of copper or manganese that can be tolerated by natural rubber vulcanisates is dependent upon factors such as the type of accelerator and of antioxidant used, it is generally accepted that more than 0.00270 of copper or 0.001'%ofmanganese based on the rubber will dangerously accelerate the deterioration rate. Copper can be present in certain forms without accelerating deterioration, and copper inhibitors have been developed which will overcome the effect of contamination by copper.
Stress at 200% Tensile strength Elon ation at break Hardness
1125 2450 360 75
1190 2450 350 75
1050 2250 360 75
1475 2350 320 79
1450 2475 310 76
1150 1375 255 79
1625 2025 260 81
1650 Tpo 2425 brittle 290 totest 79 90
After 3-Week Aging" Stre438 at 200% Tensile atrength Elon ation at break Hardness Appearanoe a
1250 2650 380 77
1200 2600 300 77
1250 2550 300 77
After 4-Week Aging" OK OK OK
All stocks press-cured 30 minutes at 287O F.
Brittle
t
OK
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
356
TABLE IV. AGINQOF GR-S Compound Elastomer % S on elastomer Azinn neriod Stress a t 100%
-
B Rubber 1.0
.550 .
I .
300%
Tensile strength Elongation a t break Hardness Winkelmann tear, 80' F. 70OC. Heat build-up
TABLE V.
2200 570
... 100 ..... ,
RUBBER VULCANIZATES IN 121 C. AIR OVEN
AND
C D GR-S GR-S 2.0 1.5 3 davs 1100 1000
.. .
2150 135 77 100
40 54
2600 200
71 135 90 53
E
E GR-S 1.0
1900
D GR-S 1.5 6 davs1600
100 81 60 30 52
2200 140 80 70 25 52
2025 180 72 125 60 50
GR-S 1.0
C GR-S 2.0
700
-- 7
...
... i~oo
1825 200 76 180 100 49
...
1000
AGINGOF GR-S AND RUBBER VULCANIZATES IN AIR PRESSURE HEATTEST
Compound Elastomer % S on elastomer Aging period Stress a t 1009' 3008 Tensile strength Elongation a t break Hardness Winkelmann tear, 80' F. 70°C. Heat build-up
A Rubber 2.0 7
...
625 1575 470
...
80
.. .
...
C D GR-S GR-S 2.0 1.5 8 hours 550 475
.. .
2250 265 71 225 140 54
2660
350 70 255 225 50
E GR-S 1.0
C GR-S 2.0 -24 400 1575 2150 ... 2750 1700 360 110 69 81 295 85 285 30 52 48
.
D GR-S
1.5 hours1175
...
1650 140 80 125 55 48
ing tendency to utilize these tests as a means of evaluating the age resistance of rubber stocks. Therefore GR-S vulcanizates were tested in both the 121" C. oven and the air pressure heat tests. Additional data on the heat aging of GR-S vulcanizates are reported by Sturgis (6). AIR OVEN AT 121' C. The results obtained are given in Table IV and plotted in Figure 3. The truck tube stock containing only 1 % sulfur was used as the rubber control. The GR-S stock retains its tensile strength to a greater extent than does natural rubber, just as it does in the oxygen pressure test. The effect of simultaneously decreasing sulfur and increasing accelerator content is not definite, but there is some evidence t o indicate that higher sulfur content is deleterious. The effect of this test on change in modulus is striking; the normal sulfur GR-S stock shows an 850% increase in stress at 100~o during the test. As found with the oxygen pressure test, decreasing the sulfur and increasing the accelerator content reduces the increase in modulus, although the GR-S stock containing 1% sulfur still shows a 400% increase. The air oven test at 121' C. is much more severe on the ultimate elongation of GR-S stocks than the oxygen pressure test but the direction of the change is the same. The effect of this test on the tear resistance of GR-S stocks is entirely different from that of the oxygen pressure test. In the air oven test the change is similar to that occurring in natural rubber, and there is a marked loss in tear resistance. Increasing the accelerator while simultaneously decreasing the sulfur improves the resistance to this type of deterioration. The effect of this test on the dynamic properties of GR-S stocks is worthy of note. This method of accelerated aging does not seriously affect the resilience of a GR-S stock as measured by heat build-up on the Goodrich flexometer. The reason is obscure although it is probably associated with the tremendous increase in modulus. However, this cannot be the full answer since the modulus also increases in the oxygen pressure test which is exceptionally severe in its effect on the resilience of GR-S stocks. ' AIR PRESSURE HEATTEST. The results obtained at 127" C. and 80 pounds per square inch pressure are listed in Table V and plotted in Figure 4. I n this test the passenger tube stock containing 2y0 of sulfur was used as the rubber control. The following conclusions can be drawn: The GR-S stock6 retain tensile strength better than natural rubber; low-sulfur
E GR-S 1.0
Vol. 36, No. 4
stocks are superior to higher-sulfur stocks. GR-S stocks stiffen continuously throughout the test, and the amount of stiffening is comparable to that shown in the air oven test; low-sulfur stocks show a lower amount of stiffening. GR-S stocks lose elongation rather rapidly during this test, but as with the oxygen pressure test, the effect is about the same in GR-S as it is in rubber. Changes in tear resistance of GR-S stocks combine the effect noted in the oxygen pressure and in the air oven tests; there is an increased resistance to tear in the early stages of the test followed by a marked decrease in resistance. This test, like the air oven test, shows little effect on the resilience of GR-S stocks as measured by heat build-up on the Goodrich flexometer. C O M P A R I S O N OF ACCELERATED A G I N G TESTS
750
From the data presented, the order of severity ... 2100 of the various aging tests is approximately the 240 same for GR-S vulcanizates as has been found for 76 natural rubber. The oxygen pressure test ap210 130 pears to be the mildest, folIowed by the air oven 46 a t 121' C. and the air pressure heat test in that order. Attempts t o correlate these tests in natural rubber vulcanizates have never been successful when applied t o a variety of stocks-i.e., stocks prepared with different accelerators or accelerator-sulfur ratios or those containing different antioxidants. Similar difficulties can be anticipated in attempts to correlate the various tests on GR-S vulcanizates. A comparison of the severity of the tests is given in Table VI; it shows complete lack of any real correlation of these tests. However, if its application is limited to carbonblack-loaded GR-S stocks, accelerated with an activated thiazole and having approximately the accelerator sulfur ratios used-in our stocks, it will be of some interest.
COMPARISON OF SEVERITYOF AGING TABLE VI. APPROXIMATE TEST^^ Oxygen Bomb
41/n weeks 3 23/r
1210 c. Oven
Air Pressure Heat Test
To Reduce Tensile Strength 30% 7 hr. 48 hr. 10 .. 18 88
% Sulfur on GR-S 2.0%
1.5
1.0
5
To Reduce Elongation 20% 4 hr. 20 hr. 6 28 7 32
2.0% 1.5 1.0
2 weeks 3 8
To Increase Modulus 10070 20 hr. 6 hr. 24 8 12 40
2.0% 1.5 1.0
la/& weeks
2'/r
LITERATURE CITED
(1) Am. Soo. for Testing Materials, Standards on Rubber Products, Methods of Testing, Specifications, 1943. (2) Koch, Albert, Rubber Chem. Tech., 10, 17 (1937). (3) Naugatuck Chemical, Synthetic Rubber C m p o u n d i n g BulZ.J3 (1943). (4) Stocklin, Paul, Kautschuk, 15, 1-7 (1939). (5) Stocklin, Paul, T r a m . , Inst. Rubber Ind., 15, 51-75 (1939-40). (6) Sturgis, Baum, and Vincent, IND.ENG.CHEM.,36, 348 (1944). (7) Vanderbilt Co., R. T., India Rubber World, 106, 453, 572 (1942) : 107, 36 (1942). PRBSENTED before the fall meeting of the Division of Rubber Chemistry,
AM~RICA CHBMICAL N SOCIETY, New York, ti. Y.,1943.
