Correlation of Room Temperature Shelf Aging with Accelerated Aging R. A. YOUMANS] AND G. C. MAASSEN R . T . Vanderbilt Co., Znc., 230 Park Aue., New Y o r k , N . Y .
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N 1931, a series of compounds was mixed to obtain data for publication purposes. After the necessary data for use at
that time had been obtained, the remaining vulcanized test slabs of each compound were tied together in a bundle and stored in a desk drawer without wrapping. The compounds used for this series of tests contained varying amounts of antioxidant. After being stored for 22 years, the several stocks were analyzed to determine whether there had been any migration of antioxidant from stocks containing antioxidant to those containing no antioxidant. A definite migration was found; compounds that had originally contained no antioxidant definitely showed its presence. It was, therefore, decided in this work to use a compound t h a t had originally contained antioxidant, so that the results would not be affected by the gradual acquisition of an antioxidant. The selected compound had the following formulation:
The tensile and elongation properties (1) depreciated a t a fairly uniform rate during the entire 22-year period. The modulus showed a very rapid increase during the early stages of storage, a maximum at the end of about 12 years, and a gradual decline to 22 years. The phenomenon known as reversion in natural rubber had not yet occurred. Although the modulus showed an indication of dropping, the elongation had not yet begun t o increase; it can be assumed that at the end of 22 years this compound had not begun to revert, as reversion is ordinarily known in the rubber industry.
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After 22 years, the antioxidant content of this compound had dropped from 2 parts t o 1 part on the rubber hydrocarbon. A similar stock originally made without antioxidant had, during the 22 years of storage, acquired enough antioxidant so that it contained 0.2% based on the rubber content. This amount of antioxidant evidently had been acquired by volatilization and migration from stocks containing the antioxidant.
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Figure 1. Change in physical properties with natural aging
At intervals during the course of the 22 years, the physical properties of these stocks were determined. The percentage change in physical characteristics of the compound is shown in Figure 1. 1
Present address, Kirkhill Rubber Co., Brea, Calif.
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Change in physical properties with natural and accelerated aging
To try to correlate these data with accelerated aging tests, the compound was remixed and subjected to several types of accelerated aging tests-the air oven a t 70' C. (4), the oxygen bomb at 70" C. and 300-pound pressure (S), and the air bomb a t 127" C. and SO-pound air pressure ( 2 ) . To avoid the possibility of migration of antioxidants, all the accelerated aging was done in individual containers. The information, for both natural shelf-life aging and accelerated aging, is shown in Figure 2. The curves on the left are very much cramped and i t is difficult to make any definite correlation. By selecting convenient time intervals along the horizontal axis for each set of aging conditions, i t is possible to spread the curves so that correlation is possible. Figure 3 shows the curves for tensile depreciation. From these curves Table I, giving per cent tensile depreciation equivalents, was developed. From these data it can be seen that given a sufficient exposure in the oxygen bomb or air bomb, i t is possible to cause tensile depreciation equal to that obtained on natural aging for almost any period. The air oven curve changes slope and tends to level off. Unfortunately, in their work the authors did not continue the oxygen bomb or air oven tests long enough to get correlations to 21 years. However, i t is seen that 6 days in the oxygen bomb is the equivalent of 14 years of natural aging, 21 days in the air oven is equivalent to 14 years of aging, 97 1487
INDUSTRIAL AND ENGINEERING CHEMISTRY
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minutes in t h e air bomb is the equivalent of 14 years, and 173 minutes in the air bomb is the equivalent of 21 years. Gsing similar time intervals on the horizontal axis, curves were drawn for elongation properties (Figure 4). From these curves a correlation can be had among natural, air bomb, and oven aging. However, because of t h e change in slope in the curve for oxygen bomb aging, the correlation is limited.
Table I. Natural Aging, Years
Tensile Equivalents
70" Oxygen Bomb, Days
70' Air Oven, Days
.4ir Bomb, 3Iin.
mate tensile or elongation. Most rubber parts become nonserviceable because of changes in modulus. T h e modulus change t h a t takes place under different conditions of aging is depicted in Figure 5. It is obvious t h a t the compound aged under natural conditions continues t o increase in modulus for approximately 12 years, a t the end of -which time there is a very gradual decrease in modulus. When aged in t h e oven there is a fairly rapid change in modulus for t h e first 7 or 8 days, after which time a definite leveling takes place, so t h a t even after 21 days the increase in modulus is equal only to t h a t of approximately 6.5 years of natural aging. I t is difficult to predict whether with continued oven aging the modulus increase would ever reach the values obtained for 12 years of natural aging.
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Elongation Equivalents
70' Oxygen
Katural Aging. Years 1 2 3
70' Air Oven, Days 0.75 1.b 2.3 3.0 3.75 7.5
Bomb. Days 0.25 0.50 0.92 1.25 1.63 4.13
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From the curves in Figure 4, Table 11,showing per cent elongntion depreciation equivalents, has been developed. From these d a t a it is evident t h a t the oxygen bomb can be use to predict the elongation changes due to natural aging up t o 10 years. At t h a t point, the curve starts to level off to such an extent t h a t it is difficult to obtain further correlation. T h e air oven deteriorates the elongation properties in 21 days to about the szmc extent as 22 years of natural aging, or 220 minutes in the air bomb.
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Elongation changes with natural and accelerated aging
In the oxygen bomb there is a rapid increabe in modulus for the first 2 days, when there is a definite leveling, and after about 4 days there is a tendency to drop, so t h a t in this instance the oxygen bomb can pre4ict changes in modulus up to about 4.5 years. Beyond that point further aging in the oxygen bomb is of no &ue in malting correlations. T h e air bomb causes the modulus to increase for about the first 1.25 hours, and exposure for longer periods causes a rapid deterioration in modulus. According to these figures, the compound ~vouldincrease in modulus for about 3.5 years of natursl aging, after which time the-e would be a decline in modulus. Continuing the evposure in t h e air bomb for 4 hours causes the per cent modulus increase t o return to zero. This, of couree, is not true in natural aging. By comparing the various accelerated aging curves in Figure 5 with the natural aging curl'e, Table I11 is developed.
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Modulus Equivalents
Oxygen Bomb, Days
Oven Aging, Days
Air Bomb, Min.
Tensile depreciation with natural and accelerated aging
T h e air bomb and air oven depreciation curves approximately parallel the natural aging depreciation curve for elongation. However, the oxygen bomb curve tends to become level and correlation is limited to 10 years. I n evaluating the aging characteristics of a rubber compound, the changes in ultimate tensile and ultimate elongation properties are generally noted. Yet fieldom, if ever, are rubber parts in service used a t their ultimate tensile or elongation, and few rubber articles are taken out of service because of failure of ulti-
From the figures shown, it is obvious t h a t not all the characteyistics of a compound are depreciated at the same rate under accelerated conditions, as they are under natural conditions. Therefore, a set of time intervals along the horizontal axis m s
INDUSTRIAL AND ENGINEERING CHEMISTRY
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devised which n-ould permit suFerimposing the per cent tensile depreciation curves for all types of aging, as shown in Figure 6. Using the same time intervals, the curve for elongation mas developed and is shown in Figure 7 . No longer can these curves be superimposed on one another-an indication t h a t the rate of tensile depreciation is not the same as the rate of elongation depreciation under all conditions of aging.
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The oven test as introduced by Geer ( 6 ) was the first of the accelerated aging test methods to find wide acceptance. A t that time (1916) 2 and 4 days in a 70" C. oven caused considerable depreciation of the tensile proreyties of a normal compound. As compounding techniques improved, longer and longer oven aging was necessary to deteriorate physical properties. T o reduce the time necessary to deteriorate a compouIid, Bierer and Davis ( 5 ) introduced the oxygcn bomb, which was recommended for use a t 60" or '70" C. and 300-pound oxygen pressure. Exposure for hours under these conditions caused deterioration apparently equal to that requiring days in the air oven a t 70" C. This represented a saving in aging time and the method found considerable favor. Later it was shown that the time to cause a given deterioration could be reduced roughly 50% by raising the temperature to 80" C. This procedure found wide acceptance.
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Modulus changes with natural and accelerated aging
Similarly, the differences in rate of change in modulus are much more pronounced than the changes in elongation, as shown in Figure 8. The air bomb causes the least modulus increase, the oxygen bomb more, and the oven the greatest, yet none approximates the change in modulus t h a t takes place on natural aging. Of the three types of accelerated aging, the oven most nearly approximates the change that takes place in natural aging.
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Elongation changes with natural and accelerated aging
In the late 1920's, the air bomb ( 7 ) was introduced. This test conducted a t 127" C. and 80-pound air pressure was designed to simulate inner tube service conditions. The conditions prescribed deteriorated compounds in hours to an extent which would normally take days in the oxygen bomb and possibly weeks in the air oven. It represented a further saving in aging time.
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From the data reported, the following general statements can be made: During natural storage not all physical properties change a t the same rate. During accelerated aging not all properties change a t the same relative rates as during natural aging. Xone of the usual accelerated aging tests predicts accurately all of t h e physical changes t h a t will take place on long-time storage. Of the three accelerated heat aging tests, all will predict n.jth some degree of accuracy the trend in breaking tensile and brea!iing elongation during limited long-time storage. Of the three types, the air bomb is least reliable for predicting modulus change at' normal temperatures. The 70" oxygen bomb exposure is somewhat more reliable. The best method is the 7 0 circulating air oven. For this particular stock, it could be uLe for predicting shelf life up to 6 years.
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Modulus changes with natural and accelerated aging
Despite the fact that this test was developed to simulate inner tube service, it was adopted because it was a time saver and not necessarily hecause it had any bearing on the ultimate service of the article. Several brancLes of the rubber industry use this test method as a means for predicting service life, whereas the actual service of the article has no relation to the test. An extreme case is use of the air bomb for testing foam mattresses and pillows; this is only a heat deterioration test and not a measure of service life.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Any of the three aging methods can be used for predicting changes in breaking tensile and breaking elongation. However, most rubber items do not fail in service because of changes in ultimate tensile or ultimate elongation b u t rather because of other changes such as modulus. Yet, all too often, modulus changes are not given their proper consideration in evaluating anticipated service life of a compound. For the compound being discussed, the air oven predicted natural shelf-life modulus changes for 6.5 years, the oxygen bomb for 4.5 years, and the air bomb for only 3.5 years. Each piece of apparatus, in the order named, reduces the time necessary to cause a certain percentage deterioration in properties. However, as the speed of deterioration increases, the length of time for which modulus changes can be forecast is reduced. So, the effort t o become efficient b y saving testing time defeats its purpose; eventually it becomes wasteful t o run accelerated aging tests, the results of which do not forecast the service life of the article. In selecting accelerated aging conditions, service conditions should be considered and aging conditions t h a t most nearly approximate service conditions should be used. From the data
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presented, it appears that the air oven is better than either the air bomb or oxygen bomb for predicting modulus changes. T h e more nearly service conditions are simulated, the more meaningful the tests become. ACKNOWLEDGMENT
The authors wish to thank the R. T. Vanderbilt Co. for permission t o present this paper. LITERATURE CITED
(1) Am. SOC.Testing Materials, Philadelphia, Pa., “24STll.1Stand-
ards for Rubber,” Designation D 412, 1951. (2) Ibid., D 454. (3) Ibid., D 572. (4) Ibid., D 865. CHEM.,16, 711 (1924). (5) Bierer, J. M., and Davis, C. C., IND.ENG. (6) Geer, W. C., I n d i a Rubber W o r l d , 55:‘127 (1916). (7) Vanderbilt Co., R. T., New York, Vanderbilt Rubber Handbook,” Rogers, ed., p. 454, 1948.
s. s.
RECEIVED for review September 16, 1954. ACCEPTED January 25, 1955. Presented before the Division of Rubber Chemistry at the 126th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, N. Y., 1954.
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