INDUSTRIAL AND ENGINEERING CHEMISTRY
1008
Vol. 21, No. 11 __
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SYMPOSIUM O N AGING Papers presented before the New York Group of the Division of Rubber Chemistry of the American Chemical Society, June 21, 1929
Foreword W. B. Wiegand BIXXEY
& SMITH
COMPANY,
41
EA3T 4 2 N D ST., N E W
kORK, N 1 '
T H E last few years have seen a striking increase in the attention devoted to the prolongation of the life of rubber goods. To the classical Geer oven there has been added the Bierer-Davis oxygen bomb, thus providing another important laboratory aid to the rubber compounder. There can be no doubt that through the use of these agencies there has been a striking improvement in the durability of rubber goods. At the same time, some doubt has existed in the minds of many as to the extent to which artificial aging represents the course of natural aging. The subject, in spite of its importance, cannot as yet be regarded as rationalized. We therefore felt justified in holding a symposium calculated t o present a cross section of the views of representative workers in this field. The authors have made a real attempt to present their data in a quantitative manner, where such treatment seemed at all warranted, and the present writer entertains the hope that a close study of the papers presented will enable the reader to reach a clear idea as to the possibilities and the limitations of artificial or accelerated aging as now practiced.
Relation between Artificial Aging Tests and Natural Aging J. M. Bierer and C. C. Davis BOSTON
WOVEN HOSEAND RUBBER CO., CAMBRIDGE, MASS.
H E more varied the uses to which rubber is put, the more varied are the conditions under which it ages, and the less can it be expected that any single aging test will duplicate these various conditions. The most important factors which promote the deterioration of rubber goods are oxidation by atmospheric oxygen, after-vulcanization, heat effects, cracking and other changes from exposure to sunlight, and wear or deterioration due to mechanical work. No two types of rubber products are exposed in just the same way to these various influences; in fact, one product may be exposed to a marked degree to only one of the influences whereas another may deteriorate from all five influences. Thus, automobile tires undergo oxidation, aftervulcanization, heat effects, cracking from exposure to sunlight and abrasion, and deterioration from mechanical work. On the other hand, steam hose tubes deteriorate predominantly from after-vulcanization and heat effects, oxidation, mechanical work and sunlight playing almost no part. Aftervulcanization and oxidation are the most important factors in the deterioration of air-brake hose, while auto-topping and bathing caps undergo severe exposure to sunlight. I n brief, each kind of rubber goods is exposed to different con-
T
ditions. and to attempt to duplicate the different combinations of light, heat, oxygen, and mechanical work would lead to a different test for every type of rubber goods manufactured. Obviously, this is impracticable and undesirable. The few artificial aging tests which have been developed up to the present time-including the Geer oven test, the Rlarzetti oxygen test, the oxygen bomb test, the ultra-violet light test, and the Fadeometer test, as well as numerous mechanical tests-have been based on the opposite principle, that is, to determine the resistance of R rubber product t o the one most important influence to which it is evposed in service. Thus, when bathing caps or auto-toppings are exposed to a Fadeometer or to ultra-violet light, the idea is only to intensify greatly the action of sunlight, while oxidation, after-vulcanization, and other effects are ignored. Similarly, when inner tubes are aged in the oxygen bomb, the purpose is to determine their resistance to oxidation and t o disregard the influence of heat and mechanical work. Because most rubher goods deteriorate from several influences rather than from one, and because present-day aging tests do not intensify each of the factor.: to the same degree, it cannot be expected that any of the aging tests duplicates exactly natural aqing. Nevertheless, where the predominant effect in the artificial test is also the predominant effect in natural aging, almost the same results from a practical point of view are obtained. Tests which intensify oxidation, heat effects, and mechanical work have been developed to a high degree of practical utility, but it still remains for some one to develop a test for simulating the effects of sunlight. The Fadeometer and ultra-violet light tests represent attempts to duplicate the effects of sunlight, but evidence thus far shows that thcse tests do not duplicate the effects of sunlight and only give misleading results. There is accordingly an urgent need for an aging test that will approximate the effects of sunlight. Limitations of Oxygen Bomb Test
As mentioned above, all aging tests thus far developed intensify one factor disproportionately, and therefore come closest to duplicating natural aging when this factor is t h e predominant one in natural aging. The oxygen bomb, under the conditions used in most experimental and routine work a t the present time, intensifies oxidation to a very high degree. When vulcanized rubber is exposed a t 60" C. to oxygen under a pressure of 300 pounds per square inch (31 kg. per sq. cm.), oxidation is so rapid, compared with after-vulcanization, for instance, that for some types of rubber mixtures, such as undercured ones or those containing a high proportion of Sulfur, it may not duplicate natural aging closely enough, even when the particular mixture deteriorates chiefly from oxidaiion in natural aging. I n cases like this it is better to carry out the oxygen bomb test under milder conditions-for example, a t 50" C. and a lower pressure of oxygen. When the oxygen bomb test was first designed, there wap no intention of specifying any particular range of pressure or temperature and it must be understood that this test does not in any way imply a test a t 60-70" C. and a pressure of 300 pounds per square inch (21 kg. per sq. cm.) or more. Essentially the oxygen bomb test is a method of acceler-
November, 1929
INDUSTRIAL! AND ENGINEERISG CHEXISTRY
ating the rate of deterioration of rubber caused by oxidation by increasing the concentration and the temperature of oxygen in the surrounding medium. Judgment must be used in choosing the particular conditions of pressure and temperature for a particular type of rubber mixture, but whatever intensities are chosen, the test is still a n "oxygen bomb test" if the presrure of oxygen exceeds that in the atmosphere. The test might better be known as a compressed oxygen test, for the bomb is only an incidental feature for safety. The question coiitiiiues to be asked-how many hours in a n oxygen bomb under given conditions correspond to one year of natural aging? $ssuming that oyidation is the predominant influence causing natural deterioration in the rubber mixture in question, there should be a certain number of hours in oxygen a t a definite pressure and temperature wliicli correspond to one year of natural aging in dry air in darkness a t a definite and constant temperature. But the difficultylies in the fact that in natural aging the conditions, particularly the temperature, are not constant. l'lost chemicsal reactions proceed a t least twice as fast TT hen the temperature is raised 15" F. (8.3" C.), and assuming thc oxidation of lubber to be a typical chemical reaction, it is obvious that, nith vulcanized rubber which deteriorates chieflj from oxidation, deterioration should be a t least tnice as fast a t 75" F. 123.9" C.) a< at GO" F. (15.6" C.). In other words, a certain number of hours in an oxygen bomb might represent one y t w at 73" F., but two years a t 60" F. There i, so much variation from place to place, and betnei,'11 summer and n-inter in each place, that the average temperature of rubber goods during a year in one place may varv far more than 15" I?. (8.3" C.) from tlie average temperature of rubber goods elsewhere. Therefore, from a practical point of view it is impoisible to designate so many hours in an oxygen bomb as the equivalent of so many yearb of natural aging. Because of this impossibility it is necessary, as 111 so many other laboratory tests, to run control tests. In other words, instead of assuming that an oxygen bomb test of a certain number of hours represents definitely a certain period of natural aging, it is better to determine the relative aging of the sample in question u i t h a vulcanizate of known aginq properties. For example, an unknown sample is aged for 200 hours in oxygen under 150 pounds (10.6 kg. per sq. cm.) pressure a t 50" C., and a t the same time a control, the aging of which is known to be satisfactory for the use concerned, is also aged. The comparative aging in oxygen will then give the needed information about the unknown sample If in the oxygen bomb the aging of unknown samples is compared with the aging of known samples, then all the practical information which is ordinarily desired I $ obtained without the use of a fallacious comparison of number of hours in a bomb with number of years of natural aging. In this way the oxygen bomb test may be depended upon t o give reliable information about the aging properties of rubber products nhich deteriorate chiefly from oyidation. Its great utility is well illustrated by the story of the company which found discordant results between the 70" C. oven test and the oxygen bornh test in developing 3 new inner tube. The oven test indicated an excellent stock, while the oxygen bomb test gave warning that the tubes would deteriorate too soon. Trusting the mole favorable oven tebt, tlie inner tubes were sold, and nithin one year tuhcs valued a t $150,000 were returned because of had aging Another concern, d i i c h supplieq inner tubes to certain buq :ompanies, received the complaint that its tubes became wit and tacky after 5000 miles. By the aid of the oyygen bomb test this company was able to develop inlier tubes nliich after 20,000 miles were still in p o d condition.
1009
I t is unnecessary to cite further examples to show the value of the oxygen bomb test in foreseeing the natural aging of rubber products which deteriorate chiefly from oxid at ion. ' But this test is not sufficient, for the importance which sunlight, for instance, plays in the aging of some of the most important of products demands that an artificial test be developed for this kind of aging. Until tests other than those involving oxidation in darkness are developed, the rubber industry will remain without any means of foreseeing the natural aging of some of its most important products.
Natural vs. Artificial Aging Stanley Krall
A
COSSIDERABLE number of types of n.rtificial aging are being used t'oday in an endeavor to determine thc aging properties of rubber stocks in a short period of time without nrxitiiig for natural aging result,s. The dnta reported here \yere taken from some thnt are being obtniiied by Sub-Committee XY,Committee D-11 of the Ainwican Society for Testing 1Iaterinls. Experimental
Three types uf pneumatic tire stocks were aged by two natural and two artificial methods: (1) Slabs were hung separately in the dark. (2) Slabs were hung separately exposed t o the weather. (3) One-inch (2.5-cm.) wide strips were hung separately in the Geer ( 2 ) oven a t 158" F. (70" (2.). (4) One-in-h (2.5-cm.) wide strips were hung separately in the Bierer ( 1 ) oxygen bomb a t 1%" F. (70" C.) and 300 pounds (21 kg. per sq. cm.) oxygen pressure.
The st'ocks tested were a pure gum stock, a first grade, and a reclaim tread st'ock as follows: Smoked sheets Whole tire reclaim D. 0. T. G. Sulfur Zinc oxide Carbon black Mineral rubher Pine tar
STOCK1 100.00
STOCK2
STOCK3
100.00
0.75 3.00 5.00
1.25 3.50 5.00 40.00 5.00 2.00
60.0 66.0 1.0 3.5 5.0
....
....
,...
35.0 5.0 2.0
.... . .. . --_
-__
_-
108 73
156. 75
177.5
The stocks were cured 45 and 60 minutes a t 287' F. (141.7' C.). The stoclcs were tested after 6 and 12 months' dark aging; 3, 6, 9, and 12 months' weather exposure; 3 and 7 days in the oven; 12 and 24 hours in the bomb. Results
The results were plotted on a scale ol 3 clays in the oven and 12 hours in the bomb, equivalent to one year's natural aging. Charts 1 and 2 show the load a t break values; 3 and 4 the load a t 500 per cent for stock 1 and a t 300 per cent for stocks 2 and 3; 5 and 6 the percentage elongation :it ljreak; 7 and 8 the tensile product. Chart 9 shows the relation between the days in the oven and hours in the bomb versus months in the dark. These results IT-ere obtained from the load a t break curves on Charts 1 and 2. The results indicate that one year of dark aging is equivalent to approximat,ely: STOCK
1 >
3
~ S - A I I N U TCURE B Days in oven Hours in bomb 2 15 3 9 6: 10
BO-AZINUTI~ C~JRF: Days in oven Hours in bomb 3 16 3 X 5 11
