I,VDUSTRIAL A N D ENGINEERING CHEMISTRY
870
low sulfur content and of relatively poor quality, but that further time will be required before it is possible to judge its true value for most high-quality stocks. The use of the oxygen bomb is to be continued.
Behavior of Tire Stocks by Various Aging Methods By
w.w. vogt
394 452 479 532 495 498
Vol. 17, No. 8
MOMHSOF NATURAL ACINGFOR DBTBRIOPERCENTAGE OF ORIGINAL R ~ T I O N EQUIVALENT TENSILE PRODUCT TO OVEN OR BOMB After TEST After natural After oven bomb 16 hrs. aging at 75-85OF. test test 1 dayin inthe in the dark at 70'C. 16 hrs. the oven bomb STOCK 6 mo. 12 mo. 3 da. 6 da. at 60' C. a t 70' at 60' C. 69 (Coat) 86 72 86 78 3.3 13 (Breaker) 79 92 100 78 3.0 27 65 (Coat) 78 87 74 86 2.5 90 6 (Sidewall) 78 92 86 80 4.5 74 22 28 (Tread) 49 68 41 71 3.7 6 37 (Tread) 67 91 6.7 71 78 13
...
AVERAGE. 4 . 0
THE GOODYEAR TIRE & RUBBER CO, AKRON,OHIO
I
14.5
HE present paper deals exclusively with high-grade tire stocks, and presents the behavior of this type of stock when subjected to natural aging, to the Geer oven test, and to the Bierer-Davis bomb test. Experimental Procedure NATURALAGING-The test sheets were hung in a cabinet, which was substantially light-tight, in such a manner that there was a liberal clearance between the sheets. The average temperature was 24' to 30' C. (75' to 85' F.). GEER OVEN TEST-The test sheets were placed in a large, electrically heated and controlled oven supplied with fresh air and maintained a t 70" f0.5' C. (158' f 1' F,). The test strips were dried out after aging and tested after 24 hours. BIERER-DAVIS BOMBTEST-The test strips, weighing about 5 grams each (for pure gum stocks), were placed in a bomb having a capacity of 1200 cc. Ordinarily about thirty strips were aged a t one time. The bomb was heated by means of an electrically heated and controlled water-filled thermostat where a temperature of 60' f 0.1" C. (140" i 0.2' F.) was maintained. The test was carried out under a pressure of oxygen of 400 pounds per square inch (about 28 atmospheres). The strips were tested 24 hours after removal from the bomb. Experimental Data The table gives the results of the best cure only on six representative tire stocks, with a comparison of the three aging tests. The data for the natural and oven tests were obtained on the same stocks, whereas the stocks for the bomb test were com-
Cure a t Ltoa~
Stocks 394 and 479 were accelerated stocks containing about 90 per cent rubber by weight. Stocks 452 and 532 were compounded with about 10 and 16 volumes of zinc oxide, respectively, and were accelerated stocks. Stocks 495 and 498 were black tread stocks containing about 20 volumes of gas black. The last two columns give the number of months of natural aging necessary to produce a deterioration equivalent t o the two artificial aging tests, Theoretically, this figure should be a constant, but practically i t is not with the diversified stocks considered, for the figures of the oven test vary from 2.5 to 6.7a range of 270 per cent based on the smaller figure-and those of the bomb test vary from 6 to 27-a range of 350 per cent based on the smaller figure. Natural aging tests show a wide divergence in the aging properties of black tread stocks 495 and 498viz., 41 per cent and 71 per cent, respectively-the 3-day oven t e s t 4 9 per cent and 67 per cent-checking fairly closely 18 months of natural aging. The 16-hour bomb test shows 71 per cent and 78 per cent, respectively, which are not in agreement with the natural aging tests. Stocks C o n t a i n i n g Antioxidants Figures I and I1 give the original (0) tests, 6 months' natural aging tests (6 M), 3-day or 6-day (3 D or 6 D) Geer oven tests, and the 16-hour a t 60' C. bomb test (B) for a series of tread stocks (over a range of cures) containing one part of antioxidant by weight on the rubber. 1
pounded and cured about two years later. However, the resulting stocks gave original physical properties which checked closely with those of the stocks on which natural and oven tests were made. The data are recorded as percentage of the original tensile product, because it is believed that the tensile product takes into account the changes in tensile and elongation, both of which are of significance.
2 3 4
STOCK Control Benzidine Hydroquinol Tolidine
5
6 7
8
STOCK 9-Aminophenol Control (another tread stock) Hydroquinol Benzidine
I n Stocks 6 and 8 the 6 months' natural aging test shows up considerably better than would be expected on the basis of either the bomb or the oven tests. In Stock 4 the bomb checks the 6 months' natural aging tests, and the oven test is misleading. In the other cases the bomb and oven test are comparable with the 6 months' natural aging tests and also with each other.
August, 1925
I S D USTRIAL L4i\rD ENGIiVEERISG CHEMISTRY
Effect of Different Pigments on Aging Properties Figure I11 gives a comparison of the oven and bomb tests of a series of stocks containing 20 volumes per 100 volumes rubber of several representative pigments compounded in a base stock containing smoked sheets 50, pale crepe 50, zinc oxide 5, sulfur 4, and diphenylguanidine 0.75. The aging tests are reported for the best technical cure, which varied from 30 to 45 minutes for all stocks except the magnesium carbonate stock, which cured in 15 minutes, and the gas black stock, which cured in 60 minutes. The data are given in terms
871
strength without any material change in the stress-strain curve. Antioxidants, therefore, aid in preserving the original tensile strength but the stress-strain curve changes materially.
A Low-Temperature Combustion Method for Oxidation of Rubber By Webster N. Jones THE B. F. GOODRICH Co., AKRON, OHIO
HE principal reaction involved in the accelerated aging test of Geer and Evans is that of oxidation. This was proved conclusively by sealing strips of vulcanized thread, inner tube, and side-wall stocks in evacuated tubes (3 mm. pressure) and heating them in the oven along side of test strips. The following data were obtained: STOCK Inner tube Sidewall Thread
-CURETensile Time Temperature strength Min. ' F. Lbs./so. .~ in.
45 SO 80
Origi?tai 292 294 294
AFTER 1 4 DAKSAT
Aging 158' F.
Ultimate elongation Per cent
2885 1951
810 557
2063
943
AFTER EXPOSURE FOR
14 D.im AT 15S0 F.IK OVEN IN SEALED TUBE Tensile Ultimate Tensile Ultimate strength elongation strength elongation Lbs./sq. in. Per cent Lb./sq. in. P e r cent I n n e r tube 2S12 842 1968 675 Sidewall 1992 558 1318 483 106 157 Thread 1504 958
of percentage of the original tensile product for the 6-day oven test a t 70' C., the 16-hour bomb test a t 60' C., and as per cent by weight of oxygen taken up by 100 parts by weight of rubber, assuming that the gain in weight of the samples during aging in the bomb was due to combined oxygen. Gas black, Thermatomic carbon black, and ground natural barytes gave the greatest decrease in tensile product and also the largest amounts of combined oxygen. The other pigments behaved in much the same fashion as the base stock. The amount of combined oxygen is practically a linear function of the time of cure over the range of cure studied, which includes pronounced technical overcures for all stocks with the exception of gas black. Cures a t 75 and 90 minutes were obtained for the gas black stocks, which gave oxygen absorption figures of 1.05 per cent and 1.20 per cent, respectively. These are not plotted for considerations of space, but they give a straight line projection on the curve when plotted. The 90-minute cure was definitely overcured. The two tests on the whole gave parallel results.
Aging with and without Antioxidants By H . A. W i n k e l m a n n THE B F. GOODRICH CO.,A K R O N ,
OHIO
SERIES of compounds with and without antioxidants was A aged in direct sunlight from June to October, 1924. The samples were exposed to the south a t an angle of 45 degrees. The aging of these compounds was also followed in the Geer aging method. The compounds that gave the best results by the Geer aging method also gave the best aging in sunlight. A study of the stress-strain curves taken every 2 weeks on the compounds exposed to direct sunlight showed that the addition of an antioxidant has a very beneficial effect in maintaining the original tensile strength, but in nearly every case there was a regular increase in the stiffness of the stress-strain curve. The rubber compounds without an antioxidant decreased in tensile
After 14 days a t 158" F. there was no appreciable change in the tensile strengths and elongations of the strips that were in the evacuated tubes, while the exposed strips had noticeably deteriorated. These experiments also indicated that heat (158' F.) in the absence of oxygen does not have a marked effect upon the aging of these compounds. The evidence in the table is so conclusive that a low-temperature combustion method with oxygen was developed to follow the rate of oxidation of rubber and to study the factors which affect this oxidation process. The method employed was to pass dry oxygen over a weighed sample of rubber maintained a t a constant temperature. The oxygen was passed a t the rate of approximately 0.5 liter per hour through a drying train consisting of a potassium hydroxide solution, a tower filled with soda lime, and a tower filled with calcium chloride. The reaction chamber was a specially constructed U-tube of Pyrex glass of 35 cc. capacity, with side arms extending a t right angles on the same side of the tube. The tube was heated in an oil bath in an automatically controlled electric oven and the oil was stirred with an electric stirrer. The arms of the U-tube extended through holes in the side of the oven. One arm was attached to the drying train, the other arm was connected to a series of three Fisher absorption bottles, in which the products of oxidation were collected. The first Fisher bottle contained calcium chloride, the second, ascarite, and the third, calcium chloride. Some organic decomposition products were formed during the oxidation. A small amount of reddish oil generally collected in the arm of the U-tube and in the first calcium chloride bottle. It had the odor of oxidized rubber and gave the pyrrole test for levulinic aldehyde. The increase in weight of sample, of the calcium chloride bottle, and of the ascarite bottle were obtained at different intervals of time and a t different temperatures of the oven. The increases in weight were plotted against time. There was an induction period during which the rate of oxidation was slow. After the reaction was once started it proceeded rapidly. Pure rubber oxidized very rapidly and the rate of oxidation was retarded by the addition of antioxidants. Compounded rubbers which showed the best aging in the low-temperature combustion, method also showed the best aging in the Geer oven,
