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
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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 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
A
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,
