600
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
loadings. It is also significant that the bound rubber level in these stocks is below the level given by carbon black alone. DISCUSSION
The mechanism involved in this inhibiting effect of carbon black on the oxidation reactions of rubber appears to be similar to that proposed by Rhodes and Goldsmith ( 5 )as an explanation for the retarding effect of carbon blacks on the drying of oil paints. They ascribed this retarding effect to the adqorption of intermediate oxidation products on the carbon This action
Vol. 46, No. 3
carbon accelerates the decomposition of the benzoyl pc’yosidt:. The action of carbon on benzoyl peroxide in t,he natui,iil rubiicr mixes is probably similar, but increased scission rathor iiiaii increased gelation apparently is the result. SUMMARY
The effect of carbon black on the autocatalytic slage of ositiation of unvulcanized rubber is shown to be specific for the lypc of carbon, the reaction conditions, and t,he polymer. Carbon black is an effective antioxidant for cold rubber \\hen the rubber is heated in air or oxygen. The effectivene5. or rarbon in inhibiting oxidation increases as t,he volatile contcnt of the carbon increases. The special case of low loadings of liiglily oxidized carbons, which completely represred gelation during Banbury mixing, is of particular interest. The opposite behavior of carbon black during rubber gclalion is observed when benzoyl peroxide is used as catalyst. With cold rubber carbon black promotes gelation, while with natural rubber increased scission results. Alteration of any one of t’hese variables produces profound changes in the course and extent of the oxidation reactioriP. LITERATURE CITED
(1) (2) (3) (4) (5) 21phr Mioronex W - 6
+ 3.3%B m o y l Peroxide
(6) (7) (8) (9) (10)
TEMP HEATING, *C.
Figure 9. Effect of Carbon Black on Natural Rubber Gelation Initiated hg Benzoyl Peroxide D q adsorption method
is somewhat different from the well-known function of cai bon black as a light-screen to prevent the deterioration of various organic polymers ( 2 , 4, 9). The mechanism in the case of rubber appears to be the inactivation of hydroperoxides or their decomposition products by adsorption on the carbon surface. On the basis of studies to date, the primary characteristic of the carbon surface, responsible for the adsorption of these intermediate products, appears to be its state of oxidation. Decreasing the combined surface oxygen results in a lowering of the selective adsorption of intermediates, with this selectivity in adsorption largely disappearing as the combined oxygen is reduced t o minimum levels. The inhibition effect of carbon black on the oxidation of unvulcanized cold rubber a t relatively high temperatures is in contrast with its catalytic effect on the oxidation of vulcanized cold rubber at lower temperatules. The oxygen solubility-temperature relationship shown by van Anierongen-Le., smaller differences between loaded and unloaded stocks with increasing temperature-provides a possible explanation. At low temperatures, carbon substantially increases oxygen concentration in the rubber stock, thereby increasing the reaction rate. Since at high temperatures this solubility effect is no longer a factor, carbon inhibits the oxidation. There is the possibility, however, that the curatives used for vulcanization mag exert pronounced changes in the oxidation reactions of cured stocks. The opposite behavior of carbon black in benzoyl peroxide initiated gelation of cold ruhber mav he due t o the fact that
(11)
Amerongen, G . J. van, I h i d . , 45, 377 (1953). Biggs, B. S.,Bell System Tech. J . , 30, 1078 (1951,. Kohman, G. T., J . Phys. C’hevz., 33, 226 (1929). Lister, W.S . . Trans. Inst. R t ~ h b vI d , , 8, 241 (1932). Rhodes, F. H., and Goldsmit!i. If. E., ISD. EKG.CHI,:M..18, Xi6 (1926). Shelton, J. R., and Winn, H., I b i d . , 38, 71 (194fl). Sweitzer, C. W., Ruhber &e, 72, .i5 (1952). Sweitzer, C. IT., and Lyon. I?,,1x11. I
