Effect of Absorbers in Rubber A. A. SOMERVILLE
R. T. Vanderbilt Company, New York, N. Y.
CST
HE oxidation of vulcanized rubber and its deterioration as a result of oxidation are old and well-known phenomena, the ontward characteristics of which are loss- of tensile strength, hrittlena%, surface cracking under tension, and an increase in the m o u n t of acetone-soluble material. The rubber increases in weight as oxygen is absorbed. In this species of oxidation, or aging, the oxygen is mostly derived from the atmosphere.
Oxygen Content of Rubber Much lesb is known about the oxygen contained in the interior of a piece of vulcanized rubber. That such oxygen exists in rubber goods is generally known. Williams and Neal (6) have determined the amount of oxygen which remains dissolved in pure gum vulcanized rubber under different conditions of temperature and pressure. For example, a t 760 mni. and 29" C . (somewhat higher than average room temperature) rubber can hold dissolved 12.5 cc. of oxygen per 100 grams. according to these authors. This is equivajentto about0.018 per cent by weight. Besides this, the compounding of robber with certain pigments and fillers may introduce still greater proportions of oxygen. Carbon black, for instance, is known to contain considerable quantities. The amount of oxygen held by carbon black varies with the source and treatment of the material, but Johnson (Z)has shown one sample of carbon black to contain 5.76 per cent. The oxygen in carbon black appears to be held in a peculiar state which can be considered neither gaseous adsorption nor chemical combination with the carbon. It is therefore clear that manyrubber compounds made with ordinary rompounding ingredients, and particularly with carbon black, may contain appreciable quantities of oxygen.
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The cracking of vulcanized rubber, in particular rubber compounded with carbon black, when subjected to often repeated dynamic strain is discussed, and a new, chemical means of reducing flexcracking tendency is described. The new method consists in the addition to the rubber compound of organic materials which absorb oxygen in the presence of a base and is founded on the theory that oxygen dissolved in the rubber and adsorbed or otherwise held by compounding ingredients such as carbon black plays a part in the causation of flex-cracking and should be removed. The conditions under which these oxygen absorbers are most effective are described, and the results obtainable from pyrogallol-ethanolamine and pyroqallolquinol-ethanolamine combinations are shown. Factors which influence the tendency to flex-crack, such as the amount of carbon black and the sulfur ratios used, are discussed, and a laboratory method of evaluating flex-cracking is described.
The function and fate of this oxygen in vulcanization and during the subsequent life of the rubber is obscure and speculative but it is not unreasonable to suppose that it must have a bearing on the aging properties and service obtainable from the rubber article. It is well known that carbon black compounds age more rapidly than similar compounds made with other fillers, or with none. It is probable that this poor aging property of carbon black, which offsets to a considerable extent 11
VOL. 28, NO. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
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