Alternating Behavior of Fatty Acids in Rubber Compounds

It has been shown (Bayer, Ber., 10, 1286 (1877); Garner and Randall, J. Chem. Soc., 125, 881 (1924)) that the alternations in the melting points and heats of crystallization of normal fatty acids were due to differences in the crystalline structure. Two distinct curves of behavior corresponding to the acids with odd and even numbers of carbon atoms have been shown for the heats of crystallization. McKee (Naturwissenschaften, 11, 938 (1923)) discussing the fats used in the diet of diabetics, states that in the case of glycerides of acids with an uneven number of carbon atoms these are tolerated and their use leads to gradual reduction in acidosis. Glycerides having an even number of carbon atoms break down during metabolism to form butyric, oxybutyric, and acetoacetic acids. Smith and Boone (Ind. Eng. Chem., 19, 398 (1927)) have experimented with the addition of purified fatty acids to standard mixings of different kinds of raw rubber. They employed molar proportions of the acid to conform with the recommendations of Kratz, Flower, and Coolidge (Ind. Eng. Chem., 12, 317 (1920)) that comparisons of organic accelerators should be made with molecularly equivalent amounts of the substances in question. They determined the tensile strength and elongation of the vulcanized products and found no relations between the elongation and the number of carbon atoms present in the acids, but in the case of the tensile strength an alternating effect was evident, the extent of the differences being affected by cure. The tests were carried out on two kinds of rubber, namely, blanket crepe and pale crepe, and the alternating effect of the acids on the tensile strength was more marked in the former case. As pointed out in the paper, the difference in behavior of the two kinds of rubber when treated with fatty acids was presumably due to the difference in the kind and proportions of acids which they originally contained. In fact, in the case of pale crepe the fatty acids already present in the raw rubber masked the alternating behavior to a marked extent, and the curve obtained was in most respects quite different from that obtained with blanket crepe.