It is generally recognized that the protein adsorbed on the rubber particles in Hevea latex plays a predominant róle in connection with stability and coagulation phenomena. Although the function of proteins as protective substances in colloidal dispersions is well known, very little exact information, is available regarding the mechanism of their protective action.

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The low tensile strength of Buna-S gum stocks is generally believed to be due to failure to obtain effective cross-linkage as the result of vulcanization with sulfur and accelerators. Combined with this is the complete absence of crystallization of Buna-S on stretching which, in the case of natural rubber, reinforces and strengthens the vulcanized gel structure. The absence of crystallization in Buna-S can be explained on the basis of nonsymmetry along the polymer chains. Strictly speaking, Buna-S is not a true polymer, for ozonolysis shows that the styrene units are not spaced evenly in the chain but are grouped together in some locations. Ozonolysis also has proved the presence of vinyl groups attached to the chain, resulting from the polymerization of butadiene in the 1,2 instead of the 1,4 position. These vinyl groups must be unevenly spaced along the chain, and mixed trans and cis isomers must be present. Figure 1 illustrates the chemical units present in Buna-S, rubber, and gutta-percha hydrocarbons. In a Buna-S copolymer containing 24.5 per cent of styrene, there are six butadiene to one styrene units. It appears that about one butadiene in five polymerizes in the 1,2 position in the chain. It should be emphasized that, in Buna-S, ozonolysis has shown that no regular order exists in the location of A, B, and C units in the polymer. An entire lack of symmetry in the positioning of these units in the chain would be expected in view of the nature of the polymerization reaction.

1. Three protein fractions have been isolated from fresh and ammoniated latex serum. The usual color and precipitation reactions have been carried out, and they place these proteins in the classification of conjugated glycoproteins. Proteins from fresh latex differ in composition from, and are less sensitive to, coagulating influences than protein from ammonia-preserved latex. 2. Coagulation tests using latex coagulants were performed on the protein hydrosols. It was found that, as the concentration of electrolytes decreased, coagulation of the proteins was inhibited, whereas coagulation readily took place when salts were present in sufficient concentration. 3. Normal ammoniated latex has been subjected to extensive dialytic treatment in specially constructed apparatus. As the ash content of the total solids from the latex approached a value of 0.1 per cent, the mechanical stability decreased and the chemical stability increased. Addition of excess acid or alkali to dialyzed latex yielded stable latices which were highly resistant to spontaneous coagulation. 4. Quantitative coagulation studies, using methyl alcohol and salts of varying types and made on dialyzed latex adjusted to various pH conditions, show that small amounts of a dissolved salt greatly aid coagulation. Considerable variation in the effectiveness of different salts was found. In acid latex of pH 2.5 the charge on the latex particles was reversed from negative to positive, and the salt effect was shown to be different from that experienced in the case where the pH was on the negative side of the isoelectric point. 5. The amount of alcohol required to coagulate dialyzed latex depends on the pH of the latex. It is shown that the amount of alcohol required for coagulation increases on each side of the isoelectric point, which was determined to be at a pH of 4.15. 6. The protective mechanism of proteins has been discussed on the basis of the electrical charges induced on the surface of the latex particles by the presence of the protein layer. The zwitter-ion effect induced by the presence of free amino and carboxyl groups in the rubber proteins is of fundamental importance in determining the colloidal characteristics of latex.