Film from Mixtures of Natural and Synthetic Rubber Latex. Physical Properties

When tested in a single standardized procedure for cast latex films, the type of synthetic-rubber latex employed in latex blends containing 70 per cent or more natural-rubber latex had little effect on the stress-strain properties of the mixture. Cold-rubber latexes imparted higher stress-strain values to blends with natural rubber than did the corresponding hot-rubber latexes. The improvement was particularly noted on comparison of tensile product values. Low-conversion synthetic polymers produced higher stress-strain properties than high-conversion polymers in blends with natural rubber, even though their tensile strengths in 100 per cent synthetic stocks were approximately equal. Optimum physical properties were obtained by use of blends with synthetic polymers of medium Mooney viscosity. It is believed that the appearance of an optimum Mooney viscosity is tied in with the necessity of having quite high molecular weight on the one hand, and, on the other, the ability of the particles to knit well, the latter in turn requiring a comparative freedom from tight gel. Tensile product values increased with increasing styrene content in the synthetic polymer, but, correspondingly, the low-temperature stiffening increased. The physical properties of a natural rubber stock are far superior to those of any of the synthetic-rubber latexes tested to date. Cold-rubber latexes now in production are an improvement over high-temperature latexes, for example, in wet gel strength but do not approach natural rubber latex in stress-strain properties.