Effect of Storage on Milled Crude Rubber

Abstract

Abstract Smoked sheets which have been milled to different degrees of plasticity and stored in bale form for periods up to 9 months show a decided increase in modulus, plasticity, and recovery values. The increase in recovery value is the most noticeable, the change being 180 per cent of the original, if the rubber is baled at 40–50° C. and stored at 10–20° C. for 9 months. When the aged, milled rubber is mixed in a tube stock and processed on a tubing machine, the stock is rougher and the speed of extrusion is slower than a similar stock containing freshly milled crude rubber. The plasticity of stocks which are subjected to tubing operations is shown best on an extrusion type plastometer in preference to the compression type. Milled crude rubber “freezes” at temperatures below 0° C. and thaws at room temperature of 15–25° C. It may be permanently frozen by being placed under slight pressure for several months, freezing temperature being unnecessary. In either sheet or milled form this type of frozen rubber requires a temperature of about 50° C. to thaw, whereas temporarily frozen rubber will thaw at room temperature.