Effect of Milling on Properties of Hevea and Buna Rubbers

1. The superior cut-growth resistance of Hevea tread stock is partly ascribed to its pronounced relaxation or lengthening when subjected to repeated severe cyclic extensions. Under such conditions the breaking elongation increases from 700 to 900%. GR-S under similar treatment fails to develop any lengthening. 2. As a consequence of this failure to lengthen under repeated severe strains, such as those antecedent to cut-growth failure, GR-S exhibits higher modulus, i.e., at 500 or 600 per cent elongation, or higher retractive forces than does Hevea. This also promotes cut growth. 3. In Hevea, progressive milling (at normal temperature) of tread stocks tends to develop a tensile maximum. At this point of optimum milling, d.c. resistivity reaches a minimum. Overmilling, characterized by a decrease in tensile, shows a sharp rise in log R. 4. With Perbunan, highest tensiles and elongations are realized only after several remillings, at which point modulus and Shore hardness have already shown marked decline. 5. With Buna-S (GR-S), tensile and elongation reach their maxima at about one remilling (at normal temperatures). Milling beyond this point is reflected in sharp degradation of elongation. Log R provides no indication of optimum conditions, since it is rising at this point. 6. In Butyl B, tensile and modulus reach a simultaneous maximum between two and three remillings. Log R in this region is declining and reflects retardation due to the softness of Butyl stocks. 7. Hot milling of GR-S shortens and reduces heat and flex properties. Cold refining lengthens GR-S and improves these properties, but does not protect against subsequent damage from heat. GR-S shortened by heat cannot be restored by subsequent cold refining. The benefits of cold refining apply also to the completely formulated tread. 8. Extreme development of the cold-refining technique produces the following results (based on treatment of complete tread mixing) : Modulus declines continuously to less than 50 per cent of control. Elongation increases continuously to 125 per cent of control. Tensile strength increases to 110 per cent at 20 passes and then remains steady. Hot-iron life (thermal shortening) improves continuously to 310 per cent at 80 passes. Cut-growth resistance improves progressively to 370 per cent of control at 160 passes, and is still rising. 9. The fact that such striking all-round quality takes place at about fifty cold refinings with rolls at 80° F (stock at about 120° F) suggests the importance, in the fabrication of GR-S tires and other products subject to dynamic usage, of (a) avoidance of elevated temperatures at all stages, and (b) further study of the remedial effects of strenuous mastication of refining of GR-S tread type, carbon-reinforced stocks under low-temperature conditions. 10. Attempts to improve GR-S should be oriented in two opposite senses: to decrease its hysteresis at low extensions (under 100 per cent) but to increase its hysteresis, or capacity to relax, at the higher extensions antecedent to cut growth.