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Abstract
Abstract A quantitative relation has been established between rate of crack growth and temperature rise, as determined in the Goodrich flexometer test, for GR-S tread stocks compounded with normal quantities of EPC or MPC blacks. This is shown in Equation 1. The relation is valid for all variations which affect the state of cure—that is, time of cure, sulfur or accelerator variation, retarding or activating softeners, etc. For other variations in the tread composition, such as an increase or decrease in the loading of the EPC or MPC black, Equation 2 holds. This relation is valid for all other known recipe modifications which do not involve carbon blacks other than EPC or MPC. Improvements in the balance between flex-cracking and rise in temperature, obtained by compounding or by an improvement in the polymers, may be expressed as the ratio of the observed flexing life to the calculated flexing life using the equation appropriate to the case. This ratio is termed the quality index. Coarser blacks substituted for the EPC or MPC blacks give an improvement in the balance between temperature rise and crack growth. The slope of the curve relating crack growth to heat rise is the same for a variety of flexing machines and for various testing temperatures. Knowledge of this relation has made it possible to evaluate, with some confidence, compounding changes intended to improve the resistance to crack growth without a sacrifice in temperature build-up, and also to evaluate small quantities of experimental polymers without the necessity for precise adjustments in either the proportions of curing agents or curing time.
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