Softening and Freezing of Crepe

Abstract The outstanding results may be summarized as follows: (i) Heating gristly crepe to 60° C. or above causes softening, a change characterized by a large fall in tensile strength and rigidity and by increase in breaking elongation and permanent set. With some crepes there is a transient initial rise in tensile strength. Subsequent storage at 0° C. produces changes in the reverse direction, which, however, are rapidly eliminated by heating at 34° C. or above. It appears, therefore, that heating above 60° C. produces: (a) a permanent change not reversible by freezing; and (b) a temporary change reversible by freezing. (ii) Heating gristly crepe below 50° C. only moderately increases the extensibility and flexibility, true softening being absent. These changes are reversible by freezing, the recovery being quicker than with softened crepe. (iii) Prolonged storage at 0° C. (beyond 20–30 days) causes a gradual decrease in strength and stiffness in both gristly and softened crepes. With gristly crepe these effects are more than counteracted by thawing at 25° C., giving a tougher, stronger product than the original. (iv) The statement that golden crepe does not harden is incorrect; as, however, this is a softened crepe the hardening is only temporary and does not restore the original gristly qualities. (v) Uniform temperature and rate of elongation are essential in tensile tests on raw rubber. The permanent set of broken rings becomes sensibly constant in 2 hours.