Studies in hard rubber reaction. Part I. Heat of hard rubber reaction

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Abstract
A method for the determination of heat of hard‐rubber reaction by the application of differential thermal analysis is reported. The heat of reaction was determined with stocks containing different rubber/sulfur ratios and also with a 68/32 stock, preheated to contain different amounts of combined sulfur. Heat evolution is observed first with samples containing about 7% sulfur and therefrom the amount of heat evolved shows a nearly linear increase up to 30% sulfur. With increasing combined sulfur in the 68/32 stock, the quantity of exothermic heat gradually diminishes; so also does the temperature of initiation, i.e., the temperature at which heat evolution appears to begin. Initiation of exothermic reaction appears to be a function of composition and temperature of the mass. An increase in the rate of reaction was observed when the composition reached 0.5 g.‐atom of sulfur per isoprene unit. An endothermic dehydrogenation reaction is observed at the end of the hard‐rubber reaction. This, however; does not affect the determination of exothermic heat, because there is similar dehydrogenation taking place in the reference material (ebonite) which almost balances this heat loss. The final product has a lower sulfur content due to loss of sulfur as hydrogen sulfide.

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