Heats of Reaction of Natural Rubber With Sulfur

An adiabatic copper calorimeter was used to determine the heats of vulcanization of pale crepe natural rubber with sulfur for mixtures varying in composition from 0 to 32 percent added sulfur. The side reaction that produces hydrogen sulfide was avoided by using reaction temperatures near 155 °C. Heats of reaction at 25 °C and at 155 °C are reported. The enthalpy change at 25 °C for compounds containing up to about 18 percent sulfur is given in joules per gram of vulcanizate by the equation, ΔH ₂₅ = -21.1 · S with a standard deviation of 11 J/g. Here S is the percentage of combined sulfur. Above 18 percent sulfur the heat of reaction at 25 °C remains approximately constant at 380 ±8 J/g. A comparison is made between the heat of vulcanization and the volume change on vulcanization, both as functions of combined sulfur, by making use of data in the literature.