Compressibility of Natural Rubber at Pressures Below 500 kg/cm²

The specific volumes of unvulcanized natural rubber and of a peroxide-cured vulcanizate of natural rubber were measured at pressures of 1-500 kg/cm² at temperatures from 0 to 25 °C. Observations on mercury-filled dilatometers were made through a window in the pressure system. No time effects or hysteresis phenomena were observed. The specific volume V in cm³/e over the range studied can be represented by where P is the pressure in kg/cm², and t the temperature in °C. The constants for the unvulcanized and for the peroxide-cured samples are: V ₀,₂₅= 1.0951 and 1.1032 cm³/g;10⁴ A = 6.54 and 6.36 per degree;10⁶ α ₂₅= -50.5 and -50.4 (kg/cm²)^-1;10⁶ k ₁ = -0.227 and -0.203 per degree;10⁹ β ₂₅= 10 and 11.5 (kg/cm²)^-2;and 10⁹ k ₂=0.048 and 0.073 per degree, respectively. The compressibility of unvulcanized natural rubber at 25° and 1 kg/cm² is thus 50.5×10^-6 (kg/cm²)^-1 falling to 40.6×10^-6 (kg/cm²) ^-1 at a pressure of 500 kg/cm². It is concluded that a low degree of vulcanization produces no significant changes in the constants listed. The values are not far different from those obtained by extrapolating to zero sulfur content the observations of Scott on the rubbersulfur system. Calculations of values of compressibility (and its reciprocal the bulk modulus), "internal pressure", bulk wave velocity, difference between specific heats, and several other physical properties are in reasonable agreement with those obtained by direct observation by other workers. For the prediction of values at pressures above 500 kg/cm² the use of the Tait equation is recommended.