Compressibility of Natural Rubber at Pressures Below 500 kg/cm(2)

The specific volumes of unvulcanized natural rubber and of a peroxide-cured vulcanizate of natural rubber were measured at pressures of 1–500 kg/cm(2) 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 cm(3)/e over the range studied can be represented by [Formula: see text] where P is the pressure in kg/cm(2), and t V(0),(25)= 1.0951 and 1.1032 cm(3)/g; 10(4)A = 6.54 and 6.36 per degree; 10(6)α(25)= −50.5 and −50.4 (kg/cm(2))(−1); 10(6)k(1) = −0.227 and −0.203 per degree; 10(9)β(25)= 10 and 11.5 (kg/cm(2))(−2); and 10(9)k(2)=0.048 and 0.073 per degree, respectively. The compressibility of unvulcanized natural rubber at 25° and 1 kg/cm(2) is thus 50.5×10(−6) (kg/cm(2))(−1) falling to 40.6×10(−6) (kg/cm(2)) (−1) at a pressure of 500 kg/cm(2). 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/cm(2) the use of the Tait equation is recommended.