Specific Heats of Hevea, GR-S, and GR-I Stocks

Abstract

1. A calorimeter, suitable for determining the specific heats of various rubber compounds (with a precision of 1 per cent or better), is described. Although the results reported are for room temperature (actually 26–27° C), the calorimeter could be operated with minor changes at other temperatures. 2. Employing values of the specific heat at room temperature as given by the National Bureau of Standards for various raw rubbers, it is shown by direct measurement on various gum and loaded stocks that the specific heats of these materials can be calculated from a knowledge of the specific heats of the ingredients. 3. A 5 per cent deviation from linearity in the stress-temperature curves at constant length can cause a total variation in cL of only about 0.00014 calorie per cc. per 1° C over an elongation range of 0 to 200 per cent. This result holds, provided an equation of state exists and provided the region of temperature and elongation considered is one in which phase changes do not occur. Subject to these restrictions, we may neglect variations in cL with elongation and may interpret cL (in calories per gram per ° C) as being identical with cp (in calories per gram per ° C) at the same temperature. 4. Accurate knowledge of cL is needed for the correlation of the reversible heat developed when rubber is stretched rapidly with the entropy component of the stress-strain curve. It is also needed to correlate calculated energy losses with rise in temperature in flexometer experiments.

Specific heats of rubber compounds

“…The specific heats of rubber compositions is an important factor for thermal calculations in various thermal processes, such as mixing, extrusion, calendering, and vulcanization, as well as for predicting the temperature increase of rubber vulcanizates which affects the service life of the products under repeated stressing conditions, such as tires and belts. For this reason, a number of investigations have been made of the measurement of the specific heats for rubbery materials2–14 and various molten polymers 15, 16…”

“…1 The specific heats of rubber compositions is an important factor for thermal calculations in various thermal processes, such as mixing, extrusion, calendering, and vulcanization, as well as for predicting the temperature increase of rubber vulcanizates which affects the service life of the products under repeated stressing conditions, such as tires and belts. For this reason, a number of investigations have been made of the measurement of the specific heats for rubbery materials [2][3][4][5][6][7][8][9][10][11][12][13][14] and various molten polymers. 15,16 Bekkedahl and Matheson 2 determined the specific heats at temperatures sufficiently low to permit an accurate extrapolation to absolute zero in order to use the values for obtaining the free energy of the formation of rubber based on the third law of thermodynamics.…”