Rise of Temperature on Fast Stretching of Butyl Rubber

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Abstract
The rise of temperature on fast stretching of natural and synthetic rubber stocks was investigated recently by Dart, Anthony, and Guth (D.A.G.). Since then the experimental technique was considerably improved and the taking of data greatly simplified thereby. By D.A.G. emphasis was placed to record temperature rises of up to 15°C for extensions up to 700 percent by a fast galvanometer of moderate sensitivity. In the present work this high heat was investigated. In addition, however, a less fast but more sensitive galvanometer was employed to record changes of temperature as small as 0.001°C for 0–80 percent extension. In accordance with early work by Joule on natural gum (1859) it was found that also Butyl gum stocks show an initial cooling effect. This passes at a thermoelastic inversion point into a heating effect. In agreement with the recent theory by James and Guth the thermoelastic inversion point was found to depend solely upon the thermal expansion coefficient of the unstretched stock. The coefficient of thermal expansion was measured for Butyl stocks in the present work. Butyl tread stocks also show the thermoelastic inversion point. The work of D.A.G. showed that the rise in temperature on extension is a slowly rising function of the extension with a steep upward turn and almost linear continuation after the onset of crystallization. The samples were kept extended for a minute and then the cooling arising on retraction also measured. The negative of the cooling on retraction plotted against extension fall, according to the second law of thermodynamics as it should under the extension curve, but crosses it at the onset of crystallization. For Butyl gum stocks this crossing takes place at rather high (600 percent or more) extensions in agreement with x‐ray work. More similarity was found between Butyl and Hevea tread stocks than between the corresponding gum stocks. Loading shifts the onset of crystallization to a range of smaller extensions. In addition to measuring the change of temperature on extension and retraction, the residual rise in temperature after an extension and immediate fast cycle retraction was also observed. This quantity is a measure of internal friction in Butyl rubber and is closely connected with rebound and free vibration tests. Summarizing, the method described has a twofold application for development work: 1. Changes of the temperature on extension and delayed retraction indicate in a simple manner the onset and progress of crystallization. 2. Changes of the temperature in a fast cycle give an estimate for internal friction. Both these applications will facilitate attempts to improve present synthetics.

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