Some Physical Properties of Elastomers at Low Temperature

Little has been published concerning the effect of low temperature on the elasticity of rubbers. Since in many applications this is the only property which justifies their use, some attention must be paid to this problem. Rubbers can be characterized by the temperature at which they become brittle and by the width of the transition region. The sharpness of the transition between the elastic and the glassy states is greatest when the testing time is long compared to the molecular relaxation times. Other physical factors such as sample shape, stress at which the modulus is calculated, and the previous history of the sample affect the absolute nature of the elastic properties measured. A test is described which gives the values of the elastic constants at slow rates of extension, at moderate extensions, on previously flexed samples. For these conditions the superiority of gum stocks at low temperature decrease in the order polybutadiene, natural rubber, Butyl, Neoprene FR. The comparative values of the stocks can be changed by compounding variations. Addition of plasticizer shifts the relative modulus vs. temperature curve down the temperature scale without producing a significant change in shape. Addition of reinforcing materials decreases the sharpness of the transition without changing the position of the curve.