Abstract
The thermomechanical analyzer (TMA) is eminently suitable for the measurement of the thermal expansion coefficient of rubbers, provided the equipment is calibrated with materials of similar expansion coefficient. FKM or other thermally stable and amorphous polymers (of independently determined thermal expansion characteristics) can be used as calibration standards. The relative simplicity and precision of the TMA method for the determination of the coefficient of isotropic linear expansion, and hence, of volumetric expansion of rubbers, makes this important thermodynamic parameter easily accessible experimentally. Because of the frequently observed anisotropy of molded elastomeric compounds, measurements in all three dimensions are required to define linear, isotropic expansion and shrinkage which, to an excellent approximation, are represented by arithmetic averages of the three unidirectional values. Good agreement has been found between the average (isotropic) shrinkage determined experimentally and that calculated from the average (isotropic) coefficient of linear thermal expansion, α. The effect of fillers, such as carbon black, on α is additive. This makes it possible to predict α and shrinkage of all filled compounds from a single value of α of the unfilled rubber. The linear thermal expansion coefficients of the tested rubbers (FKM, NBR, EPDM) increase linearly with temperature, the increase being generally stronger for FKM than for the other elastomers. The appreciably higher mold shrinkage of FKM relative to NBR is directly due to the higher α of FKM for comparable levels of fillers. This must be kept in mind when expedience dictates the use of the same molds for FKM and NBR compounds.
Temperature-dependent structural behaviour of samarium cobalt oxide
