ABSTRACT

Strain-Induced Crystallization (SIC) of unfilled and filled elastomers is investigated by mechanical and thermal analysis during stretching and retraction. The expected heating of the sample due to mechanical work done on the samples is compared to on-line measurements of the surface temperature by IR camera. SIC is quantified by taking into account that crystallization is an exothermal process leading to an additional heating besides entropy caused reversible heating and dissipative heat losses. From the measured excess temperature and the known crystallization enthalpy the degree of crystallinity in the stretched rubber sample is estimated. It is found that the crystallinity of unfilled and filled rubbers shows a pronounced hysteresis, which correlates with the mechanical hysteresis. By comparing carbon black—with silica/silane-filled NR, the influence of filler type on SIC and reinforcing properties is analyzed. A pronounced SIC is also found for carbon black filled EPDM, which can be related to the crystallization of ethylene sequences. For carbon black filled SBR composites no SIC is detected in the range of experimental errors indicating that SBR is not able to crystallize under strain. The characterization of SIC by temperature measurements is a promising technique to investigate synergetic interactions between SIC and filler reinforcement on a broader experimental scale. This will probably deliver a better understanding of the worse fatigue and wear properties of silica/silane filled NR composites in comparison to carbon black filled systems.