The Effect of Strain-Induced Crystallization on the Thermomechanical Behaviour of Rubbers

Most of phenomena involved in deformation of rubber depend on temperature and have distinguishable thermal and calorimetric signatures. However, since the pioneer investigations being those conducted by Gough and Joule, studies were dedicated more to mechanical response, and the thermal aspects of the deformation of rubber were not really explored experimentally. Revisiting the rubber deformation using experimental thermomechanics should offer therefore new perspectives to better understand damage and deformation mechanisms.

In the present study, we focus on strain-induced crystallization (SIC), which is mainly responsible for the reinforcement of the mechanical properties and the high resistance to fatigue and crack growth. SIC induces a specific thermal sensitivity in natural rubber, which can be investigated by using quantitative calorimetry. This technique is based on infrared thermography. The heat sources produced or absorbed by the material due to deformation processes are deduced from the temperature variations by using the heat diffusion equation.

The calorimetric signature of the SIC has been characterized in the case of uniaxial cyclic tensile test and in the influence zone of cracks submitted to cyclic loadings. Under uniaxial tensile test, no mechanical dissipation (intrinsic dissipation) is detected during the deformation of unfilled natural rubber. SIC leads to significant heat production, whereas the melting of crystallites absorbs the same heat quantity with different kinetics. Furthermore, relaxation tests show that crystallite melting does not systematically occur instantaneously. At the crack tip of natural rubber, crystallization acts in two opposite ways: the crystallization process produces additional heat, but crystallites act as fillers, which increases material stiffness in the crack tip zone and therefore reduces the stretch. Moreover, when the natural rubber is filled with carbon black aggregates, the heat sources in the crack tip zone remain positive and small during unloading. This phenomenon is due to the production of mechanical dissipation due to fillers and probably a continuation of the crystallization process. The results attained are compared with those recently obtained in non-crystallizing carbon black filled styrene butadiene rubber.

These results, obtained under uniaxial loading and at the crack tip of a natural rubber, provide information of importance for the understanding and the modeling of SIC.