Strain-induced crystallisation of reinforced elastomers using surface calorimetry

Abstract

The crystallinity of rubbers which are amenable to crystallisation upon stretching is classically measured using X-ray diffraction (XRD) techniques. Recently, an alternative method for evaluating the crystallinity in unfilled natural rubber (NR) during stretching was proposed. This method is based on surface calorimetry principles (Le Cam, 2018) and was validated by XRD measurements in Le Cam et al. (2018). For reinforced rubber composites, the calorimetric response is not only due to the elastic couplings and the crystallisation/melting process, but also due to the intrinsic dissipation and additional viscoelastic dissipation resulting from the presence of fillers. This makes estimating the crystallinity of carbon black (CB) reinforced rubber compounds from calorimetric techniques typically more challenging. In this paper, however, the calorimetric approach for measuring crystallinity for CB reinforced rubber compounds was investigated. Crystallinity measurements were performed using both an unfilled and three CB reinforced NR compounds. A comparative study was performed using the XRD technique on the same four compounds. The results confirmed the appropriateness of the calorimetric technique for determining the crystallinity in the unfilled NR. The combination of the XRD and the calorimetric techniques allowed the different heat sources under strain to be identified and therefore, the thermal energy associated with crystallisation to be determined. This was done initially using one of the CB reinforced NR compounds and validated using the two other CB reinforced NR compounds. The results show reasonable correlation between the crystallinity measured using the novel calorimetric technique and the classical XRD technique for CB reinforced NR compounds. The development of this methodology has potentially significant impact. The calorimetric approach does not require the measurement of the mechanical response during testing. This is useful for measuring crystallinity gradients from infrared thermography coupled with full kinematic field measurements, typically at crack tips, where estimating the local mechanical response is very difficult or inaccurate.