Influence of nanosilica particles on hysteresis and strain induced crystallization of natural rubber as investigated by a real time true stress-true strain birefringence system

In this paper, the influence of nanosilica particles on hysteresis and stress induced crystallization of natural rubber compounds is described. For this purpose a newly developed uniaxial stretching system that monitors birefringence, true stress and true strain was used in real time while stretching and retraction is applied to the nanocomposite sheets. This allowed us to link the continuous structural measurements - as represented by birefringence - and the true mechanical properties during the actual stretching and retraction stages as influenced by the presence of nanosilica particles. The birefringence measurement system was also shown to be quite robust in detecting very low levels of retardation prevalent in rubbers. The stress optical behavior of unfilled NR compounds was compared with those filled with 5 phr and 10 phr nanosilica particles. This study allowed the determination of a critical birefringence value above which strain induced crystallization occurs in the NR compounds. The results indicate that the addition of nanosilica reduces the stress optical constant of natural rubber compounds. In addition, the critical birefringence, above which the strain - induced crystallization occurs, decreases in the presence of 5 phr nanosilica thus indicating the enhancement of the crystallization process at this loading. However, the nanosilica loading beyond 5phr was not found to be a further benefit to strain - induced crystallization mostly because of the introduction of large surface area nanosilica agglomerates that act as a physical barrier between the chains that also most likely suppresses the crosslinking reaction during the preparation.