Research of strain induced crystallization and tensile properties of vulcanized natural rubber based on crosslink densities

Natural rubber (NR) has excellent mechanical properties than synthetic rubber for its strain induced crystallization (SIC) property. It is known that crosslink density has significant influences on SIC, but mechanism of SIC enhanced tensile properties based on crosslink densities has not investigated clearly. In this study, vulcanizates of NR and NR devoid of non-rubber components, characterized by varying crosslink densities, are prepared. Mechanical and crystallization properties and the structure evolution of the systems occurring during stretching are analyzed by conducting tensile tests and using the in-situ (WAXD) technique. Results reveal that the crosslink density significantly affects the mechanical properties and SIC of the NR materials. The maximum tensile strength is recorded when the crosslink density is 1.84 × 10−4 mol/cm3. The structural evolution of the NR vulcanizates characterized by varying crosslink densities is reported, and the strengthening mechanism associated with the systems is proposed. The stretching process associated with the NR vulcanizates can be divided into three stages, namely stages I, II, and III, based on the crystallization process. The NR vulcanizates characterized by low crosslink densities are primarily reinforced by a series of crystallites in stage II of stretching. Stress upturn is realized following the formation of a large number of crystallite networks in stage III. NR vulcanizates characterized by high crosslink densities form parallel reinforcements with the surrounding network post crystallization in stage II, and this can be attributed to the presence of a large number of crosslink chains in the system. Stress upturn can be realized even in the presence of a small number of crystallite networks in stage III. Furthermore, the results indicate that the process of crystallization significantly affects the elongation at break. The influence of crystallization on the elongation at break has different results divided by a critical crosslink density 0.75 × 10−4 mol/cm3. When the crosslink density is lower than the critical crosslink density, the ideal elongation at break is smaller than real elongation at break. While that is higher than the critical crosslink density, the ideal elongation at break is higher than the real. This is because crystallization in stages II and III determine the final elongation at break.