The impact of mastication-induced molecular weight reduction on the strain-induced crystallization and mechanical properties of natural rubber

Natural rubber (NR) owes its exceptional mechanical properties to strain-induced crystallization (SIC), a phenomenon strongly influenced by molecular structure. Mastication, a crucial processing step, reduces raw NR molecular weight via chain scission, yet its effects on SIC and mechanical performance remain incompletely understood. This study systematically investigates how mastication-induced molecular weight reduction modulates SIC and the mechanical properties of NR. We varied mastication degrees (via controlled passes on a two-roll mill) to obtain NR samples with different molecular weights. SIC behavior was characterized using in-situ wide-angle X-ray scattering (WAXS). Mechanical properties, including static tensile, hardness, and tear strength, were also evaluated. Results of cure characteristics showed that reducing molecular weight via mastication prolongs scorch time T S1 and cure time T 90 while regulating cure rate index. Results of WAXS showed that reducing molecular weight hinders SIC. Specifically, samples with higher molecular weight (less intensive mastication) exhibited more extensive and rapid SIC under tension, correlating with superior tensile strength, tear resistance, and modulus. In contrast, lower molecular weight (more intensive mastication) improved processability but compromised SIC-driven mechanical performance. This work establishes a clear mechanistic link between mastication-induced molecular weight changes, SIC, and mechanical properties of NR. These findings offer practical guidelines for optimizing mastication processes to balance NR processability and mechanical performance, thereby advancing the design of high-performance NR products for diverse industrial applications. • Mastication reduces molecular weight, hindering SIC in NR. • Mastication reduces molecular weight, extending scorch/cure times and lowering cure rate index. • Lower molecular weight from severe mastication compromises tensile and tear strength. • Severe mastication improves processability but weakens SIC-driven mechanical properties. • A link between mastication, SIC, and NR's final performance is established.