Natural reinforcement in Hevea rubber: Functional roles of non-rubber components from biosynthesis to performance

Natural rubber (NR), derived from the industrial crop Hevea brasiliensis, serves as a unique plant-based elastomer that underpins numerous bio-industrial applications. NR possesses exceptional mechanical and dynamic properties not replicated in synthetic rubbers. These properties are largely attributed to non-rubber components (NRCs)—such as proteins, phospholipids, lipids, carbohydrates, and inorganic ions—which serve as intrinsic stabilizers, reinforcing agents, and vulcanization accelerators, providing a bio-based, low-cost alternative to synthetic additives. This review comprehensively examines the structure, composition, and functional roles of NRCs in NR, with particular emphasis on their influence on molecular architecture, thermal and oxidative stability, viscoelastic and mechanical properties, vulcanization kinetics, and filler dispersion. Numerous studies are presented demonstrating that proteins and phospholipids contribute to the formation of physical and sacrificial molecular networks, which enhance strain-induced crystallization (SIC), energy dissipation, and mechanical reinforcement. The removal or modification of NRCs significantly alters the colloidal stability, crosslink density, and chain relaxation dynamics of NR. Furthermore, NRCs affect interactions between rubber matrices and reinforcing fillers such as carbon black (CB) and silica, thereby influencing reinforcement efficiency and compound performance. By integrating insights from molecular structures, colloidal interactions, and material performance, this review provides a cohesive framework for understanding NRC-governed structure–property relationships in NR. A comprehensive understanding of the multifaceted roles of NRCs is essential for optimizing NR formulations and advancing the development of next-generation high-performance, sustainable rubber materials.