Treasuring waste lignin as superior reinforcing filler in high cis-polybutadiene rubber: A direct comparative study with standard reinforcing silica and carbon black

There has been ever raising concern in last few decades about the utilization of biomass for different commercial applications such as filler materials in rubber composites. In this context, an interesting pathway has been proposed to develop such composites by introducing waste lignin as a reinforcing constituent in high cis-polybutadiene rubber (BR). With a judicious selection of rubber curing ingredients and, simultaneously, adopting suitable solid-state mixing protocols, particularly, a relatively high-temperature multi-steps melt-mixing process (above the glass transition temperature of lignin), rubber composites with an outstanding mechanical performance were prepared. The reinforced rubber composites with 50 (weight) parts lignin loading per hundred parts of rubber (phr) offer ∼10 MPa tensile strength (TS), ∼276% elongation at break (EB), and ∼3.51 MPa tensile stress at 100% elongation (so-called rubber modulus M100). These values are superior when compared with the composites comprised with standard reinforcing carbon black (∼8.5 TS, ∼224% EB, ∼2.79 M100) and even with a silica-silane system (∼7.34 TS, ∼229% EB, ∼2.44 M100) with same filler loading. The unique combination of the curing packages and four-stage mixing process allowed us to establish a homogeneous and fine dispersion of lignin. Furthermore, this is the first time that available models of rubber reinforcement are applied to the description of the reinforcement mechanisms of lignin in a soft elastomer involving various aspects like filler-filler interaction, rubber-filler interactions, critical strains for destroying the filler-filler network, effective filler volume fractions, shape factor, etc. The developed compounding methods for BR and their characterization and modeling can be easily applied to other commercial rubbers facilitating a real breakthrough in developing cheap and bio-based high-performance rubber composites.