Bio-based sugar polyols as multi-functional interfacial modifiers in sustainable carbon black/kraft lignin NBR-based composites

This study investigates the multi-phase viscoelastic behavior and physical reinforcement mechanisms of acrylonitrile–butadiene rubber hybrid composites reinforced with fixed amounts of carbon black (25 phr) and sustainable kraft lignin (30 phr). To optimize processability without conventional additives, three bio-based polyols—erythritol, xylitol, and sorbitol—were incorporated up to 20 phr via direct compounding without chemical modification. Dynamic rheology proved that molten polyols act as efficient internal lubricants, significantly lowering compound minimum torque and dynamic complex viscosity, which is suggested to occur through the disruption of the hydrogen-bonded networks of the biopolymer and physical couplings within the rubber matrix. Conversely, tensile and dynamic mechanical analyses revealed a unique anti-plasticization effect upon cooling. Instead of causing matrix softening, the solidified polyols act as rigid secondary crystalline fillers. For the erythritol series, in situ formed needle-like microcrystals establish a physical armature across filler–rubber boundaries, yielding an anomalous mechanical upsurge at 10 phr of erythritol where tensile strength peaked at 21.5 MPa and elongation at break reached almost 770% independently of the chemical cross-link density. Dynamic mechanical analysis confirmed a distinct damping depression and a systematic shift of glass transition temperature toward higher temperatures (up to −9.7℃). High-temperature storage modulus confirmed that the primary covalent network remains dominant above the polyol melting points, while ambient temperature compression set values consistently below 10% (alongside stable hardness of 64 – 66 Shore A) verified excellent elastic memory and network integrity under room-temperature conditions, confirming these polyols as multi-functional processing and reinforcing co-additives. This study introduces a new concept for designing sustainable rubber composites by exploiting the temperature-dependent multi-functionality of bio-based polyols. Acting as internal lubricants during processing and as reinforcing microcrystalline fillers after solidification, the polyols overcome the conventional trade-off between processability and mechanical performance. The proposed anti-plasticization mechanism provides a new strategy for developing high-performance lignin-containing elastomers with reduced dependence on fossil-derived additives.