Cold-pressed hybrid lignin composite reinforced with beech and spruce wood for automotive applications

Abstract

Kraft lignin is an underutilised raw material with the potential to be used in many kinds of composite materials. Currently, lignin is suggested by many researchers to be used for the production of adhesives. However, because of the low reactivity of kraft lignin, these developments rarely reach a high technology readiness level. Therefore, in the presented study, we hypothesised that kraft lignin could act both as a binder and reinforcing element without chemical activation. Composite pellets containing 90 % pulverised kraft lignin and 10 % beech or spruce wood powder were produced by pressing at 16 kN and 22 degrees C. We quantified filler particle size distribution, assessed composite microstructure by SEM, and evaluated viscoelastic properties using compression DMA, including frequency sweeps, strain sweeps, and quasi-static force ramps. Spruce-based samples showed generally higher mechanical strength, while the beech composite with the coarse fraction (B1) exhibited an E ' about 35 % higher than spruce composites (S1, S2) and roughly 60 % higher than fine beech (B2); its loss factor tan delta was nearly half that of the spruce-filled samples. SEM images showed only small surface cracks in wood-filled composites, compared to numerous deep cracks in the lignin-only reference. These findings suggest that wood-lignin compatibility can be tuned to enhance stiffness, elasticity, and structural integrity, and may benefit further from pressing near lignin's glass transition temperature. The single-step, low-temperature process avoids chemical curing, minimises energy input, and relies on standard pressing equipment.