Fully Biobased Thermosetting Adhesive from Enzymatic Saccharification Residue

Abstract

This study was aimed at using the solid residue of enzyme-based cellulosic sugar production (saccharification) to formulate a fully biobased heat-curable wood adhesive. The novelty of such utilization lies in three implemented strategies: (1) valorizing lignin in the hardwood saccharification residue without chemical prerefining, (2) increasing reactive surfaces by wet-grinding the lignin-rich residue, and (3) blending citric acid (biobased cross-linker) as an adhesive component to enhance bond performance of the residue. Single-lap shear specimens were prepared and tested. Results showed that saccharification improved the wood bonding ability of the resulting residue. Grinding the residue into smaller particles also led to stronger bonding. Without citric acid (only saccharification residue), the lap shear strength value attained 80% (or 6.25 MPa) of that of a commercial phenol-formaldehyde (PF) wood adhesive. With 33.3% w/w citric acid (66.7% w/w saccharification residue), the dry and wet bonding properties achieved values comparable to those of PF. Ester cross-linking was verified to account for such enhanced bonding. This formaldehyde-free, competitive adhesive product signifies waste valorization realized via an organic solvent-free process. Upon optimizing the grinding step, this adhesive could contribute to the viability of the cellulosic sugar-based biorefinery system, as a collateral benefit and a win–win strategy for utilizing plant biomass.