Fig. 4: Adhesion mechanism of lignin adhesives for bonding wood.

a, Yields of resultant aromatic monomers from hydrogenolysis of FPLs before (25 °C) and after hot pressing. The H⁺ shown in a,d indicates the addition of sulfuric acid as a crosslinking catalyst to lignin adhesives. b,c, Side-chain (b) and aromatic (c) regions in heteronuclear single quantum coherence spectra of FPL before and after hot pressing at 190 °C (δ, chemical shift; the chemical structure in the left panel of c represents an acetal-protected lignin monomeric unit). d, Optical microscopy images of the glue lines in the plywood products prepared at different hot-pressing temperatures. e, Scanning electron microscopy (left) and Fourier transform infrared (FTIR) microscopy (right) images of the glue-line region in the plywood prepared at 190 °C (the FTIR image was recorded at 1,597 cm⁻¹, a characteristic absorption peak of lignin; Extended Data Fig. 8f). See Methods for details of sample preparation and characterization.