Barley - a yet un-tapped feedstock for improved vegetable protein-based wood adhesives

The use of tri-formaldehyde adhesives (urea-formaldehyde resin adhesive, phenol-formaldehyde resin adhesive, and melamine resin adhesive) could endanger human health and damage the environment. Meanwhile, soy protein adhesive has received wide attention in wood-based panel preparation due to its green, sustainable, and abundant attributes. However, pure soy protein isolate (SPI) adhesives still lack competitive advantages compared to aldehyde adhesives. Herein, inspired by dietary pairing, a strategy was developed to obtain barley brewer's grain protein (BGP) from industrial waste for building the structural network of SPI adhesives. The globular protein of the SPI provides adhesion and hydrophilicity, and the BGP's alcohol-soluble protein and glutenin chain segments were beneficial for wet adhesion and water resistance. In addition, molecular restructuring, protein denaturation and chemical cross-linking were employed to prepare pure bio-based adhesives with desirable properties by denaturing SPI/BGP by adding papain (PA), sodium dodecyl sulfate or alkali modification and subsequently cross-linking with a bio-derived green cross-linker (TGA). Compared to the pure SPI adhesive, the in situ formulated SP/BGP-PA-T adhesive increased the wet strength, debonding work, wood breaking rate, and residual rate by 192.5% (1.55 MPa), 186.7% (0.453 J), 79%, and 83.4% (from 70.6%), respectively. Also, it decreased the viscosity and moisture absorption rate by 98.0% (5.07 Pa s) and 6.85% (from 11.84%), respectively. The modulated amino acid ratios, protein denaturation, molecular recombination and green chemical cross-linking provide a feasible strategy for developing desirable protein-based materials.