Preparation of Lignin-Based Adhesives via Pre-Hydrolysis and Study of Their Bonding Mechanism

Abstract

To meet the green development demands of the current wood-based panel industry, Mason pine (Pinus massonianaLamb.), poplar (Populus tomentosaCarr.), and wheat straw (Triticum aestivumL.) were used as raw materials. A series of milled wood lignins (MWL) were prepared using an environmentally friendly pre-hydrolysis technology, and the MWL was used as an adhesive for the preparation of three-layer plywood. The results found that the phenolic hydroxyl content and reactivity of pre-hydrolyzed Mason pine MWL were significantly improved, and the bonding strength of the prepared plywood increased. Further optimization of hot-pressing process parameters, under the optimal conditions of a hot-pressing temperature of 150°C, hot-pressing time of 4.17 min/mm, and single-side adhesive spread of 100 g/m², resulted in a plywood dry strength of up to 4.34 MPa. Compared with unhydrolyzed lignin under the same hot-pressing process conditions, the dry strength of the adhesive increased by 214%. Mechanistic studies showed that during the hot-pressing process, lignin formed a uniform and dense adhesive layer on the plywood surface through depolymerization and re-condensation reactions, and constructed a three-dimensional cross-linked structure inside the plywood, thereby imparting excellent mechanical properties to the plywood. Activating MWL through pre-hydrolysis technology provides new ideas for the preparation of high-performance lignin-based adhesives.