Heat Treatment of Industrial Alkaline Lignin and its Potential Application as an Adhesive for Green Wood–Lignin Composites

An industrial alkaline lignin preparation and poplar particles were mixed and hot pressed under different conditions. The alkaline lignin and the lignin isolated from the poplar particles were thoroughly investigated by quantitative nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and elemental analysis techniques. For the first time, it was found that the content of β-O-4′ linkages decreased accompanying with the formation of β-β′, β-5′, and β-1′ linkages at mild heat treatment temperatures (130–170 °C). However, it should be noted that most of the β-O-4′, β-β′, β-5′, and β-1′ linkages nearly disappeared at a higher temperature (180 °C). Cross-linking reactions were predominant during the hot-pressing process as the molecular weight of lignin increased at elevated temperature. Owing to the self-bonding between lignin fragments during the hot-pressing process, a green poplar wood–lignin composite was successfully prepared with poplar particles and a small amount of alkaline lignin (∼20%, w/w). Internal bond strength with 0.47 MPa was surprisingly achieved under the conditions of pressing temperature 160 °C, pressure 5 MPa. An in-depth understanding of the concerted reactions between fragmentation and cross-linking reaction in lignin during hot pressing was beneficial to a better development of self-bonding green wood–lignin composites in future.