Phenolation to Improve Hardwood Kraft Lignin for Wood Adhesive Application

Lignins, naturally occurring aromatic polymers with phenylpropane units, are promising bio-based alternatives for petroleum-based products. Resole-type phenol formaldehyde (PF) adhesive is commonly used in wood composites requiring durability and weather-proofness. However, PF adhesive is a petroleum-based product. The objective of this study is to transform the low-reactivity hardwood kraft lignin (KL) as the phenol substitute in the PF adhesive formulation by acidic phenolation. The variations in the molecular weights, chemical structures, and functional groups in lignins were investigated before and after the phenolation. The results indicate that the KL can be cleaved, and phenols are crosslinked onto KL to produce phenolated kraft lignin (PKL) under the suitable phenolation condition, heating 3/5 (w/w) of KL/phenol at 90 °C for 2 h with 5% H₂SO₄ as the catalyst. Resole-type PKL-PF adhesives can be directly synthesized after the phenolation in the same reactor. Plywood laminated with this adhesive obtains satisfactory strength and low formaldehyde emission. This not only reduces the usage of petroleum-based phenol but also increases the reactivity and applications for hardwood KL.