Use of lignin in polyurethane-based structural wood adhesives

Lignin is one of the main components of wood and the second most abundant natural polymer. It is appreciated as a synthesis starter due to its aromatic and hydroxyl functionalities. Adhesive systems (PF or PUR) comprising lignin as a raw material are recently on the rise as this natural product could replace petroleum derivatives in the industry. Lignin extraction from wood is a complex procedure, and purification treatments are required to ensure an efficient reactivity in most applications.One-component polyurethane systems (1C PUR) are used in the wood panel industry due to the good compatibility between MDI and wood, which generally favours resin penetration. However, this family of adhesives does not always fulfil the requirements of applications where thick glue lines are needed, such as structural wood bonding, where absorption of the resin and reaction with moisture can cause poor glue intimacy or foaming, respectively.In this study, differently extracted lignins have been dispersed in 1C PUR and used for the structural bonding of beech wood. The dispersion methodology was a key parameter to obtain homogeneous systems with small particle size, which improved stability. The mechanical properties of glued specimens have all been enhanced by the incorporation of lignin in this way.