Protein-mediated bonding of lignin-containing fibres at high solids content

A drastic improvement in the energy efficiency of papermaking requires new sustainable solutions to bind lignocellulosic fibres together without an excess amount of water in the web structure. In this work, protein-mediated fibre bonding was studied at high solids content (ca. 85 %) using lignocellulosic wood fibres with varying lignin content and two proteins, gelatin and casein, with known adhesive properties. Pulp fibres were mixed with 5–10 wt% of dry protein powder, forming a sheet with a type of airlaid technology, and finally heat treatment was applied to soften the protein and bind it with the surface lignin of fibres. This procedure was investigated for chemithermomechanical pulp (CTMP) and oxidised CTMP, with Kraft pulp used as a low-lignin-containing reference. The hydrated protein additive reacted with the fibre surface during the heat treatment, providing inter-fibre bonds and improving the strength and breaking strain of the formed samples. The strongest adhesion was achieved when gelatin and fibres with oxidised lignin were used. The applied temperature of 120°C was clearly lower than that of conventional hot-pressing operations, which could reduce the amount of thermal energy needed in an industrial production process.