Competition issues with food industry have recently boosted the exploration of alternative solutions capable to replace starch in paper making to a certain extent. Covalent grafting of quaternary ammonium groups onto LignoBoost® Eucalyptus kraft lignin has recently shown promise in this regard as the cationic products feature excellent water-solubility across the entire pH scale. Considering these suitable properties, cationic kraft lignin (CL) was applied and evaluated, for the first time, in the perspective of partial substitution of cationic starch as reinforcement agent in papermaking using bleached Eucalyptus kraft pulp. Based on assessment of key paper properties, such as mechanical (tensile, bursting, tearing indexes), structural (roughness, capillarity rise, air resistance, internal strength, water contact angle) and optical ones (brightness, opacity, relative color change), it is safe to conclude that partial replacement of conventional cationic starch by cationic lignin does not compromise these features. The results also show that properties can be fine-tuned by varying the degree of lignin derivatization, providing paper-specific solutions for replacing starch by lignin potentially available at large-scale at the pulp and paper industry, closing the loop within the circular economy concept. Beyond these results it has been demonstrated for the first time that cellulosic substrates can be coated by all-lignin formulations using a layer-by-layer approach to deposit alternating layers of cationic kraft lignin and lignosulfonate (LS). The electrostatic attraction between the excellent water-soluble positively charged CL and negatively charged LS leads to the formation of hydrophobic, water-insoluble self-organized macromolecule aggregates, whose structure depends on the order and mode of application. Both replacement of wet-end cationic starch by cationic lignin and all-lignin polyelectrolyte complex coating can impart a series of interesting properties to paper.

Cationic hardwood kraft lignin for improving circular economy in papermaking

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