Biobased and mechanically stiff lignosulfonate/cationic-polyelectrolyte/sugar complexes with coexisting ionic and covalent crosslinks

Abstract

We prepared moldable materials from lignosulfonate, an industrial lignin derivative, using a combination of ionic crosslinking between lignosulfonate and cationic polyelectrolytes and covalent crosslinking via the Maillard reaction. The mechanical properties of the lignosulfonate/cationic-polyelectrolyte/sugar complex at the optimal composition (stress at break: 55.1 MPa; Young’s modulus: 2791.8 MPa; strain at break: 3%) were comparable to those of poly(phenylene sulfide), which is used as a high-performance engineering plastic. In addition to the good mechanical properties, the lignosulfonate/cationic-polyelectrolyte/sugar complex was water-insoluble, in contrast with the high water solubility of the complex without the reducing sugar. Furthermore, the addition of a reducing sugar (fructose) to the complexes increased adhesion to a metal substrate. These improvements in the mechanical properties, water resistance, and adhesive strength of the lignosulfonate complex will expand the applications of lignosulfonate under high mechanical stress conditions and in water and biobased adhesives. Lignosulfonate is one of the major industrial products from wood. To utilize this compound as a moldable and strong material, we prepared a complex of lignosulfonate with cationic polyelectrolytes and reducing sugars. This complex contains two different crosslinking structures, i.e., ionic crosslinking between lignosulfonate and cationic polyelectrolytes and covalent crosslinking via the Maillard reaction of amino groups with reducing sugars. The coexisting of two different crosslinking greatly improved mechanical strength, Young’s modulus, water resistance, and adhesive strength compared with solely crosslinked complexes composed of the same ingredients.