Kraft lignin as wet-strength and wet-stiffness additives for molded pulp materials

Derived from renewable resources, cellulose based materials are gaining new importance due to their recyclability and biodegradability. Still, one fundamental challenge is their high sensitivity to water. The addition of wet strength agents (WSA) is hence necessary to maintain strength and integrity in humid or wet conditions. In this article, technical lignin was used as WSA in bleached kraft pulp, which was thermopressed to materials with the potential to replace plastics. Cationic starch or a cationic flocculant (PCB 20) was used as a retention aid during the filtration process. The effect of moisture during thermopressing and lignin particle size were also studied. The results showed that elevated moisture during pressing had the biggest impact both on dry and wet strength. Wet strength (tensile test), up to 9 MPa, and wet strength retention, up to 12 %, were obtained when moisture was present during pressing. However, the type of flocculant and the size of the lignin particles also had a limited effect on the strength. Wet strength improvement was most probably due to the plasticization of lignin at high temperatures, which was further aided by water. The cellulose-lignin network was strengthened by the melting of lignin, consolidating the network after cooling. The wet stiffness of the cellulose substrates was also increased from 200 to 938 MPa in the presence of lignin, while the elongation was maintained and no embrittlement was observed. The results in this article might hence pave the way for new developments in molded pulp and cellulose based plastics replacement.