Polyacrylate-based Water-absorbent Hydrogels Prepared with Lignin-related Compounds: Process Conditions and Performance

Utilizing biomass resources to synthesize water-absorbent resin has attracted global interest as a new research direction. Lignin, as the second most abundant renewable biopolymer in nature, is a strong candidate for use in renewable materials. In this study, water-absorbent resins were synthesized from lignosulfonate (LS) and acrylic acid (AA) by grafting copolymerization with an aqueous solution of potassium persulfate (KPS) as the initiator and N’N-methylene-bis-acrylamide (MBA) as a crosslinking agent. The optimum process parameters of lignosulfonate-grafted-polyacrylic acid resin (LS-PAA) were obtained via a single-factor method: AA 30 wt.%, LS 2.67 wt.% (to AA), cross-linker 0.03 wt.% (to AA), initiator 0.43 wt.% (to AA), and neutralization 65%. The optimum reaction temperature was 60 °C. Although lignin is a hydrophobic polymer, it has many reactive functional groups that can react with AA. It has been expected that adding lignins can change the cross-linking density by their three-dimensional structure, thus increasing the water absorbency. The LS-PAA resins were compared to other kinds of lignin derivatives including alkali lignin, lignocresol, lignoresorcinol, and lignopyrogallol under the same reaction conditions. The results showed that changing the types of lignin can change the resulting water absorbency. Lignopyrogallol-grafted-polyacrylic acid resin (LP-PAA) showed the highest water absorbency (2137 g/g).