Assessment of green approaches for the synthesis of physically crosslinked lignin hydrogels

Lignin is an excellent candidate to be used as a starting material for hydrogel synthesis due to its highly functional character. The exhaustible character of the fossil resources linked to the increase of plastic residues in the environment encourages an intensive research on biorenewable and biodegradable polymers to synthesize new materials. Taking into account this current scenario, this work searches for new green routes to elaborate physical hydrogels with excellent capacity of swelling and suitable consistency. To this end, lignin and poly(vinyl alcohol) were blended in different proportions following a three-level-two-factorial design and using six different routes of crosslinking and drying for each set of experiments. The hydrogels formed under the optimal conditions were characterized by FTIR, SEM, XRD, DSC and TGA and their mechanical properties were also evaluated by compression tests. The selected optimum synthesis routes enabled the obtaining of physically crosslinked hydrogels with up to 800% water retention ability. FTIR spectra confirmed the interactions between lignin and PVA showing shifts and modifications on the characteristic bands of the raw polymers. Compression tests showed that all the hydrogels kept complete integrity even compressing them up to an 80% of their initial thickness.