Influence of fractionation and phenolation of Lignoboost – Kraft-lignin on formation and properties of lignin-hydrogels

Technical lignin from Kraft pulping process of softwood was used for the formation of lignin-hydrogels by crosslinking with poly-(ethylene glycol) diglycidyl ether (PEGDGE) in alkaline water. Crosslinking reaction mainly takes place at phenolic hydroxy groups. Products with different molecular weight contributions and thereby different contents of phenolic hydroxy groups results by fractionation of lignin in various acetone/water mixtures. As another method for increasing phenolic hydroxy groups phenolation of lignin at aliphatic side chains was carried out. Phenol and catechol were used as phenolation agents. Changes in structure were analyzed by 31P-NMR, SEC, IR and thermal analysis like TGA/DSC and pyrolysis GC/MS. The molecular weight of the fractions increases with increasing acetone concentration. Molecular weight distribution influences the glass transition temperature Tg, the Klason-lignin content as well as the content of functional groups. Thus, also the phenolation reaction is affected.
Crosslinking was investigated for all modified lignin products. Lignins with low molecular weight don’t form hydrogels. The swelling capacities depends on molecular weight and amount of cross linker. Hydrogels of phenolated lignins show lower swelling capacity at comparable amounts of cross linker. But, the necessary amount of cross linker for formation of hydrogels is slightly lower than for unmodified lignin.
The lignins were investigated for adsorption and complexation of environmentally critical ions like copper(II), lead(II) or arsenate for water purification and enrichment of rare-earth elements. Phenolation affects the adsorption capacity of special ions.