Structural characterization of technical lignins for the production of adhesives: Application to lignosulfonate, kraft, soda-anthraquinone, organosolv and ethanol process lignins

The aim of this study is to completely characterize technical lignins so they can be incorporated into phenolic resins such as phenol–formaldehyde. Technical lignins such as Kraft (KL), lignosulfonate (LS), soda-anthraquinone (SAL), organosolv (OSL) and ethanol process lignins (EPL) were characterized by elemental analysis, functional groups analysis, molecular weight distribution, Mannich reactivity and other techniques.Results from elemental analysis (% nitrogen), ash content and polysaccharides show that all these lignins have a high level of purity after treatment except for lignosulfonate, which has a high ash content, and ethanol process lignin, which has a high nitrogen content. We discuss the results of the functional analysis and molecular weight distribution (Mw, Mn, Mw/Mn) for all lignins in order to select a good lignin for adhesive production. After characterizing the lignins, we provide an approximate C9 formula and active reaction sites for each lignin.Our study of lignin reactivity by Mannich reactivity and UV-spectroscopy shows that Kraft and soda-anthraquinone lignins are more reactive towards modification than the others studied. These lignins may be good raw material for lignin derivatives such as lignin-based phenol–formaldehyde resins.