Reactivity enhancement of organosolv lignin by phenolation for improved bio-based thermosets

• Combination of 1H, 13C, 31P and HSQC NMR shows high degree of mono-phenolation.
• Intense enhancement of aromatic moieties, protons and hydroxyl groups.
• Effect of various reaction parameters on the degree of phenolation is evaluated.
• Correlation of carbon content and degree of phenolation is established.

Lignin can function as a structural backbone in a variety of resin systems such as phenol formaldehyde, epoxy and polybenzoxazine. However, the low number of reactive sites hinders effective condensation reactions. This article presents the phenolation of beech organosolv lignin to increase the number of potential cross-linking sites. By varying reaction conditions, their influence on the extent of phenolation is deduced. The results reveal that the degree of phenolation differs significantly with regard to phenol/lignin feed ratio, catalyst content, reaction time and temperature as well as water and solvent content. By comparison of 1H, 13C and 31P NMR spectroscopy, comparable trends and an up to 13-fold increase of functional groups prone to cross-linking are attained. Structural features are supported by 1H 13C HSQC NMR spectra. Based on the quantification by NMR spectroscopy, a quick and simple method to determine the degree of phenolation by elemental analysis is presented. The altered properties of the highly activated lignin including decreased Mw and dispersity (from GPC), and improved solubility will facilitate application in various thermosetting resins.

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