Towards phosphorylated lignin-based epoxy resins: An integrated technological route to obtain a macromonomer with enhanced thermal and potential flame-retardant properties

Abstract

The growing search to replace petrochemical derivatives with materials having renewable origin has increased the prospection for biomolecules from lignocellulosic biomass. The aim of this work was to develop epoxy resins based on kraft lignin (KL) with flame retardant properties by phosphorylation of this lignin followed by its glycidylation. The phosphorylation of KL was carried out using 85 % wt. H₃PO₄ in THF at 40 °C for 30 min, yielding phosphorylated KL (PKL). Glycidylation of KL and PKL were carried out using epichlorohydrin in an alkaline medium without and with TBAB at room temperature for 24 h, obtaining glycidylated KL (GKL) and PKL (GPKL), respectively. Lignins were characterized by FTIR, ¹H, ³¹P and HSQC NMR, TGA, DSC, MCC and wet analyses. Spectral analyses showed that the grafting of phosphorous occurred in the form of monoester phosphate groups and that the use of TBAB significantly increased the quantity of grafted glycidyl groups. GPKL presented enhanced thermal, thermo-oxidative, and solubility properties in comparison with KL. The results of the proposed integrated route showed it was possible to develop bio-based material with enhanced thermal and flame-retardant properties, having great potential for use in various technological applications.

Keywords: Chemical modification; Lignin valorization; Lignochemical platform.