Lignin demethylation for modifying halloysite nanotubes towards robust phenolic foams with excellent thermal insulation and flame retardancy

The demethylation of lignin was conducted under HBr catalysis and microwave irradiation resulting in a demethylation rate of 21.4% as calculated from ¹H NMR. The demethylated lignin (DL) was applied to modify halloysite nanotubes (HNTs) forming a nano-hybrid containing 16.5 wt% of DL according to TGA results, and showing rough surfaces dotted with DL particles as observed through TEM. HNTs-DL was introduced into resol resin synthesis where DL molecules participate in phenol-formaldehyde polycondensation, functioning as a binder to connect the inorganic HNTs framework and the organic resol resin molecules, giving an increase in flow activation energy as calculated from rheological results. The prepared phenolic foam containing 0.4 wt% of HNTs-DL shows a significant increase by 37.7% in compressive strength and a decrease by 27.3% in pulverization ratio compared to the neat phenolic foam, and exhibits uniform cells with the highest cell density as observed in SEM, explaining its improved performance in thermal insulation and flame retardancy.