Synthesis and characterization of phenol–furfural resins using lignin modified by a low transition temperature mixture

Abstract

A new modification method based on an oxalic acid/choline low transition temperature mixture was used to activate the chemical groups of lignin as a substitute for phenol in phenol–furfural resins. The optimum modification conditions were 100 °C and 6 h at a molar ratio of oxalic acid to choline chloride of 1 : 2. The content of phenolic –OH in the modified lignin increased from 1.75 to 3.01 mmol g⁻¹ and the content of –OCH₃ decreased from 10.86 to 8.57 wt%, increasing the reactivity of lignin. The modified lignin was partially substituted for phenol at various substitution rates in the synthesis of phenol–furfural (PFU) resins. When the substitution rate of lignin to phenol was 50%, the lignin–PFU had a high bond strength (up to 1.84 MPa) and a low free phenol content and the lignin–PFU showed a higher curing rate and thermostability than PFU alone. The oxalic acid/choline chloride low transition temperature mixture was shown to be an efficient and green lignin modification method for the synthesis of PFU.