Demethylation of sulfobutylated kraft lignin and its application as phenol-formaldehyde adhesives

Abstract

The shift toward sustainable and eco-friendly adhesives has led to increased research into renewable alternatives for petroleum-derived components in phenol-formaldehyde (PF) resins. Lignin, a natural phenolic polymer, presents a promising option due to its structural similarity to phenol. However, its high molecular weight, structural heterogeneity, and low reactivity hinder its direct incorporation into adhesive formulations. To overcome these limitations, this study investigates a two-step chemical modification—sulfobutylation followed by demethylation—to improve lignin’s solubility and performance in PF resins. The sulfobutylation (SB) step resulted in a decrease in methoxy and hydroxyl groups while increasing molecular weight, sulfur content, solubility, and charge density, significantly improving lignin’s aqueous compatibility. Demethylation of sulfobutylated lignin (DSB) further decreased methoxy content, with a slight increase in hydroxyl groups compared to SB lignin. Additionally, demethylation led to higher molecular weight and a reduction in sulfur content back to levels observed in kraft lignin (KL) while maintaining solubility. These changes were attributed to an increase in β-O-4 interunit linkages, which contributed to improved reactivity. The demethylated sulfobutylated lignin-PF (DSBPF) resins retained the PF resin’s molecular structure but exhibited more reactive formaldehyde adducts. Increasing the lignin content led to higher MW and viscosity, which enhanced bonding strength but reduced thermal stability. The β-O-4 linkages in DSB contributed to improved adhesive properties, increasing bonding strength by 19% in DSBPF20 and 25% in DSBPF60. However, the resins also exhibited higher free formaldehyde emissions, exceeding safety limits. Additionally, while pH and non-volatile content remained stable, water absorption increased, potentially impacting long-term durability. Despite these challenges, the modified resins showed enhanced fire resistance and adhesion performance compared to conventional PF resins. Future research should focus on optimizing resin synthesis parameters, including NaOH catalyst amount, reaction time and temperature, and curing conditions, to improve performance while reducing emissions. Furthermore, incorporating enhancers such as melamine, urea, or furfural could help maintain or improve adhesive properties while minimizing formaldehyde content. This study provides valuable insights into lignin-based PF resins, contributing to the development of more sustainable, high-performance adhesives and reducing reliance on petroleum-based materials.