Noncovalently functionalized Kraft lignin/chitosan adhesive with enhanced wood bonding and water resistance property

Abstract

The majority of research on chitosan (CHS) and Kraft lignin (KL) in wood adhesives has focused on covalent functionalization strategies, which often involve toxic chemicals, high costs, and require elevated temperatures and pressures for effective crosslinking. In contrast, non-covalent functionalization offers a simpler and more environmentally friendly approach to incorporating biopolymers into wood adhesives. Despite its potential, there has been limited exploration of non-covalent functionalization, particularly involving polyelectrolyte complexes (PECs) and KL. No studies have assessed the noncovalent functionalization of KL in adhesive applications. To address this gap, this thesis investigates non-covalent functionalization strategies for biobased adhesives using CHS, poly(sodium 4-styrene sulfonate) (PSS), and KL. The study reports the successful formation of PSS-KL hybrids via ultrasonication, followed by the synthesis of CHS-PSS-KL PECs through self-assembly. The stability of the hybrids was confirmed using zeta potential measurements, while FTIR analysis revealed peak shifts indicative of hydrogen bonding. Wide-angle X-ray scattering (WAXS) confirmed the formation of a semicrystalline adhesive with increased order of the lamellar structure and evidence of π-π stacking interactions between PSS and KL. Thermogravimetric analysis (TGA) demonstrated that the adhesive containing 2.5 wt.% KL exhibited the highest thermal stability, likely due to the effective and homogenous incorporation of KL. Rheological studies showed a rapid increase in viscosity upon the formation of the CHS-PSS PEC, while the addition of KL led to a decrease in viscosity, suggesting that KL disrupts the PEC network formation. Wood bonding tests revealed that, under dry conditions, the adhesive with 10 wt.% KL (C-S-10L) achieved the highest tensile strength with a value of 2.39 ± 0.30 MPa, while under wet conditions, the adhesive with 5 wt.% KL (C-S-5L) exhibited the highest tensile strength of 1.40 ± 0.25 MPa. This study provides valuable insights into the use of non-covalent functionalization strategies for developing sustainable biobased adhesives.