Lignin-Containing Cellulose Micro/Nanofibril Reinforcement of Sodium Silicate Adhesive: Rheological Structuring and Performance Optimization

The conversion of paper-based industrial residues into lignin-containing cellulose micro/nanofibrils (LCMNFs) provides an opportunity to combine resource valorization with improved adhesive formulations. This study investigated LCMNF produced from alkaline-pretreated unbleached paper tube residues as reinforcement for formaldehyde-free sodium silicate (SS) adhesive, using polyethylene glycol 400 as a dispersing agent. LCMNF was incorporated at 0.5–2.0 wt%, and the rheological, mechanical, and fracture behavior of the modified adhesives was evaluated. Increasing LCMNF content increased viscosity, yield stress, and storage modulus, indicating increased structural resistance within the SS matrix, while all formulations retained the shear-thinning behavior required for processing. Mechanical performance was strongly concentration-dependent. At 0.5 wt%, LCMNF significantly increased strain at break by approximately 40%, whereas 1.0 wt% increased shear modulus by 41%. Both concentrations significantly increased energy to failure without compromising shear strength and exhibited more tortuous fracture morphologies than the neat SS adhesive. Higher LCMNF contents increased microstructural heterogeneity and reduced shear strength. Overall, 1.0 wt% LCMNF provided the most favorable balance of rheological, mechanical, and fracture properties, demonstrating the potential of residue-derived LCMNF for reinforcing SS adhesives and for reintegration into the paper tube production chain.