Engineering flame-retardancy in water-based adhesives: a structure–performance driven approach for enhanced safety and functionality

Abstract

There is a need to shift away from conventional formaldehyde and organic solvent-based adhesives toward sustainable water-based adhesives, as they pose a serious threat to the environment. Poly vinyl alcohol (PVA)-based adhesives are widely employed for bonding cellulosic substrates; however, their poor flame resistance restricts their utility in wood and related applications where fire safety is critical. Herein, we report the synthesis of a flame-retardant adhesive through performance driven modification in each step to get an optimum formulation, giving the best results for adhesive strength and flame-retardancy. TA was functionalized with sodium hexametaphosphate (SHMP) to form a phosphorylated complex (TS), which blended with PVA, enhanced the dry and wet adhesive strengths to 5.2 and 1.5 MPa, respectively. Further optimization was achieved by dispersing aluminium hydroxide (ATH) in PVA to form a stable PAH dispersion prior to TS addition, producing the PAH@TS adhesive system. Despite the hydrophilic nature of PVA-based systems, this formulation exhibited substantial performance, with dry and wet adhesive strengths of 8.2 and 6.1 MPa, respectively. The flame-retardant properties were significantly improved, as evidenced by a limiting oxygen index (LOI) of 47.31%, a vertical flame test (VFT) char length of 3.3 cm, and cone calorimetry results showing reduction of 93.9 and 91.5% in heat release rate (pHRR) and total heat release (THR), increased residue and 95.9% decrease in FIGRA. These findings confirm the synergistic effect of phosphorus, phenolic and hydroxide functionalities. Overall, PAH@TS represents a sustainable, formaldehyde-free adhesive with excellent bonding, durability, and fire safety, The results suggest that PAH@TS holds strong potential for application in wood-based materials and construction-related systems where enhanced fire safety and moisture resistance are required.