Zinc ion cross-linked sodium alginate modified hexagonal boron nitride to enhance the flame retardant properties of composite coatings

Abstract

Herein, an efficient boron nitride (BN)-based inorganic flame retardant was introduced into epoxy resin to enhance its fire resistance. Specifically, polyethyleneimine (PEI) was firstly grafted on the BN surface (BN/PEI) through Lewis acid-base interaction, which can greatly improve its water dispersibility. Then, sodium alginate (SA), as a synergistic flame retardant, was combined with BN/PEI (BPS) through electrostatic action to further improve the flame retardant properties of the composite coating. To promote the formation of the carbon layer during the combustion process, zinc ions (Zn2+) were adsorbed on the SA surface through ion exchange. The reinforcement effect of the resulting BPS@Zn hybrids on the epoxy (EP) coatings was explored by fire performance tests. The experimental results confirmed that the BPS@Zn-based EP composite coating exhibited the lowest backside temperature (165.2 ℃), maximum expansion height (23.6 mm) and expansion ratio (18.43) during the combustion process. The TGA-DTG curves proved that the BPS@Zn/EP composite coating had the highest carbon residue (31.7%) compared with other samples. Meanwhile, BPS@Zn/EP exhibited the lowest smoke density rating (41.2%), indicating its best smoke suppression effect. These enhancements can be mainly attributed to the optimal thermal barrier effect of BN, the intrinsic flame retardancy of SA, and the catalytic carbonization effect of metal ions. Further, the carbon residue analysis illustrated that the metal oxides (ZnO) were uniformly distributed in the char layer, which was beneficial to the improvement of its strength and thermal stability.