ZnP-H2Im glass for flame retardant wood: In situ vitrification and synergistic mechanisms

Abstract

A novel method for enhancing wood flame retardancy by in situ vitrifying ZnP-H₂Im coordination polymer within wood was developed. ZnP-H₂Im crystals were synthesized via double impregnation and then converted into a continuous and largely continuous glass film that fully coats the wood fibers using melting-quenching. The vitrification process was confirmed by SEM-EDS, XRD, and DSC. The cell wall thickness of the modified wood (ZIW39 glass) was 58.96 % greater than that of untreated wood. The plastic and glass transition peaks of ZnP-H₂Im in the modified wood were detected. The heat treatment caused the ZnP-H₂Im to melt, as evidenced by the disappearance of its characteristic XRD peaks. The molten phase permeated the cell walls, forming a glass film that increased the cell wall thickness. The flame-retardant mechanism involves forming a physical barrier, melting to absorb heat, generating phosphoric acid to catalyze carbonization, and releasing inert gases. As a result, the total heat release in 300 s of ZIW39 glass was reduced by 72.38 % and the limiting oxygen index (LOI) was increased by 69.78 %. Notably, ZIW39 glass exhibited an exceptionally low leaching rate of only 2.65 % after 120 h of immersion. This approach, highlighting in situ vitrification and multiple flame-retardant mechanisms, overcomes limitations of conventional flame retardants. It offers an efficient, eco-friendly strategy for wood modification with industrial potential.