Silica-based coatings of cellulose substrates for improved flame retardancy properties

Halogens, commonly found in conventional flame retardants (FRs), can emit harmful gases during combustion. Consequently, increasing attention has been directed toward the development of halogen-free FRs that are safer for human health and the environment. In this study, environmentally friendly and cost-effective copper (Cu)-containing silica-based coatings were developed using Copper(II) acetate dihydrate as the Cu precursor, while poly(vinyl alcohol) (PVA) was investigated as a binding polymer to improve coating adhesion. The precursor sols were prepared by the sol–gel method and applied to cotton fabric by immersion, then dried at 80 °C and cured at 170 °C. The structure, morphology, and thermal behavior of the coated fabrics were characterized using thermal analysis (TG-DSC), attenuated total reflectance infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), and UV-Raman spectroscopy. XRF and UV-Raman analyses confirmed the incorporation of Cu species within the silica matrix. TA results revealed significant improvements in thermal stability of the coated cotton fabrics compared to the uncoated ones. The incorporation of Cu further influenced the degradation behavior, suggesting a catalytic effect during thermo-oxidative decomposition. SEM observations confirmed the formation of continuous coatings on the cotton fibers, while ATR-FTIR and XRD analyses indicated interactions between the silica network and the cellulose structure. Vertical flame resistance (VFT) test showed that the Cu-containing silica coatings enhanced the flame resistance of the cotton substrates. UV-Raman analysis of the char residues revealed the formation of carbonaceous structures characterized by D and G bands, indicating the development of a protective char layer during combustion. The results demonstrate that Cu-containing silica coatings have significant potential as a halogen-free strategy for enhancing the thermal stability and flame retardancy of cotton fabrics.