Surface hardness and flammability of Na₂SiO₃ and nano-TiO₂ reinforced wood composites

The objective of this study was to explore an effect of the combined inorganic materials on the wood hardness and flame-retardancy properties in a concept of sustainable material management. Herein, the reinforcement of Scots pine (Pinus sylvestris L.) sapwood with sodium silicate and TiO₂ nanoparticles via vacuum-pressure technique is reported. Pyrolysis of modified wood was studied by TG-FTIR analysis; the results showed that maximum weight loss for the modified wood was obtained at 40-50 °C lower temperatures compared to the reference untreated wood. The Gram-Schmidt profiles and spectra extracted at maxima absorption from Gram-Schmidt plots indicated chemical changes in wood-inorganic composites. SEM/EDS analysis revealed the presence of Na-O-Si solid gel within the wood-cell lumen and showed that TiO₂ was homogeneously distributed within the amorphous Na-O-Si glass-forming phase to form a thin surface coating. EDS mapping further revealed the higher diffusivity of sodium into the cell wall compared to the silicon compound. The presence of amorphous sodium silicate and nano-TiO₂ was additionally confirmed by XRD analysis. FTIR spectra confirmed the chemical changes in Scots pine sapwood induced by alkalization. Brinell hardness test showed that the hardness of the modified wood increased with the highest value (44% increase in hardness) obtained for 10% Na₂SiO₃-nTiO₂ modified wood. The results showed good correlation between TG and flammability test; limiting oxygen index (LOI) values for the wood-inorganic composites increased by 9-14% compared to the untreated wood.