Thermal properties of silica-coated cellulose fibers for increased fire-resistance

In this paper, two precipitation methods of silica, i.e., sol–gel method and silica polymer precipitation, were compared in terms of their impact on the thermal properties of impregnated cellulose fibers. Additionally, the pre-treatment process consisting in dipping the cellulose fibers in NaOH solution to increase the fibers porosity was examined. Three molar phosphoric acid and Flodur 3 (ester hardener based on ethylene glycol diacetate) were used as the sol–gel precipitation agents and 20 wt% aqueous solution of sodium acrylate was used as the polymer precipitation agent. The silica provider was sodium water glass type D. The obtained results have shown that the silicate layer fully covered the cellulose fibers, as confirmed by the scanning electron microscope microphotographs and mid-infrared spectra. Even after thermal treatment at 700 °C, the silicate species still remained in the ash as confirmed by the infrared bands characteristic for sodium silicates, i.e., at 960 and 880 cm⁻¹. Thermogravimetric analysis revealed no significant difference in the weight loss of the cellulose samples treated with different precipitation mixtures, as well as between the raw and pre-treated samples. All weight losses oscillated between 40–50 wt% at 700 °C. Differential thermal analysis curves showed that the pre-treatment process shifts the temperature range in which the different phenomena occur towards higher values. We believe that these results could contribute to the development of cellulose materials with enhanced thermal properties and the process could be an attractive substitute for coatings based on orthosilicates and their derivatives.