Study of gelation processes in flame retardant compositions of the SiO2 sol system - a phosphate-containing additive

Abstract

One of the promising directions for ensuring the fire resistance of wooden building materials is the development of silicon phosphate compositions, thanks to which the formation of a protective layer is possible, which effectively prevents the spread of fire. The paper examines the possibility of using orthophosphate acid as a modifying additive for silicic acid sols. The influence of H3PO4 concentration on the rheological characteristics of the studied sols was determined. Sols with small amounts of orthophosphate acid (1–2%) lost their fluidity in the interval of 45–70 minutes, increasing the content of the additive to 8% led to a sharp increase in optical density values and extended the survivability interval of the sol to 24 hours. Coatings based on gels with 1–2% orthophosphate acid were elastic, but increasing the content of the additive led to the formation of hard, inelastic coatings. According to the results of IR spectroscopy, the effect of orthophosphate acid on the nature of polycondensation was confirmed. The degree of polycondensation was estimated by the ratio of intensity (I) and band width (d) at 1050 cm–1, which corresponds to vibrations of Si-O-Si bonds. According to the results of the comparison of the I/d indicator, the fact of the formation of linear siloxane chains was confirmed in the case of the use of small H3PO4 additives. Linear polycondensation ensures greater homogeneity of the gel, which was confirmed by IR spectroscopy of gels heat-treated at 800 °C and by the microscopic method. The coating on the wood surface, based on a fire-resistant silica-containing composition with small additions of orthophosphate acid, had the most uniform and continuous structure.