This article is devoted to the study of an organomineral thermal insulation composite based on a rigid polyurethane foam (PUF) matrix and a flammability-reducing silicate component-sodium metasilicate pentahydrate (Na2SiO3•5H2O, 0.9-1.25 mm fraction). The physicochemical regularities of the interaction between the silicate component and isocyanate, the fire-retardant mechanism of the silicate component, and its effect on the thermal properties and flammability of the composite have been investigated.
A chemical interaction between the isocyanate functional groups of the reactive mixture and the water contained in the silicate component was established to occur at the phase boundary.
The fire-retardant mechanism of the silicate component consists in the endothermic release of crystallization water (70-170 ℃), which induces intumescence of the metasilicate, resulting in the formation of a porous heat-insulating barrier.
The silicate component does not alter the fundamental decomposition mechanism of the polyurethane, yet it shifts the stages of thermo-oxidative degradation to lower temperatures (by 15-85 ℃). The final residue increases up to 8.1 times (from 3.6 to 29.1 wt%), which directly corresponds to the contribution of the dehydrated silicate.
Flammability tests demonstrated that when the silicate component content exceeds 45%, the material achieves a UL 94 V 0 rating, and the limiting oxygen index (LOI) increases from 17.8 to 26.6 at a 90% silicate component loading.
The obtained results substantiate the effectiveness of using sodium silicate crystalline hydrate as a fire-retardant additive, and it can be recommended for the development of composites based on other polymer matrices.