Organic-Mineral Thermal Insulation Composite: Formation of a Porous Structure

Introduction. This article examines an organo-mineral thermal insulation composite based on a reactive mixture for the synthesis of rigid polyurethane foam, modified with a silicate component—sodium metasilicate pentahydrate (Na₂SiO₃·5H₂O) of two fractions (0.9–1.25 mm and 0.1–0.315 mm)—to reduce flammability and polymer capacity. The study investigates the effect of this additive on the formation of the composite’s pore structure by analyzing the processes of foaming and polymer formation (nucleation and growth of gas bubbles, their uniform distribution, and stabilization within the polymer matrix). Materials and methods. The following components were selected as raw materials for the studies: Isocyanate component (IC), isocyanate – a liquid, reactive oligomer containing NCO functional groups. Polyol component (PC), polyol – a liquid, reactive oligomer containing hydroxyl functional groups (OH). Silicate component (SC) – crystalline hydrate of sodium metasilicate (sodium metasilicate pentahydrate)- Na2SiO3·5H2O)). The method for producing the organo-mineral thermal insulation composite consists of free foaming of the components of the reaction mixture. The component ratio for all compositions of the reactive system for producing rigid polyurethane foam is constant and equals IC:PC = 1.8:1. The silicate component was introduced into the composition in amounts of 15, 30, 45, 60, 75, and 90% relative to the content of the reactive composition for producing rigid polyurethane foam. Results. It was found that the silicate component accelerates foaming and polymer formation, and varying its dispersion allows the degree of this influence to be controlled by altering the spatial distribution of the additive throughout the material volume and the interfacial contact area between the silicate component and the reactive oligomer mixture. The isocyanate functional groups of the mixture can interact with the surface-adsorbed and crystal hydrate water of sodium metasilicate pentahydrate, releasing carbon dioxide and modifying the direction of chemical reactions. This directly affects the morphology of the cellular structure, pore size, and cell wall thickness. Modification with the fine fraction (0.1–0.315 mm) of the silicate component leads to an explosive foaming behavior, which, combined with accelerated polymer formation, results in a compromised structure prone to shrinkage. At a loading of up to 90%, the average pore diameter decreases from 343 µm to 186 µm (a 46% reduction), and the cell walls become thinner, decreasing from 21.3 µm to 9.3 µm (an average reduction of 55%). Modification with the coarse fraction (0.9–1.25 mm) of the silicate component exerts a more balanced effect on the acceleration of foaming and polymer formation. Consequently, the impact on the pore structure of the composite is less pronounced, making it possible to incorporate the silicate component into rigid polyurethane foam compositions at up to 90% loading—the amount required to effectively reduce its flammability. Conclusions. The silicate component in the reactive mixture accelerates the processes of foaming and polymer formation, and its varying dispersion makes it possible to regulate the degree of influence on these processes by altering the spatial structure of dispersion throughout the material volume and the interfacial contact area between the silicate component and the reactive oligomer mixture. The isocyanate functional groups of the mixture can interact with the chemically bound water of sodium metasilicate, releasing carbon dioxide and changing the direction of chemical reactions both in the peripheral zone and overall, which directly affects the morphology of the cellular structure, pore size, and pore wall thick-ness.