Potentials of silicate-based formulations for wood protection and improvement of mechanical properties: A review

Silica or silica-precursor systems are attractive for the protection of wood against biotic and abiotic damages and for improvement of the fire resistance. Alkali metal silicate solutions, also known as water glasses, colloidal silica (nanosilica dispersions) and other inorganic–organic hybrids resulting from the sol-gel chemistry of alkoxysilane compounds, are products available for this purpose. These chemicals are increasingly considered to formulate wood modification products or to develop surface coatings. This review article is focused on in-depth treatments of wood through dipping, soaking or vacuum-pressure impregnation methods. The techniques used to convert monomers and low molecular weight silicate species in water glasses into less soluble and leaching-resistant silica particles, such as heat treatment, acid treatment and reactions with multivalent metal cation salts, are discussed. The similarities and differences between the various raw-impregnation materials and the properties of the final products are highlighted. Water glasses after appropriate curing, colloidal silica and tetraalkoxysilane-based formulations all lead to deposition of silica particles (SiO2) at the surface of the cell walls, in lumens and pores. Low molecular weight organosilanes and other organo-modified formulations that are able to penetrate the wood cell walls and react with wood components are good dimensional stabilizers. The treated wood exhibits, in general, increased mechanical properties (strength, hardness) and improved resistance to biodegradation and fire retardancy. The efficiency of the treatments can significantly be enhanced to a level fulfilling the requirements for industrial applications by the addition of biocides, ultraviolet absorbers or antioxidants, fireproofing compounds (boron or phosphorus-based compounds, multivalent metal salts) and hydrophobic alkylalkoxysilanes. Silica acts as a barrier/support to many of these additives preventing them from leaching.