Differences among SiO2, TiO2, and Ti in the anti-foaming mechanism of chemically bonded silicate coatings

Potassium silicate coating foams seriously at high temperature. SiO2, TiO2 and Ti particles were added to solve this problem. Micro-morphologies, phase composition, mass change kinetics were analyzed, porosity and volatilizable water contents (κ0) of coatings were calculated to reveal the effects of adding SiO2, TiO2, and Ti particles on the foaming behavior of potassium silicate coatings at 500 °C. The higher the κ0, the more severe the foaming becomes. Results shows that addition of fillers improved the anti-foaming resistance due to volume occupier effect. Among them, SiO2 particles did not alter κ0 of the as-prepared coatings due to similar tetrahedral structure with silica, addition of TiO2 particles increased κ0 because titania can form more -Ti-OH groups than silica, Ti particles decrease κ0 because dissolved Ti4+ ions can decrease the number of the pending -Si-OH bonds. Combined with the volume expansion occurred due to the oxidation of Ti particles during heat treatment, potassium silicate coating modified by 30 wt.% Ti exhibited the best anti-foaming performance with the lowest porosity, at 1.8%.