Thickness-dependent intumescent properties of thick alkali silicate coatings obtained through solution route

Thick and homogeneous coatings with thickness gradients have been developed from aqueous Na-, K- and Li-silicates, also commonly referred to as waterglasses. Thermal treatment of such coatings to 250 °C has allowed to evidence the existence of a critical thickness (CT) below which the coating homogeneity stays intact while intumescence/foaming, crystallization or cracking may be observed above the CT, depending upon the starting composition. Intumescence is observed above CT especially in the case of Na- and K-silicates. Below the CT, water issued from condensation may diffuse outside the film resulting in a reduced thickness of the coating, evidenced from SEM images. Above CT, condensation leads to a non-permeable coating so that water cannot diffuse outside and, thus, creates a foam. Raman spectroscopy depth slice measurements on a flat 20 µm thick coating suggest that the non-permeable coating formed at 150 °C has a reduced amount of water in comparison to the as-deposited coating dried at 55 °C. (Figure presented.)