The silica particles in silica sol are spherical, and its particle size is a very important parameter of silica sol. The particle size of silica sol is generally between 6nm-100nm. The particle diameter of silica sol for conventional casting is 8-20nm. The particle diameter has a great influence on the stability (gelation) of silica sol. Since the stability of silica sol benefits from the balance of repulsion and attraction between particles. The repulsion between particles is proportional to the particle diameter, so the smaller the particle, the smaller the repulsion, and the more unstable the silica sol. 5nm silica sol is still very unstable even at a very low concentration. In addition, the particle size of silica sol has a certain influence on the pH value of silica sol.
Another point is that the effect of particle size on gel structure and performance will result in different results: if the particles are large, the structure will inevitably be loose after gelation, the strength will be low, the silica sol coating will shrink greatly, and the relative stress will also be large. If the particles are small, the structure will definitely be tight after gelation, the strength will also be high, the silica sol coating will shrink less, and the stress will also be small. This is why the topcoat and backcoat do not use the same silica sol.