After the drying reaches the critical point, the evaporation rate of water begins to decrease further. At this time, the contraction of the solid skeleton has stopped. At this stage, the transfer of liquid in the pores is still mainly carried out in a flowing manner, accompanied by the diffusion transfer of steam. Because most of the evaporation of the liquid is still carried out on the surface of the gel, that is, the surface of the shell.
When the gel is at the critical point, the skeleton strength mainly depends on the bridging effect of a large number of hydroxyls and the effect of a small amount of silicon-oxygen atomic bonds. The conversion of sol and gel is reversible. If the next layer is dipped at this time, a "re-dissolution" phenomenon will occur. This is because the rigid structural skeleton that has just been established has turned into a soft gel containing water. As aging proceeds, more hydroxyls dehydrate to form silicon-oxygen bonds. Since the process of generating silicon-oxygen bonds is irreversible, by the end of this stage, the skeleton has been sufficiently "aged". At this time, if the next layer is dipped, there will be no "re-dissolution" phenomenon. It can be seen that sufficient "aging" of the previous layer of coating is a prerequisite for dipping the next layer of coating.