Because the surface tension of water in the water system is too large, it is difficult to fast wetting the substrate (especially the substrate with low surface tension), so in the water-based coating system, the use of substrate wetting agent is particularly important. Silicone based wetting agent has been widely used in waterborne coatings because of its wide applicability.
Formula (2) is the structure of a typical water-based substrate wetting agent.
It can be seen from Equation (2) that the substrate wetting agent is actually a special polyether modified silicone oil, whose relative molecular weight is usually very small. Because the rapid wetting of the substrate is a dynamic process, and according to the principle of physical chemistry, the principle of wetting agent is to quickly migrate to the surface of the system, reduce the surface tension of the system, so as to achieve the effect of substrate wetting. According to the viewpoint of molecular motion theory, the migration rate of molecules depends on the relative molecular mass to a large extent, the smaller the relative molecular mass, the faster the migration rate. Therefore, polyether modified silicone oil with relatively small molecular weight is often used as a substrate wetting agent in water system. Further studies have found that the polyether modified silicone oil used as the substrate wetting agent is used in the water-based system. Due to its special structure [see Equation (3)], the organosilicon chain segment can be fully extended to extend a large amount of -- CH3, thus greatly reducing the surface tension of the system. In general, the surface tension of polyether modified silicone oil with high relative molecular weight can only be reduced to 25 ~ 26 N/m in water system, while the surface tension of the substrate wetting agent with small molecular weight can usually be reduced to less than 22 N/m, and the lowest can be reduced to 20 ~ 21 N/m. With such low surface tension, water-based coatings with substrate wetting agents can wet most substrates with low surface tension, even ptfe substrates. At the same time, because the substrate wetting agent can reduce the surface tension of the water-based system to a very low degree, the substrate wetting agent can give the system a good anti-shrinkage performance and spray construction of the atomization effect.
Because the relative molecular weight of the substrate wetting agent is small and the organosilicon segments are very short, it is not possible to give the paint film surface good smoothness (usually the smoothness comes from the longer organosilicon segments). When there is a requirement for the smoothness of the film, the substrate wetting agent is usually used with high molecular weight of organic silicon leveling agent, which is the difference between the substrate wetting agent and ordinary organic silicon leveling agent.