In water-based systems, due to the high surface tension of water, it is difficult to quickly wet the substrate (especially low surface tension substrates). Therefore, in water-based coating systems, the use of substrate wetting agents is particularly important. Silicone substrate wetting agents are widely used in waterborne coating systems due to their wide applicability.

The substrate wetting agent is actually a special kind of polyether modified silicone oil, and its relative molecular weight is usually very small. Since the rapid wetting of the substrate is a dynamic process, and according to the principles of physicochemistry, the principle of wetting agent is to quickly migrate to the surface of the system to reduce the surface tension of the system, so as to achieve the effect of substrate wetting. According to the molecular kinetic theory, the migration speed of molecules depends to a large extent on the relative molecular mass, and the smaller the relative molecular mass, the faster the migration speed. Therefore, polyether-modified silicone oils with relatively small molecular weights are often used as substrate wetting agents in water-based systems.

Further research found that when the polyether-modified silicone oil used as a substrate wetting agent is used in a water-based system, the silicone chain segment can be fully stretched, and a large amount of -CH3 can be stretched outward, thereby greatly reducing the surface tension of the system. Generally, polyether-modified silicone oils with medium and high molecular weight can only reduce the surface tension to 25-26 N/m in water-based systems, while small-molecule substrate wetting agents can usually reduce the surface tension to below 22 N/m. The lowest can reach 20-21 N/m. With such a low surface tension, the water-based coating added with a substrate wetting agent can wet most low surface tension substrates, 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 level, the substrate wetting agent can endow the system with good anti-crater performance and atomization effect during spraying.
 
Due to the relatively small molecular weight of the substrate wetting agent, its silicone chain segment is very short, so it cannot impart good slipperiness to the surface of the paint film (usually the slipperiness comes from a longer silicone chain segment). When the slipperiness of the paint film is required, the substrate wetting agent is usually used in conjunction with a high molecular weight silicone leveling agent, which is also the difference between the substrate wetting agent and the ordinary silicone leveling agent .