Each material (substrate and liquid) has its own surface tension. The difference in surface tension between a material and a liquid determines how the liquid behaves on the material. Below is a clear example:
In this image, the surface tension of each liquid is shown in the unit mN / m. The image clearly shows that the lower the surface tension of the liquid, the more it will spread over the substrate. This is also called “wetting”. As a rule, a liquid will spread / wet out if the surface tension of the material is higher than that of the liquid. A liquid will run off if the surface tension of the material is lower than the liquid.
The lower the surface tension of the material, the better the beading effect. To be oil, grease and graffiti resistant, the surface tension must be extremely low!
The lowering of the surface tension is caused by the chemistry of the impregnating agent. Two of the most common types of impregnation are silicone and fluorine. Below you can see the differences in surface tension:
Silicone = 24-30 mN / m
Fluorine = 10 – 20 mN / m
If you compare this with the image below, it is logical to explain why a silicone-containing impregnation is only water-repellent and why a fluorine-containing impregnation repels water and oil!