Wetting occurs, if a liquid spreads on the surface of a solid and tends to cover the entire surface. In some cases the opposite occurs. A liquid my remain in a spheric shape when deposited on a surface without spreading. Reason for the latter behaviour is a strong surface tension of the liquid which keeps the drop in shape. A liquid remains in spherical shape without wetting a surface, if the surface tension of the liquid is stronger than the surface energy of the solid. Surface energy (or "surface tension") is the tendency of surface molecules to attract molecules of different chemical character. If the surface energy of the solid is above the surface tension of the liquid, the liquid spreads on the surface of the solid an wetting occurs.
A substance can only adhere to a substrate if it wets the substrate in its liquid phase. Therefore any glue, painting, printing or coating only bonds to a surface, if it can wet the surface.
Very often the surface energy of plastics is very low, so that no glue or painting substances can be found to bond to their surface.
In these cases the surface energy can be increased by plasma treatment. A first important factor is cleaning the surface from separating substances like grease by plasma cleaning, the second factor is plasma activation in order to create reactive links on the solid substrate. On some special plastics like PTFE plasma etching helps to wet even those.