WETTABILITY & SURFACE CHEMISTRY

Surface energy is very important, particularly when bonding materials or printing onto plastic surfaces. In order to achieve optimum conditions for these processes, the liquid adhesive or ink should be able to wet the surface of the material. Wettability, which is the “ability of a liquid to maintain contact with a solid surface” depends on this surface energy of the solid and the surface tension of the liquid.

This directly affects how well a liquid wets the surface and is measured by millinewtons per metre (mN/m). The wettability is also influenced by the angle of contact. This is the angle between the tangent line at the point of contact and the horizontal line of the solid surface. If a drop of liquid is placed onto a smooth, solid horizontal surface, it may spread out over the substrate. If a complete wetting takes place, the contact angle will be circa zero. However, if only a partial wetting occurs, the resulting contact angle reaches equilibrium in the range of zero to 180 degrees.

To summarise, the higher the surface energy of the solid substrate along with the surface tension of the liquid, the better it’s wettability and the smaller the contact angle. Typically, when a proper bond takes place between a liquid and substrate surface, the solid surface energy should exceed the liquid’s surface tension by at least 2-10 mN/m.