Factors that influence the adhesion - Wetting of the surface

To enable the adhesive bonds between the adhesive and the surface, the adhesive must first wet the surface; in other words, it must be applied in the liquid form (as a solution, dispersion, or hot-melt).

A measure for the wettability of a surface is the angle of contact that forms between a drop of liquid and a smooth, plain surface.

A good wetting occurs when the angle of contact () between the adhesive and the substrate is inferior to 90? Complete wetting occurs when the molecular attraction between the liquid and solid molecules is greater than that between similar liquid molecules. Whether or not a given liquid will wet a solid depends on the surface tension of both substances, eg polymer and substrate.

The contact surface formed during wetting depends on the surface tension and the viscosity of the adhesive, and also on the structure (shape and size of the pores) of the surface. The size of the effective surface is generally smaller than the true surface of the substrate, because the pores and uneven parts of the surface are not completely filled by the adhesive.

Pressure may also help enhance the adhesion. Generally, bonds that have been set under pressure have higher adhesive strength. Pressures imparts better wetting and consequently a more complete interfacial contact.

The viscosity of the adhesive is critical to wetting, e.g.: the lower the viscosity, the more easily it will wet the substrate.
It is obvious to say that the rheological properties of the adhesive must be adapted to the application conditions (substrate's surface, curing time, pressure, temperature).