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.
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).