Page 94 - Handbook of Adhesives and Sealants

62 Chapter Two

It is also easy to see why silicone and fluorocarbon surfaces provide
good mold release surfaces. Most resins will not easily wet these sur-
faces. It is also easy to see why silicone and mineral oil provide weak
boundary layers. If a very thin layer of such oil is on the substrate,
the adhesive will want to spread over the oil rather than the substrate.
Most adhesives would not wet a surface contaminated by these oils.
It is also interesting to note that by making a coating (or adhesive)
more likely to wet a substrate (by lowering its surface tension), you
may be inadvertently making it more difficult for any subsequent coat-
ing or adhesive to wet this new material once it is cured. Graffiti re-
sistant paints work in this manner.
After intimate contact is achieved between adhesive and adherend
through wetting, it is believed that permanent adhesion results pri-
marily through forces of molecular attraction. Four general types of
chemical bonds are recognized as being involved in adhesion and
cohesion: electrostatic, covalent, and metallic, which are referred to as
primary bonds, and van der Waals forces that are referred to as sec-
ondary bonds.

2.3.2 Mechanical theory
The surface of a solid material is never truly smooth but consists of a
maze of peaks and valleys. At one time, adhesion was thought to occur
only by the adhesive flowing and filling micro-cavities on the sub-
strate. When the adhesive then hardens, the substrates are held to-
gether mechanically. According to the mechanical theory of adhesion,
in order to function properly, the adhesive must penetrate the cavities
on the surface, displace the trapped air at the interface, and lock-on
mechanically to the substrate.
One way that surface roughness aids in adhesion is by increasing
the total contact area between the adhesive or sealant and the adher-
end. If interfacial or intermolecular attraction is the basis for adhe-
sion, increasing the actual area of contact by a large amount will in-
crease the total energy of surface interaction by a proportional
amount. Thus, the mechanical interfacing theory generally teaches
that roughening of surfaces is beneficial because it gives ‘‘teeth’’ to the
substrate, and by virtue of roughening increases the total effective
area over which the forces of adhesion can develop. Exceptions to this
rule will be described in the following sections.
The mechanical theory also teaches that joint designs that have
large bonding areas are better than joint designs having a smaller
area. However, there is a point where increasing the joint area, for