Wetting, Adsorption, and Cleaning Processes - Surface and Colloid Chemistry: Principles and Applications - page 139

sense, to wet a solid surface, the adhesive should have a lower surface tension than
the adherend.
In some systems with charged surfaces, the electrostatic forces will have to be
considered. Electrostatic forces may also be a factor in the bonding of an adhesive
to an adherent. These forces arise from the creation of an electrical double layer of
separated charges at the interface and are believed to be a factor in the resistance to
separation of the adhesive and the adherend. Adhesives and adherends that contain
polar molecules or permanent dipoles are most likely to form electrostatic bonding,
according to this theory, which has been developed to explain the curious behavior
of the failure of bonded materials. Upon failure, many adhesive bonds break not at
the adhesion interface but slightly within the adherend or the adhesive, adjacent to
the interface. This suggests that a boundary layer of weak material is formed around
the interface between the two media. In the following text, some mechanisms of
adhesive failure are described.
The two predominant mechanisms of failure in adhesively bonded joints are adhe-
sive failure or cohesive failure. Adhesive failure is the interfacial failure between the
adhesive and one of the adherends. It indicates a weak boundary layer, often caused
by improper surface preparation or adhesive choice. Cohesive failure is the internal
failure of either the adhesive or, rarely, one of the adherends.
Ideally, the bond will fail within one of the adherends or the adhesive. This indi-
cates that the maximum strength of the bonded materials is less than the strength
of the adhesive strength between them. Usually, the failure of joints is neither com-
pletely cohesive nor completely adhesive. It is thus obvious that, for good bonding,
the surfaces need to be clean. Any dirt, grease and lubricants, water or moisture,
and weak surface scales need to be removed. Solvents are used to clean the soil
from the solid surfaces of the adherend by using an organic solvent without affect-
ing any physical property. The following are different procedures that have been
found to be useful:
a. Vapor degreasing is a good procedure for surface cleaning.
b. Solvent wiping or immersion or spraying is a useful method.
c. Ultrasonic vapor degreasing is used as an effective method.
The most convenient method is ultrasonic treatment with a subsequent solvent rinse
of the surface. Other intermediate procedures are abrasive scrubbing, filing, or deter-
gent cleaning.
Cleaning can be a chemical treatment that includes acid or alkaline etching of the
adherend surface. The etching process, especially, removes stubborn oxides (such as
on aluminum or iron) and roughens the surface on a microscopic scale.
Priming is the process of applying a dilute solution of the adhesive mixed with an
organic solvent on the adherend to a dried film thickness between 0.0015 and 0.005
mm (0.00006 to 0.002 in). Priming protects the surface from oxidation, improves
wetting, helps prevent adhesive peeling, and serves as a barrier layer to prevent unde-
sirable reactions between the adhesive and the adherent.
An adhesive is the basic substance that, when brought into contact with the hard-
ener, after a chemical reaction, former becomes an effective adherent. The rate of