Page 230 - Handbook of Adhesives and Sealants
P. 230
Surfaces and Surface Preparation 199
When a supposedly smooth solid surface is examined closely under
a microscope, it is found to contain irregularities. It is not flat and
smooth but contains many surface asperities, such as peaks and val-
leys, with a certain degree of roughness. A rough surface provides
more bonding area than a smooth one of the same gross dimensions.
The greater effective surface area offers a larger area for the forces of
adhesion to operate, thereby providing a stronger joint. However, a
greater degree of surface roughness could also contribute to stress con-
centrations in the adhesive joint, which reduces its strength, similar
to a notch effect in metals. This effect depends on how well the ad-
hesive wets the surface and penetrates into the surface roughness.
Surfaces are full of surprises, and they are seldom what they seem.
Often they contain constituents that are very different from the bulk
material. For metals and alloys, these surfaces may consist of oxides
and adsorbed gases. For many nonmetals, they may be moisture, mi-
grating additives, or adsorbed films, such as shop contaminants. These
outer layers can either be loosely bound or tightly adhered to the base
material, and they may have high or low cohesive strength. Two sur-
face characteristics that can hurt adhesion are when: (1) the chemical
nature provides a low surface energy; and (2) the surface is either
cohesively weak or weakly attached to the base substrate. When either
one of these conditions are present, the substrate surface must be
treated in some manner to either increase the surface energy or
strengthen the surface layer.
It does not take much contamination to affect adhesion. A single
molecular layer of contaminant can prevent proper wetting of the sub-
strate by the adhesive or sealant. The adhesive or sealant will try to
wet the contaminant surface layer rather than the substrate itself.
Since most contaminants (oils, greases, fingerprints, mold release,
etc.) have a low surface energy, the adhesive will not wet the surface
nor will it form a continuous film.
Certain surfaces also may have weakly attached surface or bound-
ary layers. Examples of these are contaminant films, oxide layers,
rust, corrosion, scale, and loose surface particles. A weak substrate
boundary layer can provide the ‘‘weak-link’’ for reduced bond strength
or premature failure as shown in Fig. 6.1.
In an ideal bonded assembly, the substrate should be the weakest
link. In most assemblies that are properly bonded, the adhesive is the
weak link because the forces of adhesion are greater than the forces
holding the adhesive material together. Usually, the internal strength
of the substrate and adhesive or sealant system is well understood
and can be controlled. However, when the surface region becomes the
weakest link, it may result in low failure strength and an inconsis-