Page 229 - Handbook of Adhesives and Sealants

198 Chapter Six

surface with the components in the adhesive or sealant. Certain
boundary layers can also form after the adhesive or sealant is cured,
depending on the nature of the bond and the type of aggressive en-
vironment to which the joint is exposed. The boundary layers that
form after the assembled joint is in service may be the most perplexing
because they are often unexpected and may lead to catastrophically
early bond failure.
As discussed in Chapter 2, wetting of the adherend surface is a
required and important process in establishing adhesion. There will
be various degrees of wetting, dependent on the chemistry of the sur-
face that comes into contact with the adhesive. Along with the wett-
ability of the surface, surface roughness and topology also influence
the strength of bonded joints. The suitability of the bonding surface
will also depend on the type and degree of cleaning or surface treat-
ment that was performed before application of the adhesive or sealant.
This chapter introduces the critical surface factors that affect the
formation, strength, and durability of adhesive and sealant joints. It
is important to have a general knowledge of substrate surfaces and
the factors that cause them to change. The surface conditions will
dictate the success of most bonding operations. This chapter will also
describe various commercial methods that have been developed to con-
trol the surface and to provide a suitable foundation for attachment
of the adhesive or sealant. Different cleaning and surface treatments
are described as well as their common applications. Appendices C-1
through C-4 offer recipes and processing procedures that have been
reported to be effective surface treatments for metals, plastics, elas-
tomers, and other substrates. These surface treatments have proved
successful in many adhesive and sealant applications. Whereas this
chapter looks at surfaces in a general way, the surface characteristics
and recommended surface treatments for specific substrates are cov-
ered in detail in Chapter 16.

6.2 Nature of Substrate Surfaces

The term ‘‘surface’’ in adhesive science is usually defined as that por-
tion of the adherend with which the adhesive interacts. The surface
is defined by both area and depth of interaction. For a freshly cleaved
single crystal, this interaction region might be only one or two atomic
layers in depth. For anodized aluminum, a low viscosity adhesive
might reach a depth of several hundred nanometers or more. For a
very porous surface, such as wood, the interaction region may be sev-
eral millimeters in depth.