Page 238 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 207

To make an economical and practical joint, the surface preparation
methods must also meet several other requirements. They must be
safe to handle and should not be flammable or toxic. They should be
inexpensive and provide fast processing times. The processes should
be easy to monitor and control in a production situation. In addition,
the surface treating process should not in itself leave a weak boundary
layer. If chemical solutions are used, they should rinse off easily and
not continue to react with the surface. The surface treating process
should allow for practical time between preparation and application
of the adhesive or sealant. Finally, the surface provided by the treat-
ment should not change once the assembled joint is made and placed
into service.

6.3.1 Importance of the surface treatment
Surface preparation of substrates prior to bonding is one of the most
important factors in the adhesive bonding process. Prebond treat-
ments are intended to provide cohesively strong and easily wettable
surfaces. The strength of an adhesive joint is significantly increased
when loose deposits such as rust, scales, flaking paint, and organic
contaminants are removed from the surface so that the adhesive can
more easily wet the substrate. Table 6.1 shows the effect of surface
preparations on adhesive-joint strength of several metallic adherends.
The bond strength provided by the surface preparation is dependent
on the type of adherend as well as the type of adhesive or sealant
used.
Surface preparations enhance the quality of a bonded metal joint by
performing one or more of the following functions: remove contami-
nants; control adsorbed water; control oxide formation; poison surface
atoms which catalyze polymer breakdown; protect the adhesive from
the adherend and vice versa; match the adherend crystal structure to
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the adhesive molecular structure; and control surface roughness. Sur-
face preparations enhance the quality of polymeric joints in a similar
manner. However, polymeric surface preparations may also chemically
alter the surface to raise the surface energy of the substrate.
Surface treatments control and protect the substrate surface before
bonding, and they protect the surface from changing after the assem-
bly is placed in service. Thus, surface preparations affect the perma-
nence of the joint as well as its initial strength. Figure 6.4 illustrates
the effect of surface treatment on the performance of aluminum alloy
epoxy adhesive joints after various times of exposure to water at 122 F.
Plastic and elastomeric adherends are even more dependent than
metals on surface preparation. Most of these materials have complex