Page 257 - Handbook of Adhesives and Sealants

226 Chapter Six

types of grits are available with a relatively wide hardness range. The
grit hardness is usually sufficient to remove paint coatings and other
organic contaminants from the surface, but it is not hard enough to
cause abrasion of the base metal or damage to relatively delicate
parts. Although polymer blasting is generally thought of as a coating
removal process rather than a surface preparation process, it has
found use as a prebond process in electrical and electronic applications
where a conductive blast medium is not acceptable.

6.5 Active Surface Treatments
Active surface treatments are chemical or physical processes that not
merely clean the surface or remove weak boundary layers, but they
also transform the inherent surface chemistry. They either improve
wetting or modify the boundary layer to be more receptive to bonding.
Acid etching, oxidation, anodizing, and pickling processes are exam-
ples of active surface treatments.
Active surface treatments of metal substrates are usually chemical
treatments that cause the formation of a predetermined type of oxide
layer or surface structure that is strong, stable, and receptive to ad-
hesives or sealants. Active surface treatments for polymeric surfaces
are usually chemical or physical treatments that alter the chemistry
of the surface to make it more wettable. Tables 6.11 and 6.12 sum-
marize the active surface preparations that are commonly used for
these substrates.
Active surface treatments are usually the last step in the sequence
of surface preparation processes and are only used when maximum
strength and permanence are required in a joint. It is always preceded
and followed by surface cleaning via one of the passive processes de-
fined in the last section.
Active surface treatments provide improved bond strength and du-
rability through several processes.

1. Remove the weak boundary layer or alternately modify the bound-
ary layer to provide a cohesively strong layer that is well bonded
to the bulk, stable, and receptive to common adhesives.
2. (Primarily for polymeric surfaces) Increase the surface energy of
the natural surface so that it is greater than the surface energy of
the adhesive used.
3. Improve the surface topography to enable the capillary action of
the adhesive to maximize joint strength.