Page 237 - Handbook of Adhesives and Sealants

206 Chapter Six

release films, such as cellophane, have also been used. It is essential
that these surface layers are removed before bonding. Just as the mold
release agents provide release of the composite from its mold, if not
removed from the part before bonding, they will also provide release
from the adhesive or sealant. Polymeric composites may also be fab-
ricated to have a resin rich surface for a glassy appearance called a
gel coat. This will provide a weaker surface layer than the material
in the bulk of the composite.
Elastomeric surfaces are very similar to plastic surfaces. However,
the more fluid nature of the elastomer’s molecules allow easy diffusion
of contaminants and low molecular weight fragments to the surface
region. These could find their way to the interface and cause a weak
boundary layer. Elastomeric substrates are especially susceptible to
this problem, because the formulations are generally more complex,
and they have more low molecular weight constituents than plastics.
Like plastics, many elastomeric surfaces are low energy surfaces and
require treatment to raise the surface energy prior to bonding.

6.3 Surface Treatment

The main purpose of surface preparation is to ensure that adhesion
develops to the extent that the weakest link in the joint is either in
the adhesive or sealant or in the adherend. With optimum surface
treatment, failure should not occur at the interface because of a weak
boundary layer or insufficient wetting. As a general rule, all sub-
strates must be treated in some manner prior to bonding if not to
remove or prevent the formation of weak boundary layers then to pro-
vide a consistent surface. Certain low energy surfaces must be modi-
fied chemically or physically prior to bonding so that the adhesive or
sealant can adequately wet the surface and provide the attractive
forces necessary for good adhesion. Surface preparation can range
from simple solvent wiping to a combination of mechanical abrading,
chemical cleaning, and acid etching.
Surface preparation can provide several principal functions:

Remove weak boundary layers that impede wetting of the substrate
and create ‘‘weak links’’ at the interface. Common weak boundary
layers are greases, oils, scale, rust, tarnish, and other oxides.
Protect the substrate surface so that weak boundary layers do not
develop during processing of the joint or during aging in service.
Influence the surface energy of the substrate so as to reduce the
contact angle between the adhesive and substrate.