Page 298 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 263

Epoxy based primers are commonly used in the aerospace and auto-
motive industries. These primers have good chemical resistance and
provide corrosion resistance to aluminum and other common metals.
Polysulfide based primers have been developed for applications where
a high degree of elongation is necessary. These systems are used where
the joint is expected to encounter a high degree of flexing or thermal
movement. Resins, curing agents, and additives used in primer for-
mulation are much like adhesive or sealant formulations except for
the addition of solvents or low viscosity resins to provide a high degree
of flow.

7.2.3 Primers for polymeric substrates
Primers can also be an effective way of protecting the surface of
treated polymeric substrates until an adhesive can be applied and a
bond formed. With polymeric substrates, the permanence of the sur-
face treatment before adhesive application is of critical importance.
Because of the dynamic and mobile nature of the polymer molecule,
the treated surface molecules can turn inward into the bulk polymer
and become ineffective. Conventional plastic surface treatments, such
as flame treating and corona treatment, have especially short expo-
sure times during which the treatments are effective. A primer can be
used to protect the treated surface and lengthen the production win-
dow between surface treatment and application of the adhesive or
sealant.
Primers generally cannot be used as a substitute for surface prep-
aration. However, there are several instances with polymeric surfaces
that primers have provided excellent adhesion without having to go
through the process of surface preparation. This is a distinct advan-
tage because surface treatment methods may be hazardous, inconven-
ient, time consuming, and often expensive. The use of a surface pri-
mer, although an extra step in the bonding process, is a desirable
alternative for use on the production line.
Cyanoacrylate adhesives (or super-glues) do not wet or adhere well
to polyolefins. The surface tension of the adhesive is much higher than
that of the substrate. However, polyolefins can be primed for adhesion
to cyanoacrylates by certain chemical compounds normally considered
to be activators for cyanoacrylate polymerization. Materials such as
long chain amines, quaternary ammonium salts, and phosphine can
be applied in either pure form or in solution to the surface of the
polyolefin. These primers are simply sprayed or brushed onto the
substrate. After drying of the primer, the cyanoacrylate adhesive is
conventionally applied and bonds extremely well to the substrate. 10,11
Several companies have discovered primers that interact with
cyanoacrylates. 10,12,13 Triphenylphosphine or cobalt acetylacetonate