Page 291 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 257

solvent. Like the adhesive formulation, the primer may also contain
wetting agents, flow control agents, and toughening compounds. If the
primer is for a metal surface, corrosion inhibitors such as zinc and
strontium chromate and other inorganic chromate salts may also be
added to the primer formulation.
The application of corrosion resistant primers has become standard
practice for the structural bonding of aluminum in the automotive and
aerospace industries. The adhesive/primer combinations are chosen
to provide maximum durability in severe environments in addition to
providing higher initial joint strength. Improved service life is typi-
cally achieved by establishing strong and moisture resistant interfa-
cial bonds and protecting the substrates’ surface region from hydration
and corrosion.
Primers can also be used to protect both treated metal and non-
metal substrates after surface treatment. The use of a primer as a
shop protectant may increase production costs, but it may also provide
enhanced and more consistent adhesive strength. The use of a primer
also greatly increases production flexibility in bonding operations.
Usually primer application can be incorporated as the final step in the
surface preparation process. The primer is applied as soon as possible
after surface preparation and usually no more than a few hours later.
The actual application of the adhesive may then be delayed for up to
several months.
With steel, for example, the maximum safe surface exposure time
(SET) interval between mechanical surface preparation and bonding
is 8 hours. Many other substrates have maximum SETs that are less
than this. By utilizing an adhesive primer, the SET may be extended
to days and even months depending on the particular adhesive/primer
system used and the storage conditions prior to bonding. This process
allows a shop to prepare the surface of a large number of parts, prime
them, and store them for relatively long periods prior to bonding. It
also enables an assembly shop to outsource the more hazardous sur-
face preparation processes. The primer provides protection of the
treated joints during transportation between the treating shop and the
bonding shop. With primers, scheduling of the entire assembly oper-
ation is not dependent on the type of surface preparation.
As with metallic substrates, primers may be used to protect treated
non-metallic substrates. After surface treatment, a high energy sub-
strate has an active surface that will readily adsorb atmospheric con-
tamination. The primer protects the treated surface until the time
when the adhesive or sealant is applied. Primers are especially useful
for the protection of polymeric parts that are treated by flame or co-
rona discharge. Primers also find benefit on polymeric substrates in