Page 290 - Handbook of Adhesives and Sealants

256 Chapter Seven

ological properties at the interface or strengthening weak substrate
regions)
Being lower in viscosity than the adhesive or sealant, primers can
be used to penetrate porous or rough surfaces to provide better me-
chanical interlocking and for sealing such surfaces from the environ-
ment. Primers are often applied and appear as protective surface coat-
ings. Application processes and equipment for applying primers are
similar to those used in applying a paint coating to a substrate.
The application of a primer is an additional step in the bonding
process, and it comes with associated costs and quality control require-
ments. Therefore, primers should only be used when justified. The
most likely occasions when a primer is needed are: when the adhesive
or sealant cannot be applied immediately after surface preparation,
when the substrate surface is weak or porous, or when the adhesive-
adherend interface requires additional protection from environments
such as moisture.

7.2.1 Application and use
Unlike substrate surface treatments described in Chapter 6, primers
always add a new organic layer to the surface and two new interfaces
to the joint structure. Most primers are developed for specific adhe-
sives, and many are developed for specific adhesive/substrate combi-
nations.
Primers are applied quickly after surface preparation and result in
a dry or slightly tacky film. It is generally recommended that they
have a dried coating thickness range from tenths of a mil to approxi-
mately two mils. It is necessary to control the primer thickness, since
if the primer layer becomes too thick its bulk properties may predom-
inate, and the primer could become the weakest part of the joint.
Primers usually require solvent evaporation and several curing
steps before the adhesive or sealant can be applied. Adhesive primers
are usually not fully cured during their initial application. They are
dried at room temperature and some are forced-air dried for 30-60 min
at 150 F. This provides a dry, nontacky surface that can be protected
from contamination and physical damage by good housekeeping prac-
tices until the substrate is ready to be bonded with an adhesive. Full
primer cure is generally achieved during the cure of the adhesive.
Primers developed to protect treated surfaces prior to bonding are
generally proprietary formulations manufactured by the adhesive pro-
ducer to match the adhesive. These usually consist of a diluted solu-
tion (approximately 10% by weight) of the base adhesive in an organic