Page 293 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 259

problems can be detected soon after the primer is applied and before
great expense is incurred in the bonding operation. Ideally the primer
is a bridge to transfer stress between the adhesive and the adherend.
With properly applied primers joint failures should be cohesive within
the adhesive or sealant material.
Examples of some commercial primers for structural adhesives are
shown in Table 7.2. It should be noted that these are formulated for
specific adhesives and applications (e.g., aluminum bonding), and they
have recommended curing processes and coating thickness that must
be followed for optimum benefit. The substrate surface treatment,
which provides the base for the primer, is generally one of the pro-
cesses commonly used for adhesives or sealants as described in Chap-
ter 6.

7.2.2 Primers for metal substrates
When a corrosive medium contacts the edge of a bonded joint, and
finds an extremely active surface such as that produced by a fresh
acid treatment to improve adhesion, corrosion at the metal-adhesive
interface can occur. This initial corrosion and its subsequent penetra-
tion can take several forms. These are discussed more fully in Chap-
ters 16 (bonding aluminum) and 17 (effects of the environment).
Some primers will inhibit the corrosion of metal adherends during
service. By protecting the substrate’s surface area from hydration and
corrosion, these primers suppress the formation of weak boundary lay-
ers that could develop during exposure to wet environments. Primers
that contain film forming resins are sometimes considered interfacial
water barriers. They keep water out of the joint interface area and
prevent corrosion of the metal surfaces. By establishing strong, mois-
ture resistant bonds, the primer protects the adhesive-adherend in-
terface and lengthens the service life of the bonded joint. However,
moisture can diffuse through any polymeric primer, and eventually it
will reach the interface area of the joint. Therefore, the onset of cor-
rosion and other degradation reactions can only be delayed by the
application of a primer unless the primer contains corrosion inhibitors
or it chemically reacts with the substrate to provide a completely new
surface layer that provides additional protection.
Representative data are shown in Fig. 7.2 for aluminum joints
bonded with an epoxy film adhesive and a standard chromate con-
taining primer. Until recently standard corrosion resistant primers
contained high levels of solvent, contributing to high levels of volatile
organic compounds (VOCs), and chromium compounds, which are con-
sidered to be carcinogens. As a result, development programs have
been conducted on water borne adhesive primers that contain low