Page 296 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 261

Figure 7.2 Effect of primer on lap-shear strength of aluminum joints exposed to 5 per-
cent salt spray. 2

VOC levels and low or no levels chrome. Data are presented on several
of these newer primers in Table 7.3.
Recent advances in primer development have included water-based
primer systems, primers that can be applied by the electrodeposition
process, and primers with more effective corrosion resistance proper-
ties. The water-based systems have been the result of recent devel-
opments in water soluble polymers such as epoxy and phenol/resor-
cinol formaldehyde novolac polymers. Low and zero level VOC primers
have been developed which meet the requirements of the aerospace
industry. 3–5 An electrodeposited primer system has been developed pri-
marily by the automobile industry. This is pollution-free and offers
uniform film distribution, controlled film thickness, and rapid appli-
6
cation. The U.S. military has developed improved primers for struc-
7
tural bonding of aluminum and titanium. These include environmen-
tally safe primers.
Plasma applied coatings have also been evaluated as surface treat-
ments for aluminum substrates being prepared for adhesive bonding.
Plasma sprayed aluminum-silicon/polyester primers gave results su-
8
perior to those of etched or anodized specimens. Plasma spraying has
also shown excellent high temperature bond performance with tita-
9
nium. The plasma spray process involves the rapid heating of pow-
dered material to the molten or semi-molten state and then propelling
it against the substrate at high velocities. These treatments eliminate