Page 292 - Handbook of Adhesives and Sealants

258 Chapter Seven

that their solvents will soften the surface, and some of the primer
resin will diffuse into the bulk of the substrate, thereby increasing
adhesive strength by molecular diffusion.
Primers also have production advantages with bonded assemblies
having many sub-sections. The nature of the assembly may not be
suitable for immersion in pretreating solution as a single structure.
With primers, individual sub-sections can be treated, primed, and
then fit into place before the bonding step without regard to time. This
allows the entire assembly to be bonded at one time.
Sometimes primers can take the place of surface treatments. Two
examples of this are with porous substrates and with certain plastic
substrates. With weak porous substrates, such as wood, cement, or
porous stone, the primer can be formulated to penetrate and bind
weakly adhering material to provide a new, tightly anchored surface
for the adhesive. Chlorinated polyolefin primers will increase the ad-
hesion of coatings and adhesives to polypropylene and to thermoplas-
tic olefins. The chlorine atoms on the outer surface of the primer in-
crease surface energy and enhance adhesion. Examples of these are
discussed in the next sections.
Low viscosity primers can also easily fill the irregularities on the
substrate surface and displace air and fill hollows. This can improve
the wetting of the adhesive ‘‘system’’. For example, if the adhesive is
a hot melt applied to a bare, roughened metallic surface, the adhesive
will gel before it gets a chance to efficiently wet the surface and me-
chanically interact with any surface roughness. However, if a dilute
primer is first applied to the substrate and dried, the hot melt adhe-
sive could bond directly to the primer which in turn has bonded to the
interstices of the substrate, thus providing excellent adhesion.
There are several possible reasons for a primer to fail when a joint
is placed in service. The most common causes of primer failure relate
to production issues as shown in Table 7.1. Fortunately many of these

TABLE 7.1 Possible Reasons for Primer Failure
Single layer application which is too thick
Too long an open time (time between application and curing) allowing hydrolysis
and contamination
Frothy coating and porous layer caused by too fast a heat-up curing rate
Low crosslink density allowing plasticizer or low molecular weight agents to
migrate from the primer to the interface
Too high a crosslink density causing a brittle primer that cannot take flexing or
thermal changes
Incompatibilty with the substrate surface or the adhesive
Undercured or overcured primer
Attack by solvents in the adhesive
Attack by temperature of the adhesive curing process