Page 289 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 255

strength and modulus can be significantly improved. Virtually all
glass fibers used in glass fiber reinforced composites are silane
treated, and the resistance to deterioration by moist environments is
greatly enhanced in this way.
Classifying primers and adhesion promoters is as difficult as clas-
sifying adhesives. Usually, these materials are grouped by their chem-
ical composition, such as silane, titanate, epoxy/amine, phenolic, and
chlorinated polyethylene. The selection of a primer or adhesion pro-
moter is normally less of a problem than selecting the adhesive since
the adhesive supplier can usually offer a complete package or make
recommendations. Often a primer is chemically similar to the adhesive
material; only the primer has a much lower viscosity so that it can be
painted or brushed on the substrate in thin coatings. Adhesion pro-
moters are generally already included in the formulation or they are
supplied as an additional ‘‘primer’’ material along with the adhesive
or sealant.
In this chapter, we will first consider primers and how they can be
used to improve the performance of bonded joints. Examples will be
provided as to their application and use on various substrate surfaces.
The discussion of adhesion promoters will be relative to their function
and use. Various chemical classifications will be described that are
commonly used for both surface application and ‘‘in-situ’’ application
within formulated adhesive and sealant products. Several common
uses of these systems in adhesive and sealant application and in poly-
mer material reinforcement will be provided.

7.2 Primers

Primers are liquids that may be applied to a substrate prior to appli-
cation of an adhesive or a sealant. The reasons for their use are varied
and may include, either singularly or in combination, the following:

Protection of surfaces after treatment (primers can be used to ex-
tend the time between preparing the adherend surface and bonding)
Adjusting the free surface energy by providing a surface that is more
easily wettable than the substrate
Dissolving low levels of organic contamination that otherwise would
remain at the interface as a weak boundary layer
Promoting chemical reaction between adhesive and adherend
Inhibiting corrosion of the substrate during service
Serving as an intermediate layer to enhance the physical properties
of the joint and improve bond strength (e.g., adjustment of the rhe-