Page 287 - Handbook of Adhesives and Sealants

Chapter
7

Primers and Adhesion Promoters

7.1 Introduction

Some adhesives and sealants may provide only marginal adhesion to
certain substrates. This could be due to the low surface energy of the
substrate relative to the adhesive (e.g., epoxy bonding polyethylene)
or to a boundary layer that is cohesively weak (e.g., powdery surface
on concrete). The substrate may also be pervious, allowing moisture
and environmental chemicals to easily pass through the substrate to
the adhesive interface, thereby degrading the bond’s permanence.
Generally, attempts are made to overcome these problems through ad-
hesive formulation and by substrate surface treatments. When these
approaches do not work, additional bond strength and permanence
may possibly be provided by primers or adhesion promoters.
Primers and adhesion promoters work in a similar fashion to im-
prove adhesion. They add a new, usually organic, layer at the interface
such as shown in Fig. 7.1. The new layer is usually bifunctional and
bonds well to both the substrate and the adhesive or sealant. The new
layer is very thin so that it provides improved interfacial bonding
characteristics, yet it is not thick enough so that its bulk properties
significantly affect the overall properties of the bond.
Both primers and adhesion promoters are strongly adsorbed onto
the surface of the substrate. The adsorption may be so strong that
instead of merely being physical adsorption, it has the nature of a
chemical bond. Such adsorption is referred to as chemisorption to dis-
tinguish it from reversible physical adsorption.
The main difference between primers and adhesion promoters is
that primers are liquids that are applied to the substrate as a rela-
tively heavy surface coating prior to the application of the adhesive.
Adhesion promoters are liquids that form a very thin (usually mono-
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