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 cohesively weak boundary layer (e.g., a powdery surface on concrete). The substrate may also be permeable, allowing moisture and 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 adhesive formulation and by substrate surface treatments. When these approaches do not work, additional bond strength and permanence may possibly be provided by:
1. Primers or
2. Adhesion promoters.
This paper will focus primarily on the use of primers with adhesives and sealants, although sometimes the boundary between primers and adhesion promoters becomes rather blurred. Examples will be provided as to the application and use of primers on various substrate surfaces.