Certain surfaces are more difficult to bond than others. There are many factors that influence the ability of an adhesive to bond a surface. One factor is wettability which is commonly defined as the tendency of a liquid to spread on or adhere to a solid surface. In order for an adhesive to form a strong bond, it must be able to spread over the substrate’s surface. The wettability of the surface is the reason why adhesive works on paper but will peel off when used on glass. The wettability of a substrates material is the direct result of its dyne level or surface energy. The lower the dyne level, the more likely an adhesive will clump into droplets instead of spreading out. This makes the surface difficult to bond. Such is the case with glass surfaces.
Another reason why some substrates are difficult to bond involves the shape and cleanliness of the surface. An object with irregular surface can be difficult to coat with adhesive. A dirty surface prevents the adhesive from properly bonding with the substrate.
Some difficult surfaces require special adhesives such as polyurethanes. These adhesives provide excellent bonding strength for various types of substrates such as ceramics, glass, metals, plastics, rubber and wood. The fast curing and availability in different viscosities of polyurethane adhesives make them well-suited even for surfaces with low dyne level.
But the polyurethane adhesives are just one aspect of solutions to difficult surfaces. Surface preparation like heating and alteration of the surface finish helps make difficult surface easier to bond.