I have recently heard of a problem in adhering urethane to an internally applied frit. What we mean by an internally applied frit is one that is applied on surface 2 or 3 instead of surface 4 where it is usually applied. The frit is the paint band around the perimeter of the glass and acts as an aesthetically pleasing finishing application, but also acts as a protective coating for urethane’s biggest enemy, ultraviolet light. The frit applied to surface 4 also acts as a bonding enhancer. The frit is a rough surface that increases the bonding surface urethane can grip. So, the rougher the frit surface is, the more enhanced bonding there is.
Glass has a very smooth surface and adhesion is enhanced when there are certain conditions present. They can include a rough surface, a clean contaminant free surface and a primed surface that can interact with different substrates. The issue I see could be all or part of the options above.
If the frit is applied to the inside surface instead of an exposed surface, then an abraded surface must be created to enhance the bonding surface. This is why there are products or procedures that abrade surfaces. Dow’s Betabrade, Sika’s PowerCleanAids and other adhesive company’s wet scrub procedures are used for contaminant removal but also adds an abrasion that would also add adhesion success with increasing bonding surfaces. The more surface to bond to, the better adhesion strength is accomplished.
The next question would be, how well was the surface cleaned? Were the proper procedures used to clean the edge of the glass? Time and again during my training sessions I see technicians cleaning the glass incorrectly, even from technicians that have years of experience. They use improper cleaners, cleaning towels, and procedures that add contaminants rather than eliminating them. Add those improper procedures to a smooth glass surface and you have adhesion failure. Technicians and companies should be seeking cleaning instructions from their adhesive company representatives to fine tune their procedures and insure proper cleaning procedures.
Finally, I have no problem with primerless urethanes. Primerless urethanes work very well and produces a safe and proper installation with the elimination of a step that can save time and effort. However, primerless urethanes demand a clean surface to bond well. There is no back-up and there is no amount of forgiveness if the surface is not clean or if the surface is too smooth to adhere to aggressively. Primerless urethanes may very well demand abrasion for maximum adhesion when applied to interior applied frits. I suggest that you check that possibility with your adhesive representative for confirmation or instruction for use.
Primable urethanes add an application of primer that also adds a level of increased bonding surface by its application to the glass. When it cures (dries), it leaves behind peaks and valleys that have walls to increase the bonding surfaces even on smooth interior applied frits.
My opinion is that if you use interior applied frit glass, use a product or procedure that abrades the bonding surface of the glass or use primer whether the urethane asks for it or not. Either way more bonding surface is created and the mechanical bond is enhanced.