We have discussed the fact that all adhesives go through three stages of bonding until strength has reached its peak. Last week we started with the first stage, hydroxyl bonding. This week we want to explain and discuss the second stage, mechanical bonding.
Most people think of mechanical bonding as physical attachment, usually with nuts and bolts. To some extent, there are similarities. However, the definition of mechanical bonding is “the use or creation of more bonding surfaces.”
How many times during your career did you learn to abrade a surface to create a more effective bonding platform? If you went to any of my classes, or classes given by most adhesive companies, you have been told that many times. In glass preparation, in paint delamination preparation and in any circumstance where you need a little more strength in a bond, mechanical bonding creates more service area for a tighter seal. Abrade it, prime it and bond it, sound familiar? That is what mechanical bonding is.
When it comes to the glass bonding surface, some of the preparation of the surface is already done for us. The paint band around the bonding edge of the glass (the frit) is many times rougher than the glass surface itself and provides a good surface on which to bond. Of course it must be cleaned and made contaminant-free, but the “peaks and the valleys” in the paint are there to promote mechanical bonding. Mechanical bonding is so important that many adhesives companies are now offering new products or recommending procedures to assure the roughing–up of the surface.
What about the body surface? If you think about the instructions given for bonding, you will understand that the chemistry is the same there as well. If paint delamination occurs, we abrade the surface, clean off debris and prime before bonding. If the vehicle was recently painted, we are instructed to abrade off the paint, clean off debris, and bond to the body primer underneath. If we have some say in the aftermarket paint job performed before installation, we ask the painters to tape off the pinchweld after the primer coat is applied to assure a perfect bonding surface. And of course, the normal bonding is done by trimming back the existing bead of urethane, leaving 1-2 millimeters, and bonding to the remaining bead. All of the preparation above leaves a rough “peak and valley” surface for bonding. The bond will be enhanced by roughening up the surfaces because adhesives work best when they have more surface to bond to.
Let’s put this together with the hydroxyl bonding we talked about last week. When you “wet out” an adhesive to a surface (hydroxyl bond) the liquidity of the adhesive seeps into the “peaks and valleys” of the surface and creates the mechanical side of the adherence (mechanical bond). When the hydroxyl and mechanical bond work together, you have a remarkably strong bond almost instantly. Every tick of the clock that goes by makes the bond between the two surfaces that much stronger until, with the help of the next stage of bonding, it reaches the ultimate strength attainable.
