by Bob Beranek
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The other day I asked the class I was teaching how they handled outside rearview mirror replacement. Most said they bought pre-cut mirrors, stuck them to the mounting plate in the mirror assembly and then taped it up until the adhesive cured. However, one installer said he used flat stock mirror and cut it to pattern. This really surprised me because I thought cutting mirror stock to pattern was a thing of the past.

I remember instructing my students how to pattern and cut stock mirror to fit mounting plates in the past, but I don’t typically teach that today. Sure, flat stock is slightly cheaper and cutting the part is a convenience to the customer because it can be done immediately rather than ordering and waiting for shipment, but please keep in mind it does not meet Federal Motor Vehicle Safety Standard 111 on the passenger side of the vehicle.

The mirror on the passenger side of the vehicle is required by law be convex and display the words, “Objects in Mirror Seem Closer than They Appear.” As a repair facility we cannot render a safety device inoperative. By changing an outside rearview mirror from convex to flat we are changing a safety device to be ineffective. The driver will not be able to see the blind spots on the passenger side of the vehicle. There is an exception for large, over-the-road, heavy haulers. The rule is that if the mirror has over 19 square inches in surface area, the mirror can be flat stock if the vehicle owner so requests but other than that exception the mirror must be convex.

However, for those of you who still cut mirror stock for the driver side of the vehicle here is the way I used to teach patterning for the part. I learned this from a manager friend named Ed Landerud. The hardest part of cutting the mirror stock to size was to get the pattern right. If it was cut too small, all the profit was lost to scrap stock. If it didn’t fit close to perfectly, the customer complained and a re-cut was in order. So here is how I did it.

1.            Get a white sheet of paper and hold it up to the mounting plate placing one finger in the center to hold it in place.

2.            I then took my other index finger and dragged it on the floor to collect dust or dirt. I used my finger to then outline the pattern on the paper using the edge of the mounting plate. It created an outline by which to estimate the shape and size of the pattern.

3.            I then cut the paper with scissors to create a template that I would use to trace the pattern to the flat stock mirror with a felt tip pen or wax pencil.

4.            The actual pattern would be slightly larger than the actual size due to the depression of the paper during the finger tracing of the plate.

5.            Once the pattern is cut, I would then finish the edging until it fit perfectly.

6.            Here is another word of advice. Some mirror stock will not perform as well as others when adhered to the mounting plate with urethane. Some mirror stock backing will separate from the glass when urethane is used to adhere it. I would suggest using silicone or epoxy when adhering the mirror to the mounting plate.

Obviously, the dirty finger method of pattern making may not be the most professional way of creating a pattern, but it worked well. Now, as then, we have to get the job done, please the customer and do it safely and according to standard and regulation.

We have talked about hydroxyl and mechanical bonding the last two weeks and now I want to explain the last and most important bonding step, entanglement bonding. Entanglement bonding means that the molecules intertwine to become stronger. It is the last step an adhesive goes through to permanently bond two surfaces.

Do you ever wonder why some adhesives stick better to some surfaces than others? The answer is in the molecular chemistry of adhesives. A chemist will add molecules to the adhesive that are similar to or will exactly match the surface molecules he wants it to adhere to. He will leave out the ones he doesn’t want it to adhere to. When the molecules locate each other they intertwine (entangle) and bond together.

Going back to the tape experiment we discussed a couple of weeks ago to illustrate the “wetting out” of an adhesive, we can use the tape to illustrate entanglement bonding. Use any tape you would like. What is the backing of the tape, cellophane, paper, fabric? Whatever it is, you can assume that the molecules in the adhesive is also found in the backing. So, masking tape has paper molecules, cellophane tape has plastic molecules, and duct tape has fabric and vinyl molecules. Now stick the tape to a table top and remove. You will notice that it has some stickiness to it but it can be removed without tearing. This is demonstrating hydroxyl bonding. Now stick the tape to itself but make sure you can grasp both ends. Separate the two pieces. You will notice that the two sides of the tape will be harder to separate but they will come apart. Finally, stick the two ends of the tape together and wait 24 hours. What do you think will happen when you attempt to separate the two ends? The chances are that the backing will tear, or not separate at all. This is because you gave the molecules time to entangle and build strength.

Adhesives stick best to themselves because all of the molecules will entangle. If there is a molecule on one surface but not the other, the adhesion will be less. The weakest bond is the one where the fewest molecules are entangled.

Let’s look at this in terms of automotive glass. All urethane company instructions say to leave 1-2 millimeters of existing urethane to the body and bond to it. They say that the best bonding surface for bonding the glass to the body is a freshly exposed, uncontaminated bead of existing urethane. When the fresh urethane molecules are given the time to entangle with the existing urethane molecules the result is a strong and unfailing bond. It can’t get any stronger. In my 30-plus years of automotive glass installation experience, I have never seen urethane separate from another bead of urethane, even under the stresses of a collision; unless there were obvious contaminants hindering the entanglement of the two beads. I have seen adhesive failure between the glass and the urethane but never between the two urethane beads on the body.

Why does urethane stick well to glass and metal but not so well to plastic and rubber? Simply put, there are no plastic molecules in urethane and only few similar rubber molecules.

So, when you are tempted to use “liquid clips” on that plastic “A” pillar moulding, be prepared to come back and replace it with a new one because it won’t stick unless both surfaces have molecules to share. It will blow off. Understanding your adhesive will help you in your everyday installations in many ways. Adhesives work well when you know the materials they stick to, not so well if you try to use them where they aren’t supposed to be used. Remember the rules, adhesives stick best to themselves and without similar molecules the surfaces will not stick together.

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.