by Bob Beranek
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“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.” —Bob Beranek

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.

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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.

“What is hydroxyl bonding? I thought I would Google the term and give you the definitive answer. However, when I searched for a definition, I consistently got pages of explanation written in the language of chemical engineers. I’m afraid I don’t speak “chemical-ese.” I found Internet explanations difficult to read and essentially unable to translate and explain the intricacies of chemical bonding. So, having a blog article to write, I thought back to a visit I had with a doctor of chemistry I met at Tremco Adhesives in Toronto many years ago.

At the time of our meeting, I did not realize how often I would use his explanations and illustrations or I would have framed his card and put it in a place of honor. He was truly a great teacher. I use his teachings in my explanations of bonding because he was able to put difficult explanations into easy-to-understand examples. This discussion will be using the good doctor’s method of explanation.

Every adhesive goes through three levels of bonding: hydroxyl, mechanical and entanglement, to achieve maximum adhesion. Hydroxyl is the first of these steps. Let’s make an example of the tape we used last week (that I used to discuss adhesion). The tape is made of a material on which a liquid adhesive is applied. That material could be paper like masking tape, fabric like duct tape or plastic like cellophane tape. The liquid adhesive when applied to a surface must be “wetted out” to begin the bonding process. Typically we do this by setting the tape onto a surface and then we use pressure by our fingers or hands to “wet it out” or “stick it” to hold it in the place desired. This gives you a certain but relatively weak initial bond. It is a weak bond because it is designed that way for repositioning if need be.

Now let’s put this into the realm of automotive glass installation. When you apply a bead of urethane on the glass or pinchweld, set the glass into the opening and then press it into place (decking), you have initialized the first step of bonding, hydroxyl bond. A liquid between two substrates (surfaces) creates a bond that can be felt and observed. You can do it with water between two pieces of glass, you can do it when you set a cold sweating glass on a hard surface coaster or you can do it with peanut butter between two pieces of bread. The effect is the same.

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Why is it important for a technician to understand hydroxyl bonding?

I am sure that most technicians have set a piece of glass into position and onto a bead of urethane and then had to immediately remove the glass for one reason or another. Every one of us has felt the strength of hydroxyl bonding at that point. If evenly distributed outward pressure was not applied, the glass would surely break. The thicker the liquid between the two substrates, the stronger that initial hydroxyl bond would be. In some cases, the new high viscosity urethanes have more initial hydroxyl tensile strength wet than butyl tape ever had at its strongest point.

When a technician uses protective adhesive tape to hold up the glass while the urethane cures, he is counting on the initial hold that is made when the tape is “wetted out” and the hydroxyl bonding is initialized. The longer the tape is allowed to stay attached, the stronger the bond will become and the harder it will be to remove. This is why removal of the tape in a timely basis is so important to protecting the paint on which it is attached.

Anytime an adhesive is used, a wetting out action is implemented to start the bond. That action starts what’s called hydroxyl bonding.