Hydroxyl Bonding

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.

07102014 Bob Photo

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.