by Bob Beranek
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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.

I was once told by a chemist that all adhesives work the same, but adhesives stick better to some materials than others because of molecular manipulation. As a technician I wasn’t sure what that meant. Put the glue down, set the glass, wait for it to dry and then release the vehicle, right? If you buy the right material and follow manufacturers’ instructions, you can assume the job is completed correctly and go about your business. Unfortunately, this is not all you need to know to be an effective and competent technician.

In the next couple of posts, I plan to discuss the three types of bonding, hydroxyl, mechanical and entanglement. Why? Because the technician that understands how adhesives work will make more accurate bonding decisions which will result in better sealing and performance.

The first rule that must be understood before we can begin discussing the types of bonding is “that an adhesive will always stick best to itself.” We have all heard this statement before, yet there are still installers who believe that the more primer they use between the existing urethane and the fresh urethane the better it is. This is wrong. There are some brands of urethane that suggest a wipe-down of the existing bead with a product that is a “prep,” not a primer. However, keep in mind the difference. Preps clean and prepare while primers promote adhesion with unlike surfaces.

To illustrate this fact and other concepts we will discuss over the next few weeks, I suggest that you get yourself a roll of tape. Any tape will do, masking, moulding, cellophane, it makes no difference. Once you have the tape, pull a length of tape off the roll and stick it to different surfaces; countertop, desk, chair, plastic, fabric, skin and feel the differences in how it adheres. On some of the surfaces the tape sticks stronger than on others. Once you have applied and peeled off the tape on a number of surfaces, stick it to itself and try to separate it. Substantially more difficult isn’t it? First rule proved, adhesive sticks better to itself than to other surfaces.

The second rule is that the surfaces to be bonded must be contaminant free for the adhesion to be effective. Now, take another piece of tape and try the same experience as before, with a contaminant between the tape and the surface you are trying to stick it to. It can be dust, dirt, grease, oil or wax or anything that would not be considered “a clean surface.” You will notice that the ability to stick to the surface is greatly reduced and in some cases it won’t stick at all. Moral of the story, make sure that the surfaces you need to bond are clean and contaminate free.

Reading this, you may be saying, “duh, I know that.” Really? All too often I see technicians sweeping off the existing bead with a dirty old brush, brushing it off with their dirty safety gloves, or worst yet, with their sweaty fingers after they just stripped it down to the 1-2 millimeter recommended by the adhesive manufacturer. Do you think they contaminated the bonding surface? Of course they did.

Another habit I see all too often are technicians who wash the new glass part with un-approved glass cleaner, wiping the glass off with a dirty shop towel and then setting it with their hands. If you ask why they don’t use the cleaning procedures recommended by the adhesive company, they may say those products are too expensive or the time needed to complete the process takes “too long.” Do you think there might be areas where the adhesive may not stick? You know there are. The fact is that nothing will stick if the surfaces are not clean. How expensive and time-consuming are callbacks?

Installing glass is not brain surgery, so sterile utensils and “scrubbing up” is not necessary to form an effective bond. However, remember that by attempting to clean the surface with the wrong materials, you will be just adding additional contaminants.

Next week we will discuss hydroxyl bonding.