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.

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.

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.