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.