by Bob Beranek
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Let me begin and end this post with one request. Invite your adhesive rep to visit and provide training for your technicians on what’s new with their products. Although I routinely preach the importance of cleaning glass properly to insure solid bonding, today that advice is more important than ever. There are several new procedures and products that should be introduced and put into practice as soon as possible.

Adhesive representatives and chemical engineers tell me that the number one reason for adhesive failure is improper cleaning of the surface. Adhesive companies have stepped up their training on this topic recently for good reason.

We all know that the most important area on the windshield for care in cleaning is the frit area. The frit is the black painted band around the interior perimeter of the glass that protects the urethane from ultraviolet (UV) light. It’s usually applied to the number four surface of the glass. The frit not only protects the urethane from UV light, but it also provides an additional mechanical bonding surface for the urethane to bite to. The peaks and valleys created by the frit paint application in the manufacturing process provides more bonding surface for better adhesion.

Until now, techs didn’t worry about internal frit preparation. What is an internal frit? It’s the black band of ceramic paint applied to the number 2 surface of the glass, not the number 4 surface. There is at least one manufacturer that applied a frit to both the 2 and 4 surfaces for aesthetic reasons. However, we are more concerned with the former type frit and not so much on the latter. We’ve seen internal frits for years, but until recently we didn’t know they needed special preparation.

Internal Frit

Traditional Frit

 

The preparation of the internal frit is very important to bonding success. Some use additional products to abrade/clean the surface. While others may use special procedures, such as “wet scrubbing” to clean and prep the surface for bonding. Either way, there have been changes you should be aware of and trained on accordingly.

For those of you that use primerless products, please make sure that the cleaning and prepping procedures are followed and practiced daily on every installation. Primerless products are very convenient because they eliminate a step in the replacement process which saves time and money. However, primerless adhesive installations allow no room for error when it comes to preparation of the glass surface. Any contaminants or “shortcuts” in preparation of the glass surface can result in poor adhesion, injury to your customers, and a liability for your shop.

As promised, I’m ending this post by encouraging you to invite your adhesive rep for a visit and ask them to train your techs on what’s new about their products. Adhesion standards do change and your installation procedures must change too. This is important stuff, guys. Do it.

A few years ago, I was asked to judge an auto glass installation competition at a large western glass company. That company planned on having the competition at a race track in their area so spectators would be able to watch. The temperature that day was going to exceed 100°F and the vehicles were parked outside in full sunlight.

When the judges arrived, we checked the adhesive instructions which stated that the application temperature should not exceed 110°F. Obviously, we were concerned about the high temperatures surrounding the competition. However, when the instructions said “application temperature” did they mean product temperature, surface temperature or ambient temperature?  What would be the ramifications of applying the adhesive in extreme temperatures?

We had an adhesive representative who was present for the competition, so we asked him for advice. Unfortunately, he did not know the exact answer, and due to the fact that the competition was occurring on a weekend, we were unable to contact a technical advisor at the adhesive company’s technical lab.

Through some serious discussion between the replacement company owner, the adhesive representative and the judges, we decided to take steps to cool down everything involved with the installation. We placed the glass and adhesives into air-conditioned service vehicles to make sure that the glass surface and the adhesives were well under the temperature limit of 110°F. We also started the vehicles and ran the air-conditioning in advance of the competition to assure that the vehicle was cooled down before the competition proceeded. We weren’t sure at the time what the upper temperature limit of the “Application Temperature” meant, but we were certainly not going to install glass under conditions contraindicated by the sealant manufacturers.

Since that incident occurred, I have researched this issue and found that there are different consequences for applying adhesives beyond their temperature limits. They can include bonding deficiencies, application problems and storage concerns. Obviously, the most important of these consequences is the bonding problem. If the sealant does not bond properly, the result could be injury or death caused by improper glass performance in a crash.

If the adhesive loses its viscosity, the adhesive pre-maturely cures, and waste is the obvious result. If the adhesive loses its thickness due to higher than normal temperatures, it can drip onto the customer’s interior causing vehicle damage. Under extreme heat conditions, the weight of the glass can flatten the windshield to the pinchweld’s metal and cause a stress fracture. Lastly, if sealants are stored improperly, the product cannot be expected to be to perform as promised.

The moral of the story and the research we conducted says one thing, temperature matters and caution must be exercised to insure proper performance of your adhesive products. Check your adhesive’s technical data and installation instructions for temperature limitations.  If for some reason the data is missing or not clear, check with your adhesive rep. If the rep is unsure, check with the adhesive company’s technical expert for clarity. This is important. It is not enough to go through the procedures perfectly if you don’t know your products limitations. Do it right every time and think before you proceed.

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.