So, we've got three different adhesives, what are they used for? 3M have one structural adhesive and then two panel bonding adhesives. What are the differences? we'll address that in a second, but it may help to know what the similarities are first. So, these are very chemically similar in a lot of ways.
The Differences between 07333, 08115 and 08116
- Number 1: The chemistry is epoxy, so all three of these products are epoxies. Nice thing about epoxies is they handle heat very well. So, there's a number of things we can take advantage with because they handle the heat well.
- Number 2: they all contain glass beads. These glass beads are in that product to guarantee us a minimum bond line thickness, so we can't squeeze all the adhesive out if we happen to over clamp.
- Number 3: they all have corrosion inhibitors. So, you should be very confident about using these products and if they're applied correctly, and you cover all the bare metal, you should not have any corrosion issues, even years down the road.
As I mentioned, we have one structural adhesive (07333) and two panel bonding adhesives, and there's a large difference between those two (08115 + 08116). So, a structural adhesive is designed only to be used where the manufacturers tell us to use it. So, you're going to need to specifically go to each procedure, for each vehicle, and specific year, and find out where they recommend a structural adhesive. You don't want to use that just anywhere we feel is suitable. We have to follow the OEM recommendations, and one of the reasons is that it's a completely different style of adhesive. So, it's got a lot of technology in adhesive, including that its impact-resistant or crash toughened. And what that means is it's got a little more give to it than a traditional bonding adhesive, and when you run a test on these two adhesive you see a noticeable difference in how they separate on impact. So, this adhesive here will absorb a lot more crash energy and it'll separate a little bit differently.
Now, the 08115 + 08116 adhesives, are panel bonding adhesives, are mainly used for outer body panels. There's an exception nowadays with the Ford F-150, the aluminum vehicle using 08115 non-structural panels. But other than that they are not intended for use on structural panels. So always follow the instructions very closely, don't just toss them aside. It's a really good idea to review these occasionally and make sure you're following these instructions. So, those are the main differences between the two products, and really when you look at this it's pretty obvious after these mock crash tests that there's a large difference between the two. You've got a panel bonding adhesive that collapsed in more of an uncontrolled manner and easier, and as you can see did not absorb the same amount of energy as this crash toughened or impact resistant adhesive. So that's where the science comes into this, where it's designed to absorb energy and slow that crash event down. And you can see how much more energy this absorbed by the way it collapsed in a much more predictable manner. So really those are the main differences between these products.
When do I use 08115 bonding adhesive versus 08116? Here are the differences?
So, the main difference is 08116 was specifically designed to meet General Motors and Chrysler specifications. For this product GM and Chrysler wanted to see cohesive failure should the seams ever come apart or separate, and that allows for better corrosion protection versus if you have adhesive failure and their separation. You would only have adhesive on one side and not on the other which is prone to corrosion. This is the the main difference. There is a slight difference as far as clamping time as well. There's a slightly longer clamping time for the 08116, but the main difference again is in that failure mode. So now, the other parts of that question is, how do I know when to use one or the other? And obviously, we're going to follow OEM recommendations. For the 08116, Chrysler wants it used for all their bonding operations. General Motors also specifies 08116, but only for the door skin bonding operations. General Motors are now recommending the structural adhesive for bonding operations.
Why do we need to equalize the cartridge and purge the cartridge?
What we're trying to do is eliminate any air pockets that may arise inside these plungers inside these cartridges. So, by plunging the cartridge it drives the air to the top and that'll come out in the beginning. We also need to inspect the ports on the end of the cartridge to make sure there's nothing dried up or hardened on the inside. It's always a good idea to have a little pick where you can pull anything out of there that hardens up, especially once the cartridge has been used and stored, there's a good possibility that happened. So, we need to get rid of that material as well so we have a good flow of material going through the nozzle.
Why do we need to purge some material out after we've equalized the cartridge?
The reason is that when this tip is empty and we're pushing product through it there's no back pressure on the material down here. It's just air above it, so it tends to flow through without going through that mixer very thoroughly. Once filled about halfway, there's now back pressure that forces it through that mixer a lot more efficiently. This is why we want to just run out a couple of inches of product before we start to apply it to the vehicle.
Do I have to remove the E-coat or galvanized coatings on the surface of the parts?
And the answer is again follow OEM recommendations. Many of the OEMs will actually in their instructions state to defer to the adhesive maker's instructions for the specific procedures only because the OEM does not always know which adhesive you're going to select for that particular vehicle, so the instructions may be slightly different. So again, familiarize yourselves with these instructions. This information is in the instructions and it'll tell you whether or not you have to remove that coating. Now this sample here shows the difference between a surface that just has the primers removed compared to one where it's been abraded aggressively enough to remove all the galvanizing or zinc coating. So, you can see a drastic difference between the two. Some vehicle makers refer to that as bright steel. Some will say remove the coatings right down to bright steel. This means you abrade until you get that nice shiny surface and you know all the galvanizing has been removed. But there's no simple solution and one answer. It's always going into those procedures whether it be the OEM or the adhesive maker's procedures to find out what they recommend as far as removing the coatings.
As far as removing the E-coat, as far as the 3M 08115, 08116 and structural adhesive are concerned, always remove the E-coat with one exception, and that's for 08115 and 08116 where they're used on door skins. It's allowable to leave the OEM E-coat intact, as long as it's scuffed and properly cleaned, but only on the door skins. So really not very complicated, but go back to the OEM and adhesive maker's instructions for the correct information.
Can I use heat to accelerate the cure?
As stated earlier, because of the epoxy chemistry of these products they handle heat very well, so it's absolutely acceptable to use heat to force cure these products. As always, we want to observe those maximum temperatures, the recommendations are about 150 degrees for 40 minutes, which will get you very close to full cure, roughly 80-85 percent of full cure in as little as 40 minutes. So, we want to observe that maximum temperature, we don't want to abuse it and overheat, you want to monitor that temperature closely. It's a good idea to use something like an infrared thermometer where you can actually get the actual temperature of the panel rather than ambient temperature. So, feel free to use heat, just follow the instructions to do so.
Can I put the product in the refrigerator to slow that cure time down?
And a simple answer is yes. However, its recommend maybe something a little less aggressive, like an air-conditioned office rather than refrigerator, but as long as you're monitoring how cold it gets, you absolutely don't want it to freeze. But just cooling that product down will give you a little more cure time. For climates like northern Australia where it gets extremely hot, it's not a bad idea to cool the product down some, but keep in mind as it gets colder the viscosity gets thicker, so just take care when you're extruding it out of the cartridge that you don't force it too hard. It may not go through the mixer correctly and get a thorough mix.
As far as when you see equivalence in a repair procedure, for example, it may say use “3M 08115 or equivalent.” Who do I listen to?
This is a tricky question. In order to avoid any liability issues it's always a good idea to use the actual called-out product rather than an equivalent. The problem with equivalence is, it takes an engineering degree to really decipher whether it is an exact equivalent or not. It may not have certain chemicals in it, it may have chemicals it shouldn't have in it. So, in order to say it's an exact equivalent is very difficult to say. So simply put, I would just recommend using the adhesive that's named in that procedure, and stay away from the equivalents as much as possible.
Why do we weld bond?
A great question. So, weld bonding is the combination of adhesives and welds in the joint. Why do we need both? Well, the adhesive adds a lot of benefits to that joint. One of the things is it prevents corrosion because you've got a completely sealed joint that you would not normally have with just welds. So, it seals that joint to keep any moisture or air from passing through there and causing corrosion. Another reason is in a case of the impact resistant structural adhesive, it actually allows the part to collapse in more of a predictable manner and absorb energy as it collapses. Another benefit is you've got a continuous bond. It's continuous so you get really good lap shear strength. The weld in there prevents the adhesive from ever going into peel mode and starting to peel. That's kind of the weakness of any adhesive is peel mode, and having that weld in those areas will prevent that from happening. So, a lot of benefits to combining the welds and the adhesive together in weld bonding.
Can you weld through a cured adhesive?
This is not recommended after its cured. Follow the work time on the adhesive cartridge. In this case it says 60-minute work time. You really want to get your welding complete within that work time. Actually, having that liquid adhesive in there helps with the conductivity between these two panels when we're trying to spot weld through them. Once that adhesive cures it actually acts as an insulator, almost like a gasket, between those two pieces of metal which do not allow for that conductivity from one side of the spot welder to the other. So, the short answer is do not weld through cured adhesives.
How do I keep my bonded flanges from corroding?
Now if you had done 3M’s replacement procedure and preparation perfectly, there really shouldn't be a chance of corrosion in that joint, as long as we've got all the bare metal covered. The adhesive has corrosion inhibitors and that should not happen. However, there's no guarantee that during the replacement of that panel we could have maybe damaged or scraped off some of the adhesive in certain areas. A great example of that are quarter panels. When you hang a quarter panel, oftentimes you need to force it a little bit down onto that wheel arch area, and in the process of that it is possible we could scrape some of that adhesive off and leave a void. So, insurance against that would be Cavity Wax . So as long as you are follow all bonding operations with Cavity Wax which will get into those areas that might be exposed and cover them and creep in, you're not going to have that corrosion in those joints.
How long will a cartridge last after it's been opened?
The answer to that is as long as you leave the nozzle on when you're finished with the product and allow it to harden up and seal this cartridge, you should get very close to the original stated shelf-life. It's also important that you know the shelf-life of these products. It generally varies between 1 and 2 years. So as long as you seal it up correctly, leave that nozzle on, you should get close to that 1 year or 2 year, whatever it stated, so don't just toss the product away after it's been used once. There's still a long ways to go with it.