Best adhesive to mount solar panels with?

C

ckent42

New member
After some researching on various forums, it looks like a common method for attaching solar panels to vehicle roofs is via adhesive, either Sikaflex or 3M VHB. I'm looking for the best way to attach aluminum unistrut to the painted aluminum roof of my ambulance, and I'd like to limit the amount of roof penetrations as much as possible. Since there's various formulas for both of those adhesives, I reached out to both Sika and 3M for guidance on the best one to use for my application. Sika responded and said:

"Sika does not support the use of our products as the primary fastening system for solar panel systems on moving vehicles. We cannot provide you recommendations for your application."

3M replied with:

"Unfortunately, neither division is able to provide a recommendation on your structural application. We suggest contacting the solar panel manufacturer or consulting with a certified repair facility where a trained technician can view the application firsthand and provide a suitable direction forward. "

Neither of which was helpful in the least, so I'm wondering what builders here have used successfully. 3M, Sikaflex, mechanical fasteners, a combination of both? I ran a structural engineering wind calculation on the panels, assuming a max 120 mph wind speed, and the uplift and shear experienced by the panels was surprisingly low, so it doesn't look like much holding force is required. On the other hand, there will be lots of environmental factors working against exposed adhesive, and the last thing I want is a 40 lb residential panel flying off the roof at freeway speeds.
 
SquirrelZ

SquirrelZ

Member
Its hardly surprising that either company wants to assume any sort of liability with specific guidance. But at least they answered your inquiry.

I attached a rigid residential solar panel to the roof of my van using a combination of mechanic fasteners to the roof rack and angled aluminum fastened to the roof with 3M 5200. Either method probably would have been adequate, but I like some redundacy. Like you, I wasn't thrilled with the possibility of having a panel fly off the top of my van. 5200 has good shear strength and its flexible through a wide range of temperatures. Its pretty much the gold standard for a marine adhesive/sealant so its a tried and proven product. https://multimedia.3m.com/mws/media/158782O/adhesive-sealant-5200-tech-data.pdf

Putting a small wind foil in front of the panel would also significantly reduce the force on the panel. You could put that in place first with your chosen adhesive and see how it performs before putting the panel in place.
 
H

headingwest

Member
For my next build I'm going to glue down parallel t-track then bolt into that. The problem with adhesive is if the solar panel fails I might need to replace it with a different size panel.

Sent from my SM-N975F using Tapatalk
 
S

Steve321

New member
W

Wrathchild

Active member
AM solar’s rocker mounts are with VHB on the bracket base with a sikaflex flood around the feet. I was skeptical but there’s lots of folks that have had success and after a few months I’m confident it’s not going anywhere.

the same principle could easily be applied to unistrut or Ltrack.
 
Alloy

Alloy

Well-known member
ckent42 said:
After some researching on various forums, it looks like a common method for attaching solar panels to vehicle roofs is via adhesive, either Sikaflex or 3M VHB. I'm looking for the best way to attach aluminum unistrut to the painted aluminum roof of my ambulance, and I'd like to limit the amount of roof penetrations as much as possible. Since there's various formulas for both of those adhesives, I reached out to both Sika and 3M for guidance on the best one to use for my application. Sika responded and said:

"Sika does not support the use of our products as the primary fastening system for solar panel systems on moving vehicles. We cannot provide you recommendations for your application."

3M replied with:

"Unfortunately, neither division is able to provide a recommendation on your structural application. We suggest contacting the solar panel manufacturer or consulting with a certified repair facility where a trained technician can view the application firsthand and provide a suitable direction forward. "

Neither of which was helpful in the least, so I'm wondering what builders here have used successfully. 3M, Sikaflex, mechanical fasteners, a combination of both? I ran a structural engineering wind calculation on the panels, assuming a max 120 mph wind speed, and the uplift and shear experienced by the panels was surprisingly low, so it doesn't look like much holding force is required. On the other hand, there will be lots of environmental factors working against exposed adhesive, and the last thing I want is a 40 lb residential panel flying off the roof at freeway speeds.
Click to expand...

Sika's urethane does not bond well to aluminum. 3M urethan is much better

Don't bond to the paint remove it and bond to cleaned /sanded aluminum within 4 hours of the prep.

There is always a chance of banging a panel with a branch. When I inspect our roof for leaks I give the panel mounts a pull.

I worked with some Henkel Methyl Methacrylate (MMA) on aluminum to aluminum that was very strong.

 
Joe917

Joe917

Explorer
Steve321 said:
Interesting @Joe917 ...something like this?

www.homedepot.com

Everbilt 1-1/2 in. x 14-Gauge x 72 in. Zinc-Plated Slotted Angle-800517 - The Home Depot

This metal angle is commonly used for constructing bicycle racks, bed frames, motor mounts, and many other utilitarian functions. Common types of angles include: Solid, punched whole, slotted whole and
www.homedepot.com

How do the panels bolt to the angle - do the panels have holes that facilitate that, or do you have to drill into them?
Click to expand...
IMG_20200610_143022.jpg
There is one piece of angle attached to the roof with 3m 5200. A second piece of angle is then bolted to the panel and first piece of angle.
IMG_20200610_143118.jpg
 
T

tacomabill

Active member
+++ for Sikaflex 221 as recommednded by AM Solar which is where I bought my panel and installed it myself. Their rocker feet come with a peel off VHB sticky back tape and the Sikaflex is used to provide a waterproof caulk around the feet. I have put 21K miles on them and some very harsh desert roads with no problems. You would need a crow bar to get them off.
 
billiebob

billiebob

Well-known member
The best all around construction adhesive I have found is PL Premium. And I've spent a small fortune on the high tech stuff. PL Premium is like 10 bucks a tube. Altho I wouldn't fault anything 3M.



adhesive-construction-brown-pl-premium.jpeg
 
billiebob

billiebob

Well-known member
Another option, ask at a local body shop, they often glue on replacement body panels.

Heres a depressing story.

In a head on with a pickup which crossed the center line, the owners of a Honda Fit won a $42M settlement, 25% paid by the pickup driver, 75% by the body shop. The previous owner had brought the Honda Fit into John Eagle Collision Center after the roof had been damaged by hail. But rather than weld on a new panel, as Honda requires, the shop instead opted to use epoxy adhesive. An industry standard but only allowed if the manufacturer does not specify otherwise.

Ask the guys who do it every day.
 
C

ckent42

New member
I hadn't heard of 3M 5200 before, that stuff looks pretty good. After doing a little more digging into data sheets, and doing some calculations, this is what I've come up with:

Wind Force, Fw = 1/2ρv^2A

Where ρ = air density
V = air velocity (in this case I chose 120 mph, thinking 60 mph highway speeds coupled with a 60 mph headwind gust.)
A = frontal area of solar panel

The head on shear force per panel (assuming installed long edge facing forwards) comes out to be Fw = 463 lbs.

I then checked the panel at a 10 degree angle of attack, which increases the vertical area from 1" to 6.25". This is assuming that somehow wind has gotten under the panel (I plan on installing a front fairing to prevent this from happening)

With that condition, the force on the panel becomes 510 lbs of uplift, and 2934 lbs of shear. This sounds like a lot of force, so lets see what it translates to in terms of needed adhesive strength.

First, find the resultant of the two forces, which is 2,978 lbs. Divide that by the number of strut sections beneath the panel, to find the force per strut. (For this example, 3)

Then, find the tension on due to the shear trying to overturn the strut. It's Fshear*(StrutHeight)/(StrutWidth). Since the strut is as wide as it is tall, the shear is just converted into tension. It doesn't stop working as shear though, so add that additional tension into the force per strut. So we end up with 1,503 lbs of combined shear and tension on each strut length.

I'm going to assume that I'm not the best glue-er, and that my surface prep isn't A+, so I'll go ahead and decrease my adhesive strength by 20% (in this case increasing the force by 20%, since I'm working backwards), which bumps things up to 1,803 lbs. After reading up on those data sheets, it seems like the industry standard for adhesive is to use a safety factor of around 5, so lets go ahead and multiply that number by 5. That gets us all the up to 9,016 lbs, or just over 4 and a half tons of force.

The final step is to find out the working area of our strut, so we can figure out how the necessary psi for our adhesives. I'm assuming a 5' long stick is about the longest I'll want to work with, so that's 60" long by 1.625" wide, or 97.5 square inches. Dividing the 9,016 lbs by 97.5in^2 gets us 92.5 psi minimum required bond strength.

Looking at 3M 5200 (393 psi) thats only a 23.7% utilization. Sikaflex 252 is a little weaker (360 psi) so 25.8% utilization. Unfortunately, using these extremely conservative calculations 3M VHB RP 25 F isn't strong enough at 80 PSI. However, if you can eliminate letting air under the panels the forces are decreased considerably and the utilization rates drop to 2.4%, 2.6%, and 11.9% respectively.

After running the numbers I feel a lot better about using only adhesive....but also apprehensive about the provided strength numbers from manufacturers. It seems crazy that a single square inch of adhesive could suspend nearly 400 pounds. Fully bonded, my 5' stick of strut would have a minimum tensile yield strength of 38,318 lbs before the safety factor. That's insane.


Disclaimer: I'm not an adhesives engineer, nor do I have much experience with fluid forces on objects. I simplified the problem as much as I could, and made the assumptions as conservative as possible. Please note that the wind force equation is a simplified version of the drag force equation, just missing the coefficient of drag. I wasn't sure what it would be for a solar panel, but probably less than 1, which would reduce the force, so I left it out. If you're looking at these numbers and thinking they look way too high because you've driven with stuff on your roof before and no way was it exerting literal tons of force on your vehicle, this and the 150 mph speed is why. Most shapes have a drag coefficient below 1, and in the wind force equation the velocity is squared, which means that force increases linearly with area, but exponentially with velocty. In my example, at a much more reasonable 60 mph the force felt by the panel head on is only 116 lbs.
 
H

headingwest

Member
Great breakdown, thanks. My experience is that many of the adhesives, bolts and other fasteners are massively over engineered for what we use. It's not having a clean surface for adhesives, pull through bolts, poorly seated rivnuts etc that fail. The floor of my camper is secured to the truck by 8 small bolts, but the specs show that just one bolt would hold in perfect conditions. I still worry about it though.

Sent from my SM-N975F using Tapatalk
 
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