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Old 06-29-2017, 11:10 PM   #1
woundedpig
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Default Unexpected response from 3M about VHB 4950 tape for rigid solar panels

I contacted 3M trying to get some help figuring out how to get appropriate width of VHB to get full coverage of some solar panel mounts I plan to use for some compact 40X20 inch 100W Renogy panels (not flex panels). I want to have the option to remove the panels in the future and feel they have advantages in terms of durability, failure risk, and output at high panel temps. The panel mounts have a base that is almost 4 inches long by 1.34 inches wide.The panels will be 1.2 inches off the roof surface when mounted. Not as stealthy as flex panels but not bad. I'd like to avoid screws, but would consider them - there are already screws up there.

I got the response below. ? Is this a CYA response? Maybe I should have expected it. I don't know which 3M tape # or how much they use to hold trucks together but it is not 4 sq inches per pound of weight supported. AMSolar's site says they use 4950 for mounting panels on Airstreams as well as fiberglass roofed RV's (but not rubber roofs).

Thoughts?

David

__________________________________________________ ____________

Thank you for contacting 3M, where we apply science to life.

I am not sure where you heard the 4950 VHB tape is frequently used for this type of application. We do not suggest that particular VHB, and in fact, VHB can be used for this application, but we suggest it in conjunction with mechanical fasteners. VHB was not intended for this application as a sole means of bonding it due to many factors. The 4950 tape is mostly used for metal to metal applications. We have other VHB tapes that will bond much better to fiberglass. I would suggest to test our VHB tape #5962 along with our adhesion promoter 111. We suggest to use 4 square inches of tape, per pound of weight it will support. I have attached the data page for review. Please read over all the application instructions as surface prep, how much tape to use, set up time, etc. is all listed. You won't need to get an exact width of tape for the width of the panels, as long as you can get the 4 sq. inches per pound around the diameter. Also, use the mechanical fasteners in addition to the tape. This is suggested to support the wind loads it will see going down the highway.

You can find these products on Amazon.com, R S Hughes, or Grainger, usually in smaller rolls or broken cases.

If you need further/additional assistance or prefer to speak directly with a specialist please call 1-800-362-3550 for the first available representative.
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Old 06-30-2017, 12:52 AM   #2
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Reading the data sheets, the 4950 certainly looks the better way to go.
The only thing 5962 has going for it is a thicker, more conformable layer between the two adhesive surfaces.

The 4950 has a 140 pounds per sq inch straight pull rating, whereas the 5962 is only 90 pounds.
The peel ratings are about the same (25 vs 22 lbs per inch). The overlap shears are the same (80 psi).

4950 family data sheet: http://multimedia.3m.com/mws/media/1...alty-tapes.pdf
5962 (5952 family) data sheet: https://multimedia.3m.com/mws/media/...-vhb-tapes.pdf

... i'd still put bolts and nuts through the leading edge's feet.

Your panels are 5 and a half square feet of (potential) sail... if they start to lift at all at 60 mph, you've got LOTS of relatively little(??) force involved.
Cranking through lift and drag calculations for a flat plate (and ignoring the fact the Sprinter is under the panel) shows only 30 pounds of upward force (lift) for a 5 degree angle of attack at 60 mph on your panel. The drag force is only about 18 ounces.

--dick
(there's another thread of someone having his flexible panels peel off, i don't recall the adhesive or tape he used)
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Get YOUR Sprinter's full configuration datacard by entering your VIN to https://www.datamb.com/
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((as always: this post may go through a couple of post-posting edits... so maybe give it ten minutes before commenting))

Last edited by autostaretx; 06-30-2017 at 02:08 AM.
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Old 06-30-2017, 01:28 AM   #3
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

There is quite a large thermal expansion mismatch between fiberglass and aluminum. In addition, the solar panel mounts will experience quite large thermal cycles each day.

I wonder if their recommendation for a version with a thicker, more compliant version is related to buffering this cyclical thermal expansion effect?

About 15 years ago I was experimenting with mechanically and thermally bonding the LED thermal mount points to a combination electrical / thermal substrate. I tested several adhesives fairly aggressively and two things became very obvious:
- The surface preparation and pre clean effect was greater than the difference between the adhesives I tested.
- The lower spec but more mechanically compliant adhesive performed substantially better than the stronger, more rigid one - in mechanical shock testing.

Last edited by HarryN; 06-30-2017 at 01:30 AM.
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Old 06-30-2017, 01:43 AM   #4
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Yes, I'm working my way through the 3M data sheets - wish I was a materials chemist. There was a person whose flex panel came off who used Sikaflex 221 as his adhesive. He said that this version of Sikaflex is more sealant than adhesive. His flex panels had a polyethylene coating on the rear that was intended to be protective, but was a very poor bonding surface. He ended up using Eternabond tape around the edges.

It seems hard to argue with the success of AM Solar's use of VHB/no screws, and all the installers who are out there full-time RV'ing and installing systems.

David



Quote:
Originally Posted by autostaretx View Post
Reading the data sheets, the 4950 certainly looks the better way to go.
The only thing 5962 has going for it is a thicker, more conformable layer between the two adhesive surfaces.

The 4950 has a 140 pounds per sq inch straight pull rating, whereas the 5962 is only 90 pounds.
The peel ratings are about the same (25 vs 22 lbs per inch). The overlap shears are the same (80 psi).

4950 family data sheet: http://multimedia.3m.com/mws/media/1...alty-tapes.pdf
5962 (5952 family) data sheet: https://multimedia.3m.com/mws/media/...-vhb-tapes.pdf

... i'd still put bolts and nuts through the leading edge's feet.

Your panels are 5 and a half square feet of (potential) sail... if they start to lift at all at 60 mph, you've got LOTS of force involved. (math may appear here in a bit...)

--dick
(there's another thread of someone having his flexible panels peel off, i don't recall the adhesive or tape he used)
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Old 06-30-2017, 02:09 AM   #5
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

A few of us beat this horse to a very bloody pulp over in the Winnebago Itasca area. Do look up wind uplift forces and what causes them. Also please realize VHB is not some miracle adhesive. It's tensile strength is comparable to silicone. And yes, AM solar seems to have an excellent track record.

Last edited by rollerbearing; 06-30-2017 at 02:52 AM.
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Old 06-30-2017, 02:15 AM   #6
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Math happened:

... you've got LOTS of relatively little(??) force involved.
Cranking through lift and drag calculations for a flat plate (and ignoring the fact the Sprinter is under the panel) shows only 30 pounds of upward force (lift) for a 5 degree angle of attack at 60 mph on your panel. The drag force is only about 18 ounces.

--dick (quite surprised by the result)
p.s. the Sprinter under the panel increases the lift ... quite like flying in "ground effect"
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Old 06-30-2017, 02:47 AM   #7
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Wind up lift forces for buildings - caused by vortexes rolling over the corners creating a low pressure over the flat surface - can be as high as 50 psf at the edges and around 20 psf in the middle of the roof area. These are usually just spot type loads - not uniformly applied over the entire roof area. But, mulitply 50 psf times just 1/4 of your panel area to get an idea of what kind of loading you might see on one footing. Up around 100 pounds!
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Old 06-30-2017, 02:51 AM   #8
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

Wind uplift on building mounted solar panels:

http://files.engineering.com/getfile...sfp=1236935214
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Old 06-30-2017, 03:05 AM   #9
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

It should be noted that my lift/drag calculations were based upon 60 mph driving speed in still air.

If you hit a 60 mph headwind (let's say driving into a rain squall), the force goes up by a factor of four (lift and drag are V-squared beasties)

If your panels are mounted at the front of your roof: look at the slope up from your windshield... that's going to be vectoring the wind to a greater angle of attack.

--dick
p.s. i notice that that solar panel wind load report specifically mentioned that they (also) did NOT take into account the close-to-subsurface "ground effect" contribution
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Get YOUR Sprinter's full configuration datacard by entering your VIN to https://www.datamb.com/
http://diysprinter.co.uk/reference <-- lots of service documentation, Thanks to Jens Moller and Arnie_Oli
((as always: this post may go through a couple of post-posting edits... so maybe give it ten minutes before commenting))

Last edited by autostaretx; 06-30-2017 at 03:08 AM.
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Old 06-30-2017, 03:42 AM   #10
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Default Re: Unexpected response from 3M about VHB 4950 tape for rigid solar panels

VHB also tends to creep when constantly loaded - that is why they recommend 4 sq. inches per pound for constant loads. It does much better under intermittent load/unload situations where it does not creep. But even then it does experience some small level of cycle fatigue. Don't get me wrong, the stuff works for many people - just be sure to have PLENTY of safety margin and possibly consider mechanical fastener back up (AM Solar does reccomend screws - at least in some places on their web site)
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