Unexpected response from 3M about VHB 4950 tape for rigid solar panels

woundedpig

2018 Unity MB
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.
 

autostaretx

Erratic Member
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/1267925O/vhb-tape-specialty-tapes.pdf
5962 (5952 family) data sheet: https://multimedia.3m.com/mws/media/67100O/3mtm-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(??:wtf:) 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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HarryN

Well-known member
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.
 
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woundedpig

2018 Unity MB
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



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/1267925O/vhb-tape-specialty-tapes.pdf
5962 (5952 family) data sheet: https://multimedia.3m.com/mws/media/67100O/3mtm-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)
 

rollerbearing

Well-known member
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.
 
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autostaretx

Erratic Member
Math happened:

... you've got LOTS of relatively little(?:wtf:?) 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"
 

rollerbearing

Well-known member
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!
 

autostaretx

Erratic Member
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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rollerbearing

Well-known member
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)
 

woundedpig

2018 Unity MB
I still don't get the 4 sq. inches per pound or load spec. It seems incredibly conservative. When I first heard about VHB, I went online and especially looked at youtube videos of industrial applications and demonstrations of sheer strength etc. These demos used nothing approaching the above spec of 4 sq in/lb load. VHB tapes are used in aircraft flying at tremendous speeds experiencing huge forces and heavy trucks bouncing down the road. I just looked at the aircraft and automotive applications sections on the 3M site.
 

rollerbearing

Well-known member
It has to do with that slow creep problem for constant static loads. Especially in shear. Things like picture frames. It is still a problem in static tension as well. You could hang a big old light fixture over your dinning room table and find it crashed to the floor the next morning.

Dynamic tension loading is a different matter - so much stonger! This is the kind of loading you are seeing on the truck trailer sidewall panels - if it doesn't have to hold the panel's weight up - ie the bottom edge of the truck sidewall is resting on a frame. In this case you'll see VERY much lower reccomendations.

They can't expect the general public to always know the difference so they are quoting you the shear static load number. That would apply if you wanted to stick panels on the side of your RV (without anything else supporting their weight.) Having them sit flat on the roof supported from below they are mostly going to experience tension loading some of which will be dynamic - from vortexes and some may be constant from angle of attack uplift as Autostartex calculated.
 
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ablock

Member
What about adding a strip of Eternabond over the mounting foot, with say 1.5" of overlap on all sides?

I don't see what #8 screws into a FRP skin panel is going to do for you. I assume the roof core has essentially zero resistance to pullout.
 

220629

Well-known member
...

I don't see what #8 screws into a FRP skin panel is going to do for you. I assume the roof core has essentially zero resistance to pullout.
Some ss screws might do more than you might think.

Is the roof plastic or composite - fiberglass? If composite the fasteners should lock in quite well. If plastic, not quite as well.

My thought is that the fasteners will clamp the parts together and help prevent the VHB or sealant from losing the bond.

I'm no expert.

I do like the idea of the Eternabond tape for weather seal. I doubt it will add much bonding strength.

I've mentioned before that Plexus adhesive has been used to bond boat decks to the hulls for quite some now. It is very effective.
http://www.itwplexus.com/UserFiles/File/Guide_To_Bonding.pdf


:2cents: vic
 
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rollerbearing

Well-known member
What about adding a strip of Eternabond over the mounting foot, with say 1.5" of overlap on all sides?

I don't see what #8 screws into a FRP skin panel is going to do for you. I assume the roof core has essentially zero resistance to pullout.
A bit better than screws are through bolts backed with load spreading plates. My fasteners on the interior are hidden in cupboards and behind ceiling AC vents. They are covered on the exterior with self leveling sealant. Agree with Vic about marine adhesives. Look up tensile strength of 3M 5200 and compare to VHB. Look up tensile strength of hardware store Permatex Silicone and compare to VHB.

Nonetheless, MANY people have used just VHB. If you go that route - really understand what you are doing - have plenty of safety margin (like 5X) and consider backing up with mechanical fasteners. The closest engineering data you will find is from the building trade - look up roof decking screw fastening schedules and wind uplift values. VHB is cheap and this is a case of a little seems to work and more is probably better (and sure wouldn't hurt anything or anybody.)
 
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ablock

Member
A bit better than screws are through bolts backed with load spreading plates.
I would say that would be much better, as in, that would be the marine industry standard. But also impossible in the Unity as the interior roof panels cannot be removed AFAIK.

5200 would be a good choic if you never, ever expect to need to remove the solar mounts.
 

woundedpig

2018 Unity MB
This is AMSolar's view: On fiberglass roofs they use VHB without screws with mounts that have just 2.5 sq inches of base area each. The Renogy mounts have twice that surface area each. 80 pounds of shear force per square inch of VHB = 800 pound of shear force using the AMSolar mounts, and twice that using the Renogy Z brackets. I tend to over analyze things and I'm in that phase right now.......... Still pondering. I don't know how thick the fiberglass shell is for the Leisure Travel Unity and don't know exactly what is underneath (4 inches thick) but wonder what the resistance to screw pullout really is, like Ablock.

I just heard from Garret, an engineer at AMS. He said that 3M's response looks like a "CYA" to him and again, no failures. He suggested adding an extra mounts when the panel is mounted near the leading edge of a vehicle where there might be laminar airflow and said that AMS tends to add extra L-Feet to increase the contact area in such a setting.
 

autostaretx

Erratic Member
There are so many issues at play here... if you simply #8 or #10 sheet-metal-screw it to the fiberglas, i'd really be worrying about pull-out (if the VHB failed/crept). But you'd certainly be helping in the straight-to-the-rear shear issues.

Next up would be to drill a bigger hole, and insert/install a Molly-like T- or expansion "nut" ... there's a WingIts brand for ADA handrails which uses two coupled broad-spreading plastic tripod back supports ... good enough to exceed 600 pound support in 5/8" wall board, so quite likely decent for fighting lift effects of a panel.

A problem with putting any hole in the fiberglas is the potential for fatigue/stress cracks spreading from it as the roof flexes up and down from the winds buffeting the panel. Bigger, smoother holes help minimize that.

Me? I'd accept the minor cosmetic damage of cutting through the ceiling inside the van to reach one or two of the forward feet to provide a load plate (my reason for saying "bolts and nuts" earlier... i'd certainly include load-spreading fender washers (at least)). You can always hide the ceiling hole with a new LED light fixture or the like.

--dick
p.s. when i installed my MaxxFan, i went the route of installing a wooden frame beneath my steel roof to receive its mounting screws, instead of just trusting the 12 or 16 screws' threads in the sheet metal alone. Belts and suspenders.
 
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chromisdesigns

New member
Based on boat experience I would make wooden or aluminum mounting pads, glue them down with 3M 5200 polyurethane and screw or bolt the panels to the pads.

Using 5200 you can probably lift the van by the pads! That stuff sticks.

And if you ever have to remove them for some reason you can cut them off with a length of thin wire fishing leader
 
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Bone Head

2014 LTV Unity MB
Math vs. REAL WORLD:

My Renology solar panels are attached with VHB tape (don't know which number) for a couple of years now and have been from Wisconsin to Texas to California and Alaska without as much as a whimper. I do go up on the roof every once and a while to check, but for my money, no penetrations is a good thing.
I understand the math and the concern, but let's face it folks, the VHB tape just works.

Bob
 

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