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LAT39

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Reply with quote  #1 
I'm mostly a lurker, just learning for now until I have time to devote to doing a build.

I've seen several comments about people wanting the keep the weight of the solar collector low, a desire that I share.  Wood tends to be heavy,  steel stud track less heavy, and obviously if one could get away with glued together pieces of polyiso that would be ideal.  The downside of 100% polyiso is durability.  Recently I saw a YouTube video that sparked an idea (one that should have occurred to me sooner.)  The idea is to build the box out of polyiso, then laminate on fiberglass/epoxy layer(s) to protect the polyiso and to build strength and rigidity.  The technique may end up being the lightest yet.

I still don't have time to experiment with the idea, but I thought I should share it here so others could give it a try.

Here is the video that sparked the idea:



It's a good YouTube channel that many fellow DIYers may find interesting:
https://www.youtube.com/user/TechIngredients/videos

stmbtwle

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Reply with quote  #2 
Great idea! That should be an easy build, covering a polyiso box with fiberglass, and you could make it any depth you want. Does anyone know if polyester resin (cheaper than epoxy) will dissolve polyiso?
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Willie, Tampa Bay
LAT39

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Reply with quote  #3 
Yes, the depth is no longer constrained by whatever some standard size construction material happens to be.

You can easily laminate plywood in places that need to be stronger for mounting points.  It's really a versatile approach to building a box.

A detail that may not be immediately obvious is that when laminating over a large span, like a 4'x8' sheet of polyiso, it's good to tie the two layers together somehow.  Imagine cutting cylinders out of the polyiso, wrapping them with a few layers of fiberglass/epoxy and stuffing them back into the holes.  Then apply the outer layers of fiberglass/epoxy as mentioned above.  The cutouts don't need to be uniformly located and it's probably better if they're not. The picture below illustrates the concept.

[BasicBox]

Regarding polyiso compatibility with polyester resin, I believe they are but I've never tried it.  I know that polyester and vinylester resins will both attack EPS insulation.

One advantage epoxy resins have over polyester resins is the odor.  Epoxy is very low odor and polyester resin stinks.
gbwillson

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Reply with quote  #4 
LAT39-

Thank you for posting this. I have been thinking of using this on my next collector build. But I thought of the mess and smell was off-putting. I also never found a source to purchase the various cloth types in bulk rolls. Neither hobby nor auto parts stores carried the fabrics in bulk. They could special order, but it costs a lot. 

I like your idea of poking plugs of sorts in the foam to help bond the foam to the fiberglas resin. As it is now, the bond is only as strong as the polyiso foam to the thin foil. I think you could also effectively build up strong mounting points  as needed instead of using a block of wood glued to the surface.

Greg in MN




stmbtwle

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Reply with quote  #5 
Greg try the boating supply or repair businesses in your area. If all else fails contact https://www.fgci.com/products . I know they'll have it.

I think a wallpaper scoring tool could perforate the paper or foil and give the resin better adhesion.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
jjackstone

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Reply with quote  #6 
Quote:
Thank you for posting this. I have been thinking of using this on my next collector build. But I thought of the mess and smell was off-putting. I also never found a source to purchase the various cloth types in bulk rolls. Neither hobby nor auto parts stores carried the fabrics in bulk. They could special order, but it costs a lot. 


Agree with boating supply companies. West Marine usually carries a good stock. Their epoxy is fairly expensive but is hi quality. I often can find good deals on Ebay even with shipping costs. 

A couple other sources:

http://sweetcomposites.com/Fiberglass.html

http://www.uscomposites.com/cloth.html


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LAT39

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Reply with quote  #7 
JJ, thanks for posting the links to glass suppliers.  I haven't purchased that stuff for more than 30 years and didn't have any idea of the cost now days.  Last time I bought it I was able to get it from a local (at the time) repair shop.  They didn't normally sell materials, but for whatever reason they were happy to sell stuff to me.

This approach to building a box may not be the cheapest way.  It's a matter of what's important to an individual.  For some people it may be worth some extra cost to be able to easily maneuver the collector into position by themselves.  That's especially true for people who need to "set it up" and "put it away" every year.  There is also the "easier build" factor.  As stmbtwle mentioned earlier, it should be a pretty easy way to build a box, no special skills required.

Regarding connecting the inner and outer layers, I have no idea how many "plugs" is the right number or even how large they should be.  For scale, in the picture of the box I posted earlier, the outside dimensions of the box are 48" x 96" x 5" (1" thick polyiso), the intake and exhaust openings are 6" in diameter and the plugs are 1" in diameter.  Larger diameter plugs probably make more sense, 2" or maybe 3" perhaps.

As Greg points out, the bond to the foam is only as good as the bond to the foil.  Even if the foil were peeled off and discarded, the bond is still no stronger than the cohesive strength of the foam, plus the foam would absorb more resin which would drive up the weight and cost without increasing the strength appreciably.  With thermal cycling it's likely the laminated layers would separate from the foam eventually.  When that happens it's probably good to have the layers connected to each other... with plugs of some sort.
gbwillson

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Reply with quote  #8 
I like Willie's suggestion of using a wall paper removal tool to poke a few thousand tiny holes in the foil. Seems reasonable, might be worth a try on a scrap of polyiso. Otherwise I think dozens of pokes with a nail, with the nail inserted at an angle to help prevent pullout might work. The resin would ooze into the holes, grip the foam, and hopefully hold the foam and fiberglas mat together as one. 

In the video, he mentioned several types of cloth or mats available. Assuming the resin and mats are for the polyiso panels, is there a preferred mat or mesh? Is there much of a difference in cost, strength, and performance between the various styles?

Greg in MN
LAT39

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Reply with quote  #9 
It may be worth an experiment on some scrap foam.  The great thing about experiments is that you always learn more not less.

Quote:
Originally Posted by gbwillson
Is there much of a difference in cost, strength, and performance between the various styles?


Generally speaking...
Ultimately it comes down to how strong you need it to be and how much glass is required to achieve that strength.  It's always a trade-off in time, effort and cost (both material and labor.)  Thicker glass will build strength faster, requiring fewer layers at the expense of being more difficult to form around corners and stay where you want it.  Thinner glass will be easier to work with, but will require more layers to achieve the desired strength and more layers usually means more time.

So, which glass to choose and how many layers?  I don't know, maybe someone else with more experience will chime in. 

The video never said how many layers of "boat cloth" (I've never heard that term before) were used.  It may have been just one.  If it was just one layer over two sides of EPS foam, the results were pretty impressive.  The video did say 6 to 8 oz, which if compared to the options on the US Composites site linked above, that suggests 7 1/2 oz per square yard and a thickness of 0.0107" (about 1/4 mm).

Personally I feel like two layers of that stuff would be a minimum, but I'm just guessing.
stmbtwle

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Reply with quote  #10 
For strength I use "biaxial" when I can get it. It's easier to work with than cloth, thicker, and stronger. Some call it 'stitch mat'. However if all you're trying to do is waterproof the foam, then the cheapest cloth is fine. You can double up any joints with fiberglass tape.

Don't overthink it, you're building a solar collector, not an ocean-racing powerboat.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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