OT: Plastic Tube Materials

I know this is OT, but there's so many knowledgeable people here, I was thinking there's probably someone who would know something about this. I've been using PVC pipe as a structural component in something that's used outdoors for a number of years. What I recently discovered is that the pipe has a tendency to expand on the sunny side of the pipe, thereby making it bend due to the difference in expansion rate between the sunny side and shadow side. This is kind of a nightmare, because we have a bunch of these things out there, and I'm now thinking it was a mistake to use PVC. The problem is that the tube/ pipe has to be completely non conductive, otherwise I would obviously just use metal.
So just wondering if there's any plastics experts out there with any ideas... Is there a type of PVC or other affordable plastic that this wouldn't happen with? I've thought about fiberglass, but having to finish it would be a huge time consuming thing. Plus, it's a lot more expensive.
Thanks for any help,
Dave

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Come on, Dave. You already know the answer. ALL material will expand when heated! Plastic will just expand more. Even metal pipes will move when heated unevenly.
The other problem with any plastic, PVC in particular, is they become very brittle when exposed to sunlight.
Paul

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Completly no conductive, or simply a section on top and bottom that are non conductive?
'In the first place, we should insist that if the immigrant who comes here in good faith becomes an American and assimilates himself to us, he shall be treated on an exact equality with everyone else, for it is an outrage to discriminate against any such man because of creed, or birthplace, or origin. But this is predicated upon the person's becoming in every facet an American, and nothing but an American... There can be no divided allegiance here. Any man who says he is an American, but something else also, isn't an American at all. We have room for but one flag, the American flag... We have room for but one language here, and that is the English language.. and we have room for but one sole loyalty and that is a loyalty to the American people.' Theodore Ro osevelt 1907

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Unfortunately, the whole thing has to be non conductive, ie: RF transparent.

Wood?
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Unfortunately, the whole thing has to be non conductive, ie: RF transparent.

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Ok, Dave. The answer is to use structural fiber glass angle, like angle iron. Use structural epoxy to join the sections. The flat sides of the angle will be strong enough to keep the structure from moving in the sun. You may have to redesign the unsupported lengths in order to keep them from twisting.
no, I don't know where you can find the material, but know it is available.
Paul

Artemus wrote:
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Try clear polycarbonate if it isn't too expensive.
http://polycarbonate-tubing.com/polycarbonate-tubes.htm

Dave D wrote:
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Is it an antenna?
Are you using white, gray or black. If not white, try it or paint it white.
IMHO, anyone using PVC for a structural application has probably made the first mistake right there. You probably picked it because it was cheap and easy to cut and glue. If you've priced your product based on that, you've pretty much boxed yourself into a corner.
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On Mon, 03 Aug 2009 13:54:43 -0700, Dave D wrote:
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I don't think there's _anything_ that has a completely zero coefficient of expansion with temperature unless it's some weird composite.
You ought to be able to get much lower, though.
Search Matweb -- www.matweb.com -- and see what you find.
If the expansion must be absolutely positively zero you may want to consider some sort of a gridiron, like they used in clocks, but then you'll (a) be juggling two different materials, (b) you'll have transient effects as the thing comes into and out of the sun, and (c) if you don't like the time spent finishing fiberglass...

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I think fiberglass is probably your best bet. I've pulled down a lot of antennas with various insulating materials, nylon just crumbled in the weather, filled ABS held up better, but the ones with fiberglass held up for 15-20 years. Surface got fuzzy, but mechanically was sound. The aluminum tubing failed before it did. Steatite worked very well, was like new when taken down, but wouldn't handle much mechanical load. PVC would have been my last choice, the stuff deteriorates in sunlight so much that pieces of tubing will just crumble in the hands after several years of exposure to UV. Wouldn't even use it for tomato stakes.
Stan

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I think fiberglass is probably your best bet. I've pulled down a lot of antennas with various insulating materials, nylon just crumbled in the weather, filled ABS held up better, but the ones with fiberglass held up for 15-20 years. Surface got fuzzy, but mechanically was sound. The aluminum tubing failed before it did. Steatite worked very well, was like new when taken down, but wouldn't handle much mechanical load. PVC would have been my last choice, the stuff deteriorates in sunlight so much that pieces of tubing will just crumble in the hands after several years of exposure to UV. Wouldn't even use it for tomato stakes.
Stan
=====================================================Just looking at the thermal expansion problem, most plastics aren't very good in that department. But glass-reinforced plastics can be:
http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html
-- Ed Huntress

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I suspect S glass has a lower expansion than E glass. I did not find any good info in a quick search on google. But did find this.
ddfiberglass.com/new-high-performance-fiberglass.html -
Dan

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Dunno, Dan. The information should be available somewhere -- probably from one of the larger composite materials suppliers.
Who are the big manufacturers of S glass these days?
-- Ed Huntress

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Why not use wood dowels. If they have to be RF invisible, wood dowels work. All the FCC testing houses that I worked with had wooden benches to put the equipment on.

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The OP says the device is used outdoors.
-- Ed Huntress

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i was gonna suggest putting wood dowels inside the pvc but figured i'd be hounded off the list as an idiot. thought probably the wood would distort as much as the pvc or the wood wouldn't be able to resist the distortion of the pvc. trying to think of some very stiff RF transparent material to put inside the pvc, can't.
b.w.

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As several folks have mentioned, the PVC itself is a lousy material for full-time outdoor exposure. And if you're going to stuff something inside of it, the chances are that any cost saved by using PVC in the first place is going to go out the window. Wood's big problem is distortion due to humidity changes.
I don't know what to suggest, but the fiberglass-reinforced plastics that do well outdoors (like fiberglass-reinforced polyester used in boats) are a place to start.
-- Ed Huntress

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Even laying up a fiberglass tube by hand isn't that impossible a job. All sorts of cardboard tubing out there for forms. Doing it in a production environment probably won't be cheap, though. I've seen fiberglass tubing in various plastics suppliers catalogs, either spiral laid or straight-rolled. Might not have the wide variety of sizes you get with plumbing PVC. It's fairly widely used for telescope tube construction in the larger sizes. It's out there, a supplier just needs to be found. Thomas Directory time?
Stan

On Wed, 05 Aug 2009 01:12:41 -0500, William Wixon wrote:
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How about using transparent plexiglas or polycarbonate tubes? Being transparent, they should heat fairly uniformly, and less than gray or black PVC. And PVC isn't really noted for its rigidity in the first place.
Cheers! Rich

Ed Huntress wrote:
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The gold standard of outdoor test ranges is made of wood.
www.epg.army.mil/Pages/Facilities/ATF.htm
Kevin Gallimore