Question about quality and cost of plastic

Hello, I'm designing a simple rainwater harvesting device, and it involves a lot of plastic. So I have some questions about the different qualities of
plastic on the market. I'm a novice here.
Let me quickly describe the environment in which the plastic should survive.
The rainwater harvesting platform will be permanently located in a mid- ocean environment, somewhere near the equator. It's basically a few square kilometres of plastic film floating on the sea-surface, with tubes sucking in the rainwater into plastic baggies. The platform has to withstand UV- radiation, the salty environment of the water, and must be moderately strong (it must capture rain, and fill itself up with a few centimetres of this fresh water, so it will basically float above sealevel since inflatable tubes push it upward when they suck the water into the baggies, exposing the upper layer of the platform to wind, which can be quite heavy at times). The platform should withstand moderate storms (since it merely floats on the waves, this should be no problem).
So knowing all this, which kind of plastic would be most appropriate, you think?
I've found several commercial polymers (like Hypalon and Hypalon based Neoprene polymers). But they're all very expensive.
The plastic (film) should cost less than US$ 10 per square metre.
I'd be glad to hear your advice!
[Excuse me for my bad English, I'm not a native speaker. I'm french, I hope my American friends don't care! :-) ]
Yours, Lou.
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Have you ever thought about Polyethylene? If colored black it is UV resistant and withstands the salt water. Depending on the thickness of your film a square meter will cost between 2 and 3 US$.
I would be worried about waves going over the edge of the film pouring salt water onto the floating platform. Furthermore, strong winds could lift up the whole platform.
Mark

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Lou wrote:

Your film is going to be shredded and rolled up by the first storm that comes along. Anything floating on the surface shares the mass motion of the surface water, so large breakers will destroy it immediately.
I don't think it will last even that long, assuming you can deploy it at all. The stretching involved with just ordinary wave motion will be well beyond the elastic limit of your polymer--ordinary waves can easily have heights that are a significant fraction of their wavelengths.
Next time you're at the beach, try stretching a dry cleaner's bag on the water surface near the shore, and see what happens.
Cheers,
Phil Hobbs
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My first thought was polypropylene or polyethylene, but you would have to stabilize them against UV degredation and they are both quite stiff... Then there's polyether polyurethane, which is not cheap - but for a price/area raito you need to establish first the thickness, which would depend on your physical and mechanical design What stresses, tensile and tearing would this have to endure ? it's possible to use a coated scrim for added strength, if needed. Polyurethane is used in potable water storage tanks.
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