polypropylene for "tray-ponics"-type "rafts"

[Paul Trudeau]

Glenn Martinez has described a nice small raft system where he uses 24 inch x 36 inch x 8 inch-deep plastic masonry mixing tubs (trays) (available at Home Depot) as plant troughs.  Instead of floating foam rafts, he lays a 24 inch x 36 inch piece of plywood on the top edge of the tray, an inch or two above the water level.  I built my system last year on this model.  As he did, I stained the plywood with some white stain.  It's worked OK for me, but I've had a few problems with the plywood:

(1) fairly labor-intensive to drill and stain

(2) warping

(3) not easy to clean plant debris, mold, or algae off the stained wood

The morning after the US election in November, I collected as many plastic campaign lawn signs as I could find and have been experimenting with using them instead of plywood.  They are made of 4 mm corrugated (aka "fluted") polypropylene (same material used in a number of food packaging items yogurt cups).  Some of these signs are exactly 1/2 the size of one of those masonry trays, so two of the signs cover one tray perfectly.  Each half is supported on 3 of its 4 edges by the rim of the tray.  The 4th edge needs a little support in the middle, so I use a 7 inch scrap of PVC pipe standing on end in the tray.  So far it's working out great.  These re-purposed signs do of course have printing on them, and I'm finding that the printing can scrape off.  So I'm planning to buy sheets of new (unpainted) corrugated polypro and cut my "rafts" out of that.  Good news is that I just found out my local independent plastics store sells this stuff for about US$12 per 4 foot x 8 foot sheet--that's less than 1/2 the price of a sheet of plywood.  No staining is required, there's no warping, and the material wipes clean very easily.  Also lots easier to drill with a hole saw, and the material is easily cut with a utility knife.  Lots lighter than plywood too, so it's a cinch to handle.

Any thoughts on all this, anyone?

[Paul Trudeau]

thanks for the tip, Vlad, will keep my eye out for some E5200.

Vlad Jovanovic said:

Paul, you might want to try some sort of polyurethane combo adhesive/sealant like 3M's E5200 or E4200. They now make a version (E5200) that cures in 24 hours instead of the standard stuff (like I have) that takes 7 days to fully cure. It has been my experience that regular old silicone "doesn't stick for shit". Though I'm sure that there may be some specialty types that adhere better.

The E5200 marine sealant/adhesive has worked real well for me so far... on a variety of materials, including ones that usually don't bond well (HDPE, LDPE etc) and normally have to be thermally bonded.

[Vlad Jovanovic]

No problem Paul. I think many aquapons stateside have found some at one of the "big-box stores" ...

[Alan McKnight]

Paul, I just I/M a buddy of min who has been a chemist for 40 years and has his own business in pharmaceutics. I asked him about the silicon and his  reply was "

1. silicon will seal it but not against pressure. A polyurethane sealer may a bit.  BUT he reccomends

    a plastic welding. They make plastic welding kits. hot air blow torches
   

   1. 1. 1. But you can try it with a regular torch. Just evenly heat the two pieces and clamp together until cool. The plastic has to melt together. Poly pro melts at about 220 degrees

[Pat James]

I have been scavenging a similar sheet from my operating room packs. Mine are only 14" square though. I'd thought of linking them with loops of plastic ties and using a bottom support of maybe even wire. They would be strong enough for even a heavy plant, at least for a while. I also collect the 14 inch square tubs the materials come in. Cheap plastic that gets brittle after a time but very useful. My scavenging is an amusement for my compadres in the OR..

[Vlad Jovanovic]

Yup, silicone makes a good sealant...but a crappy adhesive. Iv'e somewhat successfully created "welds" with polyethylene and polypropylene with a torch and a metal roller. It does work, just be careful...there's a "sweet spot" where it's hot enough to press together, but not getting fried...so play around a bit if you go that route.

220 degrees...Thanks Alan, that's good to know. 

Alan McKnight said:

Paul, I just I/M a buddy of min who has been a chemist for 40 years and has his own business in pharmaceutics. I asked him about the silicon and his  reply was "

1. silicon will seal it but not against pressure. A polyurethane sealer may a bit.  BUT he reccomends

    a plastic welding. They make plastic welding kits. hot air blow torches
   

   1. 1. 1. But you can try it with a regular torch. Just evenly heat the two pieces and clamp together until cool. The plastic has to melt together. Poly pro melts at about 220 degrees

[Alan McKnight]

Heat sealer 

http://www.abbeon.com/store/item.cfm?code=715&catid=211

[Paul Trudeau]

Pat, Alan, Vlad and everyone else:  what a wealth of knowledge you are!  Thanks.

PT

[Jon Parr]

Paul, I can't see troubling with sealing the open ends of the corrugation, unless you are also going to seal all the openings in all the holes drilled as well. That chore alone strikes the whole deal to me. Perhaps heat-welding would make sealing the round holes more practical. But at $12 per sheet, plus trimming and drilling and sealing, I kind of think you may be better off starting with the EPS lettuce rafts like I use. I'll sell you some for $8 each (2' x 4', 18 holes on 8" CL) on Saturday when we meet up, if that sounds decent to you.

And if you're interested in 3M 5200, they stock it at West Marine, and it's pricey.

[jim mckee]

I saw a Murray Hallam vid.  He is touring a greenhouse (may have been his?)  and about mid/vid he shows a grow table full of prime bouquet leaf lettuce.  It looks to me like they are using a shallow tub for the main box, then laying a sheet of white plastic over top of the bed .   The plants are in plastic pots no medium just inserted through a hole in the plastic cover.  My guess is it's flood and drain with the plastic held up off the bottom of the grow bed a wee bit.  When Murry removes the lettuce you can see the plastic sheet move like a water bed moves.

jim

[Pat James]

Why bother to even attempt to seal the channels in that polypropylene? They have to be suspended anyway, either by bottom supports or overlapping the sides. They don't have enough air space inside to hold much more than their own weight as far as floating.

I am planning a modified flood and drain/raft system myself at least in some of my growbeds.

I'm thinking I will keep some small minnows under the rafts in the beds that I keep full and maybe some Louisiana crawdads in the ones that I flood and drain. Those will have a small layer of media across the bottom as well to increase surface area and to catch material.

[Paul Trudeau]

Hi Pat,

I'm sealing the edges so moisture does not accumulate in the currugations and promote growth of mold or algae.

Pat James said:

Why bother to even attempt to seal the channels in that polypropylene? They have to be suspended anyway, either by bottom supports or overlapping the sides. They don't have enough air space inside to hold much more than their own weight as far as floating.

I am planning a modified flood and drain/raft system myself at least in some of my growbeds.

I'm thinking I will keep some small minnows under the rafts in the beds that I keep full and maybe some Louisiana crawdads in the ones that I flood and drain. Those will have a small layer of media across the bottom as well to increase surface area and to catch material.

[RW]

Thanks for starting this discussion Paul.  I had the same thoughts and was happy to find someone else doing the same thing with the polypro sheets.  I'm just starting up a raft and am going to give it a try.  I also got some HDPE sheets from a local to me plastics supplier http://www.regal-plastics.com/hdpe-high-density-polyethylene-sheet....

The solid HDPE sheet avoids the issue with the open ends of corrugation and should last a long time.  My concern is the UV stability of the HDPE and polypro plastic. My raft is outside in direct sunlight.  Also, is it translucent enough that algae will start to grow in the raft.  Any thoughts?