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I tried two adhesives which I use for boatbuilding.
I cut 6" x 6" squares from a coroplast sign and glued the squares together with about a one inch overlap. I applied the glue and then drove 2 deck screws through the joint to clamp it together. Then left it without touching it overnight.
1. Two part epoxy, similar to WEST System. Cured 24 hours. I was able to pull the pieces apart, but it took quite a bit of effort. The plastic bent much easier than the force needed to pull the two parts apart. Where there was paint from the sign graphics, the epoxy pulled the paint off of the coroplast. The pieces would probably hold together in most high speed wind conditions. Using a fastener like a bolt or rivet on the ends would likely help keep the pieces together in high stress conditions. I did not wipe the joint with acetone first, that might help improve adhesion. I did not scuff the joint with sandpaper, that might help too.
2. PL Concrete and Masonry Sealer. This is a polyurethane caulk which is used by boatbuilders as a glue because it is relatively cheap and works as well as other polyurethane glues like 3M 5200. After 24 hours, the joint had not cured completely. I believe that this is because polyurethane needs moisture in the air to cure, or maybe it just needs air. Perhaps misting the joint with water would help cure time. Perhaps using only a small amount of adhesive in a thin layer would work best. I put the two parts back together and let it cure an additional 48 hours. It held together quite tightly and took a lot of effort to pull apart, similar to the epoxy. The center of the joint was not cured completely. This adhesive would also work although you would probably need to allow it to cure for a week or more. I also tried an end to end joint, applying the glue to the end of one piece and pushing another piece against it in a butt joint. With no over lap and just the 4mm width of the two pieces touching, it was actually a decent join, not too difficult to tear apart but surprisingly tough. I tested it after 36 hours of cure time.
I tried a pop rivet on the coroplast, and it would work if you used a washer or some hard material on either end of the rivet. The rivet would pull through when trying to rivet two layers of coroplast with no backing. I don't like pop rivets much because they have an ugly side to them on the inside, but it would be one way to fasten a joint. Another type of fastener that would work with coroplast is to use zip ties, but they are quite flexible and probably would not work well together with adhesive. Deck screws with large threads worked well but then you have the points on the inside to deal with. Maybe light nuts and bolts with plastic washers might be the ideal fastener to use together with adhesive.
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