What kind of plastic?

[Mark Ries]

I just got done cutting my synmat UL 7 in half and sealed the top piece back together so I now have a 36"x20" synmat. I will also seal the bottom half and I bought a thermarest repair valve to install to give me another mat for my grand daughter. I would like to make a plastic block like thermarest uses in the corner of their mats that the valve glues into. What kind of plastic would you use? Something that the valve would glue to and that would bond to the mat. What kind of adheasive would you use. I have seen in other threads that some just bond the round part of the valve into the mat but I would much rather do it like thermarest does as i think it will stay in better. Hopefully someone has some experience with this. Also I would to bond some straps or velcro to the head of the mat to hold a stuff sack pillow right where i want it and also at the leg end of the mat to hold the closed cell pad in place or my pack depending what pack I will be using. What would you adheare the straps to the mat with? Ive got some ideas but would rather see if anyone else has had success with the repairs/mods I want to do. Thanks in advance.

[Roger Caffin]

PE and PP are absolute failures for bonding.
Silicone rubber is also most unlikely.
Solid PU could be cast (eg Shoe Goo) in a PP mold, with the valve embedded. Then bond that to the fabric with more PU. No guarrantees though.

Cheers

[David Gardner]

Polyethylene and Polypropylene can be bonded with special two-step epoxies. In addition to mixing the two epoxy components you have to lightly torch the PE/PP first to open up the surface molecules. After torching, the plastics are bondable for about an hour. TAP Plastic stores here in the SF Bay Area sell "Plastic Weld" epoxy, which I have used successfully. Kinda pricey ($15 for an ounce, which is plenty for your purposes).

For the straps and velcro Shoe Goo might work because it stays flexible.

[Colin Krusor]

I have to disagree about the recommendation to try PP or PE with flame treatment and specialized epoxy. Flame treatment introduces oxide and hydroxyl groups into the surface which can interact with the epoxy, but the bond strength will never approach that achievable with high surface energy plastics. The bond strenghs achieved with flame or corona treated PP and PE are an order of magnitude lower than high surface energy plastics like nylon or polyurethane.

Here is a good surface energy table for common plastics:

http://blog.tstar.com/bid/33845/Surface-Energy-of-Plastics

Higher surface energy results in higher bond strength, in general. Nylon, polycarbonate, epoxy, ABS, acrylic, and polyurethane are at the high end (>40 dynes/cm).

I agree with Roger's suggestion to consider polyurethane. Some polyurethane resins foam when they cure (Gorilla Glue is one of these), which means you could produce a relatively light foam block strongly bonded around the valve, cut it to the desired shape, and glue it into the pad with polyurethane glue. The heat sealable inner surface of your synmat is polyurethane, and the glue that Thermarest sells for replacing their valves is polyurethane. The Hysol glue that many people use for bonding cuben is also polyurethane.

[Mark Ries]

Hey thanks for all this info. The first mats are for me but if this works I am going to cut down some old t-a-r full length for some kids/ scouts that i know are in need. These combined with a ridge rest or blue pad when colder should be comfy and cheap and fairly light. So should I be able to cast or mold shoe goo or gorilla glue around the valve then cut file sand to perfection then glue in place with the same?

[David Gardner]

I didn't mean to recommend using PE/PP, just saying it can be done.

[Colin Krusor]

Mark, you could try making a mold. But if it were my project, I wouldn't bother with a mold having the desired final shape. I would just put a mass of polyurethane around the valve, protect the opening from occlusion somehow, and cut it to the desired shape once cured. A polyurethane resin that foams will give you a lighter finished part. I think you could glue it into the mat with the same stuff that you use to make the part. If you glue in the part with a foaming polyurethane, keep in mind that the expansion of the glue might cause the part to move a bit during curing.

[Sam Farrington]

Mark,
I have had good luck making larger saddles for guitar bridges out of epoxy. These were about 3" long, 1/4" wide and 3/8" high with a flat bottom that just rested on a filled and flattened top of the brige and stayed in place from pressure of the strings and friction. Once the epoxy set, I found it very easy to file and sand to the exact shape desired.

There are epoxy putties also, but many of them dry to a rock hard consistency. It might hold the bond better if a little flexible. You would have to look at the expoxy products in your area and make choices.

For bonding the block between the other plastic, I agree with the suggestions to use polyurethane, and make shims so you can clamp the materials to the block while the PU sets.

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