PSAS/ news/ 2003-07-31 - Tests of Tap Plastic's X-30 expanding polyurethane (EPU) foam

Who: Andrew

We'd like to use EPU as the dielectric for our cylindrical patch antennas. We think since it's pretty much air it'll have a low dielectric constant - say 1.01 - 1.2 or something like that. So we did some tests to see how it behaves.

It's a 1:1 mix, and in my basement (~75 degrees on a hot sunny day) here's what happened:

Time State
0 - 10 s Thick but very pourable amber liquid - think hot maple syrup. On contact, the two chemicals begin to produce microbubbles which turns it a yellowy tan.
10 - 20 s The mix changes from a liquid to a still-pourable foam. It's starting to thicken up.
20 - 30 s Now the mix is really foammy, and getting harder to pour
30 - 60 s The foam turns into a sticky mess - it can slowly drip, but even that stops after a while. If you cut it with a knife, the edges peel back like the "7-up jello mold" my grandmother used to make us... if we were good.
60 - 600 s The foam moves from squishy and cutable to soft but firm. Then it really firms up, and for a while acts like that closed-cell polyurethane packaging foam - stiff but bendy.
> 1 hr It gets progressively stiffer until about 2-3 hours after you start, it's a very brittle, stiff foam.

First we just mixed some stuff up. It stuck to glass, ABS and styrene plastic.. it's nasty stuff to take off, too. Acetone doesn't touch it.

Second we made a polyethylene mold out of a 6 x 6 x 1/8 in piece of polyethylene with three 6 x 0.5 x 1/8 in strips. This worked very well; it stuck to the polyethylene but not badly so - a little bit of bending the mold and peeling took it out just fine. So we got our first piece of 5 x 5 x 1/8 in EPU successfully. Fun. We tested one of the pieces for density and found it to be 31.7 mg/cm^3, which is 1.97 lb/ft^3: very, very close to the 2 lb/ft^3 specification.

Just for fun, we decided to try making a cylindrical mold which was roughly the right size for LV2 (5.25 in. OD airframe). We used the only available material, 1/32 in. thick clear polycarbonate (Lexan), and cut it into two 6 in. wide sheets, one 16.75 in. long and one ~ 16.31 in long. We rolled them up, taped the edges together with Kapton tape (what else would look as cool, really?) and then slipped them inside each other. We inserted a strip of 1/16 in. thick protoboard to seperate the cylinders and then clamped the interface with the polyethylene sticks and some C clamps. It was a bit of a challenge to pour the foamy liquid into the 1/16 in. spacing, so we just used a stick to open up the gap near where we were pouring the foam. It actually worked quite well, considering - if we had had a funnel with a <= 1/8 in. tube I think it would have been very easy (and not so messy!).

Note the closeup of the bubbles - this stuff is very low density foam. After about 15 hours, we unclamped it and cut the Kapton tape. With a bit of coaxing, it opened along the split and stood about 1" open. That's pretty impressive given that the polycarbonate sheets had quite a bit of springyness to them... the foam did a nice job of keepin it in a cylindrical shape.

I played with it for quite a while, flexing it open and closed, torquing it this way and that. It was pretty hardy, but the occasional "crack" noise meant that stuff was breaking.

I then really flexed it open, and got the first visible failure. The foam broke right down the center between the two sheets of polycarbonate - i.e. delaminated. Not surprisingly, it separated at the only section that didn't have a thick layer of foam on top. If we could seal the edges with a stiff but rubber glue, like a silicone rubber, I think it would have done better - still cracked, though.

Finally I peel tested the edge. It had a good peel strength (sort of surprising, actually). The EPU broke down the center of the thickness and came right off the polycarbonate in an equal amount.

Conclusion: It just might work. It's a little more brittle than we want, but if we don't over-flex it we'll be fine. it would be nice to have something more plyable, like a polyethylene or polypropylene, but this stuff will do. Note that we could actually save ourselves quite a bit of hassle by making a a straight, thick-walled polyethylene mold, and then removing the EPU from the mold after about 20 minutes, before it's fully cured. It would easily wrap around a 5" cylinder at that point, so it would save us the headache of making a cylindrical mold.

Update 2003/11/10: EPU didn't work since it expands too much after it dries. We've since moved to 1/16 inch double-sticky polyethylene foam tape.