# Testing XPS foam for epoxy adhesion/bonding

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#### Vigilant1

##### Well-Known Member
I don't live near a large body of water, and I'm finding out that means that Dow Buoyancy Billet foam isn't readily available near me (unless I'm willing to buy a LOT of it and pay for that partial truckload to come to my house). I do know a guy who is building a Long-EZE, I may ask him for a few scraps of whatever he's using. If I were building plane with it now, I'd probably just bite the bullet (or use it as an excuse to take a road trip to the Great Lakes), but I just want to get some experience with laminating to foam and also do some impact tests.
I can get high-strength Dupont (was Dow) blue XPS foam--60, 80, and 100 PSI compressive rating (and that's for 5% compression, not the oft-cited 10% criteria). I've heard that some foams just don't bond well to epoxy, whether due to additives (silicone?) or a very fine cell structure--maybe too fine for the epoxy to get a grip on?

Stansilavz posted a link recently (I can't find it) with an informal test a guy did--prepped some EPS and XPS in various ways (sanding, no sanding), then did an open layup (fiberglass?). After curing he peeled them off and used a fish scale to measure peel strength, together with an examination of whether the bond failed or the foam failed.
I'm thinking of doing much the same with various brands of XPS foam available to me. I'll hot wire all of them to get rid of any facings, test some with no prep, some with a few passes of 80 grit sandpaper, and also test the effects of wiping with isopropyl alcohol. Then layup/vacuum bag some strips of fiberglass to each of them, let cure thoroughly, then try to peel them up. The fish scale didn't seem to give a very precise result for the guy in the video, maybe I'll hang weights and see when the strips zip off. I suspect the type of failure (foam or epoxy-to-foam, and the consistency of that failure) will be at least as telling as any measurements.

So--
1) Does this sound like a worthwhile approach to gaining useful information?
2) Is peel strength a good stand-in for shear adhesion strength?
3) Any opinions on whether a good bond today is likely to remain a good bond for decades? We know that the right foam stays stuck to epoxy--is it likely that any silicone, flourocarbons, etc in the foam might only affect the bond a long time down the road?
4) What does "right" look like? Are there any published tests of peel strength of fiberglass/epoxy to Dow Buoyancy billet XPS? Rutan was good about offering "confidence tests" to builders of his planes as far as weighing and testing coupons of layups. Are there any field tests of adhesion to core foams?
5) Are there other types of foam that are known to bond well to epoxy? Dow/Dupont blue Styrofoam XPS insulation? Corning pink Foamular XPS sheets? Kingspan Greenguard XPS sheets?

Thanks for any assistance, criticism, lewd links, etc.

Mark

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#### Hephaestus

##### Well-Known Member
PL300 has been my go-to - but that's for layout / testing not man carrying. And I needed to run glue tests so - please continue

I'm pretty sure high lorimer had some strong opinions on glue. His aircraft were mostly XPS...

#### Marc Zeitlin

##### Exalted Grand Poobah
1) Does this sound like a worthwhile approach to gaining useful information?
Not to me, particularly
2) Is peel strength a good stand-in for shear strength?
Don't know, but I doubt it.
3) Any opinions on whether a good bond today is likely to remain a good bond for decades? We know that the right foam stays stuck to epoxy--is it likely that any silicone, flourocarbons, etc in the foam might only affect the bond a long time down the road?
Well, when using the Bouyancy billet foam sold by Wicks and Aircraft Spruce (and a zillion other vendors), we have over 4000 instances of airplanes that have no major issues with bond aging. Some that are over 40 years old, again with no issues. The only time I've seen disbonds between the foam and the glass layers is when prep work (micro application) wasn't great, and/or the layup was too dry. While Rutan derivative aircraft generally are not vacuum bagged, I've never seen disbonds on a bagged part over foam. Doesn't mean it doesn't happen, but it seems to be pretty rare if it does.
4) What does "right" look like? Are there any published tests of peel strength of fiberglass/epoxy to Dow Buoyancy billet XPS? Rutan was good about offering "confidence tests" to builders of his planes as far as weighing and testing coupons of layups. Are there any field tests of adhesion to core foams?
If the micro and glass are applied correctly, when you peel it off, the FOAM fails, not the bond line. This indicates that if done right, the bond line is substantially stronger than the foam. Which is as good as it gets - stronger won't get you anything else. And if a peel test breaks the foam, you're not going to fail the bond line in shear.
5) Are there other types of foam that are known to bond well to epoxy? Dow/Dupont blue Styrofoam XPS insulation? Corning pink Foamular XPS sheets? Kingspan Greenguard XPS sheets?
All three of those are crap, and I don't allow them near any airplane. This:

is what works. This:

Is basically what those three you pointed to are, and the properties are not appropriate. It's softer, with tiny cells that are difficult to get micro to fill, and I wouldn't go near them.

Thanks again!

#### stanislavz

##### Well-Known Member
HBA Supporter
But, they'd want to know for sure that the epoxy/fiberglass would adhere well enough to allow safe parts to be built
Exactly. Now, in next two days and two decades

And it was End of the story for me.

#### Protech Racing

##### Well-Known Member
Glad to see that I'm not the only one use Gorrilla glue .
Anyway, the best surface prep would be a mini rake that cuts square edged channels into the foam . Putting the epoxy joint a little more into shear , rather than tension.