Testing XPS foam for epoxy adhesion/bonding

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,867
Location
US
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
 
Last edited:

Hephaestus

Well-Known Member
Joined
Jun 25, 2014
Messages
2,129
Location
YMM
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
Joined
Dec 11, 2015
Messages
864
Location
Tehachapi, CA
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.

My $0.02
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,867
Location
US
Marc, thanks. But don't sugar coat it!
Well, when using the Bouyancy billet foam sold by Wicks and Aircraft Spruce (and a zillion other vendors),
I think that solved my problem right there. Aircraft Spruce will sell me a little bit of the buoyancy billet. I'll get some now to play with. If I need a lot more later, it's a good excuse to take the trailer to Cedar Point and ride the roller coasters.

I appreciate the information and tips.

Mark
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,780
Location
Saline Michigan
I contacted Dow-du Pont directly, they found me a wholesaler who does regular business with local lumberyard (and the usual homebuilder suppliers) and would supply one or two billets. Worked for me. Thinking on it, I remember the wholesaler might have been Lumbermen's.

Listen to Marc above. All my peel tests with billet foam fail rather energetically in the foam. And our airplane structures are designed to avoid peel loading. For massive foam, that has a great field history except when folks have skipped lamina, built with wrong fiber orientation, or built really dry layups. When someone leaves out crucial structure like that, bets are off.

For hollow wing, I only know of Mike Arnold's AR5 using styrene. I still do not get his arrangement - I seriously doubt he saved any weight over massive core.

Billski
 
Last edited:

daveklingler

Well-Known Member
Joined
Jan 22, 2013
Messages
128
Location
Albuquerque
1) Does this sound like a worthwhile approach to gaining useful information?
I think any information at all is useful, and whatever numbers you come up with would be a great contribution.

Dow/Dupont blue Styrofoam XPS insulation? Corning pink Foamular XPS sheets?
Blue and pink XPS are extensively used by surfboard builders, and they have a few techniques for getting good results, the most important one being to sand before bonding anything to them. Evidently one or both sides is often coated with something.
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
4,947
Location
NJ
the experience of the fabrication (playing with it) will surly help with future projects.....and engineers love tests to destruction on test panels the real fun is to calculate the expected load failure point and see if it matches within 10%
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,867
Location
US
What are your requirements?
From other answers in this thread, it appears the ability of epoxy to bond to this foam is not as good as it is to XPS foams with larger cells (rougher texture).
But similar XPS foams have been used, unfaced, in the wing ribs of some other designs (e.g. Flying Squirrel, SD-1). It all comes down to what the foam is being asked to do.
 
Last edited:

Protech Racing

Well-Known Member
Joined
Jul 10, 2020
Messages
404
Copy, Right. my glue test showed epoxy as a fail. ( It passed with epoxy run in the center and GG around the perimeter. )
I use fibreglass drywall tape on the rib edge , glued on with gorrila glue and stippled with little tiny holes to increase surface area.
The coverings get glued on. to the tape.
Thankyou.
 

JohnBouyea

Member
Joined
Nov 10, 2019
Messages
21
Location
Hillsboro, Oregon
...
But similar XPS foams have been used, unfaced, in the wing ribs of some other designs (e.g. Flying Squirrel, SD-1). It all comes down to what the foam is being asked to do.
...
The Flying Squirrel is a good reference point. Though only one had been flown so far, a couple of us (at least) are making progress to get our projects into the air. XPS foam is used extensively throughout. Ensuring no facing film remains on the surface is a requirement to successful epoxy bonding and for glass skin layups.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,867
Location
US
The Flying Squirrel is a good reference point. Though only one had been flown so far, a couple of us (at least) are making progress to get our projects into the air. XPS foam is used extensively throughout. Ensuring no facing film remains on the surface is a requirement to successful epoxy bonding and for glass skin layups.
John,
Yes, I wish the Flying Squirrel design had a longer service history and/or that Marvin Barnard had left a bit more documentation about his design decisions. You guys that have taken up the M-19 torch have your work cut out for you. The plane looks like it might be a lot of fun.
Is there a particular procedure used to prepare the XPS sheet inter-rib wing skin panels for epoxy/fiberglass bonding? Do you know of other designs that use 'small cell" XPS sheet in this way?

Thanks,
Mark
 

JohnBouyea

Member
Joined
Nov 10, 2019
Messages
21
Location
Hillsboro, Oregon
John,
Yes, I wish the Flying Squirrel design had a longer service history and/or that Marvin Barnard had left a bit more documentation about his design decisions. You guys that have taken up the M-19 torch have your work cut out for you. The plane looks like it might be a lot of fun.
Is there a particular procedure used to prepare the XPS sheet inter-rib wing skin panels for epoxy/fiberglass bonding? Do you know of other designs that use 'small cell" XPS sheet in this way?

Thanks,
Mark
Quoting the manual on the section for practice;
"If you bought plastic clad, it’s not easy but this plastic MUST! be peeled off otherwise your fiberglass will not adhere. Using 60 grit sandpaper, lightly sand the surfaces in a circular motion, fully round the corners and edges. Brush the dust away with a hand broom.
"Using a 1" brush dipped in the MGS Epoxy resin, coat one side of the Styrofoam and position on the cloth. Coat the foam edges and stretch the cloth over lapping 1 inch, then epoxy the top and cover with the top flap. Ideally you want the fibers straight, taught and fully wetted, stuck to the-board with the minimum amount of resin necessary."
I am not familiar with other types using XPS and my other experience is with the KR-series of homebuilts.
Cheers.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,867
Location
US
Quoting the manual on the section for practice...
Thanks very much. As you progress with the your Flying Squirrel project, any observations on adhesion of epoxy to "regular" XPS sheet would be of interest. There are also apparently some parts of the world where the DuPont large cell XPS buoyancy billets are just not available.
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
927
Location
Lt
Hello for all.

My link which was mentioned, come from surfboard builders.

I think it was in this topic : XPS/Glassing/delamination + some videos..

On my background - do cut eps/xps with cnc hot wire for ~ 10 years. For some years it was cutted as bussiness for a load-bearing cases for some hvac units - good insulation for heat and noise. Started from eps, gradually come to xps.. And by building code, for epx/xps wall insulation, you must use mechanical fixture too.

I do test some samples available on our part of globe - all were with much smaller cells than dow billets - you see only mechanical bonds, and damaged cells do improve bonding. Sanding or scratching worked, but was too unstable to consider it as viable solution. And my gut thinking - it was damaged layer which come-off with glass. Similar peeling force, but different view..

For me it would be go solution only if i can hotwire needed shapes, and cover it without any additional preparation.

Btw - gorilla glue was much much better on this. For plywood cowered wing - it is a go solution

And my suggestion - aviation is so horrible time consumer, that trying to use improper materials is just too expensive on time. And could lead to bad habit.
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
927
Location
Lt
There are also apparently some parts of the world where the DuPont large cell XPS buoyancy billets are just not available.
It is. But too expensive to be viable.. You may buy it and i will get a shipping quota + taxes of 5 or 10 times xps price. Shipping a car in a container is ~ 1000 usdAnd you need more than that for one airplane.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,867
Location
US
And this video :
Thanks, that's the video I couldn't find. It would have been ideal to see how well the fiberglass strips stayed stuck to the large cell Dow buoyancy billet, too, and compare it to the other foams he used.

I gotta wonder if/why only this one DuPont Styrofoam product has these large cells.

My link which was mentioned, come from surfboard builders.
I think it was in this topic : XPS/Glassing/delamination + some videos..
In that discussion, some of the surfboard builders were saying that they have problems with epoxy/fiberglass delaminating from XPS foam (I'm assuming they are using the "small cell" XPS that is most commonly available in hardware stores).

The guy who claims to be having great results and no delaminations says this (post 15):
Extruded polystyrene is stronger (pound for pound) than polyurethane, or expanded polystyrene (most surfboard polyurethane is about 3lb/cu.ft.). If you build with this foam using conventional methods, you are much more likely to have delamination problems.
Please allow me to insist you do the following:

  1. Use an epoxy glue joint/s (in place of a stringer/s) - one is o.k., two is better.
  2. Finish the blank by sanding lightly with #20, leaving hundreds scratches and small pits.
  3. Add about 5% more resin to the laminate after dragging the excess resin out of the cloth.
The extra resin will fill in all the pits and scratches left on the rough blank. I realize it is hard to scratch and tear the foam of a blank you worked so hard to perfect, but do not skip this step. The pits and scratches greatly increase the bond, and with the glue joint/s, will minimize the delamination problem to a level equal to other foams.
20 grit sandpaper is very coarse. The average size of the particles on 20 grit sandpaper is 905 micrometers (aka "microns" = .0353") in diameter. In most reports I've seen of people sanding the XPS in preparation for epoxy (including the video at the top), 80 grit was used. 80 grit particles average 192 micrometers (.00749") in diameter. If we assume the particles are leaving channels in the foam based on the sandpaper particle diameter squared, then the average abrasive particle on the 20 grit sandpaper is leaving a channel 22 times as voluminous as the average particle on the 80 grit sandpaper. So, a lot more epoxy going into the foam. Now, presuably there would be a lot >more< of the 80 grit "channels", but when woodworkers want to remove a lot of stock they use very big grit abrassives, and there's a reason for that. And, deeper channels=a deeper foam layer that would be affected/need to be ripped off.

I don't know how any of this compares to the surface of the preferred DuPont Buoyancy Billet foam with its large cells. My >>guess<< is that the open bubbles of the large cell foam would be hard to beat, since the individual pockets would have a lot of surface area compared to gouged channels. Also, the shape of many of the little pockets would be 3D "dovetails" rather than a simple "V" shaped 2D channel that would have less pullout strength.

We know the DuPont Buoyancy Billet works well in service--short term and long term.

I don't know:
1) How XPS foam prepared with very aggressive, deep sanding/cuts would compare to the bond provided by the large cells of the buoyancy billet.
2) How much peel/shear strength is required in the laminate/foam bond in our normal use. Stronger is better, but I suppose it is possible the buoyancy foam bond is much stronger than it needs to be.

And my suggestion - aviation is so horrible time consumer, that trying to use improper materials is just too expensive on time. And could lead to bad habit.
I hear you and understand. On the other hand, I'd think there may be people somewhere in the world who wouldn't mind spending two hours (or maybe a lot more) to sand/prepare a wing foam blank if it saved them $1000 in shipping costs and another $1000 in taxes and allowed them to avoid a 3 week shipping delay. But, they'd want to know for sure that the epoxy/fiberglass would adhere well enough to allow safe parts to be built.

Thanks again!
 

Protech Racing

Well-Known Member
Joined
Jul 10, 2020
Messages
404
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.
 
Top