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Testing XPS foam for epoxy adhesion/bonding

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Vigilant1

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Remember in the monolithic foam thread where I mentioned varying skin thickness? If you layup root/tip/Dtube in a heavier, then finish with a single ply of <100gsm over top pf all. 80gsm/10mm XPS backing is enough to require not-accidental force to push through a surface.
The 80 gsm/10mm XPS skin will deflect a lot compared to 2 sided composite sandwich skins unless the ribs underneath are really close together.
And, from all I've heard, 80mm over soft XPS will get a lot of dings compared to more substantial layups over stiffer foam.

Noted.

Part of the problem, I think, is some apples-to-oranges comparisons. Sure, a really light composite skin on XPS is less damage tolerant than a heavier composite skin, or one with a stiff PVC foam core, or aluminum. But, is it less tolerant to bumps than a fabric skin? Does it deflect less than a fabric skin at the same rib spacing? Folks seem to accept the fragilty and special handling rqmts of rag and tube without complaint.

The impact tests may provide some useful information.
 
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Hephaestus

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Yeah I have no idea as to individual tolerances. Mine is if I'm handling a wing section to put it in storage I don't want to have to use kid gloves and preplan every movement / hand hold.

Because that's when the wind comes up and things go to hell.

But my intended use is different than yours too... Which doesn't help.
 

Protech Racing

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Protect,
A few questions:
1) What is your wing skin/covering that is attaching to these ribs?


Is this the wide-open grid, relatively stiff fiberglass drywall tape, or the silky tape often used for reinforcing basement crack waterproofing? I can see GG working with the former, it would seem too thick for the later.

What coverings are you using, and what glue?


? "Glass tape on the covering surface"? Is this the glass tape you bonded to the edge surface of the rib, or did you attach more tape directly to the wing covering where it touches the top of the rib?
When you glassed the large "faces" of the ribs with epoxy/FG, did you remove the factory facings/skin? How did you prep the foam before glassing? When the ribs failed in your tests, did the FG come off at the bondline, or did it rib off a lot of foam with it? Do you remember if it was easy to pull the FG off the foam?

In any of your tests, did you take a wider tape (say 3") centered on the rib edge and then folded over to wrap down the rib faces about 1" on each side? That would seem to take all the peeling load off the foam due to the uplift of the wing covering and instead give a more moderate shear loads to a larger portion of the rib face. As a bonus, if a stiff adhesive/matrix (e.g epoxy) is used, you'd also get a stiffening "C" channel around the rib's perimeter.
When the rib collapsed in your tests, could you tell how it failed?

What brand XPS did you use for the ribs (pink, blue, or green, and compressive psi claimed)?

Thanks much.
The 3 in tape over the side may be useful. But the test showed we were well over strong as is.
The glass tape used prior is conventional 1 in glass tape . The tape I am using on this wing is the drywall tape cut to 1in wide .
Only one layer of taper on the covered surface. The Gorilla Glue was by far the best covering glue. The foam is Dow pink on the prior .
For best retention of the covering (1.6 oz dacron) , Stipple the rib surface( woodpecker), run some water on same, add GG sparingly, add cloth . Stretch and staple to hold it . I ruffed up some long staples and left them in on the top surface . Pretty crude .
Some blue high density on this wing. Not over joyed with it but may work .
The test was/is clamp the rib to a door frame between 2x4s. Rag on the pivot 2x4 on the floor, press the rear section with weight and a scale . Most fails were the result of lateral distortion and the load being off center. I actually chose the foil because it had a fatter rear section , 43015.
The inner ribs that were faced with glass ; The perimeter was GGed and the center area was epoxy . Yes, the epoxy peels off far to easily . The prep was 36 grit. But no chemicals . Vacuum the dust off .
 

Protech Racing

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The current wing has the bottom surface, between the spars, a solid sheet of 1in foam. The sheet is in tension/compression. IE the end of the foam buts to the alloy spars. The rear ribs, I hope to make geodetic and set upon the foam sheet. If Ican figure out how to make a geodetic foil form .I expect to make the normal foils shape and hand make the angled form .
This way I can glue the wing center on the shop floor. Then set it up on the stands and square it up as I glue the ribs on. The center section is 24in by 12 ft and the nose ribs will be 18in +- .
 

TLAR

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I know this is old but I’m bored.
Under a hand held magnifying glass, have a look at any foam cut by razor blade, cut by razor blade and sanded, and Hotwired.
looking at cut foam before bonding is a good idea
What will you discover?
I have experimented with six different Epoxies, and have found the higher dollar epoxies bond better, and when bonding carbon to foam I don’t add micro and will gladly accept the weight penalty.
 
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Vigilant1

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Under a hand held magnifying glass, have a look at any foam cut by razor blade, cut by razor blade and sanded, and Hotwired.
looking at cut foam before bonding is a good idea
What will you discover?
It will sure be worth a look.

I have experimented with six different Epoxies, and have found the higher dollar epoxies bond better,
Care to share any names? I'm about to buy some goo for some tests. Some of the low priced stuff is tempting (US Composites, etc) and every brand of epoxy seems to have its fans and adherents (sorry, couldn't resist).
when bonding carbon to foam I don’t add micro and will gladly accept the weight penalty.
IIRC, Billski does the same thing, at least when vacuum bagging. I believe he found the weight penalty wasn't crazy and he also liked the idea of saving one cure cycle, another layup session, etc.
 

Marc Zeitlin

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IIRC, Billski does the same thing, at least when vacuum bagging. I believe he found the weight penalty wasn't crazy and he also liked the idea of saving one cure cycle, another layup session, etc.
All I can say here is that at Scaled and Icon (as well as other folks with whom I've worked), when using foam rather than nomex core, always micro'ed the foam and bagged the fiberglass or carbon over the micro'ed foam with only one cure cycle. Never had an issue, if the micro is squeegeed well just to fill the pores and not leave a thick coating that can migrate into the cloth plies.

While a pre-cure of the micro will prevent any migration at all, it also causes the bond between the foam/micro and the cloth ply matrix to be a mechanical bond only, rather than a chemical one as well. Unless there's evidence of a sandwich structure strength hit by having a minuscule amount of micro migrate into the cloth plies, I don't see a reason to have multiple cure cycles.

But that's just me, and I may be misunderstanding Billski's process.
 

Voidhawk9

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While a pre-cure of the micro will prevent any migration at all, it also causes the bond between the foam/micro and the cloth ply matrix to be a mechanical bond only, rather than a chemical one as well.
I did a limited series of tests to this end, and found that even with minimal post-cure sanding of the micro, the failure point was the foam, not the glass/micro join. And the unsanded one wasn't much worse, would probably be fine as well, though a few minutes to quickly scuff the surface is peace of mind.
 

Vigilant1

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All I can say here is that at Scaled and Icon (as well as other folks with whom I've worked), when using foam rather than nomex core, always micro'ed the foam and bagged the fiberglass or carbon over the micro'ed foam with only one cure cycle. Never had an issue, if the micro is squeegeed well just to fill the pores and not leave a thick coating that can migrate into the cloth plies.

While a pre-cure of the micro will prevent any migration at all, it also causes the bond between the foam/micro and the cloth ply matrix to be a mechanical bond only, rather than a chemical one as well. Unless there's evidence of a sandwich structure strength hit by having a minuscule amount of micro migrate into the cloth plies, I don't see a reason to have multiple cure cycles.

But that's just me, and I may be misunderstanding Billski's process.
Marc, thanks. I'm probably out of bounds to be citing Billski's preferences based on my quite fallible memory. I do remember comments by folks who worried about migration of the micro into the layup.
If I'm reading your comments correctly, you do a thorough job of getting off any excess micro and then proceed with the layup, no need to wait for the micro/epoxy to reach a certain state of cure.

Mark
 

TLAR

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Marc thanks for chimin in. Micro is great in its place, but I personally don’t want to add “little Christmas tree bulbs” in the resin that I will depend on in an area like a spar cap. Knowing full well others do it
 

wsimpso1

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IIRC, Billski does the same thing, at least when vacuum bagging. I believe he found the weight penalty wasn't crazy and he also liked the idea of saving one cure cycle, another layup session, etc.
When you do open wet layups micro slurry on the foam is a weight save, as you will fill the broken cells with neat epoxy if you do not apply slurry. Section one of these and the layer of resin is substantial. This is standard Rutan style stuff, and you are doing well to have equal weights of glass and resin over the weight of your foam on these parts.

Go to vacuum bagging with wet layups, and resin fraction is substantially lower than with open layups. You basically have two ways:
  • Hard shell and layup - two cure cycles - resin/micro over the foam and let it cure, then sand it, and then do the structural layup with the vacuum bag debulking it and removing excess resin. This is theoretically lighter as the surface cells are filled with a fairly dry micro;
  • Single cure cycle layup - no micro, accept that resin will fill those broken cells instead of a drier micro. I have found that the layup layer is thinnest this way - the vacuum bag squeezes the cloth down on the foam.
I have hardshelled test parts and there are places where it is the practical option. Cutting through test pieces, this makes a thinner layer of resin than with open wet layups, but thicker than with just doing a straight layup and bag it. It looks like the straight layup and bag process squeezes things down more than hard shell does.

The single cure cycle process allows you to layup both sides of panels like fuselage walls and wing skins in one session - big time save and the parts are terrific and light. I routinely get sandwich part weight that checks out as weight of foam, cloth, and another 35-40% of cloth weight that can only be resin. I'll take that all year long...

Billski

Post Script - Long before I got definitive word on the effectiveness of well squeegeed micro in vacuum bagging, I had eliminated it - I had gotten the willies looking (with magnification) at test pieces I made where it looked like the micro had traveled into the cloth when vacuum bagged. Combine that with my usually enthusiastic but inexperienced help for these large pieces, and I decided not to chance the micro. I had difficulty telling the difference weight-wise on my test parts done with and without micro. For my process and parts, I voted for robust over a modest weight reduction I might have gotten. As always, YMMV.
 
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Yellowhammer

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The 3 in tape over the side may be useful. But the test showed we were well over strong as is.
The glass tape used prior is conventional 1 in glass tape . The tape I am using on this wing is the drywall tape cut to 1in wide .
Only one layer of taper on the covered surface. The Gorilla Glue was by far the best covering glue. The foam is Dow pink on the prior .
For best retention of the covering (1.6 oz dacron) , Stipple the rib surface( woodpecker), run some water on same, add GG sparingly, add cloth . Stretch and staple to hold it . I ruffed up some long staples and left them in on the top surface . Pretty crude .
Some blue high density on this wing. Not over joyed with it but may work .
The test was/is clamp the rib to a door frame between 2x4s. Rag on the pivot 2x4 on the floor, press the rear section with weight and a scale . Most fails were the result of lateral distortion and the load being off center. I actually chose the foil because it had a fatter rear section , 43015.
The inner ribs that were faced with glass ; The perimeter was GGed and the center area was epoxy . Yes, the epoxy peels off far to easily . The prep was 36 grit. But no chemicals . Vacuum the dust off .


DRY WALL TAPE? PLEASE EXPLAIN?
 

Protech Racing

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2in wide sawed to 1 in fiberglass tape run down the ribs. Stiffens the foam ribs and spreads the tension load. Also the glue likes it . I'l have to double check the cloth over it tho. Dont want too much ugly to show through. .
 

TLAR

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In a related matter, I have been getting schooled up on a topic called “Matrix Microcracking”.
I have not seen this topic addressed on HBA
 
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