I’m admittedly new at this, but something doesn’t add up. The whole theory of XPS gassing causing delamination doesn’t completely make sense to me. I get the bonding limitations due to lack of absorbtion, but that can’t be all of it otherwise eps would still delam 1/2 inch below the surface and it would be apparent when looking at broken boards. There are other discrepancies too, notably
Xps and eps are both polystyrene.
Manufactures claim blowing gasses vent from the product within weeks
I usually read that it starts with pressure dents, which does make sense because xps isn’t bouncy like eps. When it compresses, it stays compressed. you also hear about dark colors and leaving in the sun, which also makes sense whether eps or xps because of the low melting point, but usually xps is made out to be more problematic.
I hear there’s a lot of success using skins because they help prevent shearing. And it occurred to me that this is probably why eps has fewer delams. Eps is bouncy/springy, because of the little balls. i.e. it’s elastic , when compressed it goes back to the original shape, which means less shearing . That doesn’t only happen with compression, the elastic properties are still there when heat is present and expansion occurs. It seems this is the real cause of delams in xps to me - is lack of elasticity.
It seems to me that a sandwich construction is theoretically better. I know sandwich had been talked about to death, but all the threads I’ve seen put eps in the middle. why not put 5/8 eps under the deck to protect from pressure dent delams? Or add to the bottom to allow a little more heat tolerance when is sitting on the beach?
It seems you would get the benefits of xps (water resistance, strength, compression resistance) and minimize the drawbacks (delams )
The xps has one problem that over shadows all its other problems. Epoxy doesn’t stick to it. Not just from heat, epoxy just doesn’t stick to it. So under EPS or not, it just has problems. You can get the same bounce, or not bounce, or whatever with some thought out carbon or glass or whatever laminations on the rails, or stringer choices or wood or whatever. There is reason why most stay away from or have given up on XPS.
Just a side thought. There are lots of unexplored builds to try out. Why go with a proven difficulty? Someone said insanity is doing the same thing over again, and expecting a different result. Stay sane and enjoy!
XPS is “closed cell” foam. It absorbs very little water (virtually no water). The gas bubbles in the foam are very small and closed (no interconnecting air passages like EPS has). The only way gas can be released is if the cells are ruptured. Even after sanding, the surface is not porous, just a thin surface layer of very small ruptured cells. As result the epoxy has very little surface area to bond with and virtually no foam penetration (no soaking in like in EPS). This is why those who use it rough the surface before glassing with 40-60 grit sandpaper – increases bonding surface area. Poking holes in the foam does little to increase surface area for bonding.
Foam strength/shear for either foam will be related to density (pcf) and minimum compressive strength. In general, low density foams typically have low minimum compressive strength. The lower the minimum compressive strength the easier it is to damage the foam (pressure dings, delams, etc.).
XPS is mostly polystyrene but has a small amount of additives. Currently DOW is adding nitriles to the mix. In the 80s they were adding polyethylene. Not much bonds to polyethylene. To see what additives and how much are in the foam, check the MSDS sheet for your foam.
how do they make it work? i’ve felt their sample blocks of foam in the store and it seems to resist the ol’ finger push pretty well. do they have some special version of it to take resin better?
don’t overthink it, if you want to try xps, just do it. I build a few xps boards and they work great, no delam issues. Even if epoxy might not bond as well to it as it does to PU, who cares when in reality the boards do fine and have no issues? They dont pull epoxy, but they don’t pull water as well if you have dings.
Leave the surface rough or roughen it up after shaping, and you’ll have no problems.
Two examples of boards that hold up fine: (lam schedule: 2 4oz +big deckpatch on top, 2 x 4oz + finpatch on bottom), green resin tint, almost no pressure dings!
Score used xps foam as skin sandwich. I made some boards with XPS on rails and XPS as foam of deck sandwich on eps. In both case it works until it was dinged, then appear small delams that increase if not repaired, but now water intrusion.
That was colored XPS for home build, contain fire retardant and realese adgent (mostly polyethylene), cause of bad bond.
A White foam called IXPS was (is?) sold as surfboards blank, claim there is no realese adgent.
FCD/ Patagonia uses XPS, but they glass really heavy. To the point where even with the XPS core, they are heavier than a standard PU/ PE board. Less crushing means less gas released and less bubbles. But epoxy still won’t stick to it. They have their niche in the market doing something different that gets them sales. But even still, there is a bubble disclaimer on every board.
Sounds like your miss-reading the problems associated with XPS…it bonds quite good with epoxy if you do it right - it just surface-bonds , rather than penetrating the surface…a small indentation from a heel mark will compress the foam underneath . The foam stays compressed , and the skin seperates , causing a small “blister”…the blister grows with expansion and contraction , and eventually becomes a much bigger delaminated area . There was a company a few years back that used a spiked roller to created many anchor points on the deck and bottom , to prevent separation of the skin - seemed to work OK … IMO , a durable composite build occurs when the skins , after reaching full cure , cease to rely on the substrate for support to hold their contours. Basically , it means the board is like a hollow self supporting monocoque construction , just filled with foam to drastically reduce the effects of air expansion. …if you achieve this , then the core is not anywhere near as likely to suffer from fatigue , through years of use , and the board doesn’t go stale in performance.
My last dozen or so boards have been XPS. It works fine. If you leave the foam finished at 60 grit you will get lots of adhesion. I like what Kayu posted about the skins and core. I’ve felt the same way about how the core is mainly a way to get the shape done accurately and the skin is where the strength is. My hangup with hollow and chambered boards is the air expansion and the problem with water filling it if you ding it badly. I recently stripped an older EPS/woven bamboo skin compsand that had delamed badly, but was very water tight. I found the delam between sheets of EPS and not between the skin and the foam. I didn’t have problems with the board, but it was too wide to comfortable carry under my arm, and it had the delam, so I decided to cut a new outline and fix the delam. Interestingly the skin was very strong and the glass was well saturated, so it was very water tight. It was very air tight and that is what caused the delam. I left in the sun to post cure and ballooned up badly.
With solid rails and a strong skin you don’t need the bonding of the skins to core material to be perfect. If your rails hold, the board won’t buckle. I think that flex is probably what adds to boards buckling, but most people like to have flex.
The XTR foam is a very good surfboard foam.
I’ll add that if you strip a block EPS board there will be lots of tearing of the foam so the foam itself is not a strong material. XPS will not tear that way except at the rail, and even then it tears differently than EPS, more like PU foam. The weak point will be where the bond between the foam and skin are, the foam itself stays together very well.
