OK, I reviewed this entire thread to make sure I wan't missing anything.
My interest in XPS is surfcraft construction (surfboards, body boards, etc.). Not soaking up water is a huge advantage for surfcraft when they get dinged while in use. XPS densities are similar to the PU foam used for traditional construction.
Surfcraft take a lot of punishment in waves (often equivalent to class 4-5 whitewater). Surfcraft are also exposed to rapidly changing temperature extremes (hot sun – beach & car racks – then straight into cool-cold water). As much as 50 F temperature differential within a few minutes (expansion and contraction). Pressure dings on the decks from heels on take-offs (foam crushing: minimum compressive strength). All of this magnifies problems with delamination.
Delamination is the big problem. Some builders claim no problem with XPS delaminating. Others report the XPS delams almost immediately. My observation has been that those who report instant delamination are using XPS with 15 psi min. comp. strength.
Some of those who report successful use of XPS swear by cleaning the (printed) surfaces of retail XPS with high purity denatured alcohol first, before doing any other surface work.
I found 3M 78 adhesive bonds XPS sheets well if you don't need to sand or hotwire bonded seams of foam. However, to get max bond strength, I had to let the bonded sheets cure for 2-3 days.
The following is some of the useful information I gleaned over the past several years:
Regarding strength for internal 90 degree joints, a fillet of epoxy/flocked cotton (flox) can be used as a fairly cost effective way to strengthen the fiberglassed internal surface of the joint. For a chemical bond, the epoxy-flox mix needs to be applied before the epoxy cures. Amine blush on epoxy cured in humid environments will affect bond between epoxy coats.
Regarding composite sandwich construction, an engineer who manufactured EPS surfboard blanks told me, “Fiberglass and other composite skins should be viewed as surfaces rather than part of the core. The composite skins are minimizing parallel surface movements relative to the core.”
Regarding stiffnes/flex, I played with building longboard skateboard and mounatinboard decks. Stiffness significantly affects performance. The critical lesson for me was, “Stiffness is directly proportional to the cube of thickness.”
Regarding bonding of materials, material surface energy is the determining factor. All compounds with surfaces have their own unique surface energy value (polyethylene = 32; polystyrene = 34; 6,6 nylon = 42; glass = 250-500; etc.). The take away is that epoxy adheres better with solid surfaces that have surface energy values equal to or greater than 45 dyne/cm.
Where I am going next with WPT perforation is orientation of perforation slits/cuts relative to the axes of flexing.
I have been determined to make retail XPS a viable core material for surfcraft. I am fairly confident I have found my answers. My next build, when weather warms, should be the final proof.
