I have been meaning to do some XPS testing for a while now. A cold autumn day seemed like a good time to do it.
BSD’s thread about sealing XPS for PE motivated me to start the tests today. I only had time for the first half of the first test.
My objective has been to improve epoxy bonding with XPS. A thin, low viscosity coating is what I was thinking about.
Simple tools and common materials. Things you might find in the shed.
I cut a small XPS test panel 8" x 24" from a low density sheet of pink residential insulation (Owens-Corning); marked off four 8" x 6" quadrants and sanded with four different grits (150, 100, 60 & 40).
I added the first coat of Secret Sauce a couple hours ago. The sealer disappeared nicely into all the sanding grooves. I will add another coat of sauce later tonight. Looks good so far…
Epoxy test glassing will have to wait until next weekend.
I’ll be curious to see what you get if you try some pressure and some heat, as my experience, and understanding of others experience is: it is the out-gassing from crushing/heat which causes delam.
A strong shell with higher densitiy foam should be better for XPS rather than using low density foam.
First I want to do the peel test to see how much “low density foam” pulls loose with the glass.
Hopefully low viscosity sealant will penetrate the surface layer of small ruptured cells, and the higher surface area of finer grit sanding grooves, better than the higher viscosity epoxy resin does.
The first coat penetrated all roughed surfaces nicely.
I will add another coat then sand with 150 grit and do the test lamination with epoxy.
I will need to get some of the new Dow blue mix – less than 10% styrene polymer with 60-90% propenenitrile ethenyl benzene. The O-C pink is about 80-95% styrene polymer.
After that, side-by-side comparison with sealed-sanded and unsealed-sanded XPS.
I applied the second coat of Secret Sauce last night.
Observations:
- The first coat dried nicely after a few hours. All surfaces looked good.
- The foam applicator worked well for adding a second coat to the 100 & 150 grit surfaces.
- Adding a second coat with the foam applicator to the 40 & 60 grit surfaces felt like I was sealing coarse sandpaper. I was worried I was going to rip out small chunks of applicator foam. I did not notice any small pieces in the finish.
I may need to look at applying the sauce with a 0.25" nap roller for coarser surfaces. I decided not to for this first test because it looked like the roller might lay down too much sauce.
something that was recommended to me by people that made way more boards then me was to use a razor blade and make slices, not very deep, in the surface, in addition to sanding.
Both boards with xps foam I did, have had cork or balsa over, and have that vacuumed on. I think the vacuum process helps in getting a better bond also.
I have seen several methods mentioned for improving epoxy-XPS bonding, including razor cuts. This guy claims he tried several methods. He says he got best bonding by "scoring" foam with a steel-toothed pet brush -- using 30 psi min. comp. strength O-C XPS:
As with histology (internal surface area of the small intestine) and oxygen transfer in water (bubble size), surface area to volume ratio increases as object size decreases and number increases. Hopefully, this will hold true for some given groove size (peaks and valleys) from sanding.
The trick is finding a bonding agent that will fully penetrate these grooves. Maybe exterior, water-based polycrylic concrete/tile sealer will do that, creating a thin intermediate layer between epoxy and foam sanding grooves.
I just re-read this and realized what I said. The “groove penetration” argument is bovine manure. Barring differences in inter-atomic/inter-molecular forces, there should be no difference in epoxy penetration for grooves of the same size (polystyrene vs. polycrylic).
The big difference is porosity, “closed” cell vs. open (XPS vs. EPS), and resulting epoxy penetration. Another possibility would be a difference in the strength of bond type (hydrogen, dipole, ionic, covalent, etc.) between XPS and polycrylic vs. epoxy.
Surface roughness (epoxy surface penetration) is an issue with XPS. 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 a result, the epoxy has very little surface area to bond with and virtually no foam penetration (no soaking in like in EPS).
First delamination of XPS is related to foam density and epoxy penetration
Lower density foam crushes (compression) and tears (shearing)…
I do not want to write a treatise on this. I will try to keep it brief. Epoxy bonding is related to the material surface energy (adhesion/wetting) and epoxy penetration into the surface profile (roughness/porosity). Surface roughness increase bonding surface area and more mechanical interlocking with the surface.
Epoxy has a surface energy around 45 dynes/cm. Epoxy will bond better with materials have surface energy values greater than or equal to 45 dyne/cm.
The greater the viscosity of the coating the lower the surface penetration.
I am using the lower viscosity polycrylic sealer because it can penetrate deeper into the XPS surface – into smaller grooves. pores and cells. The idea is to get a layer that has deeper penetration and more anchoring than the higher viscosity epoxy would have. Then bond the epoxy to this thin, anchored intermediate layer. I am assuming the sealer has better strength than the XPS.
I have heard of the DOW surfboard foam but have never seen a source for it. Your link lists it but has no data sheet for it. I thought I read somewhere that DOW does not sell it anymore.
I have a little Highload 40 and 60.
I am just expermenting to see if I can get better bonding with less scoring. I am using the low density pink because it does tear so easily. I figured a peel test with low density foam would be a good simiple indicator of bond strength because it tears easily. I can get the low density pink locally for experimenting.
It is an abstract concept that I have a limited grasp of. It is related to the energy needed to create and maintain a surface – like the formation of a water drop. All materials with surfaces have it. For fluids, it indicates how well a fluid will spread/wet-out on solid surfaces. Here is a definition from Wikipedia:
Surface energy, or interface energy, quantifies the disruption of intermolecular bonds that occur when a surface is created. Surface energy is conventionally defined as the work that is required to build a unit area of a particular surface.
I was not really aware of it until I started learning about how epoxy bonds with surfaces. All compounds with surfaces have their own unique surface energy value (polyethylene = 32; polystyrene = 34; 6,6 nylon = 42; glass = 250-500; etc.).
I have some links about epoxy surface energy and its bonding that I could give you.
The take away is that epoxy adheres better with solid surfaces that have surface energy values equal to or greater than 45 dyne/cm.
This feature is adjusted by epoxy glue elasticity. Mechanical energy is force x displacement.
Why skin delam ? because forces between skin (+glue) is higher than adhesion forces between glue and foam. 2 parts in this problem.
Adhesion forces, you always accurately speak of it. Sandwich principle is to have strong skins that take load, protecting light weak foam that keep shape. Under load thin skins bend, distorsion causes shear in glue join, up to delam here. Increase skin stiffness reduce shear in glue and foam, increase delam strengh. Scouring foam, like in your link, act as shear keys that both increase bending stiffness of skin and glue surface (for adhesion forces capability), a good way to go. Well documently in sandwich research, works well in industrial panels… and in my surfboards LOL.
I will have to see if I can find some Dow surfboard foam.
Lavarat --I was looking online for a "carpet seam roller"supplier and found this. It looks like it might work well. I like the narrow and pointed spikes.
A bit pricey but if you were making a lot of XPS boards, it might be worth the investment.
I read somewhere that others have used low viscosity epoxy for the first coat to get better surface penetration and then use higher viscosity epoxy for the second coat. I do not think they were board builders.
It seems like the keying done (below) with that steel-tooth pet brush could add a lot of epoxy weight to the build.
Playing with viscosity and elastisity of epoxy is a key of building improvement. Shear key add weight but because they add buckling strengh effectively need less glass over…
