Cork & XPS

General Discussion
1

My next experimental build tech project for this summer is a cork deck skin over an XPS foam core, then cover with FG.

I feel confident that cork skins over XPS cores, under glass, are the path to a durable XPS surfcraft.

IMO Cork Skin advantages are:

  • Impact absorbing

  • Compressive strength greater than foam

  • Low thermal conductivity

  • Does not soak up water

  • Increases surfcraft skin thickness without adding a lot of weight

  • Lighter than composite cloth saturated with epoxy.

So I scoped out some retail suppliers of Cork sheet.  Found it at Staples, Office Max and WidgetCo (photo below).  (They have cork underlayment at HD and Lowes but min. thickness is 1/8" on longer rolls.)

As sheet thickness increases (2 mm, 3/32" and 1/8"), the particles and sheet surface become coarser.  The 2 mm looks like it would be easiest  to work with and will get a cleaner glassed surface (smoother surface).  The 3/32" might be a better deck thickness though.  I will use the 1/8" for a different project.

 

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2

I use some, 2mm 200kg/m3, find that sandwiched between fiber it’s no as stiff as pvc foam. I punch holes al the way before glue, saturated after glue with resin+micro+flox mix and lam over, good weight/stiffness/toughness, with veneer and glass over it will be my next greenwashing deck skin when need strength.

It can be a good idea over xps, may be vac bag with glass under and foaming epoxy… 

3

Interesting idea Lemat.

My main interest in cork is/was its impact absorbing properties (compression and rebound) combined with decent compressive strength.  And it’s fairly cost effective.

Supposedly, “Its cells have a unique 14 sided polyhedron structure that can be compressed to 15% of their normal volume and then regain most or all of its size and shape…”

4

Yes, when sandwiched  between fiber it’s by far not as stiff as pvc foam and heavier. By increase resin bridge you can increase stiffness and keep some toughness properties. 

5

PM BB30, Charlie made a lot of cork under glass boards in the past. My brother did a couple, but I’ve only used cork to build up rails. I think Charlie was getting corecork from Drew Baggett and may have gotten a lot of input about the build process. Pretty sure it was all done in a vac bag. I have a beautiful cork under glass Gemini board from Charlie, but it’s in my brother’s storage locker.

I would use goilla glue if you go with XPS, but still do it in a vac bag.

6

Thanks Harry. 

Corecork is cheaper by the foot if you buy a 4’ x 130’ roll (minimum quantity).

The plan for the 42" channel-bottom BB with Charlie is to use cork skins under glass – probably EPS.

For my own cork skin/XPS project, I plan to start with 3M 78 for bonding the skins directly to XPS.  

I will move forward from there and look at some other lay-up possibilities.

Bill

7

i have done a number of projects with cork over xps.  there used to be a bunch of pics of my older work on here before one of the server implosions years back. 

your mileage may vary, but based on my experience, you’re going to need something underneath the cork.  my best solution came to be a very thin layer of nonwoven veil (sourced as a landscape barrier cloth) beneath, bedded in resin research epoxy and topped with 1.5 or 2 mm corecork–  double 4 oz aerialite on top.  fill/hotcoat by hand, but for the lams, resin control and finish thickness means everything, so vac for sure.  bleeder, barrier, and all the goodies go a long way toward it needing little more than razor work and edge cleanup coming out of the bag, otherwise you’re shaping it twice.

test lams can be good, but i will say that lots of things that seemed fine on the test bench scale resulted in boards that would blow the skins clean off the bottoms and deck in a full scale board.  of course, most of those boards never took water with xps cores, but it’s little consolation.   they live in the barn rafters now with all the other oldies and frankensteins. 

where i live it gets 100F much of the year, so gas expansion forces are extreme.  bonding energy to the xps is very low, so it’s mostly mechanical grip.  one seldom if every blows rail bubbles. your spray adhesive may get you somewhere, but i’d almost think of doing entire deck or bottom sheets on flat glass, releasing them, and using spray 90 or contact cement or whatnot to stick down those sheet-lams like formica on a countertop base.  then you could glass the rails with tape.  you just can’t have much deck dome like so, but…

i have seen a great deal of sheet cork material but have only used the real corecork on boards.  it is very well fused, has very consistent cell and void size/distribution, and drapes/tapes fairly well without fragging.  again, to build your sandwich skin, resin/core ratios are going to be critical to produce a consistent lam.  lots of the floor underlayment and bulletin board material seems to fail on this level, but i haven’t tested.  you can waste hundreds of dollars worth of expensive resin to save a few dollars on known materials and suppliers, but i only speak of what i learned myself the hard way.

i want to part with a few statements.  drew is a super good dude, but he’s a businessman who has to take care of his family, so he does so.  corecork is out there but can be difficult to source in the continental US.  i’ll also say that i don’t make many boards these days, but i have personally given up on xps for my projects in this climate.  that said, i currently own a custom rusty that he did for me in the torsion spring construction-- javier’s perforated lam epoxy/xps technology.  it would have ballooned that big ugly black R right off the deck on the first summer day in any XPS/epoxy i’d ever been familiar with.  now it’s still dingless and bright white despite some considerable deck denting under my front foot.  i’m a big dude that surfs my boards hard though.  i’m very impressed, but my personal shapes will be 2 lb eps or more likely PU core.

 

-cbg

 

 

8

I appreciate the constructive input CBG.  What density XPS did you use?

Over the years, I have noticed that many who have  problems with XPS use low density (1.3 pcf) XPS housing insulation – O-C 150 or Dow Residential Sheathing.  

Air/gas content of low density polystyrene foams – XPS or EPS – is high, causing significant expansion and contraction with wide temperature ranges. 

Low density EPS needs vents and stiff veneer skins.  Because XPS is closed cell, there is no way to vent expanding/contracting gas. 

Like EPS, higher density XPS has less gas content and less volume change. One option to reduce post-build expansion might be to allow the shaped blank to heat up in the garage on a hot summer day, allowing the shaped blank to expand before glassing.

Like EPS, XPS is made from polystyrene.  As you mentioned, polystyrene has a relatively low surface energy that doesn’t form super strong bonds with epoxy.  EPS bonds better because it is porous, not “closed cell” like XPS (very small gas cells).

My test panels are bodyboards.  Four years ago, I threw together a quick unglassed XPS bodyboard for the grandkids.  Shaped it from 3 sheets of XPS housing insulation bonded with 3M 78 (still holding together).  It occurred to me that bodyboard builds were the perfect “test panels” for XPS build tech experiments.  Before that, I did standard small test panels.

There are multiple variables affecting delam and bonding.

Polystyrene surface energy.

Appropriate adhesives.

XPS minimum Compressive strength (correct foam density).

Lack of XPS porosity – “small” closed cells.  Perforation number, size, shape, orientation, surface area (internal & board surface distribution).  Large (relative) circular perforations have low surface area to volume ratios.  You want high surface area to volume perforations, many very small holes (Topflite Woodpecker tool, Wartenberg 7-row pinwheel).

Glassing schedule.

Load and impact dispersion (deck skins:  EVA, Cork, etc).

Foam temp (internal gas volume) at time of glassing – core and surface layer – affecting post glassing expansion and contraction.  XPS does not out gas (closed cell foam cannot).  Which is more likely to cause delamination, expansion or contraction?

Hand oils on the surface of non-porous foam, from handling during shaping and glassing.  Wear clean or new dishwashing gloves.

Board color – opaque white pigment to reflect solar radiation.

Design and construction must consider these variables.

There are many generalized assumptions when discussing XPS for surfcraft.

(BTW WidgetCo sent me a free cork sample on request.  Seemed pretty durable/tough so I bought 2’ x 30’ roll of 2mm for $60 (free shipping.)

9

There is xps foam called “spyder foam” that’s used by hobby aircraft builders that work well with coposits skin. Bigger cells no polyethylen content. May be what was used for Salomon blue boards or some tufflite surf tech when they moved to pvc to wood sandwich they make some with about 5mm thick blue xps.

10

this was quite a while back with a lot going on since then, and it also represents a longer sequence of experiments over a period of time.  the lines and times blur in the distance.  i’ll try to keep it coherent, but there’s not one single plotline.

i had sampled some blue dow xps dock billet material as well as some of what was referred to as dow high density XPS freezer floor insulation.  both were superior but challenging to source economically at development quantities.  as a result, the majority of my projects did use the blue dow residential sheathing for practical availability reasons.  most of this came from my interest after seing some of the old salomon s-core boards that my brother had when he was on oahu.  i think chuck andrus shaped them for him.  they were built like a semi hollow balsa with deck and bottom sheets of what looked like blue dow and rib frames with voids between.  my variation was to thickness and contour a 1 lb eps core piece and put it between two bread slices of dow blue.  they were glued with foaming pu glue applied with toothed spreader and vac-bagged into sled blanks weighted into a rocker form, toothpicks helping registration where necessary.  once they went off in the bag/form, i could cut very close tolerance blanks out of the sleds using masonite outline templates.

those would get final shaped and then a number of approaches were used.  best practices came to be the nonwoven, corecork, and double 4.  if i remember correctly, all these lams would have been laid out on a sheet of bleeder, taped down over a sheet of visquene, taped down on a wetout table (slab door).  saturation but little excess of the resin with the veil or glass only taking what it will hold .  the lams would have been rolled up on a big pvc pipe, trash bagged and stuck in the chest freezer for a serenity pause before building the final package.  plus it slows things downall around and stiffens things up to make it easier to manipulate and tape before you seal up and suck it down.  double sided in one pull is possible but you better be good or have a skilled helper. 

bodyboards are very doable in a single pull, but you have to decide where you want to fade your seam.  it may not be worth it.  if there’s no tuck and edge in the rail like a booger might be, it’s easier to wrap up  to a pinline termination and leave the deck raw.  i have one like that.  i think i put an extra 4 oz under the cork with the nonwoven to account for the open sandwich.  cork deck is obviously drew’s legal ip, which i respect completely, but i’m never selling any and probably never making any more.  i think biolas has bought out exclusivity on the mainland, but drew probably still does all those boards in house.  i was doing cork deck on some downhill skateboard blanks from stacked corecork and maple lams at the time and went with it.  that’s how i had originally come to the source the corecork.

heavy blabber here, and i’m sure i’ve lost the melody somewhere, but that’s the underlying backstory.  i’ll come back later to figure out what i said wrong or left out.

oh yeah, javier at epoxypro has the foam and the perforated lam and the patented needle roller tool thing really dialed in for production, but you’re not going to get your diy on with those foams.  you use what the market provides at hobbyist volumes and you takes your chances.

-cbg

ps…i think you could shape a mean booger sandwich core out of white cedar or spruce that would define your rocker, thickness flow, as well as provide structural rigidity.  you can shape moderate concaves into that core as well.  encapsulate it with epoxy and bag your xps sheets over top, then final shape and bag your composite skins in place or just top and bottom bread and tape the rails out of the bag.  you need to do fill and finish by hand to ever get rid of pinholes and get decent finish surface.  it would probably stay together if light colored or not in FL/TX.

pps…i’ve used a lot of spray adhesives and hate most of them, though i have used them a lot to hold cloth still in dry sandwiches.  the ones with decent hold throw boogers and are messy.  it’s hard to control distribution with the spraycan.  there may be a contact adhesive solution which works well and could be put down with a very finely toothed spreader that would control thickness better.  i used to love the old toluene-reeking contact adhesive flashed off with a lighter, but boy would it eat some foam!

 

11

2020 was a hot summer here. I have an open air shed with boards and I store boards under my house. One of my XPS boards developed a massive delam sitting outside in the shed. That shed is just a rack for my boards, but with panelling on 2 sides and a roof. The sun doesn’t have direct contact with boards. I gave a friend an XPS board I wasn’t using, and told him about the heat sensitivity because he leaves his board in the back of his truck and drives across the island to surf. His board also developed a large delam, but he loves it so much he keeps riding it.

I think with XPS it’s better to have pin holes in the skin to let it breath, the way XTR was glassing

One of my Home Depot EPS boards stored in the outside shed also developed a large delam, but I think it is delaminated between layers of foam and not between the glass and foam. The EPS foam boards made from a single block all survived the heat, but I left an old polyurethane foam glassed with epoxy outside and it has numerous bubbles.

I hope you don’t have the same problems.