Tyvek over foam, applied with polyurethane glue?

[Bob Hammond]

Thanks for the comments so far. They've provided ideas for consideration. Keep them coming!

One thing is, that it might be a good idea to scour/roughen a smooth-finished foam panel, to make mechanical 'keys' for the glue to grip. Another is to scrap the idea of removable panels -- a window, door, and vent should provide plenty of air leakage for water vapor to pass.

I think framing of some amount of wood framing will be desirable if not necessary, and I think spruce is the wood of choice. It has a high strength to weight ratio, and shouldn't be too expensive (it would be great to buy it at Aircraft Spruce, but likely that's not in the budget!).

Another thing that concerns me, is stability of a very light trailer while towing on the road. Here in Michigan, the highway speeds are generally 70 mph (minimum), and there's talk of raising the limit to 80 mph. I suppose that keeping the center of gravity as low as possible will help, but I think the body shape, height, and cross-sectional area will have the greatest effects.

[Bob Hammond]

The Titebond III test is underway, and after 4hrs, I think it's the best choice. It has not bled through the Tyvek as the PU glue did, and it was very easy to clean up the excess with a damp rag. Adhesion seems to be very good too, although I can tell that it's not fully cured & dry because the Tyvek surface is still cool to the touch from the water vapor passing through the Tyvek and then evaporating.

[KCStudly]

How did you apply the glue, brush or roller?

In my testing I found that the roller worked best for applying a thin even coat (makes sense, huh?), whereas the brush left thicker puddles that had a harder time drying when capped.

[Bob Hammond]

KCs-

Since this was a small scale test, I just smeared the glue over with a stiff notecard, but if I were going bigger then I'd use a roller too.

Here's the pic of the Titebond III tear-off test. The result was better than the PU, I think. The glue adhered very well to the Tyvek (the Tyvek delaminated upon tear-off), and it bonded very well mechanically to the styrofoam. I think that the application technique had some effect on this. I applied a medium/generous coat on the stryrofoam and spread it around, and then applied the Tyvek and pulled it back to see that there was an even coat on it as well. Then I laid it on again and rubbed it down smooth. I think this action forced the glue into the surface crevices of the styrofoam. I was very happy with the easy cleanup, and that the glue did not bleed through and discolor the Tyvek's outer surface.

There is something else that i noticed that might become an issue on a large scale glue-up: the Tyvek had a tendency to creep at first because the glue film acted as a lubricant until it started to develop a tack. On a sloped or vertical surface, I'm concerned that might become a problem. But maybe not, or it might be necessary to put on some thumbtacks or other mechanical fastener until the glue goes off.

I think it's time to do a larger test assembly, and leave it outside in the weather for a month or so. I might also do some joining tests of the 2" thick styrofoam panel. I'll use the Titebond III in a simple butt-joint, and make another with a reinforcing spline of 1/4" or 1/8" plywood that is embedded about 1.5 - 2" into the edges of the styrofoam (as soon as I think about how to make the slot - on table saw?).

[KCStudly]

Or a router with a biscuit slot cutting blades.

But you shouldn't need it. GG does a great job butt gluing foam. The bigger problem is that the thicknesses between sheets can vary a bit so it is important to either match parts or fair over a broad area.

[Bob Hammond]

I didn't know that there wourld be such a variability in thickness. Is only between sheets or does it happen within a single sheet? I wonder if sheets from a single lift/production run are the same thickness, and if so it would be a good idea to buy all that's needed (and maybe an extra sheet or two).

[KCStudly]

I don't know the answer to that.

For my walls I bought two 4x8 and one 2x8, all 1-1/2 thick. I cut the 2x8 in half across the length to get two 2x4 and glued them up to get two 4x10 blanks.

IIRC it was the 2x8 that was thicker, but it should not have been. Since it was all 1-1/2 nominal, and others have commented about this as well, I just assumed that the variation was typical.

I'd say that the biggest difference I saw was 3/32 inch fat.

[Bob Hammond]

Thanks for the info. I think it might be a good idea to look carefully at the sheets to see if they all bear the same production code # , and then buy a more sheets than are needed, and also save the receipt to return any unused material. That would probably be less time-consuming than fairing down to match the thicknesses.

[Westcliffe01]

Are you really saving anything ? Tyvek seems to run close to $1/sq ft (based on buying over 1000sq ft which you would not be doing for a trailer. By comparison, Style 1522, 3.7 oz./yd.² X 50" boat cloth (plain weave) is $5.32/yard which if you do the math is $0.426/sq ft (12.5 sq ft per running yard). So even if you glass both sides of your foam, you still are not at Tyvek pricing.

Since the weight of the glass is 3.7oz/9sq ft, that is 0.411oz/sqft and that is also the target amount of epoxy you would need (by weight) or 0.0257lb/sqft.. Epoxy runs about $7.02/lb so that works out to $0.185/sq ft per face.

So, my cost analysis is that glassing both sides of your foam with 3.7oz boat cloth and epoxy resin is going to cost just a little more than one side coated with Tyvek and no adhesive cost yet. I suggest that the fiberglassed foam is going to be considerably more durable. Yes, one would probably waste some foam and epoxy, but I doubt one would be a lot more efficient with the Tyvek. The fiberglass offcuts can always be saves for small repairs and bonding foam edges together.

[Bob Hammond]

Westcliffe,

At Materialconcepts.com, a 5'x30' roll of Tyvek is $65, which comes down to $0.43/sq ft, which seems reasonable enough. Moreover epoxies are not pleasant to work with, as compared to a water-soluble, non-toxic adhesive such as Titebond III. I don't know how much Titebond III costs in the larger amounts that would be necessary for this project (at ~$33/gal), but I'd think that it would be as cheap, or cheaper, than epoxy. Sanding out an epoxy/fiberglass finish wouldn't be much fun either, whereas the Tyvek would only require careful smoothing down at the time of application.

If I bought a 5'x300' roll of Tyvek, then the price would drop to $0.15/sq ft, but I can't imagine how I would need/use that much.

[Westcliffe01]

This was the link I used to check the Tyvek price. http://www.diyhomecenter.com/dupont/tyvek/homewrap-10-x-150-4-rolls-d13406453.aspx?gclid=COb_l4THl7oCFURp7AodQl4AUA Apparently I missed the fact that it was 4 rolls. So that definitely changes the price of the tyvek facings.

But lets face it, Tyvek was never intended to be a structural material, just a vapor barrier.

There are epoxies available which are closer to low VOC, thus safer to use. EZ Poxy is the replacement to the old Safe T Poxy http://www.aircraft-spruce.com/catalog/cmpages/ezpoxy.php

Plenty of builders have told me that most older houses that had tyvek installed, the tyvek had turned to dust over the years, whereas the old tar paper was still intact and functioning. If the system you use to hold everything together fails, it is going to cost more than just the cost of the facing (potentially the cheapest material in the entire build).

[Bob Hammond]

thanks for the link to the epoxy info (isn't Aircraft Spruce an interesting site? I've made guitar tops from their grab-bag of spruce lumber.)

About the useful lifetime of the trailer, I think the storage conditions will determine that, if my useful lifetime doesn't determine the outcome first.

[Westcliffe01]

Yes, many of us are approaching our "use by" dates....
I have mainly used the West system epoxy and it is also low odor compared to the cheap industrial resins used for sealing concrete floors and the like. But the West System price has steadily increased while other brands have stayed more or less the same price for a long time. Usually the manufactures offer a small kit and I would test it first for odor and any allergic reactions. I always use gloves and mix the epoxy in a well ventilated area since the hardener is generally quite corrosive and the more dangerous of the components.

Once you have it mixed and it sets up it is basically plastic.

Several years ago, while living in Germany I was interested in building one of the Rutan Canard airplanes. Most of the structure is hot wire cut blue foam with glass shear webs and spars and typically 2 layers of bid cloth over the external surfaces. To see what it was like to work with composites, I laminated foam sheet between 2 facings of glass fiber and then bonded it together to make an insulated dog house with a picture window and in floor heating (I had to insulate the electric blanket from the structure and lay tile over the top). It worked out OK, but I did learn that one needs a working space that is temperature controlled. Once the mercury dips below 60F things really slow down and below 50 it about stops completely.

When I moved to MI in 2003 I visited the local EAA chapter in Jackson but the group there was not into composites at all, they were all working with aluminum and the environment did not suit composites at all (hardly any heat). In the meantime support for the Canards has massively dwindled and very few kit planes are made via this method anymore. It is becoming a factory only deal. I made the mistake to buy a home on a 1/3 acre lot and no possibility of having a barn / shop space. I don't know what I was thinking, I guess my landlord was a real jerk and I couldn't take any more of his BS... Consequently, still have no sensible place to work.

[Bob Hammond]

I think that 1/3 acre isn't much space to land a plane on anyway. Maybe you could consider a helo.

Over here on the east of Detroit, it's pretty cold too in winter. Like most people, half my garage is filled with 'stuff' of my own and some of my boomerang kids. The other half is for my car in the wintertime. I'm getting too young to dig it out of the snow to go to the job. So I might suspend trailer operations until spring, or plan to start heating the garage on Thursday night or Friday to work over a weekend. A small trailer should be manageable.

BTW, there is also a site for DIY boatbuilders that sells plans and materials, bateau.com and bateau2.com They have epoxies, plywood, and honeycomb ($$$, but feather-light)

[Westcliffe01]

A good site for quality lumber / marine grade ply is http://www.theworkbench.com/marine.php They have 3 locations in Michigan.

Another interesting site is http://www.nauticallumber.com/virtuemart?page=shop.browse&category_id=6 This place has marine plywood in 5x10' sheets. Sometimes it can be worth while to pay a little more to get a single sheet to do the job.