silica fabric windscreen

[Colin Krusor]

I experimented several years ago with using silica and alumina (ceramic, not fiberglass) woven fabrics for stoves. When I went back to graduate school I had to put that project away, but this weekend I found a few hours to pull those materials out again and do a bit of sewing (with inconel wire-reinforced ceramic fiber thread). I made a windscreen to use with my BPL 900ml pot and MYOG mini-whitebox alcohol stove.

The windscreen weighs 0.65 oz (18 grams). It is made with a rectangular piece of 3 oz/yd tightly woven silica fabric and two hoops of "superelastic" nitinol (nickel-titanium alloy) wire. The fabric will tolerate temperatures up to 1800F. The fabric is very stiff and it is sturdy standing up on its own without vertical pieces of wire (only hoops at the ends). Unlike most aluminum or titanium windscreens, this one is taller than the pot, so it offers more protection from wind and traps the stove exhaust against the pot sides like a caldera cone. It is also lighter than the 4" tall aluminum tooling foil windscreen I have been using (which is 27 grams, or 0.95 oz). On the other hand, the fabric is probably pretty IR transmissive. It doesn't reflect radiant heat against the pot the way a polished aluminum windscreen would.

By squishing it down, flipping one side to form a figure-eight, and folding it over, it becomes a compact "puck" that fits neatly inside the pot.

This windscreen has a vertical space at the ends of the fabric that can be brought together or pulled apart (by sliding the fabric along the wire hoops) to control airflow to the stove. When this opening is made wide to admit a lot of air, though, the sides of the windscreen start to bunch up and it looses its cylindrical shape. I'd like to have air holes around the bottom, but I wasn't sure how to do this. The fabric is very stiff and slippery (like fiberglass). Holes cut out of it would rapidly unravel and small holes would be impossible to hem (especially with the stiff, thick, high temperature thread I used). I considered grommets but metal grommets large enough and numerous enough to let in sufficient air would probably double the overall weight.

So I decided to try making "soft grommets". I have a small amount of high temperature silicone gasket compound made by Cotronics called Duraseal 1531. A blue pigment was added to this batch for an industrial customer (I have a friend at that company). Cotronics conservatively advertises a maximum continuous use temperature of 650F, but the product actually tolerates temperatures much higher. My friend used it in a furnace that peaked at 1039F and stayed above 950F for an hour and the silicone gasket was still soft and flexible but it became lighter in color. His company bought this product after trying a dozen other "ultra-high temperature" silicones that deteriorated above 700F.

I painted some of the blue Cotronics silicone onto the silica fabric, covered it with LDPE plastic sheet, clamped it lightly to force the silicone into the fibers of the fabric, and let it cure. The next day, I cut holes out of the silicone-impregnated area.

I plan to make another windscreen, using lighter gauge wire hoops, and add a bunch of these "soft grommet" air holes around the bottom. The silicone couldn't tolerate direct contact with a flame, but it will be on the bottom, on the outside of the fabric, exposed to cool incoming air, so I expect it to hold up just fine (I'll post updates once I have given it a test). I'm also considering making the next windscreen out of 3.5 oz/yd basalt fiber fabric rather than silica, because the weight increase would be negligible but the basalt fibers are stronger, more flexible, less brittle, and I like the brown color better than the bright white of the silica. The air holes and the softer fabric might make vertical wires necessary for a sturdy cylindrical shape. I don't think the extra wires would be a problem for packing it into the pot, though.

Also, the aluminum dish under the stove, in case anyone is curious, is actually a 4" aluminum foil dust cap for a speaker. I thought it's bowl-like shape might help reflect radiant heat toward the underside of the pot, and it protects the ground. It packs neatly inside the pot and it weighs 0.15 oz (4 grams). I got it here: http://www.ebay.com/itm/Aluminum-4-Dust-Caps-For-JBL-Speakers-/150709557033?pt=Vintage_Electronics_R2&hash=item2316fd5729

Any ideas or feedback are welcome.

[Dale Wambaugh]

Where does the line start? Twisting up into a puck in the bottom of the pot is genius!

[Roger Caffin]

Hum — free-standing? Works on sheet rock? Interesting!
How long does the fabric stay stiff enough to keep its shape?

Cheers

[Colin Krusor]

"How long does the fabric stay stiff enough to keep its shape?"

Roger, I'm not sure I understand your question. Do you mean how many packing and unpacking cycles before the fabric softens? Or how long would it keep its shape once erected for use around a stove?

I don't know how many packing and unpacking cycles the silica fabric would endure before it started to fuzz a bit and become less stiff. Many, I would guess. Right now, the fabric will stand on its own and hold its shape over time without any sagging. I've had the windscreen unfolded, in a cylinder, on a shelf for several days now and it hasn't sagged at all. The stiffness of this fabric is similar to midweight (~3 oz) cuben, or 6-8 oz heavy-tow (maybe 8k) plain weave carbon fiber fabric. I can roll it around on the floor without very much change in the shape, and it wants to spring back open when I fold it up.

Si02 is a very high-modulus fiber, and this fabric is "preshrunk", which means it has more short-range order (more "quartz-like"), and a higher modulus, than the more common amorphous silica fabrics.

[chris Mcfarland]

Can you add a spring to the screen to make it pop up if the stiffness degrades? If you made one with the same diameter as the hoops and placed it on the inside instead of two loops. If the fabric was only attached at the top and botom, you could still adjust it.

[Ryan Smith]

Very interesting post Colin. You're always thinking outside the box and it's that kind of thinking that really moves UL forward. Keep us updated!

Ryan

[Daryl and Daryl]

Thanks for the info. All new to me and very interesting. I wrote everything down in case I start on a project where it applies.

[Roger Caffin]

Hi Colin

> how many packing and unpacking cycles before the fabric softens?
Yup. I would expect some fracturing of individual fibres with each folding – but I might be wrong. Life testing needed!

The free-standing bit is very nice.

Cheers

[Colin Krusor]

Roger, I have concerns about the brittleness of the silica fibers, too. I agree that some breakage of fibers from creasing is likely to happen. Folding and unfolding of the fabric only occurs several times per day during trips, and perhaps a hundred times in a year, and the fabric is never subject to any tensile load. So, hopefully fiber breakage and weakening of the fabric will occur slowly.

I think the basalt fiber fabric would probably suffer far less fiber breakage. It has mechanical properties similar to S-glass, and tolerates flexion very well.

[Roger Caffin]

Hi Colin

You know, you can get conveyor fabric for baking ovens etc. A combination of PTFE and glass, or silicone and glass. And it is designed to handle some flexing as it goes around rollers. Temp range -150 C to +260 C. I wonder how that would go?
See for instance Chemfab or
http://www.chemfab.com/uploadedFiles/SGchemfab/Documents/Top%2030%20Brochure%20English.pdf

Cheers

[Colin Krusor]

Roger, thanks for that link. One concern about a completely siliconized textile for a windscreen is flame exposure on the inside surface. My little alcohol stove occasionally has flare-ups that send flames up the sides of the pot a little bit. Even very refractory silicone like the Cotronics stuff might not withstand direct flame for very long. Also, most commercially available silicone-impregnated fabrics for high temperature applications are very heavy. I like my silica windscreen because it is lighter (and bigger) than the aluminum tooling foil windscreen I have been using.

[James Marco]

Yeah, I think that would be a bit heavy.
I am wondering if just plain fiberglass might work. Most of the 6 and 9 oz cloths would be out because they have larger fiber sizes, hence could break if folded sharply. Even some of the 4oz is just looser woven, but the same grade. They do make a 3.2oz (varries a bit in this range) with finer fibers.

I have a bit left over from canoe building and will have to try it. The volan coating may burn off leaving it a bit brown, but shouldn't bother, otherwise.

I think baffeling can be accomplished with a simple spring like lower loop. It should just lift off the ground slightly, on one side.

It could be that a simple coating of silicone/mineral spirits around the bottom will let me spread the fibers, holding them in place, without having to cut them. If I have some caulk around that is any good, I can try that, too.

[Dan Yeruski]

Nice idea, I like it. We hope it stands up after 10-20 uses. A side opening will cause a draft to blow the flame off to one side. Holes on bottom much better. Great idea using the epoxy silicone, I like.

Be sure to use a torch to fuse the ends of the fabric after sewing. You don't want those 1/8" long fibres sticking to the walls of your pot that might eventually stick to the walls of your stomach. I work with a lot of 6oz fiberglass cloth and I see how the fibers get every where. Have sun light reflect off your cutting board so you can see the millions of tiny fibers stuck to it.

[Roger Caffin]

Hi Jim

> I think that would be a bit heavy.
Dunno. I got some samples from Dotmar in Australia, and they were quite light. Chemfab start at 130 gsm and go upwards.

Cheers

[James Marco]

Dunno. I got some samples from Dotmar in Australia, and they were quite light. Chemfab start at 130 gsm and go upwards.
Yeah, that *is* light.
Thanks!

[James Marco]

OK. The fiberglass did NOT work. it had trouble supporting itself at 8" high. NOT a good idea.

[Colin Krusor]

Yes, that was my experience with fiberglass, too (both E and S types).

Roger, 130gsm is pretty light. Thanks for that tip. I've been considering trying to make a material like that. I have various fiberglass, kevlar, kevlar/PBI, kevlar/carbon, and carbon fabrics in the 1-4oz weight range that I could impregnate with silicone using my 12" x 24" screw press. I don't know if the silicone would cure in the press, though. I might need a bleeder layer to allow volatiles to escape. I still have some concerns about exposure of the silicone to flame, but it might be worth a try.

[Jan Rezac]

A while ago, I have accidentally found an interesting material that might work well in this application. It is sold as a reusable baking paper, it is a thin teflon foil with some loosely weaved fibers in it. I weighted the piece I have (i bought it without any particular use in mind, so far I found it very useful for ironing a seam tape onto a fabrics), it is 140g/m2 what seems lighter than metal foil windscreens. The hoops will be needed, but the material is stiff enough to stand without any vertical supports. I like the idea of folding a windscreen this way, I may give it a try if I find some light but springy wire.

[Roger Caffin]

Hi Colin

Not many silicones have volatiles. It is usually a polymerisation process, often using moisture from the air as a catalyst.

Cheers

[Roger Caffin]

Hi Jan

> some light but springy wire.
You can often get 'piano wire' or 'music wire', which would do fine.

But sometimes you can use humble copper wire instead. Yes, it bends, but you can straighten it many times.

Cheers

[Brad Walker]

"Hi Colin

Not many silicones have volatiles. It is usually a polymerisation process, often using moisture from the air as a catalyst.

Cheers"

Most RTV silicones are acetoxy-cure and put off acetic acid as they cure, which is catalyzed by moisture (as you mention).

There are other curing systems though, like 2-part platinum-catalyzed systems and 1-part oxime cures. Silicone is fun!

[Colin Krusor]

Roger and Brad, thanks for the information about curing of silicones. Will a thin layer of silicone cure if it is pressed tightly between two sheets of plastic? If I want to spread silicone on a piece of fabric, then force it into the fibers with my screw press and cure it under pressure between sheets of LDPE film, should I very slightly dampen the fabric first? Or should I just leave the piece exposed to humid air for a few minutes after painting the silicone onto it, then put it in the press?

[Brad Walker]

Assuming your silicone is typical RTV stuff, the following will be true:
-The thinner the layer, the faster it will cure
-Access to atmospheric humidity drives the curing (20-50%ish RH), liquid water not quite
-Heat won't help

Since you're working with such a thin layer, it might cure pretty well in just about any condition. If you can put it in a jar or something with some wet paper towels to up the RH, that could help too.

Why do you need to clamp the part while it dries? Once impregnated, the silicone probably won't run out of the fabric, especially since it will "skin over" quickly as the outside cures (a few minutes max.).

[Brad Walker]

Oh, and I guess I didn't mention it, but your windscreen is great. Awesome use of materials, and the rubber holes are very inventive.

[Roger Caffin]

Hi Colin

> Will a thin layer of silicone cure if it is pressed tightly between two sheets of plastic?
Do it on a humid day. Breathe on the LDPE sheets. By the time you have spread the silicone out to a thin layer and clamped it flat, the process should be well under way.

Yeah, silicones can be fun.

Cheers

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