How does expanding foam work ?

That may sound like a stupid question but...
For reasons beyond this topic, I wanted to make the cardboard tube from the inside of a paper towel roll a bit more rigid. Ahah, thought I, fill it with expanding foam. Just so it didn't spill out all over the shop, I shoved a Sainsburys carrier (I'm sure other supermarkets are available) in the middle and squirted said foam into that, so the tube acted as a former. Result: Only the foam at the open end expanded and set properly.
Just in case it was the carrier bag, I lined it again with some thin plastic sheeting and repeated with the same outcome (except that after leaving it overnight, the unexpanded fraction had set).
Squirted a bit onto a piece of cardboard and it swelled up etc as normal (so it's not a duff can).
It's not low expansion foam or anything, just bog standard Screwfix stuff.
So, what is it lacking that it gets normally - oxygen, nitrogen, water vapour? Or is it that it doesn't like the plastic? I never seem to have a problem getting it to expand when I squirt it into quite deep holes in brickwork etc. (ie it it always makes more than I want/need/expect)
I know this isn't the most important question ever asked, but I'm intrigued.....
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GMM wrote:

It needs moisture to cure - usually provided by a combination of the atmosphere and the surrounding materials.
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On 06/05/2013 21:08, GMM wrote:

Water is usually the key. For best expansion spray the surfaces lightly with water first.

Some of the best discussions come out of the daftest questions ;-)
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n
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I'll try spraying some water inside the plastic bag tonight then - and see what happens! Strange that expansion doesn't seem to be a problem when squirting foam into dry brick cavities. On that basis, I should be able to start some excellent discussions (!)
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On 07/05/2013 12:12, GMM wrote: \=================================================================/

I think I would put the plastic bag *outside* and possibly just moisten the inside of the tube.
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newshound wrote:

:-)
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Adam



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On 2013-05-07, John Rumm wrote:

conversely.... ;-)
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On May 6, 9:08 pm, GMM <GlMiMa-AT-yahoo.co.uk> wrote:

Heh heh. It relies on atmospheric moisture to make it go off. So if you put it inside something (near) airtight you just end up with a dribble of gunge in the bottom. So if you want to box it in, you have to spray the internal surfaces with some water before you squirt the foam an close it off.. Even this sometimes results in some largish voids.
Also large masses of the canned foam result in a huge hollow bubble.
For guaranteed results in an enclosed space, you have to use two component foam (two liquids you mix and it foams up )
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On 06/05/2013 21:08, GMM wrote:

This reminds me of the idea I had for creating a portable crash helmet.
Basically a plastic bag placed unopened on head then adding expanding foam into bag. The final shaping could be made by hand (so to speak).
(Maybe sealing the bag first with duck tape etc.)
For obvious reasons I never tried this myself, and no-one I suggested it to has ever agreed to try it.
I'll follow this thread with interest.
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I believe racing car seats are thus adapted to the driver's arse.
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On 07/05/13 20:11, Zapp Brannigan wrote:

I had a pair of ski boots fitted by injecting expanding foam into the liner.
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On Tue, 07 May 2013 23:46:35 +0100, djc wrote:

SWMBO once had a pilonidal sinus (look it up). Basically, to remove it they cut a wedge out of the top of her buttock, then it was essential that it healed from the inner end outwards or another sinus would form.
The dressing technique (every couple of days AFAIR) was to use a form of expanding foam to fill the remaining gap.
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Things have moved on, a pal had one many years ago and it had to be packed with saline soaked wadding twice daily until the cavity healed out. All rather avoidable as when he approached his doc he had a pimple sized lump but by the time he reached the top of the waiting list for surgery it was golf ball sized and healing took six months.
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On 7 May 2013 23:46:08 GMT

Yikes! We learn something every day, and it's not always nice.
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Davey.

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Sadly, that wasn't it. Nasty, aren't they?
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Much clenching of buttocks !
Rob
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wrote:

How did you subsequently get them off? Angler grinder? Or are you still wearing them?
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On 07/05/2013 23:46, djc wrote:

I had some like that too, which was (I suppose) the basis of my presumption that foam would work in a bag ....It must be another species of foam I suppose...
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Well, a bit of water definitely did the job: Plastic bag inside the tube, a splash of water into that, squirt and done!
Or, for Adam's benefit: Once the tube's nice and wet, what you inject swells right up and stiffens nicely...
There's certainly a lesson there. I must 'invest' in a plant spray for future foaming jobs. Amazing how putting a bit of foam somewhere becomes much simpler with a gun.
Cheers chaps.
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