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.....