It originally looked like this :-
It is about 3mm thick. Most of that is the plastic backing. On the front is printed paper, stuck on with a double sided sticky film and covered with single sided sticky film. This can be peeled off to just leave the back material so both sides of that can be used.
Here is a bit I destroyed developing rafts: -
It will bend a little but then it snaps, as you can see, when I pulled a raft off that was stuck too well.
I normally use this flowchart to identify plastic but it fails with this material because I think it is a polymer with a fibrous filler, possibly paper.
- It melts with a low temp soldering iron so is not a thermoset.
- It floats very well but it is not PE or PP, so that is probably due to a filler.
- It does not burn well and is self extinguishing.
- It gives black smoke and if anything has a yellow flame.
- It does not drip.
- The smoke has some odour but I don't recognise it. I don't know what phenol smells like though.