Possible chlorine plastic?

There are plastic sheets that someone used in the laser that turned yellow and had a smell. Does anyone know their origins?

That could be polycarbonate, which does not cut well at all.

The test for chlorine that I know is to heat up a copper wire in a flame, then touch it to the plastic to get a glob of it on the wire. Then put it back in the flame and see if it burns green. That indicates chlorine.