My dad works at a printing company and as a result, there's quite a lot of various materials that have accumulated in my garage. Just junk he's happened to bring home every now and then. Most of it is foamcore, but there's also a big piece of some kind of plastic, although I don't know what it is. It's about 6mm thick, and 3' x 4'. I can tell it's been cut along one of the 4' sides, so it could have originally been 4' x 8' and just trimmed down, so I'm thinking it is sintra. Is there any way to tell for sure?