What material is an Inkjet transparent printing film (slide) made of?

I don't mean [photographic film](https://en.wikipedia.org/wiki/Photographic_film)/[film base](https://en.wikipedia.org/wiki/Film_base) but rather the A4/Letter sheets of transparent film that were used in slide projectors during lectures. For example see [this](http://www.ebay.com/itm/Inkjet-Laser-Printing-A4-Transparency-Film-20-Sheets-for-Screen-Plate-Making-/181396274376).


