Cellulose triacetate

Cellulose triacetate, or TAC, is a transparent film made from plants. It's optically very clean: it barely changes the polarization of light passing through, which is rare for a plastic and ideal for protecting polarizers. It was also the 'safety film' that replaced flammable nitrate film in photography and cinema from around 1950.

It starts as cellulose, usually purified wood pulp or cotton linters, the short fibers left on cotton seeds. Each glucose unit in a cellulose chain has three -OH groups. These are reacted with acetic anhydride, made from acetic acid, with a sulfuric acid catalyst. Acetyl groups replace the hydrogen on nearly all three -OH groups, hence tri-acetate. The result dissolves in solvents that plain cellulose won't. It's dissolved, cast onto a long polished belt or drum, and dried into film tens of micrometers thick.

In a polarizer, one TAC film goes on each side of the fragile iodine-dyed PVA layer, shielding it from moisture and scratches and keeping it flat. Some modern polarizers swap one or both TAC layers for acrylic or cycloolefin films, which resist moisture better; it varies by manufacturer.