Movie Projection started using Polyester in the 1950 era, and it is needed to support the "Platter Drives" that hold enough film to run fro a couple of hours at 90 feet a minute. The prints are spliced together with Tape, as you cannot cement splice polyester. Movie polyester is thinner than acetate, to exhibit the same stiffness. Light piping is not a problem as the print stock is used in the labs and so is loaded in the darkroom. Projectors have been set up with shear pins and such to avoid broken parts.

Movie Camera neg is GENERALLY on acetate. before digital intermediate, the camera negative was cut and spliced in the editing shop. Things like IMAX Require the strength of Poly in the Camera.

Still 35mm film is generally acetate, but things like tech pan for instance have been Poly. The last of the EFKE film was on poly. The Agfa aviphot stock is all poly. (Rollei/Maco) The short lived Ilford HP5 Motorwinder film was on poly.

Poly can be curtly, attract Static, light pipe and is almost impossible to tear by hand. Often the are are Back Coatings and grey dyes used to minimize this problem. Sheet film is often made on Poly for the extra stiffness.

Poly will not get vinegar syndrome.