You are correct! Eastman Chemical does make PET (polyethylene terephthalate). As my book shows PET is delivered to Kodak Park in small pellet form. Kodak liquefies the pellets and then drafts and tenters (stretching lengthwise and widthwise) creating a uniform sheet that is rolled up as 54 inch wide by 10,000 foot long rolls.

Cellulose Triacetate (CTA) looks much the same when it is delivered to Kodak but the manufacturing process is quite different. The CTA is dissolved in a solvent. Then the "dope" is continuously extruded on a 60 inch wide 18 feet diameter wheel. Just before a revolution is completed it is striped off and rolled up in rolls 54 inches wide by as long as 11,000 feet.

This description greatly simplifies the process for making both supports. There is a lot of technology in the process. The resulting sheets (in roll form) have to be flat, smooth, straight, and free from imperfections when enlarged several diameters.