As Kodak's Book of Film Care puts it, film is animal, vegetable and mineral. When film was invented it was made from cellulose nitrate, but modern day film is made from either a cellulose acetate or polyester base with a coating of light sensitive minerals - namely silver salts.
In color films the light activated emulsion is laid onto the base in three layers each containing color dyes - magenta, yellow, and cyan. The medium used to bind the emulsion to the film is made of gelatin, a natural polymer made from animal bones and hides.
Well we have minerals in the form of salts and animal in the form of gelatin, so what about the "vegetable" part? Well, polyester is made from petrolium, which is basically decomposed vegeable matter found below the surface of the earth. But there's more. We've all heard film referred to as "celluloid" - a popular term for cellulose nitrate, though it has come to refer to film in general. Now mainly cellulose acetate is used to make film bases. However, due to its durability and stability, polyester has become more widely used for distrubution prints and other films that will not need to be edited or permanently spliced together.
Cellulose acetate is formed from plant material (cellulose... we all learned about the "cell walls" of plant cells in high school biology.) The cellulose in cellulose acetate is either wood pulp or short cotton fibers. These are natural polymers. Films made of acetate are still common becasue they can be fused together in the editing process.
So which one is better? Well, each type of film base has its advantages and drawbacks. And each, as well, has its appropriate uses.
Polyester film is stronger, more resistant to tearing, amd less brittle and can therefore be made thinner and lighter than acetate. It is also more durable and resistant to degradation or decay. This strength and durability makes it great for distribution prints. These are the films that you watch at the theaters. They are run through the projecotr over and over again and often visit more than one theater before they return to the distributor. These prints can be kept for long periods and archived with less concern about the elements such as moisture. Polyester also does not tend to "decay" or give off odors after sitting for a long period without use.
On the downside, polyester film is often so thin that it can create difficulties with older projection and processing equipment which does not have the kinds of tolorances required for the newer thinner films. It can also get bound up in the film path or wander from the film path in self-threading machines. It also cannot be spliced together with a solvent the way acetate film can. Splicing polyester film together requires a heat fusion process which does not lend itself to editing. This also renders it impossible to cement splice the two types of bases together. For these reasons, negative film stocks - the camera originals which are spliced together at the end of the editing process - must be made from acetate base so they can be easily put together for printing.
And the refrative qualitites of polyester base are greater than those of acetate, sometimes causing "halo's" of light around bright areas in the image. To prevent this manufacturers have added a thin "gray" layer in film between the emulsion and the base to dim the light as it reaches the clear base material. Actually, this "anti-halation layer" is used in general to prevent refraction in acetate films as well. Polyester film also tends to hold "coreset" longer than ecetate. "Coreset?" you ask. This is the curl found in film that has been wound on its core for a long period of time. But despite the fact that polyester does not relax as easily as acetate, it also takes it longer to retain the shape of its core.
On a final comparative note, the two films are found to be equally resistant to scratches and general wear. And each type of film serves it's assigned purpose well. Libraries, distributors and amateurs can benefit from the strength of polyester, while editors and filmmakers can work easily with the more traditional acetate stocks.
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