Wikipedia
Film base
A film base is a transparent substrate which acts as a support medium for the photosensitive emulsion that lies atop it. Despite the numerous layers and coatings associated with the emulsion layer, the base generally accounts for the vast majority of the thickness of any given film stock. Historically there have been three major types of film base in use: nitrocellulose (cellulose nitrate), cellulose acetate ( cellulose triacetate, cellulose diacetate, cellulose acetate propionate, and cellulose acetate butyrate), and polyester ( polyethylene terephthalate (PET) (Kodak trade-name: ESTAR)).