Cellulose Acetate Films
Properties
Cellulose acetate (CA) is the most important cellulose ester. It is a tough and easy to process thermoplastic of excellent clarity and gloss and little to no odor. It has exceptionally low haze, high moisture vapor transmission but extremely low water permeability, and is easy to cut and tear. It also has good chemical resistance to organic and inorganic weak acids, hydrocarbons, vegetable oils, and the like. Often plasticizers are added to further increase its flexibility or mixed ester of cellulose like butyrate-acetate and propionate-acetate are produced which have improved flexibility, toughness, and moisture resistance.
Like cellophane, CA is made from cellulose but it has very different properties. Unlike cellophane, it is thermoplastic, that is, it will soften and melt when heated and therefore is easier to process.
The most common source of cellulose is cotton linters. The fibers are mixed and reacted with glacial acetic acid and/or acetic anhydride with sulfuric acid as a catalyst. Acetylation converts the hydroxyl groups of the cellulose to acetyl groups which results in a much more soluble product. In a subsequent step water is added to stop the reaction and to partially hydrolyze the triacetate.
Cellulose acetate can be broadly classified into cellulose diacetate (CDA) and cellulose triacetate (CTA) with a degree of acetylation (degree of substitution of hydroxyl groups) of about 2.4 and 2.9 respectively.
The two types of CA have different physical and mechanical properties. In general, with increasing acetyl content, the permeability to gas and moisture decreases whereas the chemical resistance, glass transition temperature and modulus increases.
The most important mixed ester of cellulose is cellulose acetate-butyrate (CAB). Films made of this resin have improved flexibility, toughness, and moisture resistance. CAB films also have outstanding clarity, scratch and UV resistance, and good dimensional stability. Because of these properties, CAB can be used as a scratch resistant film or as a surface protection layer in laminate film structures.
Applications
Cellulose acetate (CA) was orignially used for photographic film as a replacement for the unstable and highly flammable nitrocellulose film. Today, CA and mixed ester of cellulose are used for many other application such as anti-fog goggles, optical films for LCD technology, window cartons, scratch resistant films, and foot wear. Due to their good permeability and bio-compatibility, acetate films are also widely used in pharmaceutical products for controlled drug release (osmotic drug delivery systems) and for taste masking. Acetate films are often an excellent choice for semipermeable film applications such as food and drug packaging because (pure) CA is biodegradable and generally recognized as safe.