Cellulose+acetate

Cellulose acetate is produced by modifying cellulose and thus is a semi-synthetic polymer. A major component of plant cell walls, cellulose is found in virtually all types of plants. Chemically, cellulose consists of D-glucose units linked by β(1->4) glycosidic bonds and is classified as a polysaccharide. In nature, it mostly exists in semi-crystalline fibre forms. Along with other polymers, such as lignin and hemicellulose, cellulose forms what is commonly known as the wood. The exact composition of wood varies tremendously between species.

 Figure 1: Structure of cellulose

 To produce cellulose acetate, cellulose is firstly extracted from plant sources such as wood pulp and cotton. Purified cellulose is then esterfied under acidic conditions, followed by partial hydrolysis. As the name suggests, cellulose acetate contains acetyl groups in place of the hydroxyl groups found in natural sugars. Cellulose is mixed with acetic acid and acetic anhydride to induce acetylation. Acidic conditions are provided by adding catalytic quantities of sulfuric acid. The level of acetylation can be varied through partial hydrolysis, which re-creates free hydroxyl groups. This changes the degrees of substitution, which in itself is an indicator of the amount of free hydroxyl groups. Along with the desired product, partial hydrolysis produces acetic acid, which can be used in the previous esterification step.

 Figure 2. Esterification of cellulose with acetic anhydride

 Various plasticizers can be used to produce thermoplastic with cellulose acetate. Materials such as diethylene diacetate, sucrose octaacetate and diethyl phthalate were tested for their use as plasticizers. The characteristics of resulting plastics were found to depend on the plasticizer used and the volume of plasticizer used. Currently, phthalate-based plasticizers are commonly used but, unfortunately, these plasticizers are not biodegradable. Consequently, more environmentally friendly plasticizers are under consideration. An example of such alternative is triethyl citrate.