Cellulose Acetate

Synthesis of Celluose Acetate

Cellulose acetate is a thermoplastic made from natural materials, usually cotton or wood pulp. Potentially, it could also be made from household or agricultural wastes high in cellulose such as corn stover. It has a melting point of 260oC, and softens at around 97oC , which is higher than the conditions most plastic products will be used in.

The conventional method of synthesizing cellulose acetate involves reacting purified cotton linters with acetic anhydride.

1. The cotton linters are first washed then dried.
2. They are then soaked with glacial acetic acid to open up the cellulose structure and to allow for a better reaction with the acetic anhydride.
3. Acetic anhydride is added with a small amount of concentrated sulphuric acid as catalyst.
4. The mixture is constantly stirred for hours. As the reaction takes place, heat is produced, so the temperature must be kept constant or else the reaction would go out of control and a useless product would be produced.
5. When the mixture has become a viscous, yellow paste, it is removed from the mixing chamber and a small amount of water is added to allow it to mature.
6. After several days, the mixture is precipitated in water to form cellulose acetate flakes.
7. The flakes are washed to remove any residual acid, and then dried.

Originally, we attempted to synthesize cellulose acetate ourselves using the lab at our school. However, we found that this was not possible without professional equipment to control the temperature of the mixture and constant stir such a viscous mixture for extended periods of time.

At first, we attempted to synthesize cellulose acetate using methyl cellulose by mixing:

• 1.50g methyl cellulose
• 6.0mL acetic anhydride and
• 6 drops of 3.0M sulphuric acid in a 250mL beaker.

A violent reaction occurred and we observed that

• A sour smelling gas rose from the beaker
• The mixture increased rapidly in temperature, warming the sides of the beaker
• The mixture turned into a dark purple-brown coloured solution.

After leaving the solution overnight, we precipitated it in 400mL of cold water. At first, no precipitate appeared to form, but after approximately 3 hours, the mixture became cloudy and we were able to filter out a small quantity of white flakes. We did not yield enough precipitate to be able to dissolve it in acetone and create an actual piece of plastic.

We then attempted to synthesize cellulose acetate using cotton fibres. However, the cotton never completely reacted and no suitable results were produced. The likely reason for the failure of our attempts was the lack of laboratory equipment we were able to work with. The temperature of the acetylating mixture must be carefully controlled at all times, or else a useless product will result. As well, the acetylating mixture must be mixed thoroughly for hours, which we were unable to do because the mixture was far too viscous for our school's magnetic stirrers.