How cellulose acetate is made

LEGO bricks were made of this wood-based plastic until 1963. Your eyeglass frames may still be.

stations: 6
things upstream: 23
raw sources: 8
steps from the ground: 6

The short version

Purified wood pulp or cotton is reacted with acetic acid and acetic anhydride, turning a fiber that will not melt into a plastic that will dissolve. A controlled partial reversal then gives the acetone-soluble form. From there it is spun into fiber for cigarette filters and playing cards, or mixed with plasticizer for sheets and molded parts.

Fluff the purified pulp

Special dissolving pulp, or the short linters on cotton seeds, is broken down into a purified fluffy white cellulose. Ordinary pulp reacts unevenly and spoils the product.

Goes in
Wood pulp
Acetylate the cellulose

Cellulose goes into acetic acid and then acetic anhydride, with sulfuric acid as a catalyst. Each repeating unit has three hydroxyl groups, and the reaction turns these into acetate groups.

Goes in
Acetic anhydride
Partly undo the reaction

Controlled hydrolysis takes off the sulfate and some of the acetate. The common form ends up with an acetate on about two of every three hydroxyls. George Miles found around 1903 that this makes it soluble in cheap solvents.

Goes in
Water
Dry into flakes

The material is recovered and dried as flake. In this form it can be stored and shipped, and later turned into fiber, film or molding powder.
Dissolve and spin fiber

Flakes dissolve in acetone to make a thick solution that is pushed through spinnerets. Warm air evaporates the solvent and leaves fine filaments, the same dry-spinning idea used for spandex.

Goes in
Acetone
Or plasticize for sheet

For eyeglass frames and molded parts, flake is mixed with plasticizers into a moldable powder or cast into film. It melts at 230 °C, lower than the fully acetylated form.

Goes in
Plasticizer

End of the lineCellulose acetate Next it goes into sunglasses, dice, shoelace and 1 more.

Worth repeating

Old acetate film breaks down and releases acetic acid, a decay that archivists call vinegar syndrome.
Camille and Henri Dreyfus did their early research on cellulose acetate from 1904 in a shed in their father's garden in Basel.
During the First World War, acetate dope was used to coat the fabric on airplane wings.