How polyvinyl alcohol is made

Its building block does not exist in a usable form. So chemists make a different plastic and rewrite it afterward.

stations: 5
things upstream: 11
raw sources: 5
steps from the ground: 4

The short version

Vinyl alcohol, the obvious starting molecule, is unstable and rearranges into acetaldehyde. Plants instead make vinyl acetate from ethylene, link it into polyvinyl acetate, dissolve that in alcohol, and use a catalyst to swap the acetate groups for hydroxyl groups. The chain survives intact, and the finished plastic dissolves in water, which is why it wraps laundry pods.

Make vinyl acetate

Ethylene, acetic acid and oxygen react over a palladium catalyst. This route has replaced the older one based on acetylene. About 80% of this monomer goes on to make the polymer in this line.

Goes in
Ethylene Oxygen gas
Link up the monomer

A free-radical reaction joins the molecules into polyvinyl acetate, the same polymer found in white wood glue. It has long been valued as an adhesive in its own right.
Dissolve in alcohol

The polymer is dissolved in an alcohol such as methanol or ethanol, ready for the swap. An alkaline catalyst such as sodium hydroxide is added.

Goes in
Methanol Caustic soda
Swap off the acetate

The catalyst removes the acetate groups and leaves the long chain undisturbed. Running this to completion gives a product that dissolves freely in water. Stopping early gives grades less soluble in water and more soluble in some organic liquids.
Dry into beads

The finished polymer is supplied as white, odorless beads or as a solution in water. Buyers use it for textile sizing, paper coatings, adhesives and water-soluble films.

End of the linePolyvinyl alcohol Next it goes into polyvinyl butyral, banknote.

Worth repeating

Laundry detergent pods use polyvinyl alcohol because its film dissolves in the wash.
Water absorbed in humid air acts as a plasticizer in polyvinyl alcohol, weakening it but letting it stretch further.
World consumption was over one million metric tons in 2006.