How polyvinylpyrrolidone is made

The polymer that holds a hairstyle in place was first put to work as a stand-in for blood plasma.

stations: 8
things upstream: 8
raw sources: 3
steps from the ground: 3

The short version

Better known as PVP or povidone, this water-soluble polymer starts as acetylene from burned methane. A chain of plant reactions builds a five-membered ring from it, using formaldehyde, hydrogen and ammonia, and a second dose of acetylene gives that ring a vinyl group. Radical initiators then link the monomer into long chains in water, and spray drying yields a flaky powder.

Burn methane to acetylene

Partial combustion of methane is the main way the US and much of the EU have made acetylene since the 1950s. This gas supplies carbon twice along the line.

Goes in
Natural gas
Build the butanediol chain

In one industrial route, each acetylene molecule reacts with two of formaldehyde to give butynediol. Hydrogenation then saturates it into 1,4-butanediol, a four-carbon chain with an alcohol group at each end.

Goes in
Formaldehyde Hydrogen
Close the lactone ring

A dehydrogenation unit pulls hydrogen off the chain and curls it into a five-membered ring, the liquid intermediate gamma-butyrolactone. It goes straight on to the next reactor.
Swap in nitrogen

The lactone and ammonia pass as vapor through a tubular reactor packed with solid magnesium silicate. Nitrogen replaces the oxygen in the ring, giving 2-pyrrolidone. Distillation lifts it to 99.5% purity.

75 to 85% yield

Goes in
Ammonia
Attach the vinyl group

Acetylene returns and reacts with the ring under a base catalyst, adding the carbon double bond that can later link up. The result is N-vinylpyrrolidone, a colorless liquid that often looks yellowish in commercial lots.
Link up the monomer

Monomer and water, sometimes with alcohol, go into a stirred reactor, and a free radical initiator such as hydrogen peroxide starts the chains. More alcohol means shorter chains, which is how the plant sets the grade, reported as a K value.

K 10 to K 100

Goes in
Water Hydrogen peroxide
Strip out leftovers

Distillation or vacuum removes the alcohol, and water often takes its place so the product can leave the reactor as a solution. In one patented process, acid breaks down remaining monomer until only 0.1% is left.
Spray dry the solution

The aqueous solution ships as it is or goes through a spray dryer to become a light, flaky powder. Hair care makers dissolve it again, because it dries from solution into a film.

End of the linePolyvinylpyrrolidone Next it goes into hairspray.

Worth repeating

Dry PVP powder can pull up to 40% of its own weight in water out of the air.
Betadine, the disinfectant, is a complex of iodine and this polymer.
The wine industry uses it as a fining agent for white wine, and some brewers do the same for beer.