How polypropylene is made

Giulio Natta found in 1954 that the right catalyst makes propylene line up its molecules like teeth on a zipper.

stations: 5
things upstream: 6
raw sources: 3
steps from the ground: 4

The short version

Polypropylene is made by linking propylene gas molecules into long chains in a reactor, using a catalyst that lines every unit up the same way. The powder that results is melted, mixed with stabilizers, pushed through a die and chopped into pellets. Other factories mold, spin or extrude the pellets into finished parts.

Purify the propylene

Propylene is cleaned of water, sulfur and other poisons that would foul the catalyst. The feed has to be clean for the chains to grow long.

Goes in
Propylene
Grow chains on a catalyst

In the reactor, propylene meets a Ziegler-Natta catalyst. The catalyst makes each new unit attach in the same orientation, which gives isotactic chains that pack tightly and make a stiff, partly crystalline plastic.
Strip off leftover gas

Unreacted propylene is flashed off and sent back to the reactor. The powder left behind is dried.
Melt in stabilizers

An extruder melts the powder and mixes in heat stabilizers and other additives. Without stabilizers the plastic would degrade in sunlight and heat.
Cut it into pellets

The molten strands are chopped into pellets as they leave the die. Pellets are cooled, dried and bagged for sale.

End of the linePolypropylene Next it goes into toothbrush, deodorant, mascara and 20 more.

Worth repeating

Polypropylene is the second most widely produced commodity plastic, after polyethylene.
Giulio Natta and Karl Rehn discovered the stereoselective polymerization of propylene in March 1954, and Montecatini began large-scale production in 1957.