Propylene
Ethylene's more useful sibling and its perpetual shortage — one carbon longer, and the source of polypropylene.
Propylene is ethylene with an extra carbon, and the difference is not cosmetic: the methyl group hanging off the chain is what makes polypropylene stiffer than polyethylene and gives it a melting point some fifty degrees higher. It is the second-largest petrochemical by volume.
Its defining commercial fact is that it has historically been a by-product. A naphtha cracker makes it alongside ethylene whether or not the market wants it; an ethane cracker barely makes it at all. So when North American crackers switched to cheap shale ethane, propylene became scarce and dedicated plants — propane dehydrogenation — were built to make it on purpose.
Processing
As a steam-cracking co-product, as a by-product of refinery catalytic cracking, and increasingly on purpose by dehydrogenating propane. The third route exists because the first two are tied to demand for something else.
Uses
Polypropylene takes about two thirds. The remainder goes to propylene oxide for polyurethane foams, to acrylonitrile for ABS and acrylic fibre, to cumene and on to phenol and acetone, and to the acrylic acid behind superabsorbent polymers.
History
Polypropylene resisted polymerisation into anything useful until the mid-1950s: the side groups arranged themselves at random and the product was a gum. Giulio Natta, building on Karl Ziegler's catalysts, found how to line them up, and the resulting regular chain crystallises into a genuine engineering plastic. The two shared a Nobel Prize for it in 1963.
That the same monomer gives either a useless gum or one of the world's most-used plastics, depending only on the geometry the catalyst enforces, is the clearest demonstration there is that a polymer's properties are not simply its composition.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is produced by
- Steam cracking — process · a co-product from naphtha and barely a product at all from ethane, which is why dedicated propylene plants exist
is an input to
- Polymerisation — process · into polypropylene, but only with a catalyst that controls the side-group geometry; without one the product is a useless gum
is a source for
- Polypropylene — material · polymerised with a Ziegler-Natta or metallocene catalyst that lines the methyl groups up
contains
is commonly confused with
- Ethylene — compound · one carbon apart, made in the same plant, and the difference decides whether the product is polyethylene or polypropylene
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)