PVC: One Man’s Trash …
If ever the old adage that "one man's trash is another man's treasure" held true it just had to be the development of polyvinyl chloride. PVC is so useful in modern society it isn't worth mentioning h...
If ever the old adage that “one man’s trash is another man’s treasure” held true it just had to be the development of polyvinyl chloride. PVC is so useful in modern society it isn’t worth mentioning how many consumer goods use this commodity polymer, but what amazes me is how some people (especially chemists) can see lemons and others lemonade.
PVC, for example, was discovered not once, but twice. But before I relate that odd tale, just what is PVC anyway?
Regular readers of this column will recall that the polymers I’ve described over the last several months are made up of long chains of carbon atoms with either nothing unusual dangling from the chain (polyethylene), a small extra side carbon atom (polypropylene) or a fat hoop earring of carbon atoms (polystyrene). In each case, the story is essentially carbon, with a few minor hydrogen atoms hanging around for good measure. The carbon, by the way, is the reason for the black residue when plastics burn, the same carbon that results from the burning of other carbon-containing chemicals like table sugar.
The carbon is essential because it forms a strong backbone to the polymer chain, but what about the side groups? It turns out that they don’t have to be carbon, and in the case of PVC, they’re atoms of chlorine.
Chlorine has a couple of interesting properties that give PVC its useful properties. In side groups, size matters (remember polystyrene) and chlorine is pretty fat by atomic standards–about three times the mass of carbon. It’s also loaded with reaction provoking “valence electrons”, unlike carbon, which is the reason why chlorine is useful at disinfecting pools and pairing off with other chlorine atoms to form deadly chlorine gas.
Safely bound to the carbon polymer chain, however, there’s not only no problem, but the chlorine adds an unusual property: set fire to PVC, and freed chlorine atoms inhibit the combustion reaction trying to recombine the carbon with oxygen. Try this in your backyard with a piece of scrap siding, and you’ll see that the stuff is essentially self-extinguishing, making PVC a natural for consumer products and construction extrusions. The flame retardancy mechanism of the chlorine is complicated, but it works. A little of the freed chlorine also combines with other elements, but I’ll leave that chemistry lesson to Greenpeace.
And why was this wonder polymer discovered twice? PVC emerged in Germany in 1912 when a chemist named Fritz Klatte was experimenting with acetylene. Acetylene chemistry was hot at the time, especially since the then-new electric grid had knocked the bottom out of the acetylene lighting business, leaving literally tons of the stuff laying around. Klatte mixed the acetylene with hydrochloric acid and discovered the gooey mess called vinyl chloride. Without knowledge of “free radical polymerization”, the vinyl monomer stayed in its pre-polymer form, and Klatte literally put it on the shelf. In time however, polymerization occurred in the flask, and Klatte had PVC. His company, Greisheim Electron, patented PVC in Germany only, then let the patent expire in 1925.
A year later, B.F. Goodrich chemist Waldo Semon, working on ways to bond rubber to metals, rediscovered PVC independently, and successfully plasticized it, making it useful for a huge variety of consumer and industrial goods. Semon’s company knew a good thing when they saw it and patented it immediately. And the rest, of course, is history.
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