Polycarbonate: Shatterproof Stuff
Think you could sell a polymer material with the clarity of glass that's more impact resistant than PMMA ("acrylic") without the high cost of advanced composites? That's polycarbonate, and like many o...
Think you could sell a polymer material with the clarity of glass that’s more impact resistant than PMMA (“acrylic”) without the high cost of advanced composites? That’s polycarbonate, and like many other “miracle resins” that make up consumer products in 2003, its origin goes back half a century. This year is in fact the 50th anniversary of polycarbonate, which was developed in 1953 by Dr. D.W. Fox at GE and Dr. H. Schnell at Bayer AG, Germany. Like many commodity plastics, the European and American researchers worked independently, yet co-discovered a complex and relatively difficult-to-produce polymer that would form the basis for an industry with a global demand approaching 2 million tonnes today.
Why is the stuff so popular?
Put simply, it has excellent transparency, is very shatter resistant, resists heat and electrical flashover, and is dimensionally stable and biologically inert. And it’s readily recyclable. PC is typically processed by extrusion or injection molding, and applications are numerous. These include glazing, electrical and electronics, optical media, medical devices, appliance housings, auto parts and packaging. If you measured cost on a price-versus-modulus scale, the stuff would be incredibly cheap, but the complexity of the structure of polycarbonate means that production won’t replace polystyrene in my coffee cup any time soon. Here’s what it looks like compared to polypropylene in the chemist’s shorthand. Note that a carbon atom is assumed to be at every intersection or apex of the straight lines.
The (relatively) massive six sided hexagons that chemists call “phenyls” strongly affect the way the chains tangle and pack together and are a major factor in the crystallinity and strength of PC. By the way, polycarbonate is named for the special group of atoms that connect the fat phenyls together and form the long polymer chains. These carbonate groups (yes it’s the same stuff that’s in the sodium bicarbonate in Alka-Seltzer) uses oxygen instead of the more conventional carbon atoms to hook up the chain’s backbone. The difference is very important, because the carbon-oxygen bonds are a little more flexible than carbon-carbon bonds, and that reduces the stiffness that results from the rigid packing forced by those fat phenyl hoops trying to fit together like spoons in a drawer.
The flexibility of the carbonate linkages combined with the rigidity of the main chain creates a “bend but don’t break” defense that gives PC terrific impact resistance. That’s great for crash helmets, but why not PC-everything?
The answer, again, is cost. There are a couple of ways to make polycarbonate resin, and neither is easy. I won’t bore you with the messy details, but with starting reagents like bisphenol A, deadly phosgene and activated diphenylcarbonate, no one is going to make polycarbonate resin in their kitchen sink.