Inside PEEK

Ever wish you could blow the budget and design that ultimate spatula? You know, the seven-dollar swizzle stick or the two thousand-dollar hammer handle? Most of us will never operate with those Government-type cost-no-object targets, but if you need something in a thermoplastic resin that does the unusual, engineering plastics are your material. And amongst this rapidly expanding pool of pricey plastics, PEEK is especially interesting. PEEK stands for polyetheretherketone, and it looks like this:

Regular readers will recall how the commodity thermoplastics discussed in this column have one thing in common: long chains of carbon atoms, expressed in the diagrams as capital “C” or often as the apex of every angle in the confusing geometric shapes that polymer chemists use to simplify things. Each of the hexagons above, for example is actually a ring of six carbon atoms. Polyethylene, by comparison is a simple string of carbons. PE is a little more complicated in other ways, so you commodity guys, don’t think I’m downplaying the chemistry in the pennies per pound trade, but look at the PEEK molecule. It’s huge, complicated, and not easy to synthesize, which partially explains its Cadillac price. If you’re used to thinking in terms of eighty or ninety cents a pound for resin, sit down. Current U.S. pricing converted to Canadian dollars works out to about 45 DOLLARS a pound.

And what do you get when you spend fifty times the price of a low-grade commodity resin? Heat resistance is a major benefit, something for which PEEK is so well known that some regard it as a high-temperature TP rather than an engineering grade. I think it’s both. PEEK is a semi-crystalline linear aromatic polymer, which is chem doublespeak for “tough”. You can put PEEK parts into environments of 260C, with typical heat distortion temperatures of over 300C. That’s over 570F. Impressed? Lead melts at 621F, so this stuff is the gold medallist in heat-resistance among widely available commercial polymers. It’s also slippery, so bearing applications are possible with the right grade, and PEEK is also resistant to water, a wide range of solvents and most acids. About the only commonly encountered substance that degrades it is concentrated sulfuric acid. With an appropriate masterbatch, it can be coloured, and it’s processable using most common techniques like I/M and extrusion. It can survive high-pressure environments including steam, and is naturally flame retardant. When it does burn it doesn’t outgas badly and produces little smoke. And you can use common adhesives like silicones, epoxies, and cyanoacrylate “super” glues. Shrinkage and molding pressures won’t scare anyone who’s worked with nylons, and PEEK melts at 633F with a recommended processing temperature range of something like 650-750F, again, a little higher than you might be used to, but hardly rocket science.

Polyetheretherketone resins were developed by British chemical conglomerate ICI (which has since departed for greener, or more profitable pastures … mainly biotech) in 1982, and like most molders in this country, I’ve never had the chance to work with the stuff. But if I had the money, I’d create the world’s most expensive spatula and revolutionize my barbecue technique. In my world, however, a gaylord of resin doesn’t cost as much as the mold it’s shot into. CPL