PET: The polymer of the masses TPE for overmolding on urethane

As polymers go, it’s as ubiquitous as Coca Cola. It has to be, since that and most other popular soft drinks are packed in it. It is, of course, PET or, more properly, polyethylene terephthalate, and like most commodity polymers, PET is more interesting than you might imagine based on the humble soda bottle. It’s also a lot older than you think.

PET may have been observed by Wallace Carrothers at Dupont in the 30s but the father of nylon (and a tragic figure who would later commit suicide) was preoccupied with his amide research and never developed it.

The official “discovery” belongs to two chemists named Rex Whinfield and James Dickson, who worked for the unlikely-sounding “Calico Printer’s Association” in Manchester, England. The year, however, was 1941, and there were other things on the minds of most Englishmen, mainly World War II, so the polymer passed to the giant ICI chemical concern, which in turn patented their early polyester as “Terelene”. Dupont had missed their chance, but bought U.S. rights to the technology in 1945 and by 1950, a pilot plant in Delaware was producing “Dacron”. By 1953, full-scale production was underway and various forms of PET “polyester” found its way into everything from sportswear to arterial transplants.

There are a couple of ways to make PET, but the main process is what chemists call a “condensation” reaction that combines terephthalic acid (PTA) and ethylene glycol, the main ingredient of automotive antifreeze. The process clips a few atoms off each molecule to form the familiar long chain of atoms that form all plastics. It’s called a “condensation” reaction because the fragments left over combine to form, among other things, water.

Unlike all the other commodity polymers discussed in this column over the past few months, however, the differences between PET and the simpler resins, like polypropylene or polystyrene, isn’t about what atoms or molecules dangle off the carbon chain like charms on a charm bracelet. In the case of PET, the enormous (as molecules go) six-sided ring of carbon that gives polystyrene its properties is incorporated inside the carbon chain. It’s the difference between that charm bracelet and a string of beads, with every third or fourth bead being much larger than the others.

The result? A crystal-clear resin that can be injection molded at over 250C to form consistent, repeatable screw threads, frozen quickly, then blown at about 100C into the familiar soda bottle. And it has excellent barrier properties, keeping oxygen out and carbon dioxide in. Form it into biaxially oriented film and the result is another useful product, commonly known by Dow’s trademark, Mylar. Weave it into tubes, and it can be implanted in place of human arteries. The body will form a thin lining of living cells inside, fooling the body into thinking it has a natural blood vessel.

There are more interesting things that you can do with PET and other polyesters than there’s room to mention, but it all flows from the unusual structure of its chainlike atomic “backbone”. It’s the stuff of the cheap bowling shirt, and the surgical implant. In my book, that’s pretty cool.