Polyethylene Terephthalate (PET)

The biggest secret of 1942 was being built under a squash court at the University of Chicago, where a team led by Enrico Fermi stacked graphite blocks and uranium into a crude pile that, on 2 December, sustained the first controlled nuclear chain reaction in history. Almost nothing about it could be photographed, published, or even discussed outside a small circle with clearance; the whole apparatus existed, for the public record, nowhere at all.

Plate I

Britain was keeping a much smaller secret of its own, for a much more modest reason. Wool and cotton were both rationed by 1942; every adult’s clothing allowance ran on a coupon book, and the Board of Trade’s “Make Do and Mend” campaign was teaching housewives to unpick old sweaters rather than buy new ones. The previous year, two research chemists at the Calico Printers’ Association in Manchester, John Rex Whinfield and James Tennant Dickson, had condensed ethylene glycol with terephthalic acid and pulled from the reaction flask a polymer that could be drawn into a strong, resilient fiber: everything wool and cotton were, without a sheep or a cotton field anywhere near it. Wartime secrecy rules closed over the discovery almost as fast as they closed over Fermi’s pile: the patent was filed in July 1941, but the government sat on its publication until 1946, and Whinfield himself was seconded to the Ministry of Supply before the year was out, leaving Dickson to carry the work forward at Calico Printers.

Plate II

Whinfield had gone looking for exactly this. A decade earlier, Wallace Carothers at DuPont had shown that condensing an acid with an alcohol built a fiber-forming polyester, but Carothers had used aliphatic diacids, and the polymer that came out melted too low and hydrolyzed too easily to compete with the nylon that grew out of the same research. Whinfield and Dickson’s insight was to reach for a rigid, ring-shaped acid instead: terephthalic acid gave the chain a stiff, flat unit at every other link, which raised the melting point far enough, and lowered the water-sensitivity far enough, to make a genuinely useful textile fiber. They called the polymer nothing in particular at first; it would later become Terylene in Britain, Dacron in America, and, stretched into film instead of fiber, Mylar.

The Molecular Marvel

PET’s success lies in its molecular structure. Picture a chain built from two alternating pieces: a short, flexible hinge contributed by ethylene glycol, and a flat, rigid brace contributed by terephthalic acid’s benzene ring. The flexible sections let the chain fold and pack; the rigid sections resist that folding just enough to let regions of the chain line up into crystals when the material is cooled or stretched under tension. That balance (flexible enough to process, rigid enough to crystallize on demand) is the entire reason PET can be blown into a bottle, drawn into a fiber, and cast into a film, each one relying on a different degree of that same crystallization.

Properties that Changed the World

PET sits right at the density of a material that just barely sinks in water, and which of its two forms you are holding matters: quenched quickly, it stays glassy and clear, close to as transparent as the glass it was designed to replace; cooled slowly or oriented under stretching, it crystallizes into an opaque, much stiffer material used for fiber and engineering parts. Either way, its glass transition sits comfortably above room temperature, so a PET bottle or a polyester shirt keeps its shape in ordinary use, and its melting point is high enough that boiling water alone will not touch it. It is also a notably tough plastic for something so rigid, absorbing an impact without the dramatic embrittlement of a poured resin, and hard enough on the surface to resist scuffing.

What made PET a packaging material rather than only a textile one is how little passes through it. Its film is a remarkably tight barrier to oxygen and water vapor, tight enough that a carbonated drink stays carbonated for months in a PET bottle, a property no earlier lightweight plastic offered. It resists dilute acids, greases and everyday hydrocarbon solvents comfortably, absorbs almost no moisture on its own, and holds up as an electrical insulator good enough for film capacitors. Its one real chemical weakness is alkali (strong bases attack the ester linkages directly, hydrolyzing the backbone), which is why PET bottles and fiber are dried thoroughly before melt processing and why the material has no place in contact with strong cleaning alkalis over the long term.

From Molecule to Material

PET is produced through a two-stage process: first, the esterification of terephthalic acid with ethylene glycol, creating bis(hydroxyethyl) terephthalate; this intermediate then undergoes polycondensation under vacuum at a higher temperature, where the molecular chains grow longer as ethylene glycol is driven off. The process requires precise control of temperature, pressure, and catalysts (typically antimony or titanium compounds) to reach the molecular weight each end use needs, since fiber, film and bottle-grade resin are not the same product off the same line.

A Material That Changed Everything

For three decades after Whinfield and Dickson’s discovery, PET was a fiber and, cast as thin film, an electrical and photographic material; DuPont’s Mylar dates to the early 1950s. Its move into rigid packaging came from a single DuPont engineer, Nathaniel Wyeth, who spent much of the late 1960s solving the problem of how to blow-mould a PET bottle strong enough to hold a carbonated drink without bursting; the patent for a biaxially oriented PET bottle was granted in 1973. That patent is the reason a plastic soda bottle exists at all, and it opened PET’s largest market by volume.

Plate III

Today PET’s range spans from carbonated beverage and water bottles to polyester apparel, thermal wear and automotive upholstery, from aluminized film for food packaging to glass-fiber-reinforced grades used in electrical components. It is, by weight, one of the most manufactured polymers on Earth.

The Recycling Question

PET carries resin identification code 1, and of the common plastics it is the one curbside recycling has actually managed to handle well: infrared sorting picks PET containers out of the mixed stream, and the flakes that result are clean enough to spin into new fiber or, after further processing, back into food-contact bottles. Mechanical recycling, melting and reforming the flakes, is the workhorse route; a newer chemical route, enzymatic or catalytic depolymerization back to the original monomers, is being run at industrial scale by companies including Carbios, and promises virgin-quality output regardless of how mixed or coloured the input stream is.

Plate IV

None of that erases the scale of the problem: most PET still goes to landfill or the environment after a single use, and the material that survived a dive to the floor of the Mariana Trench survives just as well in a hedgerow. But among plastics it is the one the recycling system was actually built to work with, which is more than can be said for most of what shares a supermarket shelf with it.

From Fiber to Fashion

In textiles, PET is simply polyester, melted and extruded through fine holes, then stretched to align the molecules along the fiber axis, which is what gives polyester its wrinkle resistance and durability. It has gone from a wartime wool substitute to the material behind moisture-wicking sportswear and, increasingly, fabric spun from recycled bottles rather than virgin resin. This is a fiber that has come, eight decades on, roughly back to where its own raw material began.