Polyethylene (PE) was invented by accident 75 years ago. One reporter traces the world's most ubiquitous plastic and the role her great uncle played in its discovery
I WAS vaguely aware that I had a great uncle who had been involved in the invention of polyethylene (PE). So I jumped at the chance to chart the development of today's highest-volume commodity plastic.
My great uncle, Edmond Williams, joined UK chemical company ICI in 1934 and became part of a small team of researchers working on a speculative project to find out more about the properties of a white, waxy solid that had been discovered a year earlier.
The waxy solid had been formed during high-pressure experiments conducted in 1933 by ICI scientists Reginald Gibson and Eric Fawcett. They had heated a mixture of ethylene and benzaldehyde to 170°C (338°F), using apparatus that could submit materials to a pressure of 1,900 atmospheres (1,925 bars). But the reactions were explosive and safety concerns prompted the now defunct ICI, which merged into Dutch-based Akzo Nobel, to halt the research.
Fawcett had been disappointed that the research was not allowed to continue, and his attempts to make the scientific community recognize his and Gibson's achievements led in September 1935 to what became known as the "Fawcett disclosure," records Carol Kennedy in her book ICI: The Company that Changed Our Lives.
At a major conference, attended by some of the world's most eminent scientists, Fawcett told delegates that he had made a solid polymer of ethylene, with a molecular weight of about 4000. But the consensus at the time was that ethylene could not polymerize because the double bond could only be activated at very high temperatures, explains Valentina Brunella, a polymer chemistry scientist at Italy's University of Turin.
In December that year, Williams and colleagues Michael Perrin and John Paton reinvestigated the experiments of Gibson and Fawcett using ethylene alone. Under similar experimental conditions - but with better equipment - they observed a pressure drop, and when the reaction finished there were 8.5g of white PE powder.
Williams, Perrin and Paton had been lucky. The vessel had leaked and, it was later confirmed, a trace of oxygen was present in the fresh ethylene that had been added to the reaction vessel to replace the leaked gas. The fresh ethylene contained, by chance, the right amount of oxygen to act as an initiator.
"When it first happened, it was a fluke," recalled Frank Bebbington, a laboratory assistant working on the experiment. He was speaking at a Royal Society of Chemistry meeting in 2004 to commemorate the discovery of PE, also known as polythene.
A SWEET DISCOVERY
"I'll never forget the day we realized we'd got something pretty important," Williams told UK magazine Woman's Own in 1961. "We'd been heating a certain substance to well over boiling point. For once it didn't explode - usually it did - and we thought something must be wrong. So we left it to cool overnight. And when I looked inside the metal container the next day, I found what looked like a lump of sugar. In fact, that 'sugar' was polythene."
A member of ICI's dyestuffs division, Bernard Habgood, recognized that PE could supersede gutta-percha, a natural material, for insulation of submarine cables. This provided the impetus to proceed to commercial scale production. The first full-scale PE plant, with a 100 tonne/year capacity, went into production on September 1, 1939, the day Germany invaded Poland and war became unavoidable for Britain.
ICI's work on PE changed during the Second World War, when the material was used to insulate airborne radar equipment. During the development of radar in the early war years it had proved difficult to insulate the equipment to prevent power loss and thus preserve the strength of the signal. PE's electrical insulation properties enabled the British forces to reduce the weight of radar equipment and allowed them to place radars inside fighter planes. This provided an enormous technical advantage in long-distance warfare, most significantly in the Battle of the Atlantic against German submarines. The Germans were obliged to use a bulkier insulating material for their radar, which was less effective.
Williams worked with UK government experts and the cable and electrical industries during the war to develop the use of PE in radar, and travelled in secret to the US to liaise with the Americans. "One of the greatest difficulties in those days was the problem of leakage at the points where the cables joined the various parts of the radar system," he said in an ICI newsletter article published in 1965, following his appointment as chairman of the plastics division. "Much of my work was therefore concerned with developing molding techniques for the new material."
PE was top secret during the war but emerged shortly afterward as a commercial product. Williams recalled proposing with colleagues that PE could be used to make bowls and plates. "They threw it back at us," he told Woman's Own. "'Useless,' they said. 'You can't possibly make bowls from a flexible material.'"
Uses for early PE were limited, as the material was soft and had a low melting point. This was because under the high pressure polymerization process the ethylene molecules did not always add in a regular chain.
German professor Karl Ziegler found a way to polymerize ethylene at low pressure using catalysts in 1953. With colleagues, he discovered it was possible to make ethylene molecules join up in a more disciplined manner on the surface of particles of "Ziegler catalyst," and without the high pressures and temperatures previously required. This produced a PE that was more rigid than high pressure PE and could handle boiling water.
In contrast to the poor reception of Gibson and Fawcett's work, the scientific and business communities embraced Ziegler's low-pressure PE, known today as high density polyethylene (HDPE).
US group Phillips Petroleum was also working on PE in the early 1950s, and discovered that supported reduced chromium oxide catalysts also produced the polymer at low pressures. Phillips quickly commercialized the product as Marlex and licensed the technology. The Phillips catalyst is cheaper and easier to handle than that of Ziegler, but requires medium pressure and therefore more engineering, observes Brunella.
Many chemical companies rushed to manufacture the new PE, but as the plants were coming onstream problems with the polymer started to emerge, says Brunella. Even at room temperature, cracks appeared in bottles or pipes after several months.
The solution was to create PE with a small amount of side branches in the chain (less than in ICI's high pressure PE, known today as low density polyethylene [LDPE]) by adding small amounts of other gases to ethylene. It was named medium density polyethylene (MDPE).
HULA HOOP KEEPS FIRMS ALOFT
In the meantime, companies were facing economic disaster - until the hula hoop craze in the late 1950s came to the rescue. Toy firm Wham-O started manufacturing the product, an extruded tube bent into a circle that is twirled around the waist, out of Marlex in 1958. Production of the toy rapidly consumed the otherwise useless PE stocks and gave companies breathing space to make the necessary changes to their processes, says Brunella.
PE has come a long way since Gibson and Fawcett's exploding experiments 75 years ago. New metallocene catalysts have opened up many opportunities for the polymer. Today it is used today in an array of applications, from supermarket bags to bullet proof vests, and as flexible packaging it has helped change the nature of food storage.
A true thermoplastic, it can be injection molded, extruded or blow molded. But it was perhaps the polymer's critical role in the development of radar that my great uncle, who died in 1972, would be most proud.