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Polyethylene: discovery and growth
Article · January 2003
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Polyethylene: discovery and growth
Luigi Trossarelli and Valentina Brunella
Dipartimento di Chimica IFM dell’Università di Torino
Via Pietro Giuria 7, 10125 Torino (Italy)
Correspondence addresses:
Prof. Luigi Trossarelli
Dipartimento di Chimica IFM
Via Giuria 7
10125 Torino Italy
luigi.trossarelli@unito.it
Introduction
Among the many important developments in chemical technology during the last century,
one of the most important has been the use on a large scale of synthetic polymeric materials, or
more popularly plastics, and polyethylene is one of them.
In less than five years, and more precisely starting in 1930, three new polymeric materials
that since then had a large impact on our existence, were discovered as unexpected results of a
research project. They are:
•
polychloroprene (commercial name Neoprene), 17 April 1930, the first synthetic
elastomer more similar to natural rubber industrially produced (E. I. DuPont de Nemours and
Co., U.S.A.);
•
polyethylene, 27 March 1933 (Imperial Chemical Industries Ltd., U. K.);
•
nylon, 1st March 1934, the first totally synthetic fiber industrially produced (E. I.
DuPont de Nemours and Co., U.S.A.).
The use in so many fields of polymeric materials has given the plastic industry an important
place in the economy of any industrialised nation. Since approximately fifty years, polymeric
materials are no longer considered as substitutes of the traditional ones, but they have
recognized specialistic uses of their own. Today polymeric materials can be synthesised
deliberately with a molecular composition and structure designed in advance to give peculiar
required properties.
The first solid polymer of ethylene (liquid polymers of ethylene were known since 1869),
which is the first and the simplest member of the vinyl polymer family, the so called high
pressure polyethylene, was discovered as the unexpected result of researches initiated with no
such an objective and carried out by a team who had no particular knowledge of the
macromolecular field. The same holds true in the case of the discovery of the low pressure
polyethylene twenty years later.
As we will see, chance has played an important role in the polyethylene story. As time
passes, however, there is a natural tendency to idealise the story and to present it in a form
suggesting a logical growth from the start of the general research programme to the discovery
and development of the product. Making the due allowance for the element of chance,
polyethylene can indeed be regarded as a successful outcome of research work carefully planned
and properly executed and it can be taken as an example of the benefits coming from the
collaboration between those working in industrial research laboratories and those in academic
ones.
7
High pressure polyethylene
The story of polyethylene starts in the early 1930s in Great Britain where, as in the whole
industrialized world, there was the deeply recession time due to the Wall Street crash of the
previous year and the research management of the research laboratories of I.C.I. in Winnington
(Cheshire), opened in 1928, suggested to the physical chemistry group the pursuance of work
involving special techniques. This suggestion led to a project to investigate what happens at high
temperatures, under high vacuum or at high pressures.
Two researchers, Eric William Fawcett and Reginald Oswald Gibson began to investigate,
under the direction of John Cuthbert Swallow, the phenomena occurring in the field of high
pressures, namely pressures of 1000 atm and higher and, among them, the possible effect of high
pressure on chemical reactions. The reactions to be studied were not selected because they might
be expected to be strongly affected by pressure on theoretical grounds, but rather it was hoped
that compounds, that normally do not react, might react or that reactions, which normally need
catalysts, might occur without them at high pressures. Fifty different chemical reactions were
studied without any success, but one of the failures resulted, through a series of coincidences, in
the discovery of polyethylene.
Figure 1. Eric W. Fawcett (left) and Reginald O. Gibson (right).
In one experiment involving toluene and ethylene, toluene became noticeably turbid and
Fawcett succeeded in isolating traces of a white powder, but only some months later he knew it
was polyethylene. On Friday 24 March 1933, Fawcett and Gibson started a reaction of ethylene
with benzaldehyde. The temperature was 170 °C, the pressure 1900 atm and the apparatus was
left overnight. On Saturday morning the pressure was over 1800 atm, and since there was the
suspect of some loss of gas, it was raised again to over 1900 atm and the apparatus was left over
8
the week-end. On Monday 27 March 1933 in the morning, it was found that the pressure had
fallen right down because a leak and all the benzaldehyde had blown out of the reactor in the oil
of the thermostat. When the bomb (Figure 2) was dismantled, Fawcett observed that the tip of
the steel U-tube (B in Figure 2) was coated with a waxy material and Gibson recorded in his
rough notebook: Waxy solid found in reaction tube. It is this rough note (Figure 3) which has
been accepted as the first recorded observation of the formation of polyethylene.
Figure 2. Scheme of the reaction vessel in which polyethylene was first discovered.
Fawcett, who was an organic chemist, collected this substance (about 0.4 g). By two
microanalyses the empirical formula CH2 was established and by determination of the elevation
of the boiling point in benzene a molecular weight of 3700 or higher was found. The substance
did not contain oxygen and, as a consequence, benzaldehyde did non react with ethylene. This
substance was indeed the first solid polyethylene obtained.
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Figure 3. Rough notes of the experiment in which polyethylene was first discovered. In the right page at about
2/3 from the top one reads: Waxy solid found in reaction tube.
Attempts to repeat the experiment as well as to increase the yield were unsuccessful, and
often explosive decomposition of ethylene occurred. Such accidents might have caused severe
damages in the laboratory and the Research Manager at Winnington decided that no more
experiments with ethylene should be done until more suitable equipment was ready and had
been installed in a laboratory with adequate safety arrangements. On 7 April 1933 it was
reported to the Dyestuff Group Research Committee that the work on the reaction between
ethylene and benzaldehyde at 2000 atmospheres has been abandoned. The first experiment gave
a wax-like substance, probably polymerised ethylene, but a repetition resulted in an explosion
which smashed the gauges. Polyethylene slept for about three years.
When the Faraday Society announced that it was to hold a General Discussion in Cambridge
in September 1935 on the “Phenomena of Polymerisation and Condensation”, the first major
conference on polymer science to be held in UK, Fawcett obtained permission to attend and to
make it known that at Winnington a high polymer of ethylene was made. Fawcett noted that H.
Staudinger in his paper to be presented on the first day described ethylene as a stable compound
which polymerises with difficulty giving only low molecular weight mixtures of hydrocarbons
10
and he thought it would be courteous to tell Staudinger about the I.C.I. work before he presented
his paper. Staudinger did not believe Fawcett and would not discuss the matter. In the discussion
on Staudinger’s paper the next day, H. Mark invoked some theoretical arguments to explain why
ethylene does not polymerise. Then Fawcett got up and told the Conference that he made a solid
polymer of ethylene, with a molecular weight of about 4000, by heating ethylene to170 °C at
about 2000 atm. This disclosure elicited no reaction from the people present, the cream of
England and world polymer scientists, and Staudinger, even when prompted by the chairman,
declined to comment.
Late in 1935 the experiments of Fawcett and Gibson were reinvestigated by a different team
in a safer laboratory and with better equipments. On 20 December 1935 M. W. Perrin and his
associate J. P. Paton set about their first experiment with ethylene alone under the same
experimental conditions (170 °C and 2000 atm) used by Fawcett and Gibson. The course of the
event was rather different. After some time the pressure started to fall slowly but steadily. This
might have been due to ethylene polymerisation, as Perrin hoped, or to ethylene escaping
through a leaky joint as their assistant F. Bebbington, who had the job of raising the pressure
back to 2000 atm, suspected. When all the ethylene in the secondary compressor was finished,
the reactor was cooled and opened, and there were 8.5 g of a white powder, namely
polyethylene.
Perrin and his associates were lucky, just as Fawcett and Gibson had been three years before
in their first experiment. Much later it was realized that there must have been oxygen in ethylene
which initiated the polymerisation. There was a leak as well, since the yield of polyethylene did
not account for all the ethylene used, but that too was fortunate because the ethylene fed in
probably brought with it an extra oxygen needed to sustain the reaction.
In the second experiment by Perrin and co-workers, when the pressure was raised to 3000
atm, there was a decomposition, but they carried on and in some other experiments they had
yields up to 30 g.
The first provisional patent was filed on 4 February 1936 and others followed as they
recognized the importance of removing the heat of reaction to avoid decomposition, of varying
the pressure to control the reaction rate and the molecular weight of the polymer and, most
important of all, the role of oxygen. It was the oxygen present as an impurity in the ethylene that
had acted as the initiator of the polymerisation, but too much oxygen gave rise to explosive
reactions.
Having more polymer available than Fawcett and Gibson, Perrin and his associates easily
confirmed that it was indeed a crystalline high polymer of ethylene, with the properties
11
established with a lot of difficulties by Fawcett two years before: it could be moulded, made into
fibre and films and, because of its chemical structure, to have interesting electrical properties
and to be chemically inert.
The trade name Alketh, later changed to Alkathene, was registered and all groups of I.C.I.
who might be interested in or could advise on potential applications were visited. By the end of
1936 enough was known about the polymerisation reaction to suggest that it could be scaled up
and controlled. The discovery phase of polyethylene for the types that could be made by free
radical polymerisation under high pressure was over.
After polyethylene re-discovering the commercial aspects were fully considered and
prospective customers were approached as material became available. Within the first two years
practically every use had been demonstrated except domestic articles. The concept of a tough
but flexible material was too new, although lightweight unbreakable battery cases for
submarines were investigated.
Remarkable luck helped in finding a large potential market for polyethylene at a critical point
of its development. A member of the I.C.I. (Dyestuff) staff, B. J. Habgood, who recently had
first-hand knowledge of the cable industry, recognized that polyethylene could supersede guttapercha for insulation of submarine cable and this gave the impetus to proceed to the commercial
scale. An assessment showed that polyethylene could be produced competitively with guttapercha. In fact the cable for which the first order (100 tons, September 1938, delivery by the
middle of 1939) was placed was never laid because of the beginning of the II World War 19391945 (1st September 1939 Germans invaded Poland and the next day Great Britain and France
declared war on Germany). The outbreak of war, however, had a great effect on the subsequent
story of polyethylene, both in regards to development and production, since its application in the
high frequency equipment used in radar, for which its electrical and mechanical properties made
it particularly suitable, and radar applications absorbed most of the output almost to the end of
the war. The availability of this insulator allowed the allies to use airborne radar, which gave an
enormous technical advantage in long distance air warfare, most significantly in the Battle of the
Atlantic against german submarines. Because of this, polyethylene became a top secret during
the war.
Polyethylene emerged shortly afterwards as a commercial product and rapidly found many
uses, but it was soft and low melting (you could not put boiling water into a polyethylene jug, or
it would tend to collapse). The reason for this is that, ideally, the polyethylene chain would be a
long chain of carbon atoms (Figure 4).
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Figure 4. Ideal polyethylene chain.
Under the conditions of high pressure polymerisation, sometimes the ethylene molecules did
not add on in a regular fashion (intramolecular chain transfer reactions), and so put short
branches (2-5 carbon atoms) in the polymer chain (Figure 5):
Figure 5. Short branches in polyethylene chain.
This stops the chains packing together regularly and, as a consequence, crystallinity, density
and melting point decrease. Few long side chains are also formed as a consequence of chain
transfer reactions to the polymer and these have influence on melt viscosity and consequently on
polymer processing.
Low pressure polyethylene
In 1898 a son was born to a clergyman called Ziegler who lived near Kassel (Germany) and
this child was named Karl.
Karl Ziegler received his doctorate in 1923 from the University of Marburg and held
academic appointments in the Universities of Frankfurt am Main and Heidelberg. In 1936
Ziegler’s wandering in search of a professorship came to a successful end with the headship of
the Department of Chemistry at Halle.
Ziegler’s reputation grew steadily, and in 1943, when the directorship of the Kaiser Wilhelm
Institute für Kohlenforschung at Mülheim an der Ruhr (later the Max Plank Institute für
Kohlenforschung) fell vacant, he was chosen. At first he refused saying “I do not know anything
about coal, I never did anything with coal in my life and I do not want to”. It is an astonishing
fact that in the middle of the 1939-1945 war, when all Germany’s resources were committed to
the war effort, Ziegler agreed to become Director at Mülheim on his own terms, which were that
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he had complete personal freedom in the direction and choice of research and in publication, and
he retained ownership of all inventions which were not concerned with coal. Ziegler shared with
Giulio Natta the Nobel Prize for chemistry awarded in 1963 for his discovery of coordination
catalysts which greatly enhanced the progress of polymeric materials. Karl Ziegler died in
August 1973 in Mülheim and der Ruhr.
Figure 6. Karl Ziegler.
The discovery of coordination catalysts for the low pressure polymerisation of ethylene by
Ziegler and his co-workers is again one of the most fascinating stories in the history of polymer
science.
Still pursuing his interest on metal-organic compounds, in 1950 Ziegler discovered what he
called the “Aufbau” reaction in which ethylene was converted by aluminium alkyls into a
mixture of linear unbranched α-olefins whose chain length depends on the number of ethylene
molecules which were jointed together before the aluminium complex broke away from the
growing molecule to start another one.
When his friend H. Mark called on him, Ziegler told him about the “Aufbau” reaction and
asked Mark to tell him something about really high molecular weight polymers. This time Mark
did not ignore the story or try to explain why aluminium alkyls could never make high
polymers. He was very interested and when he got to London he told his friend Sir Robert
Robinson all about it. Sir Robert was at the top of his career: a Nobel laureate, Past President of
the Royal Society, honoured nationally by the award of the Order of Merit. Since he was on the
Board of Petrochemicals Ltd. as a nominee of the Finance Corporation for Industry, he
14
persuaded the company to seek and obtain exclusive licence in the UK under Ziegler’s patents
and future developments.
It was late at the end of 1953 that linear, high molecular weight polyethylene was first made,
at normal temperature and pressure in Ziegler’s laboratory in Mülheim an der Ruhr.
These are the events. Three of Ziegler’s doctoral students were assigned to study the factors
limiting chain propagation (the “Aufbau” reaction) in the interaction of trialkylaluminium with
olefins. One of them, E. Holzkamp, embarked on a study of the reaction of tripropylaluminium,
(CH3CH2CH2)3Al, with ethylene. In this case the insertion of ethylene into the aluminiumcarbon bond should lead exclusively to chains with an odd number of carbon atoms (propagation
reaction), whereas the displacement of an alkyl chain by ethylene (displacement reaction) should
give chains with an even number of carbon atoms. Holzkamp found that at 70 °C only propylene
and butene were obtained, indicating that the displacement reaction was the dominant process.
This completely unforeseen and surprising result was eventually traced to the use of one specific
autoclave for the reaction and a painstaking investigation led to the implication of traces of
nickel salt which had been dissolved from the stainless steel when it was cleaned with acid and
were reduced by the alkyl aluminium compounds to colloidal nickel. Holzkamp concluded that
this nickel catalyst of the displacement reaction was responsible for the earlier failure to obtain
very long alkyl chains in the reaction of triethylaluminium with ethylene and suggested to
investigate as many other metals as possible.
Ziegler, who was particularly interested in catalysts converting ethylene into butene in the
presence of triethylaluminium and not in polymer formation, asked one of his students, H. Breil,
to systematically test, as his thesis problem, the entire periodic system of element. It is funny
that this investigation aimed at the discovery of catalysts favouring displacement over chain
propagation should have lead to the much more important opposite finding that trietylaluminium
in the presence of certain transition metal compounds catalyses the polymerisation of ethylene to
high polymers. This was observed first with zirconium acetylacetonate and later with the more
effective titanium compounds which allowed the polymerisation to proceed even at atmospheric
pressure and room temperature.
According to its infrared spectrum, Breil observed that the polyethylene produced by his
catalyst contained a very small number methyl groups indicating a mostly linear structure for the
polymer, and he also found that this material softened at 130-150 °C, a temperature consistently
higher than that of polyethylene prepared by the high pressure method, but he did not relate this
property to the structure of the chain.
15
The difference in the reaction of the world’s scientific and business communities, compared
with 20 years earlier when polyethylene was first discovered, could hardly have been greater:
researchers all over the world took up Ziegler chemistry and companies beat a track to Ziegler’s
door for licenses, and all the license fees that accrued belonged to Ziegler personally.
The polymers of ethylene produced at low pressure and relatively low temperature with the
catalyst based on titanium halides and aluminium alkyls (Ziegler catalyst) were substantially
different from those produced at higher temperatures under high pressure being essentially linear
in structure (see Figure 4) and with densities of 0.95 to 0,96 g/cm2 compared with about 0,92
g/cm2 for high pressure polyethylene. They were more rigid than high pressure polyethylene and
could handle boiling water. For what is concerned with the polymerisation processes the low
pressure process does not require such a lot of expensive engineering as the high pressure one.
At about the same time, Phillips Petroleum in the USA found that supported reduced
chromium oxide catalysts also produced high density polyethylene at low pressures. Phillips
quickly commercialised this product and licensed their technology. The Phillips catalyst is a
cheaper and easier to handle catalyst than the Ziegler one, but it requires medium pressure and
therefore more engineering. However, the gain on one side and loss on the other, relative to the
Ziegler process, were about equal.
Originally, one sometimes referred to the old and new types of polymers from ethylene as
high-pressure or low pressure polyethylenes, according to the polymerisation process. Now the
old type of polyethylene, namely the one produced through the high pressure process, is referred
to as Low Density PolyEthylene or LDPE. This is in contrast to the Ziegler and Phillips
polyethylenes, which because of their high crystallinity have a much higher density, are referred
to as High Density PolyEthylene or HDPE.
Many major chemical companies rushed to put the new polyethylene into production, and
their plant were just coming on line, when problems with the new polymer started to show up. If
exposed to hot air for a few hours, it would crack and fall apart. Even at room temperature
cracks appeared after several months in bottles or pipes, trouble enough if carrying water,
disastrous if it was gas.
The solution to this problem was to make a polyethylene with a small amount of side
branches in the chain (not so many as in LDPE) in order to create small regions of rubbery
material to hold the hard stuff together. This was achieved by adding small amounts of other
gases to ethylene. This type of polyethylene was given the name Medium Density PolyEthylene
or MDPE.
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In the meantime, the companies faced economic disaster. Rescue came with the Hola-Hoop,
introduced around 1958 by the Wham-O-Toy Company, a circular piece of polyethylene, about
1 m in diameter, that teenagers gyrate on their hips in order to get fit and be in fashion. This toy
rapidly used all the otherwise useless polyethylene in the warehouses and gave companies
breathing space to make the necessary changes to their processes.
Around 1980, with advances in catalyst technology, a linear polyethylene of low density,
intermediate between the HDPEs and MDPEs on one hand and the LDPEs on the other was
realised. These materials were called Linear Low Density PolyEthylene or LLDPE.
Since polyethylene is still the major insulator for electric cables, a form in which the polymer
molecules are lightly crosslinked to prevent them from going liquid if cable overheats has been
realized. This kind of polyethylene is known as Crosslinked Linear Polyethylene or XLPE.
Once better catalysts were available, it has been possible to control the molecular weight of
polyethylenes. Commercially available polyethylene now range from medium molecular
weights to ultra-high molecular weights, and manufacturers are now able to prepare grades
specially tailored to a given application.
Ultra-high molecular weight polyethylene (UHMWPE) is now the major material use in
artificial replacement of hip and knee joints. How this happened is another fascinating story but
it is out of the aims of this paper.
Acknowledgements
The Author is greatly indebted to I.C.I. (Imperial Chemical Industries Ltd) for the access to
records, to Mrs. Yvonne Joy (ICI Wilton Record Management Centre), to Dr A. H. Willbourn
for the text of his lecture (Ref. [7]) and to E. G. Hill for the many helpful contributions.
References
[1].
GIBSON R. O., The Discovery of Polythene, The Royal Institute of Chemistry,
Lecture Series 1964, Number 1, 1-30.
[2].
WILLBOURN A. H., The Origin and Discovery of Polythene, Lecture at the Golden
Jubilee Conference, POLYETHYLENES 1933-1983, 8-10 June 1983, London, UK.
[3].
PERRIN M. W., The Story of Polyethylene, Research, 6 (1953), 111-118.
17
[4].
Polyethylene-the First Fifty Years, Plastic and Rubber International, 8 (1983), 127133.
[5].
Polythene: Discovery and Early Development, Plastiquarian, Summer 2002, 2-5.
[6].
MORAWETZ H., Polymers. The Origin and Growth of a Science, John Wiley and
Sons, New York 1985.
[7].
ZIEGLER K., Consequences and development of an invention, Nobel Lecture, 12
December 1963
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