The Story of Polythene
 

by Robert H. Olley

University of Reading Polymer Physics Centre
J. J. Thomson Physical Laboratory, Whiteknights, Reading RG6 6AF, England
last update: 29/Apr/2002
 
the World's No. 1 Plastic
 
I was first prompted to write this article after reading a student thesis on the use of Intermediate Technology (IT) in Building Design. One of the pressures forcing the business to use IT is the increasing number of materials and varieties of any given material that are available for construction. The greatest single change since the Second World War is the introduction of Plastics. However, few people appreciate the great variety of plastics there are. Even among those who are aware of the difference between polyethylene (better known as polythene), PVC, and polystyrene, very few indeed realize how many different varieties of polyethylene there are. 

(My own interest in this is that the major part of my own research is devoted to preparing specimens of polyethylene and similar materials for examination of their micro-crystalline structure under the electron microscope. Before I continue, I must apologize to chemists for over-simplifying matters, but I would encourage economists to read on. Do not let the little bit of simple chemistry put you off, for there is much about the market as well!)

The story of polyethylene really starts in 1932. Britain, along with the whole industrialized world, was in deep recession following the Wall Street Crash of 1929. It was difficult to find money for large-scale research, and yet something new was needed. In ICI, there was suggested a research program to look for new reactions under extreme pressure. Fifty different reactions were tried, all without success - but one of the failures resulted in the discovery of polyethylene through a remarkable series of coincidences.

One of the suggested mixtures had included ethylene, a very light gas prepared from petroleum. The reaction hoped for had not occurred, but instead there was a white waxy solid on the walls of the reaction vessel. Analysis showed that this must have formed from the ethylene alone. In 1935, the reaction was tried again without the other component, but this time the vessel leaked; nevertheless, some more polyethylene was obtained. At this time, ICI management made the very bold decision to start a major development programme, on the basis of only 8 grams obtained of the promising product! So they tightened up their procedures, and as a result - no polyethylene! It was only after months of work that they realized that oxygen had to be present in some form, either from air leaking in, or, in the first experiment, indirectly from having reacted with the other component of the original mixture. These two "happy accidents" had allowed polyethylene to be produced.

The first proposed application for the polyethylene was in submarine telecommunication cables. This eventually proved unsatisfactory, but it was a fruitful failure, because polyethylene was ready to be used for the critical job of insulating radar cables. Based on the estimated demand for submarine cables, a production plant had been started, and it came into operation in September 1939, on the same day that the Germans invaded Poland. (The next day, Britain and France declared war on Germany). The availability of this insulator allowed the allies to use airborne radar, which gave us an enormous technical advantage in long-distance air warfare, most significantly in the Battle of the Atlantic against the U-boats (submarines) with which threatened to starve Britain of supplies of food and raw materials.

Because of this, polyethylene became a top secret during the war, but emerged shortly afterwards as a commercial product. The early polyethylene rapidly found many uses, but it was soft and low melting, and so its uses were limited - for example you couldn't put boiling water into a polyethylene jug, or it would tend to collapse. The reason for this is that, ideally, the polyethylene molecule is based on a long chain of carbon atoms, as follows:

          

Each ethylene molecule, as it adds on, contributes two more carbon atoms to the chain. However, under the conditions of high pressure used, sometimes the ethylene molecules did not add on in a regular fashion, and so put short branches in the chain, as follows:
          

This stops the chains packing together regularly, and forming a nice stiff, higher melting material. Such as material is known as a branched polyethylene, in contrast to the desired straight chain molecules, which would give what scientists call a linear polyethylene. 

A large research effort was going in to try to improve the process, and late in 1953 in Germany, Karl Ziegler made the crucial discovery, by which one could make the molecules of ethylene join up in a more disciplined manner on the surface of particles of "Ziegler catalyst", and without the high pressures and temperatures previously required, which also entailed a lot of expensive engineering. This produced polyethylene with the linear chains everyone was hoping for. It was much more rigid, and could handle boiling water. Meanwhile, Robert L. Banks and J. Paul Hogan were developing the Phillips process which used a cheaper and easier to handle catalyst, but required 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, and so both processes are widely used to this day.

(Ziegler shared the Nobel prize for his discovery with Giulio Natta, who simultaneously discovered how to discipline other monomers in order to produce other plastics, of which polypropylene is by far the most important. That is why, in general, these are referred to as Ziegler-Natta catalysts).

Originally, people sometimes referred to the old and new types of polyethylene as high- or low-pressure polyethylene, according to the process, and you might still find these terms if you are studying early literature. But (industrially at least) the old type of polyethylene is now 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 and are referred to as High-Density Polyethylene or HDPE.

Many major chemical companies rushed to put the new polyethylene into production, and their plants were just coming on-line, when problems with the new polyethylene started to show up. If exposed to hot air for a few hours, it would crack and fall apart; even at ordinary temperatures, cracks would appear in bottles or pipes after several months - trouble enough if the pipes were carrying water, disastrous if it was gas. The chemists had done their job too well - overcoming one set of difficulties had created others.

It took a few months to come up with the solution, which was to make a polyethylene with a small amount of branches in the chain - not so many as in the old "rubbery" polyethylene, but enough to create small regions of "rubbery" material to hold the "hard" stuff together. This was done by adding a small proportion of other gases to the ethylene. However, in the meantime, the companies faced economic disaster, and the people involved faced the prospect that "heads would roll".

Rescue came in the form of a toy - the Hula-Hoop which was introduced by the Wham-O Toy Company (around 1958). This was a circular piece of polyethylene pipe, about 1 metre in diameter, and teenagers especially would gyrate this on their hips in order to get fit and be in fashion. This toy rapidly used up all the otherwise useless polyethylene in the warehouses, and gave the companies breathing space to make the necessary changes to their processes.

This type of polyethylene was given the name Medium Density Polyethylene or MDPE, and is what scientists call a lightly branched polyethylene.. However, only a limited quantity of other monomers can be incorporated this way. It was still desired to produced a material of low density, intermediate between the HDPEs and MDPEs on the one hand, and the LDPEs on the other. This was achieved around 1980 with advances in catalyst technology, and these materials were called Linear-Low-Density Polyethylene or LLDPE. (This is because industrialists refer to all Ziegler and Phillips polyethylenes as "linear", which is not what scientists understand by the term. (I have attempted to sort out the confusion in Table 1).

Since the 1960s, polyethylene has gone from strength to strength, as new and varied types are produced. Once "serious" catalysts were available, it was also possible to control the molecular weight of the material. Commercially available polyethylenes now range from medium molecular weight to ultra-high molecular weight distributions, and manufacturers will sell you a grade specially tailored to your application. Examples of its widespread use in the modern world are:

It is the highest tonnage agricultural chemical (commercially, glass greenhouses have been replaced by poly-tunnels).

Polyethylene is now the major insulator for electric cables. This is a form in which the molecules have been cross-linked to prevent it from going liquid if the cable overheats. It is know as Cross-Linked Polyethylene or XLPE. This was first achieved by cooking ordinary PE at high temperatures with an organic peroxide, but this rather messy process is being superseded by ones where a polyethylene with silane (organo-silicon) containing side groups is processed in a more ordinary way, and the cable is then steam-cooked to link the silane groups on neighbouring molecules.
 
 

Several companies are now producing spun polyethylene fibres, which take advantage of the intrinsic strength of the carbon backbone of the polymer chain.

Ultra-high molecular weight polyethylene (UHMWPE) is the major material used in artificial replacement hip joints.

And many more ...... 


 
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Polyethylene - was it bound to happen?

Our industrial world today would indeed be very much poorer without polyethylene. But one may well ask - was it bound to happen sometime, given the onward development of science as it then was? One man who has researched the history thoroughly (Frank M. McMillan, in his book "The Chain Straighteners") has listed six factors which operated together to make this possible. I have condensed these as follows: 

1. The discoveries were made in huge industrial laboratories which had the "muscle" to carry them through to the commercial product;

2. Most of the researchers were actually looking for something different at the time, but the unexpected result was more important than what they were looking for;

3. Fortuitous ("lucky") events seemed to operate time and again; but people had flexibility to modify their targets, alertness to note strange happenings, curiosity to pursue unforeseen results, and intelligence to interpret the unexpected. In addition, they had the courage to take an intelligent gamble with their careers and resources;

4. There was good communication up-and-down and sideways within the organizations, and also between organizations. Not only "top-dogs", but technical people, could communicate at scientific conferences;

5. Timing was right. The scientific knowledge and industrial basis to make straight chain PE were available 20 years earlier, but science and industry were not ready to recognize the value of the product. In the 1950's there was also a mood for expansion in research which has not occurred since.

6. Key people were involved again and again. Personalities are important. In this story the two stars are Ziegler, and in Italy Natta who made the equivalent discovery for making regular polypropylene (the world's No.2 plastic, I think). These two shared the Nobel Prize for Chemistry in 1963.

In regard to our dependence on timing, McMillan is careful to say that the provision of "lucky events" does not mean that we should not plan carefully, budget our resources, and target our research; and the scientist should train and discipline himself. No one can command discovery, but as Louis Pasteur says,

"Chance favours the prepared mind".

But McMillan concludes that all that we can do is still not enough - the additional requirement, uncontrollable but vital, is "a little bit o'luck".

or, as Solomon said, 3000 years ago -

Again I saw that, under the sun, the race is not to the swift, nor the battle to the strong, nor bread to the wise, nor riches to the intelligent, nor favour to the men of skill, but time and chance happen to them all.

Ecclesiastes 9:11