We will begin our description of the polymer, polyethylene, with an informative introduction, which will provide a brief description of the origins of thermoplastic polymers, as well as the origins polyethylene. This introduction will show some advancements in polymer technology that lead to the need for the development of new and better polymers. These new and better polymers, including polyethylene, were used to sustain the development of more new technologies in the polymer industry. Also this introduction will show how the discovery of polyethylene aided in the development of the polymer industry.

The uses of naturally occurring polymers, including tortoise shell and horn, are referenced throughout history. These compounds are natural cellulose materials that are produced by many animals. These compounds are unique because they possess special processing capabilities. The most important capability of these polymers is the fact that they can be heated to a softening temperature, and while at this temperature the material can be formed to a desired shape, and once the material is allowed to cool the desired shape is retained. The early uses of these materials have been shown as household utensils, including spoons, goblets, and bowls. These discoveries can be called the beginnings of thermoplastic polymers.

The first non-naturally occurring thermoplastic polymer was discovered and produced in 1868, by John Wesley Hyatt. Hyatt was looking for a synthetic material to replace the ivory used in piano keys and billiard balls. This was needed because about this time there was a shortage of natural ivory. It was about this time when many governments had restricted the import of the ivory materials. Many manufacturers while looking for replacements for these materials held public contests to find cheap replacements for the natural materials. This was the case for Hyatt, who was a journalist by trade. His material given the trade name 'Celluloid' allowed the technology used on the early naturally occurring polymers to be used in processing. The only problem with this material was that there was a thermal instability when the temperature of the material was increased significantly. This thermal instability led the search for better polymers.

In the early 1870's another cellulose derivative, Cellulose Acetate, was discovered. This new polymer had many of the same properties of the Celluloid material discovered by Hyatt. This material however did not possess the same thermal instability that the Celluloid polymer. Because of this fact the development of several thermal-processing technologies developed. Included among these technologies was injection molding. With the discovery and development of the Cellulose Acetate polymer, molding industries were allowed develop and prosper in the early 1900's.

The next significant advancement in polymer technology came with the introduction of polystyrene. It was possible for Polystyrene polymer to be processed in the same methods as those early cellulose polymers. This polymer was allowed to prosper in the late 1920's and throughout the end of WWII. This was because a by-product of the U.S. Military's Rubber Program, which was in production before and during WWII. It was this governmental program that made the styrene monomer to be readily available and probably most important it was cheap. This caused many manufactures to get into the polystyrene processing. The toy industry was a major purchaser of polystyrene. Polystyrene polymer had one major disadvantage, which was the fact that it was brittle.

These early polymers aided in the development of many polymer technologies. Each of those early polymers had their disadvantages, and some of these were discussed above. Due to the huge amount of polystyrene introduced into the public a general dissatisfaction with the plastics industry as a whole. The market was ready for more durable polymers.

History of Polyethylene Polymers

Ethylene has been used in chemical reactions for as long as it has been in existence. It is a very useful and reactive substance due to its double bond. It was not until 1898 however that the first experiments to create long chain molecules with ethylene had commenced. These first reactions however had several problems. First, solid polyethylene had not been able to be produced. That is all the products of these early reactions had been waxes and greases. These non-solid polyethylene polymers had little use at the time.

It wasn't until 1933 when the first solid polyethylene polymer had been produced. Two scientists working for the Imperial Chemical Company had made this discovery. These scientists, E. W. Fawcett and R. O. Gibson, had produced this solid polymer while experimenting with pure ethylene gas at extremely high pressure and temperature. The ICI didn't wait long before obtaining a patent for this new material and began to market it.

The first applications of this early polyethylene polymer were for use as coatings. The primary coating that the ICI marketed polyethylene for was as insulation on electrical wires. The Telegraph Construction Maintenance Company had invested a lot in order to coat their submarine cables with this new polymer. It was about this time when the British Military had begun to use this polymer in the coating of their high frequency cabling and wires. This allowed the British to make great technological strides in the radar field. History has stated that it was because of the polyethylene polymer used here that the British had possessed the best radar capabilities in the world.

It was these military advancements by the British that interested many of the world powers at the time. Included among these powers was the U.S. government. The U.S. government was then able to persuaded two American based companies (Dupont Corp. and Union Carbide Corp.) to seek licenses from the ICI so that the production of polyethylene could begin stateside. Before the end of the Second World War the production of U.S. polyethylene had surpassed that made by Britain.

It was not until after WWII when extensive chemical studies began to be performed on the polyethylene polymer. The manufacturers of the polymer quickly marketed these new properties. New methods for processing polyethylene had been discovered during these physical tests. Film extrusion and injection molding were two of the new processing techniques used on polyethylene after WWII.

Another important discovery concerning polyethylene polymers came at about the 1940's. It was about this time when the IR spectroscopy technology had begun to develop. Substances including polymers were often subjected to this test to discover the compound's structure. An important discovery that was made when polyethylene was subjected to this test had shown that there were more than two methyl groups present in a single polyethylene chain. This lead to the natural conclusion the polyethylene polymer was not a straight chain polymer as previously thought, for this spectroscopic result to occur the polyethylene polymer must contain branches along its carbon backbone.

It was also about this time when work commenced on discovering ways of lowering the reaction conditions to produce polyethylene polymers. A polyethylene polymerization mechanism were the reaction conditions were lowered was accomplished in 1951 by Karl Ziegler. Ziegler was able to produce the solid polyethylene polymer at low pressures and temperatures by employing a catalyst. Testing on Ziegler's polyethylene polymer had shown that it had a larger density then the original high-pressure polyethylene process. It was after this discovery that characterization of polyethylene polymers were made on their density.

Shortly after Ziegler had discovered HDPE (High Density Polyethylene) another group of researchers working at the Phillips Petroleum Inc. had discovered other catalysts that produced the same results as Ziegler's process. These HDPE polymers were characterized as having fewer branches than those early high-pressure polyethylene polymers (LDPE - Low Density Polyethylene).

In 1977 researchers working at Union Carbide Corporation discovered a method to produce a new polyethylene. This new polyethylene was made from monomers other than ethylene gases, and as a result contained few short branches. Because of this property they were termed as LLDPE (Linear Low-Density Polyethylene) Polymers.

New Polyethylene polymers are still being developed today. This polymer contains a rich history rooted back more than a century ago. We will now present the individual polyethylene polymers giving summaries of the reaction chemistry and physical/mechanical properties of each type of polyethylene polymer.

Low Density Polyethylene (LDPE)

LDPE was the first solid polyethylene polymer produced. It was first produced in 1933 in the research laboratories of the ICI in England. The two scientists accredited for the development of this new polymer were E. W. Fawcett and R. O. Gibson. These two scientists were able to produce this polyethylene polymer only at extremely high pressure and temperature using pure ethylene gas. Unfortunately, ethylene gas doesn't react on its own at high temperature or high pressure. These two scientists needed to inject a small amount of oxygen gas into the reaction vessel in order to cause a reaction. The reaction they discovered was very rapid and also quite exothermic. The thermoplastic resin they formed in this reaction was noticed to have very remarkable and different properties from the early polyethylene waxes and pastes. The typical results of this method and the earlier methods can be shown below₍₂₎:

After some study by ICI scientists, it was determined that the high-pressure reaction to form polyethylene followed a Free Radical Polymerization mechanism. This classical polymerization mechanism is one that follows the three main steps including; initiation, propagation, and termination. The LDPE mechanism is described below in detail:

The initiation and propagation steps in this reaction are as follows₍₃₎:

The Termination of this polymerization reaction is as follows₍₃₎:

 

One can see from the above mechanism equation (2) the initiation step is a function of pressure. The effect of varying the pressure of the reaction has been verified by experiment. These experiments have shown that the properties of the polyethylene polymer formed by the above mechanism change as the pressure is varied. The fact that the properties of the polymer can change as a function of pressure makes polyethylene polymers very desirable to manufacturers. The manufacturers then have the ability to make specific polyethylene polymers with the customer's specific requirements. This mechanism was the primary method responsible for the production of all polyethylene polymers from 1933 to 1955, when Ziegler made his famous discovery.

 

A representation of a typical high-pressure polyethylene polymerization process₍₂₎:

 

As described above the polyethylene polymers produced by the above method use pure ethylene monomer in the reaction. Is this the only monomer that can be used in the above mechanism? The answer to this question is no, there are other monomer that can be used. There are many other monomers that can be used to produce a carbon backbone polymer. A few of these reactions can be seen below₍₁₎. The reason why ethylene is the primary monomer used to produce polyethylene polymers is purely economic. The other two reactions below do produce polyethylene polymer, but in the other reactions there are by-products of reaction. The by-products of these reaction can be found in the polymer produced in significant fractions. Because of this a purification step must then be used to purify the polyethylene polymer produced before it can be processed to a desired product. Another benefit of using the purified ethylene polymer as the primary monomer is that many of the chemical companies who produce polyethylene polymer also produce petroleum products, and the ethylene gas is by-product of petroleum cracking process.

 

Why would changing the reaction conditions have an effect on the properties of the LDPE polymers? It was in the 1940's when Infer Red Spectrum analysis of chemical compounds began to appear as an important method for determination of chemical structure. Polyethylene, as shown above can have different properties depending on the reaction conditions. When the polyethylene polymers were subjected to IR Spectroscopy the results were startling. The results of the testing had shown that the polymer contained a large number of methyl groups in a singe molecule. These results are explained because the molecule has many branches of varying length along its backbone chain. The amount of chains and length of the chains are what varies the properties of the polymer. Branching in the polymer must then be a function of pressure. In fact as the reaction pressure is increased the branching in the polymer is decreased.

What causes the branching in the polyethylene polymers? The branching can be explained through one of the two types of chain transfer. Short chains consist of branches with lengths varying from 1 to 3 carbon atoms. These short carbon branches can exist along the backbone of the polyethylene molecule in numbers from 10 to 60 branches per 1000 backbone carbons. Intermolecular chain transfer explains the creation of the short branches in the polyethylene polymer. See the below picture₍₃₎. There are not only short branches along the carbon backbone of the polyethylene. The long chain branches present in the polyethylene have a length longer than 5 carbon atoms. These branches can be present in the polymer in numbers of 1 to 4 branches per 1000 backbone carbon atoms. The cause of the long chain branches intermolecular chain transfer. See the below picture₍₃₎:

Since one can control the properties of the polyethylene polymer, methods of controlling the branching amount in the polymer had to be developed. Two commercial methods for controlling the amount of branching in the polymer that were developed for the manufacture of polyethylene are based on the reactor type. The two commercial reactor methods to produce the polyethylene are using an autoclave reactor or a tubular reactor. The design of the reactor has an influence on the amount of intermolecular chain transfer (which is the important type of chain transfer in this reaction). Tubular reactors typically operate at pressures of 2000atm and 350 degrees Celsius, and the typical conversion rates for this type of reactor are 15-20 percent. Autoclave reactors typically operate at pressures of 1400atm and 250 degrees Celsius, and the typical conversion rates for this type of reactor are 16-23 percent. The results of the reactor type methods are as follows₍₃₎: