Polyethylene: A Crisp Overview | Sciencemonk.com

Polyethylene or simply polythene is a type of polymer and also known as a thermoplastic which means that it can be melted to a liquid state and remoulded back to its solid state. It is chemically synthesized from ethylene which can be obtained mainly from the petroleum or natural gas source. It is most often abbreviated as PE. Put it in the simplest manner; polyethylene is nothing but a composition of several monomers called ethylene molecules.

Readers may be wondering what the n represents in figure-1. In chemistry, n acts as a placeholder for a number, that gives you some idea about the potential chain length of the polymer. As polyethylene molecules can be very long, this type of information is very helpful to understand the whole structural pattern. Generally, a typical polyethylene molecule can contain more than 500 ethylene monomeric units.

polyethylene or polythene
Figure1: Schematic diagram for the synthesis of Polyethylene

 

Polyethylene or polythene
Figure 2: Spacefill model of the synthesis of Polyethylene

Some Background:

Polyethylene was first synthesized by a German chemist Hans von Pechmann, who prepared it by accident in 1898 while investigating diazomethane. When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long –CH2– chains and termed it as polyethylene due (to repeating –CH2 group).

Polyethylene
Figure 3: First accidental synthesis of Polyethylene

Eric Fawcett and Reginald Gibson discovered the polymerization of ethylene on the 24th of March 1933 at the Imperial Chemical Industries (ICI), Northwich, England. This was the first industrially practical polyethylene synthesis, and interestingly it was also by accident. The heating of the diazomethane route was avoided as CH2N2 is a notoriously unstable substance.

Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde(C7H6O), they produced a white, waxy material. Since this was an extremely high-pressure process, which was considered dangerous and was not repeated for two years. Later in 1935 another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial low-density polyethylene (LDPE) production beginning in 1939.

 Polyethylene or polythene
Figure 4: First industrially practical Polyethylene synthesis
Ziegler-Natta Catalyst : Polyethylene
Figure 5: Description of Ziegler-Natta Catalyst

 

In the 1950s a German chemist, Karl Zeigler developed catalysts that polymerized ethene at lower pressures and produced polythene with fewer branches than the high-pressure process. This is called high-density polythene, and it is denser, stronger, and harder to melt than low-density polythene. Ziegler shared the 1963 Nobel Prize with the Italian Giuglio Natta who also worked on catalysts for polymerization.

Synthesis of Polythene using Ziegler-Natta Catalyst: Polyethylene
Figure 6: Synthesis of Polythene using Ziegler-Natta Catalyst
Giulio NattaandKarl Zeigler : Polyethylene or polythene
Figure 7: The great scientists Giulio Natta and Karl Zeigler – 1963 Nobel Prize winner

Metallocenes or Organic-inorganic hybrid catalysts are the latest addition to the ethylene (in general olefin) polymerization to give polyethylene (in general olefin) to the catalyst family. A metallocene is a compound typically consisting of two cyclopentadienyl anions (C5H5-, abbreviated as Cp) bound to a metal centre (M) in the oxidation state of II, with the resulting general formula (C5H5)2M. Depending on the metal M and the associated ligands, the respective catalysts can be used for the polymerization. The following example is given to understand better.

Metallocene catalysts for the synthesis of polythene: Polyethylene
Figure 8: Metallocene catalysts for the synthesis of polythene

Background of Polyethylene (Flow Chart):

 

The 1890’s: Synthesized by accident while heating diazomethane

The 1930’s: First Industrial polymerization of ethylene at Imperial Chemical Industries. Discovery of LDPE.

The 1950’s: Ziegler-Natta Catalyst for the synthesis of HDPE

The 1970’s: Metallocene Catalyst for the synthesis of HDPE

Physical and Chemical Properties of Polyethylene:

Physical Properties:

The melting point and glass transition temperature for the polyethylene are dependent on the type of plastics they belong to. For common commercial grades of medium- and high-density polyethylene, the melting point is typically in the range of 120 to 180 °C. The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C.

These temperatures vary strongly with the type of polyethylene. Polyethylene is of low strength, hardness, and rigidity, but has high ductility and impact strength as well as low friction. It shows strong creep under persistent force, which can be reduced by the addition of short fibres. It feels waxy when touched.

Chemical Properties:

Polyethylene consists of nonpolar, saturated, high molecular weight hydrocarbons. Therefore, its chemical behaviour is similar to paraffin. Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning they can not be attacked by strong acids or strong bases, and are resistant to gentle oxidants and reducing agents.

Polyethylene (other than cross-linked polyethylene) usually dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene. Polyethylene absorbs almost no water. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar to candle flame). The material continues burning on the removal of the flame source and produces a drip.

Classification and Uses of Polyethylene:

Polyethylene or polythene is classified by its density and branching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight. There are several types of polyethylene; however, with regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE. Some of the important ones are as follows:

  • High-Density Polyethylene (HDPE):

HDPE is defined by a density of greater or equal to 0.941 g/cm3. The mostly linear molecules pack together well to create HDPE. It can be produced by chromium/silica catalysts, Ziegler–Natta catalysts or metallocene catalysts. HDPE has a high tensile strength. It is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, and water pipes. One-third of all toys are made from the HDPE category of polyethylene. The global HDPE consumption reached a large volume of more than 30 million tons as reported in 2007.

Molecular structure of HDPE where branching is absent: Polyethylene
Figure 9: Molecular structure of HDPE where branching is absent

Low-Density Polyethylene (LDPE):

LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short- and long-chain branching, which results in lower tensile strength and increased ductility. A free-radical polymerization reaction makes LDPE. It is used for both rigid containers and plastic film applications such as plastic bags and film wrap. In 2013, the global LDPE market had reached a large volume of almost US$33 billion.

 

Molecular structure of LDPE where branching (asterisked carbon) is present: Polyethylene
Figure 10: Molecular structure of LDPE where branching (asterisked carbon) is present

Linear Low-Density Polyethylene (LLDPE):

LLDPE is defined by a density range of 0.915–0.925 g/cm3. It is structurally similar to LDPE. It is a linear polymer with significant numbers of short branches. It is made by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene, and 1-octene) by using Ziegler-Natta or metallocene catalysts. Suitable for a variety of film applications such as general-purpose film, stretch film, garment packaging, agricultural film, etc. In 2013, the world’s LLDPE market reached a volume of US$40 billion.

Molecular structure of LLDPE: Polyethylene
Figure 11: Molecular structure of LLDPE

Ultra-High-Molecular-Weight Polyethylene (UHMWPE):

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.5 and 7.5 million amu. The high molecular weight makes it a very tough material. Generally, these kinds of plastic are made through Ziegler-Natta catalysts technology. Due to its unique toughness and characteristic properties, these are used in a diverse range of applications like canned bottle-handling machine parts, moving parts on weaving machines, bearings, gears, and butchers’ chopping boards. It is commonly used for the construction of particular portions of implants used for hip and knee replacements. Like fibre, it competes with aramid in bulletproof vests. It is also used in the form of large sheets instead of ice for skating rinks.

Figure 12: Pictorial representation of the molecular structure of UHMWPE

Manufacturing Process:

As described in the above section monomeric ethylene or olefin is being converted to polyethylene or polyolefin by using a suitable catalyst and optimum temperature and pressure. Generally, metal chlorides or metal oxides are used as a catalyst. The most common catalysts are the Ziegler–Natta catalysts, metallocene catalysts, and Phillips catalysts (prepared by depositing chromium(VI) oxide on silica). Polyethylene can also be produced through radical polymerization, but this route has not preferred much due to its limited utility and a drawback of the requirement of high-pressure apparatus.

Figure 13: General diagram for the common manufacturing process of Polythene

Environmental Impact:

The use of polythene has been increased in many folds what it was used in the last decades. For instance, by 2023 India’s polyethene demand is expected to be increased by 129%. It affects almost every aspect of the environment including terrestrial as well as an aquatic biome. The large stockpile of plastics in the environment becomes the breeding ground of mosquitoes and flies that further imposes health threat on humans.

The death of animals like the cow is often reported by consuming plastic. In the United Arab Emirates camel and endangered desert animals are reported dead by eating polythene bags along with the food. Marine debris of plastics disturbs the marine ecology as well. It has been reported that due to debris, there has been a decrease in marine fauna population.

Polyethylene is not readily biodegradable and thus accumulates in landfills. Incineration may result in harmful gaseous emissions, polluting the environment. Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually bacteria. There are a number of species of bacteria and animals that are able to degrade polyethylene.

In the year 2008 Canadian young man aged only 16, discovered that the bacteria Pseudomonas fluorescens, in combination with Sphingomonas can degrade the plastics. In the year 2014, a Chinese researcher found holes in the plastic bags which he subjected to the experiment with Indian meal moth larvae, inferred that the hungry larvae digested the plastic. In 2017, researchers reported that the caterpillar of Galleria mellonella eats plastic garbage such as polyethylene.

Concluding Remarks:

The role of plastics in many important aspects has already been discussed in the above section, and that cannot be ruled out in any way. Therefore, in spite of environmental pollution, we still need to see the other part of the coin as well. We need to use plastics that are mostly biodegradable. Screening of more and more microbes are required to decompose the plastics.

Awareness programs should be organized often to make people educated, and the plastic wastes can be collected and try to utilize them more in the synthesis of bitumen that is a typical road construction material. In a nutshell, nothing will work if the mass of the people does not come forward, so we should start working to make this world a better place for living.


References:
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