Polyethylene


Polyethylene or polythene is the most commonly produced plastic. It is a polymer, primarily used for packaging., over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.
Many kinds of polyethylene are known, with most having the chemical formula n. PE is usually a mixture of similar polymers of ethylene, with various values of n. It can be low-density or high-density and many variations thereof. Its properties can be modified further by crosslinking or copolymerization. All forms are nontoxic as well as chemically resilient, contributing to polyethylene's popularity as a multi-use plastic. However, polyethylene's chemical resilience also makes it a long-lived and decomposition-resistant pollutant when disposed of improperly. Being a hydrocarbon, polyethylene is colorless to opaque and combustible.

History

Polyethylene was first synthesized by the 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 polymethylene.
The first industrially practical polyethylene synthesis was again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson at the Imperial Chemical Industries works in Northwich, England. Upon applying extremely high pressure to a mixture of ethylene and benzaldehyde they again produced a white, waxy material. Because the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was difficult to reproduce at first. It was not until 1935 that 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 production beginning in 1939. Because polyethylene was found to have very low-loss properties at very high frequency radio waves, commercial distribution in Britain was suspended on the outbreak of World War II, secrecy imposed, and the new process was used to produce insulation for UHF and SHF coaxial cables of radar sets. During World War II, further research was done on the ICI process and in 1944, DuPont at Sabine River, Texas, and Union Carbide Corporation at South Charleston, West Virginia, began large-scale commercial production under license from ICI.
The landmark breakthrough in the commercial production of polyethylene began with the development of catalysts that promoted the polymerization at mild temperatures and pressures. The first of these was a catalyst based on chromium trioxide discovered in 1951 by Robert Banks and J. Paul Hogan at Phillips Petroleum. In 1953 the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminium compounds that worked at even milder conditions than the Phillips catalyst. The Phillips catalyst is less expensive and easier to work with, however, and both methods are heavily used industrially. By the end of the 1950s both the Phillips- and Ziegler-type catalysts were being used for high-density polyethylene production. In the 1970s, the Ziegler system was improved by the incorporation of magnesium chloride. Catalytic systems based on soluble catalysts, the metallocenes, were reported in 1976 by Walter Kaminsky and Hansjörg Sinn. The Ziegler- and metallocene-based catalysts families have proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including [|very-low-density polyethylene] and linear low-density polyethylene. Such resins, in the form of UHMWPE fibers, have begun to replace aramids in many high-strength applications.

Properties

The properties of polyethylene depend strongly on type. The molecular weight, crosslinking, and presence of comonomers all strongly affect its properties. It is for this structure-property relation that intense effort has been invested into diverse kinds of PE. LDPE is softer and more transparent than HDPE. For medium- and high-density polyethylene the melting point is typically in the range. The melting point for average commercial low-density polyethylene is typically. These temperatures vary strongly with the type of polyethylene, but the theoretical upper limit of melting of polyethylene is reported to be. Combustion typically occurs above.
Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning that they are not attacked by strong acids or strong bases and are resistant to gentle oxidants and reducing agents. Crystalline samples do not dissolve at room temperature. Polyethylene usually can be 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. The permeability for water vapor and polar gases of is lower than for most plastics. On the other hand, non-polar gases such as Oxygen, carbon dioxide, and flavorings can pass it easily.
Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin. The material continues burning on removal of the flame source and produces a drip.
Polyethylene cannot be imprinted or bonded with adhesives without pretreatment. High-strength joints are readily achieved with plastic welding.

Electrical

Polyethylene is a good electrical insulator. It offers good electrical treeing resistance; however, it becomes easily electrostatically charged.
When pure, the dielectric constant is in the range 2.2 to 2.4 depending on the density and the loss tangent is very low, making it a good dielectric for building capacitors. For the same reason it is commonly used as the insulation material for high-frequency coaxial and twisted pair cables.

Optical

Depending on thermal history and film thickness, PE can vary between almost clear, milky-opaque and opaque. LDPE has the greatest, LLDPE slightly less, and HDPE the least transparency. Transparency is reduced by crystallites if they are larger than the wavelength of visible light.

Manufacturing process

Monomer

The ingredient or monomer is ethylene, a gaseous hydrocarbon with the formula C2H4, which can be viewed as a pair of methylene groups connected to each other. Typical specifications for PE purity are <5 ppm for water, oxygen, and other alkenes contents. Acceptable contaminants include N2, ethane, and methane. Ethylene is usually produced from petrochemical sources, but is also generated by dehydration of ethanol.

Polymerization

Polymerization of ethylene to polyethylene is described by the following chemical equation:
Ethylene is a stable molecule that polymerizes only upon contact with catalysts. The conversion is highly exothermic. Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of titanium chloride, the so-called Ziegler–Natta catalysts. Another common catalyst is the Phillips catalyst, prepared by depositing chromium oxide on silica. Polyethylene can be produced through radical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.

Joining

Commonly used methods for joining polyethylene parts together include:
Pressure-sensitive adhesives are feasible if the surface chemistry or charge is modified with plasma activation, flame treatment, or corona treatment.

Classification

Polyethylene 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:
  • Ultra-high-molecular-weight polyethylene
  • Ultra-low-molecular-weight polyethylene
  • High-molecular-weight polyethylene
  • High-density polyethylene
  • High-density cross-linked polyethylene
  • Cross-linked polyethylene
  • Medium-density polyethylene
  • Linear low-density polyethylene
  • Low-density polyethylene
  • Very-low-density polyethylene
  • Chlorinated polyethylene
With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE.

Ultra-high-molecular-weight (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, but results in less efficient packing of the chains into the crystal structure as evidenced by densities of less than high-density polyethylene. UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. Because of its outstanding toughness and its cut, wear, and excellent chemical resistance, UHMWPE is used in a diverse range of applications. These include can- and bottle-handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks, steel cable replacements on ships, and butchers' chopping boards. It is commonly used for the construction of articular portions of implants used for hip and knee replacements. As fiber, it competes with aramid in bulletproof vests.