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Polyethylene is a thermoplastic polymer of ethylene, belonging to the class of polyolefins. It is a white mass (thin sheets are transparent and colourless). Sometimes in the popular mind it is identified with cellophane.

History

The inventor of polyethylene is the German engineer Hans von Pechmann, who first accidentally received this product in 1899. However, this discovery was not widespread. The second life of polyethylene began in 1933 thanks to engineers Eric Fawcett and Reginald Gibson. Initially, polyethylene was used in the production of telephone cables and only in the 1950s did it begin to be used in the food industry as packaging.

According to another version, more widely accepted in scientific circles, the development of polyethylene can be traced back to the work of Imperial Chemical Industries employees on the creation of industrial production technology, which began in the 1920s. The active phase of development began after the installation of a synthesis unit, which was operated by Fawcett and Gibson in 1931. They produced a low-molecular-weight paraffin-like product with a monomeric linkage similar to polyethylene. Fawcett and Gibson's work continued until March 1933, when it was decided to upgrade the high-pressure apparatus to obtain better results and greater safety. After the upgrade, the experiments were continued in collaboration with M. W. Perrin and J. R. Patton and in 1936 were successfully completed, with a patent for low-density polyethylene (LDPE) being granted. Commercial production of LDPE began in 1938.
The history of high-density polyethylene (HDPE or LDPE) dates back to the 1920s, when Karl Ziegler began work on creating catalysts for ion-coordination polymerisation. In 1954, the technology was mastered and a patent was granted. Later, industrial production of HDPE was launched.

Types

Different types of polyethylene are usually classified by density.

Despite this, there are many common names for homopolymers and copolymers, some of which are listed below.

• Low-density (high-pressure) polyethylene - PENG, LDPE, LDPE, LDPE (Low Density Polyethylene).
• High-density polyethylene (low pressure) - LDPE, HDPE, HDPE, HDPE (High Density Polyethylene).
• Medium-density polyethylene (high-density polyethylene) - PESD[7].
• Linear medium-density polyethylene - PESP[7], MDPE or PEMD.
• Low-density linear polyethylene - LLDPE[7], LLDPE or PELLD.
• Very low density polyethylene - VLDPE
• Ultra-low density polyethylene - ULDPE
• Metalocene linear low-density polyethylene - MPE
• Cross-linked polyethylene - PEX or XLPE, XPE.
• High molecular weight polyethylene - HMWPE, HMWPE or PEHMW or VHMWPE.
• Ultra-high molecular weight polyethylene – UHMWPE

Chemical properties

It burns with a bluish flame, with a weak light, and emits the smell of paraffin, i.e. the same as that of a burning candle.
It is resistant to water, does not react with alkalis of any concentration, with solutions of neutral, acidic and basic salts, organic and inorganic acids, even with concentrated sulfuric acid, but decomposes under the influence of 50% nitric acid at room temperature and under the influence of liquid and gaseous chlorine and fluorine. Unlike unsaturated hydrocarbons, it does not discolour bromine water and potassium permanganate solution.

At room temperature it is insoluble and does not swell in any of the known solvents. At elevated temperatures (80 °C) it is soluble in cyclohexane and carbon tetrachloride. Under high pressure, it can be dissolved in water heated to 180 °C.

Over time, it undergoes degradation with the formation of transverse interchain bonds, which leads to an increase in brittleness against a slight increase in strength. Unstabilised polyethylene undergoes thermal oxidative degradation (thermo-aging) in air. Thermal aging of polyethylene is carried out by a radical mechanism, accompanied by the release of aldehydes, ketones, hydrogen peroxide, etc.

Production

Polyethylene is supplied for processing in the form of granules from 2 to 5 mm. Polyethylene is produced by polymerisation of ethylene:

Production of high-pressure polyethylene
High-density polyethylene (HDPE), or low-density polyethylene (LDPE), is produced under the following conditions

• temperature 200-260 °C
• pressure 150-300 MPa
• the presence of an initiator (oxygen or organic peroxide); in autoclave or tubular reactors. The reaction is based on a radical mechanism. The polyethylene produced by this method has an average molecular weight of 80,000-500,000 and a crystallinity of 50-60%. The liquid product is then granulated. The reaction takes place in the melt.

Production of low-pressure polyethylene

Low-density polyethylene (LDPE), or high-density polyethylene (HDPE), is produced under the following conditions:

• temperature of 120-150 °C;
• pressure below 0.1 - 2 MPa;
• the presence of a catalyst (Ziegler-Natta catalysts, for example, a mixture of TiCl4 and AlR3);

Polymerisation takes place in suspension by the ionic coordination mechanism. The polyethylene produced by this method has an average molecular weight of 80,000-300,000 and a crystallinity of 75-85%.

Applications

• Plastic film (especially packaging film, e.g. bubble wrap or adhesive tape)
• Containers (bottles, cans, boxes, canisters, garden watering cans, seedling pots)
• Polymer pipes for sewage, drainage, water and gas supply
• Electrical insulation material
• Polyethylene powder used as a hot melt adhesive
• Armour (armour panels in body armour)
• Hulls for boats, all-terrain vehicles, parts of technical equipment, dielectric antennas, household items, etc.
• Foamed polyethylene (polyethylene foam) is used as a heat insulator
• Low-density polyethylene (LDPE) or high-density polyethylene (HDPE) is used in the construction of waste treatment sites, storage facilities for liquid and solid substances that can contaminate soil and groundwater
• Low-pressure polyethylene is widely used in the improvement of adjacent territories and playgrounds, pushing plywood and wood into the background, as the service life of HDPE slopes is more than 15 years, while the service life of "wooden analogues" is 10 years, and after 3-5 years the wood loses its marketable condition.