Polyethylene

Chemical structure of polyethylene, sometimes represented only as (CH₂-CH₂)_n

The polyethylene (PE) is chemically the simplest polymer. It is represented by its repeating unit (CH₂-CH₂)_n. It is one of the most common plastics due to its low price and simplicity in its manufacture, leading to a production of approximately 80 million tons per year worldwide. It is chemically inert. It is obtained from the polymerization of ethylene (with the chemical formula CH₂=CH₂ and called ethene by the IUPAC), from which it derives its name..

This polymer can be produced by different polymerization reactions, such as: free radical polymerization, anionic polymerization, ion coordination polymerization, or cationic polymerization. Each of these reaction mechanisms produces a different type of polyethylene.

It is a straight chain unbranched polymer, although branching is common in commercial products. Polyethylene chains are arranged in amorphous and semi-crystalline regions when it is below the glass transition temperature, which is its usual state in service.

History

Polyethylene was first synthesized by German chemist Hans von Pechmann who accidentally prepared it in 1898 while heating diazomethane in the stove. When his colleagues Eugen Bamberger and Friedrich Tschimer investigated the white, greasy substance they created, they discovered long chains made up of -CH _{2 - and named it polymethylene.}

On March 27, 1933, in England, it was synthesized as we know it today, by Reginald Gibson and Eric Fawcett working for ICI Laboratories. They achieved this by applying a pressure of approximately 1,400 bar and a temperature of 170°C in an autoclave, producing the off-white, highly viscous material known today as "low-density polyethylene" (PEBD or, in English, LDPE).

In 1953, Karl Ziegler and his collaborators at the Max Planck Institute for Carbon Research, based on the work started by the Italian Natta, studied the low-pressure polymerization process. The reaction with a catalytic complex of aluminum alkyl and titanium tetrachloride gave rise to the manufacture of polyethylene with a higher density and melting temperature, as a consequence of its greater regularity. This polyethylene was called high-density polyethylene (HDPE or its acronym in English HDPE), making mention of its properties, or low-pressure, due to its method of obtaining it. This gave rise to the Ziegler-Natta catalysts, for which Karl Ziegler and Giulio Natta were awarded the Nobel Prize in Chemistry in 1963.

In 1955, the first factory for this material was opened in Germany. Simultaneously, the Phillips Petroleum Co. in the United States was developing an industrial process for obtaining high-density, highly crystalline polyethylene using medium pressures, and chromium oxide supported on silica as a catalyst. The first industrial plant was set up in Pasadena in 1957.

Process

Polyethylene is used for different types of final products, for each of them different processes are also used, among the most common are the following:

• Extrusion: Film, cables, threads, pipes.
• Co-Extrusion: Multilayer films and foils.
• Injection moulding: Parts in third dimension with complicated forms.
• Injection and blowing: Bottles of different sizes.
• extrusion and blowing: Slim caliber bags or tubes.
• extrusion and blowing of hollow bodies: Bottles of different sizes.
• Rotomoldeo: Large hollow deposits and shapes.

Polyethylene has a translucent milky color, this color can be modified with three common procedures:

• Add pigment powder to PE before processing.
• Color all PE before processing.
• Use a color concentrate (known in English as masterbatch), which represents the most economical and easy way to color a polymer.

Depending on the final function of the product, it is necessary to add certain additives: antioxidants, flame retardants, antistatics, bactericides, etc.

Recycling

• In countries such as Germany and Norway more than 90% of the polyethylene teephthalate used for beverages is recycled with great success.

Applications

• Bags of all kinds: supermarkets, boutiques, panification, frozen, industrial etc.
• Coverage of acequia;
• Automatic packaging of food and industrial products: milk, water, plastics, etc.;
• Transparent film (stretch film);
• Base for disposable diapers;
• Serum bags;
• Domestic hermetic containers;
• Tubes and pomes: cosmetics, medicines and food;
• Water pipes;

• Packaging for: detergents, bleach, automotive oils, shampoo, dairy;
• Bazaar and menage;
• Cajones for fish, soda, beers;
• Packaging for paint, ice cream, oils;
• Tambors;
• Gas pipes, telephony, drinking water, mining, drainage sheets and sanitary use;
• Chain guides, mechanical parts.
• It is also used to cover lagoons, canals, neutralization pits, water tanks, indoor tank coatings, water treatment plants, artificial lakes, foil canals, etc.;
• Baby bottles;
• Toys;
• Cubes.

Modern apps

Polyethylene can form a three-dimensional network when it is subjected to a covalent reaction of vulcanization (cross-linking in English). The result is a polymer with a memory effect. The memory effect in polyethylene and other polymers is that the material has a stable or permanent shape and at a certain temperature, known as the shutter temperature, either Tg or Tm, or a combination, you can get a temporary form that can be modified. The thermal memory effect in polymers is different from the thermal memory effect in metals, found in 1951 by Chang and Read in which there is a change in the crystalline arrangement by means of a martensitic rearrangement. In polymers, this effect is based on entropic forces and points of physical (knots between chains) or chemical (vulcanized) stability.

Other polymers that exhibit the thermal memory effect include: poly(norbornene), polyurethanes, modified polystyrene, and almost any polymer or copolymer that is crystalline or amorphous that can form a three-dimensional network.

Polymers with problems for the thermal memory effect: polypropylene.

Rotomolding is a young process that uses polyethylene powder as raw material. It is placed in a mold, where by means of bi-axial rotation and heating the polyethylene adheres to the wall to form a hollow piece; an example are the water tanks.

Other new applications of PE include the compound of wood flour and PE in percentages ranging from 10% to 70% wood by weight. The result is a stable compound with a higher density than PE.