Polyethylene / PE

The most outstanding property of polyethylene is its low water vapour permeability. Its permeability (diffusion) for gases and aromas, on the other hand, is relatively high.

Due to its higher density, the permeability to water vapour, gases and odorous substances is lower for PE-HD than for PE-LD. Polyethylene is insensitive to water, acids and bases as well as common chemical solvents. It is milky cloudy when uncoloured, but becomes transparent when processed into thin films. Polyethylene is tough and elastic, cold-resistant down to -50 °C and thus practically unbreakable. With regard to high temperatures, the limit for PE-LD is 70 to 80 °C, for PE-HD over 100 °C due to its higher density. The range for welding film is 120 to 135 °C for PE-LD and somewhat higher for PE-HD.

In general, however, the mechanical properties such as hardness at room temperature, toughness, but also transparency and melting range depend very much on the different manufacturing processes, catalysts and monomers and copolymers used, which in turn determine the chain length and degree of branching of the polyethylene.

Since the monomer ethylene is non-toxic, polyethylene is considered harmless to health. However, the added additives can be toxicologically questionable. Polyethylene burns without residue to form carbon dioxide, carbon monoxide and water. It is largely resistant to environmental influences, becomes brittle when exposed to UV radiation and is only very slowly degradable in landfills.

Pure PE can be easily recycled and reused. Its good properties and low raw material costs justify its widespread use and the high proportion of polyethylene in the field of plastic packaging. Polyethylenes are among the most important thermoplastics for the production of films. Due to the variations in production, there are a large number of PE products, some of which differ considerably from one another. Based on the density and structure of the polymer chains, a distinction is made between 3 main groups of polyethylenes, which are manufactured using different processes and each have specific properties. However, the many different PE products also allow overlaps and transitions between these main groups.

On the one hand, polyethylene is processed into monofilms, but is used to a large extent in the production of composite films and composites. PE films are mainly produced using the blown film process. This process makes it possible to improve the mechanical material properties and to combine different polyethylenes.

A small proportion of polyethylene films are produced using the flat film extrusion process. Since no stretching of the film is carried out here, the films obtained in this way have a low strength. However, they have better optical properties and a softer feel. By embossing, soft protective packaging can be produced as wrapping material.

LD means "Low Density" and refers to a low density polyethylene. It is produced under very high pressure and very high temperatures in so-called tubular reactors or autoclaves (the so-called high-pressure process). PE-LD was the first polymer to be obtained from ethylene. The properties of PE-LD are determined by the strong branching of its polymer chain. This structure results in low crystallinity, which leads to a low melting temperature and low strength / stiffness of the films. Films made of PE-LD have a rather soft and waxy feel. They are processed, for example, into sacks and agricultural films with a film thickness between 150 and 250 my. In the production of carrier bags (shopping bags), the film thicknesses are in the range between 30 my and 65 my.

In the blown film process, PE-LD films can be stretched considerably, i.e. they are pulled apart in the longitudinal and transverse directions. This results in film thicknesses of 10 my and below, which are used, for example, as packaging bags in self-service shops.

Due to the stretching, these have a translucent matt appearance and a rather paper-like, crackling feel. PE-LD films have a low water vapour permeability and a relatively high oxygen permeability. In the field of hollow containers, PE-LD is used to make smaller-volume containers such as bottles, crates, barrels, canisters, tins, trays or lids. means "Linear Low Density" and refers to a linear low-density polyethylene. It is produced in solution or gas phase (the so-called gas phase process). Due to the linear branching of its polymer chain, it has similarities to PE-HD, but at the same time has a low density like PE-LD. It thus combines the softness and suppleness of PE-LD on the one hand and the high mechanical strength of PE-HD on the other.

The water vapour permeability and oxygen permeability are similar to those of PE-HD and PE-LD. Due to this combination of suppleness and high mechanical strength, PE-LLD films can generally be manufactured thinner than PE-LD films. Film thicknesses of up to 5 my are possible. However, due to its lower melting point compared to PE-HD, PE-LLD places much higher demands on film extrusion, as film tearing and clogging of the extruder are more likely to occur.

Continuous learning processes, appropriate adjustments to the extruder designs and constant modifications to the polymers have meanwhile overcome initial problems in the production of PE-LLD films. Especially in the field of laminated films, PE-LLD has increasingly replaced PE-LD, as its specific properties enable the production of thinner and thinner films. Continuous research is constantly leading to new PE grades.

The gas phase process enables the production of copolymers, i.e. the combination of other monomers such as propene, butene or hexene with the ethylene monomer. The use of new catalysts also continuously expands the wide variation of polyethylenes and its properties.

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