In terms of volume, polyethylene is the most used and important type of plastic. This is partly due to PE’s incredible resistance to water and moisture, as well as its good resistance to virtually all chemicals and solvents.
Polyethylene is a polymer produced by high-pressure or low-pressure polymerization of ethylene. PE is a partially crystalline thermoplastic, meaning that polyethylene becomes malleable when exposed to heat, and it has a glass transition temperature of -120 °C, meaning that if exposed to temperatures above this, it loses its shape stability. Before heating, polyethylene appears as an opaque, tough, and flexible material.
Variants of Polyethylene and their Properties
Polyethylene exists in several different variants – certain variants of PE plastic are more flexible and malleable than others. The most common types of Polyethylene are HDPE (High Density Polyethylene) and LDPE (Low Density Polyethylene). HDPE is the most shape-stable variant and is therefore often used for products such as buckets, water and drainage pipes, bottles, and containers. Whereas HDPE is the most shape-stable, LDPE is the toughest, although it is not quite as strong. This variant is often used for products such as plastic bags, bottles, cable insulation, and coating in cartons, such as milk cartons, which are lined with PE plastic on the inside.
In addition to these two variants, there are also several others, but it only applies to these two variants that they can be injection-molded or extruded. The remaining variants are VLDPE (Very Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), MDPE (Medium Density Polyethylene), and UHMWPE (Ultra High Molecular Weight Polyethylene) – these can only be mechanically processed.
Composition of Polyethylene
For polyethylene, it applies that there is a great difference in the degree of molecular order, which describes the crystallinity of polyethylene – this helps to show the nature of polyethylene, but at the same time also helps to determine the density and properties of polyethylene. The density, properties, and nature of polyethylene help determine what each variant can be used for.
Variants with low density have many branches in the molecular order, which means that the same amount of molecules cannot be present in the composition, and therefore the density is lower, unlike variants with high density, where the molecular order contains fewer branches, but instead many linear molecules, and therefore the density is higher.
For the variants with low density (LDPE, LLDPE, VLDPE), it applies that these are tougher, due to the flexibility of the branched molecules, as well as the low density and therefore weight of the variant. Variants with high density (HDPE, UHMWPE) are instead much stronger, more rigid, and more malleable, due to the large density and the linear molecules.
Use of Polyethylene
Variants with low density are often used for products that require high flexibility and toughness, such as food foil, carrier bags, and other plastic bags.
Variants with high density, on the other hand, are often used for products with special requirements for strength properties – these products could, for example, be crates, drums, transport tanks, as well as a wide range of consumer products.
