Generally speaking, polyethylene is a general term for a large class of ethylene homopolymers and copolymers. According to the difference in molecular structure and density, polyethylene is roughly classified into low density polyethylene, high density polyethylene, and linear low density polyethylene. Even the three most common types of polyethylene have different properties. Let’s take a closer look at each of them.
Low density polyethylene, the abbreviation LDPE, is the first commercial polyethylene produced. In the 1930s, it was first commercialized by ICI. Low density polyethylene is a homopolymer synthesized by free radical polymerization of ethylene monomer under extremely high pressure. Although there is only one monomer of ethylene, the low density polyethylene molecules have a highly branched structure due to the severe reaction conditions. A large number of long chain branches make it very good in extrusion properties and melt strength. It is also subject to a violent reaction process, and there are few conditions that can be controlled in the reaction, so the density of the low-density polyethylene is narrow. Common low density polyethylenes are mostly between 0.915 and 0.930. Most low density polyethylenes have a melting point of around 105 degrees.
High-density polyethylene, the abbreviation HDPE, is the first polyethylene to be synthesized using a catalyst. The first commercial production was in the 1950s. Due to the introduction of the catalyst, the reaction conditions are not as harsh. The polymerization can be carried out at a lower pressure, so it is also called “low pressure polyethylene” in China. Also due to the action of the catalyst, the polymerization process can be carried out gently and orderly. Thereby a regular single-chain structure is obtained without any long chain branching. In the polymerization process, only a small amount of comonomer (butene or hexene) is added, even without comonomer, so that the high-density polyethylene has only a very small short-chain structure on the molecule. This gives it a high degree of crystallinity and a higher density. Polyethylene having a density exceeding 0.940 is generally referred to as high density polyethylene. Of course, typical high-density polyethylenes have a density of 0.950 or higher and a melting point of more than 130 degrees. High crystallinity gives high density polyethylene a good stiffness and moisture barrier, but it also reduces its transparency and gloss.
Linear low density polyethylene
With the development of polymerization technology, linear low density polyethylene (LLDPE) was successfully developed in the 1970s using Ziegler-Natta catalysts. As the name implies, linear low-density polyethylene is characterized by a linear single-chain structure with a lower density. The most common linear low density polyethylene has a density around 0.920. In practice, however, the density of linear low density polyethylene can be well regulated by controlling the amount of comonomer in the polymerization. Theoretically any density between 0.855 and 0.960 can be produced. However, common specifications are between 0.915 and 0.940. Due to the difference in comonomers, linear low density polyethylene can be further divided into four carbon linear (butene copolymer), six carbon linear (hexene copolymer) and eight carbon linear (octene copolymer). In general, the larger the comonomer, the better the physical properties.
Metallocene catalysts were invented and commercialized in polyethylene in the 1990s. The polyethylene obtained by catalytic polymerization of metallocene is called metallocene polyethylene. Strictly speaking, it is actually a linear low density polyethylene. However, due to the more precise control of the molecular structure of the metallocene catalyst, the metallocene polyethylene can provide very excellent toughness and heat sealing properties. At the same time, it is also possible to produce products in a wider range of densities. From elastomers to plastomers at low temperatures, the most common 0.920 density or even medium to high density can be produced using metallocene technology.