Polyethylene (PE, Polyethylene) is the most used material in the plastic industry. Polyethylene is sometimes known as Polythene. In 2017, this material took about 34% of the entire plastic market. Its primary use is in packaging (plastic bags, bottles, etc.).

Polyethylene is a thermoplastic polymer (a polymer that melts by giving heat and solidifies again by cooling it) consisting of ethylene gas monomers (C ₂ H ₄ ). Although there are different types of polyethylenes, the main base of all of them is C ₂ H ₄ monomer .

Chemical composition and synthesis

Polyethylene is obtained from the polymerization of ethylene. Ethylene is a gaseous hydrocarbon that is usually produced by cracking ethane, one of the main components of natural gas that can also be extracted from oil. Ethylene molecules are actually composed of two methane molecules that are connected to each other by a double bond. By being exposed to the catalyst in the act of polymerization, the double bond is broken and the ethylene molecule can form a bond with the adjacent ethylene and a polymer chain is formed.

This polymer can be formed in a linear or branched structure. Branched structures are known as low-density polyethylene (LLDPE) or linear low-density polyethylene (LLDPE). Linear structures are known as high-density polyethylene (HDPE) or ultrahigh molecular weight polyethylene (UHMWPE).
The composition of polyethylene can include elements or other chemical groups, which adds to the diversity of its compositions. More about these compounds will be discussed below.

History

Light polyethylene was discovered in England in 1933 while studying the effect of high pressure on polymerization. In 1937, the patent of its synthesis process was registered. Its first use was in World War II as an insulator for radar cables.

Types of polyethylene

Low-density polyethylene

Light polyethylene (LDPE) is formed from ethylene gas under very high pressure (about 350 MPa) and high temperature (about 350 degrees Celsius) in the presence of initiator oxides. This process leads to the production of a polymer structure including short and long branches. Because the branches prevent ethylene molecules from coming too close to each other, LDPE is a very flexible structure. Its approximate melting temperature is 110 degrees Celsius. Its uses are in packaging films, garbage, shopping bags, agricultural mulch, wire and cable insulation, bottles, toys and home appliances.

Linear low-density polyethylene

Linear light polyethylene (LLDPE) is structurally similar to LDPE. This structure is the copolymerization of ethylene with 1-butene and smaller amounts of 1-hexene and 1-octene, which is synthesized using Ziegler-Natta and Metallocene catalysts. The final structure has a linear base but has uniform and short branches that, like the longer branches of LDPE, prevent the polymer chains from approaching each other.

In general, LLDPE has the same properties as LDPE. The main advantage of LLDPE is that its polymerization conditions require less energy than LDPE and that its properties can be changed by changing the amount of other components.

High-density polyethylene

Heavy polyethylene (HDPE) is synthesized at low temperature and pressure. It uses Ziegler-Natta and Metallocene catalysts or active copper oxide (Phillips catalyst). Its lack of branches allows the polymer chains to stick to each other. The result is the creation of a dense and highly crystalline structure that has high strength and moderate stiffness. Its melting point is 20 degrees Celsius higher than LDPE and it can withstand several stages of being exposed to a temperature of 120 degrees, as a result of which it can be sterilized.

The uses of HDPE include milk bottles, shopping bags, metric and construction plastics, agricultural mulch, syringes, doors and toys.

Ultrahigh-molecular-weight polyethylene

Linear polyethylene can be synthesized with high molecular weight, from 3 to 6 million atomic units. Meanwhile, HDPE has about 500,000 atomic units. These polymers can be spun into the fibers and give them a very high tensile strength. This polymer is used in bulletproof vests.

Ethylene copolymers

Ethylene can be copolymerized with many other compounds. For example, ethylene-vinyl acetate copolymer (EVA) is synthesized by copolymerizing ethylene and vinyl acetate under pressure and using free radical catalysts. Different grades are produced from it, in each of which the weight ratio of vinyl acetate varies from 5 to 50%. EVAs are more permeable to water and gases than polyethylene, but they have less crystallinity and more transparency. They also show better resistance against oil and grease. EVA is used in packaging sheets, adhesives, toys, sealants, covering wires and carpets.

Ethylene acrylic acid and ethylene methacrylic acid copolymers are formed through polymerization in suspension or emulsion using free radical catalyst. Acrylic acid and meta acrylic acid usually make up 5 to 20% of the structure. These copolymers are used in car parts, packaging sheets, shoes, surface coatings and carpets.

Polyethylene problems

Polyethylene is actually not biodegradable and does not decompose easily. Of course, there are bacteria and organisms that can break it down, but in general, one of the problems of using polyethylene is its non-biodegradability.

Of course, polyethylene can be recycled, and if the recycling process is done well, it can greatly reduce the costs of its re-production.
It should not be forgotten that polyethylene is extracted from oil or natural gas, which is considered a non-renewable resource.