What is Polyethylene?

Polyethylene, also known as polythene or polyethene, is one of the most commonly used plastics in the world. Polyethylenes usually have a linear structure and are known to be addition polymers. The primary application of these synthetic polymers is in packaging. Polyethelyne is often used to make plastic bags, bottles, plastic films, containers, and geomembranes. It can be noted that over 100 million tonnes of polyethene is produced on an annual basis for commercial and industrial purposes.

The general formula of polyethylene can be written as (C2H4)n. Most types of polyethylene are thermoplastic (they can be remoulded by heating). However, some modified polyethylene plastics exhibit thermosetting properties. An example of such a class of polyethylene is cross-linked polyethylene (often abbreviated to PEX).

Preparation of Polyethylene

The primary constituent of polyethylene is ethylene (an organic hydrocarbon with the chemical formula C2H4; IUPAC name: ethene). For the production of polyethylene, the typical specifications involve less than 5 parts per million of oxygen, water, and other alkenes. However, other compounds can be present during the polymerization reaction as contaminants. Some commonly accepted contaminants during the production of polythene include nitrogen, methane, and ethane.

Since ethene is a relatively stable molecule, its polymerization requires suitable catalysts. It is important to note that the conversion of ethylene into polyethylene is highly exothermic in nature. One of the most commonly used catalysts for the polymerization of ethylene is titanium(III) chloride (which is sometimes referred to as a Ziegler-Natta catalyst).

What are the Physical and Chemical Properties of Polyethylene?

Physical Properties

  • The mechanical strength of polyethylene is relatively lower than other plastics. The rigidity and the hardness of these polymers are also relatively low.
  • Polyethylene is known to be highly ductile. Furthermore, this plastic is known to possess very high impact strength.
  • This synthetic polymer exhibits strong creep when placed under a persistent force.
  • Polyethylenes usually have a waxy texture.
  • The melting points of commercial grades of high-density polyethylene (HDPE) and medium-density polyethylene (MDPE) lie in the range of 120 – 180 degrees Celsisus.
  • The melting point of the commercially available low-density polyethylene (LDPE) usually lies in the range of 105 – 115 degrees Celsius.
  • Polyethylene is known to be a very good insulator of electric current since it offers high electrical treeing resistance.

Chemical Properties

Polyethylene is made up of nonpolar saturated hydrocarbons with very high molecular weights. This is believed to be the reason why the chemical properties exhibited by polyethylene is quite similar to those of paraffin. It can be noted that the individual polyethylene macromolecules are not linked via covalent bonds. However, these molecules crystallize due to their rather symmetric molecular structures. Therefore, polythene can be considered as a partially crystalline plastic. The greater the crystallinity of the polymer, the greater its density and chemical stability.

It is important to note that most types of polyethylene have very high chemical resistance towards acids and alkalis (including LDPE, MDPE, and HDPE). These plastics are also resistant towards weak oxidizing agents and weak reducing agents. Most polyethylenes are known to be soluble in aromatic hydrocarbons like xylene or toluene under elevated temperatures.

Types of Polyethylene

Polyethylene can be classified into several different types based on the density of the plastic and the degree of branching in its structure. The type of branching and the extent of branching has a direct impact on the mechanical properties of the plastic. Therefore, different types of polyethylene exhibit different mechanical properties. Some important types of polyethylene are listed below.

  • High modulus polyethylene, also known as ultra-high molecular weight polyethylene (UHMWPE)
  • Ultra-low-molecular-weight polyethylene (or ULMWPE)
  • High-density cross-linked polyethylene (HDXLPE)
  • High-density polyethylene (HDPE)
  • High-molecular-weight polyethylene (HMWPE)
  • Cross-linked polyethylene (XLPE)
  • Medium-density polyethylene (MDPE)
  • Low-density polyethylene (LDPE)
  • Linear low-density polyethylene (LLDPE)
  • Chlorinated polyethylene (CPE)

It can also be noted that the low-density polyethylene exhibits lowe crystallinity than high density polyethylene. The crystallinity of polythene is known to range from 35% for low-density polyethylene to 80% for high-density polyethylene.

Uses of Polyethylene

  • The most important application of polyethylene is in packaging products. This plastic is often employed for the production of plastic bags, plastic films, bottles, geomembranes, and containers.
  • Polyethylene is also used in crates, trays, jugs that carry milk or fruit juices, and other food packaging products.
  • High-density polyethylene is used in toys, garbage containers, ice trays, and other houseware. The versatility of this plastic makes it ideal for a wide spectrum of applications.
  • HDPE is also used in ropes, fishing nets, agricultural nets, and industrial fabrics. It is not uncommon for this plastic to be used in wirings and cables as well.
  • Low-density polyethylene (LDPE) is widely used in the production of squeeze bottles, garbage bags, laminations, and food packaging due to its high flexibility and low cost.
  • LDPE is also used in pipes and fittings. It is ideal for such applications due to its low water absorption and also due to its plasticity.
  • Polyethylene is also used for cable jacketing since it is a good insulator of electric current.

Frequently Asked Questions on Polyethylene

What is the chemical composition of polyethylene?

Polyethylene is primarily made up of the monomer ethylene. Ethylene is a chemical compound with the formula C2H4. It is a gaseous hydrocarbon which can be generated by ethane cracking. Ethylene molecules are essentially made up of two methylene units which are linked together by a double bond between the two carbon atoms. This structure can be represented by the formula CH2=CH2. This double bond can be broken by placing the molecule under the influence of polymerization catalysts. The resulting extra single bond can be employed to link another carbon atom to the ethylene molecule. Thus, the ethylene molecule can be made into a large, polymeric molecule.

What are the uses of polyethylene?

The primary uses of polyethylene are in packaging film, garbage bags, grocery bags, insulation for wires and cables, agricultural mulch, bottles, toys, and houseware. Polythene is also used in trays, fruit juice containers, milk containers, crates, and food packaging products.

What is cross-linked polyethylene?

Cross-linked polyethylene, often abbreviated to PEX or XPE, is a type of polyethylene that features a cross-linked chemical structure. The primary application of cross-linked polyethylene is in the construction of pipework systems and radiant heating and cooling systems. It is also used for domestic water piping and as insulation for high voltage electric cables.

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