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 as PEX).
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).
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 are 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 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.
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.
It can also be noted that low-density polyethylene exhibits lower 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.
Frequently Asked Questions on Polyethylene
Q1
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.
Q2
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.
Q3
Cross-linked polyethylene, often abbreviated as 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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