Polyethylene, falling under the classification of thermoplastic, is a polymer type that can be melted to liquid form and be physically changed or assume a new form when it solidifies. Polyethylene, as the name suggests is chemically synthetic molecules that have poly or many or long chains of ethylene. Ethylene is a monomer with a characteristic to double bond with other monomers, which are carbon-based, in order to form polymers. Polyethylene is also known popularly as polythene in the United Kingdom or simply abbreviated as PE.
In 1898, polyethylene was first created by in a laboratory by Hans von Pechmann. It was created by accident while heating another compound called diazomethane which he also previously discovered. After 35 years, the synthesis of polyethylene by undergoing extreme heat and pressure in industrial setup was discovered again by accident. After few more years, then a chemist of the same company based in England have come up with a process to manufacture polyethylene under the same conditions. With all these, in the year 1939, polyethylene has become the main source of low-density polyethylene (LDPE) production.
Polyethylene is a substance found in various household items like plastic shampoo bottles, milk containers, toys, plastic bags, food wrappers and many other plastic products. It is also used in manufacturing of artificial knee and hip replacement components, Kevlar vests, and even for glassy flooring of ice skating rinks. The aforementioned are just a few examples of the significance of polyethylene to the plastics industry.
Melting point and glass transition may be noticed but are dependent on certain factors like crystallinity and molecular weight. The temperature for the melting point and glass transition are distinct for every type of polyethylene under consideration. The temperature range of 120 to 130 degrees Centigrade (248 to 266 degrees Fahrenheit) is the typical melting point for commercial grades of medium and high density polyethylene. While for low density polyethylene, the temperature range is only from 105 to 115 degrees Centigrade (221 to 239 degrees Fahrenheit).
Due to the crystallinity property of the polyethylene, low-, medium-, and high-density PE have excellent chemical resistance. Moreover, they do not dissolve at room temperature. They can only be dissolved at high temperatures in aromatic hydrocarbons like toluene or xylene, or with chlorinated solvents like trichloroethane or trichlorobenzene.
Classification of Polyethylenes
There are several classifications of polyethylene. Each is distinguished by the molecular weight and branching properties, which are, in turn, influenced by its crystallization. For example, low-density polyethylene is branched polyethylene. This is because its carbon molecules are connected to long chains of polyethylene and not with hydrogen. If a linear structure consisting of carbon and hydrogen results, this is referred to as high-density polyethylene. Aside from low- and high-density PE, other variants may also be produced like ultra high molecular weight PE, medium-density, and very low-density PE.
Due to the fact that plastics have been a global environmental concern because they are not biodegradable and remains in landfills for long years, scientists are formulating solutions to make plastics biodegradable. They are experimenting on using Sphingomonas, a certain type of anaerobic bacteria which can decrease the length of time to degrade polyethylene in a matter of months. Also, bioplastics are being developed to aid in synthesizing polyethylene from ethanol which is a derivative of sugarcane.
Two companies, Braskem and Toyota Tsusho Corporation formed joint marketing activities to obtain from sugar cane a by-product called green polyethylene. In Triunfo, RS, Brazil , Braskem have built an industrial facility that has a capacity to produce 200,000 short tons (180,000,000 kg) of High Density Polyethylene and Low Density Polyethylene from sugarcane’s derivative, bioethanol.
Polyethylene terephthalate is commonly abbreviated as PET, PETE, or PET-P is a thermoplastic polymer resin. It is part of the polyester family and finds application in synthetic fibers, food and beverage containers, thermoforming, and engineering resins. This is oftentimes coupled with glass fiber.
World’s PET production is primarily for synthetic fibers which is more than 60%. 30% is for bottle production in a global demand basis. For textile industries, PET is known as “polyester”, but for packaging applications, it is known as “PET”.
PET comes in some trade names. Dacron, Diolen, Terylene, Tergal, Trevira, Clearcut, Eastman PET, and Polyclear bottle resins are just a few trade names. For films, some trade names are Hostaphan, Melinex, and Mylar. For injection molding resins, some examples are Arnite, Ertalyte, Impet, Rynite, and Valox. 18 percent of world polymer production is polyester industry and ranked third following polyethylene (PE) and polypropylene (PP).
Uses of Polyethylene Terephthalate
PET has some characteristics. It can be rigid and semi-rigid, dependent on its size or thickness. Another characteristic of PET is its being lightweight. It finds application as a barrier to moisture, alcohol, and other solvents. It is also colorless and highly transparent.
Another main application of PET is for soft drink bottles. That’s why soft drink bottles are often called PET bottles. For specialized applications requiring reduced permeability to air or oxygen, PET includes additional polyvinyl alcohol inside.
Another common application of PET is in films. Mylar film is known for it. PET can be made reflective and opaque by evaporation of a thin film of metal onto it. This is done in order to reduce its permeability and made reflective. Some examples are flexible food packaging, and thermal insulation.
PET is also used in tape applications for its high mechanical strength. Examples are magnetic tapes and pressure adhesive tapes.
Possible Toxicity of PET
PET may have some ill effects on human. As published in Environmental Health Perspectives in April 2010, a commentary suggested that PET may induce endocrine disruptors as an effect of constant or common use of products containing it. However, there have been some proposals like leaching of phthalates or antimony as mechanisms. There are however some which say that PET does not cause what some publishers fear in the absence of evidence. The possible toxicity of PET are still the subject of researches, and evidence of its harmful effect are still under consideration.