Polyethylene is the most common form of plastic that is used in our world. It is normally used in plastic bags ,plastic bottles, plastic films and many more. Most of them have the chemical formula of (C2H4)nH2. The IUPAC name for polyethylene is polyethene or polymethylene.
Physical properties.
Polyethylene is a thermoplastic polymer. They consist of a long hydrocarbon chain. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 180 °C (248 to 356 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (221 to 239 °F).
Chemical properties.
Polyethylene have a strong chemical resistance. Strong acids and strong bases can hardly affect them.
They can usually be dissolved at elevated temperature in aromatic hydrocarbons or chlorinated solvents.
There are 2 types of process in this polyethylene.
One of them is monomer.
The ingredient of monomer is ethylene . It is a gaseous hydrocarbon with the formula C2H4. The ethylene has a high purity because it is highly reactive. The specifications for this to happen are <5 ppm for water, oxygen, as well as other alkenes. Ethylene is usually produced from petrochemical sources, but also is generated by dehydration of ethanol.
Polymerization
Ethylene is a more stable molecule compared to other molecules . It polymerizes only upon contact with catalysts. This conversion is highly exothermic. So we can say this conversion gives out a high amount of heat. Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of titanium(III)chloride, the so-called Ziegler Natta catalyst. Another common catalyst is the Phillips catalyst, prepared by depositing Chromium(VI)oxide on silica. Ethylene can be produced through radical polymerization, but this route has only limited utility and typically requires high pressure apparatus.
Classification
Polyethylene can classed into different several groups depending on its density and its branching. Its mechanical properties mainly depends on its branching like molecular weight and crystal structure. The most important polyethylene grades are HDPE, LLDPE and LDPE
High-density polyethylene (HDPE)
-Has density of greater or equal to 0.941 g/cm3.
-has a low degree of branching so have a low intermolecular forces and tensile strength.
-can be produced by chromium/silica catalysts, Ziegler Natta catalyst or metallocene catalysts.
-used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers and water pipes.
-One third of all toys are manufactured from HDPE. In 2007 the global HDPE consumption reached a volume of more than 30 million tons.
Linear low-density polyethylene (LLDPE)
-has a density range of 0.915–0.925 g/cm3.
-has higher tensile strength than LDPE.
-has better environmental stress cracking resistance but is not as easy to process.
-is used in packaging, particularly film for bags and sheets. Lower thickness may be used compared to LDPE. Cable covering, toys, lids, buckets, containers and pipe.
-In 2009 the world LLDPE market reached a volume of almost US$24 billion (€17 billion)
Low-density polyethylene (LDPE)
-has a density range of 0.910–0.940 g/cm3.
-has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. Therefore it has less strong intermolecular forces .
-is created by free radical polymerization.
-the high degree of branching with long chains gives molten LDPE unique and desirable flow properties.
-is used for both rigid containers and plastic film applications such as plastic bags and film wrap.
-In 2009 the global LDPE market had a volume of about US$22.2 billion (€15.9 billion).
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