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Polyethylene

Polyethylene has become an integral part of our life and the development of this compound has become a life saver for many application that we get to see around us. It was produced without any specific purpose until 20th century when this compound became the darling of many of the chemists around the world. In the initial period the polyethylene was found to be highly branched material with very low density and very specific set of physical properties the commercialisation of the compound paved way to get this compound as inherent part of the war scenario and many of its application was in fact considered as an answer to war material. 

It’s only after 1960s the new methods adopted for polymerisation led to production of linear polymers with high density and hence enhance the life cycle of the compound. In recent times the use of resin as additives to this wonder compound rejuvenated the application field of this synthetic compound and more so for specific daily use applications.

 

Polyethylene Definition

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The preparation of polyethylene on record belongs to Von Pechmann in 1898 and then quickly followed by Bamberger. In both the cases polyethylene was prepared by decomposition of diazomethane but the commercialisation didn’t take place till the world war. The decomposition of diazomethane gives polymethylene but its difference with linear polyethylene is with the number of carbon atoms. The polyethylene must have even number of carbon atoms whereas the polymethylene can have as many number of carbon atoms. 

The first polyethylene was slated for output and used in submarine insulation for communication cable but with outbreak of war, the priorities changed and diverted for use in radars as cable insulation. The polyethylene high dielectric strength and low loss factor made this compound the ideal one for such applications. The use of polyethylene as an insulator and as enabled component are made much smaller than the traditional insulation. The early formulations for this compound polyethylene was carried out by adding polyisobutylene and make this flexible at low temperature and also increase the plasticity characteristics. 

In its simplest form the polyethylene molecule consist of long backbone of an even number with linkage of covalent carbon atoms and pairs of hydrogen atoms attached to each of these carbon atoms. Each of these chains ended with methyl groups. The pure form of polyethylene resins contains the polymerised alkanes turned into a polymer. 

And quite unlike the usual organic materials the polyethylene does not consist of identical molecules. These polyethylene resins comprise chains with range of backbone lengths. As and when the polyethylene’s degree of polymerisation is less than 8 the compounds exist as gases or liquids at room temperatures and normal pressure. The moment the degree increases beyond that the material turns solid. The polyethylene molecules can be classified into different categories based on various degrees along with small amount of unsaturation. 

Many types of polyethylene exist in many forms of branches which ranges from simple alkyl groups to more complex esters and acid functionality. Variation arise only due to defects in polymer backbone which contains vinyl groups found at the end of chain. 

Polymer Backbone
 
The higher the concentration of branches the lower is the density of the solid. There are few specific classes of polyethylene like high density polyethylene, low density polyethylene, linear low density polyethylene and very low density polyethylene. The high density polyethylene is the closest to pure form of polyethylene as far the structure is concerned. The high density polyethylene resins with very less branching is termed as linear polyethylene. The low density polyethylene is named as such because it contains huge concentrations of branches which hinder the crystallisation process and this results in low density. The numerous branches characterises the low density polyethylene molecule which results in ability of crystallisation. 

The linear low density polyethylene resins consists of molecules with linear polyethylene backbones to which are attached short alkyl groups at random intervals. These are produced by copolymerisation of ethylene with alk-1-ene. The very low density polyethylene is a specialised form of linear low density polyethylene that has much higher concentration of short chain branches. The high level of branching inhibits the crystallisation in an effective manner. 

The molecular composition of the polyethylene resin consist of molecules exhibiting molecular length of distribution and branching characteristics. The size of the polyethylene molecule is described in terms of its molecular weight and all polyethylene resins contains a mixture of molecules with a range of molecular weights. The properties of polyethylene completely depends upon the average molecular weight and the distribution of chain lengths. 

The molecular weight distribution within a polyethylene can be attributed to various molecular weight averages. This molecular weight characteristic has deep impact on the physical properties of Polyethylene resins and effects the viscosity, impact strength and cracking. 

Polyethylene Glycol

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The Polyethylene glycol is one of the derivatives of Polyethylene produced by reacting ethylene oxide with water. The catalyst present are either acidic or basic in nature. The chain length of this derivative of Polyethylene depends upon the reactant ratio. The reaction can either be cationic or anionic but it’s the anionic mechanism which is preferred as it helps in producing Polyethylene glycol with lesser polydispersity. 

Polyethylene Glycol
 
The Polyethylene glycol is mainly used as laxative base, and also as the preparatory material to clear the bowel before major surgeries. Due to its lower toxicity it is used as lubricant in many useful items. The use of Polyethylene glycol as polar stationary phase in many gas chromatography tests is well documented. The use in electronic testers as fluid of heat transferring is also well know. 

How is Polyethylene Made?

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The main two forms of polyethylene are low density polyethylene and high density polyethylene. The preparation for these forms also differ along with their overall characteristics. The low density polyethylene or LDPE is prepared by polymerisation of ethylene monomers in presence of initiator like peroxide which helps in the formation of free radicals. These radicals helps in the initiation of polymerisation. The pressure of the reaction system is kept at 1000 atmosphere and temperature of 300 C. 

The high density polyethylene is prepared by combining ethylene monomer but at a very low pressure and temperature of 60 C. The catalysts used can be of organic origin but the reaction takes place on the surface of the catalyst. 

Polyethylene Properties

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The polyethylene with so many spectrum of physical properties is used in multiple discipline and application. The adaptability lies mainly in its highly tuned semi crystalline morphology. These are controlled by manipulating molecular and various processing methods. The manipulation of polyethylene before and after crystallisation is also responsible for all kinds of solid state properties. 
  • The high density polyethylene has high density allowing very high degree of crystallisation and provides high stiffness in turn. 
  • The high density polyethylene has lowest permeability 
  • The high density polyethylene has relatively high melting point
  • The high density polyethylene has high tensile properties 
  • The high density polyethylene are chemically resistive.
  • The low density polyethylene with short chain has low density and hence lower crystalline properties 
  • The low density polyethylene has lower melting point 
  • The low density polyethylene has low viscosity 
  • The linear low density as well as very low density polyethylene are non-crystalline in nature 
  • These linear low and very low density polyethylene have superior strength 
The very low density polyethylene are used where the clarity of product is essential as well as the toughness to prevent damages. The bulky acetate side groups inhibits sliding of chains past one another during any kind of deformation.  

Polyethylene Uses

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  • Because of its varied forms and crystalline properties these are widely used in packaging industries
  • All kinds of adhesive tapes are made up of these polyethylene films
  • The coating of cardboards to prevent exposure to moisture is carried out by these polyethylene
  • Food packaging is done mainly by high density polyethylene
  • The rain coat materials are made compatible with these high density materials to prevent water seepage
  • Long distant transport of perishable materials like meat products are made from polyethylene
  • The stretching tapes to pack materials during transport are made up of these low density polyethylene
  • The blister packs to prevent damage to breakable items are carried out by these very low density polyethylene 
  • These are used as water storage tanks due to their excellent low permeability properties 

Polyethylene Terephthalate

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The polyethylene terephthalate or PET are nothing but modification of the normal polyethylene polymer as these polymer films need additional surface treatments to help modify surface properties. These are done for better wettability and adhesion activities. This polyethylene terephthalate has a variable density and it varies from amorphous form to crystalline form. The crystalline properties are attributed to its crystal defects which are caused by crystallisation conditions while the amorphous density variations are formed from conformational restrictions. 

The polyethylene terephthalate is produced by the ethylene glycol polymerisation along with terephthalic acid. The colourless ethylene glycol are heated together in presence of catalysts which forms a viscous, molten matter known as polyethylene terephthalate. 
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