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Polythene (polyethylene):Properties, Production & Uses

Properties and Uses

Property	Low Density Polyethylene (LDPE)	High Density Polyethylene (HDPE)
Melting Point	~115^oC	~135^oC

Crystallinity	low crystallinity (50-60% crystalline) Main chain contains many side chains of 2-4 carbon atoms leading to irregular packing and low crystallinity (amorphous)	highly crystalline (>90% crystalline) contains less than 1 side chain per 200 carbon atoms in the main chain leading to long linear chains that result in regular packing and high crystallinity

Flexibility	more flexible than HDPE due to lower crystallinity	more rigid than LDPE due to higher crystallinity

Strength	not as strong as HDPE due to irregular packing of polymer chains	strong as a result of regular packing of polymer chains

Heat Resistance	retains toughness & pliabilty over a wide temperature range, but density drops off dramatically above room temperature.	useful above 100^oC

Transparency	good transparency since it is more amorphous (has non-crystalline regions) than HDPE	less transparent than LDPE because it is more crystalline

Density	0.91-0.94 g/cm³ lower density than HDPE	0.95-0.97 g/cm³ higher density than LDPE

Chemical Properties	chemically inert Insolvent at room temperature in most solvents. Good resistance to acids and alkalis. Exposure to light and oxygen results in loss of strength and loss of tear resistance.	chemically inert

Schematic diagram

Uses	sandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction	freezer bags, water pipes, wire and cable insulation, extrusion coating

Production of LDPE

Production of LDPE by addition polymerization requires:

temperature range of 100-300^oC
very high pressure 1500-3000 atmospheres
oxygen or an organic peroxide such as dibutyl peroxide, benzoyl peroxide or diethyl peroxide as initiator.
An initiator is a substance which is added in small quantities and is decomposed by light or heat to produce a free radical (R^.). A free radical is formed when a covalent bond is broken and a bonding electron is left on each part of the broken molecule. Since the O-O covalent bond is weak, free radicals are easily formed from oxygen or peroxides.
benzene or chlorobenzene used as the solvent since both polymer (polythene) and monomer (ethene) dissolve in these compounds at the temperature and pressure used.
Water or other liquids may be added to dissipate the heat of reaction as the polymerization reaction is highly exothermic.

CH₂=CH₂ ethene	+	R^. initiator	----->	^.CH₂-CH₂-R
CH₂=CH₂	+	^.CH₂-CH₂-R	----->	^.CH₂-CH₂-CH₂-CH₂-R
Process continues to form polythene (polyethylene) [-CH₂-CH₂-]_n

Production of HDPE

Production of HDPE by addition polymerization with a supported metal oxide catalyst requires:
- temperature ~300^oC
- 1 atmosphere pressure (101.3kPa)
- aluminium-based metal oxide catalyst (metallocene catalyst)
  The catalyst can be used in a variety of operating modes including fixed-bed, moving-bed, fluid-bed or slurry processes
- The ethene (ethylene) monomer is fed with a paraffin or cycloparaffin diluent (diluting agent).
  After polymerization the polymer (polythene) is recovered by cooling or by solvent evaporation.
Production of HDPE by coordination polymerization requires:
- temperature 50-75^oC
- slight pressure
- a coordination catalyst is prepared as a colloidal suspension by reacting an aluminium alkyl and titanium chloride (TiCl₄) in a solvent such as heptane (C₇H₁₆).
- The polymer (polythene) forms as a powder or granules which are insoluble in the reaction mixture. When the polymerization is completed, the catalyst is destroyed by adding water or alcohol to the reaction mixture. The polymer (polythene) is then filtered or centrifuged off, washed and dried.