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IntroductionIntroductionPolyethylene or polyethene is aPolyethylene or polyethene is a
thermoplastic polymerthermoplastic polymer
Basic monomer unit is Basic monomer unit is etheneethene
ClassificationClassificationDesignationDesignation AcronymAcronym Density g/ccDensity g/cc
High Density High Density PolyethylenePolyethylene
HDPEHDPE >0.941>0.941
Low Density Low Density PolyethylenePolyethylene
LDPELDPE 0.910 0.910 –– 0.9410.941
Linear low Density Linear low Density PolyethylenePolyethylene
LLDPELLDPE 0.915 0.915 –– 0.9250.925
Ultra High Ultra High Molecular weight Molecular weight
polyethylenepolyethylene
UHMWPEUHMWPE 0.930 0.930 –– 0.9350.935
Schematic Molecular StructureSchematic Molecular Structure
Polymerization MechanismPolymerization Mechanism
LDPE LDPE –– Radical polymerizationRadical polymerizationLLDPE,HDPE LLDPE,HDPE –– Coordination polymerization Coordination polymerization using catalystsusing catalysts
Back Biting – short branching
CoCo--ordination Polymerizationordination PolymerizationInitiationInitiation: In the initiation reaction the first monomer unit is added to : In the initiation reaction the first monomer unit is added to an active centre. It an active centre. It proceeds through the insertion of ethylene into a metal (e.g. Tiproceeds through the insertion of ethylene into a metal (e.g. Titanium) carbon bond. tanium) carbon bond. TiTi--C2H5 + C2H4 C2H5 + C2H4 TiTi--CH2CH2--CH2CH2--C2H5C2H5..PropagationPropagation: The chain grows by the successive insertion of ethylene molecu: The chain grows by the successive insertion of ethylene molecules.les.
MM--polymer + C2H4 polymer + C2H4 MM--CH2CH2-- CH2CH2--polymerpolymerM = titanium (or chromium)M = titanium (or chromium)
Chain TransferChain Transfer::Transfer with Monomer: Transfer with Monomer: MM-- CH2CH2-- CH2CH2--polymer + C2H4 polymer + C2H4 MM-- CH2CH2--CH3 + CH2=CHCH3 + CH2=CH--polymerpolymer..
Transfer with organometallic compounds:Transfer with organometallic compounds:MM--polymer + (C2H5)3Al polymer + (C2H5)3Al MM--C2H5 + (C2H5)2AlC2H5 + (C2H5)2Al--polymerpolymer..
Spontaneous Chain Transfer:Spontaneous Chain Transfer:MM--CH2CH2-- CH2CH2--polymer polymer MM--H + CH2=CHH + CH2=CH--polymerpolymer..
This is the principle chain transfer reaction with ChromiThis is the principle chain transfer reaction with Chromium based catalysts. Its importance um based catalysts. Its importance increases with increasing temperature and it provides an easy meincreases with increasing temperature and it provides an easy means of controlling molecular ans of controlling molecular weight.weight.Chain Transfer with Hydrogen: Chain Transfer with Hydrogen: MM--polymer + H2 polymer + H2 MM--H + polymerH + polymer--HH
This reaction is the most important chain transfer reactioThis reaction is the most important chain transfer reaction allowing the control of molecular n allowing the control of molecular weight when using weight when using Ziegler Ziegler –– NattaNatta catalystscatalysts
CatalystsCatalysts
Catalysts used for HDPE and LLDPECatalysts used for HDPE and LLDPEChromium oxide catalysts (Phillips catalysts)Chromium oxide catalysts (Phillips catalysts)
Ziegler Natta CatalystZiegler Natta Catalyst
MetalloceneCatalyst
Manufacturing ProcessesManufacturing ProcessesPolymerization in Supercritical EthylenePolymerization in Supercritical Ethylene ––polymerization is carried out at high pressure and polymerization is carried out at high pressure and temperature (110 temperature (110 –– 14014000C and around 120 C and around 120 –– 300 300 MPa). Generally used for LDPEMPa). Generally used for LDPEPolymerization in solution or slurryPolymerization in solution or slurry –– polymer is polymer is dissolved in the solvent which has to be stripped off dissolved in the solvent which has to be stripped off in the last step. Temperature is around 120 in the last step. Temperature is around 120 –– 15015000C C and pressure is 5 and pressure is 5 –– 10 MPa. Generally used for 10 MPa. Generally used for HDPE and LLDPEHDPE and LLDPEPolymerization in the gas phasePolymerization in the gas phase –– polymerization polymerization carried out by agitation by a mechanical stirrer or by carried out by agitation by a mechanical stirrer or by the fluidized bed technique. Temperature is around the fluidized bed technique. Temperature is around 10010000C and pressure is around 2 MPaC and pressure is around 2 MPa
Control of PE propertiesControl of PE propertiesDensity: 2 factors used to control densityDensity: 2 factors used to control density
1. Amount of co1. Amount of co-- monomer (e.g. 1monomer (e.g. 1--butene, 1butene, 1--hexene, hexene, 11--octene, 4octene, 4--methylmethyl--11--pentene). Thispentene). Thisis generally used for catalytically produced is generally used for catalytically produced PE (HDPE,LLDPE)PE (HDPE,LLDPE)
2. Temperature used to control density of LDPE 2. Temperature used to control density of LDPE produced by free produced by free –– radical polym.radical polym.Molecular weight:Molecular weight:
1. Phillips catalyst 1. Phillips catalyst –– reaction temperature reaction temperature 2. Radical polymerization, Ziegler catalysts and 2. Radical polymerization, Ziegler catalysts and
metacellone catalysts metacellone catalysts –– chain transfer agentschain transfer agents
Comparison of propertiesComparison of propertiesPropertyProperty LDPELDPE HDPEHDPE LLDPELLDPE UHMWPEUHMWPE
Molecular Molecular massmass
87,00087,000 96,00096,000 96,00096,000 4.5x104.5x1066
Yield Yield Strength Strength
MPaMPa
12.412.4 26.526.5 10.310.3 2222
Rupture Rupture Strength Strength
MPaMPa
1212 21.121.1 25.325.3 4444
Impact Impact energy energy kJ/mkJ/m22
6161 55 6363 210210
USESUSESLDPELDPE
Film for food packagingFilm for food packagingSealants and insulatorsSealants and insulatorsAdhesivesAdhesivesCarry out bagsCarry out bagsSqueeze bottlesSqueeze bottlesmoldedmolded laboratory equipment laboratory equipment
USES (contd.)USES (contd.)
HDPEHDPEWater pipesWater pipesContainers (milk, laundry detergent etc.)Containers (milk, laundry detergent etc.)Electrical wire insulationElectrical wire insulationOther plastics packaging (caps, trays etc.)Other plastics packaging (caps, trays etc.)Trash BagsTrash BagsIndustrial drumsIndustrial drums
USES (contd.)USES (contd.)
LLDPELLDPEplastic bagsplastic bags and sheets (where it allows using and sheets (where it allows using lower thickness than comparable LDPE)lower thickness than comparable LDPE)plastic wrapplastic wrapflexible tubing flexible tubing toys toys buckets and containers buckets and containers
USES (contd.)USES (contd.)
UHMWPEUHMWPEhigh high modulusmodulus fibers for bulletproof vestsfibers for bulletproof vestsorthopedic orthopedic implantsimplants ((artificial hipsartificial hips, knees), knees)Due to its low friction and wear resistance it is Due to its low friction and wear resistance it is used in industrial impact, wear, and sliding used in industrial impact, wear, and sliding applications in both normal and corrosive applications in both normal and corrosive environment environment Coatings for grain silos Coatings for grain silos