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