Polyethylene

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Polyethylene
Ken Anderson Polyethylene RD The Dow Chemical
Company Freeport, Texas Invited Lecture for Chem
470 Industrial Chemistry Prof. Michael Rosynek,
Texas AM University April 7, 2006
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• My background
• B.S. Chemistry, Tarleton State Univ.,
  Stephenville, TX, 1978
• Ph.D. Polymer Science, Univ. of Southern
  Mississippi, 1984
• Joined Dow Chemical in 1983 in Epoxy Products RD
  then moved to Polyethylene Product Research in
  1996
• My present role at Dow
• Product Research Leader for Solution PE
  technical mentor to younger members of Product
  Development group
• Design of molecular architecture for new product
  development and development of structure-property-
  performance interrelationships
• Interface with catalysis, characterization,
  material science, intellectual property, process
  development, pilot plants, fabrication,
  Manufacturing, TSD, and Marketing, with
  occasional customer interaction to execute
  product development
• RD rep on North American Films Market Management
  Team

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Part of The Ethylene Chain
Natural Gas Liquids (Ethane, Propane) or Naphtha
(from Crude Oil)
Steam Cracking
Ethylene, Propylene
Other Polymers
Chemicals
POLYETHYLENE
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-(-CH2-CH2-)n-
Ethylene
Polyethylene
Any Questions?
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Polyethylene The Largest Volume Thermoplastic
2004 Annualized Capacity Billions of Pounds
151
92
90
75
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PE Demand by Region 2004 Global PE Demand 136
Billion Pounds
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Markets/Applications for PE

• Rigid and flexible packaging
• Films, Bottles, Food Storage, Shrink film
• Hygiene and medical (nonwovens)
• Pipe, Conduit, and Tubing
• Fibers
• Consumer and industrial liners
• Automotive applications
• Stretch film and heavy duty shipping sacks (HDSS)
• Agricultural films silage, mulch, bale wrap
• Elastomers, Footwear
• Wire and Cable
• Durables, Toys

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Fabrication Versatility

• Film (blown and cast) extrusion
• Injection molding
• Blow molding
• Sheet, profile, or pipe extrusion
• Thermoforming
• Rotomolding
• Extrusion coating - Lamination
• Foaming
• Fiber spinning
• Wire Cable

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PE Demand by Conversion Process 2004 Global PE
Demand 136 Billion Pounds
Food Packaging Hygiene Medical Consumer
Ind. Liners Stretch Films Agricultural
Films HDSS
Film
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World Leaders in Polyethylene Production

Dow ExxonMobil SABIC Sinopec Innovene Chevron
Phillips Basell Lyondell/Equistar Borealis Total
Formosa Plastics NOVA Chemical Polimeri
Europa PetroChina
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Types of Polyethylene
HDPE (0.940-0.965) High Density
LLDPE (0.860-0.926) Linear Low Density
O
O
O
C-OH
O
O
O
O
O
O
O
O
LDPE (0.915-0.930) Low Density
High Pressure Copolymers (AA, VA, MA, EA)
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Other Ethylene-Containing Polymers

• EPDM rubber
• Ethylene-Propylene rubber
• Impact copolymer polypropylenes
• Random copolymer polypropylenes
• Chlorinated PE
• Maleic Anhydride-grafted PE
• Ionomeric salts of EAA or EMA

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Classification of PE by Molecular Architecture

• PE resins can be distinguished by their unique
  combinations of the following attributes
• molecular weight distribution (MWD)
• short chain branch distribution (SCBD)
• interrelation of SCBD across MWD
• degree of long chain branching
• comonomer type and level
• These are dictated by polymerization chemistry
  and reaction conditions.

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Classification of PE by Polymerization Chemistry

• Free radical polymerization
• LDPE
• Coordination Polymerization via Catalyst
• HDPE and LLDPE

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Classification of PE by Polymerization Chemistry

• Free radical polymerization LDPE
• extremely high pressures, using organic peroxides
• formation of both long short branches by side
  reactions
• can utilize polar comonomers, e.g. AA, VA
• first practical form of PE, discovered in 1930s

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Discovery of LDPE Reaction

• Date March, 1933
• Company Imperial Chemical Industries (ICI)
• Location Winnington, England
• Inventors R. O. Gibson and E. W. Fawcett
• High pressure research program (effects on
  reaction rates)
• Ethylene/benzaldehyde system at 170 deg C and
  29,000 psi
• Unexpected loss of reaction pressure
• Obtained minute quantities of waxy, white solid
  (LDPE)
• Two years of research and explosions to reliably
  reproduce result
• Trace oxygen initiated ethylene polymerization
• First commercial autoclave train started up in
  1939 in England.
• Tubular reactor technology developed by UCC
  during WW II

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Free Radical Polymerization of LDPE
Typical Propagation Mechanism
H
H
.
CH2 .
CH2-CH2-CH2
CC
H
H
The active center is transferred from the end of
the growing chain to a position on one of the
ethylene carbons and the process continues
forming longer and longer polyethylene chains
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Free Radical Polymerization of LDPE
Back-biting Mechanism Short Chain Branching
The active center is transferred from the end of
the growing chain to a position along the back of
the chain and chain growth proceeds from this
position.
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Free Radical Polymerization of LDPE
Chain Transfer to Polymer Long Chain Branching
The active center is transferred from the end of
the growing chain to a position on a dead chain
that allows that chain to begin forming a long
chain branch.
Your class notes have these reactions illustrated
in greater detail.
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Typical High Pressure, Low Density PE Process
Low pressure recycle
Purge to LHC
High pressure recycle
CTA
Reactor
HPS
(16-39,000 psi)
Compressor
LPS
Extruder
Secondary or Hypercompressor
Ethylene
Compression ? Reaction ? Devolatilization ?
Extrusion
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Example of Autoclave PE Reactor
Ethylene
Peroxide
Peroxide
Peroxide
Peroxide
To HPS
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Classification of PE by Polymerization Chemistry

• Coordination Polymerization via Catalyst
• Used for
• HDPE
• LLDPE, when using alpha-olefin comonomers
• Can use solution, slurry, or gas phase processes
• Much lower pressures than free radical
• Lower reaction temperatures, esp. in slurry and
  gas phase (particle-form processes)
• Must manage heat of reaction to maintain reaction
  temperature, esp. in particle-form
• Lower capital cost than LDPE

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Three major coordination catalyst types

• Chromium oxide types so-called Phillips type
• restricted to slurry and gas phase
• dominant type in conventional slurry HDPE
• can be used for LLDPE
• Ziegler-Natta conventional LLDPE
• discovered in 1950s for HDPE and PP
• effectively commercialized in 1970s for LLDPE
• still predominant type for LLDPE
• density limited to ca. 0.900 and above
• Single site catalysts
• constrained geometry and metallocene types
  (mLLDPE)
• both can be used as homogeneous (soluble) or
  supported for particle-form processes (gas,
  slurry)
• relatively recent innovation, commercialized in
  1992
• enables densities all the way down to that of
  amorphous
• enabling rapid growth in specialty polyolefins

Your class notes illustrate the catalyst
chemistry and polymerization mechansims.
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Typical Gas Phase PE Process
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Typical Solution PE Process
Solvent Recovery
Comonomer
Ethylene
Reactor
Devo 2
Devo 1
Polymer
Your class notes also illustrate the Phillips
slurry loop process.
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Linear Low Density Polyethylene (LLDPE)
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INSITE Catalyst Technology

• A novel constrained geometry, single-site
  catalyst technology introduced in 1992 that has
  transformed the polyolefins industry
• An innovation that continues to deliver new
  families of plastics offering new combinations of
  performance and processability
• Exceptional control of molecular architecture and
  polymer design sparking innovation and unique
  solutions

Trademark of The Dow Chemical Company
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LLDPE Molecular Structure Comparison
Homogeneous chain length distribution
Homogeneous short chain branch distribution
Heterogeneous chain length distribution
Heterogeneous short chain branch distribution
INSITE Technology Polymer (typical mLLDPE lacks
long chain branches)
Conventional LLDPE via Ziegler-Natta
Trademark of The Dow Chemical Company
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Semi-Crystalline Morphology
Since SCB disrupt crystallinity, more branching
means fewer and smaller crystals. Conventional
LLDPE is a mixture of small and large crystals
while metallocene LLDPE has more uniform crystal
size distribution
TIE CHAIN
INTERFACE
CRYSTAL CORE
AMORPHOUS
MATERIAL
A 3-d representation of chain-folded lamellae in
semi-crystalline PE is shown in your class notes.
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DSC Melting Endotherms
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Solid State Properties

• Solid state properties are determined by
• Percent crystallinity (density) crystal size
  distribution
• Amount of Short Chain Branching
• Tie-chain concentration (Toughness)
• Short Chain Branching Distribution
• Molecular Weight
• Orientation of both crystalline and amorphous
  phases
• Molecular Weight Distribution
• Long Chain Branching

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Engineering Stress-Strain Response - ITP resins
(Strain Rate - 2.4 min-1)
Samples were cooled at 1 oC/min.
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Decreasing the Crystallinity (Density)

• Is accomplished by...
• Increasing the amount of short chain branching by
  adding comonomer
• And results in...
• Decreasing the modulus (stiffness)
• Decreasing the yield strength
• Improving optics (haze, gloss, clarity)
• Lowering the melting softening points

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Increasing Tie Chain Concentration

• Is accomplished by
• Optimizing Short Chain Branching Distribution
• Increasing the molecular weight
• Increases
• Toughness
• Impact
• Tear (needs balance of tie chain high dens)
• Environmental Stress Crack Resistance (ESCR)

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Properties vs. Density
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What is Molecular Weight ?

• One of the most important properties of a polymer
  is molecular weight.
• The MW is simply the weight of all the atoms in a
  molecule. (The weight of the chain).
• Due to the random nature of the polymerization
  process, all of the polymer chains are not
  exactly the same length.
• This requires that molecular weight be defined as
  an average and as a distribution function (MWD).

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Molecular Weight Distribution Comparison by Gel
Permeation Chromatography
Typical mLLDPE
Mw 73800, Mn 37400, MWD 2.0
Mw 124600, Mn 33200, MWD 3.8
Conventional LLDPE
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ELUTION VOLUME (mls)
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• Melt properties are determined by
• Molecular Weight, esp. viscosity k M3.6
• Doubling Molecular weight leads to ten
  fold increase in viscosity
• Molecular Weight Distribution
• Long Chain Branching
• As molecular weight increases
• Processability becomes more difficult
• Melt strength, bubble stability improves
• Tensile strength improves
• Impact strength improves
• ESCR increases

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