polyethylene ken anderson polyethylene r&d the dow chemical company freeport, texas invited...
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Polyethylene
Ken AndersonPolyethylene R&D
The Dow Chemical Company
Freeport, Texas
Invited Lecture for Chem 470 – Industrial Chemistry
Prof. Michael Rosynek, Texas A&M 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 R&D 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, TS&D, and Marketing, with occasional customer interaction to execute product development
– R&D rep on North American Films Market Management Team
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Part of The Ethylene ChainNatural Gas Liquids (Ethane, Propane)
or Naphtha (from Crude Oil)
Steam Cracking
Ethylene, Propylene
Other Polymers Chemicals
POLYETHYLENE
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-(-CH2-CH2-)n- C=C
HH
H H
Ethylene Polyethylene
Any Questions?
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Polyethylene – The Largest Volume Thermoplastic
PE PP Polyester PVC PS
151
9075
31
92
2004 Annualized Capacity – Billions of Pounds
Source: Chem Systems – 2004
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PE Demand by Region 2004 Global PE Demand: 136 Billion Pounds
SOURCE: Nexant/Chem Systems 2005
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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
SOURCE: Nexant/ChemSystems 2005, PTAI 1/05
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World Leaders in Polyethylene Production
DowExxonMobilSABICSinopecInnoveneChevron PhillipsBasell Lyondell/EquistarBorealisTotalFormosa PlasticsNOVA ChemicalPolimeri EuropaPetroChina
SOURCE: Nexant/Chem Systems
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Types of Polyethylene
O
OOO
O O
OO
O
O
C-OH
O
HDPE (0.940-0.965)“High Density”
LLDPE (0.860-0.926)“Linear Low Density”
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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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.
Classification of PE by Molecular Architecture
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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 1930’s
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Date: March, 1933Company: Imperial Chemical Industries (ICI)Location: Winnington, EnglandInventors: 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
Discovery of LDPE Reaction
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Typical Propagation Mechanism
CH2 . + C=C
H
CH2-CH2-CH2.
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
H
H H
Free Radical Polymerization of LDPE
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“Back-biting” Mechanism – Short Chain Branching
CH
CH2
CH2
CH2H
.
CH2
CH
CH2
CH2
CH3
CH2
.
Butyl branch
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.
Free Radical Polymerization of LDPE
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Chain Transfer to Polymer – Long Chain Branching
CH2 . + R-CH2-R CH3 + R-CH-R.
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.
Free Radical Polymerization of LDPE
Your class notes have these reactions illustrated in greater detail.
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Compression Reaction Devolatilization Extrusion
Ethylene
Compressor
Secondary or Hypercompressor
Reactor HPS
LPS
Extruder
Purge to LHC
High pressure recycle
Low pressure recycle
(16-39,000 psi)
CTA
Typical High Pressure, Low Density PE Process
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Ethylene
Peroxide
To HPS
Example of Autoclave PE Reactor
Peroxide
Peroxide
Peroxide
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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 1950’s for HDPE and PP• effectively commercialized in 1970’s 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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RawMaterialHandling
To ResinStorageand Loading
PelletingSystem
ReactionSystem
ResinPurging
AdditiveAddition
VentRecovery
Catalyst
Typical Gas Phase PE Process
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Typical Solution PE Process
Reactor
Devo2
Devo1
SolventRecovery
Polymer
Ethylene
Comonomer
Your class notes also illustrate the Phillips slurry loop process.
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LLDPE is ethylene/alpha-olefin copolymer.
-olefin typically 1-butene, 1-hexene and 1-octene
-CH2-CH2-CH2-CH-CH2-CH2-CH2-CH2-
CH2
CH3
CH2
CH2
CH2
CH2
Branch length = Comonomer length - 2
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
Ti
Si
N
* Trademark of The Dow Chemical Company
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INSITE* Technology Polymer(typical mLLDPE lacks longchain branches)
Conventional LLDPEvia Ziegler-Natta
LLDPE Molecular Structure Comparison
Heterogeneous chain lengthdistribution + Heterogeneousshort chain branch distribution
Homogeneous chain lengthdistribution + Homogeneousshort chain branch distribution
* Trademark of The Dow Chemical Company
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Semi-Crystalline MorphologySince 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
AMORPHOUS MATERIAL
TIE CHAIN
INTERFACE
CRYSTAL CORE
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
40 60 80 100 120 140
0
0.5
1
1.5
2
Temperature (oC)
Heat Flow (Watts/gm)
ITP, 0.92 g/cc
ITP, 0.902 g/cc
ITP, 0.908 g/cc
ITP, 0.896 g/cc
LLDPE, 0.92 g/cc
VLDPE, 0.905 g/cc
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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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0.8630
0.8702
0.8730
0.8817
0.8960
0.9016
0.90990.9180
Strain, %
0 250 500 750 1000 1250
Str
ess
, M
Pa
0
10
20
30
40
ALLS50. 21.3.93
0.9550
Strain, %
0 50 1000
5
10
15
(Strain Rate - 2.4 min-1)
Engineering Stress-Strain Response - ITP resins
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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Modulus (stiffness), Softening point, Moisture Barrier
Density
Gloss, Clarity, Haze Impact strength, Tear strength, ESCR
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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ELUTION VOLUME (mls)
16 18 20 22 24 26 28
Conventional LLDPE
Mw = 124600, Mn = 33200, MWD = 3.8
Mw = 73800, Mn = 37400, MWD = 2.0 Typical mLLDPE
Increasing Molecular Weight
* Trademark of The Dow Chemical Company
Molecular Weight Distribution Comparisonby Gel Permeation Chromatography
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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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