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

• 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

Part of The Ethylene ChainNatural Gas Liquids (Ethane, Propane)

or Naphtha (from Crude Oil)

Steam Cracking

Ethylene, Propylene

Other Polymers Chemicals

POLYETHYLENE

-(-CH2-CH2-)n- C=C

HH

H H

Ethylene Polyethylene

Any Questions?

Polyethylene – The Largest Volume Thermoplastic

PE PP Polyester PVC PS

151

9075

31

92

2004 Annualized Capacity – Billions of Pounds

Source: Chem Systems – 2004

PE Demand by Region 2004 Global PE Demand: 136 Billion Pounds

SOURCE: Nexant/Chem Systems 2005

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

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

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

World Leaders in Polyethylene Production

DowExxonMobilSABICSinopecInnoveneChevron PhillipsBasell Lyondell/EquistarBorealisTotalFormosa PlasticsNOVA ChemicalPolimeri EuropaPetroChina

SOURCE: Nexant/Chem Systems

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)

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

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

Classification of PE by Polymerization Chemistry

• Free radical polymerization – LDPE

• Coordination Polymerization via Catalyst– HDPE and LLDPE

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

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

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

“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

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.

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

Ethylene

Peroxide

To HPS

Example of Autoclave PE Reactor

Peroxide

Peroxide

Peroxide

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

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.

RawMaterialHandling

To ResinStorageand Loading

PelletingSystem

ReactionSystem

ResinPurging

AdditiveAddition

VentRecovery

Catalyst

Typical Gas Phase PE Process

Typical Solution PE Process

Reactor

Devo2

Devo1

SolventRecovery

Polymer

Ethylene

Comonomer

Your class notes also illustrate the Phillips slurry loop process.

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)

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

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

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.

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

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

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.

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

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)

Modulus (stiffness), Softening point, Moisture Barrier

Density

Gloss, Clarity, Haze Impact strength, Tear strength, ESCR

Properties vs. Density

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).

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

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