lecture 3.1-effect of mw on pe properties

8/2/2019 Lecture 3.1-Effect of MW on PE Properties

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Case Study

Polyethylene

THE EFFECT OF MOLECULAR WEIGHT, STRUCTUREAND DENSITY ON THE CHOICE/TYPE OF APPLICATION


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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-C=C

HH

H H

Ethylene Polyethylene


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Polyethylene The Largest Volume Thermoplastic

PE PP Polyester PVC PS

151

90

75

31

92

2004 Annualized Capacity Billions of Pounds

Source: Chem Systems 2004


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PE Demand by Region2004 Global PE Demand: 136 Billion Pounds

Latin America8.4%

NA (US & Can)

23.5%MEAF

7.8%

Western Europe

21.4%

Central/EasternEurope

4.1%

Asia-Pacific

34.8%

SOURCE: Nexant/Chem Systems 2005


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Markets/Applications for PE

Rigid and flexible packagingFilms, 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 Process2004 Global PE Demand: 136 Billion Pounds

Blow Molding

14%

Sheet

2%

Extrusion Coating

4%

Injection Molding

12%

Pipe and Conduit

7%

Other Non-Extrusion

3%

Wire and Cable

3%

Film

51%

Other Extrusion

2%Rotational Molding

1%

Food Packaging

Hygiene & Medical

Consumer & Ind. Liners

Stretch Films

Agricultural Films

HDSS

Film

SOURCE: Nexant/ChemSystems 2005, PTAI 1/05


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Many types of PE

HDPE: High DensityPolyEthylene

LDPE: Low

LLDPE: LinearLow

VLDPE: Very Low

Applications dependon properties,combinations of

Density MWD (Molecular

Weight Distribution)

Comonomer andbranching


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The applications of PE are determined by:

Molecular weight (MW) or melt flow index (MI)

Density


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MOLECULAR WEIGHT DISTRIBUTION MWD

MWD distribution of polymer chains lengthsBroad MWD wide range of chain lengthsNarrow MWD narrow range of chain lengthsCatalyst and processing dependent

Narrow BroadLow warpage/shrinkage High warpage/shrinkagePoor processability Good processability

Good melt strength


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Injection Molding

Hydraulic

Pressure

Nozzle


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The Injection Molding Process


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Blow Moldinge.g. milk bottle

Extruded Parison-

Mold Open

Mold Closed and Bottle

Blown

Finished Bottle

Removed from Mold

Plastic


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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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Amorphous: density lowCrystalline : density high

Density is a measured of crystallinity/amorphous of

PE solid.


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Amorphous vs. Crystalline

HDPE - Highly ordered = crystalline(translucent to opaque)LDPE - Random arrangements = amorphous

(clear)Depends partly on polymer chain shapeDepends partly on thermal history (rate ofcooling)

As Crystallinity increases, density increases,stiffness increases, heat tolerance increases,barrier properties improve, clarity decreases

Polyethylene


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Solid State Properties

Solid state properties are determined by:

Percent crystallinity (density) & crystal sizedistribution

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

0.9099

0.9180

Strain, %

0 250 500 750 1000 1250

Stress,MPa

0

10

20

30

40

0.9550

Strain, %

0 50 100

0

5

10

15

(Strain Rate - 2.4 min-1)

Engineering Stress-Strain Response - ITP resins

Samples were cooled at 1o

C/min.


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Decreasing the Crystallinity (Density)

Is accomplished by...

Increasing the amount of short chain branching byadding 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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BIMODAL MWD

Outstanding final properties

- the low molecular weight part forming the crystallineand the high-molecular weight forming the amorphous

part with high numbers of tie molecules per volume.

Bimodal HDPE resins are used to produce thin, stiff filmsConventional process,Unimodal LDPE or LLDPE is used

for making film

- The HDPE film thickness is less than half that HDPE film

for the same application

Bimodal blow molding products, such as drums and bulk

container are superior with respect to cold impact and

chemical resistance.


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NEW CATALYST


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INSITE* Technology Polymer(typical mLLDPE lacks longchain branches)

Conventional LLDPE

via 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

lecture 3.1-effect of mw on pe properties

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