bimodal polyethylene
TRANSCRIPT
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How do bimodal polyethylene resins provideimproved pipe properties?
Sebastian Joseph, Ph.D.October 31, 2005
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Lyondell Chemical Company - Overview
Lyondell is one of the world’s largest chemical companies, with$15.9 bill ion in assets.
Lyondell companies include:
Three wholly owned businesses – Lyondell, Equistar and
Millennium
LYONDELL-CITGO Refining (LCR) joint venture
Lyondell is a major global manufacturer of ethylene, propylene,titanium dioxide, styrene, polyethylene, propylene oxide and acetyls.
Lyondell is headquartered in Houston, TX, operates on fivecontinents and employs nearly 10,000 people worldwide.
Lyondell’s consolidated pro forma 2004 revenues were $15.2 bill ion.
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Advantages of new generation bimodal
polyethylene pipes
Improved mechanical properties
Increased pressure rating
Resistance to slow crack growth (SCG)
Resistance to rapid crack propagation (RCP)
Increased service pressure
Reduced wall thickness
Increased capacity
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Today’s discussion
Describe unique features of bimodal polyethylene that
allow step-out performance in pressure rating, stresscrack resistance and rapid crack propagation ofpolyethylene pipe
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Outline
Evolution of polyethylene pipe performance
Factors affecting service life of polyethylene pipe
SCG resistance – unimodal vs. bimodal
RCP resistance
Summary
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Evolution of polyethylene pipe performance
0
200
400
600
800
1000
1200
PE1404 PE2406 PE3408 PE4710
Polyethylene Pipe Code
H y d r o s t s t i c D
e s i g n S t r e s s ( W
a t e r ) , p s i
*
* With the implementation of increased design factor
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Properties controlled by resin producers
Molecular weight
Polymer is made up of multiple repeating units of a monomer.
Molecular weight distribution (MWD)
Polymers are composed of chains of dif fering lengths, which results in a
distribution of molecular weights.
Density
Incorporation of comonomer (butene, hexene, octene) in the polymer
chain lowers density (crystallini ty) and leads to short chain branching.C
C
…-C-C-C-C-C-C-C-C-C-C-C-…
-(CH2-CH2)-n
MW = 1,000 – 1,000,000
Butene Branch
nCH2=CH2
MW =28
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Molecular weight
W e i g h t f r a c t i o n
Unimodal and bimodal polyethylenes
Molecular weight
W
e i g h t f r a c t i o n
Unimodal molecular weight distribution Bimodal molecular weight distribution
Bimodal polyethylenes are characterized by two peaks in the molecularweight distribution.
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Factors affecting polyethylene pipe service life
Field experience confirms thatpipes predominantly fail due to
slow crack growth (Stage II) ascompared to bursting (Stage I).
Pipe service life is determined byresistance to slow crack growth.
Other factors include:
Environmental
Loading
H o
o p S t r e s s
Failure Time
KneeStage I
Stage II
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Conventional property balance with unimodal PE
P i p e S e
r v i c e L i f e
S C G R
e
s i s t a n c e
H y d r o s t a t i c D
e s i g n S t r e s sSCG Resistance Density
Density
•Constraints on unimodal PEs•High SCG resistance requires low densities
•Low densities result in low HDS
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SCG resistance of unimodal and bimodal PE grades
Literature* shows that for unimodal PE grades, SCG resistanceincreases with
Increase in molecular weight
Decrease in density
Type of comonomer (for unimodal grades)
*Berthold, J. et al., Plast. Rub. Comp. Proc. Applcn., vol. 25, 368(1996)
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3 Ps of polymer design*
Polymer (Density, MI)
(Resin Manufacturer) Processing
(Extrusion at Pipe Manufacturer)
Properties
(Utility/
Municipality)
HDS
Pressure Rating
SCG
Slow Crack
Growth
RCP
Rapid Crack
Propagation* Wallace Yau, Lyondell
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SCG resistance of unimodal and bimodal PE grades
Literature* shows that for unimodal PE grades, SCG resistanceincreases with
Increase in molecular weight
Decrease in density
Type of comonomer (for unimodal grades)
Bimodal PE grades at a higher density show an order of magnitudeincrease in SCG resistance over unimodal grades
For example, a bimodal grade at 0.948 density showed failure
times > 1000 hours, while a unimodal PE grade at 0.940 densityand equivalent molecular weight had failure times of ~100 hours
*Berthold, J. et al., Plast. Rub. Comp. Proc. Applcn., vol. 25, 368(1996)
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Structure of unimodal PE resins*
Key Findings:
Most of the comonomer
is found in the low
molecular weightfraction, which
contributes very li ttle to
SCG resistance.
High molecular weightfraction has about six
orders of magnitude
higher SCG resistance
than low molecular
weight fraction.
0
1
2
3
4
5
6
7
8
0.1 - 3 3 - 8 8 - 15 15 - 41 41 - 400
Molecular weight range (104)
S h o
r t C h a i n s / 1 0 0 0 C
0.1
1
10
100
1000
10000
P E N T F a i l u r e t i m e ,
h o u r s
67% of distribution 33% of distribution
* Graph generated from Tables I and II in Lu, X. et al, J. Poly. Sci., Part B: Polym. Phys., vol. 34, 1809 (1996)
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Optimum structure for PE pipe resin*
Place comonomer in the high MW portion of the distribution and nocomonomer in low molecular weight portion of the distribution.
Difficult to implement with traditional catalyst systems in a single
reactor Most of the comonomer is placed in the low MW fraction.
Molecular weight
W
e i g h t f r a c t i o n
Comonomer
Distribution
Comonomer distribution in unimodal resins
* Brown, N. et al, Plast. Rub. Comp. Proc. Applcn., vol. 17, No. 4, 255 (1992)
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Multi-reactor approach to bimodal PE
Independent reactor control
•Molecular weight•Density
•Novel catalyst technology
Polymer property control
•Molecular weight•Molecular weight distribution
•Comonomer distribution
•Density
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How do bimodal PE resins provide improved pipe
properties?
Preferred location of comonomer
Higher crystallini ty (density) over unimodal resins at same level of SCB
Leads to higher hydrostatic design stress
Eliminates constraints imposed by unimodal PE resins
Increase in molecular weight without sacrificing processability
Molecular weight
W e i g h t f r a c t i o n
Comonomer distribution in bimodal resins
Comonomer
Distribution
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Property balance with unimodal and bimodal PE
P i p e S e
r v i c e L i f e
S C G R
e
s i s t a n c e
H y d r o s t a t i c D e s i g n S t r e s s
SCG Resistance Density
Density
•Bimodal PEs remove constraints•High SCG resistance at high densi ties
•High densities result in high HDS
Bimodal PE
Unimodal PE
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How about rapid crack propagation (RCP)?
RCP refers to the propagation of an impact event over several lengths ofa pipe.
Resistance to RCP is characterized by ISO 13478 (small scale steady-state or S4 test).
Pc , which refers to pressure at a given temperature (usually 0oC),
below which RCP will not occur.
Tc , which refers to temperatures at a given pressure (5 bar), abovewhich RCP will not occur.
North America is currently in process of implementing RCP into ASTMD2513.
Pc and Tc, in addition to inherent resin impact resistance, also dependson pipe diameter and thickness.
Not solely a resin property
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How is RCP affected by PE molecular structure?
Clutton et al* showed that Tc from the RCP test was directly correlated withTd-b, the ductile-brittle transition temperature from the Charpy test
Molecular weight has a signif icant effect on Td-b.
An increase in molecular weight reduces the temperature at which RCPcan occur
Trade-off between RCP and SCG
Homopolymer PE, which has poor SCG, is predicted to have the highestresistance to RCP!
RCP resistance of bimodal PE is obtained by tailoring the molecular weightand distribution
Pipe processability is not impaired by an increase in molecular weightfor bimodal PE resins
*Clutton, E.Q. et al., Plast. Rub. Comp. Proc. Applcn., vol. 27, 478(1998)
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Summary
Step change performance of bimodal polyethylene piperesin is a result of tailoring the polymer structure toachieve:
Appropriate placement of comonomer for SCG
resistance
Tailoring the molecular weight and distribution for
RCP resistance
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Questions?
Bimodal polyethylene
Sebastian Joseph
Bimodal polyethylene pipe processing
Adel Haddad
Ideas for webinar improvement
Bill Podborny