bimodal polyethylene

8/18/2019 Bimodal Polyethylene

1/22

How do bimodal polyethylene resins provideimproved pipe properties?

Sebastian Joseph, Ph.D.October 31, 2005


2/22

2

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.


3/22

3

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


4/22

4

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


5/22

5

Outline

Evolution of polyethylene pipe performance

Factors affecting service life of polyethylene pipe

SCG resistance – unimodal vs. bimodal

RCP resistance

Summary


6/22

6

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


7/22

7

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


8/22

8

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.


9/22

9

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


10/22

10

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


11/22

11

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)


12/22

12

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


13/22

13

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)


14/22

14

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)


15/22

15

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)


16/22

16

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


17/22

17

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


18/22

18

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


19/22

19

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


20/22

20

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)


21/22

21

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


22/22

22

Questions?

Bimodal polyethylene

Sebastian Joseph

[email protected]

Bimodal polyethylene pipe processing

Adel Haddad

[email protected]

Ideas for webinar improvement

Bill Podborny

[email protected]

bimodal polyethylene

Documents