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BLENDING OF

HIGH-PERFORMANCE LLDPEs

WITH BUTENE-LLDPEs AND LDPEs

H. Mavridis, Equistar Chemicals, LP

This article originally presented at SPE RETEC, February 25-28, 2001.

ABSTRACT

Several High-Performance LLDPE resins for film applications have been developed inthe past few years, either through Advanced Ziegler-Natta or Single-Site catalysts. These newresins improve significantly on the film properties of conventional Ziegler-Natta resins, but they

are generally more costly and/or more difficult to process. A relatively simple strategy foraddressing the cost and processability issues is via post-reactor blending, either with butene-LLDPEs in order to reduce cost, or with LDPEs in order to improve processability. Blending ofHigh-Performance resins has a generally detrimental effect on film strength properties,although there are exceptions in certain compositions. Viability of a certain blend compositionthen hinges on a cost versus performance evaluation of the properties of interest. Thepresent work focuses on the performance aspect, through a study of the effect of blendingbutene-LLDPEs and LDPEs on the optical and mechanical properties of Advanced Ziegler-Natta and metallocene-LLDPEs.

INTRODUCTION

A number of new resins have been introduced in the last decade for high-strengthLLDPE film applications. In the area of hexene-LLDPE film resins, the two main categories [1]include metallocene-catalyzed (mLLDPE) or single-site-catalyzed (SSC) resins and AdvancedZiegler-Natta resins (commonly referred to as superhexenes or High PerformanceHexenes, HPH, in the industry). A brief comparison of their relative merits in terms of filmproperties and blown film processability was recently presented in Ref.[2].

However, both families of LLDPE resins (using either single-site-catalysts or advancedZiegler-Natta catalysts) still lack the extrusion processability of high-pressure LDPE resins.Blending LDPE into LLDPE is known to improve processability but compromise film properties[3]. It may be possible, in principle, to have a certain type of LLDPE that displays a relativeadvantage in blendability with LDPE. It is an objective of the present work to investigate therelative blendability of mLLDPEs and advanced Ziegler-Natta HPH-LLDPEs, in their blendswith LDPE.

By analogy to blending with LDPE for the purpose of improving processability, a second

type of blending commonly employed in industry, is to blend standard Ziegler-Natta butene-LLDPEs so as to reduce cost (butene LLDPEs can be 10-20% cheaper than high-performance hexene-LLDPEs, thus the cost incentive). It is the second objective of thepresent work to investigate the relative blendability of mLLDPEs and advanced Ziegler-NattaHPH-LLDPEs, in their blends with standard Ziegler-Natta butene-LLDPE.

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EXPERIMENTAL

MATERIALSThe resins used in the present study are listed in Table 1, along with their fundamental

polymer properties. HPH is a High-Performance-Hexene LLDPE from Equistar, made withan Advanced Ziegler-Natta catalyst and described elsewhere [5-6]. mLLDPE is ametallocene-catalyzed LLDPE without long chain branching [7]. C4-LLDPE is a standardZielger-Natta butene-LLDPE and LDPE is a high-pressure LDPE with long chain branching.

A comparison of the molecular weight distributions, short chain branching distributions,thermal characteristics and rheological behavior, is given in Figures 1 through 4, respectively.Figure 5 also shows a certain representation of the rheological data that highlights differencesin MWD and/or LCB. The higher the Storage Modulus at low Loss Modulus in Figure 5, thebroader the resin rheologically [4], which translates to better melt extrusion processability. AsFigure 5 clearly shows, the LDPE is the broadest rheologically (also indicated by the ERnumber in Table 1, see also Ref.[4]), whereas the mLLDPE is the narrowest rheologically.

FILM FABRICATION AND TESTINGBlown film extrusion was performed on a lab line equipped with a 4 spiral mandrel die

with a 0.100" die gap and a dual lip air ring. A "hump" temperature profile was used for thebarrel temperature settings in all cases. Output rate was held constant at 40 lb/hr. Blow upratio and film gauge were held fixed at 2.5:1 and 1 mil, respectively. Each run condition wasreplicated 2 times. All films produced were conditioned and tested per the standard ASTMmethods. Film property averages and standard deviations were determined on the basis of 2-replicates.

RESULTS AND DISCUSSION

BUTENE-LLDPE BLENDSComparative plots of film properties versus blend composition for HPH/C4-LLDPE blends

and mLLDPE/C4-LLDPE blends are given in Figures 6-12. The impact strength of the blendsis shown to decrease precipitously as function of C4-LLDPE content. However, for the

mLLDPE/C4-LLDPE blends where the C4-LLDPE is the minor component (

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LDPE BLENDSComparative plots of film properties versus blend composition for HPH/LDPE blends and

mLLDPE/LDPE blends are given in Figures 14-20. The processability enhancement uponLDPE addition is shown in Figure 13, which plots the extruder amps as a function of blendcomposition. There is clearly a reduction in extruder amps with increasing LDPE content,which in turn relates to the reduction in high-shear-rate viscosity (see for example Table 1).There is no significance difference in the extrudability of the HPH and mLLDPE blends.

The impact strength of the blends is shown to decrease precipitously as function of LDPEcontent (Figure 14). The mLLDPE/LDPE blends do maintain an advantage over HPH/LDPEblends, which may relate to the lower film modulus of the mLLDPE/LDPE blends (shown inFigure 15. Film impact strength is inversely related to film modulus, generally). However, inMD-Tear strength, all differentiation between mLLDPE/LDPE and HPH/LDPE blends is lost(Figure 16): the effect of LDPE addition is to decrease the MD-Tear dramatically, as alsoobserved in the past [5].

In contrast to the catastrophic effect of LDPE addition on MD-Tear, the effect on opticalproperties is synergistically positive (Figures 18-19). Again, all differentiation betweenmLLDPE/LDPE and HPH/LDPE blends is lost, but now both types of blends have opticalproperties better than any of the pure components.

Finally, in terms of resistance to slow puncture (Figure 20), the mLLDPE/LDPE blendshave a slight advantage over HPH/LDPE blends (higher puncture energy). This may be, atleast partly, due to the difference in surface friction characteristics of the films (as illustrated bythe higher coefficient of friction, COF, of the mLLDPE/LDPE blends in Figure20). It is knownthat the puncture resistance of polyethylene films, as measured in ASTM-5748, shows apronounced dependence on COF.

CONCLUDING REMARKS

Advanced Ziegler-Natta catalyzed LLDPE (HPH) and metallocene-catalyzed LLDPE(mLLDPE) were compared in their relative blendability with butene-LLDPE and LDPE. Suchblends are practiced in industry for cost reduction and/or processability enhancement. In thisstudy we compared the relative merits of such blends, by selecting pure components of similar

melt index and density. In the case of blends with butene-LLDPE, all film properties show agradual change between the values of the pure components. The mLLDPE/C4-LLDPE blendshave an advantage over HPH/C4-LLDPE blends in film impact strength, whereas the HPH/C4-LLDPE blends have an advantage in Elmendorf tear strength, at C4-LLDPE levels below 50wt%. Therefore the choice of one type of blend over another will depend on the particular setof film properties required and the specific cost structure of the components. In the case ofblends with LDPE, the expected enhancement in processability upon LDPE addition isobserved in both the HPH/LDPE and mLLDPE/LDPE blends. LDPE causes a dramaticreduction in film strength properties (impact and tear) and a synergistic improvement in opticalproperties. There is little differentiation between HPH/LDPE and mLLDPE/LDPE blends, withthe minor exception film impact strength at

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5. H. Mavridis, TAPPI PLC Conf. Proc., 413-422 (1998)6. H. Mavridis, SPE ANTEC Tech Papers, 41-45 (1999)7. R. Shroff and H. Mavridis, Macromolecules, 32, 8454-8464 (1999)

Table 1. List of resins used in the present study

Resin HPH mLLDPE C4-LLDPE LDPE

CatalystAdvanced

Ziegler-Natta Metallocene Ziegler-Natta N/A

Melt Index, g/10min 0.66 1.1 0.94 0.77

Resin Density, g/cc 0.916 0.918 0.920 0.920

Comonomer Hexene Hexene Butene N/A

Antiblock, ppm 7500 4600 6300 0

Slip, ppm 1500 500 1600 0Rheology (190 oC)

ER, Ref.[4] 0.90 0.15 0.62 2.5

* @100 rad/sec, kpoise 18.1 23.3 17.7 6.3

* @=2*103 Pa, kpoise 114 61.8 82.5 140

MWDMw 140,000 116,000 128,000 106,000

Mz/Mw 2.6 1.8 2.7 2.8

Mw/Mn 5.3 2.9 4.3 7.4

TREF Br/1000C 15.3 16.7 15.8 21.0

DSC

Tm1 125.6 119.8 122.2 109.8

Tm2 - 109.3 - -

Tc 114.9 107.5 110.8 97.4

Heat of fusion, J/g 127.5 123.7 118.2 122.8

Figure 1. Molecular weight distributions (GPC) of the resins in this study(apparent MWD for LDPE no LCB correction).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Molecular Weight (M)

dw/dlogM

HPH

mLLDPEC4-LLDPE

LDPE HPH

mLLDPE

LDPE

4

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Figure 2. Comonomer compositions distributions (SCB by TREF) for theresins of this study.

Figure 3. DSC (melting at 10C/min) for the resins of this study.

Figure 4. Rheology data (viscosity) at 190C, illustrating differences in meltstrength and extrudability.

0.0

5.0

10.0

15.0

20.0

25.0

0 10 20 30 40 50

Short Chain Branching, SCB/1000C

dw/dlogM

HPH

mLLDPEC4-LLDPE

LDPE

HPH

LDPE

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03

Frequency (1/sec)

Viscos

ity(poise)

HPHmLLDPEC4-LLDPELDPE

Melt Strength

Extrudability

-9.0

-7.0

-5.0

-3.0

-1.0

60 80 100 120 140Temperature (deg.C)

HeatFlow(W/g)

HPH

mLLDPEC4-LLDPE

LDPE

5

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Figure 5. Rheology data (storage versus loss modulus) at 190C, illustrating

differences in rheological polydispersity.

Figure 6. Variation of film impact strength (Dart Drop) with blend composition

for butene-LLDPE blends.

Figure 7. Variation of film modulus with blend composition for butene-LLDPEblends.

6

22000

24000

26000

28000

30000

0 25 50 75 100

% C4-LLDPE

FilmMD-Modulus(psi)

HPH

mLLDPE

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+04 1.0E+05 1.0E+06 1.0E+07

Loss Modulus (dyn/cm2)

StorageModulus(dy

n/cm

2)

HPHmLLDPEC4-LLDPELDPE

0

200

400

600

800

1000

1200

0 25 50 75 100

% C4-LLDPE

DartDrop

HPH

mLLDPE

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Figure 8. Variation of film Elmendorf MD-Tear Strength with blend compositionfor butene-LLDPE blends.

Figure 9. Variation of film Elmendorf TD-Tear Strength with blend composition

for butene-LLDPE blends.

Figure 10. Variation of film optical properties (Haze) with blend composition forbutene-LLDPE blends.

Figure 11. Variation of film heat seal strength with blend composition forbutene-LLDPE blends.

7

0.0

1.0

2.0

3.0

0 25 50 75 100

% C4-LLDPE

SealStrength@110C(lbf) HPH

mLLDPE

8

0

100

200

300

400

500

0 25 50 75 100

% C4-LLDPE

MD-Tear(g)

HPH

mLLDPE

0

100

200

300

400

500

600

0 25 50 75 100

% C4-LLDPE

TD-Tear(g)

HPH

mLLDPE

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 25 50 75 100

% C4-LLDPE

Haze

HPH

mLLDPE

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Figure 12. Variation of slow puncture energy and COF with blend compositionfor butene-LLDPE blends.

Figure 13. Variation of extruder amps with blend composition for LDPEblends.

Figure 14. Variation of film impact strength (Dart Drop ) with blendcomposition for LDPE blends.

0.00

0.25

0.50

0.75

1.00

1.25

1.50

0 25 50 75 100

% C4-LLDPECOFa

ndSlowPunctureEnergy

(Joules)

HPH

mLLDPE

COF

5

10

15

20

25

0 25 50 75 100

% LDPE

Extrud

erAmps

HPH

mLLDPE

0

200

400

600

800

1000

1200

0 25 50 75 100

% LDPE

DartDrop

HPH

mLLDPE

9

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Figure 15. Variation of film modulus with blend composition for LDPE blends.

Figure 16. Variation of film Elmendorf MD-Tear Strength with blendcomposition for LDPE blends.

Figure 17. Variation of film Elmendorf TD-Tear Strength with blend compositionfor LDPE blends.

20000

25000

30000

35000

40000

0 25 50 75 100

% LDPE

FilmMD-Modulus(psi)

HPH

mLLDPE

0

100

200

300

400

500

0 25 50 75 100

% LDPE

MD-Tear(g)

HPH

mLLDPE

0

100

200

300

400

500

600

700

800

900

0 25 50 75 100

% LDPE

TD-Tear(g)

HPH

mLLDPE

10

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Figure 18. Variation of film optical properties (Haze) with blend compositionfor LDPE blends.

Figure 19. Variation of film optical properties (Gloss) with blend compositionfor LDPE blends.

Figure 20. Variation of film slow puncture energy and coefficient of friction

with blend composition for LDPE blends

11

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 25 50 75 100

% LDPE

Haze

HPH

mLLDPE

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

0 25 50 75 100

% LDPE

Gloss

HPH

mLLDPE

0.00

0.25

0.50

0.75

1.00

1.25

1.50

0 25 50 75 100

% LDPECOFandSlowPunctureEnergy

(Joules)

HPH

mLLDPE

COF