unusual enhancement in tear properties of single-site lldpe

Unusual Enhancement in Tear Properties of

Single-Site LLDPE Blown Films at Higher Draw-Down Ratio Nitin Borse, Norman Aubee, Paul Tas NOVA Chemicals Technical Centre ABSTRACT Anisotropy is generally observed in the machine direction (MD) and transverse direction (TD) tear strength properties of polyethylene blown films prepared at high draw-down ratios. This study investigates the tear strength of blown films prepared from different linear low density polyethylenes (LLDPE) at different draw-down ratios. Conventional Zeigler-Natta catalyzed (Z-N LLDPE) and single-site catalyzed (sLLDPE) resins were used. The films were analyzed using Raman spectroscopy, optical microscopy and wide-angle X-ray scattering (WAXS) for pole figures analysis. We observed that the TD tear strength correlated with crystalline phase orientation in the films, and MD tear strength correlated with orientation in both the crystalline and the amorphous phases. The Z-N LLDPE blown films showed higher MD orientation of crystalline and amorphous phases at higher draw-down ratio. The sLLDPE blown films showed higher crystalline MD orientation at higher draw-down but the overall orientation was higher in TD. This anomalous orientation behavior in sLLDPE blown films resulted in unusually high TD and MD tear properties at higher draw down. INTRODUCTION Linear Low Density Polyethylene (LLDPE) resins are mostly used in film applications. Although different film market segments have different performance requirements, superior tear, tensile and dart impact strength are always desirable. Packaging films are required to possess high tear resistance in most applications. The tear resistance of the films, along with the other physical properties, depends upon the molecular architecture, microstructure and molecular orientation. It has been recognized [1] that film performance strongly depends upon the orientation of both the crystalline phase and the amorphous chains. There have been attempts to correlate the properties of polyethylene blown films to morphology and orientation. Krishnaswamy and Sukhadia [2] found that the MD tear strength of LLDPE blown films was higher when non-crystalline chains were close to equi-biaxial in the plane of the film, while the TD tear strength was higher when the crystalline lamellae were relatively straight and close to the TD. Krishnaswamy and Lamborn [3] explained the differences in the tensile properties of LLDPE blown films between the MD and TD in terms of lamellar orientation. There are attempts to relate the orientation, morphology and properties of the blown polyethylene films to the MD/TD stress balance at the frost line [4, 5] and draw down ratio [6]. Chen et al. [1] fabricated LLDPE films at different conditions of blow up ratio, die gap, and frost line height. The films were analyzed for White-Spruiell orientation factors of crystal unit cells, amorphous chains and Herman’s orientation factors of lamellae from wide-angle X-ray scattering (WAXS) pole figures, birefringence and small angle X-ray scattering (SAXS). They found that in typical Z-N LLDPE films the amorphous chains were aligned preferentially along the MD. The results show that at higher draw down the TD tear strength was higher, but MD tear strength and dart impact strength were reduced. Zang et al. [7] studied oriented structure and anisotropy properties of polyethylene blown films. The Z-N LLDPE films with octene comonomer showed higher tear strength than those produced using butene comonomer. The butene comonomer Z-N LLDPE films showed higher anisotropy in MD/TD tear strength. However in both the films the MD tear strength was reduced at high draw-down ratio. In this study we prepared sLLDPE, Z-N LLDPE, mLLDPE and super-hexene LLDPE films at low and high draw- down conditions by varying blow up ratio, film processing speed and film gauge. The films were tested for MD and TD tear strength. The crystalline and amorphous phase orientations were analyzed for White-Spruiell orientation factors of crystal unit cells by wide-angle X-ray diffraction (WXRD). The film crystallinity was analyzed by Raman spectroscopy. Polarized optical microscopy technique was used to estimate MD/TD birefringence of the films. The unique tear behavior was observed in sLLDPE films, where both the MD and the TD tear strength were significantly

higher at high draw ratio. The tear properties of LLDPE films are explained in terms of the orientation of molecular chains in crystalline and amorphous phases in the films. BACKGROUND Development of fundamental structure–property–processing conditions relationships in polyethylene films is of great importance in understanding blown-film characteristics. Understanding morphology and different structures in blown films is the first step towards this development. A hierarchy of structure exists in commercial polymers. A well defined crystalline structure generally exists at dimensions on order of magnitude in Angstrom units [8]. A lamellar structure is found at dimensions of order 20-200 Å. Spherulites exist with dimensions of about 1 μm. Important parameters to determine in processed films are the level of crystallinity and the level of polymer chain orientation. The method first described by Herman and Platzek [9] to measure orientation in fibres was determination of birefringence by optical retardation in a polarized light microscopy. The birefringence is defined as the difference in the refractive indices between two perpendicular directions. In semi-crystalline polymers, both crystalline and amorphous phases possess different orientation levels. The measured birefringence is the sum of the contributions from both these phases. The crystalline structure of polyethylene was first determined by Bunn [10] in 1939 using a long chain branched polyethylene (LDPE) on the basis of wide-angle X-ray diffraction studies. Polyethylene is found to crystallize into an orthorhombic unit cell with dimensions of a, b and c-axes as 7.41, 4.94 and 2.54 Å respectively. The crystal growth is seen in the direction of b-axis, and c-axis is the polymer chain axis. The crystalline phase orientation is usually analyzed by wide-angle x-ray scattering (WAXS) and then constructing pole figures [1]. This allows a complete representation of the distribution of polymer chains and crystallographic axes. In blown films it is seen that a-axis shows orientation towards MD whereas b-axis orients itself in TD. White and Spruiell [11] developed the representation of a series of second moment biaxial orientation functions, which are useful for characterizing molecular orientation in films and sheets. Figure 1 shows the White-Spruiell triangular representation of orientation of crystalline axes in a film. By knowing the relative orientation of all three axes (namely a-, b- and c-axis) of a unit crystal, the overall crystalline orientation in the film could be visualized.

G

F

E

D

C

B

A

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

MD →

TD →

Figure 1: White-Spruiell representation of crystalline axis orientation in the films

A: TD only; B: MD only; C: ND only; D: MD/TD equi-biaxial; E: TD/ND equi-biaxial; F: MD/ND equi-biaxial; G: isotropic

The amount of amorphous orientation in the blown films can be estimated by assuming the simple two phase model for polyethylene [1, 12]. In this case, the total birefringence can be represented by,

MDamoac

MDaocab

cMDc

ocbcNMMN fwffwnn ,,, )1(][ Δ−+Δ+Δ=−=Δ (Equation 1)

TDamoac

TDaocab

cTDc

ocbcNTTN fwffwnn ,,, )1(][ Δ−+Δ+Δ=−=Δ (Equation 2)

where w : film crystallinity (by Raman spectroscopy) nM , nN , nT : refractive indices in Machine, Normal and Transverse direction of film fc

c, fac :crystalline orientation functions of c and a-axis of PE unit crystal (by GADDS pole figure analysis)

nceBirefringe TD/ND and MD/ND are , TNMN ΔΔ fam,MD and fam,TD are MD and TD amorphous orientation function

0.058 phase amorphous of ncebirefringe intrinsic

005.0 ,056.0

direction respectivein crystals PE of valuesncebirefringe intrinsic are ,

=Δ

−=Δ=Δ

ΔΔ

oa

ocab

ocbc

ocab

ocbc

Subtracting equation 2 from equation 1, leads to the equation in terms of MD/TD birefringence, which contains the term showing the difference in the amorphous orientation between MD and TD.

)()1()]()([ ,,,,,, TDamMDamoac

TDac

MDaocab

cTDc

cMDc

ocbcMT ffwffffw −Δ−+−Δ+−Δ=Δ (Equation 3)

where )microscopy optical(by nceBirefringe MD/TD=−=Δ−Δ=Δ TMTNMNMT nn Using equation 3 the difference in MD and TD amorphous phase orientation functions (fam,MD – fam,TD) can be calculated by knowing the overall orientation obtained from the MD/TD birefringence analysis. We obtained pole figure analysis and the MD/TD birefringence for all the LLDPE films produced under low and high draw-down conditions. The tear properties of the films were correlated with crystalline orientation functions, MD/TD difference in amorphous chain orientation functions and the overall polymer chain orientations.

EXPERIMENTAL Materials Three sLLDPE, two Z-N LLDPE, one m-LLDPE (metallocene catalyzed LLDPE) and one super-hexene co-polymer LLDPE resins were blown into films at low and high draw-down conditions. The following Table gives the material characteristics. Table 1: Material characteristics

Product Reference Namein This Paper Co-monomer Melt Index

(gm/10min) Density(g/cm3)

SURPASS FPs016-C sLLDPE1 Octene 0.65 0.9160

SURPASS FPr018-D sLLDPE2 Octene 0.86 0.9180


SCLAIR FP020-C Z-N LLDPE1 Octene 0.67 0.9210


COMP mLLDPE mLLDPE Hexene 1.18 0.918

NOVAPOL TD-9022-D Sup hex LLDPE Hexene 0.64 0.9195 Experimental Procedure All films were produced on a Macro 8” blown film line, equipped with a general purpose 3.5” single screw with barrier design and length to diameter ratio 30. A general-purpose spiral die was used; the die diameter was 8” and the die gap 50 mil. The cooling unit consisted of a dual lip air ring in combination with internal bubble cooling (IBC). Table 2: Blown film processing conditions

Film gauge (mil)

Blow-Up Ratio

Line Speed(ft/min)

Draw-DownRatio

Low DDR 1.5 3:1 130 10.4

High DDR 0.75 2:1 390 30.5 All the films produced were tested for the Elmendorf tear strength as per ASTM D1922 in machine direction (MD) and transverse direction (TD). The film crystallinity was determined by Raman spectrometry using Renishaw inVia Raman spectrometer with air cooled Argon ion 514.5 nm Laser. Birefringence analysis of all the films was done as per ASTM D4093-95 using Olympus BX51 microscope with U-CBR1 compensator for measuring the birefringence. Crystalline orientation in the films was analyzed using the pole figure from the wide angle X-ray diffraction. The pole figure method utilizes the General Area Detector Diffraction System (GADDS) manufactured by Bruker AXS Inc. operating in the wide angle X-ray diffraction mode. The X-ray source was generated using a copper anode which emits characteristic wavelength lines of Kα at 1.54 nm with nickel filter. The X-ray generator was set at 40 kV and 40 mA. The sample to detector distance was fixed at 4.90 cm. The films were rotated 180° at 5° intervals.

The diffraction pattern is then integrated for the (200) and (020) reflections. The results for the Hermans and White-Spruiell orientation functions obtained with the GADDS are represented in the form of White-Spruiell triangle plots as shown in Figure 1. RESULTS AND DISCUSSIONS Film Crystallinity, Birefringence and Tear strength Table 3 shows the results of crystallinity, birefringence and tear strength analysis of the films produced. Raman spectrometry gives the three-phase analysis which includes interphase as an intermediate phase between the crystalline and amorphous phases. In the present study, half of the interphase was taken as the part of the crystalline phase and the other half as part of the amorphous phase. The positive sign of the birefringence indicates that the overall orientation of the polymer chains in the film is in MD, and the negative sign indicates that it is in TD. Table 3: Results of film analysis

Resin Type DDR Raman film Crystallinity

(%)

Birefringence (MD – TD)

(X 10-4)

MD Tear Strength (gm/mil)

TD Tear Strength (gm/mil)

Low 37.8 – 4.63 272 445 sLLDPE 1

High 36.7 – 7.83 783 658

Low 39.2 – 1.3 313 455 sLLDPE 2

High 37.7 – 5.6 481 691

Low 38.7 0 313 502 sLLDPE 3

High 36.6 – 3.83 595 753

Low 42.0 – 11.7 288 636 Z-N LLDPE 1

High 40.2 + 16.33 116 917

Low 41.3 – 6.47 327 593 Z-N LLDPE 2

High 39.4 – 1.65 313 982

Low 40.4 + 5.63 311 383 mLLDPE

High 39.4 – 0.61 307 492

Low 38.8 – 7.31 456 592 Sup hex LLDPE

High 36.3 – 13.13 350 888 Crystalline Orientation Small and large data points are shown in Figures 2 and 3 to denote the two films prepared from each resin. The small data point represents the film produced at low draw-down ratio (10.4) and the large data point represents the film produced at high draw-down (30.5). The difference between the MD and TD orientation function is taken as the measure of orientation, the positive values indicate MD orientation and then negative values indicate TD orientation. The higher draw-down increases a-axis orientation in the MD and b-axis simultaneously shows higher orientation in the TD. For all the films a- axis always showed orientation in MD and b-axis in TD. This means the crystal growth direction in the films is always TD. As seen in Figure 3, the polymer chain axis (c-axis) shows MD orientation in most films. As the draw-down ratio increases, c-axis MD orientation decreases. In the case of Z-N LLDPE2, the c-axis orientation is in the TD at higher draw-down ratio. In the discussion that follows with respect to the crystalline orientation, we consider the orientation of a-axis as the measure of the crystalline orientation in the films.

‐0.2

‐0.15

‐0.1

‐0.05

00 0.05 0.1 0.15 0.2

f bMD‐fbTD

faMD‐faTD

sLLDPE1

sLLDPE2

sLLDPE3

Z‐N LLDPE1

Z‐N LLDPE2

mLLDPE

Sup Hex LLDPE

Figure 2: Orientations of a- and b- axes of unit crystal in the LLDPE blown films Small data point: low DDR (10.4); Large data point: high DDR (30.5)

‐0.08

‐0.06

‐0.04

‐0.02

0

0.02

0.04

0.06

0.08

0 0.05 0.1 0.15 0.2f cMD‐fcTD

faMD‐faTD

sLLDPE1

sLLDPE2

sLLDPE3

Z‐N LLDPE1

Z‐N LLDPE2

mLLDPE

Sup Hex LLDPE

Figure 3: Orientation of a- and c-axis of unit crystal in LLDPE films Small data point: low DDR (10.4); Large data point: high DDR (30.5)

Film Tear Properties and Polymer Chain Orientations Machine Direction Tear Strength: Figure 4 shows the plot of MD tear strength of LLDPE films and crystalline a-axis orientation functions (MD-TD). It is clearly seen that in all the three sLLDPE resin films the MD tear strength does not correlate with the crystalline orientation in the films. At high draw-down ratio the crystalline orientation is seen to increase in MD and the MD tear strength also increases. On the contrary all other LLDPE films show a decrease in MD tear strength at higher draw-down.

0

100

200

300

400

500

600

700

800

900

0 0.05 0.1 0.15 0.2

MD Tear(gm/m

il)

faMD‐faTD

sLLDPE1

sLLDPE2

sLLDPE3

Z‐N LLDPE1

Z‐N LLDPE2

mLLDPE

Sup Hex LLDPE

Figure 4: Relation between MD tear strength and the crystalline orientation

Small data point: low DDR (10.4); Large data point: high DDR (30.5) We observed that the MD tear strength of LLDPE blown films did not show correlation with any of the crystalline axes or with the amorphous phase chain orientation alone. But, as seen in Figure 5 and Table 3, it shows reasonably good correlation with the MD/TD birefringence. The birefringence values indicate the sum of the contributions from both these phases. In the case of sLLDPE films, the overall orientation of polymer chains seems to increase in TD at higher draw-down, which results in higher MD tear strength. In Z-N LLDPE films the overall orientation at higher draw-down (as indicated by birefringence) increases towards MD, which gives poor MD tear characteristics to the films. In super hexene-LLDPE films the birefringence at low draw-down is already significantly in TD. This gives high MD tear properties to these films even at low draw-down ratio. Increasing polymer chain orientation further in TD may not be effective in further enhancing the MD tear strength of super hexene-LLDPE films.

0

100

200

300

400

500

600

700

800

900

‐15 ‐10 ‐5 0 5 10 15 20

MD Tear (gm/m

il)

Birefringence (X10‐4)

sLLDPE1

sLLDPE2

sLLDPE3

Z‐N LLDPE1

Z‐N LLDPE2

mLLDPE

Sup Hex LLDPE

MDTD

Figure 5: Relation between MD tear strength and film birefringence (MD-TD) Small data point: low DDR (10.4); Large data point: high DDR (30.5)

Transverse Direction Tear Strength:

The TD tear strength of the LLDPE films correlated well with the crystalline orientation function of a-axis of unit crystal. As shown in Figure 6, higher draw-down ratio in all the films results in higher MD crystalline orientation, which results in higher TD tear strength.

0

200

400

600

800

1000

1200

0 0.05 0.1 0.15 0.2

TD Tear (gm/m

il)

faMD‐faTD

sLLDPE1

sLLDPE2

sLLDPE3

Z‐N LLDPE1

Z‐N LLDPE2

mLLDPE

Sup Hex LLDPE

Figure 6: Relation between TD tear strength and the crystalline orientation

Small data point: low DDR (10.4); Large data point: high DDR (30.5)

CONCLUSIONS In LLDPE resin films, we observed that MD tear strength correlates with the overall orientation of the polymer chains in the film plane, which includes the orientations in the amorphous phase and the crystalline phase. The overall MD/TD orientation can be evaluated by analyzing the films for birefringence using optical microscopy. The TD tear strength of the films depends strongly on the crystalline phase orientation as indicated by the a-axis orientation of the unit crystal of polyethylene. We observed that when sLLDPE films are blown at higher draw-down conditions, they show an increase in crystalline MD orientation as well as an increase in overall polymer chain orientation in TD. This phenomena results in higher TD tear strength as well as higher MD tear strength. Z-N LLDPE resin films when blown at higher draw-down show higher crystalline and overall orientation in MD, which results in higher TD tear strength in these films but the MD tear strength is reduced. The anomalous and counter intuitive enhancement in tear properties of sLLDPE films may be attributed to their unique resin architecture and morphology. References 1. Chen H. Y., Bishop M. T., Lands B. G., Chum S. P., “Orientation and Property Correlation for LLDPE Blown

Films”, J. Appl. Polym. Sci., 101, pp 898-907(2006). 2. Krishnaswamy R. K., Sukhadia A. M., “Orientation Characteristics of LLDPE Blown Films and their

Implications on Elmendorf Tear Performance”, Polymer, 41, pp 9205-9217(2000). 3. Krishnaswamy R. K., Lamborn M. J., “Tensile Properties of Linear Low Density Polyethylene (LLDPE) Blown

Films”, Polym. Eng. Sci., 40(11), pp 2385-2396(2000). 4. Choi K-J., Spruiell J. E., White J. L., “Orientation and Morphology of High Density Polyethylene Films

Produced by Tubular Blowing Method and its Relationship to Process Conditions”, J. Appl. Polym. Sci., 20, pp 27-47(1982).

5. Kwack T. H., Han C. D., “Development of Crystalline Structure during Tubular Film Blowing of Low Density Polyethylene”, J. Appl. Polym. Sci., 35, pp 363-389(1988).

6. Patel R. M., Butler T. I., Walton K. L., Knight G. W., Polym. Eng. Sci., 34(19), pp 1506-1514(1994). 7. Zang X. M., Elkoun S., Aji A., Huneault M. A., “Oriented Structure and Anisotropy Properties of Polymer

Blown Films: HDPE, LLDPE and LDPE”, Polymer, 45, pp 217-229(2004). 8. White J. L., Cakmak M., “Orientation, Crystallization, and Haze Development in Tubular Film Extrusion”,

Advances in Polymer Technology, 8(1), pp 27-61(1988). 9. Hermans P. H. and Platzek P., Kolloid Z., 88, 68(1939). 10. Bunn C. W., Trans. Faraday Soc., 35, p 482(1939). 11. White J. L. and Spruiell J. E., “Specification of Biaxial Orientation in Amorphous and Crystalline Polymers”,

Polym. Eng. Sci., 21, pp 859-868(1981). 12. Pazur R. J.,, and Prud'homme R. E., “X-ray Pole Figure and Small Angle Scattering Measurements on Tubular

Blown Low-Density Poly(ethylene) Films”, Macromolecules, 29(1), pp 119-128(1996).

Unusual Enhancement in Tear Properties of

Single-Site LLDPE Blown Films at High Draw-Down Ratio

Presented by:Name Norman AubeeTitle Technical Service SpecialistCompany: NOVA Chemicals

ObjectiveObjective

Study the relation between crystalline and amorphous orientations and tear properties of LLDPE blown films

MaterialsMaterials

Product Reference Name

Co-monomer

Melt Index(gm/10min)

Density(g/cm3)


SURPASS FPr018-D sLLDPE2 Octene 0.86 0.9180




COMP mLLDPE mLLDPE Hexene 1.18 0.918

NOVAPOL TD-9022-D Sup hex LLDPE Hexene 0.64 0.9195

ExperimentalExperimental

Films produced on a Macro 8” blown film line with dual lip air ring and IBCGeneral-purpose spiral die with die gap 50 milElmendorf tear strength as per ASTM D1922

Film gauge(mil)

Blow-UpRatio

Line Speed(ft/min)

Draw-DownRatio

Low DDR Film 1.5 3:1 130 10.4

High DDR Film 0.75 2:1 390 30.5

Film CharacterizationFilm Characterization

Overall crystalline and amorphous orientation: Birefringence by retardation of light through the film in MD and TD using polarized light microscopy–ASTM D 4093 – 95Crystalline Orientation: Orientations functions of three crystalline axis by GADDS Pole Figure analysisAmorphous Orientation: Calculated from overall orientation(birefringence), crystalline orientation(GADDS

Pole Figure) and film crystallinity (Raman Spectroscopy)

Polyethylene Crystal StructurePolyethylene Crystal Structure

a, b and c -axes define unit cellChain axis is aligned with c -axis

Crystalline OrientationCrystalline Orientation

General Area Detector Diffraction System (GADDS) Pole Figure analysis

G

F

E

D

C

B

A

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

MD →

TD →

White-Spruiell representation of crystalline axes orientations

A: TD onlyB: MD onlyC: ND onlyD: MD/TD equi-biaxialE: TD/ND equi-biaxialF: MD/ND equi-biaxialG: isotropic

GADDS Pole Figure GADDS Pole Figure –– ExampleExampleWhite-Spruiell representation of crystalline axes orientations for Z-N LLDPE1

a -axis : MD orientedb -axis : in TD/ND planec -axis : Isotropic

Amorphous Phase OrientationAmorphous Phase OrientationSubtract the two equations to get amorphous phase orientation (fam,MD − famTD)

TDamoac

TDaocab

cTDc

ocbcNTTN fwffwnn ,,, )1(][ Δ−+Δ+Δ=−=Δ

MDamoac

MDaocab

cMDc

ocbcNMMN fwffwnn ,,, )1(][ Δ−+Δ+Δ=−=Δ

w : film crystallinity (by Raman spectroscopy)nM , nN , nT: refractive indices in Machine, Transverse and Normal direction of filmfc

c, fac – crystalline orientation functions of c and a-axis of PE unit crystal

0.058 phase amorphous of ncebirefringeintrinsic ,

direction respective in crystals PE of values ncebirefringeintrinsic are

=Δ

−=Δ=Δ

ΔΔ

oa

ocab

ocbc

ocab

ocbc

005.0056.0

,

)Microscopy Optical(by nceBirefringe MD/TD=−=Δ−Δ TMTNMN nn

Results: Crystalline OrientationResults: Crystalline Orientation

Unit Crystal Orientation Functions

Small data point: Low DDR (10.4)Large data point: High DDR (30.5)

MD Tear and Crystalline OrientationMD Tear and Crystalline OrientationMD Tear strengths of sLLDPEs do not correlate with a-axis orientationMD Tear strength of sLLDPE films increases with increase in DDR

Small data point: Low DDR (10.4)

Large data point: High DDR (30.5)

MD Tear and Crystalline OrientationMD Tear and Crystalline Orientation

MD Tear strength of all LLDPE films correlates with MD/TD birefringence



MD Tear Strength MD Tear Strength

MD Tear strength of sLLDPE increases at higher DDRMD Tear strength of Z-N LLDPE decreases at higher DDR

TD Tear StrengthTD Tear Strength

TD Tear strength of LLDPE films correlates with crystalline a-axis orientation



ConclusionsConclusions

MD tear strength correlated with the overall orientation of polymer chains in LLDPE films as analyzed by birefringenceTD tear strength of LLDPE films correlated with the a-axis orientation of unit crystal

ConclusionsConclusions

At high DDR sLLDPE films showed higher a-axis orientation in MD but higher overall polymer chain orientation in TDAbove mentioned phenomena resulted in higher MD and higher TD tear strength of sLLDPE films at high DDRZ-N LLDPE films showed increased a-axis orientation and higher overall polymer chain orientation in MD with increase in DDR, which resulted in poor MD tear

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unusual enhancement in tear properties of single-site lldpe

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