Processability of LLDPE/LDPE blends: capillary extrusion studies.

INTRODUCTION

The processability of linear low-density polyethylenes (LLDPEs) can be improved by blending with a small amount of a low-density polyethylene [1-3]. However, because of structural differences between resins, many undesirable effects may occur such as immiscibility of the components and undesirable morphological changes [4, 5] as well as premature onset of flow instabilities [1, 6]. These effects obviously influence the economic feasibility of the processes as well as the mechanical properties of the final products [1, 7-9].

In our previous work, Delgadillo-Velazquez et al. [10] have studied the miscibility between a linear polyethylene (Ziegler-Natta hexane copolymer of LLDPE) and four branched polyethylenes (LDPEs) using differential scanning calorimetry (DSC) and several rheological methods (the exact same blends are used in the present work). It was found that these blends are immiscible at high low density polyethylene (LDPE) concentrations (typically grater than 20 wt%) and miscible at smaller LDPE weight concentrations. The effects of long chain branching (LCB) on the rheology were assessed by means of parallel-plate and extensional rheometry. It was found that shear is insensitive to additions of small amounts of LDPE into LLDPE (up to 20 wt% in many cases) particularly if the viscosity curve of LDPE is about the same or lower than that of the corresponding LLDPE. On the other hand, extensional rheometry was found sensitive to addition of only 1 wt% of LDPE into LLDPE only at high Hencky strain rates (typically greater than 5 [s.sup.-1]) and for the blends that contained the LDPE with the highest molecular weight. Such rates are not easily accessible by commercial extensional rheometers. The SER Universal Testing Platform from Xpansion Instruments is the only known extensional rheometer that can perform reliable experiments at such high Hencky strain rates [11, 12].

In this work, we study systematically the processing behavior of a LLDPE with four LDPEs that have viscosity curves which lie above, about the same and below that of the LLDPE. As discussed earlier, the same blends used by Delgadillo-Velazquez et al. [10] are also tested in this work. The processability of all blends is studied in detail in capillary rheometry to determine the effects of LCB on the onset of flow instabilities known as melt fracture. We are interested to know how the addition of various amounts of LDPE (of varying molecular weight) can affect the onset of flow instabilities such as surface, oscillating, and gross melt fracture. In addition, how the details of flow, particularly the magnitude and period of oscillations are influenced by the presence of small amounts of LLDPE. Can such flow details be used to detect small amounts of LDPE. Surprisingly, it is reported here that oscillating melt fracture flow details are extremely sensitive to such subtle changes in the structure of polymers.

EXPERIMENTAL

Polyethylene Resins and Blends

The LLDPE resin used in this study was a Ziegler-Natta, hexene copolymer, synthesized by ExxonMobil (LL3001). The LDPE resins used in this work are LD200 by ExxonMobil, EF606A by Westlake Polymers; and finally 662I and 132I, provided by Dow Chemicals. Table 1 lists all the polymers used along with their melt indices and densities. The LDPE resins have been labeled as LDPE-I to IV in order of increasing molecular weight.

The LLDPE resin was melt blended respectively with each LDPE resin in order to create LLDPE/LDPE blends having weight compositions of 99/1, 95/5, 90/10, 80/20, 50/50, and 25/75. The blending was performed as follows: the original components were mixed and grinded in a Brabender mixer in order to reduce their pellet sizes. Then, the mixture in the form of flakes was blended into a single screw extruder, at low processing speed (3-4 rpm), using a screw having mixing elements near to the end of the metering zone. The produced extrudates were then pelletized for easy handling. The blend 99/1 was produced in two dilution steps, the first being the 95/5.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Rheological Techniques

Parallel-plate rheometry was performed to determine the linear viscoelastic properties of the pure components. Details on the linear viscoelasticity of blends can be found elsewhere [10]. Here we report the rheological properties of the pure components. The measurements were performed using a Rheometrics System IV (con-trolled-strain) and a Bohlin--CVOR (controlled-stress). Experiments performed at different temperatures, namely, 130, 150, 170, 190, and 210[degrees]C. Mastercurves were obtained and most results are presented at the reference temperature of 150[degrees]C.

The pure components and their blends were rheologically characterized in simple extension using an SER Universal Testing Platform [11, 12] from Xpansion Instruments. As described by Sentmanat (2003, 2004), the SER unit is a dual windup extensional rheometer that has been specifically designed for use as a fixture on a variety of commercially available rotational rheometer host platforms. The particular SER model used in this study, a model SER-HV-B01, was designed for use on a VOR Bohlin rotational rheometer host system. Details of the experiments can be found elsewhere [10]. Here we report only the extensional properties of the pure components.

[FIGURE 3 OMITTED]

Capillary extrusion measurements were conducted at 150 and 190[degrees]C using a capillary die having a diameter equal to 0.762 mm and a length-to-diameter ratio, L/D, equal to 16. The onset of melt flow instabilities (melt fracture) was determined for the pure resins and their blends. The surface of the extrudates was analyzed with an Olympus MIC-D microscope. Selective images of the extrudates are presented here.

RESULTS AND DISCUSSION

Rheological Characterization of Pure Resins

Figure 1 depicts the complex viscosity of all polymers listed in Table 1 as a function of frequency for the pure resins at 150[degrees]C. For the case of LLDPE (LL3001), the viscosity curve approaches its zero-shear viscosity value at small frequencies and exhibits a certain degree of shear thinning at higher ones. The zero-shear viscosity of the four LDPEs was not reached experimentally, as can be seen from Fig. 1, but were calculated by determining their relaxation spectrum of these resins using the linear viscoelastic master-curves at the reference temperature of 150[degrees]C. The values are listed in Table 1. Note that while LDPE II (EF606), LDPE-III (662I), and LDPE-IV (132I) possess a higher zero-shear viscosity than that of LL3001, the presence of LCB causes significant shear thinning and thus their viscosity becomes considerably smaller at high frequencies. Such behaviors are typically observed in the shear rheology of polyolefins [13-15].

The extensional rheological behavior of all resins is depicted in Fig. 2 at 150[degrees]C. In all cases the tensile stress growth coefficient, [[eta].sub.E.sup.+], is plotted as a function of time for three different Hencky strain rates (although data are available for several other Hencky strain rates), namely 0.1, 1, and 10 [s.sup.-1]. For the sake of clarity, the material functions [[eta].sub.E.sup.+] have been multiplied by an appropriate factor (for convenience powers of 10 was used), as indicated on the plot. It can be observed that the LLDPE (LL3001) does not exhibit any degree of strain hardening at any extension rate, an observation consistent with polymers of linear architecture [15-17]. In fact, it displays very little deviation from the linear viscoelastic envelope (LVE), 3[[eta].sup.+]. The latter material function (3[[eta].sup.+]) has been determined independently from linear viscoelastic shear rheology measurements, plotted as a dashed line in Fig. 2. On the other hand, the four LDPEs show significant strain hardening (deviation from the LVE, 3[[eta].sup.+], also indicated for all resins by dashed lines) which is typically an indication of the presence of long branches [13, 15, 16].

[FIGURE 4 OMITTED]

Capillary Rheometry

The flow curves of the pure components and those of their blends, were determined at 150 and 190[degrees]C by means of the capillary rheometer as described earlier. For the first set of blends, LLDPE (LL3001.32)/LDPE (LD200), the results are shown in Fig. 3. First, a significant viscosity mismatch between the LLDPE (LL3001) and LDPE-I (LD200) can be observed (also seen in Fig. 1 from their complex viscosities). Because of this, addition of up to 20% of LDPE-I into LLDPE does not seem to have a significant effect on its viscosity. The flow curves seem to overlap. Differences can clearly be seen only at concentrations as high as 50 wt% of LDPE. Similar observations for this system were made by Delgadillo et al. [10].

The processability in capillary extrusion of the individual components and their blends are determined in terms of the onset of melt fracture phenomena, i.e. the critical shear rates and stresses at which extrudate distortions appear. In general, there are three types of instabilities which might occur in the capillary extrusion of polyolefins [18]. First, there is a critical shear stress at which small amplitude periodic distortions appear on the extrudate surface and these phenomena are known as surface melt fracture or sharkskin [19]. At higher apparent shear rate values and in spite of the fact a fixed volumetric flow is used, the pressure and thus the shear stress oscillates between two extreme values because of a combination of wall slip and melt compressibility. This phenomenon is known as stick-slip or oscillating melt fracture [20]. At higher shear stress values the flow becomes again steady, although gross distortions appear on the surface of the extrudate. The latter phenomena associated with gross extrudate distortions are collectively known as gross melt fracture [21].

[FIGURE 5 OMITTED]

As discussed earlier, the results for the first set of blends are plotted in Fig. 3. For pure LLDPE, the onset of sharkskin was observed to occur at a critical wall shear stress value of 0.19 MPa, a value consistent with other reported values in the literature [22, 23]. Table 2 lists all critical shear rate and stress values for the onset of all three types of instabilities at 150[degrees]C. It is noted that sharkskin and stick-slip phenomena does not occur in the case of pure LDPE-I, again consistent with previous reports [22]. Addition of up to 20 wt% of LDPE into LLDPE does not seem to change the critical values for the onset of surface melt fracture behavior, which completely disappears at amounts of LDPE higher than 50 wt%. The onset of stick-slip phenomena was found to gradually disappear with increasing amount of LDPE and this is discussed later into more detail. Finally, the onset of gross melt fracture of blends seems to shift to smaller critical shear stress values with increase of LDPE amount, an observation consistent with the critical shear stress values of the pure components. Similar results were obtained at 190[degrees]C as can be seen from the second part of Table 2.

[FIGURE 6 OMITTED]

Figure 4 plots the flow curves of the pure components and their blends in system II, LL3001/EF606. Again, because of significant viscosity mismatch, no effect on the flow curve can be seen up to addition of 20 wt% of LDPE-II (EF606). Table 3 lists the critical shear rate and stresses for the onset of gross melt fracture phenomena. The results are similar to those reported for blend system I discussed earlier. First, sharkskin is not eliminated with the addition of small amounts of LDPE (i.e. no effect up to 20%). The effect on stick-slip is evident even at small amounts of LLDPE i.e. the amplitude of the oscillations gradually decrease even at amounts as low as 1 wt% LDPE. Finally, the onset of gross melt fracture of blends seems to shift to smaller critical shear stress values with increase of LDPE amount

Figures 5 and 6 depict the flow curves of blends of system III (LL3001/662I) and system IV (LL3001/132I). Viscosities of the two pure components in both these blend systems are closer, however, the effect of the addition of LDPE into LLDPE on their processability are similar to those reported above for the other two blend systems. The critical shear rate and shear stress values for the onset of melt fracture phenomena are listed in Tables 4 and 5, respectively. It is noted that the onset of gross melt fracture for the LDPE III (662I) and LDPE-IV (132I) occur at very low rates, less than 15 [s.sup.-1] for both resins.

Figures 7-10 show images of the typical extrudate appearances for the four blend systems at selected apparent shear rates (15, 100, 350, and 1000 [s.sup.-1]). In all blend systems similarities can be observed. Smooth extrudates are obtained at 15 [s.sup.-1] in most cases, shark-skinned extrudates at 100 [s.sup.-1], stick-slip or oscillating melt fractured extrudates at 350 [s.sup.-1], and finally gross melt fractured extrudates at 1000 [s.sup.-1].

[FIGURE 7 OMITTED]

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[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

Stick-Slip Flow Regime

As discussed earlier, pressure oscillations were obtained in the capillary extrusion of LLDPE and LLDPE with the addition of LDPE up to about 10 wt%. In all blend systems (I-IV) these stick-slip or oscillating instabilities were eliminated with the incorporation of 20 wt% of LDPE. It is noted that pure LDPE does not exhibit such a flow regime. It was also noted that the addition of even large amounts of LDPE have little effect on the sharkskin and gross melt fracture behavior of LLDPE. Strikingly, the addition of only 1 wt% of LDPE into LLDPE has a significant effect on the oscillating melt facture of LLDPE as can be seen in Figs. 11-14. The amplitude of the oscillations gradually decreases with increasing amounts of LDPE starting from even 1 wt%.

Figure 15 plots the amplitude of the oscillations as a function of the LDPE wt% addition. In all blend systems the amplitude decreases with increasing amount of LDPE (wt%). At 20 wt% of LDPE no pressure oscillations exist as these have been eliminated because of the presence of long branches.

Such effects at small LDPE concentrations could not be detected in shear rheology [10]. For example, the blends with 1 wt% LDPE essentially shows the exact same shear behavior with the pure LLDPE in both capillary and parallel-plate rheometrical tests. Small differences could only be seen in extensional rheology only at high extensional rates i.e. greater than 5 [s.sup.-1] and only for the blends that include the LDPEs with the highest molecular weights. This is clearly illustrated in Fig. 16. Therefore, the oscillating flow regime is very sensitive to the presence of long chain branching. Since stick-slip is a phenomenon due to the combined effects of wall slip and compressibility [20], it seems that long branches suppress slip effects to a certain extend.

CONCLUSION

The processing behavior of a number of LLDPE/LDPE blends with emphasis on the effects of long chain branches was studied extensively in this work. A Ziegler-Natta, linear low-density polyethylene was blended with four low-density polyethylene LDPE's having distinctly different molecular weights. Capillary extrusion experiments revealed that the onset of sharkskin and gross melt fracture are slightly influenced with the addition of LDPE into LLDPE. However, it was found that the amplitude of the oscillations in the stick-slip flow regime, scales well with the weight fraction of LDPE. Amounts as low as 1% LDPE has a significant effect on the amplitude of pressure oscillations. These effects are clearly due to the presence of LCB. Since stick-slip is a phenomenon due to combined effects of wall slip and compressibility, the presence of LCB certainly suppresses wall slip effects. Furthermore, it was observed that the onset of this flow regime was shifted to higher shear rates with increase of LDPE content.

[FIGURE 16 OMITTED]

On the other hand, it was observed that shear rheology is not sensitive enough to detect the addition of small levels of LDPE. Extensional rheology (uniaxial extension) can detect levels of LDPE as small as 1 wt% only at high Hencky strain rates (typically greater than 5 [s.sup.-1]) and only for certain blends, particularly those with LDPE of high molecular weight (blend systems III and IV) It is suggested that the magnitude of oscillations is a sensitive method capable of detecting low levels of LCB and therefore can possibly be used for this purpose.

ACKNOWLEDGMENTS

We thank DOW Chemicals (S. Costeux) for the materials supplied and valuable comments. One of the authors (D-V.O.) would like to acknowledge CONACyT for financial support in the form of a scholarship.

REFERENCES

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21. J.M. Dealy and S. Kim, "Gross Melt Fracture in Extrusion," in Polymer Processing Instabilities: Control and Understanding, S.G. Hatzikiriakos and K.B. Migler, Eds., Marcel Dekker, New York, 207 (2005).

22. A.V. Ramamurthy, J. Rheol., 30, 337 (1986).

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O. Delgadillo-Velazquez, S.G. Hatzikiriakos

Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, British Columbia, Canada

Correspondence to: Savvas G. Hatzikiriakos; e-mail: [e-mail]

Contract grant sponsor: NSERC, CONACyT.

TABLE 1. Properties of polyethylene resins used.

                   Melt index       Density
                   (g/10 min)       (g/cc)          [[eta].sub.0] (Pa s)
Resin              (190[degrees]C)  (25[degrees]C)  150[degrees]C

LLDPE (LL3001.32)  1                0.917            24,557
LDPE I (LD200)     7.5              0.915             8,272
LDPE II (EF606A)   2.2              0.919            44,234
LDPE III (662I)    0.47             0.919            72,780
LDPE IV (132I)     0.22             0.921           132,065

TABLE 2. Critical shear rates and stresses for blend system I (LL3001/
LD200) at 150 and 190[degrees]C.

                                Sharkskin MF
Polymer         [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

Critical shear rates and stresses for blend system I (LL3001/LD200) at
150[degrees]C
LLDPE (LL3001)  0.19                    50
 1% LDPE        0.19                    50
 5% LDPE        0.19                    50
10% LDPE        0.18                    50
20% LDPE        0.17                    50
50% LDPE        0.16                    80
75% LDPE        --                      --
LDPE-I (LD200)  --                      --

Critical shear rates and stresses for blend system I (LL3001/LD200) at
190[degrees]C
LLDPE (LL3001)  0.17                    90
 1% LDPE        0.17                   100
 5% LDPE        0.17                   100
10% LDPE        0.17                   100
20% LDPE        0.16                   100

                                Stick-slip
Polymer         [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

Critical shear rates and stresses for blend system I (LL3001/LD200) at
150[degrees]C
LLDPE (LL3001)  0.40-0.31              230
 1% LDPE        0.37-0.28              250
 5% LDPE        0.34-0.26              300
10% LDPE        0.33-0.27              350
20% LDPE        --                      --
50% LDPE        --                      --
75% LDPE        --                      --
LDPE-I (LD200)  --                      --

Critical shear rates and stresses for blend system I (LL3001/LD200) at
190[degrees]C
LLDPE (LL3001)  0.42-0.39              850
 1% LDPE        0.41-0.39              950
 5% LDPE        0.42-0.41              950
10% LDPE        --                      --
20% LDPE        --                      --

                                  Gross MF
Polymer         [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

Critical shear rates and stresses for blend system I (LL3001/LD200) at
150[degrees]C
LLDPE (LL3001)  0.42                    700
 1% LDPE        0.42                    800
 5% LDPE        0.41                    700
10% LDPE        0.40                    550
20% LDPE        0.46                    700
50% LDPE        0.23                    180
75% LDPE        0.14                    100
LDPE-I (LD200)  0.13                    400

Critical shear rates and stresses for blend system I (LL3001/LD200) at
190[degrees]C
LLDPE (LL3001)  0.41                   1100
 1% LDPE        0.42                   1100
 5% LDPE        0.41                   1100
10% LDPE        0.42                    950
20% LDPE        0.41                    950

TABLE 3. Critical shear rates and stresses for blend system II (LL3001/
EF606) at 150 and 190[degrees]C.

                                 Sharkskin MF
Polymer          [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

Critical shear rates and stresses for blend system II (LL3001/EF606) at
150[degrees]C
LLDPE (LL3001)   0.19                   50
 1% LDPE         0.19                   50
 5% LDPE         0.17                   50
10% LDPE         0.18                   50
20% LDPE         0.17                   50
50% LDPE         0.17                   80
75% LDPE         --                     --
LDPE-II (EF606)  --                     --

Critical shear rates and stresses for blend system II (LL3001/EF606) at
190[degrees]C
LLDPE (LL3001)   0.16                   90
 1% LDPE         0.16                   90
 5% LDPE         0.16                   90
10% LDPE         0.15                   90
20% LDPE         0.15                   90

                                 Stick-slip
Polymer          [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

Critical shear rates and stresses for blend system II (LL3001/EF606) at
150[degrees]C
LLDPE (LL3001)   0.40-0.31              230
 1% LDPE         0.39-0.35              350
 5% LDPE         0.40-0.37              350
10% LDPE         0.41-0.40              450
20% LDPE         --                      --
50% LDPE         --                      --
75% LDPE         --                      --
LDPE-II (EF606)  --                      --

Critical shear rates and stresses for blend system II (LL3001/EF606) at
190[degrees]C
LLDPE (LL3001)   0.42-0.39              850
 1% LDPE         0.41-0.39              900
 5% LDPE         0.40-0.39              900
10% LDPE         --                      --
20% LDPE         --                      --

                                   Gross MF
Polymer          [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

Critical shear rates and stresses for blend system II (LL3001/EF606) at
150[degrees]C
LLDPE (LL3001)   0.42                    700
 1% LDPE         0.44                    900
 5% LDPE         0.42                    700
10% LDPE         0.44                    600
20% LDPE         0.48                    600
50% LDPE         0.19                    100
75% LDPE         0.15                     80
LDPE-II (EF606)  0.10                     90

Critical shear rates and stresses for blend system II (LL3001/EF606) at
190[degrees]C
LLDPE (LL3001)   0.41                   1100
 1% LDPE         0.41                   1100
 5% LDPE         0.41                   1100
10% LDPE         0.40                    950
20% LDPE         --                      950

TABLE 4. Critical shear rates and stresses for blend system III (LL3001/
662I) at 150[degrees]C.

                                   Sharkskin MF
Polymer          [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

LLDPE (LL3001)   0.19                   50
 1% LDPE         0.19                   50
 5% LDPE         0.19                   50
10% LDPE         0.19                   50
20% LDPE         0.19                   50
50% LDPE         0.19                   50
75% LDPE         0.17                   50
LDPE-III (662I)  --                     --

                                  Stick-slip
Polymer          [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

LLDPE (LL3001)   0.40-0.31              230
 1% LDPE         0.37-0.30              300
 5% LDPE         0.36-0.32              300
10% LDPE         0.37-0.34              300
20% LDPE         --                      --
50% LDPE         --                      --
75% LDPE         --                      --
LDPE-III (662I)  --                      --

                                   Gross MF
Polymer          [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

LLDPE (LL3001)   0.42                   700
 1% LDPE         0.42                   700
 5% LDPE         0.39                   700
10% LDPE         0.43                   700
20% LDPE         0.40                   300
50% LDPE         0.17                    40
75% LDPE         0.13                    25
LDPE-III (662I)  0.09                    15

TABLE 5. Critical shear rates and stresses for blend system IV (LL3001/
132I) at 150[degrees]C.

                                  Sharkskin MF
Polymer         [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

LLDPE (LL3001)  0.19                   50
 1% LDPE        0.18                   50
 5% LDPE        0.18                   50
10% LDPE        0.19                   50
20% LDPE        0.19                   50
50% LDPE        0.20                   50
75% LDPE        0.17                   50
LDPE-IV (132I)  --                     --

                                 Stick-slip
Polymer         [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

LLDPE (LL3001)  0.40-0.31              230
 1% LDPE        0.39-0.35              350
 5% LDPE        0.45-0.41              350
10% LDPE        0.40-0.36              300
20% LDPE        --                      --
50% LDPE        --                      --
75% LDPE        --                      --
LDPE-IV (132I)  --                      --

                                  Gross MF
Polymer         [[sigma].sub.w] (MPa)  [dot.[gamma].sub.A] ([s.sup.-1])

LLDPE (LL3001)  0.42                   700
 1% LDPE        0.43                   700
 5% LDPE        0.42                   700
10% LDPE        0.40                   500
20% LDPE        0.41                   300
50% LDPE        0.23                    70
75% LDPE        0.16                    30
LDPE-IV (132I)  0.10                    15