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The effect of nanoclays on the processibility of polyolefins.

INTRODUCTION

In the extrusion of thermoplastics including polyethylenes and polypropylenes to make useful products and articles such as wire coatings, tubes, bottles, and films, it is desirable that the final products have smooth and glossy surfaces [1-6]. It is also desirable for the extrusion rates to be as high as possible and the extrusion pressure to be as low as possible in order to make the most efficient use of the processing machinery and therefore reduce capital and operational costs [4-6]. However, as the extrusion rates increase, a point is reached at which the surface begins to exhibit first a "matte" appearance (loss of gloss of the surface of the extrudate) and consequently begins to roughen [5, 6]. If the extrusion rate increases further, more severe roughening appears (small amplitude periodic distortions) described as "surface melt fracture," "land fracture," or "sharkskin" [5-9]. At even higher extrusion rates, "gross melt fracture" appears, which manifests itself by distortions with magnitude of the same order of magnitude of the thickness of the extruded article, i.e., thickness of the film or thickness of the tube [5, 10-12]. These gross distortions usually exhibit no regularity or pattern as they seem rather chaotic in appearance. These phenomena and other aspects of surface distortions as the extrusion rates increase are discussed in detail in Refs. 1-4.

To delay the onset of these undesirable effects of surface distortions and roughness, additives called extrusion aids or processing aids have been developed [6, 13, 14]. Examples of processing aids are many. Fluoropolymers and fluoroelastomers are useful as processing aids in the extrusion of polyolefins (polyethylenes and polypropylenes) and other polymers [6, 13, 14]. Later improvements included the combination of fluoropolymers with polyethylene glycol and or polar-side-group adjuvants [15]. These processing aids have been reported to have an effect on surface or sharkskin melt fracture but not on gross melt fracture [6].

It has been discovered by scientists at DuPont that the addition of hexagonal boron nitride particles improves the surface smoothness of extrudates of these polymers and permits increased extrusion rates without deterioration of surface properties [16]. This discovery was further studied for the extrusion of other polymers and under different conditions and it was concluded that boron nitride is a useful processing aid [17-20]. Combining a small amount of boron nitride with a fluoroelastomer or fluoropolymer and addition of this to melt processible polymers improves even further the surface smoothness of polyolefins and permits even further increased extrusion rates without deterioration of surface properties [21-23]. The hexagonal boron nitride particles and its combinations have been reported to eliminate even gross melt fracture to a certain extend [6].

In this paper, the influence of organically modified nanoclays on the processibility of polyolefins is studied. Their morphology is similar to that of boron nitride as both exhibit a platelike structure. Consequently, these nanoclays are combined with traditional processing aids such as fluoroelastomer in order to examine any possible synergistic effects as those observed in the case of boron nitride-fluoroelastomer combinations [21-23]. The types of nanoclays used in the present work are the montmorillonites [24-27]. They are high purity aluminosilicate minerals sometimes referred to as phyllosilicates [24, 27]. They have a sheet-type or platey structured. They can be chemically modified or surface treated in order to be easily incorporated into different polymers [25-27]. Chemically modified nanoclays are also referred to as organonanoclays.

EXPERIMENTAL

Materials

Several polyolefin resins were used in this study. First, two Ziegler-Natta polyethylenes supplied by NOVA Chemicals were used, one being a film grade linear low-density polyethylene (butene comonomer) (labeled here as LLDPE PF-Y821-BP) of melt index 0.8 and the other a blow molding grade of high-density polyethylene of melt index 0.95 (labeled as HDPE 58G). A metallocene LLDPE (labeled as m-LLDPE Exact[R] 3128) was also used in this study, previously used to study the effect of boron nitride and its combinations on its processibility [17-19]. It is a film grade m-LLDPE of melt index equal to about 1. Finally, a polypropylene (PP) resin was also used in this study. Its rheological properties are reported in Ref. 28.

Five different grades of montmorillonite nanoclays were supplied by Nanocor and Southern Clay Products (see Table 1). Nanoclays PGV, PGW of Nanocor, and Cloisite Na from Southern Clay Products, Inc., are unmodified nanoclays. On the other hand, clay I.44PA and Cloisite 10A are organically modified nanoclays. This modification renders these nanoclays easily dispersible in a polyolefin matrix. Table 1 lists the various clay grades along with their physical properties [29, 30]. All these nanoclays were used to investigate their effect on the processibility of polyolefins. The fluoropolymer-based processing aid used was Dynamar[R] FX-9613 from Dyneon, a 3M Company. This processing aid is a copolymer of vinylidene fluoride and hexafluoropropylene, with 10% of inorganic partitioning agent, primarily microtalc [31]. The physical form of the elastomer is a free-flowing powder ground to a 25 mesh particle size.

The clay powders were mixed with appropriate amounts of the virgin resin that was previously pre-ground. Then, a master batch of 2 wt% clay with ground resin was prepared by using a 3/4-inch extruder equipped with a screw having appropriate mixing elements. The threads produced for the extrusion were pelletized and were ground again. A desired final concentration of a particular blend was obtained by mixing virgin ground polyethylene with the ground master batch by means of the same extruder. This procedure resulted in a uniform dispersion of nanoclays into the polymer, confirmed with scanning electron microscopy (SEM). It is noted that thorough dispersion is a necessary condition for obtaining good performance of processing aids. The fluoropolymer polymer processing aid was introduced into the host polymer by means of dry mixing. This procedure was shown to be more effective in terms of minimizing the induction time for complete coverage of the die walls with the fluoropolymer, thus resulting in minimizing the extrusion pressure [32].

Rheological Characterization and Processibility

The linear viscoelastic experiments of virgin and loaded polyethylene samples were performed using a Rheometrics System IV rheometer equipped with 25 mm parallel plates. The characterization was done by performing frequency sweep experiments in the frequency range of 0.1 rad/s to about 500 rad/s. The tests were done at temperatures in the range of 120-210[degrees]C depending on the resin. Time temperature superposition was applied as necessary in order to obtain the master curves at the desired reference temperature.

The extensional rheology of the resins was determined with the new SER Universal Testing Platform [33, 34] from Xpansion Instruments. 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 SER-HV-B01 model used in this study was used on a Bohlin VOR host and was capable of generating Hencky strain rates beyond 20 [s.sup.-1] under controlled temperatures in excess of 250[degrees]C. For more details on these experiments the reader is referred to Refs. 35 and 36.

In order to assess the effect of the nanoclay additives on the processibility of polymers, capillary extrusions were carried out. Two types of dies were used, including a standard capillary die made of tungsten carbide, with various diameters, length to diameter ratios, and entrance angles. These dies were fitted into either a ROSAND RH-2000 or an Instron capillary rheometer. The second type of die used was the crosshead die. Crosshead dies are typically used for wire-coating purposes and it has been shown that the effect of boron nitride (BN) is more evident in these types of die [25]. In this study, the crosshead die used was a Nokia Maillefer 4/6 that included a die and a tip ("tip" is the wire guide) with equal entry cone angles of 60[degrees] and die land length of 7.62 mm. The diameters of the tip and the die were 1.52 mm and 3 mm, respectively, and they were made of tungsten carbide. For the crosshead die, an Instron piston driven constant speed capillary rheometer was used.

No Bagley and Rabinowitch corrections were applied to any of the data obtained for all the resins. Hence, these experiments would yield the apparent flow curves of the resins. During the extrusion process, the piston was allowed to travel down the barrel at a preset speed corresponding to a desired shear rate. The speed was maintained until an obvious steady extrusion pressure was recorded. Polymer sample that was extruded under a steady extrusion pressure was collected. The extruded sample was then inspected visually to detect the presence of melt fracture. Sometimes, an optical microscope was used to aid in detecting the presence of melt fracture. Performance of the polymer processing aid was measured in terms of critical shear rate (CSR), which was defined as the maximum possible shear rate at which the polymer blend could be extruded without any melt fracture appearance.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Rheological Characterization

Figure 1 shows the comparison of complex viscosity, [eta]* and dynamic moduli, G', and G", of resin LLDPE PF-Y821-BP with and without clay I.44PA. It can be seen that there is practically a very small effect in the linear viscoelastic properties of the polymer with the addition of clay I.44PA. In fact, the addition of clay particles causes a small decrease in the linear viscoelastic properties of this polymer. This might be due to a small decrease of the entanglement density of the polymer as the aspect ratio of nanoclays is high, which causes a small disruption in the entangled network of polymer chains. A small effect of slip might be excluded as these are linear viscoelastic measurements and therefore the strains involved are of the order of less than 10%. It should be pointed out, however, that in many other cases this small effect was not noticeable and therefore for all practical reasons this minor effect can be neglected. This observation is in agreement with other studies that have reported that the addition of a small amount of solid particles such as BN has no effect on the shear rheological properties of polymers [6, 17-20]. Similar results have been found for all the other polymers used in this study. A complete rheological characterization for the PP has been reported in Ref. 28, for the m-LLDPE Exact[R] 3128 in Ref. 17, and for the HDPE 58G in Ref. 37.

The tensile stress growth curves as a function of time for several extensional Hencky strain rates for resin LLDPE PF-Y821-BP are plotted in Fig. 2. It can be seen that the extensional behavior for this polymer is typical for linear polymers. One common characteristic in the extensional behavior of all polymers (not shown here) used in this study is that there is no strain hardening effect, typical for linear polymers. In each of these cases, the tensile stress tends to plateau to a certain level before it suddenly decreases, which translates to the rupture of the polymer specimen. Strain hardening is usually associated with polymers having a high molecular weight component and/or a high degree of long-chain branching [38]. The addition of a small amount of clay has practically no effect on the extensional behavior of all polymer used (not shown here) except at relatively low temperatures and high extensional rates. Such high extensional rates are responsible for the cause of gross melt fracture and, as will be discussed below, the presence of nanoclays plays a catalytic role in dissipating the elastic stored energy within the polymer and as a result can eliminate gross melt fracture phenomena. This effect is similar to the one disseminated by Sentmanat and Hatzikiriakos [35] in case of the addition of a small amount of BN. These results will be presented and discussed below, where the mechanism of sharkskin and gross melt fracture elimination will be explained in the presence of organically modified nanoclays.

[FIGURE 3 OMITTED]

Capillary Die Extrusion

Figure 3 shows the capillary extrusion behavior of the m-LLDPE Exact[R] 3128 alone and with the addition of several nanoclays and one fluoropolymer, namely 0.1 wt% and 0.5 wt% of clay PGV with 0.1 wt% and 0.5 wt% clay PGW, with 0.5 wt% clay I.44PA and with 0.1 wt% Dynamar[R] FX-9613. The capillary die used had a diameter of 0.127 cm, a length-to-diameter ratio of 10, and a reduction angle of 90[degrees]. The extrusion temperature was 163[degrees]C. The results are presented in terms of apparent shear stress (no Bagley corrections) vs. apparent shear rate. The apparent shear stress, [[sigma].sub.A], is defined as [[sigma].sub.A] [equivalent to] [DELTA]P/4(L/D), where [DELTA]P is the pressure drop required for extrusion, L is the die length, and D is the diameter of the die. The apparent shear rate is defined as [dot.[gamma].sub.A] [equivalent to] 32Q/[pi][D.sup.3] where Q is the volumetric flow rate. It can be seen that most nanoclays have no effect on the flow curve of the m-LLDPE with perhaps the exception of 0.5 wt% I.44PA. Note that the decrease in shear stress in the presence of Dynamar[R] FX-9613 is significant and as expected [6].

The critical shear rates for the appearance of sharkskin melt fracture for the m-LLDPE Exact[R] 3128 in the presence of the various additives are summarized in Fig. 4. The onset of surface deterioration of pure m-LLDPE Exact[R] 3128 is about 38 [s.sup.-1]. Addition of 0.1 wt% or 0.5 wt% of clay PGV and/or PGW has no effect on the processability of the linear polyethylene as the critical shear rate for the onset of surface deterioration remained about the same or caused only a marginal improvement in the processibility of polyethylene (case of PGW). Surprisingly, the addition of 0.5 wt% of clay I.44PA causes a dramatic improvement of the processibility of polyethylene. The critical shear rate for the onset of surface deterioration increases to 375 [s.sup.-1]. This critical shear rate is about 10 times the critical shear rate for the onset of surface deterioration of pure m-LLDPE Exact[R] 3128. Dynamar[R] FX-9613 has shown the best performance as the critical shear rate for the onset of surface deterioration increases to about 450 [s.sup.-1]. Typical extrudate photos to show the usefulness of nanoclay I.44PA as processing aids are shown in Fig. 5 at the apparent shear rate of 300 [s.sup.-1]. It is noted that clay I.44PA performed better in this case than BN. The latter has been shown in the past to be an enhanced processing aid [6, 16-22]. Clay I.44PA is organically modified whereas PGV and PGW are not. This causes clay I.44PA to be easily dispersed into the polymer, a requirement for good performance of a solid lubricant as a processing aid, i.e., use of BN powders as processing aids [6, 16-22].

The improved dispersion of organically modified clays was confirmed from SEM pictures. Figure 6 shows SEM pictures of pure m-LLDPE Exact[R] 3128 and with nanoclays I.44 PA and Cloisite 10A (organically modified) and PGV and Cloisite Na (unmodified). It can be seen clearly that better dispersion was achieved in the case of organically modified clays. A recent publication by Eckel et al. [39] suggests that transmission electron microscopy (TEM) is a powerful tool to study the dispersion of nanoclay particles. In our study, we used SEM to confirm the dispersion of clay particles, instead of TEM. This is because we are interested in the dispersion of clay in the polymer matrix, not the state of dispersion (exfoliation) of the clay particle. It is expected that no exfoliation would occur during the preparation of the samples. Thus, SEM is enough to examine the state of dispersion of our samples.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Figure 7 shows another example of the usefulness of clay I.44PA as a processing aid in the capillary extrusion of the polypropylene (PP). The flow curves of PP alone and with 0.1 wt% and 0.5 wt% clay I.44PA are shown. These were obtained in capillary die extrusion by using a capillary die having a diameter of 0.127 cm, a length-to-diameter ratio of 10, and a reduction angle of 90[degrees]. All runs were carried out at 200[degrees]C. It can be seen that the use of clay I.44PA significantly decreases the extrusion pressure. The appearance of the extrudates is also significantly affected by the presence of clay I.44PA. It is noted that sharkskin/surface melt fracture is not obtained in the extrusion of PP. Instead, the onset of surface gross melt fracture for pure polypropylene was found to be about 372 [s.sup.-1]. With the addition of 0.1 wt% of clay the onset of surface deterioration is delayed up to 750 [s.sup.-1]. Furthermore, the addition of 0.5 wt% of clay delays the onset of surface deterioration up to 900 [s.sup.-1]. One example of the type of extrudate distortions obtained in the extrusion of pure PP and with that loaded with 0.1 wt% and 0.5 wt% of I.44PA at several apparent shear rate values is illustrated in Fig. 8.

[FIGURE 7 OMITTED]

The results illustrated in Figs. 5, 7, and 8 show that clay I.44PA is a useful and an effective processing aid in eliminating gross melt fracture in the capillary flow of polypropylene as well as useful in decreasing the extrusion pressure significantly. It is pointed out that in this example, clay I.44PA can eliminate volume instabilities manifested as spiral depicted in Fig. 8. Similar results were obtained with nanoclays Cloisite 10A and Na. While clay Cloisite 10A has a significant effect on the processibility of polyolefins similar to that of I.44PA, the use of Cloisite Na was found to have almost no effect. It is noted that nanoclays I.44PA and Cloisite 10A are organically modified and easily dispersed into polyolefins, whereas nanoclays PGV, PGW, and Cloisite Na are not.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Crosshead Die Extrusion

Figure 9 depicts the apparent flow curve of pure m-LL-DPE (Exact[R] 3128) with and without additives obtained from a capillary rheometer fitted with a crosshead die having a diameter of 3 mm and a tip of 1.52 mm in diameter. The apparent shear rate was calculated by using the formula, which applies to slit dies:

[dot.[gamma].sub.A] = 6Q/[0.25(D - d)[.sup.2]0.5[pi](D + d)] (1)

where Q is the volumetric flow rate, and d and D are the tip and die diameter, respectively. The apparent wall shear stress, [[sigma].sub.A], was estimated as the average of the stress at the inner and outer walls by using the following formula for a power-law fluid [40]:

[[tau].sub.rz] = [[[DELTA]PD]/[4L]]([2r/D] - [[beta].sup.2][D/2r]) (2)

where [[tau].sub.rz] is the shear stress at radius r, [DELTA]P is the pressure drop, L is the length of the die land, and [beta] is a parameter that depends on the geometry and the power law index [40]. The experiments were run at 163[degrees]C. The various additives used were nanoclays PGV, PGW, and I.44PA and the fluoroelastomer Dynamar[R] FX-9613. The onset of surface deterioration of m-LLDPE Exact[R] 3128 is 37 [s.sup.-1]. Addition of 0.1 wt% of clay PGV or clay PGW has increased the critical shear rate for the onset of surface deterioration to 164 [s.sup.-1]. Surprisingly, the addition of 0.1 wt% of clay I.44PA causes a dramatic improvement of the processibility of polyethylene. The critical shear rate for the onset of surface deterioration increases to 982 [s.sup.-1]. This critical shear rate is about 26 times the critical shear rate for the onset of surface deterioration of pure m-LLDPE Exact[R] 3128 and about six times the critical shear rate for the onset of surface deterioration that can be obtained with nanoclays PGV and PGW. These results indicate that clay I.44PA is a useful and effective processing aid in eliminating surface instabilities in the processing of polyethylenes.

A similar critical shear rate was also reached by the use of BN under identical conditions used in this set of experiments [17]. Similar results were obtained with all other polymers used in this work. Some of these will be presented below, where nanoclays are combined with fluoropolymers to produce enhanced processing aids. This parallels the enhanced processing aids developed by combining BN with fluoroelastomers and stearates [17, 18, 22, 23, 41].

Combinations of Nanoclays With Fluoroelastomers

As was previously seen, the use of certain nanoclays may eliminate not only sharkskin/surface melt fracture but also postpone the onset of gross melt fracture to much higher shear rates, without necessarily decreasing the extrusion pressure. On the other hand, fluoropolymers may eliminate sharkskin and significantly decrease extrusion pressure but have very little effect on the onset of gross melt fracture. In this section, the two additives are combined together and, as will be demonstrated, their combinations produced enhanced processing aids.

Figure 10 shows the behavior of the LLDPE PFY821-BP alone and with several combinations of additives, including 0.1 wt% Dynamar[R] FX-9613, 0.1 wt% of clay I.44PA, and the combination of 0.05 wt% Dynamar[R] FX-9613 and 0.1 wt% clay I.44PA. The extrusion is performed by using the crosshead die described above. All experimental runs were carried out at 170[degrees]C. The onset of surface deterioration (sharkskin) for the pure LLDPE PF-Y821-BP is about 250 [s.sup.-1]. With the addition of 0.1 wt% of fluoroelastomer Dynamar[R] FX-9613, the onset of surface deterioration (sharkskin) is delayed up to a shear rate of 1800 [s.sup.-1]. At this critical shear rate, sharkskin appears. With the addition of 0.1 wt% of clay I.44PA the onset of surface deterioration (sharkskin) is delayed up to a shear rate of 660 [s.sup.-1]. At this critical shear rate, sharkskin appears. When clay I.44PA is combined with 0.05 wt% of Dynamar[R] FX-9613, the critical shear rate for the appearance of sharkskin melt fracture is delayed to 2260 [s.sup.-1]. The results show that the combination of clay I.44PA with Dynamar[R] FX-9613 is a very useful processing aid and can perform better than either of its individual constituents when they are used independently. Note that in all cases a reduction in pressure results even in the case of using clay I.44PA alone. Typical extrudate photos obtained at the apparent shear rate of 982 [s.sup.-1] are shown in Fig. 11. While the use of only I.44PA leaves a few stripes of sharkskinned sections on the extrudate surface, the combination of clay I.44PA and Dynamar[R] FX-9613 produces glossy and smooth extrudates.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Figure 12 represents a similar example for the case of HDPE 58G. The flow curve of pure HDPE 58G alone, and that with several combinations of additives including 0.1 wt% Dynamar[R] FX-9613, 0.1 wt% of clay I.44PA, and the combination of 0.05 wt% Dynamar[R] FX-9613 and 0.1 wt% of clay I.44PA are plotted. The extrusion is performed by using the crosshead die described above. All experimental runs were carried out at 170[degrees]C. The onset of surface deterioration (sharkskin) for HDPE 58G is about 500 [s.sup.-1]. With the addition of 0.1 wt% of Dynamar[R] FX-9613 the onset of surface deterioration (sharkskin) is delayed up to a shear rate of 2440 [s.sup.-1], while the addition of 0.1 wt% of clay I.44PA delays the onset of surface deterioration (sharkskin) to 1330 [s.sup.-1]. When clay I.44PA is combined with 0.05 wt% Dynamar[R] FX-9613 the critical shear rate for the appearance of sharkskin melt fracture is delayed to 3600 [s.sup.-1]. The results show that the combination of clay (Nanomer[R] I.44PA) with a fluoroelastomer (Dynamar[R] FX-9613) is a very powerful processing aid. It is noted that although there is very little effect of the addition of processing aids on the extrusion pressure, the effect is significant on the extrudate appearance. To obtain a higher reduction in extrusion pressure particularly due to the presence of fluoroelastomer, one might have to extrude the blend for longer times due to a large induction time. This is easier to be observed in the continuous extrusion operations of polyolefins, while it is more difficult to see in the batch extrusions operations using the capillary rheometer.

One example of the type of extrudate distortions (surface deterioration) obtained in the extrusion of HDPE 58G with I.44PA and its combinations at the extrusion rate of 1964 [s.sup.-1] is shown in Fig. 13. It can be concluded that the combinations of organically modified nanoclays with a small amount of fluoroelastomer produces a very effective and powerful processing aid for the extrusion of polyolefins.

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Onset of Surface/Gross Melt Fracture and its Relation to Extensional Rheology

Sentmanat and Hatzikiriakos [35] have recently studied the relationship between the onset of sharkskin/gross melt fracture with the extensional rheological behavior of polyolefins. While sharkskin is initiated at the exit of the die and gross melt fracture at the entry to the die, both are due to high extensional stresses developed at these particular locations of the die [5-8, 12]. The addition of a small amount of BN has a significant effect on the extensional stress growth coefficient of the filled resins. Sentmanat and Hatzikiriakos [35] have reported that the BN-filled resins (up to 0.05 wt%) have shown a significantly lower extensional stress (up to about 30% lower) compared to the virgin resin. These results demonstrated that BN is able to suppress the increase of extensional stresses to levels that can lead to the initiation of surface/sharkskin and gross melt fracture. As a result the use of BN eliminates sharkskin melt fracture and postpones the onset of gross melt fracture to significantly higher shear rates [6, 16-21].

We report here a similar effect for the case of polyolefins filled with a small amount of an organically modified nanoclay. Figure 14 summarizes the results. The tensile stress is plotted as function of Hencky strain for the LLDPE PFY821-BP alone and that filled with 0.1 wt% of I.44PA and 0.1 wt% of BN at a Hencky strain rate of 22.6 [s.sup.-1] and temperature of 130[degrees]C using the new extensional rheometer described above. The experiment was performed at low temperature in order to emulate extensional rates closer to those at which gross melt fracture occurs. It can clearly be seen from Fig. 14 that both the clay- and BN-filled resins show a significantly lower extensional stress (up to about 30% lower) compared to the virgin resin. These results demonstrate that organically modified nanoclays are able to suppress the increase of extensional stresses. Thus, the ability of nanoclay to dissipate elastic energy associated with the rapid increase of extensional stresses at high rates of deformation is clearly apparent. In addition, this shows that clay I.44PA acts as an effective processing aid to postpone the onset of gross melt fracture. Based on these results, we believe that the mechanism of melt fracture elimination in the presence of BN and organically modified nanoclays is the same [35-37].

CONCLUSIONS

Four different polyolefins were used to study the effect of the addition of a small amount of nanoclay on their processibility. It was found that organically modified nanoclays can be easily dispersed into the polyolefins and that they can not only eliminate the onset of sharkskin melt fracture but may significantly postpone the onset of gross melt fracture to much higher rates. It was found that the melt fracture behavior of linear polyethylenes (sharkskin and gross melt fracture) is closely related to high-rate extensional melt rheology. In other words, it was discussed that the tensile stress was found to be indicative for the onset of gross melt fracture. This was also confirmed by extensional behavior of clay- and BN-filled resin, which showed significant reduction of the tensile stress growth coefficient compared to the unfilled resin. It was finally discussed that the suppression of the growth of tensile stress is mainly responsible for the ability of the additives (organically modified nanoclays and boron nitride) to eliminate melt fracture phenomena.

[FIGURE 14 OMITTED]

ACKNOWLEDGMENTS

This work was financially supported by a strategic grant provided by NSERC Canada.

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Savvas G. Hatzikiriakos, Nimish Rathod, Edward B. Muliawan

Department of Chemical and Biological Engineering, The University of British Columbia, 2216 Main Mall, Vancouver, BC V6T 1Z4, Canada

Correspondence to: S.G. Hatzikiriakos; e-mail: hatzikir@interchange.ubc.ca

Contract grant sponsor: NSERC of Canada.
TABLE 1. Physical properties of nanoclays.

Clay Color Aspect ratio Specific gravity Moisture

PGW White 200-400 2.60 12%
PGV White 150-200 2.60 18%
I.44PA White >500 - -
Cloisite Na Off white - 2.86 <2%
Cloisite 10A Off white - 1.90 <2%

 Particle size
Clay ([micro]m) Organic modifier Origin

PGW 16-22 None Nanocor[R]
PGV 16-22 None Nanocor[R]
I.44PA 15-25 Onium ion modified Nanocor[R]
Cloisite Na 2-13 None Southern Clay
 Products Inc.
Cloisite 10A 2-13 Quaternary ammonium salt Southern Clay
 Products Inc.
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Author:Hatzikiriakos, Savvas G.; Rathod, Nimish; Muliawan, Edward B.
Publication:Polymer Engineering and Science
Date:Aug 1, 2005
Words:5593
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