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Dispersion in High Viscosity Ratio Polyolefin Blends.

The dispersion of polyethylene and polypropylene in a polypropylene matrix using a co-rotating twin screw extruder has been investigated. Polymer pairs were selected to study the effect of viscosity ratio, defined as the viscosity of the minor component over that of the matrix, in the range of 0.1 to 900. The dispersion quality was defined by determining the number of "gels," i.e., large undispersed particles, present in thin films and by conventional microscopy techniques. The gel numbers were found to increase steadily with the viscosity ratio. It was also observed that particle size distributions in the high viscosity ratio blends was very broad, with particles as large as a hundred microns coexisting with much finer ones in the sub-micron range. For a given polymer blend, the re-processing was found to have an important effect on gel reduction. The effect of screw rotation speed, flow rate and minor phase feeding position was also investigated and is discussed in the paper.

INTRODUCTION

Dispersion of polymer blends is a mixing problem of industrial significance. It can be a challenging task when the viscosities of the blend components are not well matched. The dispersion of high viscosity ratio blends will be addressed in this paper. The viscosity ratio is defined as the viscosity of the minor blend component over that of the matrix. Impact modified polymers are an important class of materials in which high viscosity polymer modifiers or elastomers are often blended to a less viscous polymer material. Another area where the dispersion of a highly viscous component is central is the gel reduction in homopolymers or reactor blends. Gels are high molecular weight fractions that have not been homogenized with the bulk of the material. These are very detrimental to film appearance and to mechanical properties of all types of polymer products. Gel reduction is clearly a difficult dispersion problem since the gel particles can be several orders of magnitude more viscous than the surrounding material.

Most prior polymer dispersion studies have focused on blends with relatively well-matched viscosities. It has been shown in several polymer systems that the blend morphology evolves rapidly during mixing. The first step is the minor phase sheet formation. It is followed by growth and coalescence of holes in these sheets under interfacial forces to form a lace structure. The final step is the breakup through interfacial instabilities of the fibrils that were forming the so-called lace structure [1, 2]. When the major phase has a melting or softening point significantly higher than that of the minor phase, the latter can act as the matrix until the melting temperature of the major phase is reached. Phase inversion then occurs, leading again to a fibril structure that can then undergo interfacial instability [3]. These steps were shown to occur within a few kneading disks in the twin-screw extruder. Since the first mixing zone of the twin-screw extruder usually also serves the purpose of melting the polymers, t he melting and mixing mechanisms cannot be established independently. It has been argued that the melting mechanism is central to the dispersion process and that in many instances the phase inversion was essential for proper dispersion. However, the melting stage was shown not to be a controlling factor at least in one set of experiments where polyethylene and polystyrene were put into contact either in the melt state or in solid state and then mixed in the twin screw extruder. Similar dispersions were found regardless of the initial state of the blended components [4].

The present study focuses on the dispersion of high viscosity high-density polyethylene (PE) and polypropylene homopolymer (PP) in a PP matrix. The intent is to model the dispersion of highly viscous impact modifiers or that of gels in homopolymers. These systems typically have low interfacial tension and should be well represented by the selected model blend. The relation between blend toughness and average modifier diameter is well established and has made transmission and scanning electron microscopy the basic tools to quantify the dispersion state of a blend. Because of the small sample surfaces scanned by conventional microscopy technique, it is difficult to quantify or even to detect the presence of large undispersed particles and gels. Thus, this aspect of the dispersion evaluation has received less attention in the past, even though the presence of gels can have very detrimental effects on product performance. In this study, the dispersion quality will be evaluated by automated gel counting on thin f ilms, gels being defined as film defects greater than 50 microns. This technique was assumed to be statistically representative of the content of large undispersed particles in blends. The objective of the study is to evaluate the effect of material and processing parameters on the dispersion of a highly viscous minor phase. In addition to viscosity ratio effects, the effect of interfacial tension (PP/PP versus PP/PE blends), operating conditions, initial particle size, feeding position of dispersed phase and effect of multiple pass will be investigated.

BACKGROUND

In the classical Newtonian micro-rheology, dispersed droplet deformation and breakup depend on two dimensionless numbers: the capillary number and the viscosity ratio [lambda]. The capillary number is defined as D[sigma]/[nu] where D is the droplet diameter, [sigma] is the stress acting on the droplet and [nu] is the interfacial tension coefficient. The viscosity ratio, [lambda], is defined as the viscosity of the dispersed phase to that of the matrix. Drop deformation and breakup occur if the capillary number is greater than a critical value, which is a unique function of the viscosity ratio. The critical capillary number in pure shear flow is minimum when the viscosity ratio is equal to unity and increases to infinity for a viscosity ratio greater than 4. Thus, systems with [lambda] [greater than] 4 should not be dispersible in pure shear. In elongational flow, the critical capillary number is lower and less sensitive to the viscosity ratio. More details on the classical Newtonian micro-rheological concept s and assumptions are summarized elsewhere [5]. In summary, it is expected that dispersion will be a function of the viscosity ratio, interfacial tension and of the flow field present in the mixing apparatus.

In polymer processing, the fluids are viscoelastic and the flow is far from viscometric. There is no clear consensus on the effect of elasticity. It has been observed that in pure shear, elastic normal forces will flatten droplets into sheets especially if the matrix is more elastic than the dispersed phase [6]. Studies on viscoelastic model solutions indicate that drop elasticity tends to decrease deformability [7, 8] but could accelerate particle breakup via a non-linear effect during rapid flow transition [9]. The complex flow encountered in compounding equipment provides unsteady stresses, reorientation, mechanical splitting and phase transition, which will all relax the requirements for breakup. The increase of average diameter with viscosity ratio has been reported for different systems. For ethylene-propylene rubber/polyamide blends, a minimum critical capillary number, calculated using approximated average stress in the twin screw extruder, was found for a viscosity ratio around unity [10]. The criti cal capillary number in the extruder was found to be less dependent on viscosity ratio than the theoretical predictions for pure shear flow. Similar viscosity ratio effects were found for polycarbonate/polyethylene blended in an internal mixer. The droplet diameter, in the micron range, increased slightly with viscosity ratio, but again, dispersion was possible at viscosity ratios in excess of 10 [11]. In rapidly quenched samples of PE/PS systems, it was observed that droplets, fibrils and sheets could coexist not only in the early stages of compounding but throughout the twin-screw extruder if the viscosity of the system is high enough. This is explained by the increased morphological stability as the matrix and dispersed phase viscosity increases. In these conditions, the effect of the viscosity ratio is to increase the size of dispersed droplets but also to increase the fraction of large undispersed structures (e.g. fibers and sheets) [12]. Compounding conditions such as flow rate and screw rotation speed were shown to have little or no effect on the final blend morphology for a range of polymer pairs [5, 13, 14].

This study differs from the previous one in three particular ways. First, the dispersion quality will be examined in terms of the number of gels present in the blend, thus shifting the focus to the higher end of the particle size distribution rather than to an average diameter. Second, the blend components chosen will cover a wide range of viscosity ratio, from 0.1 up to 900. Finally, the interfacial tension in the systems studied is very low (PE/PP) or zero (PP/PP). This should enable the determination of a clear relationship between dispersion dynamics and the viscosity ratio with little or no interfacial tension effects.

EXPERIMENTAL

Materials

Polypropylene, PP, and high density polyethylene, PE, of varying viscosity were used in this study as matrix and dispersed phase, respectively. The PE/PP blend is immiscible and has a relatively low interfacial tension of approximately 1 mJ/[m.sup.2] [10]. Blends of different PP were also made to isolate viscous from interfacial effects. The materials used in the study are listed in Table 1. The polyethylenes are all of high-density type. Materials Nova 1008 and Nova 455 were supplied by Nova Chemicals. The DMDA grades were supplied by Union Carbide. The three polypropylenes were homopolymers supplied by Montell. The PH920S is in the form of reactor beads; others are in pellet form. The material's viscosity was measured in oscillatory shear at 190, 200 and 230[degrees]C in the 0.1 to 100 rad/s range. Figure 1 presents the complex viscosity as a function of frequency for the different materials at 200[degrees]C. The shape of the curves is typical for polymer melts with a Newtonian plateau at low deformation r ates and a power law region at high deformation rates. The plateau is well defined for the lower viscosity materials. For the materials in the higher viscosity range, the Newtonian plateau is established at much lower rates.

In order to calculate viscosity ratios, [lambda], between non-Newtonian fluid, it is necessary to specify a common basis for the comparison. Because extrapolation over several orders of deformation rates is needed for determining the zero-shear viscosity, this quantity can be inaccurate. To remove the need for extrapolation, materials can be compared at a given oscillation rate or equivalent shear stress, [[sigma].sup.*] = [[eta].sup.*] . [omega]. The viscosity ratios obtained at constant stress will of course be larger than those obtained at constant rate, but the observed trends should be identical. In this study, the viscosity ratios at constant stress, [[eta].sup.*] . [omega] = [10.sup.4] Pa, will be used. The complex viscosity at 200[degrees]C for a fixed shear stress of [10.sup.4] Pa and a fixed oscillation rate of 100 rad/s are reported in Table 1. The Arrhenius flow activation energies for the PP and PE used in this study were found to be around 40 and 25 kJ/mol, respectively, in accordance with typica l values reported in the literature. Since PP has a higher activation energy, the viscosity ratio ([lambda] = [[eta].sub.PE]/[[eta].sub.PP]) will increase with temperature. The lowest viscosity ratio is thus expected right after the PP melting point.

Compounding

Compounding was carried out on the two different co-rotating twin-screw extruders. The screw and process configurations are described in Fig. 2. Configuration #1 was set up on a Werner-Pfleiderer ZSK-30 twin-screw extruder. The screw configuration is composed of two mixing zones separated by a secondary feed port. A first set of experiments, carried out on this equipment, aimed at investigating the effect of viscosity ratio, flow rate and rotation speed on dispersion. The studied viscosity ratio ranged from 0.1 to 900 while the flow rate and rotation speed ranged from 6 to 18 kg/h and 150 to 450 rpm, respectively. The minor phase concentration was 5 wt%. A second set of experiments were carried out on a Leistritz 34 mm twin-screw extruder, which offered greater flexibility for minor phase feeding. These experiments aimed at evaluating the effect of the minor component feeding position. The screw configuration was similar to the first one with two mixing sections. The minor phase was fed in four different way s: i) in pellet form in the main hopper, ii) in liquid form through a 32 mm single-screw extruder at the same mid-extruder location as the side-feeder, iii) in pellet form through a side-feeder at mid-extruder and iv) in liquid form after the twin-screw extruder second mixing zone. Because of constraints on minor phase flow rate control, the standard minor phase concentration in the second set of experiments was increased to 15 wt%. The flow rate and rotation speed were 12 kg/h and 300 rpm, respectively. In all cases, 0.15 wt% of stabilizer (Irganox B-225 from Ciba Specialty Chemicals) was added to avoid material degradation during blending.

Gel Counting and Morphology Characterization

The compounded materials were extruded on a 40 mm single-screw cast film line to produce 30-micronthick film. Gels were counted on-line using an automated gel counting system. The minimum defect size detected by the counter was around 50 [micro]m, similar to the resolution of the human eye. The amount of film used for each sample was around 250 [cm.sup.2]. PP controls (with no dispersed phase) were tested to make sure that no gels were present in the PP matrix. Visual inspection of PP films revealed no apparent gels. Gel counts for the pure PP films were in the 0-0.05 gels/[cm.sup.2]. At the higher gel count end, accurate gel counts became difficult for films with more than 80 gels/[cm.sup.2] because of poor film quality. Therefore, reproducible gel counts are expected in the 0.05 to 80 gels/[cm.sup.2] range. Duplicate gel counting runs indicate reproducibility within [+ or -] 5% for a given compounding run.

It should be pointed out that the size of defects observed on a thin film is due to film surface curvature, as illustrated in Fig. 3. This Figure presents an optical micrograph taken on the cross-cut of a 100 [micro]m film. Prior to observation, the film sample was embedded in epoxy, polished with sand paper down to 3600 grits and finally polished using diamond suspensions down to 1 [micro]m size. The optical micrograph reveals the undispersed particle at the center of the film. It is around half the size of the film defect and indicates that the apparent size of the film defect can be significantly larger than the size of the undispersed particle causing the defect. Because of this and since the compounded material needs to be reprocessed in a single screw extruder for the dispersion evaluation, the gel counts are not intended as absolute numbers of undispersed particles but as a relative and statistically representative measure for comparing the dispersion quality.

The compounded materials were also examined by performing scanning electron microscopy (SEM) on extruded strands. The strands were cooled in a water bath after exiting from the twin-screw extruder. They were then cut perpendicular to the strand axis using a cryo-microtome operated at -140[degrees]C. Finally, selective etching was performed by putting the samples in a solution of 0.7 wt% KMn[O.sub.4] in a 65/35 of sulfuric acid ([H.sub.2][SO.sub.4] 98%) and ortho-phosphoric acid ([H.sub.3][PO.sub.4] 85%) for a period of 20 min.

RESULTS AND DISCUSSION

The effect of the viscosity ratio on the number of gels per unit area is presented in fig. 4. The blends were all compounded at 300 rpm with an output rate of 112 kg/h and the minor phase concentration was 5 wt%. For the blends with viscosity ratios up to around 10. the film quality was high and could not be distinguished by eye. The gel counts in the 0.03 to 0.3 gels/[cm.sup.2] range were limited by the lower detection limit of the counting system. For higher viscosity ratios, the number of gels increases steadily with the viscosity ratio up to values of 40 gels/[cm.sup.2] for viscosity ratios around 100 and to values in excess of 100 gels/[cm.sup.2] for viscosity ratios in the 500-900 range. The data for two PP/PP blends is reported on the same graph. Surprisingly, even though it does not have any significant interfacial tension, the PP/PP data fall on the same gel number-viscosity ratio relationship as the PE/PP blends. This seems to indicate that interfacial tension is not a significant parameter for gel reduction in low interfacial tension blends such as the PE/PP blend. It is also an indication that molecular diffusion in a high viscosity miscible polymer pair is too slow to provide much homogeneity. Thus, as in PE/PP blends, dispersive mixing in PP/PP must be used to decrease the dispersed phase size before molecular diffusion can contribute to blending at the molecular level.

While gels can be easily detected on thin films, finding them by SEM can be difficult because of the small area scanned and the relatively low volume fraction that gels represent. Figure 5 presents some typical micrographs obtained on the PE/PP and PP/PP blends. Acid-etching is used to enhance topological contrast as PE and PP are difficult to distinguish in the SEM. Figure 5a presents a SEM of a PP control where the crystalline structure has been revealed by the acid treatment after removal of amorphous PP. No holes or etching artifacts that could be misinterpreted as blend features are present. Figures 5b and 5c present micrographs for the PE-3/PP-2 blend that has been extruded twice and once respectively. The viscosity ratio for the blend is 65 and the number of gels associated with the materials is 0.6 and 15 respectively. PE particles can be easily distinguished after etching and are slightly protruding because of faster removal of the PP matrix during the acid-etching. The dispersed particle size is in the 1-5 [micro]m range except for a few 5-10 [micro]m particles in Fig. 5c. Even though the gel counts are very different, it would take an important image analysis effort of several micrographs in order to compare the samples in a statistically representative manner. This illustrates the usefulness of gel counting for quick and reliable evaluation of dispersion quality. Figure 5d presents the blend morphology for the PE-2/PP-1 blend, which has a viscosity ratio of 96 and a number of gel of 39. The size of the large particles is similar to that of the previous blend (note the higher magnification). However, there seem to be fewer finer particles in the 1-3 [micro]m range probably because of a shift in the dispersed phase size distribution. The largest PE particles were found in the PE-4/PP-1 blend, which has a viscosity ratio of 900. The particle shown in Fig. 5e has a diameter of 500 [micro]m. Smaller 10-20 [micro]m PE fragments can also be observed. Finally, Fig. 5f presents acid-etched microtomed surfaces of PP/PP blends. Surprisingly, the blend morphology can still be observed with dispersed PP particles as large as 20-30 [micro]m. The minor phase is the high viscosity PP-3 while the matrix is the low viscosity PP-1. It is believed that the difference between the two PPs in terms of molecular weight and crystallinity fraction enables sufficiently different etching rates for matrix and dispersed material to effectively reveal the blend structure. To the authors' knowledge, this is the first time that a dispersed blend structure (typical of immiscible blends) has been observed by SEM in a mixture of high and low molecular weight homopolymer. It clearly illustrates that in highly viscous miscible blends, micron-range homogeneity must first be obtained through a dispersion process rather than through inter-diffusion of polymer chains.

Image analysis of SEM micrographs was carried out using between 300 and 500 particles for each blend. Fig. 6 presents the cumulative particle size distributions for 5 wt% PE/PP blends with viscosity ratios ranging from 2 to 900. Linear relationship on a probability plot indicates a Normal distribution or in the present case, a Log-Normal distribution (since the abciss scale is logarithmic). Log-Normal distributions are typical for polymer dispersions and are observed in this study for the low viscosity ratio blends. As the viscosity ratio of the blend is increased, the distribution becomes broader and the median diameter increases. For the highest viscosity ratio blend, a bimodal distribution is observed with submicron particles coexisting with large particles in the 20-100 [micro]m range. Because of the broad diameter range, obtaining statistically representative distribution in high viscosity ratio blends is a challenging task, especially in the high diameter range, where the number of particles is lower. E ven though precision on the distribution function in the high diameter range could benefit from the use of larger sampling size, the trend toward broadening of the distribution with the increase in viscosity ratio seems clear.

Figure 7 presents the effect of flow rate and screw rotation speed on the total number of gels for a 5 wt% PE-3/PP-2 blend. The gel numbers are in the high range since the blend has high viscosity ratio of 65. The number of gels decreases significantly with rotation speed from over 26 to 10 gels/ [cm.sup.2]. This may be explained by increased local stress levels and better distribution at high rpm. Even though this is a relatively important decrease, the number of gels at the highest rotation speed is still an order of magnitude higher than acceptable film gels levels. The number of gels is not as significantly affected by flow rate. Surprisingly, the gel number goes through a minimum at the intermediate flow rate. At fixed rotation speed, increasing flow rate in a twin-screw compounder decreases residence time but increases the length of fully filled high stress extruder portions. These two phenomena would have opposite effects on mixing and could explain the observed dependency. Data on other blends of diff erent viscosity ratio are needed to determine if this trend can be generalized.

Figure 8 presents the effect of the number of extrusion passes on the number of gels for the 5 wt% PE-3/PP-2 blend. The number of gels is reduced from 15 to 0.6 gels/[cm.sup.2] when the material is extruded for a second time. Further reduction occurs after a third pass. In prior studies, steady-state morphologies were attained rapidly after the end of the melting zone [3] or were thought to be dependent on the final flow history encountered by the blend within the extruder (4). In this study, our dispersion evaluation technique focuses on particles that did not finely disperse. From this point of view, the blend is clearly not at a final state even after the first complete extrusion pass. Because gels represent a very small volume fraction, in the order of [10.sup.-4]-[10.sup.-6], they are often difficult to detect on a cross-cut using conventional techniques such as scanning electron microscopy on microtomed surfaces. It is thus possible that the presence of large undispersed particles has been overlooked i n prior studies on high viscosity ratio blends.

To assess the significance of the minor phase incorporation on gel content, the minor phase was introduced in solid and melt form in twin-screw setup no. 2. presented above. Gel numbers for four different minor phase feeding modes are presented in Fig. 9 for 15 wt% blends PE-1, PE-2, PE-4 and PP-3 into PP-2. These blends have viscosity ratios of 2, 15, 142 and 30, respectively. In all cases, the melt feeding of the minor phase at the end of the extruder leads to the worst dispersion, as expected, since the blend does not go through any dispersive mixing elements. The best dispersion results are observed when the minor phase is fed in solid form in the primary hopper. The feeding of the PE dispersed phase at mid-extruder in the molten PP matrix results in five-fold increase in the number of gels for the PE/PP blends. Thus, the PP matrix state seems to be an important factor for gel reduction. This could be explained by the high stresses associated with the melting stage and that could help in the deformation of the dispersed PE phase. A second interpretation could come from the melting order. As the PE phase will melt first, it may form the continuous phase until the PP melting point is reached. The phase inversion occurring at this point has been well documented in internal mixer studies and is thought to be an important factor for dispersion dynamics (2). However, the results for the PP-3/PP-2 blend presented in Fig. 9d are qualitatively similar to that obtained for the PE/PP blends (Figs. 9a-c even though the PPs have similar melting temperatures. Thus, the phase inversion mechanism invoked in the literature for explaining rapid morphological changes during the melting stage does not seem likely for this system.

For mid-extruder feeding, there is no clear trend as to whether it is better to feed the dispersed phase in the melt state or in solid state in order to minimize the number of gels. The lack of significant differences between these two very different feeding modes is direct evidence that gels are not remnants of unmelted single pellets. Clearly, gels of a high viscosity dispersed phase can form from the melt state.

CONCLUSIONS

Gel counting on thin films was shown to be a useful method for comparing the dispersion quality in high viscosity ratio polyolefin blends. The number of gels in FE/PP blends increased steadily with the blend viscosity ratio. Gel numbers for two PP/PP blends fell on the same relationship, seemingly indicating that the interfacial tension is not a controlling factor in the dispersion of polyolefin blends. Successive extrusion passes significantly decreased the number of gels, indicating that the dispersion is not in its final state after a first or even a second extrusion pass. The effect of feeding position was found to be less important than that of a second extrusion pass. Pellet-pellet feeding in the primary extruder hopper led to the lowest number of gels. No clear difference was found when feeding the minor component in solid or in the melt state, indicating that the gels were not remnants of the unmolten minor phase. Increasing the screw rotation speed resulted in significant gel reduction, while the ef fect of flow rate was more complex, with the largest gel reduction at intermediate flow rates.

ACKNOWLEDGMENTS

The authors would like to thank Mr. Kouichi Nakayama for the gel counting, Ms. Helene Roberge for the sample etching and scanning electron microscopy and Mr. Robert Lemieux for carrying out the extrusion trials.

REFERENCES

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 List of Materials Used in PE/PP and PP/PP Mixing Study.
Reference Name MFI (dg/min) [[eta].sup.*] at [sigma] =
 [10.sup.4] Pa (Pa.s)
 PE-1 DMDA 8907 8.0 2300
 PE-2 Nova 455 2.0 18,200
 PE-3 DMDA 6147 0.8 78,000
 PE-4 Nova 1008 0.5 170,000
 PP-1 PH920 S 60 190
 PP-2 SM-6100 12 1200
 PP-3 PP-6823 0.5 36,000
Reference [[eta].sup.*] at [omega]
 = 100 rad/s (Pa.s)
 PE-1 630
 PE-2 1490
 PE-3 2045
 PE-4 2420
 PP-1 128
 PP-2 340
 PP-3 1410


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Article Details
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Author:HUNEAULT, MICHEL A.; MIGHRI, FREJ; KO, GLEN H.; WATANABE, FUMINAO
Publication:Polymer Engineering and Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Apr 1, 2001
Words:4811
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