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Melt fracture and rheology of linear low-density Polyethylene--calcium carbonate composites.

INTRODUCTION

Continuously increasing requirements regarding both quality and productivity of polymeric products with a concurrent demand for lowering production costs are the most important issues in polymer processing. One of the most common methods of cost reduction is the incorporation of inorganic and organic fillers into the polymeric matrix [1-7]. Polyolefins, which are extensively used as materials for cast film production, are widely modified with high amounts of inorganic fillers, such as calcium carbonate, talc, etc. [7-9]. Increasing productivity often requires extremely high extrusion intensities which may cause melt processing instabilities.

Melt flow instabilities of polyethylene, which is commonly used for extrusion, are well described in the literature. The influence of molecular weight distribution, die material and additives have also been studied [10-21]. The sequence and origin of nascent melt flow instabilities during flow of linear low density polyethylene were extensively described in several papers [22-24]. In the case of linear low density polyethylene, five different types of flow are usually observed (Fig. 1): stable flow (1), sharkskin instability (2,3), stick-slip (4), second stable flow regime or superextrusion (5), and gross melt fracture (6). In this study, two types of sharkskin instability are also distinguished. The first type of sharkskin instability, described as "matted surface," is related to the first stage of low-amplitude sharkskin ([2] in Fig. 1). The second type of macroscopic sharkskin, characterized by a high amplitude of cyclic distortions on the surface of the extrudate, is presented in (3) in Fig. 1.

The appearance of sharkskin-type distortions on the extrudate surface often lowers the quality of cast or blow molded film, however it does not lead to an interruption in the production process. Sharkskin instability is observed as low amplitude and high frequency distortions occurring on the surface of the extrudate, which origin was widely studied by many research groups and presented in several papers [23, 25, 26]. In general, the main reason for sharkskin instability is the strong elongation stresses that form during acceleration of polymer flow near the die exit. The stick-slip is a flow instability which is characterized by alternate changing of the slip conditions above the characteristic critical value. As a result, the surface of the extrudate becomes reversibly distorted by sharkskin instability ([4] in Fig. 1). Stick-slip phenomena are assigned to the interfacial slip mechanism by disentanglement of adsorbed chains from the polymer bulk, which is accompanied by a high amplitude and low-frequency pressure drop. Being distinct from sharkskin instability, the origin of the stick-slip phenomenon is not at the die exit but in the die land. The effect is strongly connected with the compressibility of the polymer and the slip conditions at the die wall [20]. A successive increase in extrusion intensity may cause a second smooth region to occur ([5] in Fig. 1). The instability observed at the highest extrusion throughputs in linear polyethylene processing is gross melt fracture ([6] in Fig. 1), whose origin is connected with the polymer flow near the die entrance and with the macromolecular structure of the polymer. Many melt flow instability suppression and detection methods have been presented in the literature, e.g., incorporation of small amounts of boron nitride [11-14], application of polymer processing additives such as polyethylene glycol (PEG), esters of PEG [15, 16] or fluoropolymers [27], polymer blending [28], usage of induced temperature gradients [29], and application of signal processing methods allowing to control the extrusion process [30, 31],

The contribution of thermoplastic composites in the general amount of extrusion products made it necessary to gain a better understanding of the influence of filler type and amount incorporated into the polymeric matrix on rheological behavior and the melt flow instability sequence. Polyolefin-based composites filled with both modified and unmodified calcium carbonate have received significant attention. Most considerations were focused on the effect of incorporating inorganic filler into the polymeric matrix on properties such as rheological behavior, thermal stability, oxidation resistance, surface modification, crystallization, structure and mechanical properties [6, 9, 32-42]. Different case studies demonstrated that stearic acid modification of calcium carbonate leads to an enhancement of uniformity in polyolefin-based composites.

Despite the broad rheological studies presented in the literature, and also at high shear rates, it should be noted that there have only been several studies that dealt with melt flow instabilities of filled and highly filled polymer composites. Hristov presented the effect of wood flour addition on the melt fracture of polyethylene composites [43, 44]. Ariffin et al. presented a study of the influence of kaolin addition on melt fracture of polypropylene-based composites [45], Both of the abovementioned case studies considered melt fracture of polypropylene, yet the polyethylene processing sequence of nascent flow instabilities is much more complicated.

The studies presented in this paper are focused on determining the influence of mineral filler addition into a linear low density polyethylene matrix on the sequence of melt flow instabilities as well as rheological behavior of the composites. It should be noted that even small suppression or transition of the first instability type can lead to an increase in the processability of the considered composites. Two types of calcium carbonate were used in order to obtain more information about the influence of mineral filler incorporation.

EXPERIMENTAL

Materials and Sample Preparation

Commercial linear low density polyethylene C-4 (118NJ, butane copolymer) used as a polymer matrix was supplied by Sabic (Riyadh, Saudi Arabia) as a pellet. The density of the polymer was 0.918 g/[cm.sup.3] and the melt flow index was 1 g/10 min (2.16/190[degrees]C). Used polyethylene was characterized with polydispersity PD equal to 2.92 and weight average molecular weight Mw 194 019.

Two types of calcium carbonate were used as the inorganic filler, i.e., Calplex Extra (CaC[O.sub.3]) and Calplex Extra T (mCaC[O.sub.3]), which is a calcium carbonate modified with stearic acid, both produced by Calcit d.o.o. (Slovenia). Size distribution of CaC[O.sub.3] was in the range of 0.2 to 158.5 (xm. mCaC[O.sub.3] has a smaller particle size distribution, i.e., from 0.4 to 104.7 |tm, which results from the calcium carbonate being modified by stearic acid. A detailed description of the filler used in this study was presented in a previous paper [46],

Composites before being blended in molten state were preliminarily mixed by means of a high-speed rotating mixer, Retsch GM200 (= 3 min, n = 2,000 rpm), with different filler contents (0-20 wt%). Before processing, premixed blends were dried in vacuum in a temperature of 80[degrees]C for 4 h. Then all blends were mixed in molten state by means of a ZAMAK 16/ 40 EHD co-rotating twin screw extruder (T = 190[degrees]C, n = 120 rpm) and pelletized after cooling in a water bath.

Methods

Oscillatory Rheological Measurements. Rheological investigations in small-amplitude oscillation shearing mode were carried out using an Anton Paar MCR 301 rotational rheometer with 25 mm diameter parallel plates. The experiments were conducted at 180[degrees]C. To realize the dynamic oscillatory measurements the strain sweep experiments had to be proceeded. Hence, the strain amplitude sweep experiments of all the samples were performed at 180[degrees]C with a constant angular frequency 10 Hz in the varying strain window 0.01%-100%. The strain value, determined during the preliminary investigations and used during the frequency sweep experiments, was set at 1% and was located in the linear viscoelastic (LVE) region for all samples.

Capillary Rheometry. Extrusion experiments were conducted with the use of a capillary rheometer, Dynisco LCR 7000, at a temperature of 180[degrees]C. The influence of calcium carbonate addition on the range of occurrence of melt fracture instabilities and the evaluation of critical shear stresses that corresponded to them were determined with the use of a capillary die with a diameter of D = 0.762 mm and L/D = 30. The Bagley correction was omitted due to a high value of L/D, and the analyses of changes in the rheological behavior were prepared with the use of an apparent shear stress value [47]. All observations of distortions occurring on the surface of the extrudates were made using a laboratory magnifier.

The effect of filler on slip velocity of the molten polymer composites was evaluated in accordance with the Mooney method [48-50], Measurements were carried out using three capillary dies with L/D = 30 and diameter of D = 0.5, 1, and 1.5 mm.

Scanning Electron Microscopy (SEM). The samples' fractures were examined and digitally captured using a scanning electron microscope Zeiss Evo 40. An electron voltage of 12 kV was applied. Prior to the tests, both specimens were sputtered with a layer of gold. A magnification of 3,000X was used.

RESULTS AND DISCUSSION

Oscillatory Rheological Measurements

Oscillatory rheological measurements allow to assess information about rheological changes induced by the incorporation of both unmodified and modified CaC[O.sub.3] into the polyethylene matrix at low shear rates. Preliminary strain sweep experiments were focused on an evaluation of the LVE region for LLDPE and LLDPE composites. The changes of storage (G') and loss modulus (G") as a function of the strain were presented in Fig. 2. For all considered samples a constant plateau in the wide range of strain may be observed. Determination of the LVE range was based on the detection of downward deflection of presented in Fig. 2 G' (y) curves. Pure LLDPE reveal the nonlinear behavior at 40.6%, whereas the plateau region shortens with incorporation of the filler. The values of the boundary strain rates measured for composites containing 20 wt% of the filler were 7.3% for CaC[O.sub.3] and 12.2% for m CaC[O.sub.3]. The difference between filled materials was probably caused by the lower dispersion of the unmodified particles and presence of CaC[O.sub.3] agglomerates, which behave as strain-sensitive rigid structures. LLDPE composites containing 5 and 10 wt% of the filler exhibit intermediate values between pure polyethylene and composites containing maximum value of calcium carbonate. In all materials it can be seen that G" is dominating over G' values, however for composite materials higher both moduli values were noted. Further frequency sweep experiments were conducted at 1% strain which guarantee remain in the LVE range in case of all investigated materials.

Complex viscosity curves of pure LLDPE and LLDPE-CaC[O.sub.3]/mCaC[O.sub.3]composites are presented in Fig. 3. Incorporation of both types of inorganic filler results in an increase in the composites' viscosity in comparison to pure LLDPE. It should be noted that in the case of LLDPE-mCaC[O.sub.3] (Fig. 3b) the viscosity increase was slightly smaller than for LLDPE-CaC[O.sub.3] (Fig. 3a). Moreover, there were no significant differences between composites containing 5 and 10 wt% of mCaC[O.sub.3], which may suggest better dispersion as an effect of chemical treatment of the filler.

An evaluation of the relaxation time (0) and zero shear viscosity ([[eta].sub.0]) was possible due to rheological measurements in oscillatory mode. Calculations were performed using the Rheoplus 32 v.3.40 application. Zero shear viscosity was determined from the Carreau-Yasuda model fitted to the experimental data. Carreau-Yasuda model has been described by following equation'.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Where [[eta].sub.0] is zero-shear viscosity, n is power law coefficient, a is adjustable exponent (equal 2 for the simple Carreau model), [??] is shear rate and A is characteristic time [51, 52].

The relaxation time was estimated from the crossover point of the storage and loss modulus recorded during the test [53, 54]. Determining both values was essential in order to understand the changes in the melt flow instability sequence which will be presented later in this paper. The 0 and r\0 values are listed in Table 1.

Incorporation of both calcium carbonate types increased the zero shear viscosity, which is typical of composite materials, however, [[eta].sub.0] values recorded for the LLDPE-mCaC[O.sub.3] composites showed lower values at higher filler concentrations. An interesting effect was observed during the analysis of 0 values. The addition of unmodified CaC[O.sub.3] led to an increase in the relaxation time, while the presence of mCaC[O.sub.3] in the LLDPE matrix resulted in a slight decrease of this rheological parameter. On the basis of the literature, it is known that an increase in the characteristic relaxation time leads to intensifying the tendency for melt fracture occurrence, hence even small local modification of the fluid rheology directly correspond to a respective increase in the critical output rate before the onset of the sharkskin instability [29, 55-57]. Moreover, linear polyethylenes with a higher molecular weight and the associated zero shear viscosity [58, 59] revealed a strong tendency for the presence of sharkskin instability. Therefore, on the basis of the r\Q 0 values as presented in Table 1, it can be stated that composites filled with unmodified CaC[O.sub.3] are potentially more susceptible to earlier sharkskin instability occurrence than composites containing mCaC[O.sub.3].

Capillary Rheology

Figure 4 presents the results of the capillary investigations in the form of viscosity curves. The viscosity curves were cut into two sections, 50-500 and 750-2,000 [s.sup.-1], excluding the stick-slip regime. In comparison to results obtained by a rotational rheometer for a capillary flow at high shear rates, significant differences between composites containing CaC[O.sub.3] and mCaC[O.sub.3] were observed. Below stick-slip instability, thus at lower shear rates, incorporation of CaC[O.sub.3] led to a strong viscosity increase, whereas for LLDPE-mCaCO, extrusion through the capillary die almost no difference between pure LLDPE and the composite containing the highest amount of mCaC[O.sub.3] was observed. At higher shear rates, i.e., above stick-slip instability, the presence of LLDPE-CaC[O.sub.3] showed a similar tendency as was observed at lower shear rates. Increasing the unmodified filler led to a gradual increase in viscosity. LLDPE-mCaC[O.sub.3] composites demonstrated opposite rheological behavior, i.e., increasing the content of the mCaC[O.sub.3] caused a decrease in the viscosity. This effect is associated with chemical treatment of the inorganic filler and slippage properties of the derivatives of the chemical reaction between calcium carbonate and stearic acid [60].

Slip Measurements

Slip measurement determinations were carried out by using Mooney analysis according to a procedure presented in several publications [48-50]. According to Laun [61], wall slip velocities were calculated in the stationary regime below the spurt effect. The slip velocity versus shear stress curves for pure LLDPE and samples containing 20 wt% of modified and unmodified CaC[O.sub.3] are presented in Fig. 5. It should be noted that the slip velocity values of pure LLDPE and of a sample filled with mCaC[O.sub.3] exhibit similar values up to 0.3 MPa. For higher shear stresses the values of slip velocities of samples containing mCaC[O.sub.3] significantly increase, which may be attributed to chemical treatment of the filler and to the presence of calcium stearate which becomes a slippage agent [60], For the sample containing 20 wt% of CaC[O.sub.3], lower slip velocities are observed.

Melt Flow Instability Evaluation

During the capillary rheological experiments, additional observations of the extrudate surface were made. This allowed to create a map of the occurrence of melt flow instabilities during extrusion, which is presented in Fig. 6. Melt flow instabilities in the form of extrudate surface distortions successively appearing with an increasing shear rate were categorized, assigned different symbols and described in the legend in Fig. 6.

The incorporation of both inorganic fillers provided suppression of various instability occurrence, however some differences between composites containing CaC[O.sub.3] and mCaC[O.sub.3] were observed. A separate description of sharkskin instability as a "matted surface" and as "sharkskin," as was previously mentioned, is related to the intensity of the phenomenon. Sharkskin as presented in Fig. 6 is a fully developed melt flow instability with a high amplitude of surface distortions. Increasing the amount of the calcium carbonate in the polymer matrix strongly reduced macroscopic sharkskin instability, especially in the case of the chemically treated filler. What is most important from the industrial point of view is the extension of the first stable flow regime (described in Fig. 1 as "smooth"). The reduction of sharkskin instability as an effect of CaC[O.sub.3] incorporation may be attributed to three different phenomenon: an increase in melt strength of the composites [62], changes of the relaxation time of composites (Table 1) and increase of the wall slip caused by chemical treatment of the filler. Moreover, it can be stated that despite the impact of the incorporation of both fillers on the sharkskin instability sequence is comparable, their effectiveness in melt flow instability suppression is based on different mechanisms. In case of the unmodified CaC[O.sub.3], suppression of the sharkskin melt flow instability may be attributed to change of relaxation time, while in case of composites containing mCaC[O.sub.3] first type of the instability is postponed mainly due to increased wall slip. Moreover, suppressed occurrence of the sharkskin is caused by the higher melt strength of polyethylene composites in comparison to pure LLDPE [62]. Therefore, it may be stated that the rheological data obtained during the capillary experiments is in good agreement with the rheological oscillatory measurements.

The second type of instability (stick-slip) during extrusion of the LLDPE-CaC[O.sub.3]/mCaC[O.sub.3] composites was only partially suppressed. The lowest critical value of the shear rate corresponding to the appearance of stick-slip instability was not influenced by filler addition. Interesting rheological behavior of the composites may be observed at the end of the stick-slip regime. A second stable flow regime (superextrusion) was observed only in case of composites extrusion, for pure LLDPE direct stick-slip to gross melt fracture transition was denoted. The range of second stable flow regime occurrence was extended when the content of calcium carbonate was increased. However, it should be underlined that for mCaC[O.sub.3]-filled composites the second stable flow region was much wider than in the case of composites containing unmodified CaC[O.sub.3]. A very wide superextrusion regime in the case of 20 wt% addition of CaC[O.sub.3] and mCaC[O.sub.3] allowed us to formulate a hypothesis that it may be possible to conduct extrusion in industrial conditions at high extrusion rates above stick-slip instability. As it was mentioned above, the origins and differences in the range of the superextrusion were connected with changes of the interfacial boundary conditions between polymer/composite system (Fig. 5). For composites containing CaC[O.sub.3], lower slip velocities provide earlier occurrence of no slip conditions resulting as a continuous second stable plug flow. The reduction of stick-slip phenomena and the suppression of gross melt fracture in the case of composites containing the presence of mCaC[O.sub.3] may be attributed to the presence of calcium stearate which acts as a slippage agent and increases slip velocity. Moreover, significant suppression of the gross melt fracture was also noted.

Scanning Electron Microscopy (SEM)

Figure 7 presents two microphotographs of LLDPE-based composites containing 20 wt% of CaC[O.sub.3](a) and mCaC[O.sub.3] (b). It should be noted that dispersion of the inorganic filler in the polyethylene matrix is much better in the case of the composite of chemically treated inorganic filer. Better dispersion of the filler as observed in the SEM microphotographs gives logical complementation of the obtained rheological measurements. Better dispersion of the filler and its smaller size result in lowering the viscosity of the molten composites, which was caused by decreasing the reciprocal interactions between the particles of modified calcium carbonate. Additionally, calcium stearate, which is the result of chemical treatment of the inorganic filler with stearic acid, acting as a slippage agent may also be observed in Fig. 7b as white precipitates.

CONCLUSIONS

The effect of calcium carbonate incorporation into a linear low-density polyethylene matrix was evaluated. Significant differences in the rheological behavior of composites containing both untreated and chemically treated filler were noted. A strong decrease in shear viscosity during extrusion was observed in composites filled with modified calcium carbonate compared to composites based on unmodified fillers. This resulted in processability improvement of the modified composites. An evaluation of the melt fracture sequence allowed to describe a reduction in all instability types occurring during extrusion of LLDPE as an effect of calcium carbonate addition. Moreover, composites containing calcium carbonate modified with stearic acid revealed much wider flow regimes that were free from surface distortions due to increased wall slip. I

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Mateusz Barczewski, (1) Krzysztof Lewandowski, (2) Marcin Schmidt, (1) Marek Szostak (1)

(1) Institute of Materials Technology, Poznan University of Technology, Piotrowo 3, Poznan, 61-138, Poznan, Poland

(2) Faculty of Chemical Technology and Engineering, University of Technology and Life Sciences in Bydgoszcz, Seminaryjna 3, Bydgoszcz, 85-326, Poland

The article title of this article was changed on January 6. 2017, after original online publication.

Correspondence to: M. Barczewski; e-mail: mateusz.barczewski@put.poznan.pl

Contract grant sponsor: Ministry of Science and Higher Education in Poland (The presented research results, executed under the subject of No 02/25/ DSPB/4310).

DOI 10.1002/pen.24477

Caption: FIG. 1. Types of melt flow instabilities occurring during extrusion of linear low density polyethylene: stable flow (1), sharkskin instability (2,3), stickslip (4), second stable flow regime/superextrusion (5) and gross melt fracture (6).

Caption: FIG. 2. Storage (a) and loss modulus (b) versus strain of pure LLDPE and composites containing 20 wt% of the filler.

Caption: FIG. 3. Complex viscosity of pure LLDPE, LLDPE-CaC[O.sub.3] (a) and LLDPE-mCaC[O.sub.3] (b) composites. [Color figure can be viewed at wileyonlinelibrary. com]

Caption: FIG. 4. Viscosity curves of LLDPE-CaC[O.sub.3] composites obtained from capillary rheometer, LID = 30, T = 180[degrees]C.

Caption: FIG. 5. Slippage velocity versus shear stress for pure LLDPE and LLDPE composites.

Caption: FIG. 6. Sequences of forming instabilities during extrusion through capillary die presented in function of CaC[O.sub.3] content and shear rate (L/D = 30, T = 180[degrees]C).

Caption: FIG. 7. SEM microphotographs of LLDPE composites containing 20 wt% of CaC[O.sub.3] (a) and mCaC[O.sub.3] (b).
TABLE 1. Zero shear viscosity and relaxation time of LLDPE and LLDPE
composites 180[degrees]C.

                       [[eta].sub.0]    [theta]
Material                   (Pa s)        (ms)

Pure LLDPE                 20.084          9.8
5 wt% CaC[O.sub.3]         22,194          9.8
10 wt% CaC[O.sub.3]        23,788        10.05
20 wt% CaC[O.sub.3]        30,590        10.32
5 wt% mCaC[O.sub.3]        22,392          9.9
10 wt% mCaC[O.sub.3]       23,294          9.8
20 wt% mCaC[O.sub.3]       28.865          9.71
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Author:Barczewski, Mateusz; Lewandowski, Krzysztof; Schmidt, Marcin; Szostak, Marek
Publication:Polymer Engineering and Science
Article Type:Report
Date:Sep 1, 2017
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