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Rheological properties of wood polymer composites at high shear rates
Polymer Testing 51 (2016) 58e62
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Short communication: Test equipment
Rheological properties of wood polymer composites at high shear rates Krzysztof Lewandowski*, Kazimierz Piszczek, Stanisław Zajchowski, Jacek Mirowski University of Science and Technology in Bydgoszcz, Faculty of Chemical Technology and Engineering, 3 Seminaryjna Street, 85-326 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2016 Accepted 14 February 2016 Available online 3 March 2016
This paper presents the rheological properties of wood-polymer composites (WPC) with a polypropylene (PP) matrix in the corrected shear rate range from approx. 20 s1 to 150 000 s1. Tests were conducted using a capillary rheometer and a rheological head of the author's construction, for which the working element is a thermoplastic injection moulding machine. The constructed tool was found to be very useful, especially for the determination of the processing characteristics of WPC composites containing a large particle-size filler. It was observed that the rheological properties of wood-polymer composites in the shear rate range of up to several thousand s1 significantly depended on the filler content of the polymer matrix; at the same time, at higher shear rate, a clear decrease in the effect of the wood filler content on the viscosity of the composites and on the flow behaviour, as described by the power law, took place. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Wood-polymer composites (WPC) High shear rates Rheological properties
1. Introduction Polymer composites with fillers of plant origin (WPC) are an interesting research area, and products manufactured from them enjoy increasing demand by consumers, which results in great interest in these materials among polymer processing companies. The main advantage of using WPC composites is a reduction of the use of petroleum thermoplastics in the finished product, with a simultaneous reduction of production costs. In addition, composites containing a large filler fraction have very interesting aesthetic values; the users of these products appreciate especially the touch sensation and appearance of the surface, which is clearly different compared to traditional polymers. At the same time, in spite of the large wood filler content, these materials exhibit much higher resistance to atmospheric factors compared to wood. Owing to the above-mentioned characteristics, WPCs are very widely used as a wood substitute for making terraces, platforms or building façades [1,2]. Currently, one of the key problems of production process optimization and the reduction of WPC product manufacturing costs is to develop precision injection moulds and extrusion heads that will
* Corresponding author. E-mail addresses: [email protected] (K. Lewandowski), [email protected] (K. Piszczek), [email protected] (S. Zajchowski), [email protected] (J. Mirowski). http://dx.doi.org/10.1016/j.polymertesting.2016.02.004 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
operate with high-capacity machines, thus significantly contributing to an increase in productivity [3]. In the case of manufacturing semi-finished products by the injection moulding method, it is preferred to run the process at very high injection speeds, which, in the manufacture of thin-walled products, causes flow of molten polymer in the shear rate range of over 106 s1. Understanding completely the possible characteristics of molten polymer flow under such conditions in indispensable, not only for the correct design of processing tools, but also for the control of the production process [4e6]. Currently, capillary rheometers are, for example, used for determining the rheological characteristics. The measurement involves the determination of the pressure of polymer during its flow through the capillary of specified geometrical characteristics (usually cylindrical in shape) at a specified volumetric flowrate. Capillary rheometers offered on the market allow measurement in a shear rate range of up to 10 000 s1 and, in the case of special reinforced constructions, even up to 120 000 s1. These ranges apply to the tests on unfilled polymer materials, where flow through a capillary with a diameter below 1 mm is possible. This means that, in the case of composites containing large filler particles, the available measurement range is considerably narrowed, as a result of which there is no possibility of evaluating the rheological properties of that material as it flows under real processing conditions. This group of materials include wood-polymer composites, in which the filler in the form of wood particles has sizes of up to
K. Lewandowski et al. / Polymer Testing 51 (2016) 58e62
several millimetres and, at the same time, its fraction of the composite may be as large as 80%. Much more information can be obtained from measurements using in-line rheometers installed directly on the processing machine [7e11]. Several design solutions of such tools mounted directly on the polymer injection moulder machine's cylinder are described in the literature, which enable the determination of viscosity based on the measurement of pressure at the measuring nozzle inlet or the determination of the pressure drop across the measuring channel [12e14]. Another design solution is tools installed behind the plasticizing system in the injection mould location [15e17]. The purpose of this study was to determine the rheological characteristics of WPC during high shear-rate injection using a rheological head designed for operation with a polymer injection moulding machine. 2. Testing methodology 2.1. Material The matrix of the WPC composites was HP 500 N polypropylene (PP 1) (Basell Orlen, MFR (2.16/230) ¼ 12 g/10min, or HP 648T polypropylene (PP 2) (Basell Orlen, MFR (2.16/230) ¼ 53 g/10min). As the filler, Lignocel 9 wood flour (WF) (with a particle size of 0.8e1.1 mm) derived from coniferous trees (J. Rettenmaier & Solne GmbH) was used. PP and WF mixtures in a specified weight ratio were extruded using a co-rotating twin screw extruder (Zamak Mercator EPH 2 24; screw diameter, 24 mm; rotational speed, 100 rpm). The temperature in individual extrusion zones increased along the plasticizing system from 120 C in the first cylinder heating zone to 190 C in the last zone and the head. At the midlength of the plasticizing system, a degassing zone was located. Extrusion was conducted through a 5 25 mm sheet die. The extrudate so obtained was granulated using a Rapid 150 granulator. A series of composites was obtained, which contained, respectively, 30, 40 and 50 wt% wood flour.
59
segment, the polymer material in a visco-fluid state, via the injection nozzle, is delivered to the channel, in which it is oriented at an angle of 90 relative to the injection moulding machine's screw axis. Then, it is directed to the second segment with a cylindrical channel, at the end of which a replaceable measuring capillary is installed. The pressure measurement is taken using a pressure transducer, situated before the measuring capillary, connected to a computer to archive the measurement data. The tests were conducted using an ENGEL Victory 120 polymer injection moulding machine. The temperature of successive barrel zones, starting from the hopper, was, respectively: 130 C, 140 C, 190 C and 190 C. The material (dried 105 C, 4 h) was batched to the hopper, and then taken up by the injection system. The material was left in the cylinder for 5 min until it was fully melted. In the next step, the material was injected, at a fixed volumetric flow rate, through the rheological head equipped with a cylindrical capillary with a diameter of 2 mm, a length of 40 mm and an inlet angle of 120 . The measurements were conducted while keeping a constant head temperature of 190 C and recording material pressure variations upstream of the measuring capillary. The procedure was repeated five times for each material. Using the developed test equipment, it was possible to determine the mean material pressure before entry to the measuring capillary under the condition of apparent shear rate in the range from 19,100 s1 to 102,000 s1.
2.3. The capillary rheometer The rheological testes were also carried out using a Dynisco LCR 7001 capillary rheometer. The measurement temperature was 190 C. Prior to measurement, the WPC composites were melted for 5 min in the rheometer's cylinder. A 2 mm-diameter 40 mm-long cylindrical capillary with an inlet angle of 120 was used. The pressure of the molten polymer compound was determined before entry to the measuring capillary at a specified volumetric flowrate in the range corresponding to an apparent shear rate from 15 s1 to 912 s1.
2.2. The measuring head Fig. 1 shows a rheological head designed for operation with the injection moulding machine [18]. The rheological tool is mounted in place of the conventional injection mould. The employed head is composed of two basic segments heated electrically. In the first
2.4. Calculations The Rabinowitsch corrected viscosity (hw) of the molten material under the measurement conditions was determined from the relationship:
hw ¼
tw g_ w
(1)
hw e corrected viscosity, Pa$s tw e shear stress at the capillary wall, Pa
g_ w e corrected shear rate at the capillary wall, s1 where the shear stress (tw) at the capillary wall is defined as:
tw ¼
Fig. 1. The rheological head designed for operation with the injection moulding machine 1e sprue bush, 2 e locating ring, 3 e backing plate, 4, 9 e thermal insulation, 5 e platen, 6 e measuring channel, 7 e measuring capillary, 8 e segment with a pressure transducer installing capability.
Dp R ; 2Lk k
(2)
Dp e material pressure before entry to the capillary, Pa Rk e capillary radius, m Lk e capillary length, m whereas, the corrected hear rate (g_ w ) is described by the formula:
60
K. Lewandowski et al. / Polymer Testing 51 (2016) 58e62
g_ w ¼ g_ a
3n þ 1 ; 4n
(3)
g_ a e apparent shear rate at the capillary wall, s1 n e flow behaviour index The apparent shear rate (g_ a ) is defined by the relationship:
g_ a ¼
4Q : pR3k
(4)
Q e volumetric flow rate the capillary, m3/s The flow behaviour index n from Equation (3) is determined from the Ostwald power law:
tw ¼ kg_ na ;
(5)
k e consistency index which, after transformation, can be represented as the function:
logðtw Þ ¼ f ðlogðg_ a ÞÞ
(6)
then, the n is the slope of this straight line [19,20]. 3. Results and discussion Figs. 2 and 3 show viscosity curves for the PP1- and PP2-matrix wood-polymer composites determined using the capillary rheometer and the rheological head. Based on the analysis of the rheological properties of the woodpolymer composites, as determined with the capillary rheometer, it has been found that, with the increase in the wood flour fraction of the composite from 30 to 50%, the viscosity significantly increases, whereas the effect of WF on the composite viscosity decreases with increasing shear rate. This confirms the conclusions of other studies that have found that the rheological properties of wood-polymer composites in the
Fig. 2. Viscosity curves for the PP1 composites with a varying wood flour fraction, as determined using the capillary rheometer (the closed symbols) and the rheological head (the open symbols).
Fig. 3. Viscosity curves for the PP 2 composites with a varying wood flour fraction, as determined using the capillary rheometer (the closed symbols) and the rheological head (the open symbols).
shear rate range of up to several thousand s1 depend significantly on the filler content of the polymer matrix [21e24]. The decrease of the difference in viscosity between the tested composites with varying WF content is the result of the different pseuoplastic properties, in spite of the same polymer matrix being used. The observed effect is associated with the gradual orientation of the filler in the polymer matrix under the influence of the increasing linear polymer flow rate gradient within the channel [23,25]. The presence of the filler in a large amount must also influence the polymerepolymer and polymerefiller interactions [21e23]. The ability to change the viscosity with increase in shear rate is most conveniently described using the flow factor n defined by Equation (5). For WPC, in the shear rate range of up to 1000 s1, the value of n decreases significantly with increasing wood flour fraction (Fig. 4). The change in the n factor value is also determined by the viscosity of the polymer matrix. The polymer viscosity in specific flow conditions determines the shear stress, the increase of which influences the orientation of filler particles [25,26]. By plotting the flow curves, as determined with the capillary rheometer and defined as straight lines in the log ðtw Þ ¼ fðlogðg_ w ÞÞ
Fig. 4. The effect of wood flour content on the value of flow factor n, as determined for the PP1- and PP2-matrix composites using the capillary rheometer.
K. Lewandowski et al. / Polymer Testing 51 (2016) 58e62
system (Fig. 5 and Fig. 6), and then extrapolating them to higher shear rates, it can be observed that, under specific flow conditions, the straight lines intersect. The intersection point should be interpreted as the location at which the viscosity of the investigated WPC composites, under specific shear rate conditions, is independent of wood flour content. For the PP1-matrix composites, the estimated critical value of g_ w is 1.4 104 s1, while for the PP2matrix composites it is 5 105s1. The analysis of the results obtained in the higher shear rate range shows that a distinct decrease in the effect of wood filler content on the composite viscosity occurs (Figs. 2 and 3). For the PP1-matrix WPC, this difference is approx. 20% in the entire measuring range of the rheological head. In PP2, a continued decrease of the difference in viscosity between the investigated composites with a varying WF content. These observations are reflected in the values of the flow factor n, which are illustrated in Fig. 7. In the case of the PP1-matrix composites, the value of this parameter does not change significantly with wood flour content, while for the PP2-matrix composites the value of n continues to decrease with increasing filler fraction. In the case of the PP1 composites, the previously estimated critical shear rate, above which the filler orientation in the flow direction no longer takes place, has been exceeded, and the small difference in viscosity between composites containing a different wood flour fraction should be interpreted as an increase in the polymerefiller interactions. As has been mentioned earlier, the viscosity of the polymer matrix has a key effect on the orientation of the filler during the flow of molten composites. In the case of polypropylene composites with a definitely lower viscosity in the measurement conditions (PP2), the estimated critical point of the shear rate value clearly exceeds the measuring range of the designed test equipment.
61
Fig. 6. The log (tw)e log(g_ w ) relationship for the PP 2 composites with a varying wood flour content.
4. Conclusions The rheological properties of wood-polymer composites in the shear rate range of up to several thousand s1 significantly depend on the filler content of the polymer matrix. With increasing shear rate magnitude, the differences in viscosity between the examined composites decrease chiefly due to the orientation of filler particles in the polymer matrix, while after exceeding the critical shear rate magnitude point, the position of
Fig. 7. The effect of wood flour content on the value of flow factor n, as determined for the PP1- and PP2-matrix composites using the rheological head.
which depends on the composite matrix type, they depend primarily on the polymerefiller interactions. The location of the critical shear rate magnitude point can be determined by extrapolation of the viscosity curves. The method of analysis employed using a developed test equipment enables the rheological properties to be determined in a simple manner in conditions as near as possible to the processing conditions, especially in the case of composites containing large filler particles. References
Fig. 5. The log (tw)e log(g_ w ) relationship for the PP 1 composites with a varying wood flour content.
[1] A.A. Klyosov, Wood-Plastic Composites, Wiley, 2007, http://dx.doi.org/ 10.1002/9780470165935. [2] S. Zajchowski, R. Ryszkowska, Wood-polymer composites e General characteristics and their preparation from waste materials, Polimery 54 (10) (2009) 674. [3] European plastics demand, Surviving in a slow market, Inject. World 11/12 (2013) 21e25. [4] W. Friesenbichler, I. Duretek, J. Rajganesh, S.R. Kumar, Meausring the pressure dependent viscosity at high shear rates using a new rheological injection mould, Polimery 56 (1) (2011) 58. ski, Reologia W Przetwo rstwie Tworzyw, Wydawnictwo Naukowo[5] K. Wilczyn Techniczne, 2001.
62
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[6] J.M. Delay, K.F. Wissbrun, Melt Rheology and its Role in Plastics Processing, Kulwer academic publishers, 1999. [7] M. Barczewski, R. Barczewski, T. Sterzynski, Dynamic pressure analysis as a tool for determination of sharkskin instability by extrusion of molten polymers, J. Polym. Eng. 32 (6e7) (2012) 335. [8] B.L. Shah, L.M. Matuana, Online measurement of rheological properties of PVC/ wood-flour composites, J. Vinyl Addit. Technol. 10 (3) (2004) 121. [9] S.-H. Chiu, S.-H. Pong, Development of an on-line twin capillary rheometer, Polym. Degrad. Stab. 64 (2) (1998) 239. [10] W.-S. Guan, H.-X. Huang, Superimposed effects in high-shear-rate capillary rheology of polystyrene melt, Polym. Eng. Sci. 53 (10) (2013) 1563. ski, T. Sterzyn ski, Bagley correction evaluation on the basis of mea[11] A. Klozin surements in extrusion line, Polimery 50 (6) (2005) 455. [12] C. Bader, M. Diez, W. Kaiser, Rheological measurements on the injection moulding machine, Kunststoffe 81 (3) (1991) 220. [13] H. Takahashi, T. Matsuoka, T. Kurauchi, Rheology of polymer in high shear rate, J. Appl. Polym. Sci. 30 (12) (1985) 4669. [14] A. Fernandez, M. Muniesa, C. Javierre, In-line rheological testing of thermoplastics and a monitored device for an injection moulding machine: Application to raw and recycled polypropylene, Polym. Test. 33 (2014) 107. [15] W. Friesenbichler, I. Duretek, J. Rajganesh, S.R. Kumar, Measuring the pressure dependent viscosity at high shear rates using a new rheological injection mould, Polimery 58 (1) (2011) 58. [16] S.-C. Chen, W.-H. Liao, J.-P. Yeh, R.-D. Chien, Rheological behavior of PS polymer melt under ultra high speed injection molding, Polym. Test. 31 (7) (2012) 864.
[17] N. Zhang, M.D. Gilchrist, Characterization of thermo-rheological behavior of polymer melts during the micro injection moulding process, Polym. Test. 31 (6) (2012) 748. [18] J. Mirowski, J. Tomaszewska, K. Lewandowski, S. Zajchowski. Processing Tool for the Determination of Rheological Characteristics of Polymeric Materials. Pat. PL120708 (U1) [19] A.V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, 1999. ski, T. Sterzyn ski, Evaluation of corrections in rheological measure[20] A. Klozin ments of polyethylene. Part II. Power low index, rabinowitsch correction, Polimery 52 (11e12) (2007) 855. [21] S. Mohanty, S.K. Verma, S.K. Nayaka, Rheological characterization of PP/jute composite melts, J. Appl. Polym. Sci. 99 (14) (2006) 1476. [22] T.Q. Li, M.P. Wolcott, Rheology of wood plastics melt. Part 1. Capillary rheometry of HDPE filled with maple, Polym. Eng. Sci. 45 (4) (2005) 549. [23] S.N. Maiti, M.R. Hassan, Melt rheological properties of polypropylene-wood flour composites, J. Appl. Polym. Sci. 37 (7) (1989) 2019. [24] K. Lewandowski, K. Piszczek, S. Zajchowski, J. Mirowski, B. Majchrowicz, Evaluation of rheological properties of polymers at high shear rates, Polym. Process 20 (5) (2014) 410. [25] S.N. Maiti, R. Subbarao, M.N. Ibrahim, Effect of wood fibers on the rheological properties of i-PP/wood fiber composites, J. Appl. Polym. Sci. 91 (1) (2004) 644. [26] P. Wang, J. Liu, W. Yu, C. Zhou, Dynamic rheological properties of wood polymer composites: From linear to nonlinear behaviors, Polym. Bull. 66 (5) (2011) 683.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Short communication: Test equipment
Rheological properties of wood polymer composites at high shear rates Krzysztof Lewandowski*, Kazimierz Piszczek, Stanisław Zajchowski, Jacek Mirowski University of Science and Technology in Bydgoszcz, Faculty of Chemical Technology and Engineering, 3 Seminaryjna Street, 85-326 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2016 Accepted 14 February 2016 Available online 3 March 2016
This paper presents the rheological properties of wood-polymer composites (WPC) with a polypropylene (PP) matrix in the corrected shear rate range from approx. 20 s1 to 150 000 s1. Tests were conducted using a capillary rheometer and a rheological head of the author's construction, for which the working element is a thermoplastic injection moulding machine. The constructed tool was found to be very useful, especially for the determination of the processing characteristics of WPC composites containing a large particle-size filler. It was observed that the rheological properties of wood-polymer composites in the shear rate range of up to several thousand s1 significantly depended on the filler content of the polymer matrix; at the same time, at higher shear rate, a clear decrease in the effect of the wood filler content on the viscosity of the composites and on the flow behaviour, as described by the power law, took place. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Wood-polymer composites (WPC) High shear rates Rheological properties
1. Introduction Polymer composites with fillers of plant origin (WPC) are an interesting research area, and products manufactured from them enjoy increasing demand by consumers, which results in great interest in these materials among polymer processing companies. The main advantage of using WPC composites is a reduction of the use of petroleum thermoplastics in the finished product, with a simultaneous reduction of production costs. In addition, composites containing a large filler fraction have very interesting aesthetic values; the users of these products appreciate especially the touch sensation and appearance of the surface, which is clearly different compared to traditional polymers. At the same time, in spite of the large wood filler content, these materials exhibit much higher resistance to atmospheric factors compared to wood. Owing to the above-mentioned characteristics, WPCs are very widely used as a wood substitute for making terraces, platforms or building façades [1,2]. Currently, one of the key problems of production process optimization and the reduction of WPC product manufacturing costs is to develop precision injection moulds and extrusion heads that will
* Corresponding author. E-mail addresses: [email protected] (K. Lewandowski), [email protected] (K. Piszczek), [email protected] (S. Zajchowski), [email protected] (J. Mirowski). http://dx.doi.org/10.1016/j.polymertesting.2016.02.004 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
operate with high-capacity machines, thus significantly contributing to an increase in productivity [3]. In the case of manufacturing semi-finished products by the injection moulding method, it is preferred to run the process at very high injection speeds, which, in the manufacture of thin-walled products, causes flow of molten polymer in the shear rate range of over 106 s1. Understanding completely the possible characteristics of molten polymer flow under such conditions in indispensable, not only for the correct design of processing tools, but also for the control of the production process [4e6]. Currently, capillary rheometers are, for example, used for determining the rheological characteristics. The measurement involves the determination of the pressure of polymer during its flow through the capillary of specified geometrical characteristics (usually cylindrical in shape) at a specified volumetric flowrate. Capillary rheometers offered on the market allow measurement in a shear rate range of up to 10 000 s1 and, in the case of special reinforced constructions, even up to 120 000 s1. These ranges apply to the tests on unfilled polymer materials, where flow through a capillary with a diameter below 1 mm is possible. This means that, in the case of composites containing large filler particles, the available measurement range is considerably narrowed, as a result of which there is no possibility of evaluating the rheological properties of that material as it flows under real processing conditions. This group of materials include wood-polymer composites, in which the filler in the form of wood particles has sizes of up to
K. Lewandowski et al. / Polymer Testing 51 (2016) 58e62
several millimetres and, at the same time, its fraction of the composite may be as large as 80%. Much more information can be obtained from measurements using in-line rheometers installed directly on the processing machine [7e11]. Several design solutions of such tools mounted directly on the polymer injection moulder machine's cylinder are described in the literature, which enable the determination of viscosity based on the measurement of pressure at the measuring nozzle inlet or the determination of the pressure drop across the measuring channel [12e14]. Another design solution is tools installed behind the plasticizing system in the injection mould location [15e17]. The purpose of this study was to determine the rheological characteristics of WPC during high shear-rate injection using a rheological head designed for operation with a polymer injection moulding machine. 2. Testing methodology 2.1. Material The matrix of the WPC composites was HP 500 N polypropylene (PP 1) (Basell Orlen, MFR (2.16/230) ¼ 12 g/10min, or HP 648T polypropylene (PP 2) (Basell Orlen, MFR (2.16/230) ¼ 53 g/10min). As the filler, Lignocel 9 wood flour (WF) (with a particle size of 0.8e1.1 mm) derived from coniferous trees (J. Rettenmaier & Solne GmbH) was used. PP and WF mixtures in a specified weight ratio were extruded using a co-rotating twin screw extruder (Zamak Mercator EPH 2 24; screw diameter, 24 mm; rotational speed, 100 rpm). The temperature in individual extrusion zones increased along the plasticizing system from 120 C in the first cylinder heating zone to 190 C in the last zone and the head. At the midlength of the plasticizing system, a degassing zone was located. Extrusion was conducted through a 5 25 mm sheet die. The extrudate so obtained was granulated using a Rapid 150 granulator. A series of composites was obtained, which contained, respectively, 30, 40 and 50 wt% wood flour.
59
segment, the polymer material in a visco-fluid state, via the injection nozzle, is delivered to the channel, in which it is oriented at an angle of 90 relative to the injection moulding machine's screw axis. Then, it is directed to the second segment with a cylindrical channel, at the end of which a replaceable measuring capillary is installed. The pressure measurement is taken using a pressure transducer, situated before the measuring capillary, connected to a computer to archive the measurement data. The tests were conducted using an ENGEL Victory 120 polymer injection moulding machine. The temperature of successive barrel zones, starting from the hopper, was, respectively: 130 C, 140 C, 190 C and 190 C. The material (dried 105 C, 4 h) was batched to the hopper, and then taken up by the injection system. The material was left in the cylinder for 5 min until it was fully melted. In the next step, the material was injected, at a fixed volumetric flow rate, through the rheological head equipped with a cylindrical capillary with a diameter of 2 mm, a length of 40 mm and an inlet angle of 120 . The measurements were conducted while keeping a constant head temperature of 190 C and recording material pressure variations upstream of the measuring capillary. The procedure was repeated five times for each material. Using the developed test equipment, it was possible to determine the mean material pressure before entry to the measuring capillary under the condition of apparent shear rate in the range from 19,100 s1 to 102,000 s1.
2.3. The capillary rheometer The rheological testes were also carried out using a Dynisco LCR 7001 capillary rheometer. The measurement temperature was 190 C. Prior to measurement, the WPC composites were melted for 5 min in the rheometer's cylinder. A 2 mm-diameter 40 mm-long cylindrical capillary with an inlet angle of 120 was used. The pressure of the molten polymer compound was determined before entry to the measuring capillary at a specified volumetric flowrate in the range corresponding to an apparent shear rate from 15 s1 to 912 s1.
2.2. The measuring head Fig. 1 shows a rheological head designed for operation with the injection moulding machine [18]. The rheological tool is mounted in place of the conventional injection mould. The employed head is composed of two basic segments heated electrically. In the first
2.4. Calculations The Rabinowitsch corrected viscosity (hw) of the molten material under the measurement conditions was determined from the relationship:
hw ¼
tw g_ w
(1)
hw e corrected viscosity, Pa$s tw e shear stress at the capillary wall, Pa
g_ w e corrected shear rate at the capillary wall, s1 where the shear stress (tw) at the capillary wall is defined as:
tw ¼
Fig. 1. The rheological head designed for operation with the injection moulding machine 1e sprue bush, 2 e locating ring, 3 e backing plate, 4, 9 e thermal insulation, 5 e platen, 6 e measuring channel, 7 e measuring capillary, 8 e segment with a pressure transducer installing capability.
Dp R ; 2Lk k
(2)
Dp e material pressure before entry to the capillary, Pa Rk e capillary radius, m Lk e capillary length, m whereas, the corrected hear rate (g_ w ) is described by the formula:
60
K. Lewandowski et al. / Polymer Testing 51 (2016) 58e62
g_ w ¼ g_ a
3n þ 1 ; 4n
(3)
g_ a e apparent shear rate at the capillary wall, s1 n e flow behaviour index The apparent shear rate (g_ a ) is defined by the relationship:
g_ a ¼
4Q : pR3k
(4)
Q e volumetric flow rate the capillary, m3/s The flow behaviour index n from Equation (3) is determined from the Ostwald power law:
tw ¼ kg_ na ;
(5)
k e consistency index which, after transformation, can be represented as the function:
logðtw Þ ¼ f ðlogðg_ a ÞÞ
(6)
then, the n is the slope of this straight line [19,20]. 3. Results and discussion Figs. 2 and 3 show viscosity curves for the PP1- and PP2-matrix wood-polymer composites determined using the capillary rheometer and the rheological head. Based on the analysis of the rheological properties of the woodpolymer composites, as determined with the capillary rheometer, it has been found that, with the increase in the wood flour fraction of the composite from 30 to 50%, the viscosity significantly increases, whereas the effect of WF on the composite viscosity decreases with increasing shear rate. This confirms the conclusions of other studies that have found that the rheological properties of wood-polymer composites in the
Fig. 2. Viscosity curves for the PP1 composites with a varying wood flour fraction, as determined using the capillary rheometer (the closed symbols) and the rheological head (the open symbols).
Fig. 3. Viscosity curves for the PP 2 composites with a varying wood flour fraction, as determined using the capillary rheometer (the closed symbols) and the rheological head (the open symbols).
shear rate range of up to several thousand s1 depend significantly on the filler content of the polymer matrix [21e24]. The decrease of the difference in viscosity between the tested composites with varying WF content is the result of the different pseuoplastic properties, in spite of the same polymer matrix being used. The observed effect is associated with the gradual orientation of the filler in the polymer matrix under the influence of the increasing linear polymer flow rate gradient within the channel [23,25]. The presence of the filler in a large amount must also influence the polymerepolymer and polymerefiller interactions [21e23]. The ability to change the viscosity with increase in shear rate is most conveniently described using the flow factor n defined by Equation (5). For WPC, in the shear rate range of up to 1000 s1, the value of n decreases significantly with increasing wood flour fraction (Fig. 4). The change in the n factor value is also determined by the viscosity of the polymer matrix. The polymer viscosity in specific flow conditions determines the shear stress, the increase of which influences the orientation of filler particles [25,26]. By plotting the flow curves, as determined with the capillary rheometer and defined as straight lines in the log ðtw Þ ¼ fðlogðg_ w ÞÞ
Fig. 4. The effect of wood flour content on the value of flow factor n, as determined for the PP1- and PP2-matrix composites using the capillary rheometer.
K. Lewandowski et al. / Polymer Testing 51 (2016) 58e62
system (Fig. 5 and Fig. 6), and then extrapolating them to higher shear rates, it can be observed that, under specific flow conditions, the straight lines intersect. The intersection point should be interpreted as the location at which the viscosity of the investigated WPC composites, under specific shear rate conditions, is independent of wood flour content. For the PP1-matrix composites, the estimated critical value of g_ w is 1.4 104 s1, while for the PP2matrix composites it is 5 105s1. The analysis of the results obtained in the higher shear rate range shows that a distinct decrease in the effect of wood filler content on the composite viscosity occurs (Figs. 2 and 3). For the PP1-matrix WPC, this difference is approx. 20% in the entire measuring range of the rheological head. In PP2, a continued decrease of the difference in viscosity between the investigated composites with a varying WF content. These observations are reflected in the values of the flow factor n, which are illustrated in Fig. 7. In the case of the PP1-matrix composites, the value of this parameter does not change significantly with wood flour content, while for the PP2-matrix composites the value of n continues to decrease with increasing filler fraction. In the case of the PP1 composites, the previously estimated critical shear rate, above which the filler orientation in the flow direction no longer takes place, has been exceeded, and the small difference in viscosity between composites containing a different wood flour fraction should be interpreted as an increase in the polymerefiller interactions. As has been mentioned earlier, the viscosity of the polymer matrix has a key effect on the orientation of the filler during the flow of molten composites. In the case of polypropylene composites with a definitely lower viscosity in the measurement conditions (PP2), the estimated critical point of the shear rate value clearly exceeds the measuring range of the designed test equipment.
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Fig. 6. The log (tw)e log(g_ w ) relationship for the PP 2 composites with a varying wood flour content.
4. Conclusions The rheological properties of wood-polymer composites in the shear rate range of up to several thousand s1 significantly depend on the filler content of the polymer matrix. With increasing shear rate magnitude, the differences in viscosity between the examined composites decrease chiefly due to the orientation of filler particles in the polymer matrix, while after exceeding the critical shear rate magnitude point, the position of
Fig. 7. The effect of wood flour content on the value of flow factor n, as determined for the PP1- and PP2-matrix composites using the rheological head.
which depends on the composite matrix type, they depend primarily on the polymerefiller interactions. The location of the critical shear rate magnitude point can be determined by extrapolation of the viscosity curves. The method of analysis employed using a developed test equipment enables the rheological properties to be determined in a simple manner in conditions as near as possible to the processing conditions, especially in the case of composites containing large filler particles. References
Fig. 5. The log (tw)e log(g_ w ) relationship for the PP 1 composites with a varying wood flour content.
[1] A.A. Klyosov, Wood-Plastic Composites, Wiley, 2007, http://dx.doi.org/ 10.1002/9780470165935. [2] S. Zajchowski, R. Ryszkowska, Wood-polymer composites e General characteristics and their preparation from waste materials, Polimery 54 (10) (2009) 674. [3] European plastics demand, Surviving in a slow market, Inject. World 11/12 (2013) 21e25. [4] W. Friesenbichler, I. Duretek, J. Rajganesh, S.R. Kumar, Meausring the pressure dependent viscosity at high shear rates using a new rheological injection mould, Polimery 56 (1) (2011) 58. ski, Reologia W Przetwo rstwie Tworzyw, Wydawnictwo Naukowo[5] K. Wilczyn Techniczne, 2001.
62
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[6] J.M. Delay, K.F. Wissbrun, Melt Rheology and its Role in Plastics Processing, Kulwer academic publishers, 1999. [7] M. Barczewski, R. Barczewski, T. Sterzynski, Dynamic pressure analysis as a tool for determination of sharkskin instability by extrusion of molten polymers, J. Polym. Eng. 32 (6e7) (2012) 335. [8] B.L. Shah, L.M. Matuana, Online measurement of rheological properties of PVC/ wood-flour composites, J. Vinyl Addit. Technol. 10 (3) (2004) 121. [9] S.-H. Chiu, S.-H. Pong, Development of an on-line twin capillary rheometer, Polym. Degrad. Stab. 64 (2) (1998) 239. [10] W.-S. Guan, H.-X. Huang, Superimposed effects in high-shear-rate capillary rheology of polystyrene melt, Polym. Eng. Sci. 53 (10) (2013) 1563. ski, T. Sterzyn ski, Bagley correction evaluation on the basis of mea[11] A. Klozin surements in extrusion line, Polimery 50 (6) (2005) 455. [12] C. Bader, M. Diez, W. Kaiser, Rheological measurements on the injection moulding machine, Kunststoffe 81 (3) (1991) 220. [13] H. Takahashi, T. Matsuoka, T. Kurauchi, Rheology of polymer in high shear rate, J. Appl. Polym. Sci. 30 (12) (1985) 4669. [14] A. Fernandez, M. Muniesa, C. Javierre, In-line rheological testing of thermoplastics and a monitored device for an injection moulding machine: Application to raw and recycled polypropylene, Polym. Test. 33 (2014) 107. [15] W. Friesenbichler, I. Duretek, J. Rajganesh, S.R. Kumar, Measuring the pressure dependent viscosity at high shear rates using a new rheological injection mould, Polimery 58 (1) (2011) 58. [16] S.-C. Chen, W.-H. Liao, J.-P. Yeh, R.-D. Chien, Rheological behavior of PS polymer melt under ultra high speed injection molding, Polym. Test. 31 (7) (2012) 864.
[17] N. Zhang, M.D. Gilchrist, Characterization of thermo-rheological behavior of polymer melts during the micro injection moulding process, Polym. Test. 31 (6) (2012) 748. [18] J. Mirowski, J. Tomaszewska, K. Lewandowski, S. Zajchowski. Processing Tool for the Determination of Rheological Characteristics of Polymeric Materials. Pat. PL120708 (U1) [19] A.V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, 1999. ski, T. Sterzyn ski, Evaluation of corrections in rheological measure[20] A. Klozin ments of polyethylene. Part II. Power low index, rabinowitsch correction, Polimery 52 (11e12) (2007) 855. [21] S. Mohanty, S.K. Verma, S.K. Nayaka, Rheological characterization of PP/jute composite melts, J. Appl. Polym. Sci. 99 (14) (2006) 1476. [22] T.Q. Li, M.P. Wolcott, Rheology of wood plastics melt. Part 1. Capillary rheometry of HDPE filled with maple, Polym. Eng. Sci. 45 (4) (2005) 549. [23] S.N. Maiti, M.R. Hassan, Melt rheological properties of polypropylene-wood flour composites, J. Appl. Polym. Sci. 37 (7) (1989) 2019. [24] K. Lewandowski, K. Piszczek, S. Zajchowski, J. Mirowski, B. Majchrowicz, Evaluation of rheological properties of polymers at high shear rates, Polym. Process 20 (5) (2014) 410. [25] S.N. Maiti, R. Subbarao, M.N. Ibrahim, Effect of wood fibers on the rheological properties of i-PP/wood fiber composites, J. Appl. Polym. Sci. 91 (1) (2004) 644. [26] P. Wang, J. Liu, W. Yu, C. Zhou, Dynamic rheological properties of wood polymer composites: From linear to nonlinear behaviors, Polym. Bull. 66 (5) (2011) 683.