Analysis of flow and mixing in screw elements of corotating twin‐screw extruders via SPH

Due to its meshless nature, the smoothed particle hydrodynamics method (SPH) provides high potential for the simulation of free‐surface flows and mixing in complex geometries. We used SPH to analyze the flow inside five typical screw elements of corotating twin‐screw extruders, two conveying elements, two kneading elements and a mixing element. Our results show conveying capabilities, pressure generation and power input for various operation states, completely and partially filled. We conducted a detailed mixing analysis based on tracer particles, which yielded the time evolution of the intensity of segregation for different tracers. From that, we determined exponential mixing rates, which describe the relative decrease of the intensity of segregation per screw revolution and characterize the mixing performance in different operation states. This provides valuable input information for simplified models of extruders, which are relevant to industrial applications and can significantly contribute to the efficient design, optimization and scale‐up of extruders. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2451–2463, 2017     

Improving mixing characteristics with a pitched tip in kneading elements in twin‐screw extrusion

In twin‐screw extrusion, the geometry of a mixing element mainly determines the basic flow pattern, which eventually affects the mixing ability as well as the dispersive ability of the mixing element. The effects of geometrical modification, with both forward and backward pitched tips, of a conventional forward kneading disks element (FKD) in the pitched‐tip kneading disks element on the flow pattern and mixing characteristics are discussed. Numerical simulations of fully filled, nonisothermal polymer melt flow in the melt‐mixing zone were performed, and the flow pattern structure and the tracer trajectories were investigated. The pitched tips largely affect the inter‐disk fluid transport, which is mainly responsible for mixing. These changes in the local flow pattern are analyzed by the distribution of the strain‐rate state. The distribution of the finite‐time Lyapunov exponent reveals a large inhomogeneity of the mixing in FKD is suppressed both by the forward and backward tips. By the forward tips on FKD, the mixing ability is relatively suppressed compared to FKD, whereas for the backward tips on FKD, the mixing ability is enhanced while maintaining the same level of dispersion efficiency as FKD. From these results, the pitched tips on the conventional KD turn out to be effective at reducing the inhomogeneity of the mixing and tuning the overall mixing performance. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1424–1434, 2018     

3‐D non‐isothermal flow field analysis and mixing performance evaluation of kneading blocks in a co‐rotating twin srew extruder

We have developed a three‐dimensional non‐Newtonian and non‐isothermal flow analysis of the twin screw extruder (TSE) using the finite element method. This code can simulate the fully filled parts of several kinds of screw elements, such as full flight screws, kneading blocks, rotors and their combinations. A marker particle tracking analysis has also been developed to evaluate the mixing performance of the screw elements. In this paper, simulations for the kneading blocks in a co‐rotating TSE were carried out. The screw configurations are combinations of 2‐lobe kneading blocks with several stagger angles and disk widths. The effects of screw configurations on pressure and temperature distributions are examined. We also evaluate the dispersive and distributive mixing via stress magnitude and area stretch obtained by marker particle tracking analysis. Additionally, we discuss the desirable stagger, disk width and their combinations that promote the mixing performance.     

3‐D numerical simulations of nonisothermal flow in co‐rotating twin screw extruders

Three‐dimensional nonisothermal flow simulations in the kneading disc regions of co‐rotating twin screw extruders were performed using a finite element method. The standard Galerkin method and penalty function scheme were applied to the flow field. The streamline‐upwind/Petrov‐Galerkin scheme was used in the temperature field to reduce numerical oscillation. The simulations were carried out under the operational conditions of The Japan Steel Works TEX30 machine for various rotational speeds. The configuration was ten 2‐lobe kneading discs with a 90° stagger angle. Experimental observations were also performed to validate the numerical simulations under the same operational conditions. The pressure in front of the tip in the rotation direction was higher than behind the tip, and the region behind the tip sometimes had a negative value. Since variation of the pressure gradient in the axial direction causes forward and backward flows in the disc gap regions, the disc gap regions play an important role for mixing. The temperature becomes higher with increasing rotation speed due to high viscous dissipation. A high temperature was observed on the disc surface, in the disc gap, and in the intermeshing regions. The numerical results of pressure profiles with the rotation and the temperature in the axial direction were in good agreement with the experimental observations.     

Theoretical analysis of the devolatilizing performance of an intermeshing co‐rotating twin‐screw extruder with a type of square‐channel flighted element

The local film interfaces in the long vent section of an intermeshing, co‐rotating twin‐screw extruder (ICoTSE) with square‐channel flights (SFs) and the exposure time to the vapor phase were analyzed. A theoretical overall film‐interface geometry factor (OFIGF_cal) was calculated from a new devolatilization (DV) model proposed previously by the authors, and the predictions were compared to the experimental results (OFIGF_exp) for polystyrene/ethylbenzene (PS/EB). The OFIGF represents a measure of the devolatilization performance of the long vent section. The ratio of the experimental to the predicted apparent film interface areas was 5.5, which demonstrates the enhancing effect of the roughness and bubble‐forming behavior of the film interfaces on DV. The predicted concentrations of EB in the PS melt at the exit of the long vent section agreed well with experimental results.     

Investigation of the melting behavior and morphology development of polymer blends in the melting zone of twin‐screw extruders

The melting behavior and the morphology development that runs parallel to it play central roles in the processing of polymer blends. We studied the impact of speed, melt throughput, continuous‐phase viscosity, screw configuration, and disperse‐phase content on the melting behavior and morphology development in the melting zone of a twin‐screw extruder. The polymer blend used incorporated polyamide‐6 (PA6) as its disperse phase and a high‐viscosity or low‐viscosity polypropylene as the matrix phase. The melting behavior of the polymer blend was investigated with press plates. A qualitative assessment was made of the processes, on basis of the optical impression gained from the transilluminated press plates. One key result was that the PA6 granules melted very rapidly in the polypropylene melt. We took samples over the length of the melting section to permit a quantitative assessment of the morphology. The results show a finely dispersed morphology already at the start of the melting section. This did not undergo any essential change as the blend passed through the extruder, and only a limited correlation was evident with the process parameters. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1986–2002, 2001     

Preparation,characterization, and properties of polyethylene composites highly filled with calcium carbonate through co‐rotating conical twin‐screw extrusion

A series of highly filled polyethylene (PE) composites in which the calcium carbonate content was as high as 50–75 wt% was prepared by co‐rotating conical twin‐screw extrusion (Co‐RCE). The effects of Co‐RCE processing and CaCO₃ content on the morphology, structure, and properties were investigated in detail. The results indicated that the Co‐RCE processing improved the filler dispersion and thereby enhanced the melt processability of the blends. Observation by polarized light microscopy and analysis by differential scanning calorimetry and wide‐angle X‐ray diffraction showed that the crystallinity of PE decreased with the increase of CaCO₃ content. In comparison with neat PE, an increase of up to 20^oC in onset weight loss temperature was observed in the CaCO₃‐filled PE composites, as confirmed by thermogravimetric analysis. Tensile tests indicated that the elongation at break of the highly CaCO₃‐filled PE composites was much higher than that of the neat PE. Meanwhile, the highly filled composites still exhibited superior tensile strength. J. VINYL ADDIT. TECHNOL., 20:108–115, 2014. © 2014 Society of Plastics Engineers     

Numerical study of twin-screw extruders by three-dimensional flow analysis—development of analysis technique and evaluation of mixing performance for full flight screws

We have developed a method for predicting the three-dimensional flow field in the melt conveying zone in counter-rotating and co-rotating twin-screw extruders. We applied this technique to the full flight screws with thin flight width and open C-shaped channels in both rotating type extruders having the same screw configurations. We compared the details of velocity and stress fields, the flow rates of transportation, and various kinds of leakage flows for both rotating type extruders. Also, we obtained the spatial distribution of tracer particles and residence time distribution using a numerical tracer experiment. The flow rate in the transport direction in the co-rotating twin-screw was larger than that in the counter-rotating twin-screw, and this suggested that the latter has higher transport performance when the screws have thin flight width and open C-shaped channels as used in this study. As for the distributive mixing, it was found that the co-rotating twin-screw excels in the area of fluid rearrangement between the two screws and distribution in the rotational direction, while the counter-rotating twin-screw has the desirable characteristic of wide distribution in the axial direction. With regard to dispersive mixing, there was no considerable difference between calculated stress fields in both rotating type extruders.     

3‐D numerical simulations on flow and mixing behaviors in gas–liquid–solid microchannels

Three‐dimensional (3‐D) gas‐liquid–solid flow and mixing behaviors in microchannels were simulated by coupled volume of fluid and discrete phase method and simulations were validated against observations. The detachment time and length of gas slug are shortened in liquid–solid flow, compared with those in liquid flow due to higher superficial viscosity of liquid–solid mixture, which will move the bubble formation toward the dripping regime. Solid particles mainly distribute in liquid slug and particle flow shows obvious periodicity. With the increase of contact angle of the inner wall, gas slug (0–50°), stratified (77–120°), and liquid drop (160°) flows are observed. The residence time distributions of solid and liquid phases are similar because particles behave as tracers. The backmixing of solid and liquid phases in liquid drop flow is the weakest among the three flow patterns, and the backmixing of gas phase in slug flow is weaker than that in both stratified and liquid drop flows. The results can provide a theoretical basis for the design of microreactors. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1934–1951, 2013     

3‐D numerical simulations for polycondensation of poly(p‐phenylene terephthalamide) in twin screw extruder

Poly(p‐phenylene terephthalamide) (PPTA) is a typical high‐performance fiber and usually synthesized by low temperature solution polycondensation in twin screw extruder. During the reactive extrusion, flow behavior, rheokinetics, and temperature are highly coupled, which makes it difficult to ensure the quality of product. In the present work, the rheokinetics of the PPTA solution is established by experiments and its reactive extrusion is investigated via 3‐D numerical simulation. The flow pattern, molecular weight distribution and temperature have been investigated for the extruders with three different elements, i.e., full flight screw (FF), kneading block (KB), and screw mixing element (SME). The results indicate that the unique flow characteristic of different screw elements affects mixing experience of the reactants and further the PPTA qualities. By numerical simulation, it shows the temperature increase within the tested extrusion stage is 7 K and reaction heat is found the main energy source for extrusion stage. Furthermore, heat removal method is verified with consideration of industrial production process. The numerical simulation work also discusses the reaction and mixing process in the extruders with three combined screws. The results show the introduction of more reverse FF and KB elements can lead to higher quality of PPTA. POLYM. ENG. SCI., 57:1252–1261, 2017. © 2017 Society of Plastics Engineers     

Enhancement of the surface and bulk mechanical properties of polystyrene through the incorporation of raw multiwalled nanotubes with the twin‐screw mixing technique

In this article, we present the effects of incorporated multiwalled nanotubes (MWNTs) on a metal surface and the bulk mechanical properties of as‐synthesized polystyrene (PS)–MWNT composites prepared with the twin‐screw mixing technique. The MWNTs used for preparing the composites were raw compounds that were not treated with any surface modifications. The morphology for the dispersion capability of the MWNTs in the PS matrix was subsequently characterized with transmission electron microscopy. Surface mechanical property studies (i.e., wear resistance and hardness) showed that the integration of MWNTs led to a distinct increase in the wear resistance and also the micro/nanohardness with up to a 5 wt % MWNT loading in the composites. Moreover, the enhancement of the wear resistance of the as‐prepared composites, in comparison with pure PS, was further identified with scanning electron microscopy observations of the surface morphology after testing. On the other hand, for bulk mechanical property studies (i.e., the tensile strength and flexural strength), the composites containing a 3 wt % concentration of MWNTs in the PS matrix exhibited the best performance with respect to the tensile strength and flexural strength. This means that this composition of MWNTs exhibited good compatibility with the PS matrix, and this can be attributed to the π–π interacting forces existing between the aromaticity of the MWNTs and PS matrix. Furthermore, at higher MWNT loadings (e.g., 5 wt %), raw MWNTs were aggregated in the polymer matrix, as observed by transmission electron microscopy. Also, this led to an obvious decrease in the tensile strength and flexural strength. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Agglomerate size evolution in modular twin‐screw extruder: Modeling and validation with CaCO3/LLDPE compounding experiments

The screw combination of twin‐screw extruder affects the filler size in inorganic filler‐reinforced polymeric compounds. This article tried to conveniently and precisely model the agglomerate size evolution in modular co‐rotating twin‐screw extruder. The break‐up process of agglomerate was analyzed to obtain the relationship between average deagglomerating energy (M) and agglomerate size, the result presented a mathematic relationship between M and agglomerate size. Numerical simulation study was conducted to consider the effect of kneading blocks (KB) or flight elements (FE) number on dispersion capacity of screw combination. The average deagglomerating energy, calculated based on POLYFLOW simulation, was introduced as major parameter to evaluate the dispersion capacity, which presented a proportional relationship to the KB or FE number. The model was developed by combining the agglomerate break‐up process and simulation results. In validation, CaCO₃/linear low‐density polyethylene (LLDPE) compounds were prepared by modular co‐rotating twin‐screw extruder with different screw combinations. The optical image analysis showed that CaCO₃ agglomerate size decreased with the KB or FE number rising, while it achieved minimum when the KB number was 4. The model was in accordance with the experimental results, and proved to be valid for KB and FE combinations in preparing CaCO₃/LLDPE compounds. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45535.     

Effects of mixing conditions on the reaction of 3‐octanoylthio‐1‐propyltriethoxysilane during mixing with silica filler and natural rubber

The effects of mixing temperature and mixing time on the reaction of 3‐octanoylthio‐1‐propyltriethoxysilane (NXT silane) during mixing with precipitated silica and natural rubber (NR) were investigated. Results showed that the reaction between NXT silane and precipitated silica can proceed at temperatures above 130°C. Because of the blocking group of NXT silane, the silane–NR coupling reactivity is low, so that the reaction of NXT silane with NR occurs only during the curing period. There is no reaction between NXT silane and NR during mixing, which showed that the mixing time of silica‐filled NR compound containing NXT silane must be longer than 10 min at 150°C to obtain the desired silane coupling efficiency. With increasing mill temperature, the coupling efficiency increases. A high mixing temperature promotes improvement of silane–silica coupling efficiency, although the tensile strength, 100% modulus, 300% modulus, and hardness of NR vulcanizates decrease. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2295–2301, 2004     

Synthesis and characterization of thermoplastic composites filled with γ‐boehmite for fire resistance

This paper explores the use of boehmite as a fire retardant in low‐density polyethylene and polyamide 6. The structure and morphology of the filler are characterized by X‐ray diffraction and scanning electron microscopy. The water content of our boehmite is estimated about 30% by thermogravimetric analysis. The filler/matrix interactions in nanocomposites are investigated by means of thermo‐physical measurements: differential scanning calorimeter and thermogravimetry. The resulting morphology shows particles individually dispersed in the matrix. The fire tests present a fire‐resistance effect at low filler content despite a different behavior as a function of the matrix. For instance, with only 2% volume of boehmite, the burning time of LDPE composite is significantly increased by 15%. At the same filler content in PA6, the burning time is solely increased by 4.5%. On the other hand, the limiting oxygen index is increased by +7.0% (only 2.6% with LDPE composites). Copyright © 2010 John Wiley & Sons, Ltd.     

Facile growth of 1‐D nanowire‐based WO3 thin films with enhanced photoelectrochemical performance

A flame reactor embedded with a constant tungsten wire feeding system to prepare one‐dimensional (1‐D) nanostructured tungsten oxide thin film for photoelectrochemical (PEC) water splitting was developed. Photoactive vertically‐aligned nanowire‐based WO₃ thin films could be obtained with a controlled thickness via a flame vapor deposition process followed by air‐annealing. The PEC performances of WO₃ photoelectrodes for different thin film thicknesses were examined. The optimum thickness of WO₃ thin film was found to be about 7.2 μm for PEC water splitting based on incident photon‐to‐current efficiency plots and I–V curves. The WO₃ prepared with optimum thickness showed better PEC performance than those of recently reported nanostructured WO₃ photoanodes. © 2015 American Institute of Chemical Engineers AIChE J, 62: 421–428, 2016     

Are 3‐D models necessary to simulate packed bed reactors? analysis and 3‐D simulations of adiabatic and cooled reactors

Simulations and analysis of transversal patterns in a homogeneous three‐dimensional (3‐D) model of adiabatic or cooled packed bed reactors (PBRs) catalyzing a first‐order exothermic reaction were presented. In the adiabatic case the simulation verify previous criteria, claiming the emergence of such patterns when (ΔT_ad/ΔT_m)/(Pe_C/Pe_T) surpasses a critical value larger than unity, where ΔT_ad and ΔT_m are adiabatic and maximal temperature rise, respectively. The reactor radius required for such patterns should be larger than a bifurcation value, calculated here from the linear analysis. With increasing radius new patterned branches, corresponding to eigenfunction of the problem emerge, whereas other branches become unstable. The maximal temperature of the 3‐D simulations may exceed the 1‐D prediction, which may affect design procedures. Cooled reactor may exhibit patterns, usually axisymmetric ones that can be characterized by two anomalies: the peak temperature may exceed the corresponding value of an adiabatic reactor and may increase with wall heat‐transfer coefficient, and the peak temperature in a sufficiently wide reactor need not lie at the center but rather on a ring away from it. In conclusions, we argue that transversal patterns are highly unlikely to emerge in practical adiabatic PBRs with a single exothermic reaction, as in practice Pe_C/Pe_T > 1. That eliminates patterns in stationary and downstream‐moving fronts, whereas patterns may emerge in upstream‐moving fronts, as shown here. This conclusion may not hold for microkinetic models, for which stationary modes may be established over a domain of parameters. This suggests that a 1‐D model may be sufficient to analyze a single reaction in an adiabatic reactor and a 2‐D axisymmetric model is sufficient for a cooled reactor. The predictions of a 2‐D cylindrical thin reactor with those of a 3‐D reactor were compared, to show many similarities but some notable differences. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Synergistic effect on flame retardancy and thermal behavior of polycarbonate filled with α‐zirconium phosphate@gel‐silica

A new kind of flame retardant (ZS) with layered α‐zirconium phosphate disks (α‐ZrP) as the core and inorganic flame retardant (gel‐silica, GS) to shield solid acid sites on the surface of α‐ZrP as the shell, was synthesized via a facile method. The incorporation effects of ZS with silicone resin on the thermal properties and flame retardance of PC composites were investigated. The presence of ZS could improve the thermal stability of PC matrix. With the addition of ZS contents increased to 3 wt %, the limiting oxygen index (LOI) of the composite was 32.3% and the vertical burning (UL‐94) test reached a V‐0 rating. However, with more contents of ZS, the LOI value decreased, and without the GS layer, the LOI value was decreased significantly as well. The synergism between the α‐ZrP core and gel‐silica shell, also with the silicone resin were found. Based on these results, the flame‐retardant mechanism was proposed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44829.     

3D numerical investigations of the effect of fill factor on dispersive and distributive mixing of rubber under non‐isothermal conditions

Three‐dimensional, non‐isothermal, transient computational fluid dynamics simulations are conducted for rubber mixing with a set of two‐wing rotors in a partially filled chamber. The main objective is to analyze the effect of different fill factors of rubber on dispersive and distributive mixing characteristics by simulating 15 revolutions of the rotors rotating at 20 rpm. 60%, 70%, 75%, and 80% are the four different fill factors chosen for the study. An Eulerian multiphase model is employed to simulate two different phases, rubber and air, and the volume of fluid technique is used to calculate the free surface between two phases, in addition to the continuity, momentum and Energy equations. To characterize non‐Newtonian, highly viscous rubber, shear rate and temperature dependent Carreau‐Yasuda model has been used. A set of more than 3,600 massless particles are injected after a certain period of time to calculate several quantities in terms of dispersive and distributive mixing. Both the Eulerian and Lagrangian results showed that, fill factors between 70% and 80% presented the most reasonable and efficient mixing scenario, thus exhibiting the best dispersive and distributive mixing characteristics combined. POLYM. ENG. SCI., 59:535–546, 2019. © 2018 Society of Plastics Engineers     

Crystalline morphology and mechanical properties of isotactic polypropylene composites filled by wollastonite with β‐nucleating surface

Wollastonite‐filled α‐isotactic polypropylene (iPP) and β‐iPP were prepared through introduction of wollastonite (W) and wollastonite with β‐nucleating surface (W_x) in iPP matrix. The α‐ and β‐nucleating ability of wollastonite, crystalline morphology, and mechanical properties of injected iPP filled by wollastonite with different nucleating surface were compared using differential scanning calorimetry, wide‐angle X‐ray diffraction, polarizing optical microscopy, mechanical testing, and scanning electron microscopy. The results indicated that iPP filled by wollastonite with different nucleating surface has different crystalline morphology, melting behavior, and mechanical properties. The W and W_x filled iPP mainly formed α‐ and β‐phase iPP, respectively. The tensile and flexural modulus of iPP/W and iPP/W_x increased with increasing wollastonite content, and the tensile and flexural modulus of iPP/W_x were lower than that of iPP/W. The tensile property, flexural property, and impact strength of iPP/W_x were higher than that of iPP/W and β‐iPP. The synergistic effect of reinforcing of wollastonite and toughening of β‐phase leads to higher mechanical properties. POLYM. COMPOS., 35:1445–1452, 2014. © 2013 Society of Plastics Engineers