Extension-dominated improved dispersive mixing in single-screw extrusion. Part 1: Computational and experimental validation

Extensional mixing elements (EMEs) were previously developed for twin-screw extruders (TSEs) to incorporate extensional flows through stationary single-plane or double-plane hyperbolic convergence–divergence channels. In Part 1 of this work, the EME concept is extended to single-screw extruders (SSEs), which are known for their good pumping characteristics but limited dispersive mixing capability, to enhance the latter. Flow simulations are performed to optimize the EME design for SSE. Experimental validations are performed on immiscible polymer blends (varying viscosity ratio) and nanocomposites. Morphological results show tremendous improvement in mixing capability of SSE when equipped with EME without significant throughput and pressure drop penalties. Rheological and crystallinity studies are observed to be in line with the morphological analysis. Morphological and mechanical results are benchmarked with TSE operations in Part 2 of this work.     

Melt compounding of polypropylene‐based clay nanocomposites

Polypropylene (PP)‐based nanocomposites containing 4 wt% maleic anhydride grafted PP (PP‐g‐MA) and 2 wt% Cloisite 20A (C20A) were prepared using various processing devices, viz., twin‐screw extruder (TSE), single‐screw extruder (SSE), and SSE with an extensional flow mixer (EFM). Two processing methods were employed: (I) masterbatch (MB) preparation in a TSE (with 10 wt% C20A and clay/compatibilizer ratio of 1:2), followed by dilution in TSE, SSE, or SSE + EFM, to 2 wt% clay loading; (II) single pass, i.e., directly compounding of dry‐blended PP‐g‐MA/clay in TSE, SSE, or SSE + EFM. It has been indicated that the quality of clay dispersion, both at micro‐ and nanolevel, of the nanocomposites depends very much on the operating conditions during processing, such as mixing intensity and residence time, thus affecting the mechanical performance. Besides that the degradation of the organoclay and the matrix is also very sensitive to these parameters. According to results of X‐ray diffraction, field emission gun scanning electron microscopy, transmission electron microscopy, and mechanical tests, the samples prepared with MB had better overall clay dispersion, which resulted in better mechanical properties. The processing equipment used for diluting MB had a marginal influence on clay dispersion and nanocomposite performance. POLYM. ENG. SCI., 47:1447–1458, 2007. © 2007 Society of Plastics Engineers     

Flow through a convergence. Part 2: Mixing of high viscosity ratio polymer blends

Dispersive mixing of high viscosity ratio blends was studied in a converging flow using a capillary rheometer equipped with dies having different entry profiles. Three inhomogeneous bimodal polyolefin blends, with stress‐dependent viscosity ratios ranging from 8 to 450, were used in this work. Such magnitudes of viscosity ratio indicate that the dispersed droplets can be mixed only in an elongational flow field. The mixing efficiency was found to be dependent on both the profile of the convergence and flow rate. At the lowest flow rates, the dispersive mixing efficiency was very low, but it increased with an increasing flow rate until a profile‐dependent maximum. This maximum mixing efficiency was observed prior to fracture of the matrix material, after which the efficiency decreased. Stress and deformation fields within different profiles were estimated by numerical simulation using the K‐BKZ equation, and the results were used to interpret experimental results. The dispersive mixing efficiency was found to be proportional to the maximum elongational stress within the converging section, and to the length of the region where the critical conditions for elastic fracture of droplet material were met. It is shown that the dispersive mixing mechanism in high viscosity ratio blends is mainly dictated by elastic fracture of the droplet, and hence is applicable over a wide range of polymer blending processes.     

Mixing efficiency in co‐rotating batch mixer for preparation of NR/EPDM blends

An in‐house developed co‐rotating batch mixer was used to prepare the blends of natural rubber (NR) and ethylene‐propylene‐diene terpolymer (EPDM) in the present work. Phase morphology and magnitude of dispersive mixing efficiency offered by the in‐house developed co‐rotating batch mixer and a conventional counter‐rotating batch mixer were compared. It has been found that the co‐rotating batch mixer equipped with the MX2 rotor configuration could improve the dispersive mixing efficiency of NR/EPDM blends considerably. A poor state‐of‐mix in blends, particularly at high fill factor, could be overcome by the utilization of MX2 rotor configuration where the extensional flow is probably facilitated in the converging zones. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Phase Control of Polyamide 6 via Extension-Dominated Polymer Blend Reactive Extrusion

Control of the crystalline phase transition of polymers is critical to improve mechanical properties and to achieve good levels of thermal properties in polymer engineering. Current methods control the transition process by using thermal methods that is, annealing above the glass transition temperature (T_g) to get the targeting crystalline phase. It requires precise control of the temperature and process time and is limited to a small scale. In contrast, a novel processing-based method developed in our group by combining the use of reactive processing technique and extensional mixing elements (EME) allows different sizes of minor phase droplets in immiscible polymer blends to be achieved by controlling the degree of aggressiveness of the EME. The use of EME can confine polyamide crystalline structure into the γ-phase from the α-phase. The reactive processing benefits in stabilizing these droplets and in freezing the corresponding crystalline morphologies in droplets. Crystalline phase transition of the minor phase in these droplets is therefore observed as the size of droplet decreases from micro to submicron. Furthermore, phase transition can be precisely controlled by tuning additives and EME blocks. POLYM. ENG. SCI., 60:1019–1028, 2020. © 2020 Society of Plastics Engineers     

Relationship between processing method and microstructural and mechanical properties of poly(ethylene terephthalate)/short glass fiber composites

Composites of recycled poly(ethylene terephthalate) (PET) reinforced with short glass fiber (GF) (0, 20, 30, and 40 wt %) were compounded in a single‐screw extruder (SSE) and in a intermeshing corotating twin‐screw extruder (TSE). An SSE fitted with a barrier double‐flight screw melting section in between two single‐flight sections and a TSE with a typical screw configuration for this purpose were used. The composites were subsequently injection molded at two different mold temperatures (10 and 120°C), with all other operative molding parameters kept constant. The effects of processing conditions on composite microstructure, PET degree of crystallinity, and composite mechanical properties were evaluated. Appropriate dispersive and distributive mixing of the glass fiber throughout the PET matrix as well as fine composite mechanical and thermal‐mechanical properties were achieved regardless of whether the composites were prepared in the SSE or TSE. The performance of the SSE was attributed to the efficiency of the barrier screw melting section in composite mixing. The mold temperature influenced the mechanical properties of the composites, by controlling of the degree of crystallinity of the PET in the composites. For a good balance of mechanical and thermal‐mechanical properties, high mold temperatures are desirable, typically, 120°C for a mold cooling time of 45 s. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Effects of extensional flow on properties of polyamide‐66/poly(2,6‐dimethyl‐1,4‐phenylene oxide) blends: A study of morphology,mechanical properties,and rheology

In this study, polyamide‐66/poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PA66/PPO) blends with high viscosity ratio were processed by a self‐designed triangle‐arrayed triple‐screw extruder (TTSE, which simulates extensional flow) and a commercial twin‐screw extruder (TSE), respectively. Furthermore, in order to improve the mechanical properties of the immiscible PA66/PPO blends, PPO‐grafted maleic anhydride (PPO‐g‐MA) and styrene–ethylene–butylene–styrene (SEBS) block copolymer were used. The mechanical properties, phase morphology, and rheological properties of both binary PA66/PPO blends and toughened PA66/PPO/PPO‐g‐MA blends were comprehensively investigated to compare the above mentioned two processing method. Samples processed with TTSE exhibited better mechanical properties than the TSE‐processed blends. The morphologies of the blends were examined by scanning electron microscopy, exhibiting smaller particles sizes and narrower particle size distributions, which were attributed to the significant effects of extensional flow in TTSE. The toughening mechanism of compatibilized blends was investigated through morphology analysis, dynamic mechanical, and rhelogical analysis. Thus, TTSE with an extensional effect was proved to be efficient in the blending of high viscosity ratio polymers. POLYM. ENG. SCI., 57:1090–1098, 2017. © 2016 Society of Plastics Engineers     

Non-linear rheological response as a tool for assessing dispersion in polypropylene/polycaprolactone/clay nanocomposites and blends made with sub-critical gas-assisted processing

Polypropylene (PP) was blended with polycaprolactone (PCL) and nanoclay (NC) in a twin-screw extruder (TSE) using a traditional extrusion process and a sub-critical gas-assisted process (SGAP). SGAP is a new and facile processing method that injects compressed gas (CO₂ or N₂) at low pressures (~10 bars) into the barrel of the extruder to induce rapid and repetitive foaming and resolubilization as the melt travels through regions of high pressure and low pressure. Bubble expansion during foaming introduces an equibiaxial elongational flow not otherwise generated in TSE, adding to the total stress the polymer matrix can exert to break up nanoparticle agglomerates and reduce the droplet size of secondary polymers in blends. Impact, morphology, and X-ray diffraction (XRD) properties confirmed a smaller PCL phase droplet size and an increase in dispersion of the NC when SGAP was used. Standard small amplitude oscillatory (SAOS) rheological tests for the storage modulus G′ were not sensitive enough to discern the difference between the traditionally extruded samples and the SGAP samples. However, the zero-strain non-linearity parameter, Q₀, determined by the Fourier-Transform rheology, was able to distinguish the enhanced dispersive and mixing capabilities of SGAP. Practical implications of SGAP and Fourier-Transform (FT) rheology are also discussed in this paper. POLYM. ENG. SCI., 60:55–60, 2020. © 2019 Society of Plastics Engineers     

Morphology evolution of a thermotropic liquid‐crystalline polymer in a polyamide 6,6 matrix regulated by graphene

A two‐step process, thermotropic liquid‐crystalline polymer (TLCP) premixing with reduced graphene oxide (RGO) followed by blending with polyamide 6,6 (PA66), was used to prepare ternary TLCP/RGO/PA66 blends. The rheological behaviors, morphology, and mechanical properties of the blends were investigated. The results show that RGO migrated from the TLCP phase to the interface between the TLCP and PA66 phase during melt blending; this was due to a similar affinity of the RGO nanosheets to both component polymers. The dimensions of the dispersive TLCP domains were markedly reduced with the mounting RGO content; this revealed a good compatibilization effect of RGO on the immiscible polymers. The hierarchical structures of the TLCP fibrils were found in both the unfilled TLCP/PA66 blends and TLCP/RGO/PA66 blends. This supposedly resulted from the extensional and torsional action of unstable capillary flow. With the addition of RGO, the viscosities of the blends decreased further, and the fibrillation of TLCP and the mechanical performance of TLCP composites were both enhanced. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43735.     

Polymer blend containing a thermotropic polyester with long flexible spacer in the main chain

Polymer blends of nylon 66 and thermotropic polyester with long flexible spacers in the main chains were prepared by melt mixing. The samples were made as single filaments by passing the polymer blend through a small and round die of a capillary rheometer. Mechanical properties of blends showed that the modulus and strength of nylon 66 could be improved without reduction of extensibility. The morphology of fractured surfaces was observed by a scanning electron microscope (SEM). It showed that the microfibrillar structure of a thermotropic polyester was formed by extensional flow while the spherical and ellipsoidal particles in the nylon 66 (matrix polymer) were produced by shear flow. The polyester particles were occasionally covered with adhering matrix polymer because of good adhesion between these two polymers. They were highly elongated by tensile stress without loss of elongational characteristics of blends. This fact was explained by very good adhesion between the two phases.     

Morphologies and tensile properties of elastomer-modified epoxy and polycarbonate blended systems

The morphologies and tensile properties of an elastomer-modified epoxy (EME)/polycarbonate (PC) binary system and an EME/diglycidyl ether of bisphenol A (DGEBA)/PC ternary system were examined. In the EME system, a continuous elastomer-rich phase formed, while in the EME/DGEBA systems (unblended with PC), a continuous epoxy-rich phase formed. In both of these systems, two-phase structures were observed. In contrast, a microdispersed structure was observed when the PC was blended with either the EME or with the EME/DGEBA systems. It is suggested that blending of the epoxy with PC caused an increased solubility of the former into the elastomer phase. The tensile strength and tensile elongation of both the EME and EME/DGEBA systems were improved by blending with PC. In the EME/PC blend, the tensile elongation reached its maximum value (60%) at a PC content of approximately 10 p.h.r. (parts per hundred resin by weight), with this maximum being approximately one and a half times higher than that of the unblended EME. Tensile strength was also clearly increased by blending with small amounts of PC, but soon reached a steady value. In the EME/DGEBA/PC blends, the tensile properties were dependent on the weight ratio of EME to DGEBA. In the absence of PC, as this ratio increased, the tensile elongation also increased, while at the same time the tensile strength decreased. The tensile properties were also improved in this system, by blending with PC. From the results obtained, it was clear that the improvement in tensile properties was closely related to the changes in morphology. Therefore, blending of the PC induced a microdispersed structure and improved the elongation of the epoxy resin.     

Comparison of different processing methods to prepare poly(lactid acid)–hydrotalcite composites

The effect of the compounding method on the morphology and on the properties of poly(lactic acid) (PLA)–hydrotalcite (HT) composites was studied. Moreover, the influence of two different kinds of HT—organically modified (OM‐HT) and unmodified (U‐HT)—and their concentration was evaluated. The composites were prepared using either a single screw extruder (SSE), a counter rotating twin‐screw compounder (TSC) or a corotating twin‐screw extruder (TSE). The prepared materials were characterized by scanning electron microscopy, gel permeation chromatography (GPC) analysis, mechanical and rheological measurements. The results indicated that the best morphology, i.e., particles dimension and distribution, is exhibited by materials prepared by TSE while the worse ones by the samples processed by SSE. The viscosity of all the materials containing the HT is lower in comparison with the viscosity exhibited by the neat matrix, in particular, when the OM‐HT is used. These results were correlated to degradation phenomena occurring during the processing of the materials as revealed by the results of GPC analysis. The addition of HT caused only a slight increase of elastic modulus of filled materials even when 5% of filler was incorporated. However, in full agreement with morphological analyses, the best performances were exhibited by materials prepared by TSE while the worse ones by the samples processed by SSE. POLYM. ENG. SCI., 54:1804–1810, 2014. © 2013 Society of Plastics Engineers     

The effect of mixing on the modes of dispersion and rheological properties of two-phase polymer blends in extrusion

An experimental study was carried out to investigate the effect of mixing on the state of dispersion and rheological properties in the two-phase flow of polymer blends. For the study, blends of polystyrene and polypropylene were used, and two mixing devices were employed: a single-screw extruder combined with a “static mixer,” and a twin-screw compounding machine. Materials of various blending ratios were extruded at a constant temperature (200°C) through a capillary die having an L/D ratio of 20 (D = 0.125 in.). The state of dispersion in the two-phase system was investigated from pictures taken of the microstructure of the extrudate samples. It was found that different mixing devices have a profound influence on the state of dispersion of one polymer in another. Also determined were the rheological properties of the two-phase system investigated, from wall normal stress measurements. Our results show that, when shear stress is used as a parameter, the melt viscosity goes through a minimum, whereas the melt elasticity goes through a maximum. This is regardless of the type of mixing device employed, although the shapes of the curves are affected by the type employed. It is suggested that shear stress, instead of shear rate, be used in correlating the viscoelastic properties of two-phase polymer systems.     

UV fluorescence monitoring of the mixing of molten polymers in a batch mixer

A fluorescence monitoring device was developed to study the distributive mixing process inside the chamber of a batch mixer. A bifurcated UV‐fluorescence probe was implemented instead of the thermocouple measuring the temperature of the melt. Hydroxymethyl anthracene was used as fluorescence tracer and was preliminarily dispersed in the minor component. Furthermore, the main polymer of this study was a copolymer of ethylene and vinyl acetate (EVA). Structure development during mixing of miscible polymer blends, low viscosity ratio blends (plasticizer/polymer), and immiscible polymer blends was investigated by this method. It was shown that the mixing process of a miscible system depends on the shear rate, which governs the temperature of the melt and consequently the melting process of the solid masterbatch‐tracer at the early stages of mixing. On the other hand, the presence of a miscible component of very low viscosity, as, for example, mixing of DOP in EVA, delays the onset of mixing. This lubricant effect was shown to be due to the fact that DOP migrates to the wall of the chamber rather than inside the polymer, though it is well miscible with the EVA phase. The study of immiscible polymers, as, for example, polypropylene in EVA, did not show any fundamental difference with miscible polymer blends. This observation is actually inherent to the UV‐fluorescence tracer disperesed in a masterbatch system, which allows quantification of the distributive mixing process only.     

挤出加工方法对制备PP/PET原位微纤共混物微观形态的影响

分别使用双螺杆挤出机、配备不同结构螺杆或强剪切机头的单螺杆挤出机对聚丙烯(PP)/聚对苯二甲酸乙二醇酯(PET)进行熔融共混挤出,并用扫描电子显微镜观察了产物的微观形态。结果表明,使用双螺杆挤出机或使用配备三段式螺杆的单螺杆挤出机挤出PP/PET,只能制备出PET以球状形态均匀分散在PP连续相中的共混物,不含有任何微纤;使用配备有头部直槽混炼件的单螺杆挤出机挤出PP/PET,部分PET会形成短而粗的微纤;采用熔融挤出热拉伸淬冷法挤出PP/PET,可生产出微纤直径约为5 μm、长径比超过20的原位微纤共混物;采用强剪切机头及头部配备有直槽混炼件螺杆的单螺杆挤出机挤出PP/PET,可生产出微纤直径约为5~7 μm、长径比超过20的原位微纤共混物,且该方法操作简单、辅助设备少、具有工业可行性。     

Rheological Responses and Morphology of Polylactide/Linear Low Density Polyethylene Blends Produced by Different Mixing Type

Polylactide (PLA)/linear low density polyethylene (LLDPE) was produced by different mixing type, provided by combining different single screw elements. The relationship between rheological responses and morphology of the blends was investigated. It is shown from rheological curves that different morphology is reflected to be formed in blends produced by different mixing type. Han plots, van Gurp-Palmen plots and cole-cole curves show that a stronger interaction between polymer minor and matrix or co-continuous morphology appears in PLA/LLDPE (70/30) produced by high shear/chaotic mixing. This is in agreement with the results of morphology observation. In addition, the crystallization of blends is analyzed. Results show that the crystallity of blends produced by shear/chaotic mixing is decreased, due to the stronger interaction between the two phases in blends.     

Numerical simulations of partially‐filled rubber mixing in a 2‐wing rotor‐equipped chamber

Partially filled internal batch mixers are used for mixing of rubber compounds in the polymer industry. The use of mixing in such mixers equipped with a rotor is critical to the process itself, and hence, understanding of mixing is important in terms of evaluating how various operating parameters such as rpm, fill factor, and ram pressure affect distribution and dispersion of materials. The objective of the current study is to gain valuable insights on the influence of fill factor, which is the volume of the material relative to the volume of the chamber. Two‐dimensional (2D) computational fluid dynamics (CFD) simulations of rubber mixing in a 2‐wing rotor‐equipped chamber are presented here, for the first time, for fully‐filled/100% and partially‐filled/75% chambers. The volume‐of‐fluid (VOF) technique is employed to capture the interface between the rubber and air in partially filled isothermal simulations. Flow patterns are visualized to analyze the material movement. Massless particles are injected and various statistics are calculated from their positions in order to compare dispersive and distributive mixing characteristics between the fully‐filled and partially‐filled cases. Specifically, quantities such as mixing index and the maximum shear stress distribution history of particles are analyzed to obtain information about dispersive mixing, while length of stretch and cluster distribution index, also calculated from particles, are presented to investigate distributive mixing capabilities. All the results consistently demonstrated the superior effectiveness of partially‐filled mixing chambers in terms of their dispersive and distributive mixing characteristics in comparison to fully‐filled chambers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44250.     

三螺杆挤出机中剪切-拉伸交变流场对PP/PE-HD体系增韧效果的影响

利用倒三角形排列三螺杆挤出机与传统双螺杆挤出机对不同黏度比的聚丙烯/高密度聚乙烯(PP/PE-HD)体系进行混炼加工,研究了三螺杆挤出机中剪切拉伸交变流场对于高黏度比聚合物两相体系的分散混合及分布混合效果的影响;并通过高黏度比体系的混合效果证明三螺杆挤出机中剪切拉伸交变流场的作用效果及机理。结果表明,在高黏度比体系的加工中,倒三角形排列三螺杆挤出机分散与分布混合效果明显优于双螺杆挤出机,且三螺杆挤出机的增韧效果随着体系黏度比的增大而提升。     

A new composition–processing–property relationship for studying the tensile modulus‐phase plastic blends

Based on thermodynamic principles, a composition–processing–property relationship for predicting the modulus properties of multiphase plastic blends has been developed. This relationship describes the relative modulus of the blend in terms of the volume fraction and the index for the degree of mixing of an inclusion‐polymer in the matrix‐polymer. The relative modulus is defined as the ratio between the modulus of the blend and that of the matrix polymer. These blends include a nylon 6,6/polymethyl methacrylate(PMMA) system mixed using an injection molding process arid a nylon 6/ethylene‐vinyl acetate copolymer system mixed using a corotational extrusion process. Based on the values determined for the mixing index of the nylon 6,6/PMMA blends, a relationship between the mixing index and the fill time used in the injection molding has been developed. The results also imply that the degree of mixing of the blend mixed using a correlation extrusion process is better than that of the blend processed using an injection molding process. Using the above results, we now can scientifically develop new plastic blends and design optimum processing conditions for various automotive applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011