Effect of supercritical carbon dioxide on morphology development during polymer blending

Supercritical carbon dioxide (scCO₂) was added during compounding of polystyrene and poly(methyl methacrylate) (PMMA) and the resulting morphology development was observed. The compounding took place in a twin screw extruder and a high‐pressure batch mixer. Viscosity reduction of PMMA and polystyrene were measured using a slit die rheometer attached to the twin screw extruder. Carbon dioxide was added at 0.5, 1.0, 2.0 and 3.0 wt% based on polymer melt flow rates. A viscosity reduction of up to 80% was seen with PMMA and up to 70% with polystyrene. A sharp decrease in the size of the minor (dispersed) phase was observed near the injection point of CO₂ in the twin screw extruder for blends with a viscosity ratio, ηPMMA/ηpolystyrene, of 7.3, at a shear rate of 100 s^?1. However, further compounding led to coalescence of the dispersed phase. Adding scCO₂ did not change the path of morphology development; however, the final domain size was smaller. In both batch and continuous blending, de‐mixing occurred upon CO₂ venting. The reduction in size of the PMMA phase was lost after CO₂ venting. The resulting morphology was similar to that without the addition of CO₂. Adding small amounts of fillers (e.g. carbon black, calcium carbonate, or nano‐clay particles) tended to prevent the de‐mixing of the polymer blend system when the CO₂ was released. For blends with a viscosity ratio of 1.3, at a shear rate of 100 s^?1, the addition of scCO₂ only slightly reduced the domain size of the minor phase.     

Supercritical carbon dioxide assisted blending of polystyrene and poly(methyl methyacrylate)

A method for blending polystyrene and poly(methyl methacrylate), (PMMA), with the addition of supercritical carbon dioxide has been investigated. The first series of blends was a PMMA and polystyrene with similar melt viscosities. The second series of blends was a PMMA and polystyrene with a viscosity ratio (ηPMMA/ηpolystyrene) of about 20. The results show that a reduction in the size of the minor or dispersed phase is achieved when supercritical carbon dioxide is added to the blend system. A high-pressure mixing vessel was used to prepare the blends under pressure with carbon dioxide for batch blending. The solubilities of CO₂ in PMMA and polystyrene, measured in the high-pressure mixing vessel at 200°C and 13.78 MPa (2000 psi) was 5.8 and 3.6 wt%, respectively. A single screw extruder was used to study the effects of carbon dioxide on the viscosity of polymer melts. The melt viscosity of PMMA was reduced by up to 70% with approximately 0.4 wt% CO₂. The melt viscosity of polystyrene was reduced by up to 56% with a CO₂ content of 0.3 wt%. A twin screw extruder was used to study the effects of injecting carbon dioxide in a continuous compounding operation.     

The effect of pressurized carbon dioxide as a plasticizer and foaming agent on the hot melt extrusion process and extrudate properties of pharmaceutical polymers

The aim of the current research project was to explore the possibilities of combining pressurized carbon dioxide with hot melt extrusion of polyvinylpyrrolidone-co-vinyl acetate 64, Eudragit^® E100 and ethylcellulose 20 cps, to evaluate the ability of the pressurized gas to act as a temporary plasticizer as well as to produce a foamed polymeric material. Pressurized carbon dioxide was injected into a Leistritz Micro 18 intermeshing co-rotating twin-screw melt extruder using an ISCO 260D syringe pump. The physicochemical characteristics of the polymers before and after injection of carbon dioxide were evaluated using MDSC, dissolution measurements, specific surface area measurements, porosity, dynamic vapour sorption and microscopy. An extruder set up and screw configuration were configured and optimized for injection of pressurized CO₂. Carbon dioxide acted as plasticizer for all three polymers, reducing the processing temperature during the hot melt extrusion process. The specific surface area and the porosity of the polymers was increased after treatment with carbon dioxide, resulting in enhanced dissolution. The macroscopic morphology was changed to a foam-like structure due to expansion of the carbon dioxide at the extrusion die. This resulted in improved milling efficiency.     

Effect of supercritical carbon dioxide on PMMA/rubber and polystyrene/rubber blending: Visosity ratio and phase inversion

Supercritical carbon dioxide (scCO₂) was added during blending of polystyrene or poly(methyl‐methacrylate) (PMMA) and a rubber impact modifier (SP 2207). The resulting blend morphologies were compared. The compounding took place in a Leistritz ZSE‐27 twin‐screw extruder at 100 RPM, at a temperature of 200°C, and with 2.0 wt% CO₂ Injection. The viscosity reduction of PMMA, polystyrene, and SP 2207 was measured using a slit die rheometer attached to the twin‐screw extruder. A viscosity reduction of up to 84% was seen with PMMA, 70% with polystyrene and 30% with SP 2207. The solubility of CO₂ in these polymers was measured in a high‐pressure vessel at 200°C and 13.78 MPa (2000 psi). A solubility of 5.79 wt% CO₂ was seen with PMMA, 3.65 wt% with polystyrene, and 2.60 wt% with SP 2207. The injection of CO₂ reduced the size of the dispersed rubber phase in both polystyrene and PMMA. For both blends (polystyrene/SP 2207 and PMMA/SP 2207) with and without the injection of CO₂, the extruder length for phase inversion was shortened by about L/D = 4, or 10% of the total extruder length. The impact strength for a 70/30 polystyrene/SP 2207 blend was increased by 26% by the addition of CO₂. The improvement in impact strength was not as large for blends of PMMA and SP 2207.     

Static mixers as heat exchangers in supercritical fluid extraction processes

The performance of a Kenics static mixer as a heat-transfer device for supercritical carbon dioxide (CO₂) flow is studied and compared with conventional tube-in-tube heat exchangers. Measurements were carried out at pressures ranging from 8 to 21 MPa, temperatures from 283 to 323 K, and mass flowrates from 2 to 15 kg/h. The corresponding Reynolds and Prandtl numbers, at bulk conditions, ranged between 10³ and 2 × 10⁴ and between 2 and 7, respectively. The temperature increase experienced by the supercritical CO₂ stream varied between 10 and 35 K. The heat fluxes obtained with the static mixer are one order of magnitude higher than the ones observed with a tube-in-tube heat exchanger for the same set of operating conditions. The heat-transfer enhancement is caused by the cross-sectional mixing of the fluid and to a lesser extent by conduction across the metallic mixing elements. Heat-transfer is also affected by temperature-induced variation of physical properties, especially in the pseudocritical region of the fluid. From the experimental data, a correlation was developed for convective heat-transfer to supercritical CO₂ in terms of the Nusselt number.     

Extrusion of PE/PS blends with supercritical carbon dioxide

The effects of dissolved supercritical carbon dioxide on the viscosity and morphological properties were investigated for polyethylene/polystyrene blends in a twin-screw extruder. The viscosities of the blend/CO₂ solutions were measured using a wedge die mounted on the extruder. A considerable reduction of viscosity was found when CO₂ was dissolved in the blend. It was observed that the dissolution of CO₂ into PE/PS blends, regardless of the CO₂ content used, led to decreased shear thinning behavior resulting in an increase of the power law index from 0.29 to 0.34. The cell structures of foamed PE/PS blends showed a typical dependence of pressure and CO₂ concentration, with higher operating pressures and CO₂ content leading to a smaller cell size. Also, it was noted that the size of the dispersed PS phase in the PE/PS phase blends decreased by increasing the CO₂ concentration, and that the dispersed PS phase domains were highly elongated in the direction normal to the cell radius.     

Melt processing and rheology of an acrylonitrile copolymer with absorbed carbon dioxide

The purpose of this study is to determine under what conditions it is possible to use CO₂ to plasticize and, thereby, reduce the viscosity of an acrylonitrile (AN) copolymer in an extrusion process and render it melt processable. To assess whether it was possible to absorb adequate amounts of CO₂ in short residence times by injection into a single screw extruder, a slit‐die rheometer was attached to the end of the extrusion system for the purpose of directly assessing the viscosity reduction. A chemorheological analysis was performed on 65 and 85% AN copolymers to establish the temperature at which the 85% material would be as stable as the melt‐processable 65% material at its recommended extrusion temperature. This, coupled with studies correlating the degree of T_g and viscosity reduction with the amount of absorbed CO₂, and comparison to previous data obtained in batch processes allowed one to predict conditions for melt extrusion of the 85% AN. Preliminary studies using a pressurized chamber attached to the exit of the die allowed one to assess the conditions under which suppression of foaming is possible. POLYM. ENG. SCI., 2009. © 2009 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.     

Controlling the structure of a porous polymer by coupling supercritical CO2 and single screw extrusion process

A study on the extrusion of Eudragit E100 was carried out using supercritical carbon dioxide (scCO₂) as plasticizer and foaming agent. ScCO₂ modifies the rheological properties of the material in the barrel of the extruder and acts as a blowing agent during the relaxation when flowing through the die. For experiments, a single‐screw extruder was modified to be able to inject scCO₂ within the extruded material. The aim is to determine a correlation between operating conditions and foam structure. The effect of three parameters was studied: the temperature in the die and in the metering zone, the screw speed, and the volumetric flow rate of CO₂. An increase in temperature enhances the expansion rate and the average pore diameter and appears to be the most significant parameter. The effect of CO₂ concentration is significant at small concentrations only: the higher the CO₂ concentration, the lower the pore density and the higher both the pore diameter and the expansion rate. The effect of the screw speed is tricky because a variation of this speed involves a decrease of CO₂ weight ratio. This study shows that the structure of the extrudates does not evolve with a coupling of screw speed increase and a subsequent CO₂ weight ratio decrease. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

数值模拟静态混合器结构对PS/CO_2熔体温度的影响

利用专用CFD软件Polyflow对SMX型和Kenics型静态混合器中PS/CO_2发泡溶液进行数值模拟计算,分析比较不同板厚在不同元件个数条件下两种静态混合器消耗的压力损失,以及不同CO_2浓度对静态混合器压力损失的影响;并引入"离散系数"分析比较两种静态混合器出口温度均匀性的变化.数值模拟的结果表明:SMX型静态混合器冷却效果优于Kenics型静态混合器,并且SMX型静态混合器出口温度均匀性高于Kenics型静态混合器.     

Extrusion with Carbon Dioxide (CO2) and Characterization of ABS/Mg (OH)2/Nanoclay Composites

In this paper, carbon dioxide (CO₂) is used to form a high-density microcellular thermoplastic foam structure in order to reduce polymer consumption and facilitate dispersion of Mg (OH)₂ and nanoclay fillers. A twin-screw extruder system was used to predistribute inorganic fillers into the ABS polymer, resulting in composite ABS/filler pellets. This is followed by the use of a single-screw extruder wherein supercritical carbon dioxide is introduced into the formulation. Finally, the resulting foam ABS/filler/CO₂ pellets are injection- molded into test samples. The structure and properties of the composites are characterized using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Furthermore, ABS/Mg(OH)₂/nanoclay polymer composite samples are tested to obtain their yield and tensile strengths, elastic moduli, yield and tensile elongations, izod impact strengths, hardness values, heat deflection temperatures (HDT), Vicat softening points, and melt flow indices (MFI). These tests reveal that for the overall reduction in the amount of polymer in the samples, material properties did not generally deteriorate and even showed improvements in some areas. Moreover, resulting injection-molded samples have been shown to possess dimensional integrity due to the continued expansion of CO₂ during the molding operation.     

CO2‐assisted polymer processing: A new alternative for intractable polymers

CO₂‐assisted polymer processing is proposed as an alternative route for intractable and high molecular weight polymers based on the plasticization effects of CO₂ and its direct effect on the melting behavior of semicrystalline polymers. A modified processing system was used to process a variety of polymers in the presence of high‐pressure CO₂. The system includes an extruder that was modified to allow for high pressures created by the injection of CO₂. The new design includes a modified feed section that allows a given mass of polymer to interact with CO₂ before and during the extrusion process. The inherent shear mixing and the presence of CO₂ allow for a specific control over the extrudate morphology. Results suggest that this alternative design provides a new and easy route to melt process high melt viscosity polymers of commercial importance, such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene copolymer (FEP), and syndiotactic polystyrene (s‐PS). The increased processability of these systems in CO₂ is related to the plasticization effect of CO₂ that was quantified through a depression in the glass‐transition temperature according to the Chow model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1501–1511, 2004     

An analysis of liquid CO2 drop formation with and without hydrate formation in static mixers

The formation process of CO₂ drops in various types of Kenics Static Mixers was analyzed from the perspective of energy dissipation in the mixer, focusing on the formation of drop surfaces. Experimental studies on CO₂ drop formation were conducted under varying temperatures, pressure, and flow rates, with and without hydrate formation. Analysis of the CO₂ drop size and distribution at several locations within the static mixer was conducted, as of pressure drop in the mixer, to determine dissipation energies. In all the experimental conditions, by considering the surface energy for hydrate formation, the energy required for the formation of CO₂ drops correlated well with total energy dissipation by mixer flow, which is represented by a pressure drop along the mixer. This process has important applications to the formation of liquid CO₂ for ocean disposal as a countermeasure to global warming. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Effect of supercritical carbon dioxide on polystyrene extrusion

A study on the extrusion of polystyrene was carried out using supercritical carbon dioxide (scCO₂) as foaming agent. scCO₂ modifies the rheological properties of the material in the barrel of the extruder and acts as a blowing agent during the relaxation at the passage through the die. For experiments, a single-screw extruder was modified to be able to inject scCO₂ within the extruded material. The effect of operating parameters on material porosity was studied. Samples were characterized by using water-pycnometry, mercury-porosimetry and scanning electron microscopy. Polystyrene with expansion rate about 15–25% was manufactured. A rapid cooling just downstream the die is important to solidify the structure. The die temperature allows the control of the porosity structure. CO₂ concentration shows no significant influence.     

Characterization of PP/Mg(OH)2 and PP/Nanoclay Composites with Supercritical CO2 (scCO2)

In this article, supercritical carbon dioxide (scCO₂) is used to form a high density microcellular foam structure to reduce the polymer use and facilitate dispersion of Mg(OH)₂ and Nanoclay fillers. A twin-screw extruder system was used to predistribute the inorganic filler from the PP polymer, resulting composite PP/filler pellets. This followed by the use of a single-screw extruder wherein supercritical carbon dioxide is introduced in the formulation. Finally the resulting foam PP/filler/CO₂ pellets are injection molded into test samples. The structure and properties of the composites are characterized using a scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), and density measurements. Furthermore, PP/Clay/Mg(OH)₂ polymer composites are subjected to examinations to obtain their yield and tensile strengths, elasticity modulus, % elongation, Izod impact strength, hardness, Heat deflection temperature (HDT), Vicat softening point and Melt flow index (MFI).     

Measurements and modeling of PS/supercritical CO2 solution viscosities

This paper presents a technology to determine the melt viscosity of a PS/super-critical CO₂ solution using a linear capillary tube die mounted on a foaming extruder. CO₂ was injected into the extrusion barrel and the content of CO₂ was varied in the range of O to 4 wt% using a positive displacement pump. Single-phase PS/CO₂ solutions were formed using a microcellular extrusion system and phase separation was prevented by maintaining a high pressure in the capillary tube die. By measuring the pressure drop through the die, the viscosity of PS/CO₂ solutions was determined. The experimental results indicate that the PS/CO₂ solution viscosity is a senstive function of shear rate, temperature, pressure, and CO₂ content. A theoretical model based on the generalized Cross-Carreau model was proposed to describe the shear-thinning behavior of PS/CO₂ solutions at various shear rates. The zero-shear viscosity was modeled using a generalized Arrhenius equation to accommo-date the effects of temperature, pressure, and CO₂ content. Finally, the solubility of CO₂ has been estimated by monitoring the pressure drop and the absolute pressure in the capillary die.     

Modeling flow and cell formation in foam sheet extrusion of polystyrene with CO2 and co-blowing agents. Part II: Process model

In foam extrusion, process parameters, material properties, and the blowing agent have an influence on the resulting foam properties. For safety and environmental reasons, carbon dioxide (CO₂) has gained importance as physical blowing agent for the production of low-density polystyrene foam sheets. The sole use of CO₂ often leads to corrugation, open cell structures, or surface defects on the foam sheet. As an alternative, blowing agent mixtures based on CO₂ and organic solvents such as ethanol, acetone, or ethyl acetate can be used, changing solubility and flow behavior of the gas-loaded melt. A model approach for describing foam extrusion of polystyrene with various blowing agent mixtures in an annular gap die is developed. Part I of the paper describes the modeling of material properties. In Part II, the process model including nucleation and cell formation in the flow field is developed and applied to a foam sheet extrusion process. Based on the material model, melt flow and formation of cells are modeled by a step-wise calculation along the die, showing good agreement with experimental data. Dimensionless numbers are used to describe the foaming process and a parameter study based on these dimensionless numbers is presented.     

Visualisation of mechanisms involved in Co2 injection and storage in hydrocarbon reservoirsand water-bearing aquifers

Storage of carbon dioxide (CO₂) in hydrocarbon reservoirs and saline aquifers is considered as one of the promising mitigation strategies to reduce the negative impact of this greenhouse gas. The static and dynamic behaviour of CO₂ in these storage sites which are located at various depths and geographical locations, affects the efficiency of this strategy. Understanding the impact of the conditions of these storage sites on mechanisms involved in CO₂ flow, displacement and trapping is also critical for the purpose of site selection and the design of CO₂ storage projects. In this paper we report the results of a series of CO₂ injection (CO₂I) flow visualisation (micromodel) experiments conducted using high-pressure transparent porous media representing various aquifer and depleted oil reservoirs storage conditions. The impact of pertinent parameters on the interaction between the stored CO₂ and the reservoir fluids were investigated. Both sub-critical and supercritical CO₂ were used to investigate the effect of pressure (depth) of the storage site on CO₂ trapping mechanisms. A faster CO₂ breakthrough (BT) was observed in the micromodel test simulating CO₂I into depleted oil reservoirs, compared to that into aquifers, reducing the sequestration capacity of the depleted oil reservoirs. Compared to the injection of supercritical CO₂, the BT of gaseous CO₂ happened faster, adversely affecting the CO₂ displacement performance. The results of these direct flow visualization experiments significantly improve our understanding of the complex mechanisms and interactions involved in CO₂I and storage in geological formations. This knowledge is essential for identifying storage conditions that would lead to maximising CO₂ storage capacity, for better understanding the ultimate fate of the stored CO₂ and the storage safety.     

The numerical simulation of a new double swirl static mixer for gas reactants mixing

For the nitrogen oxide removal processes, high performance gas mixer is deeply needed for the injection of NH₃ or O₃. In this study, a new type of double swirl static mixer in gas mixing was investigated using computational fluid dynamics (CFD). The results obtained using Particle Image Velocimetry (PIV) correlated well with the results obtained from simulation. The comparisons in pressure loss between the experimental results and the simulation results showed that the model was suitable and accurate for the simulation of the static mixer. Optimal process conditions and design were investigated. When L/D equaled 4, coefficient of variation (COV) was < 5%. The inlet velocity did not affect the distributions of turbulent kinetic energy. In terms of both COV and pressure loss, the inner connector is important in the design of the static mixer. The nozzle length should be set at 4 cm. Taking both COV and pressure loss into consideration, the optimal oblique degree is 45°. The averaged kinetic energy changed according to process conditions and design. The new static mixer resulted in improved mixing performance in a more compact design. The new static mixer is more energy efficient compared with other SV static mixers. Therefore, the double swirl static mixer is promising in gas mixing.     

Processing of polypropylene–clay nanocomposites: Single‐screw extrusion with in‐line supercritical carbon dioxide feed versus twin‐screw extrusion

This work investigates two different melt‐blending strategies for preparing compatibilized polypropylene‐clay nanocomposites, specifically: (1) conventional twin‐screw extrusion, and (2) single‐screw extrusion capable of direct supercritical carbon dioxide (scCO₂) feed to the extruder barrel. Proportional amounts (3 : 1) of maleic anhydride functionalized polypropylene compatibilizer and organically modified montmorillonite clay at clay loadings of 1, 3, and 5 wt % are melt‐blended with a polypropylene homopolymer using the two approaches. The basal spacing, degree of exfoliation, and dispersion of organoclay is assessed using X‐ray diffraction, transmission electron microscopy, and rheology. In terms of the latter, both steady shear and small‐amplitude oscillatory shear provide information about the apparent yield stress and solid‐like terminal behavior respectively. Finally, nanoindentation is performed to determine the room temperature modulus of each melt‐blended nanocomposite. The results reveal unequivocally that the high shear of the twin‐screw process is vastly superior to the single‐screw with in‐line scCO₂ addition in generating well‐exfoliated, percolated polypropylene‐clay nanocomposites. It is likely that increased contact time between clay and scCO₂ is necessary for scCO₂ to positively affect exfoliation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 884–892, 2007