The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation

This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO₂–N₂ blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO₂–N₂ blends have been studied. It was observed that the 75% CO₂–25% N₂ blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N₂ and the high foam expanding ability of supercritical CO₂ would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO₂ increased the solubility of supercritical N₂ in PS, so the concentration of dissolved supercritical N₂ was higher than the prediction by the simple mixing rule. The additional supercritical N₂ further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO₂–N₂ composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics.     

Investigation of the nanocellular foaming of polystyrene in supercritical CO2 by adding a CO2‐philic perfluorinated block copolymer

Nanocellular foaming of polystyrene (PS) and a polystyrene copolymer (PS‐b‐PFDA) with fluorinated block (1,1,2,2‐tetrahydroperfluorodecyl acrylate block, PFDA) was studied in supercritical CO₂ (scCO₂) via a one‐step foaming batch process. Atom Transfer Radical Polymerization (ATRP) was used to synthesize all the polymers. Neat PS and PS‐b‐PFDA copolymer samples were produced by extrusion and solid thick plaques were shaped in a hot‐press, and then subsequently foamed in a single‐step foaming process using scCO₂ to analyze the effect of the addition of the fluorinated block copolymer in the foaming behaviour of neat PS. Samples were saturated under high pressures of CO₂ (30 MPa) at low temperatures (e.g., 0°C) followed by a depressurization at a rate of 5 MPa/min. Foamed materials of neat PS and PS‐b‐PFDA copolymer were produced in the same conditions showing that the presence of high CO₂‐philic perfluoro blocks, in the form of submicrometric separated domains in the PS matrix, acts as nucleating agents during the foaming process. The preponderance of the fluorinated blocks in the foaming behavior is evidenced, leading to PS‐b‐PFDA nanocellular foams with cell sizes in the order of 100 nm, and bulk densities about 0.7 g/cm³. The use of fluorinated blocks improve drastically the foam morphology, leading to ultramicro cellular and possibly nanocellular foams with a great homogeneity of the porous structure directly related to the dispersion of highly CO₂‐philic fluorinated blocks in the PS matrix. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Polypropylene‐dispersed domain as potential nucleating agent in PS and PMMA solid‐state foaming

The potential of using dispersive domains in a polymer blend as a bubble nucleating agent was investigated by exploiting its high dispersibility in a matrix polymer in the molten state and its immiscibility in the solid state. In this experiments, polypropylene (PP) was used as the nucleating agent in polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at the weight fraction of 10, 20, and 30 wt %. PP creates highly dispersed domains in PS and PMMA matrices during the extrusion processing. The high diffusivity of the physical foaming agent, i.e., CO₂ in PP, and the high interfacial tension of PP with PS and PMMA could be beneficial for providing preferential bubble nucleation sites. The experimental results of the pressure quench solid‐state foaming of PS/PP and PMMA/PP blends verified that the dispersed PP could successfully increase the cell density over 10⁶ cells/cm³ for PS/PP and 10⁷ cells/cm³ for PMMA/PP blend and reduce the cell size to 24 μm for PS/PP and 9 μm for PMMA/PP blends foams. The higher interfacial tension between PP and the matrix polymer created a unique cell morphology where dispersed PP particles were trapped inside cells in the foam. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Microcellular foaming of poly(phenylene sulfide)/poly(ether sulfones) blends using supercritical carbon dioxide

Microcellular foaming of poly(phenylene sulfide)/poly(ether sulfones) (PPS/PES) blends presents a promising approach to produce high‐performance cellular materials with tailored microstructures and enhanced properties. This study investigated the effects of multiphase blend composition and process conditions on the foaming behaviors and final cellular morphology, as well as the dynamic mechanical properties of the solid and microcellular foamed PPS/PES blends. The microcellular materials were prepared via a batch‐foam processing, using the environment‐friendly supercritical CO₂ (scCO₂) as a blowing agent. The saturation and desorption behaviors of CO₂ in PPS/PES blends for various blend ratios (10 : 0, 8 : 2, 6 : 4, 5 : 5, 4 : 6, 2 : 8, and 0 : 10) were also elaborately discussed. The experimental results indicated that the foaming behaviors of PPS/PES blends are closely related to the blend morphology, crystallinity, and the mass‐transfer rate of the CO₂ in each polymer phase. The mechanisms for the foaming behaviors of PPS/PES blends have been illustrated by establishing theoretical models. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42634.     

Diffusion and desorption of CO2 in foamed polystyrene film

Based on the existence of the pores in foamed polystyrene (PS), foamed‐non‐Fickian diffusion (FNFD) model was proposed, for the first time, to regress the desorption data obtained by gravimetric method. Results showed that FNFD model could accurately describe the diffusion behavior of CO₂ out of foamed PS, and well predict the solubility of CO₂ in foamed PS. The characterization of scanning electron microscopy indicated that there were abundant pores in the foamed PS, and the pores store most of CO₂, which would diffuse in the pores, adsorb to the wall of the pores, penetrate across walls of the pores, diffuse in the matrix of PS, and desorb out of PS. The mass of CO₂ in the pores of foamed PS was expressed as a function of foaming pressure and temperature according to foaming kinetics. Results showed that the values calculated by this function agreed well with the values obtained from the FNFD model. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45645.     

Measurement and prediction of LDPE/CO2 solution viscosity

When CO₂ is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO₂ solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO₂ was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO₂ in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO₂ concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO₂ solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO₂ solution was reduced to 30% of the neat polymer by dissolving CO₂ up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO₂ dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO₂ solutions were calculated to accommodate the effects of temperature, pressure and CO₂ content. The developed model can successfully predict the viscosity of LDPE/CO₂ solutions from PVT data of the neat polymer and CO₂ solubility data.     

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.     

Control of micro‐ and nanocellular structures in CO2 foamed PES/PEN blends

High performance thermoplastic blends based on polyethersulfone (PES) and poly(ethylene 2,6 naphthalate) (PEN) were foamed with supercritical CO₂ to develop high‐performance foams with improved heat deflection temperature, extended processing range, and controlled cellular morphology. The cellular morphology resulted to be strongly influenced by the initial morphology of the blend. The presence of small amount of PES as dispersed phase in PEN based blends acted as blowing agent reservoir and allowed to extend the processing temperature range for obtaining low density foams. The presence of PEN droplets in PES based systems extended the foaming temperatures towards lower values and allowed the development of a bimodal micro/nanocellular morphology at a foaming temperature 60°C lower than the PES glass transition temperature. Furthermore, the presence of PEN droplets compensated for the reduced capability of the host matrix to generate a fine and diffuse porosity at low foaming temperatures. POLYM. ENG. SCI., 55:1281–1289, 2015. © 2015 Society of Plastics Engineers     

Measurement and prediction of diffusion coefficients of supercritical CO2 in molten polymers

The solubility and diffusivity of supercritical carbon dioxide (sc‐CO₂) in low‐density polyethylene (LDPE), high‐density polyethylene (HDPE), polypropylene (PP), ethylene‐ethylacrylate copolymer (EEA) and polystyrene (PS) were measured at temperatures from 150°C to 200°C and pressures up to 12 MPa by using the Magnetic Suspension Balance (MSB), a gravimetric technique for gas sorption measurements. The solubility of CO₂ in each polymer was expressed by Henry's constant. The interaction parameter between CO₂ and polymer could be obtained from the solubility data, and it was used to estimate the Pressure‐Volume‐Temperature relationship and the specific free volume of polymer/CO₂ mixtures. The diffusion coefficients were also measured by the MSB for each polymer. The resulting diffusion coefficients were correlated with the estimated free volume of polymer/CO₂ mixture. Combining Fujita's and Maeda and Paul's diffusion models, a model was newly developed in order to predict diffusion coefficients for the polymers studied. Polym. Eng. Sci. 44:1915–1924, 2004. © 2004 Society of Plastics Engineers.     

Solubility and diffusion behavior of compressed CO2 in polyurethane oligomer

In CO₂-assisted polyurethane (PU) foaming, the solubility and diffusion coefficient of CO₂ is vitally important to the cell nucleation and growth. This work is aimed at the effect of molecular weights (M _n) and crosslinking densities (V _e) on the solubility and diffusion coefficient of CO₂ in PU oligomers. A series of PU oligomers with different M _n and V _e were synthesized. The solubility and diffusivity of CO₂ in PU oligomers were measured experimentally in the temperature from 80 to 140 °C and with pressures up to 15 MPa. It was shown that the solubility and diffusion coefficients of CO₂ was decreased 20.5 and 21.0%, respectively, with the M _n increasing from 5864 to 153,754 g mol⁻¹ at 80 °C, 15 MPa. The solubility and the diffusion coefficient also decreased 11.1 and 38.0% as the V _e was increased from 64 to 1493 mol m⁻³. Furthermore, the diffusion mechanism of CO₂ in PU oligomers was explored via molecular dynamics simulations. The results indicated that the calculated diffusivity of CO₂ showed the same changing trend as the experimental values, and the smaller M _n or crosslinking degree contributed to an increase in fractional free volume and stronger polymer–CO₂ interactions. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47100.     

Biopolymer foams from Novatein thermoplastic protein and poly(lactic acid)

A batch processing method is used to fabricate foams comprising of a blend of poly(lactic acid) (PLA) and Novatein, a protein‐based thermoplastic. Various compositions of Novatein/PLA are prepared with and without a compatibilizer, PLA grafted with itaconic anhydride (PLA‐g‐IA). Pure Novatein cannot form a cellular structure at a foaming temperature of 80 °C, however, in a blend with 50 wt % of PLA, microcells form with smaller cell sizes (3.36 µm) and higher cell density (8.44 × 10²¹ cells cm^?3) compared to pure PLA and blends with higher amounts of PLA. The incorporation of 50 wt % of semicrystalline Novatein stiffens the amorphous PLA phase, which restrains cell coalescence and cell collapse in the blends. At a foaming temperature of 140 °C, NTP₃₀–PLA₇₀ shows a unique interconnected porous morphology which can be attributed to the CO₂‐induced plasticization effect. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45561.     

Supercritical CO2–assisted synthesis and characterization of a polystyrene/thermal polyurethane blend

A blend of polystyrene and thermal polyurethane (PS/TPU) was prepared using supercritical (SC) CO₂ as a substrate‐swelling agent and monomer/initiator carrier. The SC CO₂/styrene/TPU ternary system was studied. Virgin TPU and synthesized blends were characterized through differential scanning calorimetry, infrared spectroscopy, rheometric measurements, and SEM. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2016–2020, 2005     

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.     

Hybrid FSC membrane for CO2 removal from natural gas: Experimental,process simulation,and economic feasibility analysis

The novel fixed‐site‐carrier (FSC) membranes were prepared by coating carbon nanotubes reinforced polyvinylamine/polyvinyl alcohol selective layer on top of ultrafiltration polysulfone support. Small pilot‐scale modules with membrane area of 110–330 cm² were tested with high pressure permeation rig. The prepared hybrid FSC membranes show high CO₂ permeance of 0.084–0.218 m³ (STP)/(m² h bar) with CO₂/CH₄ selectivity of 17.9–34.7 at different feed pressures up to 40 bar for a 10% CO₂ feed gas. Operating parameters of feed pressure, flow rate, and CO₂ concentration were found to significantly influence membrane performance. HYSYS simulation integrated with ChemBrane and cost estimation was conducted to evaluate techno‐economic feasibility of a membrane process for natural gas (NG) sweetening. Simulation results indicated that the developed FSC membranes could be a promising candidate for CO₂ removal from low CO₂ concentration (10%) NGs with a low NG sweetening cost of 5.73E?3 $/Nm³ sweet NG produced. © 2014 American Institute of Chemical Engineers AIChE J 60: 4174–4184, 2014     

Experimental study and thermodynamic model for respective solubilities of CO2 and ethanol for CO2‐ethanol‐polystyrene ternary system at elevated pressure and temperature

Foamed non‐Fickian diffusion (FNFD) model for a ternary system was proposed for the first time to regress the desorption data obtained by the gravimetric method. Results showed that FNFD model could accurately describe the diffusion behavior of CO₂ and ethanol out of foamed polystyrene (PS) and well predict total solubilities of CO₂ and ethanol in foamed PS. Meanwhile, Sanchez–Lacombe equation of state (S–L EoS) was adopted to calculate the respective solubilities (solubility of CO₂ in PS or solubility of ethanol in PS) and total solubilities of CO₂ and ethanol in PS for CO₂‐ethanol‐PS ternary system. Results showed that the total solubility of CO₂ and ethanol obtained from S–L EoS agreed well with values obtained by FNFD model. Furthermore, the respective and total solubilities of CO₂ and ethanol at 313.15, 338.15, and 343.15 K were calculated by S–L EoS. Results indicated that in the dissolving process, ethanol would be accelerated by CO₂ to dissolve into PS, and ethanol would compete with CO₂ to dissolve into PS, simultaneously. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46281.     

Modification of polycarbonate membrane by polyethylene glycol for CO2/CH4 separation

In this study, permeation of carbon dioxide (CO₂) and methane (CH₄) through the polycarbonate/polyethylene glycol (PC/PEG) blend membrane was investigated. The effect of PEG content (0–5 wt%) on the permeability and selectivity was studied. Permeability measurements were carried out at pressures of 1–7 bar and at room temperature. The membranes were characterized by Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR), X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and density measurement. The results revealed that the PC/PEG blends are miscible/partially miscible without considerable micro-phase separation. The effect of PEG content and gas pressure on the diffusion and solubility of coefficients were also investigated and analyzed. It was concluded that the most influential parameter for the permeation is the diffusion coefficient of the gases. The permeability and selectivity decrease as the operating pressure and PEG content are increased. Furthermore, the results showed that the addition of 5 wt% of PEG into PC increases the CO₂/CH₄ selectivity from 26.6 ± 0.99 to 40.9 ± 2.14 (more than 53%) at 1 bar.     

Open‐cell foams of polyethylene terephthalate/bisphenol a polycarbonate blend

Open microcellular foams of polyethylene terephthalate (PET)/polycarbonate (PC) blends were prepared by controlling their foaming behavior at the interface between these two polymers. Interface modification was a crucial factor in governing the foaming behavior and cell morphology of the blend foams: annealing at 280°C, i.e., conducting the transesterification reaction, generates a PET‐b‐PC copolymer, which lowers the interfacial tension, increases the affinity between PET and PC, and decreases the crystallinity of the PET domains. When CO₂ foaming was performed at the interface modified with the copolymer, an interesting fibril‐like structure was formed. The cell density of the PET/PC blend then increased, and its cell size reduced to the microscale while maintaining a high open‐cell ratio. The effect of heat annealing (transesterification reaction) on CO₂‐foaming was studied to reveal the relationship among the interface affinity, crystallinity, and degree of fibrillation. The optimal heat‐annealing procedure generated a fibril‐like structure in the PET/PC blend foams with a high cell density (7 × 10¹¹ cm^?3), small cell size (less than 2 μm), and 100% open‐cell ratio. POLYM. ENG. SCI., 55:375–385, 2015. © 2014 Society of Plastics Engineers     

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.     

Preparation and properties of foamed cellulose acetate/polylactic acid blends

In order to improve the foaming performance of pure cellulose acetate (CA), blends were prepared by mixing polylactic acid (PLA) in CA and foamed by supercritical CO₂ (ScCO₂) in this study. The effect of PLA content (percentage by mass of blend) on structure, thermal properties, rheological properties, foaming properties and mechanical properties of the blends was investigated. The results showed that the addition of PLA destroyed the original hydrogen bonds of CA, while the blends had good crystallization properties. At the same time, compared with pure CA, the glass transition temperature (T_g) of the blends decreased, and the initial decomposition temperature (T₀) was reduced from 349.41°C (pure CA) to 334.68°C (CA/20%PLA). In addition, the rheological properties of the blends were improved, and the viscosity was reduced, which was obviously beneficial to foaming process. The pore size and density of the foamed blends both reached the maximum value at 20%PLA. The presence of PLA could degrade the mechanical properties of the blends. However, the overall drop (1.01 KJ/m²) of impact strength of the blends after foaming is much smaller than that before foaming (12.11 KJ/m²), indicating that the improvement of foaming performance was beneficial to improve its impact strength.     

Preparation of polyethyleneglycol (PEG) and cellulose acetate (CA) blend membranes and their gas permeabilities

Miscible blend membranes containing 10 wt % PEG of low molecular weight 200, 600, 2000, and 6000, and 10 wt %, 20 wt %, 30 wt %, 40 wt %, 50 wt %, and 60 wt % of molecular weight 20,000 were prepared to investigate the effect of PEG on gas permeabilities and selectivities for CO₂ over N₂ and CH₄. The permeabilities of CO₂, H₂, O₂, CH₄, and N₂ were measured at temperatures from 30 to 80°C and pressures from 20 cmHg to 76 cmHg using a manometric permeation apparatus. It was determined that the blend membrane, which contained 10% PEG 20,000, exhibited higher permeability for CO₂ and higher permselectivity for CO₂ over N₂ and CH₄ than those of the membranes that contained 10% PEG of the molecular weight ranging from 200 to 6000. The high PEG 20,000 content blend membranes showed remarkable permeation properties such that the permeability coefficients of CO₂ and the ideal separation factors for CO₂ over N₂ reached above 200 barrer and 22, respectively, at 70°C and 20 cmHg. Based on the data of gas permeability coefficients, time lags, and characterization of the membranes, it is proposed that the apparent solubility coefficients of all CA and PEG blend membranes for CO₂ were lower than those of the CA membrane. However, almost all of the blend membranes containing PEG 20,000 showed higher apparent diffusivity coefficients for CO₂, resulting in higher permeability coefficients of CO₂ than those of the CA membrane. It is attributed to the high diffusivity selectivities of CA and PEG 20,000 blend membranes that their ideal separation factors for CO₂ over N₂ were higher than those of the CA membrane in the temperature range from 50 to 80°C, even though the ideal separation factors of all CA and PEG blend membranes for CO₂ over CH₄ became lower than those of the CA membrane over nearly the full temperature range from 30 to 80°C. © 1995 John Wiley & Sons, Inc.