Phase equilibria of poly(methyl methacrylate) in supercritical mixtures of carbon dioxide and chlorodifluoromethane

Phase behavior data are presented for poly(methyl methacrylate) (PMMA: M_w= 15,000, 120,000) in supercritical solvent mixtures of carbon dioxide (CO₂) and chlorodifluoromethane (HCFC-22). Experimental cloud point curves, which were the phase boundaries between single and liquid-liquid phases, were measured by using a high-pressure equilibrium apparatus equipped with a variable-volume view cell at various CO₂ compositions up to about 63 wt% (on a polymer-free basis) and at temperatures up to about 100 °C. The cloud point curves exhibited the characteristics of a lower critical solution temperature phase behavior. As the CO₂ content in the solvent mixture increased, the cloud point pressure at a fixed temperature increased significantly. Addition of CO₂ to HCFC-22 caused a lowering of the dissolving power of the mixed solvent due to the decrease of the solvent polarity. The cloud point pressure increased with increasing the molecular weight of PMMA.     

Thermal behaviors of polystyrene plasticized with compressed carbon dioxide in a sealed system

The thermal behavior of polystyrene (PS) plasticized with compressed carbon dioxide (CO₂) was studied using differential scanning calorimetry with a high‐pressure stainless steel pan in a sealed system. The technique proved to be a simple and convenient way to study the thermal behavior of a polymer plasticized with compressed CO₂ at pressures up to 100 atm, which covers both the gas and supercritical states. A sharp fall in the decrease rate of the glass transition temperature (T_g) under conditions near the critical point of compressed CO₂ was firstly observed, which corresponded with the solubility of CO₂ in PS. Since the system is scaled, which results in a stable pressure at a certain temperature, it is more suitable to study the effect of annealing. An endotherm was detected after the PS was annealed at a temperature below its T_g under compressed CO₂. The enthalpy of this endotherm increased linearly with increasing logarithm of annealing time under a certain pressure. The endotherm was affected by two thermodynamic equilibrations at a temperature below its T_g: (i) enthalpy relaxation of the PS; and (ii) the absorption/desorption of CO₂. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Phase behavior and density of polysulfone in binary fluid mixtures of tetrahydrofuran and carbon dioxide under high pressure: Miscibility windows

Mixtures of tetrahydrofuran (THF) and carbon dioxide (CO₂) were identified as new solvent systems for polysulfone. The miscibility and density of polysulfone in binary fluid mixtures of THF and CO₂ were investigated from 300 to 425 K at pressures up to 70 MPa. The influence of the CO₂ and polysulfone concentrations was studied, with the concentrations of the other two components kept constant. At a 4.5 wt % polymer concentration, the demixing pressures in a 10 wt % CO₂ and 90 wt % THF mixture increased with temperature (310–425 K) from 15 to 40 MPa. With increasing CO₂ concentration (from ca. 10 to 14 wt %), a significant increase (from 15 to 70 MPa at 310 K) was observed in the demixing pressures. Furthermore, with an increasing amount of CO₂, the nature of the phase boundary shifted from lower critical solution temperature behavior to upper critical solution temperature behavior. The influence of the polymer concentration was studied in the 0–5 wt % range at two CO₂ levels, with solvent compositions of 10 wt % CO₂ and 90 wt % THF and 13 wt % CO₂ and 87 wt % THF. The system with a higher level of CO₂ (13 wt %) showed highly unusual phase behavior: on pressure–composition and temperature–composition diagrams, the system displayed two distinct regions of miscibility. In the system with 10 wt % CO₂, the distinct regions of miscibility that were observed in the system with 13 wt % CO₂ partially overlapped and led to a W‐shape phase boundary. The densities of the polymer solutions were measured from the one‐phase region through the demixing point into the two‐phase region at a constant temperature. No significant change in density was found around the phase boundary; this indicated that the coexisting phases had similar densities, as is often the case with liquid–liquid phase separation in polymer solutions under high pressure. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2357–2362, 2002     

Comparative Study on Thermodynamic Analysis of CO2 Utilization Reactions

Thermodynamic analysis of dimethyl ether (DME) and methane synthesis from CO₂ hydrogenation, and dry reforming of methane was performed using Gibbs free energy minimization. The effects of temperature, pressure, and feed composition on conversion, selectivity, and yield were investigated for each process. High pressure, high H₂/CO₂ ratio, and low temperature favored DME production. The yield of methane during CO₂ methanation increased at lower temperature, higher pressure, and H₂/CO₂ ratio. The yield of synthesis gas improved at higher temperature. Comparison of the three processes demonstrated that the CO₂ conversion was highest during CO₂ methanation reaction if the fraction of CO₂ mol in the feed was less than 0.3. Above this value in the feed, dry reforming allowed the highest CO₂ conversion.     

Phase behavior on the binary and ternary mixtures of poly(cyclohexyl acrylate) and poly(cyclohexyl methacrylate) in supercritical CO2

High‐pressure phase behavior was measured for the CO₂–cyclohexyl acrylate and CO₂–cyclohexyl methacrylate system at 40, 60, 80, 100, and 120°C and pressure up to 206 bar. This system exhibits type I phase behavior with a continuous mixture‐critical curve. The experimental results for the CO₂–cyclohexyl acrylate and CO₂–cyclohexyl methacrylate system were modeled using the Peng–Robinson equation of state. Experimental cloud‐point data, at a temperature of 250°C and pressure of 2800 bar, were presented for ternary mixtures of poly(cyclohexyl acrylate)–CO₂–cyclohexyl acrylate and poly(cyclohexyl methacrylate)–CO₂–cyclohexyl methacrylate systems. Cloud‐point pressures of poly(cyclohexyl acrylate)–CO₂–cyclohexyl acrylate system were measured in the temperature range of 40 to 180°C and at pressures as high as 2200 bar with cyclohexyl acrylate concentrations of 22.5, 27.4, 33.2, and 39.2 wt %. Results showed that adding 45.6 wt % cyclohexyl acrylate to the poly(cyclohexyl acrylate)–CO₂ mixture significantly changes the phase behavior. This system changed the pressure–temperature slope of the phase behavior curves from the upper critical solution temperature (UCST) region to the lower critical solution temperature (LCST) region with increasing cyclohexyl acrylate concentration. Poly(cyclohexyl acrylate) did not dissolve in pure CO₂ at a temperature of 250°C and pressure of 2800 bar. Also, the ternary poly(cyclohexyl methacrylate)–CO₂–cyclohexyl methacrylate system was measured below 187°C and 2230 bar, and with cosolvent of 27.4–46.7 wt %. Poly(cyclohexyl methacrylate) did not dissolve in pure CO₂ at 240°C and 2500 bar. Also, when 53.5 wt % cyclohexyl methacrylate was added to the poly(cyclohexyl methacrylate)–CO₂ solution, the cloud‐point curve showed the typical appearance of the LCST boundary. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1117–1125, 2004     

Effect of dissolved CO2 on the crystallization behavior of linear and branched PLA

In this study, the crystallization behavior of polylactide (PLA) was investigated in the presence of dissolved CO₂ using high-pressure and regular differential scanning calorimeter. The isothermal and non-isothermal melt crystallization results showed that increasing the CO₂ pressure decreased the crystallization half-time. During isothermal and low-cooling-rate non-isothermal crystallization, a very high crystallinity was achieved at 15 bar CO₂ pressure by facilitating more perfect crystal formation with the plasticization effect of CO₂ while limiting the crystal nucleation rate. At higher CO₂ pressures, a larger number of less close-packed crystals were formed due to chain entanglement, and consequently, the final crystallinity was decreased. The non-isothermal results at high cooling rates showed the total crystallinity decreased for all CO₂ contents, because of less time given for crystallization. Also the effects of the CO₂ pressure and the cooling rate on T_c and T_g were investigated.     

Phase equilibrium characteristics of supercritical CO2/poly(ethylene terephthalate) binary system

The solubility of carbon dioxide in poly (ethylene terephthalate) (PET) at high pressure and elevated temperature conditions was investigated for a better understanding of the phase equilibrium characteristics of supercritical CO₂/PET binary system and useful data for the process development of the supercritical fluid dyeing. Based on the principle of pressure decaying, a novel experimental apparatus suitable to high pressure and high temperature measurement was established. The solubilities of CO₂ in PET were measured with the apparatus at temperatures of 110, 120, and 130°C and pressures up to 30.0 MPa. The results show that the solubility of CO₂ in PET increases with the increase of pressure and CO₂ density, respectively, at a constant temperature, whereas it decreases with the increase of temperature at a constant pressure. The Sanchez‐Lacombe equation of state (S‐L EOS) was used to correlate the experimental data. The calculated results are in good agreement with the experimental ones. The average absolute relative derivation (AARD) is less than 3.91%. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

Ductile‐to‐brittle transition behavior of low molecular weight polycarbonate under carbon dioxide

We investigated the stress–strain behavior of low molecular weight polycarbonate for optical disc grade (OD‐PC) under carbon dioxide (CO₂) at various pressures, and compared the results with that under ambient pressure at various temperatures. Elongation at break decreased sharply with increased CO₂ pressure at around 2 MPa, while the elastic modulus decreased gradually up to 6 MPa. These results indicate that the tensile property changed from ductile to brittle with increased CO₂ pressure, although the molecular motion is accelerated due to the plasticization effect of CO₂. Such ductile‐to‐brittle transition is similar to that observed under elevated temperatures caused by chain disentanglement due to accelerated molecular motion. Although the changes of tensile properties were similar, the craze structure obtained by the brittle behavior was different, i.e., a filamented‐craze structure was obtained under high‐pressure CO₂, while a lace‐like one was obtained under elevated temperatures. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers     

Synthesis of sago starch laurate in densified carbon dioxide

Fatty acid starch esters are potential candidates for novel biodegradable plastics. This work describes a systematic study on the synthesis of starch laurate using sago starch and vinyl laurate (VL) in densified CO₂ as a green solvent. The phase behavior of the CO₂–VL system was investigated in a high pressure view cell and the critical point of the CO₂‐VL mixtures was shown to increase with temperature. Within the experimental window, sago starch laurate with a maximum degree of substitution (DS) of 0.97 is obtained. To the best of our knowledge, such high DS values have never been reported before for reactions in densified CO₂. Moreover, the presence of laurate chains in the starch backbone has a profound influence on the degree of crystallinity, the melt and crystallization temperature, and the degradation temperature of the final products. POLYM. ENG. SCI., 58:291–299, 2018. © 2017 Society of Plastics Engineers     

Viscosity and rheological behavior of carbon dioxide-expanded fish oil fatty acid ethyl esters: Measurement using a rotational viscometer and modeling

The viscosity of fish oil fatty acid ethyl esters (FAEE) in equilibrium with CO₂ was determined at 40, 55, and 70 °C and pressures ranging from 0.1 to about 12 MPa using a rotational rheometer equipped with a high pressure cell. Viscosity of CO₂-expanded (CX) FAEE decreased due to the dissolution of CO₂ with pressure, which was temperature dependent. The viscosity of CX FAEE was correlated using a new empirical model utilizing saturation pressure and temperature with temperature-dependent parameters, which allows interpolation of viscosity data for any pressure and temperature within the envelope of this study. As well, viscosity was modeled using available data for CO₂ solubility in FAEE at specific temperatures of this study utilizing simpler models with only one parameter, such as the Grunberg and Nissan model. The rheological data suggest shear thickening behavior of CX FAEE at elevated pressures. The limitations of the rotational rheometer for measuring CX liquids at viscosities below 1 mPa s are discussed. Understanding of viscosity and rheological behavior of CX FAEE is essential for equipment and process design involving such lipids.     

Attempt to explain the changes in solvation of polystyrene in supercritical CO2/ethanol mixtures using infrared and Raman spectroscopy

The polystyrene//CO₂/ethanol system has been investigated in the supercritical domain using infrared absorption (FTIR) and Raman scattering spectroscopies. The pressure and temperature were fixed in the range 20–35 MPa at 423 K for CO₂/ethanol mixtures with proportion in weight from (70/30) to (58/42). These conditions are those used in our previous study concerning the fractionation of this macromolecule. A surprising finding reported before was that the CO₂/ethanol mixture made of two non-solvents of PS at ambient conditions is able to dissolve PS chains under high temperature and pressure. In the present article, this question will be addressed at a microscopic level from a discussion of the intermolecular interactions between the polymer and the medium and between the CO₂ and ethanol components provided by the analysis of the spectra.     

Improved resistance against coke deposition of titania supported cobalt and nickel bimetallic catalysts for carbon dioxide reforming of methane

The catalytic behavior of bi-metallic Co–Ni/TiO₂ catalysts for CO₂ reforming of CH₄ to synthesis gas was investigated under atmospheric pressure with a particular attention to carbon deposition. The catalysts with optimized Co/Ni ratios showed high catalytic stability towards the reaction with very little amount of deposited carbon at a wide range of reaction temperature (773–1123 K). The results suggest that adjusting of composition of the active metals (Co and Ni) can kinetically control the elementary steps (formation of carbon species and its removal by oxygen species) of CH₄/CO₂ reaction.     

Low temperature melting behavior of CO2 crystallized modified PETs

Differential scanning calorimetry (DSC) analysis has been performed on modified and commericial amorphous samples of poly(ethylene terephthalate) (PET) crystallized by high pressure carbon dioxide (CO²). Two endothermic peaks are present in the DSC scans of all the carbon dioxide-treated samples. A qualitatively analogous behavior has been detected in the case of amorphous samples heat treated at temperatures slightly exceeding the glass transition temperature of virgin material. Wide angle X-ray scattering analysis has confirmed the structural analogies between samples CO₂ crystallized at 50°C and thermally crystallized slightly above T_g. A differential scanning calorimeter capable of working at high pressure of CO₂ has been adopted in order to examine the effect of carbon dioxide on the crystallization temperature range.     

Preparation and properties of catalyzed polyimide/dicyanate semi‐interpenetrating networks for polymer gas membrane with suppressed CO2‐plasticization

The work presents an approach to reduce the plasticization of polymeric membranes caused by condensable gases, and particularly the effect of plasticization caused on polyimides by CO₂ at high pressure. A technical polyimide, Matrimid^®, was chosen as a reference of polyimide membrane and the approach applied consisted of incorporating reactive oligomers to have cross‐linkable mixed systems, which do not plasticize at high CO₂ pressure. Films of semi‐interpenetrating networks (semi‐IPNs) based on Matrimid^® and phenolphthalein dicyanate as cross‐linking monomer in ratios 90/10, 80/20, and 70/30, were prepared using a catalyst to lower the curing temperature from 280 to 180°C. Semi‐IPNs properties such as thermal stability, mechanical properties, glass transition temperatures, or density were measured to characterize the films and were correlated with the dicyanate monomer content. The CO₂ gas permeation behavior of the three semi‐IPNs was studied using a CO₂ feed pressure ranging from 1 to 30 atm. The gas separation properties were mainly explained attending to the density of the films, which depended on the dicyanate content used. In the three catalyzed semi‐IPNs, the CO₂ permeability coefficients remained almost constant all along the investigated range of CO₂ pressure while Matrimid^® treated at 180°C did show a clear tendency to plasticization over a critical feed pressure of about 17 bar. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of pressure and temperature on fixed-site carrier membrane for CO2 separation from natural gas

In this paper, the effect of testing temperature on the performance of fixed carrier membrane for CO₂ separation were studied. The blend composite membranes were developed respectively with a blend of PEI-PVA (polyetheleneimine-polyvinyl alcohol) as separation layer and PS (polysulfone) ultrafiltration membranes as the substrates. The permselectivity of the membranes was measured with CO₂/CH₄ mixed gas. The effect of testing temperature on membrane separation performance was investigated. The results showed that both the permeances of CO₂ and CH₄ decreased with the increase of temperature, and the permeances decreased more quickly under low pressure than those under high pressure. At the feed pressure of 0.11 MPa, the CO₂/ CH₄ selectivity of PEI-PVA/PS blend composite membrane reduced along with temperature increment. Under the feed pressure of 0.21 MPa, as well as 1.11 MPa, the selectivity decreased with the increase of temperature.     

Phase behavior on the binary and ternary mixtures of poly(isooctyl acrylate) + supercritical fluid solvents + isooctyl acrylate and CO2 + isooctyl acrylate system

Experimental cloud‐point data to the temperature of 180 °C and the pressure up to 2000 bar are presented for ternary mixtures of poly(isooctyl acrylate) + supercritical fluid solvents + isooctyl acrylate systems. Cloud‐point pressures of poly(isooctyl acrylate) + CO₂ + isooctyl acrylate system is measured in the temperature range of 60–180°C and to pressures as high as 2000 bar with isooctyl acrylate concentration of 0–44.5 wt. This system changes the pressure–temperature slope of the phase behavior curves from upper critical solution temperature (UCST) region to lower critical solution temperature (LCST) region as the isooctyl acrylate concentration increases. Poly(isooctyl acrylate) does dissolve in pure CO₂ to the temperature of 180°C and the pressure of 2000 bar. The phase behavior for poly(isooctyl acrylate) + CO₂ + 9.5, 14.8, 30.6, and 41.9 wt % dimethyl ether (DME) mixture show the curve changes from UCST to LCST as the DME concentration increases. Also, the cloud‐point curves are measured for the binary mixtures of poly(isooctyl acrylate) in supercritical propane, propylene, butane, and 1‐butene. High pressure phase behaviors are measured for the CO₂ + isooctyl acrylate system at 40, 60, 80, 100, and 120°C and pressure up to 200 bar. This system exhibits type‐I phase behavior with a continuous mixture‐critical curve. The experimental results for the CO₂ + isooctyl acrylate system are modeled using the Peng‐Robinson equation of state. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Crystallization kinetics of poly(L‐lactide) in contact with pressurized CO2

The effect of CO₂ on the isothermal crystallization kinetics of poly(L‐lactide), PLLA, was investigated using a high‐pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO₂. It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO₂ dissolved in the PLLA. The crystallization rate was accelerated by CO₂ at the temperature in the crystal‐growth rate controlled region (self‐diffusion controlled region), and depressed in the nucleation‐controlled region. CO₂ has also decreased the glass transition temperature, T_g, and the melting temperature, T_m. As a result, the CO₂‐induced change in the crystallization rate can be predicted from the magnitudes of depression of both T_g and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO₂ were also investigated using a wide angle X‐ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO₂, although the presence of CO₂ did not change the crystalline structure.     

Bubble-point measurement for the CO2+diethylene glycol diacrylate and CO2+diethylene glycol dimethacrylate systems at high pressure

Pressure-composition isotherms are measured by using a static apparatus for the phase behavior data for the CO₂+diethylene glycol diacrylate (DEGDA) and CO₂+diethylene glycol dimethacrylate (DEGDMA) systems. The experiments are performed at five temperatures of (313.2 to 393.2) K and pressures up to 28.3 MPa. The solubility of CO₂ for the two systems decreases as the temperature increases at a fixed pressure. The CO₂+DEGDA and CO₂+ DEGDMA systems exhibit type-I phase behavior. The experimental results for the CO₂+DEGDA and CO₂+DEGDMA systems are correlated with Peng-Robinson equation of state using a mixing rule.     

Phase equilibria predictions for the CO2-H2O-NaCl-bitumen system

Predictions of vapour-liquid, liquid-liquid and vapour-liquid-liquid phase equilibria are presented for the quaternary CO₂-H₂O-NaCI-Peace River bitumen system at reservoir conditions. Henry's law correlations are regressed from available data for the CO₂-H₂O, CO₂-(H₂O + NaCl) and CO₂-bitumen binary sub-systems. These correlations are incorporated into a model for predicting the phase behaviour of the quaternary system at temperatures of 80-200°C, NaCl concentrations of 0-10.5 mass% and pressures up to 10 MPa. It is shown that temperature, salt concentration and the global CO₂ concentration significantly affect the distribution of CO₂ in the liquid-liquid region. The proposed model is simple and computationally less demanding than those based on equations of state.