Properties of thin Teflon AF 2400 coatings deposited onto carbon fabric from solutions in supercritical carbon dioxide

For the design of hydrophobized conductive gas diffusion layers of the electrodes of polymercontaining fuel cells, of great interest is the method for the formation of a fluoropolymer film from fluoropolymer solution in supercritical carbon dioxide. The present work describes the systematic studies on the carbon-fabric-deposited Teflon AF 2400 coatings obtained by this method. The electrical and geometric characteristics of the coatings, their elemental compositions, and stabilities are studied. It is shown that supercritical carbon dioxide allows the deposited fluoropolymer to penetrate homogeneously into the depth of carbon fabric to form a uniform coating around single individual fibers. The optimum limits of variation in the Teflon AF 2400 amount upon production of the gas diffusion layers of polymer electrolyte, alkaline, and phosphoric acid fuel cells are determined.     

Properties of fractions of ultradisperse polytetrafluoroethylene soluble in supercritical carbon dioxide

The morphology and structure of FORUM ultradisperse polytetrafluoroethylene fractions obtained via separation of the initial sample with supercritical carbon dioxide are studied by means of scanning and transmission electron microscopy, thermogravimetric and X-ray structural analysis, and IR and NMR spectroscopy. The lowest molecular mass fractions of the ultradisperse polytetrafluoroethylene are involved in the dissolution process, while an increase in production parameters (pressure and temperature) entails gradual involvement of higher molecular mass oligomers in this process. On the basis of the experimental data, the chain length of oligomers soluble in supercritical carbon dioxide is estimated.     

Grafting of glycidyl methacrylate onto polypropylene using supercritical carbon dioxide

Free-radical grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) films has been studied using supercritical carbon dioxide (SC-CO₂) as a solvent and a swelling agent. As the reaction temperature was below the melting point, PP was modified in the solid phase. The PP film was first soaked with the monomer GMA and benzoyl peroxide (BPO) as an initiator using SC-CO₂ at different experimental conditions of pressure, temperature, and thermal treatment time. After releasing CO₂, film GMA molecules were grafted onto PP in different times. Using this method, the degree of grafting and the morphology could be controlled through the combination of pressure, temperature, and soaking time. FTIR spectra confirmed that GMA had been grafted onto PP and that polypropylene-graft-glycidyl methacrylate (PP-g-GMA) presented a high surface reactivity for conductive polyaniline anchoring. DSC measurements and TG analyses showed that the thermal profiles of the graft copolymer and virgin PP are quite similar and that the graft PP does not exhibit changes in terms of thermal degradation profile and melting temperature, respectively. X-ray data showed that a high degree of grafting leads to a lower degree of crystallinity of polypropylene.     

Grafting of methyl methacrylate onto natural rubber in supercritical carbon dioxide

The grafting of the methyl methacrylate (MMA) monomer onto natural rubber (NR) was carried out by supercritical carbon dioxide (scCO₂) swelling polymerization with benzoyl peroxide (BPO) as an initiator. Fourier transform–infrared spectroscopy (FT–IR) was used to confirm the formation of graft copolymers with the characteristic bands of symmetric C?O and C? O? C stretching vibrations at 1728 cm^?1 and 1147 cm^?1, respectively. The effects of the rubber‐to‐monomer ratio, amount of initiator, reaction time, and pressure on the monomer grafting level (GL) and grafting efficiency (GE) were investigated, and the optimum conditions for the preparation of NR‐g‐MMA were found to be 70:30 of the rubber‐to‐monomer ratio, 1.2% of the initiator content, and the reaction pressure of 23 MPa for 6 h. The thermal behavior of the NR and the different NR/MMA molar ratio grafted copolymer samples was studied by differential scanning calorimetry (DSC). The observed glass transition temperature (T_g) was consistent with the GL. The tensile strength, modulus of elasticity, elongation at break, hardness, and oil resistance of graft copolymers were determined and compared with the values of NR and that of polymerization products prepared in traditional toluene solution. The results showed that the tensile strength, modulus of elasticity, hardness and oil resistance were greatly improved after modification in scCO₂. Copyright © 2007 John Wiley & Sons, Ltd.     

Structures and thermophysical properties of ultradispersed polytetrafluoroethylene and its fractions obtained in supercritical carbon dioxide

The thermal properties and morphology of ultradispersed polytetrafluoroethylene, as well as those of its soluble and insoluble fractions isolated in supercritical carbon dioxide at 75°C and 30 MPa, are investigated. It is found that, in fact, the soluble fraction contains low-molecular-mass and high-molecular-mass fractions that have been previously obtained via pyrolysis in the temperature ranges 50–150 and 120–300°C, respectively. With the use of thermogravimetric analysis, differential scanning calorimetry, and powder X-ray diffraction, it is shown that the initial polymer and both fractions have semicrystalline structures and that the crystallinity of the insoluble fraction is weaker than that of the soluble fraction because of its higher molecular mass. The crystal lattice of the polymer is monoclinic with equal parameters а and с and angle β being approximately 120°. The soluble fraction does not show phase transitions during heating to the onset of mass loss. As a result of separation of the low-molecular-mass fraction, the phase-transition temperature increases from–33 to–20°C.     

The phase behavior of fluorinated diols,divinyl adipate and a fluorinated polyester in supercritical carbon dioxide

The use of supercritical carbon dioxide as a reaction medium for polyester synthesis is hindered by the low solubility of diols in CO₂. However, it has been previously demonstrated that fluorinated compounds can exhibit greater miscibility with carbon dioxide than their hydrocarbon analogs. Therefore, the phase behavior of fluorinated diols and divinyl adipate (DVA), an activated diester, in supercritical carbon dioxide has been investigated at 323 K. The phase behavior of equimolar mixtures of DVA with the most carbon dioxide-soluble diol, 3,3,4,4,5,5,6,6-octafluorooctan-1,8-diol (OFOD), was also determined. The solubility of a polyester synthesized from DVA and 2,2,3,3-tetrafluoro-1,4-butanediol (TFBD) was found to be less CO₂-soluble than its monomers. DVA was much more soluble in CO₂ than any of the fluorinated diols, therefore, no attempt was made to fluorinate the DVA structure. Because both substrates and polyester product were soluble in carbon dioxide, the enzymatic synthesis of a fluorinated polyester from DVA and octafluorooctandiol was performed in supercritical carbon dioxide, resulting in a polymer with a weight average molecular weight of 8232 Da.     

在超临界CO2流体中的化学反应

超临界CO2流体中的化学反应是继超临界流体应用于萃取分离过程之后进一步将其应用于化学反应的新尝试。本文重点综述了超临界CO2流体中的化学反应研究进展, 并对其发展前景作了展望。     

Drawing of ultrahigh molecular weight polyethylene fibers in the presence of supercritical carbon dioxide

The drawing behavior of ultrahigh molecular weight polyethylene fibers in supercritical carbon dioxide (scCO₂) is compared to that in air at different temperatures. The temperature substantially influences the drawing properties in air, whereas in scCO₂, a constant draw stress and tensile strength are observed. Differential scanning calorimetry shows an apparent development of a hexagonal phase along with a significant increase in the crystallinity of air‐drawn samples with increasing temperature. The existence of this phase is not confirmed by wide‐angle X‐ray scattering, which instead shows that air‐drawn samples crystallize in an internally constrained manner. In contrast, scCO₂ allows crystals to grow without constraints through a possible crystal–crystal transformation, increasing the processing temperature to 110 °C. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1375–1383, 2003     

Dissolution of uranium dioxide in supercritical fluid carbon dioxide

Uranium dioxide can be dissolved in supercritical CO2 with a CO2-philic TBP-HNO3 complexant to form a highly soluble UO2(NO3)(2).2TBP complex; this new method of dissolving UO2 that requires no water or organic solvent may have important applications for reprocessing of spent nuclear fuels and for treatment of nuclear wastes.     

Extraction of atabron residues from cabbage with supercritical carbon dioxide

Summary In this work we present a new procedure for extraction of Atabron residues from cabbage samples using supercritical carbon dioxide. The results obtained with this procedure were compared with those obtained using traditional extraction methods for Atabron residues. The extraction yields found by both procedures are similar but SFE has several advantages, especially higher speed, greater economy, better selectivity and greater reproducibility.     

Synthesis of dimethyl carbonate from methanol and supercritical carbon dioxide

The synthesis of dimethyl carbonate (DMC) from methanol and supercritical carbon dioxide over various base catalysts has been studied. Compounds of group-I elements (Li, Na and K) were used as base catalysts. The promoter and the dehydrating agent were also used to enhance the yield of DMC. The effects of the catalysts, promoter and dehydrating agent on the yield of DMC were investigated. By-products such as dimethyl ether (DME) and C₁–C₂ hydrocarbons were formed with the DMC as a main product. The yield of DMC with different alkali metal catalysts ranked in the following order: K > Na > Li. The catalysts of the metal-CO₃ compounds were more effective than the metal-OH compounds in DMC synthesis. The maximum DMC yield reached up to about 12 mol% in the presence of K₂CO₃ (catalyst), CH₃I (promoter) and 2,2-dimethoxypropane (dehydrating agent) at 130–140°C and 200 bar. The reaction mechanism of DMC synthesis from methanol and supercritical carbon dioxide was proposed.     

Monoacetylferrocene crystallization in supercritical carbon dioxide

An organometallic compound, monoacetylferrocene, was for the first time obtained as single crystals by crystallization from supercritical carbon dioxide. This offers the possibility of utilizing supercritical media for efficient crystallization and purification of organometallic compounds without using organic solvents. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 555–557, March, 2006.     

超临界二氧化碳介质中有机化学反应研究

综述了本研究小组近年来在超临界二氧化碳介质中过渡金属催化的有机化学反应的研究结果,主要包括烯烃和炔烃的羰基化反应、氧化反应、自由基反应、低聚反应。     

Olefin metathesis in supercritical carbon dioxide

Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy(3) or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO(2) as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by (1)H NMR data obtained in compressed CO(2). Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.     

超临界CO_2萃取甾醇的研究(1)

本文用超临界ＣＯ＿２对高酸价麦胚油进行选择住萃取，结果表明，萃取温度和压力不同时，萃取物中甾醇的含量不同．且甾醇混合物主要为β－谷甾醇和菜籽甾醇。萃取物的酸价也明显高于高酸价麦胚油。     

Crystallization of bisphenol a polycarbonate induced by supercritical carbon dioxide

Supercritical carbon dioxide readily induces crystallization in bisphenol A polycarbonate. Crystallization begins within one h of exposure to the CO₂ at temperatures and pressures as low as 75°C and 100 atm. The degree of crystallinity increases sharply as the CO₂ pressure is raised from 100 to 300 atm but levels off thereafter. This behavior is likely due to a minimum in the T_g of the polycarbonate/CO₂ mixture owing to the opposite effects of the pressure on the T_g of the polymer and on the equilibrium weight fraction of CO₂ absorbed. Percent crystallinities of over 20%, comparable to that achieved using acetone or other organic liquids, have been obtained after 2 h exposure to supercritical CO₂. Since polycarbonate degasses quickly and quantitatively at ambient temperature and pressure, the high T_g of bisphenol A polycarbonate can be regained in the crystallized material without further vacuum treatment.     

Characterization of BN-films deposited onto carbon fibres by a continuous CVD-process

Pyrolytic BN-films have been prepared continuously on HT-carbon fibres by conventional CVD using the reaction of BCl₃ with NH₃ at temperatures between 700–1000° C and at ambient pressure. The film thickness varied with the N: B ratio, the dilution of the reactant gases, the flow conditions in the reactor tube and the velocity of roving. Good infiltration was obtained up to a thickness of about 50 nm at a speed of roving of 5 m/h. The coatings were characterized by XPS, TEM, REM, X-ray diffraction and thermogravimetric analysis.