Supercritical impregnation of polymers - ZyXEL NSA210

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400 I. Kikic, F. Vecchione / Current Opinion in Solid State and Materials Science 7 (2003) 399–405 dissolving SC CO 2 1 Additive 3 swelling 4 plasticizing Before presenting the different processes, the behaviour of the binary system (supercritical fluid–polymer) will be considered. 2. Supercritical fluid–polymer interaction loading Supercritical fluids can interact not only with polymers at temperatures higher than the softening point but also with polymers in the glassy state. Three concomitant effects must be considered: the dissolution of the SCF (polymer sorption), the swelling of the polymer matrix and the glass transition temperature (T g ) depression, simply called plasticization. The plasticization effect is an important feature: the sorbed gas acts as a kind of Ôlubricant’, making it easier for chain molecules to slip over one another, and thus causing polymer softening. The measured depression reaches values as high as 60 °C for poly(methyl methacrylate) and poly(styrene). Measurements of glass transition temperatures at high pressures can be only indirect; recently, a new method has been developed for measuring the glass transition temperature: it is based on a supercritical fluid chromatographic technique. In this case, using the supercritical fluid at a given pressure as mobile phase and the polymer under investigation as stationary phase, the retention volumes of various organic solutes are measured at different temperatures [5,6]. Some attempts can be found in the literature to model both the sorption of a supercritical fluid in a glassy polymer and the glass transition temperature depression induced by supercritical fluids. Regarding the shift of the glass transition temperature, the most important contribution is due to Condo et al. [7]. In this approach the Gibbs Di Marzio criterion (at the glass transition the entropy of the system is zero) is used together with the Sanchez–Lacombe equation of state. The possibility of the so-called retrograde vitrification is suggested: when the temperature is reduced at constant pressure, it is possible to observe a first liquid– glass transition followed by a second transition from the glass-back to the liquid-state. A detailed description of the model and a discussion on the effects of different parameters is reported in [6,8]. 2 Polymer Substrate Fig. 1. Interactions between the SCCO 2 -assisted impregnation system. Another approach proposed originally by Wissinger and Paulaitis [9] introduces an additional variable (order parameter) that describes the thermodynamic state of a system. As the order parameter, the fraction of holes in the lattice and the number of nearest-neighbour contacts between polymer segments on the lattice sites are used. The NELF model, recently proposed by Doghieri and Sarti [10] uses as an order parameter the density of the swollen polymeric phase and the Sanchez–Lacombe equation of state for the evaluation of the chemical potential of the supercritical fluid in the gas phase (pure supercritical fluid) and in the polymeric phase. This approach can also be modified using a different equation of state for the gas phase, and does not require any binary interaction parameter. In this way, the sorption behaviour can be predicted from the swelling data alone. Since swelling measurements are, in general, more difficult to perform, it was recently [11] proposed to use the model as an empirical method to correlate and predict sorption data. Using one experimental sorption data point, and applying the NELF model, it is possible to predict the sorption at other pressures and temperatures. 3. Supercritical impregnation processes 3.1. Drugs impregnation Controlled-release products have recently received considerable attention in the pharmaceutical industry since their use significantly reduces the problems connected with excessive dosages. In the preparation of controlled-release drugs, a liquid solvent swelling the polymer matrix and serving as a carrier for the drug component, is used. An alternative is to substitute the liquid organic solvent by a supercritical fluid with the advantage that the final product is completely free of any residual solvent contamination. Guney and Akgerman [12] investigated the impregnation of a biodegradable polymer matrix, poly-dllactide-co-glycolide (PLGA), with 5-fluorouracil and b-estradiol, drugs that are used for chemotherapy and estrogen hormone therapy, respectively. The experimental set-up includes a saturation column packed with the drug, and an impregnation vessel in which the polymer is placed. Sorption experiments require the introduction of a drug-saturated CO 2 stream into the impregnation column. That is why the experiment consists of two sections: solubility measurement followed by impregnation. The apparatus is also equipped for a qualitative determination of polymer swelling. For b-estradiol at 35 °C and 207 bar, the impregnated amount (q ¼ mg drug/g PLGA) was 10.24 while at 55 °C and 207 bar, it arrives at 26.80.
I. Kikic, F. Vecchione / Current Opinion in Solid State and Materials Science 7 (2003) 399–405 401 For 5-fluorouracil at 55 °C and 207 bar, the impregnated amount was very low (0.49). For these specific applications the drug loadings obtained are even higher than necessary since the therapeutical level for these products is much lower [12]. Kazarian and Martirosyan [4] used ATR-IR spectroscopy to study the process of impregnation of ibuprofen into poly(vinyl pyrrolidone) (PVP) from SCCO 2 solution. In situ spectroscopy allows the continuous monitoring of the amount of the impregnated drug, and the process can be instantly stopped by depressurising the high-pressure cell once the desired level is achieved. It has been shown that the supercritical fluid impregnation process results in ibuprofen being molecularly dispersed in a polymer matrix where all drug molecules are H-bonded to the polymer and without the presence of ibuprofen crystals. ATR-IR spectroscopy has also revealed specific interactions between CO 2 molecules and carbonyl groups of PVP and it has been shown that a competitive interaction of impregnated ibuprofen molecules with the carbonyl groups of PVP prevents CO 2 molecules from interacting with the carbonyl groups of PVP. In addition, IR spectroscopic evidence has proved that similar interactions have an effect on water uptake into PVP. Thus, the PVP films impregnated with ibuprofen show much lower water uptake, presumably due to the competitive interaction of ibuprofen with basic carbonyl groups of PVP [4]. 3.2. Dye impregnation In the dyeing processes of the textile industry, the use of supercritical carbon dioxide as an alternate solvent instead of water-based processes has been gaining much interest for environmental reasons. The conventional dyeing process of PET fibers discharges much wastewater that is contaminated by various kinds of dispersing agents, surfactants and unused dye. It is very difficult to design a conventional biological process that treats the wastewater discharged from a conventional dyeing PET process. The environmentally friendly supercritical fluid dyeing (SFD) process does not require any water, dispersing agents or surfactants and also does not involve any drying stage after dyeing. However, supercritical fluid dyeing has not been adopted in any of the world dyeing industries yet due to the high initial investment cost. Therefore, it may be better to apply the SFD to the hard-to-dye materials such as aramid, PE and PP fibers and films. Although extensive studies have been performed by several researchers, only a limited amount of basic dyesorption data is available. The amount of dye sorption in polymers in the presence of supercritical carbon dioxide is closely related to both the solubility of dye in the fluids and the distribution of dye between the fluid and the polymer phases. The mobility of dye molecules between polymer chains is generally enhanced due to the swelling of polymers in the supercritical fluids [13]. Park and Bae [14] performed the measurement of equilibrium dye uptake in PET fiber at various temperatures and pressures using a flow-method. The distribution coefficient increases with the pressure increase, because the sorption of dye in PET fiber increases more slowly with the pressure than the dye solubility in carbon dioxide does. This tendency is weakened with increase of temperature [14]. Shim et al. [13] investigated sorption of some disperse dyes (C. I. Disperse Blue 60 (B60), C.I. Disperse Red 60 (R60), C.I. Disperse Yellow 54 (Y54), and C.I. Disperse Orange 30 (O30)) in polyethylene terephthalate (PET) and polytrimethylene terephthalate (PTT) fibers as well as difficult-to-dye fibers such as aramid and polypropylene using supercritical carbon dioxide at pressures between 10 and )33 MPa and temperatures between 35 and 150 °C. For dyeing in a co-solvent laden supercritical fluid, ethanol, acetone, or N-methyl pyrrolidone was also introduced in the dyeing vessel. PET and PTT fibers were easily dyed with Y54 and other dispersed dyes. The amount of sorption of Y54 dye in the crystalline polyester textile fibers increased with increasing pressure up to 33 MPa at various temperatures. The amount of sorption for PTT was (about 10%) larger than that for PET because PET has a high degree of crystallinity of 30% which is larger than that for PTT. Dye molecules are generally large and their diffusion rate is very small. They may penetrate only into the glassy part and not in the crystalline part of a polymeric matrix. Dye sorption increased with pressure at the same temperature and increased with temperature at the same pressure, but this increasing rate was reduced with increasing pressure [13]. Van der Kraan et al. [15] studied dyeing of synthetic and natural textiles with a reactive dichlorotriazine dye in the presence of supercritical carbon dioxide. Experiments were carried out on polyester, silk, wool, cotton and aminated cotton. Pressure and temperature were varied from 225 to 278 bar and from 100 to 116 °C. In these experiments a small quantity of water was added as an enhancer of reactivity and/or accessibility of the natural fibers. Polyester was dyed well, with fixation percentages in the order of 95% and the color yield increased with pressure, but not with temperature. Silk and wool were dyed with a color yield independent of pressure and temperature. Fixation percentages on silk (76%) and wool (70%) were almost independent of pressure and temperature. Comparison of the experimental results with literature data shows that silk can
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