TREATMENT OF CELLULOSIC FIBERS WITH SUPERCRITICAL CARBON DIOXIDE

Transcription

1 TREATMENT OF CELLULOSIC FIBERS WITH SUPERCRITICAL CARBON DIOXIDE Gregor Kraft 1, Christoph Muss 1, Christian Adelwöhrer 2, Thomas Rosenau 2, and Thomas Röder 1 1 Lenzing AG, R&D, A-4860 Lenzing, Austria, g.kraft@lenzing.com 2 University of Natural Resources and Applied Life Sciences - Vienna (BOKU), Christian-Doppler-Laboratory for Pulp Reactivity, Muthgasse 18, A 1190 Vienna, Austria Supercritical carbon dioxide has become a tool for various applications and different substrates. The introduction of active substances into cellulosic fibers with the usage of supercritical carbon dioxide is of interest in this study. Particularly, the distribution of D- panthenol all over the cross sectional area of the cellulosic fiber after the treatment with supercritical carbon dioxide was investigated closely. The impregnation and incorporation of D-panthenol was not successful for example by bathing or steaming, even when never dried Lyocell fibers are treated in a solution of the active components. Therefore further experiments were performed with supercritical carbon Introduction dioxide at NATEX Prozeßtechnologie, Ternitz, Austria. We could determine only a small increase of the absorption abilities of Lyocell fibers by supercritical drying. Contrary to our expectations there was no benefit with regard to the water retention. The incorporation of D-panthenol in cellulosic fibers, however, could be performed successfully with this method, which could be confirmed with Raman spectroscopy. Keywords: Lyocell fibers, Viscose fibers, supercritical drying, supercritical carbondioxide, D-panthenol The treatment of different compounds with supercritical carbon dioxide started as a pure laboratory method. Within the last years it has reached a high acceptance and has been implemented in different technical processes. The probably most common application is the extraction of natural essences, particularly the production of decaffeinated coffee and tea. The impregnation of wood products with supercritical carbon dioxide has been established recently. Furthermore, cellulosic material has been treated with it mostly for extraction and dyeing 1-3). This new technology has the advantage of working without any additional. Additionally there formed no waste products, which have to be disposed, by applying this method. In other words, the supercritical carbon dioxide treatment could be a key technology for an environmental friendly industrial process. The main focus of this investigation was on fibers produced by the NMMO process 4). 117

2 This method simplifies the production of cellulosic fibers by dissolving the cellulose in NMMO without any chemical modification of the cellulose. The feature of the NMMO process is a closed water circuit with an extremely high recovery rate of the. Additionally, the NMMO is a biodegradable, nontoxic substance. This study investigates the application and incorporation of D-panthenol, a widely used substance especially for wound healing 5), in cellulosic fibers with supercritical carbon dioxide and its final distribution over the fiber cross-section. Therefore the treated fibers were scrutinized by Raman spectroscopy. Supercritical drying of Lyocell fibers The adsorption of the water is a significant ability of the fiber, which is correlated to its pore structure. Unfortunately, the originally existing fiber pores collapse during the conventional drying process. Consequently, the water retention decreases during this process. It is known from aerogels, that the original porosity does widely remain unchanged when supercritical drying after exchange is performed. Normally, the water is replaced by acetone to avoid the collapse of these pores during the supercritical drying. Therefore, it should also be possible to increase the absorption abilities of Lyocell fibers clearly by the supercritical drying process. The investigations, done by P. Aldred 6), showed an increase of 30% in the water retention of Lyocell fibers comparing supercritical CO 2 drying to the conventionally oven dried Lyocell fiber. The verification of these results principally confirmed the results mentioned before. We also found an increase of the water retention ability of approximately 20% compared to a conventional oven dried fiber performed on a Lyocell fiber with different properties. Primarily, this is a very encouraging result. If the water retention ability of the supercritical dried fiber is compared with a never dried or air dried fiber the experiment, however, gives fairly disappointing and unexpected results. The increase of the relative adsorption capacity from 102% in case of air dried up to only 106% by supercritical drying and is therefore only marginal (Fig. 1). 110% 105% 100% 95% 90% 85% 80% 75% Relative water retention of Lyocell fibers after different treatments (compared to the never dried fiber = 100%) 102% air dried 88% dried at 100 C 99% - air dried 103% - dried at 100 C 106% - CO2 dried Figure 1. Relative water retention of Lyocell fibers Incorporation of biological active substances into a cellulosic fiber Introducing the technique of supercritical fluids can very likely increase the number of incorporable compounds in an environmental friendly way. The usage of supercritical carbon dioxide may open the door for several compounds, especially biologically active, hydrophobic substances, which are so far not utilizable by the classical incorporation technologies. Naturally there is an overwhelming choice of active compounds available like plant extracts, vitamins, perfumes/scents or even flame retardants. The analysis of these complex natural products is anything but simple. Therefore we started the investigations in that field by using pure active substances like D- panthenol. This compound was selected 118

3 because of its rather simple structure and the fact that it can be easily determined and well observed by Raman spectroscopy. The Raman measurements were performed with a HoloLab Series 5000 Modular Raman Spectrometer (HL5R) from Kaiser Optical Systems Inc. (USA) equipped with f/1.8 optics, transmission grating, multichannel CCD array detector (optimised for NIR), and a 785 nm diode laser (500 mw) coupled via single mode fibre to the microscope (approximately 80 mw on the sample). Between microscope and detector, a confocal fiber with a pinhole of 20 µm was used. With the 100X objective the lateral resolution was about 1-2 µm. Raman was used to track the concentration distribution of D-panthenol within a single cellulosic fiber. This method allows determining the active component within the cross section of a single fiber. Figure 2 shows the selected measurement points over the cross section of a Lyocell fiber after the supercritical incorporation of D-panthenol. rel. intensities Raman spectroscopic determination of D-Panthenol 1,E+04 8,E+03 6,E+03 4,E+03 2,E+03 0,E Distance [µm] Figure 3. Distribution of D-Panthenol along the cross section of a single Lyocell fiber The result of the Raman microspectroscopic investigation of the treated fibers showed that the incorporation of D-panthenol was successful. This, however, is depending on certain characteristics of the used fibers. Figure 3 shows the D-panthenol distribution across the processed Lyocell fiber. This is indicated by the relative intensities of the correlated Raman bands between cm -1. The amount of the active substance is clearly lower at the edge of the fiber cross section than it is in the center of the fiber. We assume that the substance is washed off at the end of the treatment, when the supercritical fluid is removed from the autoclave. Figure 2. Selected measurement points within one fiber. Figure 4. Selected measurement points within the cross section of two ZS-Viscose fibers 119

4 The incorporation can not only be performed with Lyocell fibers but the experiments can be transferred to a common ZS-Viscose fiber as well and gives similar results (Figure 4 & 5). rel. intensities 1,4E+04 1,2E+04 1,0E+04 8,0E+03 6,0E+03 4,0E+03 2,0E+03 0,0E+00 Raman spectroscopic determination of D-Panthenol Distance [µm] Figure 5. Relative intensities of the corresponding Raman band of the selected measurement points showed in figure 4 (ZS-Viscose fiber) Principally, it is possible to incorporate active substances into a cellulosic fiber by using the supercritical carbon dioxide technology. The permanence of the incorporated active compound is a key criteria for the practical use. It is necessary to investigate if the incorporated substance will be washed out or can resist the common laundry. A practical test performed in a commercial washing machine (40 C) gave the following results. The fibers contained no more D- panthenol after washing. This raised the question if this behavior is specific for D- panthenol, or universal for all possible incorporated compounds. Further experiments will be necessary to answer to this question. Alternatively, there might be an opportunity to impregnate or even incorporate the fibers with D-panthenol by treating never dried fibers with aqueous D-panthenol or Capantothenate solutions, respectively. Surprisingly, only a very small amount of D- panthenol / panthothenate could be found in the fiber. The highest concentration of the active substance, is located on the surface but cannot be determined quantitatively with the Raman analysis. Furthermore steaming was applied to the fiber, which is normally a useful way to impregnate Lyocell fibers. It, however, was not successful in this case. So far our conclusions are that the only way to incorporate D-Panthenol into a cellulosic fiber is the supercritical carbon dioxide treatment. Economical aspects The easy removal of D-panthenol from the fiber during one washing cycle predestined this product for a single use application. This one time usage could be an interesting application in the medical field of wound healing. Furthermore a slow release of the active substance could be an additional benefit for this application. First trials performed at BOKU showed a slow release effect of D-Panthenol out of the fiber, but clearly more experiments are necessary. Unfortunately, the expected costs of the supercritical incorporation of the active compounds are still in the range of 2 per kg fibers, which is far too expensive for an economically application. This process can only become interesting for industrial usage, when high value products can be manufactured or the process itself will be optimized and therefore becomes cheaper. Acknowledgements I want to thank Mr. E. Lack and Mr. M. Sova from NATEX Prozesstechnologie, Hauptstr. 2, A-2630 Ternitz, Austria, for performing the supercritical CO 2 experiments. References [1] Lindstroem, Eva et al.; Treating cellulose fibers with supercritical fluids for fibers with reduced content 120

5 of wood extractives and increased absorption rate and products comprising the fibers therefrom, EP [2] Schlenker, Wolfgang et al., Process for dyeing cellulosic textile material with disperse dyes, US [3] Schollmeyer, E. et al., Färbeverfahren, DE [4] Firgo H., Eibl M., and Eichinger D., Lyocell an ecological alternative, Lenzinger Berichte 1995, 75, [5] Hagers Handbuch der Pharmazeutischen Praxis, 4. Aufl., 9. Bd., Berlin: Springer [6] Aldred, P., internal report,