Nanotechnology launches new ways in synthetic fiber development and modification R. Hufenus, F.A. Reifler, G. Fortunato, D. Balazs, D. Hegemann, M. Halbeisen EMPA, Swiss Materials Science and Technology,
[email protected] 1. Introduction Empa is a materials science and technology research institution. It belongs to the ETH domain and as such is an important element in education, science and innovation in Switzerland. It specializes in applications, focused research and development, and provides high-level services in the field of sustainable materials science and technology. Its core tasks are innovative collaboration with industry and public institutions, ensuring the safety of people and the environment, knowledge propagation and university-level teaching. Over 800 people engage in interdisciplinary work in a large number of specialized disciplines. Their key areas of research are grouped in five programs entitled Nanotechnology, Adaptive Materials Systems, Materials for Health and Performance, Materials for Energy Technologies and Technosphere-Atmosphere. Empa's laboratory for Advanced Fibers develops synthetic fibers with distinct functionalities in new combinations. Our pilot melt spinning plant enables us to produce mono- and bicomponent fibers with diverse fiber cross-sections (Houis et al., 2007; Kaufmann et al., 2007). We use nanocomposites to enhance or add new properties to synthetic fibers. The materials of interest include functional nanoparticles, carbon nanotubes, natural nanofibers, and a variety of polymers. Distinct features can be imparted to these fibers in taking advantage of functional properties of the sheath component and mechanical properties of the core component. Ongoing projects include photocatalytic fibers, controlled-release fibers and nanofibril reinforced fibers. Novel methods to modify synthetic fibers enable us to develop complex fiber surfaces. Techniques like vacuum plasma treatment (Hegemann & Balazs, 2007), continuous dip coating (Varga et al., 2006) and 360° roll embossing (Schift et al., 2006) are applied to create micro- and nanosurfaces with given properties such as water repellency, abrasion resistance, flame retardance, biocompatibility or conductivity. Plasma technology offers a convenient way to tailor surface properties due to the possibility to design functionally gradient coatings by ablation and/or deposition processes. Dip coating and roll embossing processes have a great potential to enhance fiber properties in a simple, straightforward process utilizing machinery commonly used in textile finishing. 2. Fiber development 2.1 Photocatalytic fibers Many textiles have a distinct ability to adsorb odors from the surrounding air and act as a sink for volatile substances. This can be a problem when there are unpleasant odors leaving the textile through desorption. To meet this issue, odor control textiles are developed. In most cases, a finishing based on the adsorption-desorption abilities of cyclodextrins (Buschmann et al., 2003) is applied on the textile fabric. Cyclodextrins have a cage-like structure which can act as a "host" taking up "guest" molecules. Hence, the odor control effect is based on such host-guest complexes with the volatile substances, thus leading to delayed desorption. Regarding the practical use of odor control textiles, this can lead to unwanted side effects, as the release of adsorbed volatiles is retarded, but can last much longer than with untreated textiles. Furthermore, this odor control technique can only be applied for textiles which are regularly washed.
A far more attractive way is the degradation of the malodor itself at the surface of the odor control material through e.g. photocatalytic processes on TiO2 based photocatalysts (Fujishima et al., 2000, Kaneko & Okura, 2002). The goal of the presented work is a PET-based fiber with photocatalytic activity for the degradation of malodor in air (Hufenus et al., 2006). The photocatalytic particles shall be incorporated directly into the fiber. Photocatalytic TiO2 nanoparticles with high degree of crystallinity and surface area were obtained by flame synthesis (Figure 1).
Figure 1: TEM micrograph of TiO2 nanoparticles synthesized in an aerosol flame reactor (Akurati et al., 2007). Specific surface area (BET): 63 m2/g. The synthesis of nanoparticles with the general formula TiNxO2-x was conducted through a new process route (patent pending). Such nitrogen doped nanoparticles are known to show a good photocatalytic activity in the wavelength range between 400 nm and 500 nm and open the opportunity to realize photocatalytic fibers operating even under indoor conditions (artificial light) or in an automotive interior, i.e. behind laminated glass (Sakthivel et al., 2004). For particle incorporation and fiber manufacture experiments (monocomponent and bicomponent fibers), Degussa P25, a commercially available photocatalytic TiO2, was used. Pretreatment of the P25 particles helped to facilitate the compounding step and to prevent agglomeration problems. Analytical procedures play a central role during the fiber development, e.g. to assess photocatalytic activity and malodor degradation (Reifler & Ritter, 2006), to gain insight into size and dispersion of incorporated particles, and to determine the resistance of fibers to UV light. New methods have been developed to enable the assessment of the photocatalytic activity of a broad range of samples (particles, fibers, fabrics) using light with various spectral distributions. Fibers containing P25 were subjected to formaldehyde decomposition tests to evaluate photocatalytic activity. The results indicate that malodor degradation can be expected. Method development to verify the degradation of other malodors is ongoing. The fibers mentioned were also subjected to artificial weathering. Their degradation behavior was not severely inferior to the fibers without P25. 2.2 Controlled-release fibers Nowadays modern medicine increasingly adopts invasive diagnostic or therapy to prolong human life. Average age is rising, and elderly people have reduced resistance against infection. Above all, a lot of germs and bacteria like staphylococci or pseudomonas aeruginosa have developed antibiotic and antiseptic resistance. Thus nosocomial infections,
i.e. infections which are a result of treatment in a hospital have attracted increasing notice. Solutions for the problems mentioned are antibacterial or bioactive textiles. Textiles are used day-to-day in hospitals to protect medical team and patients. Furthermore, textiles are worn close to the body and thus next to bacterial colonies living on the skin. A possibility to achieve an antibacterial effect on textiles is the application of silver, as silver is a well known fungicide and bacteriostatic agent (Sondi & Salopek-Sondi, 2004) with low risk of antibiotic resistance (Damm et al., 2006). Due to the high surface to bulk ratio and thus ample release of silver ions, silver nanoparticles promise a substantial antibacterial effect (Damm et al. 2007). This effect decreases as soon as particles agglomerate, as this reduces the ratio of surface to volume. Although the concentration of silver needed to gain an antibacterial effect is depending on the ionic content of the surrounding fluid, as well as the bacteria treated, the amount of silver in fibers can be reduced to a minimum if particles are inhibited from agglomeration. The silver ions act by displacing essential metal ions on the bacteria membrane surface (Figure 2). Bacteria need the metal ions for interaction with their surrounding. By displacing H+, the silver ions inhibit hydrogen transfer. In addition, the binding of silver to bacterial DNA can inhibit important transport processes such as the cellular oxidation process (Kumar & Münstedt, 2004). This leads to termination of bacteria cell growth and ultimately to bacteria death. Thus silver ions show biocidal effect on around 16 species of bacteria including for example E. coli (Sondi & Salopek-Sondi, 2004).
Figure 2: Ag+ cations interact with bacteria cells as follows (Height, 2007): 1. Damage cell membrane (Sondi & Salopek-Sondi, 2004); 2. Displace Ca2+ and Zn2+ ions; 3. Interact with cell molecules containing sulphur, oxygen or nitrogen (Dowling et al., 2001). The presented project aims at the production of PET fibers loaded with silver-silica particles for antibacterial application. The 5 to 20 nm silver nanoparticles are supported in a porous, amorphous SiO2 matrix with particle diameters of 0.1 to 1 µm. Amorphous silica is widely used as a polymer filler. The silica matrix drags nanoparticles into polymers and liquids and maintains the predispersed state and the activity of the silver nanoparticles (Height, 2007). Mono- and bicomponent fibers with different particle loading have been produced. No problems occurred during spinning, independent of particle concentration or the fact whether mono- or bicomponent fibers were produced. Silver release was measured with atomic absorption spectroscopy (AAS), a technique for determining the concentration of metal elements in a sample. With a particle concentration of only 0.2 % the silver release is sufficient to render antibacterial effects, as was confirmed by antibacterial tests. A new bioactive fiber with good mechanical properties could be produced.
2.3 Nanofibril reinforced fibers Petroleum based polymers exhibit the most prominent role in fiber technology and in industrial production. The recent introduction of polylactic acid (PLA) as a commercial thermoplastic polymer offers new benefits to the textile industry. PLA is made from renewable sources like corn, it is biodegradable and compostable. However, compared to common synthetic fibers, lacks in mechanical properties have to be admitted. The melt spinning of pure PLA from different sources has been studied extensively (Schmack et al., 2004). It was shown that high crystallinity and alignment of the PLA crystalline forms could be achieved by post drawing the fibers, reaching strength values of up to 0.8 GPa, these values representing the maxima for the specific PLA polymers used. The presented project tackles these problems using both nano- and biotechnological approaches. The interest to develop nanocomposites based on renewable materials have increased considerably in recent years due to the great potential associated with this relatively new group of materials. Small crystallites or fibers can affect the polymer properties very effectively even with very low loadings. Using modern mixing techniques (e.g. extrusion including chemical surface modification of cellulose nanofibrils), novel raw materials can be obtained for further processing such as melt spinning. Cellulose nanofibrils were used as reinforcement components in synthetic polymers for the production of films and lacquers, rendering improved mechanical properties (Zimmermann et al., 2005). Mathew et al. (2006) prepared biodegradable composites using microcristalline cellulose as the reinforcing material and PLA as the matrix material. Relatively poor adhesion between cellulose and PLA occurred with unsatisfactory mechanical properties due to aggregation of the fibrils. Chakraborty et al. (2005) prepared a biocomposite out of micro-fibrillated cellulose and PLA, showing well dispersed fibrils within the matrix. Cellulose fibrils from different raw materials like cellulose powder, wood pulp or wheat straw pulp have been produced by mechanical and chemical-mechanical disintegration. The fibrils have diameters below 100 nm and a high aspect ratio (Figure 3). For compounding, solution casting and melt extrusion were applied. The solution route gave dispersed fibrils within the powdered polymer (Figure 3), whereas the melt extrusion process revealed segregation and reagglomeration of the fibrils. Only weak bonding between polymer and fibrils was obtained.
Figure 3: Left: Network of mechanically isolated cellulose fibrils (basis wood pulp). Right: Dispersed cellulose fibrils in PLA at a fracture. Up to now, the melt spinning of the compounds (PLA and fibrils from solution casted powders) revealed fibers with lower mechanical properties compared to pure PLA fibers.
In order to achieve good fibril-matrix embedding, the cellulose fibrils have to be modified to match the hydrophilic or hydrophobic nature of the polymer matrix. Strategies to be tested are water chemical surface modification of the cellulose fibrils (e.g. oxidation, esterification), soluble carrier polymers (PEO, PVP) to maintain the fibril structure prior to compounding, and solvent exchange (e.g. water to ethyl alcohol) to diminish reagglomeration of the fibrils after drying. With further improvement of the morphology and dispersion of the fibrils, as well as the fibril-matrix interaction, we expect biopolymer based fibers with equal to superior technical properties as petroleum based synthetic fibers. A respective European project proposal passed the first stage evaluation process of the 7th European framework programme "Nanosciences, Nanotechnologies, Materials and New Production technologies". 3. Fiber modification 3.1 Plasma technology Plasma technology can be used for ablation and deposition processes. While ablation enables a complete cleaning from manufacturing residuals, deposition can be controlled in the nanometer range to add new functionalities to fibers and textiles. The textile properties remain unaffected with both treatments, while being a dry and eco-friendly process. In principle, all types of textile materials might be treated within a plasma (Kang & Sarmadi, 2004). For textile industry mainly plasma systems running under atmospheric pressure seems to be of high interest to enable a continuous process. Contrary to established atmospheric plasmas, low pressure plasmas enable a variety of treatments mainly by the variation of the pressure (Oehr et al., 2005; Kull et al., 2005). Moderate pressure plasmas (100 to 1000 Pa) were found to give optimum cleaning and activation conditions since a high number of active particles contribute to chemical etching (Keller et al., 2005). Plasma polymerization can best be controlled using low pressure plasmas (1 to 100 Pa) due to well-defined plasma zones (Hegemann, 2006). In this pressure range the mean free path lengths are high enough to allow the penetration of the textile structure by energetic particles and long-living radicals. Sputtering processes, finally, are carried out in the pressure range between 0.5 and 5 Pa to avoid in-flight scattering collisions of the background gas and the sputtered atoms (Rossnagel, 2003). Therefore, at our laboratory mainly low pressure plasmas are considered to obtain nanoscaled treatments on fibers.
Figure 4: Plasma fiber coater.
We built and run several batch reactors, a semi-continuous web coater and a continuous fiber coater (Figure 4). The unique fiber coater is equipped with an inverse cylindrical magnetron enabling the uniform coating of fibers or wires (Amberg, 2004). Additionally, plasma pre-treatment (cleaning) and plasma polymerization processes can be performed. A comparison of different cleaning methods on PET fibers proved plasma cleaning to be ideal (Keller et al., 2005). The fibers are wind up and off in air and are transported through a sealing system into the vacuum chamber. Velocities up to 100 m/min can be reached with fiber diameters ranging from micrometers to millimeters. Magnetron sputtering of metals on fibers provides conductive/antistatic and - in the case of e.g. silver - also antibacterial surfaces (Amberg et al., 2004). After a plasma cleaning step the fibers are metallized in a continuous process by moving them several times through the plasma zone created inside the cylindrical magnetron. Thus, a homogeneous, all-side coating can be achieved. A coating thickness of around 200 nm is required to obtain a resisitivity below 1 Ω/mm, which is appropriate for good conducting fibers. Such silver coated fibers are suitable for the development of flexible electrical connections in smart textiles and for wearable computing. Nanoscaled coatings, however, are adequate to obtain antistatic effects that may be used for medicinal or occupational textiles and electrostatic shielding. Moreover, in contact with the human skin, silver ions are released from the ultrathin coatings, which reveal a strong antibacterial and fungicidal effect. Plasma polymerization on textiles has gained increasing interest during the last decade, since highly functional surfaces are enabled, resulting in several industrial applications (Hegemann, 2006). To obtain a hydrophobic, water repellent yarn, siloxane coatings using HMDSO discharge have been deposited continuously on fibers. Nanoscaled functional plasma polymers can thus be deposited directly on the fiber geometry within a continuously running process. In general, textile trends which are relevant for plasma treatments mainly deal with hydrophobic-oleophobic, hydrophilic functional, anti-static and conductive, anti-microbial and medicinal coatings as well as multi-functional surfaces. 3.2 Dip coating New textile materials for technical applications are often subjected to various and sometimes even contrasting requirements which cannot be solved by just one material. A possible solution to combine such complex requirements in one product or component is the separation of the surface and substrate functions (Gadow & Niessen, 2002). To create new tailor-made textile fibers, the idea was to improve the properties of the textile fibers by coating them with ceramic particles (Clemens, 2000). Ceramics are hard materials, with resistance against high temperature, oxidation and abrasion. It is believed that fibers coated with ceramic particles will combine the positive characteristics of both materials, i.e. retain the flexibility and elasticity of polymeric fibers, and obtain advantageous characteristics of ceramic materials (e.g. resistance to abrasion, heat, impact, cut, hydrolysis, light). In the presented project, fibers are coated with ceramic particles in a straightforward, simple dip coating process. Emphasis is placed on the investigation of the influence of ceramic particle size, binder content and various additional components on the abrasion resistance of the fiber coating. Al2O3 particles are used, with average particle sizes of 0.3 and 2 μm, respectively. Apart from the microparticles, the dip coating slurry contains a binder system (organic polymer), water and additives (anionic surfactants). The additives are meant to adsorb on the particles and prevent their accumulation, due to electrostatic effects. The dip coating was performed by drawing a 0.1 to 0.5 mm monofilaments (PA 6, PET, PP) through the slurry, drying it and curing the binder. The first goal was to achieve a considerable coating thickness, which can be obtained with high-viscous slurries.
A PU binder system was found to be the one with the highest viscosity, thus most appropriate for dip coating (Figure 5). To get an optimized coating, the dispersion (water, ceramic particles, and additives) must be thixotropic, i.e. the viscosity must decrease rapidly with increasing shear rate. With respect to thixotropy, ammonium citrate < 0.5 wt% proved satisfactory as surfactant. Using slurries with 20 to 40 vol% of Al2O3 particles shows that the coating thickness increases considerably with increasing particle concentration. With too high concentration, the coating gets non-uniform, brittle and loose.
Figure 5: SEM cross–section micrographs of the fiber coating containing 2 μm Al2O3 particles embedded in a PU binder system. To evaluate the coating quality, abrasion resistance tests have been conducted using a custom-built textile friction analyzer (Gerhardt et al., 2007). As friction partner, rough-textured glass was applied to cause slow wear of the ceramic coating. Change in coating appearance, as revealed by scanning electron microscopy, was chosen as abrasion resistance criteria. The coating with large particles shows a higher abrasion resistance, in accordance with (Luo, 1999). The addition of thickener, fixer and defoamer increased the wear resistance considerably (Varga et al., 2006). 3.3 Microstructuring Many of the advances of synthetic fiber technology are based on the enlargement of the fiber surface (Fourné, 1999). Mostly the fibers are profiled by adequate spinneret nozzles, which enable the realization of a surface profile during fiber spinning. However, this technology has two disadvantages: The profiling occurs only in the direction of spinning (longitudinal profiling), and the profiling fidelity is limited by the fabrication fidelity of the spinning nozzles, by the viscosity of the fiber molding material and the relaxation during the extrusion process. The fiber surface is therefore rather coarse. Consequently, many effects based on surface micro- and nanostructures can not be applied sufficiently. By generating an arbitrary topographical relief (lateral profiling), the important properties of the fiber surface can be largely improved. A roll embossing device has been developed to laterally microstructure textile fibers, using a roundabout thermoplastic molding (Halbeisen & Schift, 2004). The surface relief of a stamp (nickel shim) is transferred onto the surface of a fiber, by molding a thermoplastic material at a temperature well above its glass transition temperature. A sophisticated system with unaligned pressure rolls ensures that the embossing effectively covers the entire fiber surface (Figure 6). This process has similarities to the hot embossing of surface structures into thin polymer sheets for the fabrication of diffractive optical elements (Gale, 1997) or into thin thermoplastic resists as used in the nanoimprint process (Schift & Heyderman, 2003).
Figure 6: Principle of fiber structuring (patent granted). Roll embossing is a process highly suitable for mass-fabrication at industrial scale, process speeds can be up to some meters per second. Possible applications are textile copyright marks, dye-less fiber coloring via diffraction, or substrates for biomedical applications with controlled cell growth. Using a metal cylinder (lithographic stamp), we succeeded in transferring periodic gratings with sub-µm dimensions (sine grating of 1 µm period and 200 nm depth) onto PET monofilaments. To achieve optical effects, two kinds of microstructures were imprinted on PMMA fibers. A plasma silver coating was applied to achieve reflective surfaces required for more distinct optical effects. Periodic microstructures render rainbow optical effects, while alternating microstructures lead to sparkling optical effects. The results show that roll embossing can be applied for the lateral structuring of polymer fiber surfaces. A temperature-pressure-speed regime was found for homogeneous fibers, which allows molding a sub-µm pattern onto the surface of the fiber, while the core of the fiber is only slightly deformed. With respect to dimension, the process does not show any principal limits. Therefore, as already found for other replication processes, it can be expected that smaller structures can be replicated on fibers using roll embossing. References Akurati, K.K., Vital, A., Fortunato, G., Hany, R., Nueesch, F. & Graule, T.: Flame synthesis of TiO2 nanoparticles with high photocatalytic activity. Solid State Sciences, 9 (2007) 247-257. Amberg, M., Geerk, J., Keller, M., Fischer, A.: Design, Characterisation and Operation of an Inverted Cylindrical Magnetron for Metal Deposition. Plasma Devices and Operations, 12 (2004) 175-186. Buschmann, H.J., Knittel, D., Schollmeyer, E.: Wie funktionieren Textilien mit fixierten Cyclodextrinen? Melliand Textilberichte, 84 (2003) 988-990. Chakraborty, A., Sain, M., Kortschot, M.:Cellulose microfibrils - A novel method of preparation using high shear refining and cryocrushing. Holzforschung 59 (2005) 102-107 Clemens, F.: Entwicklung einer oxidationsbeständigen Faserbeschichtung für die Optimierung der Faser-Matrix-Grenzfläche im Faserverbundsystem Al2O3/Mullit. PhD thesis, Geowissenschaftl. Fakultät der Eberhard-Karls-Universität Tübingen (2000). Damm, C., Münstedt, H., Rösch, A.: Lomg-term antimicrobial polyamide 6/silver-nanocomposites. J Mater Sci 42 (2007) 6067-6073.
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