Linear materials with nanofiber coating. Mailing address: Technical university of Liberec, Textile faculty, Department of

Transcription

1 Linear materials with nanofiber coating Ondřej Novák, tel.: Jiří Chaloupek, tel.: Oldřich Jirsák, tel.: Mailing address: Technical university of Liberec, Textile faculty, Department of nonwovens, Hálkova 6, Liberec 1, , Fax:

2 Potential referees: Prof. RNDr. David Lukáš, CSc., Tel.: , Technical university of Liberec, Faculty of textile engineering, Department of nonwoven, Hálkova 6, Liberec 1, , Prof. Ing. Jiří Militký, CSc , Technical university of Liberec, Faculty of textile engineering, Department of textile materials, Hálkova 6, Liberec 1, , Prof. Ing. Petr Louda, CSc., tel.: , Technical university of Liberec, Faculty of mechanical engineering, Department of Material Science, Hálkova 6, Liberec 1, , Ing. Ladislav Mareš, Tel.: , ELMARCO, V Horkách 76/18, Liberec 9, , ladislav.mares@elmarco.cz RNDr. Vít Chudoba, Ph.D., Tel.: , ELMARCO, V Horkách 76/18, Liberec 9, , vit.chudoba@elmarco.cz

3 Abstract: This article deals with a possibility of a production of linear formations with nanofibers. Many authors try to achieve a production of single nanofibers or fiberbundles in which the nanofibers are more or less oriented. Higher orientation can provides needed properties as are strength, elasticity or suitability for other applications having a connection with a structure orientation. In the article the electrospinning process is described and in a short search some possibilities how to produce nanofiber structures with parallel orientation are shown, because in the process of the yarn production a higher orientation is strongly required. Further, the new electrospinning method and its modification are shown. This method is based on the discovery that it is possible to create Taylor cones not only from the tip of the capillary but also from a thin film of polymer solution. The modification of this method is based on the usage of linear collector, which simultaneously creates the career of nanofibers. In the article some examples of linear formations with nanofibers are shown and application possibilities are discussed. Methodes of nanofiber production Nanofibers are an interesting branch of nanomaterials. The name indicates that these are fibers with a diameter less than one micron. They have lots of advantageous properties for instance high specific area, high porosity, slenderness and many others. These properties can be used in various areas such as high efficiency filters, separation membranes, composite reinforcement, materials for tissue engineering and so on. Nanofibers can be produced by various techniques, which provide nanofibers with different properties. A method of a drawing1 is based on high drawing of filaments in process which is similar to dry spinning method.

4 Another method is a technology of a nanofiber extrusion. This method provides meltblown fibers with an average size of 250 nm and a range between 25 and 400 nm 2. This method uses technology Island-In-The-Sea (fibers within fibers). The number of fibers within a fiber is about Last usable method is an electrospinnig process. The basic scheme is shown in the Figure Figure 1: Description of electrospinning process. 1 High voltage power supply, 2 Syringe, 3 Grouting point, 4 Grounded collector, 5 Jet of nanofibers This term indicates that the electrostatic forces are used for the spinning. These forces are created between electrodes which form so called spinning space. Usually the electrode with a positive charge is used for bringing of a spinning solution as well. The other electrode has a negative charge; in some cases it can be grounded. The electrospinning process is based on a drawing of an electrically charged jet from the spinning solution. In the jet source some cones are formed. The cones are created in the consequence of electrostatic forces and a surface tension of the spinning solution3. The cones were named as Zeleny Taylor cones4. If the electric forces are more prominent than the surface tension, the liquid begins to form to cones with convex sides and rounded tips. If high voltage is increased a jet of liquid is emitted from the cones. The maximal half angle of a cone just before arising of the jet is 49,3. The jet produces submicron fibers. Before their deposition on the collector

5 almost all of the solvent is evaporated from the spinning solution and rising fibers are highly stretched as a result of electrostatic forces. Nanofibers are subsequently deposited onto the collector. In some cases the electrospinning process can be changed into a process where submicrone droplets with narrow distribution curve of a diameter are created. This process is called electrospraying and it is used in some applications for making aerosol. The value of viscosity and surface tension of spinning liquid determines whether droplets or nanofiberes will be created. Nanofibers can be created in three different forms. Most commonly used form of nanofibers is a nonwoven web. This type of nanofiber layer can be prepared with a basic apparatus for electrospinning, where the flat collector is used. This method provides samples with smaller thickness, typically about tens or hundreds of microns. If samples with higher thickness are required then modified electrospinning apparatus is suitable for this purpose. The modification is based on differently arranged spinning space that is created by two electrodes with opposite charges. One of the electrodes provides a spinning solution, which is attracted to the other electrode, but before achieving it, the jet is blown out from the spinning space by an air stream onto the collector, which is usually grounded. This modified method provides nanofiber layers with the thickness about millimeters 5. For some applications two or three dimensional products are not suitable and nanofibers in the linear form are required. This form is not so common, because it requires higher alignment of the structure. Methodes for production of aligned nanofibers and nanofiber yarns Usually nanofibers create a web, which has an isotropic orientation nanofibers in the web do not prefer any direction. Following is a description of several methods of

6 producing nanofiber structures with higher orientation or nanofibers in the form of yarns and those similar arrays are shown. The most important production methods are: winding with high angular speed spinning on liquid level with subsequent drawing. A method for preparing the linear system is introduced in6. It is one of the first Anton Formhals`s patents, which is focused on a production method of nanofibers in a linear form. This apparatus consists of two main parts: the first is a toothed wheel, which is inserted to a vessel with a spinning liquid. The other part of this apparatus is a metal ring or hoop. If these moveable parts are charged with high voltage of opposite charges, then nanofibers are continually produced from the toothed wheel and they are deposited on the metal ring. Nanofibers are in the form of threads, which can be removed from the ring. The process can be modified for example with an apparatus for the drawing and twisting for the production of yarns. Also a device for washing and drying of threads can be added. Another way of preparing nanofiber yarns is shown in7. Authors of this paper show a simple method of obtaining aligned polymeric nanofibers with infinite length and with a possibility to place them to the whole surface of a collector. In comparison with other methods, two needles with opposite charges are used. Spun fibers are created between them and electrostatic forces form a system similar to a yarn. The yarn is not charged and thus can be easily collected. It is an advantage, as conventional systems use grounded or negative charged collector. In the experiment, metal needles at distance of about 140mm were used. Rotating cylinders with the diameter 8, 40 and 125 mm were tested as collectors. Yarns were prepared in different concentration from polyvinylalcohol (PVA) and polyvinylpyrrolidone (PVP). It showed that higher rotation

7 of the collector causes higher degree of alignment. It was proven that higher concentration allows to achieve higher maximal winding speed of the yarn. However, very high speed causes a rupture of fibers. Similar method was used in8, where a rotating tapered and grounded wheel-like bobbin was used. Spun nanofibers are winded at a high speed on the edge of the bobbin. A parallel alignment of fibers is accounted for residual charge of fibers, which causes their repellency. Interesting method for preparation of aligned yarns from nanofibers is described in9. This method is based on modified electrospinnig method and consists of spinning on a water level. Thus, the water level is an unusual type of a collector. The apparatus is composed of a glass pipette and water reservoir collector. A spinning solution is placed on the tip of the pipette. The collector consists of a round glass Petri dish and is grounded by a copper wire electrode, which is inserted into the solution. The high voltage power supply is connected to the glass electrode. For the experiment, spinning solutions of Polyvinylidenedifluoride (PVDF), Polyvinylacetate (PVAc) and Polyacrylonitrile (PAN) were used. After a formation of a nanofiber nonwoven on the water level, fibers are manually drawn out from the water and placed on motorised take-up roller. From the results it is evident that prepared nanofiber yarns are longitudinally oriented and better performance is provided by PVAc and PAN solutions. If the PVDF at lower concentration is used, then large beads on nanofibers are created. Higher concentration results in fibers with high diameter about 1µm. A method for preparation of tubular nanofiber products is described in10. An apparatus is characterized by a rotating collector, so-called former. The former comprises a core and a removable sheet. This sheet is electrically conductive and can be made from deformable metallic foil. Nanofibers are collected on the rotating former sheet. The nanofiber layer can be removed after achieving a suitable thickness of the web.

8 Obtained structures are useful in providing novel synthetic blood vessels or components thereof. A possibility to align nanofibers on rotating drum is shown in 11. The technique is similar to the previous method, but there is an auxiliary electrode with the same charge as the collector. This invention is convenient for vascular prostheses, as aligned nanofiber structure can be more suitable then the nonaligned one. The work12 describes a technique for the controlled deposition of nanofibers. The instability of jet is characteristic for an electrospinning process. Its movement is chaotic and so lens element and collection target of opposite polarity was used for stabilization. This technique provides parallel oriented nanofiber structure. In the article13 shows a method of preparation of high-aligned nanofibers. A rectangular frame placed near to the collector is used and some nanofibers are caught onto the frame. Nanofibers have parallel alignment and their orientation can be influenced by different material of the frame. In the experiment, a wooden and aluminum frames were used, whereas the aluminum frame provided better alignment than the wooden one. An influence of the frame angle was observed. These results led to the construction of a rotating multi-frame body allowing continual creation of nanofibers. In experiment Polyethylenoxid (PEO) was used for the electrospinning. These methods and techniques show the possibilities how to create linear materials from nanofibers. All methods have in common that they create nanofiber layers directly from nanofibers, although it is not suitable for all applications. If we imagine cross section of a yarn, then we can see that a certain amount of nanofibers creates the sheet of the yarn and a bigger amount creates the core. The core can not be accessible; because it is covered by the sheet. It leads to an inaccessibility of fiber surface inside the yarn. Thus, most of the nanofibers are not utilizable and create only a filling of the yarn and in some cases it can have lower performance.

9 Electrospinning method Nanospider Revolutionary thoughts are often very simple as is the case in point of the principle of NanospiderTM technology, which is based upon the discovery that it is possible to create a Taylor cone and the subsequent flow of matter not only from the tip of the capillary but also from a thin film of polymer solution. As was mentioned above, as opposed to other methodologies, Nanospider does not utilize any jets or capillaries for the production of fibers, but on of the possibilities of a drum partially submerged in a polymer solution. Scheme is shown in the Figure Figure 2: Description of Nanospider method. 1 High voltage power supply, 2 Metal drum, 3 Vessel with spinning solution, 4 Grounded collector, 5 Jet of nanofibers Whilst the drum turns on its axis, a thin film of polymer solution is created on its surface. In the top dead center of the rotating movement of the drum, which is also the place which is closest to the collectors anti-electrodes, many focal points of Taylor cones start to form as a result of the maximum intensity of the electric field, which subsequently leads on to the process of fiber spinning. Taylor cones and the subsequent flow of matter are created in a thick net covering the top of the drum.

10 This results in the high production capacity of fiber spinning of NanospiderTM. Streams of polymer solution are then divested of solvent (as above) and become firm nanofibers just before reaching the collector. These methodologies have been known for a number of years, but none of these methodologies has to-date offered a sufficient industrial dimension with sufficient manufacturing capacity and stability. The situation has been changing from 2003, when TUL applied for a patent for a technology for the industrial production of nanofiber material. This revolutionary technology is called NanospiderTM. In principle it is a modified process for the production of nanofibers and nanofiber layers via the electrostatic fiber spinning of polymer solutions. As opposed to other methods in the public domain, nanospider technology does not utilize jets or capillaries for the production of fibers, but utilizes a rotating drum partially submerged in a polymer solution. The main advantage of this technology is the significant increase in production capacity which this methodology offers [14]. Modified Nanospider methode for linear materials In original patent, the following configuration was used: a roller and linear steel wire electrode creates a spinning space. Scheme of device is shown in the Figure

11 Figure 3: Description of modified Nanospider method for production of linear formations. 1 High voltage power supply, 2 Metal drum, 3 Vessel with spinning solution, 4 Grounded linear electrode, 5 Jet of nanofibers, 6 Linear nanofibers carrier The linear electrode was chosen because it was needed to create nanofibers all over the roller length and to place them in a narrow width (roller is placed parallel with the linear electrode). A linear formation is put as closest to the linear electrode as possible. A purpose is to hide the linear electrode by this linear formation. It is required, that the direct axes of the roller, electrode and linear formation lay in a plane. It is evident, that the cover of a whole linear formation sheet must be ensured, which is done by rotation of the linear formation. Linear formation has a relatively high rotation speed. The main problem is the impossibility to produce a uniform cover of the linear formation. The cause is mutual attraction between linear formation and the linear electrode. In the place of contact, a damage of nanofiber cover occurs. Another problem is caused by the shape of the electric field in the spinning space. Magnetic lines are not situated only in plane roller - electrode, but they are also outside of this plane. An explanation is shown in Figure 4.

12 Rising nanofibers can lead to linear electrode and simultaneously they can pass Figure 4: Shape of magnetic line field. 1 Vessel with spinning solution, 2 Metal drum, 3 Nanofiber jet, 4 Linear carrier, 5 Linear electrode linear formation and be deposited on the electrode and thus cause its pollution. Probably fiber outlets, which rise during the electrospinning process and fly out of the spinning space, have the same origin. Problems of the original solution can be summarized into the following points: non-uniformity of nanofiber cover fiber outlet Therefore, the new proposal of the device was focused on fixing the abovementioned problems. More versions have been tested, while only the final one (arrangement) is explained. The highest priority was given to the design of the linear electrode because its original solution did not guarantee the uniform cover of the

13 linear formation. The problem, as mentioned above, is the fact that linear formation makes actually a barrier in the spinning space. Due to the shape of an electrostatic field, it is not able to collect all rising nanofibers. The possible solution of this problem was to minimize the distance between the linear formation and linear electrode. It led to an idea to join these two (linear formation and electrode) and thus make one unit. In other words, the electrode was replaced by linear formation. The solution is shown in the figure 5. Figure 5: Description of modified Nanospider method for production of linear formations. 1 High voltage power supply, 2 Metal drum, 3 Vessel with spinning solution, 4 Linear nanofibers carrier, 5 Jet of nanofibers. For realization of this arrangement it is necessary to guarantee an important condition, namely to ensure an electric conductivity of the linear formation. As the use of more or less common textile materials is supposed, it is necessary to cover it with a suitable substance which can provide this conductivity. Of course, an exception exists when conductive substance is not required, but the productivity of the device is much lower. This solution is suitable for applications, where clear linear material without any additives is required. The final configuration of the device is as follows: a spinning space is created by the roller and linear formation whereas the linear formation is simultaneously the carrier of nanofiber deposition. The roller has a positive charge and the linear formation is grounded, or eventually it can have a negative charge. This provides higher productivity in case of lower conductivity of the linear formation. The distance between the linear formation and the roller is adjustable for setting of optimal conditions of electrospinning process. The roller is closed in PVC cube box. An air-condition unit can be used to adjust humidity and

14 temperature. The rotation of linear formation can be ensured by several standard methods. Cross-winding device for standard bobbins is used for winding of prepared linear formation with nanofibers. An increase in electric conductivity can be done in various ways, for example by sputtering of suitable metals, using yarns with metal wires, by plasma treatment or by other techniques. Requirements of process and parameters of linear formation with nanofibers For this process are suitable yarns or structures from multi or monofilaments. The required diameter of these structures is from hundreds of microns to millimeters. The Diameter [µm] Mean Standard deviation Spinning solution Polyvinylalcohol Polyurethane type of a material does not play a roll, but some tensile strength is required. In the experiment were tested various materials and there was not observed any difference of created nanofibers. Only the productivity of nanofibers was different. The reason of that can be seen in a different electric conductivity of nanofibers. In the experiment spinning solution of Polyvinylalcohol (PVA) and Polyurethane (PU) were spunned. From the Table I. is evident that PVA provides lower diameter with higher uniformity of nanofibers. Table I.: Diameters of nanofibers spunned from PVA and PU

15 A minimal diameter of nanofibers is in both cases just above 50 nanometers. The thickness of nanofiber cover is from tens of microns to millimeters. The higher thickness can be used for production of hollow structures from nanofibers. Some images from scanning electron microscope are attached above in Figures 6 9.

16 Figure 6: Cross-section of cotton yarn covered by PVA nanofibers, magnified x150 Figure 7: View of cotton yarn covered by PVA nanofibers, magnified x80

17 Figure 8: View of nanofiber PVA layer, magnified x1000 Figure 9: Detail of nanofiber PVA layer, magnified x5000 Applications Although the development of linear formations with nanofibers is on the beginning, there were found some possible applications. Veins are important part of human

18 body. They transport a blood without oxygen back to the heart. In some cases, for instance if an injury happens, than its replacement is required. For this purpose the vein implant exists. They are produced by knitting usually from Polyester (PL) fibers. Linear formation covered by nanofibers can replace current implants. It is supposed that a carrier creating the core of the linear formation will be removed and nanofiber sheet will creates a base of the new vein. The removing can be done mechanically the core will be pulled out from the sheet. Other variant will use a suitable solution, which dissolves the core. The possibility to create various thickness of the sheet wall is an advantage. It can be done very easily, because it is influenced by a winding velocity. The rotation speed of linear formation can provides different structures of nanofibers. The usage of biodegradable polymers offers a possibility to create temporary veins, which will be biologically removed from the body after restoration of original tissue. This reason can allowed faster patient return to normalcy. Another area of the usage is a production of scaffolds. We can imagine the scaffolds as an auxiliary system for a cell growth. Nanofibers create a structure, which can serves for a placing of cells and their growth. The nanofiber web has a high porosity. It is important for a nutrient supply. Low size of pores ensures good protection against a trough of cells. For the production of scaffolds some biodegradable polymers are suitable. We suppose that suitable setting of technologic parameters of device can ensure a higher alignment of nanofibers, which is required for the concrete sort of a tissue. For example, parallelly oriented nanofiber structure can be usable for muscle tissue characterized by transverse stripes. High specific area of nanofibers is predetermined to filter applications. One of them is usage in the area of high efficiency cigarette filters. These filters can be made directly from linear formation with nanofibers. It can provide better filtration properties due to higher filtration

19 efficiency. If a chemical treatment of polymers is used, then certain parts of a smoke can be removed more intensively then others and it can be used for removing of the tar. A production of flat or pleated filters presents another application for linear nanofiber formations. It is known that nanofiber layers have excellent filtration properties. The layers are very thin and some suitable carrier is required. But the carrier decreases efficiency and increases a pressure drop. Consequently, high breathable material has usually very low strength. Linear formations with nanofibers offer the possibility to prepare woven or knitted textiles as is shown in Figure 10 and 11. Figure 10: View of woven textile, combination of cotton uncovered warp and nanofiber covered weft. Cloth count is of 10 wefts/cm.

20 Figure 11: Detail view of woven textile. These types of structures have very good strength and nanofibers on their surface provide required filtration properties. Not only mechanical mechanism exists in the area of the filtration. A biological filtration is very important part. It used for example for water cleaning in garden lakes, aquariums or cesspits. There is used certain sort of a bacteria, which clean the water from pollutions. In these cases, water must to have very low rate flow and bacteria are kept on the same place. Bacteria are flush out if water has the high flow rate. Tests show that nanofibers in the linear form are very good carrier for bacteria and they can be used in water with high flow rate for instance in sewage plants. Conclusion This article shows the new type of nanofiber material. As opposed to other methods it is composed of two parts: a linear carrier forming the core and nanofiber layer forming the sheet. This material is produced by the modified Nanospider device. This material can be suitable for new applications, because it has some interesting properties. The area of applications involves for example special filters, scaffolds or implants.

21 References: 1. T. Ondarçuhu and C. Joachim, Drawing a single nanofiber over hundreds of microns, Europhys. Lett , Hill, Inc., HILLS, INC. announces nanofiber meltblown fabric, available on: G. Taylor, Disintegration of Water Droplets in an Electric Field. Proc. Roy. Soc. London. Ser. A 280: 383, A. L. Yarin, S. Koombhongse, S. Koombhongse and D. H. Reneker, Taylor cone and jetting from liquid droplets in electrospinning of nanofibers, Journal of Applied Physics, Volume 90, Issue 9, pp , November O. Novák, Improvement of filtration properties of nanofibre layers, Conference Autex 2004, Roubaix, ISBN A. Formhals, Process and apparatus for preparing artifical threads, US Patent , October 1934, available on: =anton+formhals 7. Huan Pan, Luming Li, Long Hu and Xiaojie Cui, Continuous aligned polymer fibers produced by a modified electrospinning method, Polymer Volume 47, Issue 14, 28 June 2006, Pages , available on &view=c&wchp=dGLbVlbzSkWW&md5=27aef75b02e8776e03f70ff0f83f5230&ie=/sdarticle.pdf

22 8. A. Theron, E. Zussman and A. L. Yarin, Electrostatic field-assisted alignment of electrospun nanofibers, Nanotechnology 12, pp , E. Smit, U. Büttner, R. D. Anderson, Continuous yarns from electrospun fibers, Polymer, Volume 46, Issue 8, 29 March 2005, Pages , available on: &view=c&wchp=dGLbVlbzSkzS&md5=36e3bd44dd2d047ef8241f1d08444a0c&ie=/sdarticle.pdf 10. A. Bornat, Electrostatic spinning of tubular products, US patent , April 1982, available on: A. Bornat, Production of electrostatically spun products, US patent , August 1987, available on: J. M. Deitzel,J. D. Kleinmeyer, J. K. Hirvonen and N. C. Beck Tan, Controlled deposition of electrospun poly(ethylene oxide) fibers, Dielectric Liquids, ICDL IEEE International Conference on Volume, Issue, 26 June-1 July 2005 Page(s): Zheng-Ming Huang, Y. -Z. Zhangb, M. Kotakic and S. Ramakrishnab, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Composites Science and Technology, Volume 63, Number 15, November 2003, pp (31) 14. Nanospider Technology benefits, available on: