IMPROVING PERFORMANCE OF POLYVINYL BUTYRAL ELECTROSPINNING. Fatma YENER, Oldrich JIRSAK

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1 IMPROVING PERFORMANCE OF POLYVINYL BUTYRAL ELECTROSPINNING Fatma YENER, Oldrich JIRSAK Nonwoven Department, Faculty of Textile Engineering, Technical University of Liberec, Studentska 2, , Czech Republic, Abstract Spinning performance (or throughput) is one of critical parameters influencing productivity and price of nanofiber webs. Besides molecular weight of polymer, kind of solvent and concentration of polymer solution, addition of some salts to polymer solution was found to be an effective way to influence both electrospinning throughput and quality of nanofiber web. Influence of the salt of complexing metal - ZnCl 2 on the throughput and web quality of polyvinyl butyral (PVB) nanofibers was studied in this work. The salt was added to the solution of PVB in ethanol after molecular weight of polymer and concentration of solution was optimized. Various salt concentrations in terms of molar ratio Zn/ OH were tested using roller electrospinning device. Addition of salt influenced throughput and web quality considerably. Molar ratio Zn/ OH = 1/6 was found an optimum salt concentration. Keywords: PVB, nonwoven, roller electrospinning, salt, nanofiber 1. INTRODUCTION Polyvinyl butyral (PVB) polymers have been extensively used in many applications, since PVB is a low-cost alternative showing flexibility, optical clarity and a good adhesion to many surfaces. In spite of the extensive literature on PVB polymer, only a limited number of studies were found involving the PVB nanofibers [1-6] produced by needle electrospinning and only one on continuous nanofiber production [7]. The main reason for this lack of knowledge consists probably in its difficult processability in the needleless electrospinning. Nanospider is one of the magnificent method to produce nanofibers continuously [8]. To benefit from PVB polymers enormous characteristics, Nanospider was used in this work. The main problem that we have to face in Nanospider technology is that some polymers are difficult to electrospinn. PVB is one of the problematic polymers which has low throughput. To overcome this problem, additional ZnCl₂ salt was used in various concentration. Addition of salt influenced throughput and web quality pretty well. Conductivity-enhancing salts can significantly affect the polymer throughput and fiber density in the final web. Even at low concentrations, the salts evoke formation of a high number of fibers during spinning process in some cases. Cengiz et. al. studied the effect of salts on roller electrospinning of polyurethane and determined that adding tetraethyleneammonium bromide to the polyurethane solution in dimethylformamide increase the conductivity, viscosity and fabric throughput. It was shown that the salt increases the entanglement of macromolecules. As a result, the polymer network is more solid [9]. Li et. al. investigated that by adding inorganic salt LiCl, fiber diameter of PAA decreased due to increasing conductivity [10]. Zong et al. showed the effect of various types of salts on the morphology of electrospun membranes. The additional salts resulted in a higher charge density on the surface of the ejected jet during electrospinning. As a result, higher electrical charge could be carried by the electrospun jet. In addition, the charge density was uniformly distributed because the charge density on the surface of the jet was enhanced by the ionic salts [11]. It was the aim of this work was to study the influence of a complexing salt (Zn process of PVB and on the quality of produced nanofiber layers. ) on the electrospinning

2 2. EXPERIMENTAL 2.1 Material Polyvinyl butyral (PVB) was purchashed from Kuraray - the grade Mowital B 60 H (mol. weight g/mol). PVB is produced by reaction of PVA with buthyaldehyde. The trade name Mowital is followed by a capital B stating the aldehyde used. In this case the products are based on butyraldehyde. The numbers refer to the degree of polymerization, the higher the number the higher the degree of polymerization and viscosity. The suffixes H indicates the degree of acetalization, H being the medium. Various grades of PVB differ in the molecular weight and in the ratio x:y:z. Ethanol was used as the solvent of PVB. ZnCl 2 salt was purchashed from Lachema. Conductivity (Radelkis OK-102/1), viscosity (Haake Roto Visco 1 at 23 C ), surface tension (Krüss K9) tests were done. SEM images were taken by Phenom FEI. 2.2 Method Ethanol and ZnCl 2 were stirred gently with a magnetic bar at room temperature and atmospheric pressure. PVB polymer was added slowly to salt-ethanol solution. 8 wt % PVB60 was determined as optimum concentration from previous works. Formula of PVB is shown in material part. The OH group of PVB reacts with Zn element. The quantitiy of ZnCl 2 was calculated according to molar ratio of Zn/-OH (-OH from PVB). 0, , , 0.125, 0.166, 0.25, 0.5, 1 molar ratios of Zn/-OH were prepared and rested over the night. Spinning conditions are on Table 1. Spinning conditions were optimized based on previous experiments. Table 1 Spinning conditions of roller electrospinning Applied Voltage (kv) Distance between Roller and Collector (cm) Relative Humidity (%) Temperature (C ) Fabric Speed (cm/min) Roller Speed (rpm)/length (cm)

3 Fig. 1 Roller electrospinning system Conductivity, viscosity and surface tension of solutions were measured. Salt is highly dissociated in ethanol which leads to high cunducivity of solutions. All conditions were kept stable. Polymer solutions were spun using roller electrospinning (Nanospider). In the Nanospider technology, there is a rotating roller spinning electrode which is partially immersed in a solution channel. A nonwoven web is moving along the collector electrode. ( Figure 1). A high voltage is applied to the rotating roller which is immersed in to a polymer solution channel. If the electrical force overcomes surface tension of polymer solution, jets are developed on the surface of roller towards to collector. A nano fiber layer covers the nonwoven web. As a result, continuous nano webs are formed. Fiber webs were collected on to polypropylene spunbond nonwoven fabric. The morphology of the electrospun nanofibres was examined using a scanning electron microscope. Fiber diameters were calculated by using Lucia 32G. The average diameter was determined by taking 100 measurements for one set of parameters.fabric throuhgput was calculated according to formula 1. P=Performance(g/min/m) G=Fabric weight (g/m 2 ) P= G*Lf*V fabric * g/min/m..(1) Lf=Length of fabric covered on the width of collector fabric (m) V fabric = fabric take up speed (m/min) Lr= lenghth of roller (m) 3. RESULT AND DISCUSSIONS Viscosity, elasticity, conductivity and surface tension of the solution are the most important and influential parameters. Changing the polymer concentration leads to various solution viscosities. The surface tension coefficient depends on the polymer and solvent. Surface tension tries to make the surface area per unit mass smaller, by changing the jets into spheres. Solution viscosity is important for an efficient electrospinning process and fiber morphology. At very low viscosities, the solution forms droplets, leading to the process called electrospraying. At very high viscosity, entanglement of polymer chains are

4 high. Increase in the chain entanglements made the stretching of the charged jet difficult as a result higher fiber diameter can be observed. Graph of conductivity, surface tension and viscosity vs. concentration were shown in Figure 2 (a,b,c). Fig. 2 a) surface tension vs. molar ratio (Zn/-OH), b) Conductivity vs. molar ratio (Zn/-OH), c) Viscosity vs. molar ratio (Zn/-OH) graphs. As shown in Fig. 2(a) surface tension of polymer solution was not significantly influenced by adding salt. However, effect of salt on conductivity is high. By adding salt, the conductivity increases significantly. Viscosty changes in a non-linear way. Viscosity mostly depends on concentration of polymer solution as we can see from literature [12,13]. Cengiz et al. found that by increasing salt concentration, viscosity and

5 conductivity inreased but surface tension was stable. We have not observed significant increase in viscosity with addition of salt in the case of PVB [9]. Zn/-OH= 0 Avr Dia: 330±120 nm Zn/-OH= 0,0312 Avr Dia: 670±230 nm Zn/-OH= 0,0625 Avr Dia: 460±167 nm Zn/-OH= 0,125 Avr Dia: 497±134 nm Zn/-OH= 0,166 Avr Dia: 387±211 nm Zn/-OH= 0,25 Avr Dia: 472±110 nm Zn/-OH= 0,5 Avr Dia: 440±201 nm Fig. 3 Fiber diameters of PVB-Salt polymer. Zn/-OH= 1 Avr Dia: 652±192 nm PVB-salt solution absorbs ambient water during electrospinning. Without adding salt fibers are more straight and thicker. ZnCl₂ supports absorbtion of humidity. At higher humidity, the absorption of water does not allow to complete the drying process during the time of flight of the polymer solution jet. This situation cause the fibers to be wet, curly and entangle to each other. At higher humidity, the nanofibres are fusing. Another important point for nano web is throughput. Fabric throughput was calculated according to formula 1. Figure 4 Shows that there is not a relationship between viscosity, fiber diameter, throughput and molar ratio of Zn/-OH groups. Fig. 4 Graph of viscosity, fiber diameter and throughput vs. Zn/-OH molar ratio. Finally, Additional salt, in different concentrations, increased fabric throughput.

6 4. CONCLUSION Nanofibers has an important role for many fields. The main problem, not every polymer is able to spin on continuous electrospinning system. Polyvinyl butyral is one of the newest polymer around the World. This polymer has many qualifications such as strong binding, optical clarity, adhesin to many surfaces, toughness and flexibility. In this work we tried to spin PVB and improved fiber quality and fabric throughput by using salt addition. Fabric throughput of 8% concentration of PVB polymer solution was increased by adding ZnCl₂ salts in the molar ratio of Zn/-OH = 1:6. This polymer solution was chosen as useful for end applications due to the low concentration of salt, nanoweb quality and fabric throughput. ACKNOWLEDGEMENTS This work was supported by Technical Universty of Liberec, Nonwoven Depertmant. The authors greatfully acknowledge to laboratory workers in Nonwoven Department for providing all device, Dao Anh Tuan, Funda Cengiz Çallıoğlu and Baturalp Yalçınkaya for their support. LITERATURE [1] Zhang, Y., Xiao, C., An, S., Yang, J., A morphological study of mullite long fiber prepared using polyvinyl butyral as spinning aids, J Sol-Gel Sci Technol, Vol.57, pp , [2] Imaizumi, S., Matsumoto, H., Suzuki, K., Minagawa, M., Kimura, M., Tanioka, A., Phenolic Resin-Based Carbon Thin Fibers Prepared by Electrospinning: Additive Effects of Poly(vinyl butyral) and Electrolytes, Polymer Journal, Vol. 41, pp , 2009 [3] Lubasova, D., Martinova, L., ControlledMorphology of Porous Polyvinyl Butyral Nanofibers, Journal of Nanomaterials, Vol. 2011, ID , 6 pages, 2011 [4] Chen, L., Liao, J., Lin, S., Chuang, Y., Fu, Y., Synthesis and characterization of PVB/silica nanofibers by electrospinning process, Polymer, Vol. 50, pp , 2009 [5] Qiu, Y., Yu, J., Rafique, J., Yin, J., Bai, X., Wang, E., Large-Scale Production of Aligned Long Boron Nitride Nanofibers by Multijet/ Multicollector Electrospinning, J. Phys. Chem., Vol. 113, pp , [6] Berutti, F. A., Alves, A. K., Bergmann, C. P., Clemens, F. J., Graule, T., Synthesis of CeO2 and Y2O3-Doped CeO2 Composite Fibers by Electrospinning, Particulate Science and Technology, Vol. 27, pp , [7] Yener, F., Jirsak, O., Gemi, R., Effect of Concentration on Roller Electrospinning, 6th Nanoscience and Nanotechnology Conference, Izmir, Turkey, [8] Jirsak, O., Sanetrnik, F., Lukas, D., Kotek, V., Martinova, L., Chaloupek, J., C.R. Patent, WO , [9] Cengiz, F., Jirsak, O., The Effect of Salt on the Roller Electrospinning of Polyurethane Nanofibers, Fibers and Polymers, Vol.10, pp , [10] Shanshan, L., Guodong, Y., Suwen, Y., The effects of Inorganic Salt (LiCl) on the Electrospun Polyimide Nanofibers, Internatıonal Conference On Advanced Fibers And Polymer Materials, Vols 1-2, pp , [11] Zong, X., Kim, K.; Fang, D., Ran, S., Hsiao, B. S., Chu, B., Structure and Process Relationship of Electrospun Bioabsorbable Nanofiber Membranes Polymer, Vol. 43, 4403, [12] Deitzel, J.M., Kleinmeyer, J., Harris, D., Beck Tan, N.C., The effect of processing variables on the morphology of electrospun nanofibers and textiles, Polymer, Vol 42, pp , 2001 [13] Chowdhury, M., Stylios, G., Effect of experimental parameters on the morphology of electrospun Nylon 6 fibres, International Journal of Basic & Applied Sciences, Vol: 10, No: 06, 2010.