The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

ROLLER ELECTROSPINNING SYSTEM: A NOVEL METHOD TO

PRODUCING NANOFIBERS

F.YENER, B. YALCINKAYA and O.JIRSAK

Nonwoven Department, Faculty of Textile Engineering, Technical University of Liberec,

Studentska 2, 46117, Czech Republic,

Tel: +420 485353 121

yenertex@hotmail.com

Abstract: Electrospinning technique has prepossessed a lot of interests recently. Needle electrospinning is commonly used to produce very thin polymeric fibers. The production rate of the electrospinning from a single jet is quite low. An alternative method to launch many jets and increase production rates are described here. In this work we discuss about roller electrospinning system. Using this method we increased productivity of nanofiber layer up to 3 g/min/m.

Keywords: Roller electrospinning system, nanofiber

1. Introduction Producing nanofiber is one of the most demanded studies for new technological applications. A single jet electrospinning process is commonly used by many research groups to create nano size fibers. This technology based on a syringe which has polymer solution or melt. Solution is ejected to needle tip by using a syringe pump. Needle tip is connected with a high voltage supplier and on the other side there is a collector which can be counter electrode or grounded electrode. Due to electrostatic field between needle tip and collector, the droplets on the needle tip create a cone form which is called as Taylor cone. The thinning jet at the end of cone is moving towards to collector. When the jet has travelled a few centimetres from the droplet, the interaction between electrical, surface, and molecular forces becomes unstable and the jet bends or disperses and forms into small droplets and fibers. Viscosity, type of solvent and polymer, concentration, net charge density (conductivity), surface tension of the polymer fluid and molecular weight can be shown as system parameters. Applied voltage, flow rate of polymer solution, distance between capillary end and collector, ambient parameters and motion of collector can be shown as process parameters. All parameters have big role on fiber morphology. This method is useful for laboratory experiments.

There are a few methods for producing electrospun nanofibers at higher mass production rate are available commercially by ELMARCO (Liberec, Czech Republic), XanoShear™ machine (Xanofi, Inc., NC) and Nanostatics (Columbus, Ohio). In this work roller electrospinning system which is under the trade name Nanospider (by Elmarco) was used [1]. In this method there is a rotating roller which is immersed in a polymer solution. Solution is connected to high voltage supplier. On the up part there is grounded collector and a nonwoven supporting material is passing through collector (Fig. 1). By using rotating roller polymer solution is fed to surface of roller. The variable which is effecting roller electrospinning system can be divided into two groups such as dependent and independent parameters. Independent parameters can be adjusted and controlled and dependent parameters depend on independent parameters. These parameters are classified in the Table 1:

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

Table 1: Summarization of electro spinning parameters [2].

Independent Parameters Dependent Parameters

Concentration of polymer [%] Density of cones (m-2) [A]

molecular weight of polymer (g/mol) Throughput (g/min/m)

Viscosity of polymer solution (Pas) Non-Fiborous area (%) [A]

Surface tension of solution (mN/m) Fiber diameter (nm)

Applied Voltage (kV) Fiber diameter distribution [A]

Velocity of Cylinder (rpm) Throughput/jet

Distance between electrodes (mm) Average current

Velocity of collected fabric (m/min) Average current/jet

Relative humidity (%) Life time of jet

Temperature (°C) etc.

Figure 1. Roller electrospinning system.

The aim of this work is increasing spinnability of solution. We used polyurethane polymer solution and different spinning conditions to improve spinnability of polymer. Cengiz et. al. [3] investigated that additional salt improve spinnability of solution. The spinning performance was increased by using tetraethylammoniumbromide salt up to 1.61g/min/m. Herein; we used another salt lithium chloride (LiCl).

2. Materials and Methods

Polyurethane (PU) (MW 2000g/m, PUR Larithane LS 1086) in dimethyl formamide (DMF) was used. LiCl was used as salt. 17.5% PU was determined as optimum concentration for spinning. 0, 0.08, 0.16 and 0.25 wt. % of salt was added 17.5 wt .% PU solution.

Solution properties (viscosity, surface tension and conductivity) were measured. After measurements solution was spun on roller electrospinning system with the conditions which is tabulated in Table 2. All conditions were kept as stable for each solution.

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

Table 2. Spinning condition of solutions

Applied Voltage (kV)

Distance Between

Electrodes (mm)

Relative Humidity (%)

Temperature (C)

Roller Speed (rpm)

Take up Fabric Speed

(cm/min)

60 120 25 19 4 10

Fibers were collected on the spunbond nonwoven fabric. SEM image was taken, fiber diameter and diameter distribution were measured and fabric performance was calculated according to Formula 1.

P=G*W*Vfabric* (g/min/m) (1)

where

P = Polymer throughput (g/min/m)

G = Nanofibre layer area weight (g/m2)

W = Width of nanofibre layer (m)

Vfabric= Backing fabric take up speed (m/min)

Lr = Length of roller spinning electrode (m)

3. Result and Discussion

In needleless electrospinning, solution jets were generated from an open solution surface. This made the jet initiation process quite different from that in needle electrospinning. In the case of roller electrospinning system solution is transported to roller surface. It is very important to keep humidity and temperature as stable as possible during spinning due to open surface. The results of polymer solutions are shown in Figure 2.

Figure 2. Surface tension, conductivity and viscosity of solutions in various salt concentrations.

It was already investigated that viscosity of solution increases with adding salt [3]. The reason can be due to interaction between salt and solvent. So, polymer-solvent-salt interaction occurs. The value for surface

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

tension did not affected by salt concentration. On the other hand, conductivity was raised by adding salt due to increase in number of ions. The result of performance is shown in Figure 3.

Figure 3. Spinning performance of PU solution in various concentration of LiCl salt.

Figure 4: Fiber diameter of PU solution in various concentration of LiCl salt.

It seems that there is a linear relationship between salt concentration and spinning performance. The increase could be changing viscosity and conductivity. Increasing viscosity increases the thickness of the layer on the surface of roller, and more solution can be transported. In another case, at high viscosity the entanglement of macromolecules are high and when the jet is stretching much polymer solution can be transported to collector. As a result number of jet increases. The same condition is valid for conductivity increase. If the conductivity is high, the electrostatic field increases between solution and collector, as a result more fibers are forming. In the case of PU without any salt, only a few cones were observed on the edge of roller. It was found that the high intensity electric field was mainly formed on the cylinder ends and much lower intensity electric field was formed on the cylinder middle surface area [4]. Figure 5 shows the electric field intensity of roller electrospinning system.

Figure 5: Electric field intensity profiles of cylinder spinneret [4].

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

It has been shown in the electrospray literature that solution concentration has a significant effect on the final size and distribution of particles [5]. Solution surface tension and viscosity also play important roles in determining the range of concentrations from which continuous fibers can be obtained in electrospinning. At low viscosities surface tension is the dominant influence on fiber morphology and below a certain concentration drops will form instead of fibers. Herein increasing viscosity and conductivity at the same time by adding salt, diameter of fibers increases too (Figure 3, 4, 6). In the condition of polymer solution with 0.25% LiCl salt, the diameter of fibers increased drastically. Consequently, spinning performance of solution increased up to 3.26 g/min/m.

Figure 6: SEM images of (a) 0%, (b) 0.08%, (c) 0.16%, (d) 0.25% LiCl concentration.

4. Conclusion Roller electrospinning is an amazing method to produce nanofibers. This method is very useful for production in industrial scales. In this work we tried to study on some parameters to improve spinnability. We observed that adding salt has a big role on final fiber morphology and spinning performance. From literature it was found that 1.27% TEAB salt increases the spinning performance up to 1.61 g/min/m [3]. On the other hand by using LiCl, we are able to increase spinning performance up to 3 g/min/m. It is a question whether high amount of salt is harmful or not. By using another salt in small amount better quality of fibers and spinning performance can be achieved (17.5% PU+ 0.16% LiCl).

It can be concluded that by using roller electrospinning system, we were able to produce nanofibers in big amount in a short time. PU was chosen due to its huge application area. As a future work the rest of parameters are planning to study and new parameters will be added to literature.

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

5. Acknowledgement The authors are thankful to “Mobility Fondu Project TUL” for their financial support and Speacial thanks to all technicians at the Technical University of Liberec,

6. References [1] Jirsak, O.; Sanetrnik, F.; Lukas, D.; Kotek, V.; Martinova, L.; Chaloupek, J., A method of Nanofibres Production from a Polymer Solution Using Electrostatic Spinning and a Device for Carrying Out the Method, European Patent: EP 1 673 493, (2004).

[2] Dao, A. T.; Jirsak, O.; The Role Of Rheological Properties of Polymer Solutions in Needleless Electrostatic Spinning, Ph.D thesis, TUL (2010).

[3] Cengiz, F.; Jirsak, O., The Effect of Salt on the Roller Electrospinning of Polyurethane Nanofibers, Fibers and Polymers, Volume 10, Number 2, 177-184, (2009).

[4] Niu, H.; Wang, X.; Lin, T., Upward Needleless Electrospinning of Nanofibers, Journal of Engineered Fibers and Fabrics, SPECIAL ISSUE – FIBERS, July (2012).

[5] Chen, D.R.; Kaufman, S.L.,"Electrospraying of Conducting Liquids for Monodisperse Aerosol Generation in the 4 nm to 1.8 µm Diameter Range," J. Aerosol Sci. 26: 963-977 (1995).