Effects of Electric Field on Multineedle ... - ACS Publications

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Effects of Electric Field on Multineedle Electrospinning: Experiment and Simulation Study Sheng Xie and Yongchun Zeng* College of Textiles, Donghua University, Songjiang, Shanghai 201620, People's Republic of China ABSTRACT: Electrospinning and melt blowing are the most commonly used processes for producing microfibrous nonwovens. In this paper, we study mass production of electrospinning with a multineedle system. To achieve uniform fibers at a high production rate, an auxiliary plate electrode has been used to be connected to a three-needle system to obtain a more uniform electric field. The spinnerets with two kinds of needle array, linear three-needle and triangular three-needle, are studied. The results of electrospinning experiments and electric field simulation demonstrate that the multineedle spinneret with an auxiliary plate can produce finer and more uniform nanofibers. And the fibers can be collected in a more concentrated area with the auxiliary plate. We also focus on the effect of the needle length protruded outside the plate. This study shows a possibility that by designing the electric field distribution, we can produce thinner fibers and more concentrated collection mats at a high production rate. needles in a linear array and in a 3 × 3 matrix, to observe the behavior of jets in multineedle electrospinning. Tomaszewski and Szadkowski5 constructed three multijet spinning heads with linear, elliptical, and circular arrangements in order to correlate the needle configuration with the production efficiency and process quality. Yang et al.6 designed a seven-needle setup, arranged as a regular hexagon with one needle located in the center, in order to get a stable spinning process. To stabilize and improve the electrospinning process of the multineedle system, Kim et al.7 used a cylindrical auxiliary electrode to concentrate the electric field of a five-jet electrospinning process. Varesano et al.8 tested several different multijet electrospinning setups with 2−16 needles, and a secondary electrode was used to obtain a reduction of the divergence of the jets. Yang et al.9 developed an equilateral hexagon distributed multineedle electrospinning spinneret and used a shielding ring to create a more uniform electric field. Recently, several studies have shown that electric field distribution influences the electrospinning process and the spinning results. Kong et al.10 studied the electric field effects on the shape of deposited webs and showed that the design of the electric field is a significant parameter in the attempt to control web formation. Yang et al.11 studied the effect of electric field distribution uniformity on the cone formation, jet path, and the morphology and the size of the resultant electrospun fibers, and their results showed that the more uniform electric field provided a suitable electric field distribution for producing thinner fibers. Angammana and Jayaram12 investigated the variation of the strength of the electric field at the tip of the needles in multijet arrangements by using experimental and simulation methods. Their investigation showed that the local field deterioration at the needle tips in multijet arrangements degraded the

1. INTRODUCTION Nonwovens refer to sheet or web structures made by bonding or entangling fibers via mechanical, thermal, or chemical means. The new applications of nonwovens require fibers with increasingly smaller diameters. In this paper, we focus on a kind of nonwoven products manufactured with the microfibers of diameter in the range from below 10 μm to around 100 nm. And it will be called “microfibrous nonwovens” in this paper. Microfibrous nonwovens find a variety of applications in areas of filtration media, life science, medicine, and industry.1,2 Electrospinning and melt blowing are the most commonly used processes for producing microfibrous nonwovens. On the one hand, electrospinning has created considerable interest for it can produce nanofibers that have diameters in the 100 nm range; in contrast the fibers produced via melt-blowing technique are usually with average diameters exceeding 1−2 μm. On the other hand, melt-blowing technique has already been in wide commercial use, while mass production lines of electrsopinning have just reached the commercial stage. The production rate of a conventional electrospining system from a single orifice is typically in the range of 10−100 mg/min.3 The slow fiber production rate for single-needle electrospinning leads to a significantly high cost for a commercial electrospinning line. In contrast to the 1500−1800 tons/year production rate of a 3 m wide melt-blowing line, production enhancement on a comparable commercial scale is still of great interest for the researchers. Several methods have been developed to enhance the electrospinning production rate. They can be classified as multineedle and needleless methods. Although the most successful commercial electrospinning line, Nanospider (Elmarco Co., Liberec, Czech Republic), is a needleless device, the multineedle system still needs study as it can be designed to both increase productivity and to produce special structure fibers such as core−shell fibers. To enhance the multineedle system, several types of needle array have been studied. For example, Theron et al.4 arranged two different arrays: nine © 2012 American Chemical Society

Received: Revised: Accepted: Published: 5336

September 11, 2011 January 8, 2012 March 12, 2012 March 12, 2012 dx.doi.org/10.1021/ie2020763 | Ind. Eng. Chem. Res. 2012, 51, 5336−5345

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematics of experimental setups: (a) three-needle system without an auxiliary plate, (b) three-needle system with an auxiliary plate, (c) configuration of a linear three-needle spinneret, and (d) configuration of a triangular three-needle spinneret.

electrospinning process significantly and produced considerable variation in the morphology of the fiber. Multineedle electrospinning is still under active investigation, since it is a simple way to enhance the productivity. Moreover, special structure fibers such as core−shell nanofibers can be produced with multineedle system. The disadvantages in a multineedle system are repulsion from the adjacent jets and the nonuniform electric field on each needle tip of the spinneret. The repulsion will lead to collecting difficulty, while the nonuniform electric field will result to processing problems. Kim et al.7 used a cylindrical electrode to make the electrospun jets stable and consequently concentrate the collection area and make it easier for fiber collection. Unfortunately, they produced coarser fibers with the additional electrode, and they considered the enlargement of fiber diameter as a result of dampening the whipping. Our earlier research13 also indicated that decreasing the whipping would produce coarser fibers. This study aims at producing thinner and more uniform fibers with a multineedle electrospinning system. For this purpose three-needle experimental setups were used. Experimental and simulation work was carried out to study the effect of electric

Table 1. Processing Parameters for Linear Three-Needle Spinneret Electrospinning and Triangular Three-Needle Spinneret Electrospinning experimental setup linear three-needle spinneret triangular threeneedle spinneret

applied voltage (kV) 12.5, 15, 17.5 11, 13.5, 16

collecting distance (cm)

solution flow rate per needle (mL/h)

17.5

0.3

20

0.3

Figure 2. Electrospun jets emitted from linear three-needle spinneret at 12.5 kV applied voltage: (a) without auxiliary plate and (b) with auxiliary plate. 5337

dx.doi.org/10.1021/ie2020763 | Ind. Eng. Chem. Res. 2012, 51, 5336−5345


Article

Figure 3. Fiber mats collected from linear three-needle systems at 12.5 kV applied voltage: (a) without auxiliary plate and (b) with auxiliary plate.

Figure 4. Fiber mats collected from triangular three-needle systems at 13.5 kV applied voltage: (a) without auxiliary plate and (b) with auxiliary plate.

Figure 5. Fiber diameters produced by linear three-needle systems with and without auxiliary plate at various applied voltages. The length of the error bar represents one-fifth of the standard deviations of the fiber diameter.

Figure 6. Fiber diameters produced by triangular three-needle systems with and without auxiliary plate at various applied voltages. The length of the error bar represents one-fifth of the standard deviations of the fiber diameter.

field distribution on fiber diameters and the collection fiber mats. To design different distributions of electric field, an auxiliary plate electrode was used to be connected to the threeneedle system. To describe the electric field distribution, a parameter which was defined as a degree of convergence of

the electric field was introduced. This study shows a possibility that by designing the electric field distribution, we can produce thinner fibers and more concentrated collection mats at a high production rate. 5338



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Figure 7. SEM images of PEO fibers produced by three-needle spinnerets at processing parameters in Table 1: (a) linear three-needle without auxiliary plate, (b) linear three-needle with auxiliary plate, (c) triangular three-needle without auxiliary plate, and (d) triangular three-needle with auxiliary plate.

2. EXPERIMENTS AND SIMULATION 2.1. Experimental Materials. Poly(oxyethylene) (PEO), with an average molecular weight (Mw) of 1 000 000 g/mol, was obtained from Shanghai Liansheng Chemical (China) and used as received. The PEO solution was prepared by dissolving PEO powder in distilled water followed by gently stirring for 5 h at about 30 °C. In this study, 7 wt % concentration of PEO solution was used for the experiments. The solution viscosity was 2120 mPa·s, and the conductivity was 50 μS/cm. 2.2. Experimental Setup. In this study, three-needle spinnerets with and without an auxiliary plate were investigated. Parts a and b of Figure 1 show the three-needle electrospinning setups without and with an auxiliary plate, respectively. The three needles were arranged in two different arrays: a linear array and an equilateral triangle arrangement. Figure 1c shows the linear three-needle spinneret, while Figure 1d shows the triangular three-needle spinneret. The spinneret with the auxiliary plate used an aluminum plate as an electrode to be connected to the high-voltage generator. The metal needles (0.84 mm inner diameter and 1 mm outside diameter) were inserted into and protruded from the 1 mm diameter holes on the plate. The polymer solution was forced by a Cole-Parmer 74900-00-05 syringe pump (Cole-Parmer Instrument Co., Vernon Hills, IL, USA) through the insulated tubes, resulting in formation of drops of polymer solution at the needle tips. A high-voltage generator that can supply direct current (dc) voltage up to 40 kV was applied to the needle and the collector. Fiber samples for characterization were prepared by adhering samples of the aluminum foil target to a platform and then sputter-coating with gold before imaging to minimize the charging effect. PEO fibers were imaged using a scanning electron microscope (JSM-5600LV, JEOL, Tokyo, Japan). For each sample, at least 100 points were randomly selected and the fiber diameters were measured. Photoshop Cs 3

(Adobe System Inc., San Jose, CA, USA) software was used to determine the fiber diameters. The areas of fiber mats were searched by scanning the aluminum foil in an EPSON scanner (Perfection V30, Nagano, Japan). 2.3. Simulation. The electric fields of the experimental setups were simulated by the Ansoft Maxwell (ANSYS Inc., Canonsburg, PA, USA) software. The Ansoft Maxwell 2D software was used for simulating the electric fields of the linear three-needle systems, while the Ansoft Maxwell 3D software was used in the simulation of the triangular three-needle systems.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. Two configurations of multineedle spinneret, linear three-needle spinneret and triangular three-needle spinneret (see the scheme in Figure 1), were arranged to enhance the spinning production rate, as well as to investigate the electrospun fibers and the collected fiber mats. A series of experiments were carried out. The processing parameters are summarized in Table 1. The needle length protruding the plate (we call it “needle length” in this paper) was set to be 20 mm. Figure 2 shows the jets resulted from the linear three-needle systems without and with an auxiliary plate, respectively, at 12.5 kV. The central jet was vertical, while the jets on the sides were pushed apart. The deviation angle from the vertical direction θ was about 32.6° for the system without an auxiliary plate, while it was about 15.2° for the system with an auxiliary plate, which was significantly smaller than the angle observed without the auxiliary plate. Figure 3 shows the fiber mats collected from the linear three-needle spinnerets without and with the auxiliary plate, respectively, under the same condition of 12.5 kV applied voltage. Due to the mutual Coulomb forces, each jet deposited fibers on the collector in separated zones. It is clear that when the auxiliary plate was used, the distance 5339



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Figure 8. Electric field distributions of line three-needle systems: (a) without auxiliary plate and (b) with auxiliary plate.

field, and their length is proportional to the strength of the electric field at that position. Parts a and b of Figure 9 show Ez (z-directional electric field strength) along the lines z = 0, 10, 40, 100, and 175 mm (as the dashed lines in Figure 1c), and parts c and d of Figure 9 show the data of Ex (x-directional electric field strength). It can be seen that the maximum Ez at the needle tip for the three needles is different. The central one has the lowest value due to the influence of the other two needles in the arrangement. Therefore the central jet undergoes lower drawing force than the jets on the sides.12,14 That is why at a low applied voltage, the amount of ejected jets at the center is relatively small while the droplet formed a big stable Taylor cone (Figure 10). The strength of the electric field of the jets on the sides is too strong to form a stable Taylor cone. From the simulation results shown in Figures 8 and 9, we can see that the system with an auxiliary plate produces a more uniform electric field with smaller electric field strength at the tip but a little larger average electric field strength along the working distance between needle tips and the collector. For the triangular three-needle systems without and with the auxiliary plate, Figure 11 shows Ez (electric field strength in the z-direction) distribution along the collecting distance at different y positions of y = 15, 25, and 65 mm in the y−z plane. It is obvious that the system with an auxiliary plate produces larger and more uniform electric field except for the area very

between the deposition zones of each jet was reduced. The concentrating effect of the auxiliary plate was also observed when the triangular three-needle spinneret was used at the applied voltage of 13.5 kV (Figure 4). Figure 5 shows the fiber mean diameter produced from the linear three-needle systems for a range of applied voltages (the fibers are obtained from the jets on the sides). We can see that the spinneret with an auxiliary plate produces a smaller diameter of fibers. The results obtained from the triangular three-needle spinnerets (Figure 6) indicate that, except for the case of 13.5 kV, the spinneret with an auxiliary plate produces a smaller diameter of fibers. In fact, in the case of 13.5 kV applied voltage, the mean diameter difference obtained from the two systems is less than 10 nm. This small difference may result from the error in observation when the fiber diameters were measured. Scanning electron microscopy (SEM) images shown in Figure 7 are the comparison of the fibers spun from the threeneedle systems without and with the auxiliary plate. Systems with the auxiliary plate produce a bit more regular fibers. 3.2. Simulation Results and Analysis. Figure 8 shows the simulation results of the electric field distributions of the linear three-needle systems (with and without an auxiliary plate) in the x−z plane calculated by the Ansoft software. The simulation conditions were in accordance with the experimental parameters. The arrows indicate the direction of the electric 5340



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Figure 9. Comparison of the electric field strength distribution for linear three-needle system: (a) Ez distribution for spinneret without auxiliary plate, (b) Ez distribution for spinneret with auxiliary plate, (c) Ex distribution for spinneret without auxiliary plate, and (d) Ex distribution for spinneret with auxiliary plate. Applied voltage in the simulation was 15 kV.

Figure 10. Electrospun jets and Taylor cone emitted from linear three-needle spinneret with auxiliary plate. 5341



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Figure 12. High-speed photographic image of the jet path in electrospinning.

Figure 11. Comparison of the electric field strength distribution for triangular three-needle systems with and without an auxiliary plate in the y−z plane along y = 15 mm, y = 25 mm and y = 65 mm. Applied voltage in the simulation was 13.5 kV.

near the needle tip. The smaller gradient of the electric field strength provides the jet with a larger and more uniform electric force which could help whipping at high speed11 and consequently produce smaller fiber diameters. To study the relationship between the electric field distribution and the whipping, the simulation for single-needle electrospinning arrangements with and without an auxiliary plate were carried out. The simulation results show that when the distance from the needle tip exceeds 40 mm, the spinneret with an auxiliary plate produces larger electric field strength. In electrospinning, the jet is first stretched in a straight path. Then it undergoes whipping instability. During the whipping process, the jet coils into a complex path, leading to a dramatic stretching. As the jet is continuously elongated and the solvent is evaporated, its diameter is greatly reduced. We investigated the jet path using high-speed photography. Figure 12 shows the image of the electrospinning process taken by the Redlake HG-100K high-speed camera. This camera has the capability of recording images at a frame rate of 1000 frames/s or up to

Figure 13. Calculated f for (a) linear three-needle systems at x = 10, 20, and 60 mm, and (b) triangular three-needle systems in electrospinning process in y−z plane along y = 0 mm, y = 25 mm, y = 65 mm.

10 0000 partial frames/s. Full frames are recorded at a resolution of 1504 × 1128 pixels. A Nikon 85 mm macrolens was used in conjunction with the camera. This image was recorded at a frame rate of 2000 frames/s and at a resolution of 512 × 512 pixels. The measured straight jet was about 5 cm. Our simulation indicates that near the bending point where the jet starts whipping, the system with an auxiliary plate produces a stronger electric field that provides a larger electric force. This may lead to a faster whipping speed which helps produce finer fibers due to the additional stretching.11 To explain the concentration effect of the auxiliary plate on the multineedle electrospinning process, a parameter f is 5342



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where Ex and Ey are the components of the electric field strength E. For the linear three-needle systems, Ey = 0. Figure 13a shows the comparison of f for the linear threeneedle systems with and without an auxiliary plate. It is noted that x = 10 mm is along the axis of needle 1 (Figure 1c). Figure 13b shows the comparison of f for the triangular threeneedle systems with and without an auxiliary plate. In this figure, y = 15 mm is along the axis of needle 1 in the y−z plane (Figure 1d). As shown in theses figures, lower values of f in the process with an auxiliary plate indicate higher convergence of the electric field to the spinning line than that of the process without the auxiliary plate. Therefore, we obtain a more concentrated fiber mat with the use of the auxiliary plate. Moreover, the system of the triangular three-needle has lower values of f than the linear three-needle system. This indicates that the needle array will influence the stability of the electrospinning process. 3.3. Effect of Needle Length on Multineedle Electrospinning. The needle length (i.e., the needle length protruding from the plate) can be changed by moving the plate relative to the needles. Different needle lengths will produce different electric fields. To study the effect of needle length on electrospinning, a series of experiments were carried out with changing the needle length in a triangular three-needle spinneret.

Figure 14. Effect of needle length on fiber diameters produced by triangular three-needle spinneret with an auxiliary plate. The length of the error bar represents one-fifth of the standard deviations of the fiber diameter.

defined to describe the convergence degree of the electric field. It is calculated as f=

Ex 2 + Ey 2 || E ||

(1)

Figure 15. SEM images of PEO fibers produced by triangular three-needle spinneret with needle length of (a) 0, (b) 10, (c) 20, (d) 30, and (e) 40 mm. 5343



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Figure 16. Fiber mats collected from triangular three-needle spinneret with needle length of (a) 0, (b) 10, (c) 20, (d) 30, and (e) 40 mm.

plate were simulated. It is observed that the system with an auxiliary plate produces smaller diameters of fibers at most cases. And with the auxiliary plate, the collected fiber mats are more concentrated, which will lessen the difficulty in fiber collection of multineedle electrospinning. A parameter, f, was defined to describe the convergence degree of the multineedle electric field. It is shown that the system with an auxiliary plate has a smaller f, which indicates less divergence of the electric field. Finally, the effect of the length of the needle protruding the plate in a multineedle system with an auxiliary plate was investigated. The results show that the reduced needle length produces smaller fiber diameters.

PEO, with an average molecular weight (Mw) of 600 000 g/mol, obtained from Sigma-Aldrich (St. Louis, MO, USA), was used to prepare a solution that was employed as the working fluid. A 5 wt % concentration of PEO solution was used for the experiments. The experiments were carried out with the applied voltage, collecting distance, solution flow rate per needle being held at 22 kV, 22 cm, and 0.3 mL/h, respectively. The needle lengths were set as 0, 10, 20, 30, and 40 mm. The fiber diameter distributions and the SEM images of the spun nanofibers are shown in Figures 14 and 15. There is a small increase in the fibers' mean diameters when the needle length decreases from 40 to 30 mm. With further decrease of the needle length, the fibers' mean diameters decrease. The smallest fiber diameters are obtained when the needle length is as small as 0, which means a uniform electric field with a flat spinneret is formed. The experiments also show that the flat spinneret produces the most concentrated collected fiber mats (Figure 16). For a flat spinneret with the needles inside the holes, we consider using a flat spinneret with holes instead of needles to produce thinner fibers and more concentrated collection mats at a high production rate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No. 10972052), Foundation for the Author of National Excellent Doctoral Dissertation of the People’s Republic of China (Grant No. 2007B54), New Century Excellent Talents Plan of Chinese Ministry (Grant No. NCET-09-0285), and Shanghai Dawning Program (Grant No. 10SG33).

4. CONCLUSIONS In this study, we first arranged single-needle electrospinning systems with and without an auxiliary electrode to study the effect of electric field distribution on fiber diameters, using experimental and simulation methods. The results show that finer fibers can be produced by the system with an auxiliary plate with an electric field of smaller electric field strength at the tip but larger average electric field strength in the working distance between needle tip and collector. Then, two configurations of multineedle spinneret, linear three-needle spinneret and triangular three-needle spinneret, were arranged to enhance the spinning production rate, as well as to investigate the stability for multineedle electrospinning. The electric fields of the systems with and without an auxiliary

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REFERENCES

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Effects of Electric Field on Multineedle ... - ACS Publications

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