Turbulent Air Flow Field and Fiber Whipping Motion in the Melt

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Turbulent Air Flow Field and Fiber Whipping Motion in the Melt Blowing Process: Experimental Study Sheng Xie and Yongchun Zeng* College of Textiles, Donghua University, Songjiang, Shanghai 201620, P.R. China ABSTRACT: Electrospinning and melt blowing are the most commonly used processes for producing microfibrous nonwoven materials. A whipping motion during electrospinning has been observed by several researchers. However, much less work has been done on the fiber whipping dynamics in the melt blowing process. In this study, a hot-wire anemometer was used to measure the turbulent air flow field below a single-orifice melt-blowing slot die. The characteristics of the mean velocity, mean temperature, and fluctuating velocity were obtained. Then, a high-speed camera was used to record the motion of a fiber below the die. The fiber whipping path was observed, and the amplitude and frequency of the whipping were obtained. It was found that the turbulent fluctuations are related to the fiber motion in the melt-blowing process. This work examines the physics of turbulent melt-blowing jets and the fiber whipping occurring during melt blowing using an experimental approach.

1. INTRODUCTION Nonwovens refer to sheet or web structures made by bonding or entangling fibers through mechanical, thermal, or chemical means. The new applications of nonwoven materials require fibers with increasingly smaller diameters. Nonwoven products manufactured using microfibers with diameters in the range from below 10 μm to around 100 nm find a variety of applications in areas of filtration media, life sciences, medicine, and industry.1,2 Electrospinning and melt blowing are two major processes for producing microfibrous nonwoven materials. Electrospinning involves applying a strong electric field to a polymer solution such that the resultant electric force attenuates the polymer jet into nanofibers. During melt blowing, air jets with high velocity and high temperature are applied to a polymer melt, and the resultant drag force attenuates the polymer jet into microfibers. The electrospun and melt-blown fibers are collected directly as nonwoven webs. Melt blowing is accomplished using specifically designed dies. Various designs for melt-blowing dies exist, with slot and annular die configurations being the most common. Annular dies use groups of polymer capillaries, each surrounded by a single annular jet. Slot dies use a row of capillaries to eject polymer melt, and a V-shaped slot to supply two jets of air. Figure 1 shows a schematic of the single-orifice slot die used in this study, which is based on the design of a typical commercial melt-blowing die. Because the polymer melt is extruded directly into hot air jets, the formation process and properties of the fibers depend on the air flow field, which, in turn, depends on the die configuration. Temperature and velocity profiles of the air flow produced from melt-blowing dies of various configurations having single or multiple polymer capillaries have been experimentally studied in the literature. Uyttendaele and Shambaugh3 measured the velocity and temperature fields below a single-hole melt-blowing annular die. Majumdar and Shambaugh4 examined seven different annular nozzles to develop a single set of correlations. Harpham and Shambaugh5,6 measured the flow fields of dual rectangular jets, the © 2012 American Chemical Society

type of jet used in industry to produce melt-blown fibers. Tate and Shambaugh7,8 examined the velocity and temperature fields below melt-blowing slot dies in order to predict an optimum die design. Bresee and Ko9 presented experimental measurements to provide air velocity and temperature information of a 600-hole slot die in a commercial-like melt-blowing system. Begenir10 carried out air velocity and temperature measurements on a 550-hole pilot-scale melt-blowing line. In these studies, process air temperatures were measured with thermocouples, and velocity measurements were made using pitot tubes. However, pitot tubes and thermocouples can report only the mean velocity and mean temperature of an air field. The air flow field of a melt-blowing die provides several complications including compressible flow and high temperatures. Moreover, free turbulent jet flow, which is one of the classical problems of turbulent fluid flow, applies to the meltblowing process. Turbulent fluctuations are fundamental characteristics of a turbulent fluid flow. To obtain the velocity fluctuations in turbulent jets issuing from a melt-blowing die, a hot-wire anemometer was used in this study. The images of a jet path in electrospinning have been successfully captured by several researchers,11−13 using highspeed photography. The images demonstrate that, after a straight segment, the jet develops a bending, spiraling, and looping path. The bending instability in electrospinning was first called “whipping” by Homan et al.,11 whereas Reneker and Yarin’s group12,13 still called it “bending instability” in their work. Compared to electrospinning research, much less has been done on whipping motion in the melt-blowing process. The online measurement of fiber motion in the melt-blowing process is not easy. Wu and Shambaugh14 used laser Doppler velocimetry to measure the fiber velocities and the diameters of the fiber “cone”. The fiber vibrations were first recorded by Received: Revised: Accepted: Published: 5346

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process will also be called whipping, as was previously done by Ellison et al.20 in their research on melt-blown nanofibers. In this study, we measured the turbulent air flow field below a melt-blowing slot die. Then, the information on fiber whipping in the melt-blowing process was captured with high-speed photography. These recorded images were processed to determine the frequency and amplitude of the whipping process.

2. MELT-BLOWING TURBULENT AIR FLOW FIELD 2.1. Air Velocity and Temperature Measurements. The measurements were carried out on the single-orifice meltblowing device shown in Figure 2. The melt-blowing die used

Figure 2. Melt-blowing device and hot-wire anemometer with the traversing unit.

in this study had the configuration shown in Figure 1. This type of die is referred to as a blunt-edge die with a nose-piece width ( f) of 1.28 mm, a slot angle (α) of 30°, and a slot width (e) of 0.65 mm. The slot length l was 6 mm, and the orifice diameter d was 0.42 mm. During the experiment, air with a pressure of 0.2 MPa and a temperature of 260 °C was compressed into the slots. The die temperature was set to be the same as the air temperature, 260 °C. The air velocity and temperature below the die were measured online with a hot-wire anemometer (Dantec StreamLine CTA90C10 and Dantec StreamLine CTA90C20, Dantec Dynamics, Skovlunde, Denmark) in the absence of the polymer stream. The anemometer probe was a metal fiber with a 5-μm diameter and a 1.2-mm length suspended between two needle-shaped prongs. The fine dimensions of the hot-wire anemometer minimized disturbance to the air flow. The hotwire anemometer was positioned with a three-dimensional traversing system, which permitted x, y, and z motions in 0.01-mm increments. The coordinate system used in this work is also shown in Figures 1 and 2. All coordinates are relative to the die face. Its origin is at the center of the die face. The x direction is along the major axis of the nose piece and slots, whereas the y direction is perpendicular to the major axis of the nose piece and slots. The z direction is directed vertically downward. 2.2. Air Velocity and Temperature Characteristics. 2.2.1. Mean Velocity and Temperature Profiles. Figure 3 shows the development of the mean velocity profiles. Figure 3a shows the mean velocity along the y direction at different z levels. It can be seen that, for the position of z = 2 cm, the profile exhibits a

Figure 1. Detailed schematic of the melt-blowing die: (a) crosssectional view and (b) end-on view.

Rao and Shambaugh.15 They recorded multiexposure strobe photographs of the melt-blowing threadline, using an exposure time of 0.25 s. They observed that there was a progressive increase in the width of the cone of fibers as the distance from the die increased. A previous work by Chhabra and Shambaugh16 looked at the online fiber motion. The fiber vibration amplitude and frequency were measured with multiimage flash photography and laser Doppler velocimetry, respectively. These experiments were performed under laboratory-scale single-hole melt-blowing conditions. In addition, their work was limited to fairly low velocities, approximately an order of magnitude below the normal operating speed of a melt-blowing die. Breese and coworkers9,17,18 recorded photographs of full-speed fibers under commercial-like melt-blowing conditions, using a high-speed (1000 frames/s) digital camera with pulsed laser illumination. Beard et al.19 used a high-speed camera (2000 frames/s) to record the motion of a fiber below both a melt-blowing slot die and a melt-blowing swirl die. The pictures of fiber vibration were recorded at a certain position below the dies. During melt blowing, the fiber undergoes bending instability, which is called whipping in electrospinning. In this study, bending instability (or fiber vibration) in the melt-blowing 5347

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spreading effect is further illustrated in Figure 3c. At positions y = 3 and 5 mm, which are outside the slot region (−0.96 mm ≤ y ≤ 0.96 mm), the mean velocity increases to a maximum and then decays with further increasing z distance until reaching a plateau. It was found that, during the measurements, because of the high velocity of the air jets, the probe of the hot-wire anemometer was easily broken when it was close to the die. Therefore, the measurement region was z = 1.75−10 cm. In contrast, for the centerline velocity, we tried a position closer to the die, and the measurement at the centerline was in the range z = 0.75−10 cm. The lower velocities obtained in the region close to the die and outside the slot region (x > 3 mm, y > 0.96 mm, and z < 4 cm) are attributable to the merging of the air jets from the two individual slots. The merging of the air jets produces complicated velocity characteristics in the region close to the die. After the merging zone, the velocities decay rapidly until they gradually become steady. It is in this first significant air-velocity-drop region of z < 5 cm where the polymer melt exiting the capillary meets with the maximum, very high air velocity at the centerline. This results in a rapid attenuation of the fiber. Figure 4 shows the mean air temperature profiles that accompany the velocity profiles in Figure 3. The trend of the temperature decay is similar to that of the velocity decay. It can be seen that, in the region around the centerline, the air temperature profiles decrease rapidly until reaching a plateau. It is this temperature drop that is believed to be the driving force for crystallization of the polymer melt and fiber formation in the melt-blowing process. 2.2.2. Turbulent Fluctuations. The measurement of turbulence quantities is of importance to the melt-blowing process. Strong velocity fluctuations can lead to operating problems, such as fibers sticking to the die face or newly formed fibers becoming entanged with themselves or with adjacent fibers. Furthermore, velocity fluctuations play an important role in fiber motion and fiber formation. The hot-wire anemometer measures the instantaneous velocity at a point. The instantaneous velocity, ut, at the centerline at z = 5 cm as a function of time is shown in Figure 5. Figure 5a shows a plot of ut for a time segment of 1 s, whereas Figure 5b shows a subset of the data from 0.5 to 0.6 s. The instantaneous velocity can be expressed as the sum of the mean and fluctuating velocities, namely, ut = u + u′. Figure 6 shows the probability density distribution of the centerline ut values at different z levels. As z increases from 2 to 8 cm, the largest probability of the instantaneous velocity decreases from around 100 to 40 m/s. Figure 7 shows the ratio between the fluctuating velocity and the mean velocity, u′/u, at the centerline. The profile has a twopeaked shape. The first peak corresponds to the location where the merging occurs. The second peak corresponds to the region where the centerline velocity drops quickly. The generation of the velocity fluctuation is proportional to the z-axial velocity gradient. It can been seen from the mean velocity profile (Figure 3) that the lowest velocity gradient occurs in the area beyond z > 5 cm, so that this region corresponds to the region of rapid decrease of u′/u in Figure 7. Earlier experiments,3 as well as our previous study on numerical modeling of fiber motion during melt blowing,22 supported that rapid fiber attenuation occurs in the region when the distance from the die is within 5 cm. The location of the second peak of u′/u is important, because it is relative to the end of the fiber rapid

Figure 3. Air mean velocity profile (a) along the y direction at different z levels (across the die face), (b) along the z direction at different x positions (y = 0), and (c) along the z direction at different y positions (x = 0).

sharp peak. As z increases from 2 to 10 cm, the velocity maximum decreases, and the profile loses its sharp look. Figure 3b shows the development of the mean velocity profile as a function of the distance, z, at different x positions. Because the slot length was 6 mm, the position of x = 3 mm represents the edge of the slots, and the position of x = 6 is outside the slot region. The profile at x = 3 mm is very similar to that at x = 0 (centerline), whereas the profile at x = 6 mm is much different from those at x = 0 and x = 3 mm. This indicates that the characteristics of the velocity are almost unchanged with x in the region within the slots (−3 mm ≤ x ≤ 3 mm). From this point of view, the air jets can be assumed to be two-dimensional in the y−z plane. It is shown that, for x = 0 and 3 mm, the mean velocity decreases rapidly with increasing distance until reaching a plateau for z > 5 cm. For x = 6 mm, the mean velocity first increases to a maximum and then decays with further increasing distance until reaching a plateau. The 5348

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Figure 5. Instantaneous centerline velocity (ut) at z = 5 cm for (a) a time segment of 1 s and (b) a small time segment of 0.1 s taken from panel a.

Figure 4. Air temperature profile (a) along the y direction at different z levels (across the die face), (b) along the z direction at different x positions (y = 0), and (c) along the z direction at different y positions (x = 0).

attenuation. We can predict that, if the location of the second peak moves away from the die, rapid attenuation of the fiber will occur at a longer distance.

Figure 6. Probability density distribution of instantaneous centerline velocity (ut) at different z levels.

done with Adobe Photoshop and the software supplied with the camera. The experiments were performed on the single-orifice slotdie melt-blowing device shown in Figures 1 and 2. The polymer used was 900-melt-flow-rate polypropylene (SK, Seoul, Korea). During the experiments, air with a pressure of 0.1 MPa and a temperature of 260 °C was compressed into the slots. The die temperature (polymer temperature) was 260 °C. The polymer flow rate was 2.6 g/min. Figure 8 shows an image of the fiber path taken by the Redlake camera during the operation of the single-orifice slotdie melt-blowing process. A frame rate of 5000 frames/s was used with an exposure time of 0.2 ms. The recording time was 0.2 s.

3. FIBER WHIPPING DURING MELT BLOWING 3.1. Fiber Path Record. The fiber motion in the meltblowing process was captured by high-speed photography. A Redlake HG-100K high-speed camera (Redlake Inc., San Diego, CA) was used in our studies. This camera has the capability of recording images at a frame rate of 1000 frames/s or up to 100000 partial frames/s. Full frames are recorded at a resolution of 1504 × 1128 pixels. The camera was equipped with a Nikon 24−85 mm, f 2.8 zoom lens. To capture the paths of the fiber issuing from the polymer capillary, the camera was placed at an angle of 4° relative to the die face. The light source was two 1300-W lamps. Image processing and analysis was 5349

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Figure 7. Ratio of the fluctuating and mean centerline velocities (u′/u) as a function of z position.

Figure 8. High-speed photographic image of whipping in the meltblowing process.

With this arrangement, the real image size was 4.7 cm × 1.3 cm. One can see that fiber bending instability sets in as soon as the polymer melt begins issuing from the capillary. The whipping in the melt-blowing process appears to be a complex fiber path. One can see that the latter loop sometimes overruns the former one. The low-speed photography with an exposure time of 250 ms used by Rao and Shambaugh15 showed that the fiber motion appears to involve splaying. However, our experimental evidence gathered with a high-speed camera reveals a whipping motion of the fiber in the melt-blowing process. We believe that the apparent splaying was an optical illusion in the form of a very fast whipping motion of the fiber. Whipping in the electrospinning process is an electrically driven bending instability, whereas in the melt-blowing process, it is an aerodynamics-driven bending instability. The theory of aerodynamically driven jet bending was described by Entov and Yarin.21 According to their theory, when the flow velocity incident on a jet exceeds a critical velocity, a small disturbance of the jet will grow and develop into bending instability. As described above, the polymer melt exiting the capillary meets with the maximum, very high air velocity at the centerline in the region of 0 < z < 5 cm. Meanwhile, turbulent fluctuations are significant in this region (see the two peaks of u′/u in Figure 7). We believe that these turbulent fluctuations are the reasons whipping sets in as soon as the polymer melt begins issuing from the capillary. 3.2. Amplitude and Frequency of Whipping. Figure 9 shows the fiber motion in the y−z plane obtained by analyzing the high-speed photographs. The data were gathered by following motion from −y to +y at a constant z position over the full recording time of 0.2 s. As z increases from 0.5 to 2.5 cm, the fiber moves greater distances in the lateral direction (i.e, in the y direction). These data verify Rao and Shambaugh’s15 observation that there was a progressive increase in the width of the cone of fibers as the distance

Figure 9. Fiber motion in the y−z plane below a melt-blowing slot die at z = (a) 0.5, (b) 1.5, and (c) z = 2.5 cm. Each data point corresponds to a fiber position determined from a frame acquired with the highspeed camera. The data were gathered by following −y to +y motion at a constant z position over the full recording time of 0.2 s.

from the die increased. It is worth noting that our view of the motion is two-dimensional, in the y−z plane. For the slot die, as the aspect ratio l/e is large enough, the flow field of the dual converging air jets from the melt-blowing device can be assumed to be two-dimensional at positions below the die center.5,6 For the single-orifice slot-die melt-blowing device used in this study, because of the small aspect ratio, fiber whipping occurs mainly in the y−z plane, although whipping in the x−y plane is stilled observed. The whipping amplitude is defined as the maximum lateral displacement at position z. Figure 10 indicates a growing whipping amplitude with increasing distance from 0.5 to 2.5 cm. For the region of z > 3.5 cm, the fiber jumps out of the frame in the lateral direction, which indicates that the whipping amplitude still grows beyond z > 3.5 cm. 5350

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Figure 10. Amplitude of whipping in the melt-blowing process. Figure 12. Simulated trajectory of whipping in the melt-blowing process.

The frequency of the fiber whipping was determined in a manner similar to determining the frequency of a sine wave. We determined the whipping frequency by calculating the peaks in the +y region and the negative peaks in the −y region in a certain period of time in Figure 9. The adjacent peaks are in the +y region and −y region alternatively. Figure 11 indicates that the whipping frequency decreases with increased distance from the die.

of the die. High-speed photography was employed to capture the fiber motion during melt blowing. The whipping motion was observed with a high-speed camera at a frame rate of 5000 frame/s and an exposure time of 0.2 ms. The fiber whipping amplitude and frequency were determined. An increase in amplitude of whipping occurred as the distance from the die increased, whereas the frequency decreased as distance from the die increased. The results indicate that the turbulent air flow can affect the dynamics of whipping in the melt-blowing process. This work examines the physics of turbulent melt-blowing jets and whipping during melt blowing, using experimental method. This experimental work is significant for our further research on the relationship between turbulent fluctuation of the air jets and the fiber motion in the melt-blowing process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 11. Frequency of whipping in the melt-blowing process.

ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant 10972052), Foundation for the Author of National Excellent Doctoral Dissertation of the People’s Republic of China (Grant 2007B54), New Century Excellent Talents Plan of Chinese Ministry (Grant NCET-090285), and Shanghai Dawning Program (Grant 10SG33).

Our previous works22,23 simulated the whipping process during melt blowing. The three-dimensional paths of whipping were calculated, and the whipping amplitude was predicted. Our simulation showed a small perturbation developing into whipping. Figure 12 shows the simulated trajectory of whipping in the melt-blowing process. Although the simulation underpredicted the whipping amplitude, the simulated trajectory of the whipping substantially agrees with the image taken by highspeed camera in Figure 8.

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REFERENCES

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4. CONCLUSIONS Online measurements of turbulent air flow field and fiber whipping in the melt-blowing process have been carried out. A hot-wire anemometer was used to measure the turbulent jets below a single-orifice melt-blowing slot die. The characteristics of the mean velocity, mean temperature, and velocity fluctuations were obtained. The measurement results show that the turbulent fluctuation is related to the way the two streams exit from the die and, therefore, is relative to the design 5351

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