Online Measurement of Fiber Whipping in the Melt-Blowing Process

Article pubs.acs.org/IECR

Online Measurement of Fiber Whipping in the Melt-Blowing Process Sheng Xie and Yongchun Zeng* College of Textiles, Donghua University, Songjiang, Shanghai 201620, P.R. China ABSTRACT: Melt blowing is a major process for producing nanofibrous nonwovens. Compared to studies on the air flow field and the fiber diameter measurement, much less has been done on the observations of whipping in the melt-blowing process. In this study, a high-speed camera was used to capture the fiber path below a single-orifice melt-blowing slot die. The behavior of loops resulted from whipping was revealed. The characteristics of the whipping amplitude, whipping frequency, and fiber velocity were obtained. Fiber attenuation contributed by whipping was calculated by measuring the perimeter of the loops. The study shows the laws of fiber whipping in a slot-die melt-blowing process and indicates that whipping plays a role in fiber attenuation.

1. INTRODUCTION Melt blowing is a major process for producing nanofibrous nonwoven materials (nonwoven products manufactured using fibers with diameters around 100−1000 nm). Nanofibrous nonwovens have found a variety of applications in areas of filtration media, life science, medicine, and industry.1,2 During melt blowing, fibers are produced by extruding a polymer melt through the spinneret and drawing down the extrudate with a jet of high velocity hot air. Compared to study on the air flow field and the fiber diameter measurement, much less has been done on the whipping dynamics in the melt-blowing process. The fiber whipping (bending instability), which was previously called fiber vibration, was first recorded by Rao and Shambaugh,3 and Chhabra and Shambaugh.4 They recorded multiexposure strobe photographs in a single-orifice melt-blowing device, using an exposure time of 0.25 s. Their works were limited to fairly low air velocities (air flow rates), approximately an order of magnitude below the normal operating speed of a melt-blowing die. They showed that the fiber motion appeared to be splaying, and the view appeared to be a bundle of jets with each jet leading toward a single fiber. This observation was similar to that from the naked eye. We believe that the apparent splaying obtained by naked eye and by low-speed photography was an optical illusion in the form of a very fast whipping motion of the fiber. Beard et al.5 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. This time they captured the motion of a single fiber. Unfortunately, the pictures of fiber vibration were recorded only at a certain position below the dies. Again, the gas rates they used were much lower than those in commercial production. Breese and co-workers6 took photographs of the fibers in a 600-orifice melt-blowing line, using a high-speed (1000 frames/s) digital camera with pulse laser illumination, but they did not capture a fiber vibration. It is worth noting that the air pressure or the primary air velocity they used was still very low. The theory of the bending instability of thin liquid jets in air was developed and described by Entov and Yarin.7 This study presents the observations of fiber whipping in a slot-die meltblowing process. We capture the fiber motion below the meltblowing slot die at an air flow rate close to that used in © 2013 American Chemical Society

commercial production, using high-speed photography. The fiber paths are described according to the recorded images. These recorded images are processed to analyze the law of fiber whipping and to determine the whipping amplitude and frequency. Meanwhile, the air flow field is simulated to explain the fiber motion.

2. EXPERIMENTS AND SIMULATION 2.1. Experimental Setups. The experiments were performed on the single-orifice melt-blowing device shown in Figure 1. The melt-blowing die used in this study had the

Figure 1. Melt-blowing device and the high-speed camera.

configuration shown in Figure 2a and 2b. This type of die is referred to as a blunt-edge 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. The coordinate system used 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 traverse to Received: Revised: Accepted: Published: 2116

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Figure 2. Detailed schematic of the melt-blowing die: (a) cross-sectional view and (b) end-on view. Panels (c) and (d) are the computational domain and boundary conditions for the air flow field simulation.

Figure 3. Evolution of fiber paths near the die. The real image size for all the images is 26.2 mm ×19.6 mm. The experiment was carried out at Pair = 1.0 atm.

1.25 atm, a polymer temperature (Tp) of 260 °C, and an air temperature (Tair) of 260 °C. 2.3. Simulation. Figure 2c and 2d show the threedimensional model of the slot die; also shown are the configuration parameters, the computational domain, and the boundary conditions. To save calculation time, we used a calculation domain of a quarter of the total air flow field. The computational domain and the corresponding grids were created by Gambit, and 1 063 500 cells were yielded. The air flow field was obtained by solving the N−S equations through the commercial software FLUENT 6.3. Different kinds of turbulence models have been used in the simulation of the melt-blowing air flow field. Krutka8 simulated the air flow field with three turbulence models (standard k-ε model, the realizable k-ε model, and the Reynolds stress model) and compared them with experimental measurements. They found that for the case of the blunt-die, the Reynolds stress model (RSM) was able to simulate the air flow field with the most success. Recently, Tan9 applied the SST k-ω model to simulate the air flow field of a Laval nozzle and found that SST

the major axis of the nose piece and slots. The z direction is directed vertically downward. The fiber motion in the melt-blowing process was captured by a high-speed camera. 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 100 000 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. The light source was two 2000-W lamps. For the high-speed camera experiments, the slot die was viewed such that the slots were parallel to the axis of the camera lens. Two regions of about 20 mm and 65 mm down from the spinneret of the die were imaged at 5000 frames/s and 3000 frames/s, respectively. The corresponding image sizes were 26.2 mm × 19.6 mm and 64.9 mm × 64.9 mm, respectively. 2.2. Experimental Conditions. The polymer used was 650-melt-flow-rate polypropylene (SK, Seoul, Korea). During the experiments, the base conditions were a polymer flow rate (mp) of 7.8 cc/min (7.1 g/min), an air pressure (Pair) of 0.5− 2117

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k-ω model can successfully model the nature/onset of lower pressure “stall” where the flow detaches from the nozzle wall. According to Krutka’s study,8 we applied the RSM to run the simulations. The turbulence parameters C1ε and C2ε were set at 1.24 and 2.05, as recommended by Krutka. The processing parameters were developed based on the experiments.

which enhances perturbations and makes the perturbations grow. Chung15 modeled the onset of whipping in the meltblowing process; their results indicate that melt inertia rather than melt rheology is the more dominant factor in controlling fiber shapes. Whipping in the slot-die melt-blowing process appears to be a fiber path with two groups of loops moving downward. Each cycle of bending instability can be described in the following steps (see Figure 3). Step 1: Segment ab which is just ejected from the capillary begins moving to the right (along +y direction) due to an initial perturbation. The movement speed of segment bc lags that of segment ab, and therefore creates an outward arc. The arc develops into a loop as segment ab moves further right. The successive process is shown in Figure 3a, 3b, and 3c. The loop bulges leftward and we call it “a left loop”, which is marked in Figure 3c. Step 2: Segment ab moves to the rightmost position (Figure 3c). When it begins moving from the right to the left (along −y direction), segment bc stays at the right due to the lag. And therefore creates a loop which bulges rightward (we call it “a right loop”). The successive process is shown in Figure 3d, 3e, and 3f. Step 3: Segment ab moves to the leftmost position and then changes direction to move from the left to the right. At the same time, a new left loop is formed (Figure 3g and 3h). Now we trace the motion trajectory of the previously formed loops. Figure 5 shows the fiber paths in a larger region at

3. RESULTS AND DISCUSSION 3.1. Fiber Paths. The region of 20 mm below the die was imaged at 5000 frames/s. Figure 3 shows the time evolution of the shape of the fiber in three dimensions. It appears that the fiber path in the region near the die is almost 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.10−13 For the single-orifice melt-blowing slot die used in this study, the aspect ratio was not as large as that of a multiorifice die. Therefore, fiber whipping in the x−z plane is still observed though whipping occurs mainly in the y− z plane near the die. During melt blowing, we observed that fiber whipping sets in as soon as the polymer melt begins issuing from the capillary. It is an aerodynamic-driven bending instability. According to Entov and Yarin’s theory,7 when the flow velocity (U0) incident on a jet, exceeds a critical velocity (U*), a small disturbance of the jet will grow and develop into bending instability. U* can be calculated by the equation U* =

α /(ρa a0)

(1)

where α is the surface-tension coefficient, ρa is the air density, and a0 is the radius of the unperturbed jet. In this study, a0 = 0.21 mm is the radius of the orifice in our experiments. ρa = 1.293 kg/m3 and a = 0.7 kg/s2 as provided by Sun.14 Under these conditions, U* is calculated as 50.8 m/s. Figure 4 shows the simulation results of the air velocity in z direction (vz) along the spinning line near the die. We can see

Figure 5. Evolution of fiber paths in a large region. The real image size for all the images is 64.9 mm × 64.9 mm. The experiment was carried out at Pair = 1.0 atm.

different time. The fiber path was imaged at 3000 frames/s. The time interval between the adjacent path profiles was 0.67 ms (1/1500 s). One right loop is marked as “R” and one left loop is marked as “L” in Figure 5. It is observed that the left loop stays on the left of the spinning line when moving downward to the collector, while the right loop stays on the right of the spinning line with its moving down to the collector. Meanwhile, the segment of the fiber in each bend elongates and the array of bends become a series of alternative left and right loops with growing perimeters. The fiber in each loop grows longer and thinner as the perimeter of the loop increases. After some time,

Figure 4. Simulation results of the centerline air velocity along z direction. The simulation was carried out at Pair =1.0 atm.

that with the inlet pressure of 1.0 atm, vz is larger than U* (50.8 m/s) except for the very small distance of z < 1 mm. This leads to bending instability as soon as the polymer jet issues from the capillary as observed. Entov and Yarin’s study indicates that, due to the jet curvature, a distributed lift force acts on the jet, 2118

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the latter loop overruns the former one and the adjacent loops begin to entangle together when they move down to a certain distance. As they move further away from the die, the motion of the loops expands to three-dimensional due to the entanglements. And the morphology of the melt-blown nonwoven web is attributed to the three-dimensional entanglement of the loops. 3.2. Fiber Attenuation Contributed by Loop Elongation. Figure 6 is derived from the measurements of a sequence

Figure 7. k Value as a function of z distance from the die.

attenuation may actually take place before whipping. The experimental studies by Yin16 and Bansal17 found that the most rapid drop in the diameter was within the first 1−2 cm from the die. Zhou18 modeled melt blowing of viscoelastic materials using a slender-jet model, and found that at larger shear stress along the fiber length, the fiber diameter decreases very rapidly near the die face and then levels off. Zhou18 and Tan19 mentioned that it was important to account for the turbulent nature of the air flow to describe the whipping phenomenon. To further understand the fiber attenuation contributed by whipping, we measured the resultant fiber diameter. Figure 8

Figure 6. Perimeter of a loop as a function of z distance from the die.

of images. Loop R was traced from the position where it forms to the position where it moves out of the image view. The perimeters at different z positions were measured with Photoshop Cs 3 (Adobe System Inc., San Jose, CA, USA). If z1 and z2 were the z-axis coordinates of the apex and the lowest point of a loop, we defined the loop locating at z = (z1 + z2)/2 level. As mentioned above, the fiber in each loop grows longer and thinner as the perimeter of the loop increases. Figure 6 shows a loop perimeter as a function of the z distance from the die. It can be seen that the loop perimeter increases with increasing distance from the die. As the perimeter of the loop increases, the diameter of the fiber forming the loop grows smaller. According to the law of conservation of volume for a certain loop, we can obtain (πd0 2/4)l0 = (πdz 2/4)lz

Figure 8. SEM image of fibers produced by melt-blowing process at the air pressure 1.0 atm. Fibers were collected at 30 cm below the die face.

(2)

where d0 is the diameter of the first loop formed from the whipping motion, which is not equal to the original diameter of d = 0.5 mm, and l0 is the perimeter of the first loop. While dz and lz are the diameter and the perimeter of the loop when it is at z level. What we focus on is to find out the relation between dz and d0. From eq 2, we have

dz =

l0/lz d0 = kd0

shows the SEM image of the melt-blown fibers imaged using a scanning electron microscope (JSM-5600LV, JEOL, Tokyo, Japan). The fibers were collected at 30 cm below the die. The average diameter of the fiber was measured as 18.6 μm under 1.0 atm air pressure, which is smaller than that used in industry. For the 0.5 mm (500 μm) initial fiber diameter, the whole diameter reduction ratio can be calculated as 26.88 (i.e., 500/ 18.6). Therefore, the diameter reduction ratio contributed by the loop elongation from z = 7 to 56 mm is about 11% (i.e., 3.03/26.88 = 0.11) of the whole diameter reduction ratio. For experimental limitation, the analysis of the loop elongation beyond z = 56 mm cannot be completed in this study. Although only part of the whipping process (i.e., 7 mm ≤ z ≤

(3)

Figure 7 shows the k value as a function of the z distance. At the distance from the die of about 5.5 cm, the diameter decreases to 0.33 of the diameter of the first loop. The diameter reduction ratio contributed by loop elongation is 3.03 (i.e., 1/ 0.33). Several papers have studied fiber attenuation during melt blowing,16−18 and it is suggested that much of the fiber 2119

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56 mm) is analyzed, it can be seen that whipping plays a role in fiber attenuation. To capture clear images, the air pressure used in this study was 1.0 atm, which is still smaller than that used in industry. We believe that the higher air velocity used in the melt-blowing process will intensify fiber whipping and increase the draw ratio. 3.3. Fiber Velocity. In this paper, fiber velocity represents the fiber velocity in the z direction. Fiber velocity was measured by analyzing the continuous images. We traced one point in a loop (such as the apex or the lowest point of the loop), measured the distances it moved in known time intervals, and calculated the velocity. If z1 and z2 were the z-axis coordinates of the traced point in adjacent images, we defined the fiber velocity as located at z = (z1 + z2)/2 level. Figure 9 shows the development of the fiber velocity along the z distance (the inset shows the fiber velocity on an

Figure 10. Process of overlapping paths from successive images. (a) An original image obtained with the high-speed camera at a frame rate of 5000 frames/s, (b) ten paths overlapped together into one image, and (c) fifty paths overlapped together into one image. The real image size for all the images is 26.2 mm × 19.6 mm. The experiment was carried out at Pair = 1.0 atm.

diameter with both multi-image flash photography and laser Doppler velocimetry. Beard and Shambaugh5 also measured the whipping amplitude by analyzing the high-speed photographic images. But the amplitudes they measured were for the region far away from the die (z > 2.5 cm). In this study, the whipping amplitude for the region of z < 2 cm, which is the starting area of whipping, was measured. Figure 11 shows the whipping amplitude with increasing distance in the y−z plane. Also shown is the amplitude in the

Figure 9. Fiber velocity and simulated air velocity at Pair = 1.0 atm.

expanded velocity scale). Also shown is the simulation result of the air velocity under the same processing conditions. The fiber velocity is very slow at the die but fibers quickly accelerate and reach around 15 m/s at about 5 cm distance from the die. Meanwhile, the air velocity decays quickly with increasing distance from the die. The drawing force created by the air drag decreases due to the decreasing of the relative velocity between the air and the fiber. We can see that at the distance beyond z = 5 cm, the fiber velocity is approaching further to the air velocity, which leads to smaller and smaller drawing force. If the fiber velocity approaches the air velocity in a certain distance, the aerodynamic force to attenuate the fiber will decrease to zero. This supports the known investigation that the fiber attenuation mostly occurs in the region when the distance from the die is within 5 cm. 3.4. Whipping Amplitude and Frequency. The whipping amplitude is defined as the maximum lateral displacement at position z. To obtain the whipping amplitude, we used a method of “path-overlap” that was carried out as follows: Photoshop CS3 was used to trace the fiber paths. Fifty successive paths from fifty successive images were overlapped into one image. Chhabra and Shambaugh4 defined a fiber cone to describe the fiber vibration during melt blowing. Our overlapped fiber path resulted in about a tapered entity, which is similar to the fiber cone they captured. We measured the whipping amplitude from this overlapped image (as shown in Figure 10). Chhabra and Shambaugh4 measured the cone

Figure 11. Amplitude of whipping in the y−z and x−z planes.

x−z plane. The amplitude in the x−z plane was obtained by rotating the camera 90 degrees around the spinning line, therefore the lens was perpendicular to the slots. It is noted that the amplitude in the x−z plane is much smaller than that in the x−y plane. It was mentioned above that whipping occurs mainly in the y−z plane due to the structure of the slot die. It is obvious that the whipping amplitude increases as z increases. The amplitude increases to around 4.5 mm when the fiber travels to around 15 mm. We adopted the method used in our previous work20 to measure the whipping frequency. Figure 12 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. The frequency was determined by calculating the peaks in the +y region and the negative peaks in the −y region in a certain period of time in Figure 12. The adjacent peaks are in the +y region and −y region alternatively. We measured that the whipping frequency is around 700 Hz at the position of z = 4 mm under Pair = 1.0 atm. This frequency indicates the rate at which new bends are generated at the onset of instability. 2120

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Figure 14. Effect of air flow rate on whipping frequency (in the y−z plane).

rates of 0.5, 0.75, 1.0, and 1.25 atm are considered. As expected, increased air flow rate results in increased whipping amplitude and frequency. The results illustrate that whipping is more strongly related to air velocity (air flow rate) near the position where bending generates. We believe that this will consequently lead to more fiber attenuation.

3. CONCLUSIONS A high-speed camera was employed to capture the fiber motion below a single-orifice melt-blowing slot die. The fiber path below the die was obtained. The measurement results show that fiber whipping motion sets in as soon as the polymer melt begins issuing from the capillary of the die. The fiber path is composed of array of loops. Segments of the jet are drawn simultaneously in many elongated loops. Each of these loops grow larger in perimeter as they move to the collector. The elongation of the loop results in an attenuation on fiber diameter. The fiber velocity is measured by analyzing successive images. The fiber velocity is very slow at the die but fibers quickly accelerate and reach around 15 m/s at about 5 cm distance from the die. Meanwhile, the air velocity decays quickly with increasing distance from the die. The fiber whipping amplitude and frequency were determined. The amplitude in the x−z plane is much smaller than that in the y−z plane. The fiber whipping amplitude and frequency increase as the air pressure increases. This work first shows the three-dimensional path of a fiber whipping below the melt-blowing slot die, which gives a useful understanding for the formation of melt-blown nanofibers.

Figure 12. Fiber motion in the y−z plane below a melt-blowing slot die at z = 4 mm for (a) a time segment of 0.2 s and (b) a small time segment of 0.04 s taken from panel a. Each data point corresponds to a fiber position determined from a frame acquired with the high-speed camera. The experimental Pair = 1.0 atm.

Our previous work21 on modeling fiber motion during melt blowing has predicted that higher air velocity creates larger whipping amplitude and smaller fiber diameter. Figures 13 and 14, respectively, show how air flow rate affects the whipping amplitude and whipping frequency in the y−z plane. Air flow

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 21 67792627. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (11272088), New Century Excellent Talents Plan of Chinese Ministry (NCET-09-0285), Shanghai Dawning Program (10SG33), Chinese Universities Scientific

Figure 13. Effect of air flow rate on whipping amplitude in the y−z plane. 2121

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Fund (Grant CUSF-DH-D-2013035), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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