Article pubs.acs.org/IECR
Mechanism of Fiber Formation in Melt Blowing Sanfa Xin and Xinhou Wang* College of Textiles, Donghua University, 2999 North Renmin Road, Shanghai, 201620, P. R. China ABSTRACT: This article describes the study of the mechanism of fiber microformation in melt blowing. Through theory analysis of the flow field of molten polymers, it is inferred that the molten polymer flows in the form of the collective motion as a unit of microlayer. There is a sliding motion between every two adjacent microlayer units, and the attenuation mechanism of the fiber formation involves different microlayers that are arranged in a longer length than that before the fiber is drawn along the axial direction in terms of their velocities. Melt blowing experiments and comparison experiments were designed and performed. After measurements of the fibers and the analysis of the experimental data, results were obtained. These results verify the inference from the principle of the shear flow and the mechanism of the fiber microformation. When the theory of the formation mechanism is used to explain the fiber tenacity, by inference the tenacity of MB fiber is lower than that of spunbond or melt spinning fiber.
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INTRODUCTION
Melt blowing (MB) is a one-step method used to convert molten polymer into fibers or fiber-webs directly. This process dates back to the 1950s and can produce fibers with an average diameter of less than about 0.5 μm,1 exhibiting high coverage and surface area. Thus, MB products find diverse applications including filtration, absorbent, upholstery, membrane separation, protective military clothing, biosensors, wound dressings, and scaffolds for tissue engineering. In this paper, we are interested in the mechanism of the fiber microformation in MB. Mainly, the study examines the form of the shear flow of the molten polymer and the origin of the microstructures in the process of molten polymer attenuation. So far, many studies in the melt-blown area have been performed, and considerable reports on MB have appeared, such as in theoretical models,1−4 in the air-flow field,5 on fiber attenuation,6 etc. However, some of the most basic questions regarding MB theory and research, such as what are the microstructures of the MB fibers, how do these microstructures form, and what are the relationships between the microstructures and the properties, have not been adequately addressed in the literature. In 2012, Xin and Wang7 investigated the flow field of the molten polymer in theory and experiment, and they pointed out that this flow field was a type of shear flow field and had great effect on the microstructures of the final fibers. In our work, the formation process of the inner microstructures of the fibers was further explored on the basis of shear flow theory.
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Figure 1. The control plane of the fiber and the coordinate system.
where ρ is the polymer density, ux is the x-component velocity of polymer flow, t is the time, and η is the dynamic shear viscosity. Because the analytical solution to eq 1 is so sophisticated and abstract that its meaning is difficult to understand, we made a simulation analysis here. To show the visual results of the numerical solution, some of the parameters were selected, MATLAB software was used, and the solution result of the eq 1 is shown in Figure 2. Shear flow is nearly an inverse parabolic profile with maximum velocity on both sides and a minimum velocity in the center. When the time t increases, the average velocity ux̅ increases gradually and the shape of the inverse parabolic profile also changes. When the partial derivative of the eq 1 analytical solution is taken, the gradient of the velocity ux along the y-axis, the shear rate, is obtained as follows.
THEORETICAL SECTION
According to Xin and Wang,7 the control plane of the fiber is depicted as shown in Figure 1 and the governing momentum equation is given as eq 1.
Received: Revised: Accepted: Published:
2
ρ
∂ux ∂u = η 2x ∂t ∂y
(1) © 2012 American Chemical Society
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Figure 2. The schematic of the numerical solution.
∂ux(y , t ) = ∂y
∞
∑ n=1
nπy nπ −n2π 2η2t / ρ2 dj 2 Cne cos dj dj
(2)
where Cn = −
2 dj
∫0
dj
φ(y)sin
nπξ dξ , dj
n = 1, 2, 3, ... (3)
From eq 2, it is clear that the gradient of the velocity increases with an increase in radius. The minimum value of the gradient occurs in the center of the polymer melt, and the maximum value occurs near the surface of the polymer melt. This indicates that the shear rate increases as the radius increases; the minimum value, zero, is in the center, and the maximum value is on the surface. Xin and Wang7 showed that the velocity distribution profiles in the longitudinal section of the attenuating polymer melt are acquired by analysis of the solution to eq 1 together with the continuity equation. The arrangement regularity of the main variable values was obtained as shown in Figure 3.
Figure 4. The distribution schematic of the velocity contours of the attenuating polymer melt: (a) in two-dimensional form and (b) in three-dimensional form.
shows the two-dimensional schematic of velocity contours in the longitudinal section. In Figure 4a, the velocity contours are not a series of straight lines, but curves, and the tangent at any part of the contour forms a small angle with the velocity vector at that point. The velocity contours in upper half part and those symmetric in lower half part form the anti-V shaped stripes. To clearly exhibit the distribution characteristic of the velocity contours, the three-dimensional schematic of velocity contours are also shown (Figure 4b). In Figure 4b, the schematic of velocity contours is a series of circular lines in the cross-section and is a series of anti-V shaped lines in the longitudinal section. Through analysis, we find that there is a different density of the contour distribution at different sites in the flow field. The density of the contours near the spinneret orifice is larger than that at a far distance, and the density near the surface is larger than that in the center. It is well-known that in the pure shear flow field (for example, the flow in the pipeline), the velocity contours are a series of straight lines, the direction of which is parallel to that of the velocity vector. Because there is no velocity gradient along the axial direction, there is no attenuation of the polymer melt. In the elongational flow field (for example, the flow in melt spinning), the velocity contours are also a series of straight lines, yet the direction is vertical to that of the velocity vector. Due to the velocity gradient in the axial direction, there is the draft and attenuation of the polymer melt. The larger the velocity gradient, the larger the rate of attenuation. Therefore, the characteristic of the contour distribution in Figure 4 is between that of the pure shear flow field and that of the elongational flow field but more like that of the pure shear flow field. In the following, the formation mechanism of MB fibers will be obtained through inference according to the principle of shear flow. To facilitate the following descriptions, we make the
Figure 3. The schematic of the regularity of distribution of the main variables in the longitudinal section.
The surface velocity in order of the numerical size: u1 ≥ u 2 ≥ ... ≥ uk ≥ ... ≥ un
(4)
The center velocity in order of the numerical size: u01 ≤ u02 ≤ ... ≤ u0k ≤ ... ≤ u0n
(5)
The average velocity of the cross-section in order of the numerical size: u1̅ ≤ u 2̅ ≤ ... ≤ uk̅ ≤ ... ≤ un̅ (6) To further display the principles of the shear flow field in the process of polymer melt attenuation, the velocity contours in the longitudinal section were drawn. Assuming that the same velocity gradient exists in every two adjacent contours, the velocity contours (see Figure 4) were plotted on the basis of the velocity distribution profiles as shown in Figure 3. Figure 4a 10622
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Figure 5. The FE-SEM images of the fibers: (a) from the melt spinning process, (b) from spunbond process, (c) from MB under conventional conditions, and (d) from MB under rapid cooling conditions.
assumptions that the whole velocity field is divided into alot of micropartitions by the contours, for which the velocity is approximately equal (see Figure 4a). If we consider any micropartition (between two adjacent contours) as a microlayer and regard the microlayer as the smallest unit, the whole flow field of the molten polymer consists of a large number of the units. The flow field of the molten polymer is a type of shear flow field in which the tangent at any part of the velocity contour forms a small angle with the velocity vector at that point and the molten polymer exhibits a collective motion as a unit of microlayer. Hence, the principle of the flow field can be described as a large number of units flowing in a moment. There is a sliding motion between every two adjacent microlayer units. The formation mechanism of the fiber attenuation mainly involves the arrangement of different microlayers in the longer length than that before the fiber is drawn of the axial direction according to their velocities. On basis of the above analysis of the polymer shear flow, we drew further inferences about the fiber microstructures so that the formation mechanism of MB fibers could be corroborated from MB experiments. Because the shear flow efficiently enhanced the crystallization kinetics,8 there might be a similar microcrystallization gradient structure to the contour gradient. For this reason, two types of microstructure forms would be generated in the MB fibers according to the formation mechanism. One type might be an annular contour distribution of the microcrystallization structure in the fiber cross-section.7 The density of the velocity contour is small in the center and large near the surface. Another type might be a contour distribution of the anti-V shaped microcrystallization structure in the fiber longitudinal section like that shown in Figure 4. The
microcrystallization structure in the cross-section has been observed7 through a special MB experimental method. The anti-V shape of the microcrystallization structure is not seen under normal circumstances possibly because of the conventional MB processing conditions. In the process, when the final MB fibers lay on the open screen, they are still at high temperature and have almost no tension. In this situation, the internal thermal motion of the macromolecules is still very fast, and disorientation and decrystallization takes place, which causes the interface between the adjacent microlayers and the differences in orientation and crystallinity of different microlayers of the fibers to weaken or disappear. Hence, this kind of anti-V shape phenomenon of the microstructure would not be observed by the instruments. To observe this phenomenon in melt-blown fibers, MB should be designed under special conditions, in which the shear flow is the only factor to affect the orientation and crystallinity, and the interfaces and the differences in orientation and crystallinity of different microlayers of the fibers could be retained after solidification. Because the main aspects of this MB experiment are similar to those in the work of Xin and Wang,7 design conditions and melt-blown fiber testing similar to those used in their study have been adopted. Furthermore, to facilitate the analysis and description of the observed results of the MB fiber production under special conditions, the related fibers produced by the elongational flow field, such as in the spunbond process and melt spinning process, are measured as a comparison.
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EXPERIMENTAL SECTION The MB experiments were carried out in a setup with only one spinneret hole and dual slot jets. The die capillary had an inside 10623
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Figure 6. The optical images of the fibers: (a) from the melt spinning process, (b) from the spunbond process, (c) from MB under conventional conditions, and (d) from MB under rapid cooling conditions.
fibers were manufactured in our laboratory. The main process conditions for the spunbond fibers were as follows: the material was the same, polypropylene, as for MB, and the polymer temperature was 260 °C. The melt spun polypropylene fibers were purchased from commercial sources. The surface smoothness of the fiber was measured with a field emission scanning electron microscope (FE-SEM S-4800, Japan). Comprehensive characteristics of the fibers were measured with a YG002C fiber detection system (optical microscope with a computer, China) and a polarized light microscope (DM750P, China). From all the images, the fiber diameter measurements were made using image analysis software (ImageJ). A fiber tensile tester (XQ-2, China) was used to test the tenacity of MB fibers.
diameter of 0.42 mm and a length of 10 mm. Each of the slots in the die was 0.65-mm wide and 6-mm long. The material in our work was polypropylene with a polymer melt index of 1500. To observe the anti-V shape phenomenon of the microstructure, we designed rapid cooling conditions as follows. First, unheated air at room temperature was used. The actual air temperature was not tested when it passed through the hot MB die before impacting the polymer stream. Second, a cool water surface replaced a traditional collector so that the polymer melt could cool quickly. Therefore, shear flow would be the main factor to affect the microstructure of fibers, to enable observation of the anti-V shape microstructure phenomenon. In addition to the air temperature and collecting conditions, the other experimental conditions were as follows: the polymer temperature was 260 °C, the polymer flow rate was 6.5 cm3/ min, and the air pressures were 2, 3, and 4 atm. An MB experiment for comparison was also conducted, the main reason for which was to determine whether the microstructural phenomenon of the anti-V shape could be instrumentally detected. Thus, the same material was used in the same MB process in the same setup with the same conditions except for the air temperature (260 °C) and the conventional collector. According to the conditions above, two MB experiments were completed to manufacture fibers for the next stage. To further analyze and describe the fibers produced by the shear flow field, the fibers manufactured by the elongational flow field, the spunbond fibers and the melt spinning fibers, were added to the testing and analysis process. The spunbond
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RESULTS AND DISCUSSION Figure 5 shows the FE-SEM images of the representative fibers. Figure 5a, 5b, 5c, and 5d present the fibers from the melt spinning process, the fibers from the spunbond process, the MB fibers under conventional conditions, and the MB fibers under rapid cooling conditions, respectively. The images show that almost all the fibers have a smooth surface despite different diameters. We knew in advance that these processes had different flow fields for the polymer melt. The flow field in the melt spinning or spunbond process is the elongational type while the flow field in MB under conventional conditions or rapid cooling conditions is a shear type. Thus, the shear flow field does not affect the fiber surface smoothness. Figure 6 shows the optical images of the representative fibers. Figure 6a, 6b, 6c, and 6d display the fibers produced by the 10624
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Figure 7. The polarized light images of the fibers: (a) from the melt spinning, (b) from the spunbond, (c) from MB under conventional conditions, and (d) from MB under rapid cooling conditions.
information about the apparent properties of the fibers. In this sense, the optical microscope usually is used to test the apparent performance of the fibers such as the fiber diameter, the surface smoothness, the surface defects, etc. On the other hand, the transmission refraction of light can offer information in regard to the inner structure, such as the uniformity. In this sense, the optical microscope can also be used to identify the inner structure, such as the hollow fibers, to some extent. Thus, the reason for the different image characteristics may come from two aspects: the surface and the inner microstructure. From the results in Figure 5, we know that all the fibers in Figure 6 are smooth. Hence, we can exclude the possibility that the surface factor has an effect on the appearance. Therefore, the cause of the different optical effect should come from the inner microstructure. Therefore, we understand that a different optical appearance indicates a different internal microstructure when the surface is smooth. On the one hand, uniform brightness indicates the same microstructure in the fibers. The uniform brightness in Figure 6c indicates the microstructure uniformity of the MB fibers under conventional conditions. Similarly, the uniform brightness in Figure 6a and 6b also indicates the microstructure uniformity of the fibers from the melt spinning and spunbond processes, respectively. On the other hand, a novel optical effect indicates a novel microstructure in the MB fibers under rapid cooling conditions, like the anti-V shape. This anti-V shaped microstructure is informative for the assumption about the form of shear flow (see Figure 4). As a result, conclude that it is the different processing conditions that result in the different
melt spinning process, the spunbond process, the MB process under conventional conditions, and the MB process under rapid cooling conditions, respectively. The images in Figure 6a, 6b, and 6c all have approximately similar appearance, smoothness, and uniform brightness, despite different diameters. They all account for most of the total product. However, the fiber in Figure 6d, also accounting for most of the total product under rapid cooling conditions, has a novel appearance. The brightness on the surface of the fiber is not uniform. Different levels of brightness appear at different locations along the radial direction. The fiber surface seem rough, and there are some stripes on the surface. All the stripes show some certain regularity, symmetric about the centerline and like that of the anti-V shape approximately. According to the image characteristics in Figure 6c and 6d only, we assume that they might be two types of fibers. In fact, they come from the same material, polypropylene, the same MB method, and different processing conditions. Why does this phenomenon occur? We used the optical microscope to observe and identify the fiber on the basis of information from light. Under the microscope, the object is between the light source and the objective lens. When we measure the fibers with the optical microscope, the light that enters our eyes is composed of two types: the light reflected by the fiber surface and the light that passes through the fiber. Thus, the images convey the comprehensive effects of the surface-reflected light and transmission refraction of light. These two kinds of light reveal different information concerning the fibers. On the one hand, the surface-reflected light of the fiber can provide 10625
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Figure 8. Effect of air pressure on fiber diameter and fiber stripes: (a) air pressure of 2 atm, (b) air pressure of 3 atm, and (c) air pressure of 4 atm. Fibers 1, 3, 5 are from optical microscopy, and fibers 2, 4, 6 are from polarized light microscopy.
that anisotropic substances in the fibers occur along the radial direction. Through the analysis of the novel structural phenomenon, we believe that these anisotropic substances are nothing but the orientation or crystallinity of the microlayers due to the shear flow. These observations from polarized light microscopy confirm that the results from the optical microscope are basically correct. Figure 8 shows the effect of air pressure on the fiber diameter and fiber stripes. All the fibers are produced by MB under rapid cooling conditions. Each of the image panels (a−c) in the figure contains one fiber from the optical microscope and one fiber from the polarized light microscope, respectively. They are, in fact, images from one fiber and the same observation point. Obviously, the stripes (anti-V shaped) under the optical microscope also emerge in the polarized light images, and they present a similar regularity of distribution. The fibers in Figure 8a, 8b, and 8c result from air pressure conditions of 2, 3, and 4 atm, and their diameters are about 25.2, 15.6, and 10.1 μm, respectively. From these images, we can make two conclusions. First, there appears to be a quick decrease in the diameter with increasing air pressure. Clearly, the increase in air pressure causes higher air velocity, which gives rise to the decrease in fiber diameter, as one would expect. Second, the stripes become thin, and the distance between two adjacent stripes decreases when air pressure increases. This indicates that the increase in air velocity brings about the decrease in thickness of the moving microlayers. Using the fiber tensile tester, we determined the tenacities of MB fibers under rapid cooling conditions: 1.20cN/dtex,
microstructures that lead to this novel optical effect. In brief, this experiment verifies the inferences about the flow principle and the formation mechanism of the fiber attenuation. These inferences are as follows. The principle of the flow field can be described as a large number of the units that flow in a moment. There is a sliding motion between every two adjacent microlayer units. The mechanism of fiber attenuation is based on the arrangement of different microlayers in a longer length than that before the fiber is drawn along the axial direction in terms of their velocities. To supplement the results from the optical microscope measurements, we also used polarized light microscopy. Polarized light microscopy can distinguish between isotropic and anisotropic substances. In this way, we can exploit the optical properties specific to anisotropy and reveal detailed information on the structure of the fibers that are invaluable for identification and diagnostic purposes. This is considered a contrast-enhancing technique. Figure 7 shows the polarized light images of the representative fibers. The fibers in Figure 7a, 7b, 7c, and 7d are produced by the melt spinning process, the spunbond process, the MB process under conventional conditions, and the MB process under rapid cooling conditions, respectively. Figure 7 exhibits phenomena similar to that of Figure 6, and thus we reach similar conclusions. The fibers from the melt spinning, spunbond, and conventional MB processes are all uniformly bright. This indicates that they have uniform microstructures as a whole. The fiber from MB under rapid cooling conditions has stripes (the anti-V shape). This indicates 10626
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conditions and the comparison experiments and measurements were carried out. Conclusions were reached after the comparison analysis between the experimental results and the theoretical prediction. First, the flow form of the molten polymer is a type of shear flow involving a sliding motion between the microlayers. This type of flow is not suitable to stretch most of the macromolecules or chain segments in the microlayer. Second, the mechanism of the fiber attenuation involves an arrangement of different microlayers in a longer length than that before the fiber is drawn along the axial direction in terms of their velocities. This type of arrangement does not contribute to the extension of most of the macromolecule chains or chain segments in the whole flow field. Third, when theory is applied to explain the fiber tenacity, by inference the tenacity of MB fiber is lower than that of spunbond or melt spinning fiber. The theory of the formation mechanism describes the fiber formation principle in MB. The mechanism of fiber formation reveals that the opportunity to improve the fiber tenacity using the MB process is extremely limited.
1.23cN/dtex, and 1.56cN/dtex, respectively, when the air pressures were 2 atm, 3 atm, and 4 atm. From ref 9, we know that the general tenacity scope of conventional MB fibers is 1.5−2.0cN/dtex. In our work, the tenacities of the MB fibers under rapid cooling are lower than the general scope. Although the rapid cooling conditions can bring about the retention of the polymer orientation, the crystallization time is reduced dramatically, which is directly related to the formation of the crystals. According to the fiber formation mechanism, we try to explain some phenomena about MB. For example, the fiber tenacity from MB is much lower than that from spunbond and melt spinning processes. Thus, a recombined technique is often used to increase the tenacity of fiber-web products and promote the applied scope of MB fiber-web products, such as SMS (a sandwich structure of nonwovens, a spunbond fiber-web on two sides, and a MB fiber-web in the middle). The mechanism of fiber formation will be used to explain the phenomenon of low tenacity in the MB fiber and its fiber-web products as follows. First, because the flow form of the molten polymer is a type of shear flow with a sliding motion between the microlayers in MB, the microlayer is the moving unit. This flow form has limited effect on the stretch of the macromolecules or chain segments of the interface between the microlayers. Yet they do almost nothing to the stretch of the macromolecules or chain segments of the inner unit. This is the main aspect to affect the tenacity of the MB fiber. If the straightness of the macromolecules or chain segments was low, the parallelism of the macromolecules or chain segments could not be high. Thus, the high crystallinity of the MB fiber could not form either. Second, from the velocity contours in Figure 4, the tangent at any part of the contour is not normal to the velocity vector at that point but forms a small angle with the velocity vector at that point. This kind of velocity distribution does not benefit the extension of most of the macromolecule chains or chain segments in whole flow field. It is well-known that the velocity contours in the elongational flow field in melt spinning are a series of straight lines, parallel to each other. These lines are vertical to the direction of velocity vector at that point. This kind of velocity distribution greatly helps to extend most of the macromolecule chains or chain segments. So, the macromolecule straightness and orientation degree in MB is far lower than that in melt spinning. Finally, after the MB fibers are deposited on the open screen, they have less tension but are still at higher temperature. In this situation, because the internal thermal motion of the macromolecules is still very fast, disorientation and decrystallization would take place, which can not only make the macromolecules straight and orientation degree even much lower but can also decrease the crystallinity rapidly. On this basis, we reach the conclusion that the MB fibers have much lower crystallinity and tenacity than the fibers from melt spinning and spunbond processes, as expected. To summarize, not only is the theoretical prediction verified by the MB experimental results, but the further inference of the theoretical application also provides an excellent fit. Therefore, the formation mechanism theory can reveal the fiber formation principles in MB.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financial supported by the National Natural Science Foundation of China (grant number 50976091).
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NOMENCLATURE dj = the width of the CP, μm dx = the length of the CP, μm Cn = coefficient n = positive integer t = the time, s uj = the surface velocity of the CP, m/s u1,u2,uk,un = the surface velocity, m/s u01,u02,u0k,u0n = the center velocity, m/s u̅1, u2̅ , u̅k, un̅ = the average velocity of the cross-section, m/s ux = the x-component velocity of the polymer flow, m/s x = horizontal coordinate y = vertical coordinate
Greek Letters
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ρ = the polymer density, kg/m3 η = the dynamic shear viscosity, Pa·s π = ratio of the circumference of a circle to its diameter ξ = integral constant φ = the initial velocity function
REFERENCES
(1) Uyttendaele, M. A. J.; Shambaugh, R. L. Melt Blowing: General Equation Development and Experimental Verification. AIChE J. 1990, 36 (2), 175−186. (2) Rao, R.; Shambaugh, R. L. Vibration and Stability in the Melt Blowing Process. Ind. Eng. Chem. Res. 1993, 32 (12), 3100−3111. (3) Marla, V. T.; Shambaugh, R. L. Three-Dimensional Model of the Melt Blowing Process. Ind. Eng. Chem. Res. 2003, 42 (26), 6993−7005. (4) Shambaugh, B. R.; Papavassiliou, D. V.; Shambaugh, R. L. NextGeneration Modeling of Melt Blowing. Ind. Eng. Chem. Res. 2011, 50 (21), 12233−12245.
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CONCLUSIONS On basis of the shear flow field theory of molten polymers, a formation mechanism was proposed and subjected to analysis and inference. The MB experiment under rapid cooling 10627
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(5) Uyttendaele, M. A. J.; Shambaugh, R. L. The Flow Field of Annular Jets at Moderate Reynolds Numbers. Ind. Eng. Chem. Res. 1989, 28 (11), 1735−1740. (6) Ellison, C. J.; Phatak, A.; Giles, D. W.; Macosko, C. W.; Bates, F. S. Melt Blown Nanofibers; Fiber Diameter Distributions and Onset of Fiber Breakup. Polymer 2007, 48 (11), 3306−3316. (7) Xin, S.; Wang, X. Shear Flow of Molten Polymer in Melt Blowing. Polym. Eng. Sci. 2012, 52 (6), 1325−1331. (8) Yu, F.; Zhang, H.; Wang, Z.; Yu, W.; Zhou, C. Prediction of the Flow-Induced Crystallization in High-Density Polyethylene by a Continuum Model. J. Polym. Sci., Polym. Phys. 2009, 47 (5), 531−538. (9) Ke, Q.; Jin, X. Nonwovens; Donghua: Shanghai, 2010.
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