656
Ind. Eng. Chem. Res. 1995,34, 656-660
Use of Oscillating Gas Jets in Fiber Processing Manoj K. magi and Robert L. Shambaugh* Department of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
Oscillating air jets were used to draw (attenuate) polymer streams in the process known as melt blowing. By oscillating the air jets, finer fibers can be produced than by using continuous air jets. Thus, there is a n economic advantage to using oscillating air. Certain applied air frequencies are “resonant” with the melt blowing system; these frequencies give optimum results.
Introduction In the process known as melt blowing, gas jets are used to attenuate molten polymer streams into fine fibers (see Shambaugh, 1988). The resultant fibers are collected upon a screen as a nonwoven mat. Melt blown fibers are used commercially in insulation, filters, and absorbent materials. Figure 1 shows a typical commercial die use to produce melt blown fibers; this configuration was originally developed by Wente (1954, 1956). Molten polymer is forced through a row of small holes; a typical hole diameter is 0.406 mm (0.016 in.). High-velocity air exits from two slots; the slots are located along both sides of the row of polymer holes. Equal amounts of air exit from each slot. Figure 2 is a cross-sectional view of a typical melt blowing die. Mechanical systems can act in consort, or resonance, with a disturbance. A classic example of this is the collapse of the Tacoma suspension bridge in 1940 (O’Neil, 1983). When the natural frequency of the bridge was matched with input frequency caused by the wind, the bridge failed. The vibration of a motor is a more commonly observed example of resonance. A fiber which is being formed in a melt blowing process can also be considered to be a mechanical system with a characteristic set of natural frequencies. If these frequencies can be utilized, the melt blowing process might be improved. In the experiments described in this study, the air supply t o the melt blowing die was oscillated and the effects of this oscillation were studied.
Experimental Equipment and Details A slot die similar to that shown in Figure 1was used in our experiments. However, the nosepiece of the die had only one polymer orifice; this orifice was located at the center of the nosepiece. The slot length was 76.2 mm (3.00 in.). The slot width, nosepiece setback (0 mm), and other dimensions were exactly as shown in Figure 2. The material of construction was type 304 stainless steel. Two 250-W cartridge heaters were placed in the die head; temperatures up to 400 “C were possible. Polymer was melted and pressurized with a Brabender extruder of 19.0 mm (0.75 in.) diameter and 381 mm (15 in.) length. After exiting the extruder, the polymer passed through a metering pump and then was fed to the slot die. Except for the use of the slot die, the polymer system was the same as that used by Kayser and Shambaugh (1990)and Wu and Shambaugh (1992). The polymer used in all the experiments was Fina Dypro polypropylene of 75 MFR (melt flow rate). This polymer is used commercially for melt blowing.
* Author to whom correspondence should be addressed.
Figure 1. (a, top) Slot melt blowing die. This style of die was originated by Wente (1954, 1956). (b, bottom) Enlarged and sectioned view of the slot melt blowing die. These drawings have been reprinted from Shambaugh (1988).
Instead of feeding air continuously to the two slots of the die, first the air was fed to only one slot for a short length of time, and then the air was fed only to the other slot for the same length of time, and this cycle was then repeated. In this manner, an air oscillation frequency was created. The equipment used to supply the air t o the die is shown in Figure 3. After passing through a flowmeter, the air can be supplied in either an oscillating or a continuous mode. If a continuous mode is desired, valve 3 is closed and valve 4 is open. Also, valves 6 and 7 are set so as to provide flow from valve
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Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995 657
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Figure 4. Effect of air oscillation frequency on fiber diameter for a gas flow of 54 slpm.
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Figure 2. Cross-sectional view of a melt blowing die with typical dimensions. The die used in our experiments had these same dimensions.
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heater 8 or the branch containing valve 7 and gas heater 9. The three-way valves (6 and 7) are set to provide flow from valve 5 to the gas heaters. Because of the length of the branches between valve 5 and the die head, there is a lag time associated with the switching of valve 5 and the time at which the switch affects the flow at the die. However, this lag time is balanced out by the matched lengths and temperature profiles of the two branches. Oscillation frequencies of 0-50 Hz are possible with the experimental system. Fiber diameters in the melt blown mat were determined with an optical microscope equipped with a micrometer eyepiece. The diameter was estimated to be the average of 20 measurements taken at 40x magnification.
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Figure 3. Experimental equipment used to provide oscillating air to the die. The numbered components are as follows: 1supply air,2 flowmeter; 3 and 4 on-off valves; 5 oscillating solenoid valve; 6 and 7 3-way valves; 8 and 9 gas heaters; 10 melt blowing die; 11 hot polymer inflow; 12 melt blown fibers.
4 through to the two gas heaters and, thence, to the two
sides of the die 10. If an oscillating flow is desired, valve 4 is closed and valve 3 is open. The air then passes
through the oscillating valve 5. This oscillating valve is an electronically controlled solenoid valve that is fired by a voltage pulse from a signal generator. After passing through the oscillating valve, the air travels either through the branch containing valve 6 and gas
Results and Discussion Effect of Oscillation Frequency. For a particular configuration of melt blowing equipment and for a specific polymer, there are these four major operating variables in melt blowing: polymer flow rate Qp, polymer temperature Tp,air flow rate Q8,and air temperature T,. To these four variables we have added a fifth variable: air oscillation frequency. The effect of air oscillation frequency on fiber diameter is shown in Figure 4. At 1 Hz, the fiber diameter is only 64 pm. The diameter increases as frequency increases until a peak is reached at a frequency of about 14 Hz. Then the diameter steadily decreases with increasing frequency until a plateau is reached at about 25 Hz. The dotted line on the figure represents the fiber diameter when there is flow continuously coming from both slots (i.e., the condition in conventional melt blowing). The solid line on Figure 4 (as well as the solid lines on Figures 5-8) are arbitrary fits to the data. Except for frequencies a t or near the peak, the oscillations increase the efficiency of melt blowing by producing smaller fibers at a given air flow rate. In oscillating flow the air emits from one slot at a time, while in conventional melt blowing the air emits from both slots. To make sure that the oscillation was producing the fiber size difference-and not the use of a single slot-a series of experiments were run wherein an identical quantity of air was run through a single slot versus both slots. Table 1 summarizes these experiments. As can be observed, the use of a single slot only slightly reduces the fiber diameter. Thus, the air oscillation is the dominant parameter that causes fiber diameter reduction.
658 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 Table 1. Comparison of Fiber Diameters Produced from Continuous Flow through Both Slots (as in Conventional Melt Blowing) versus Flow through One Slot Onlp air flow rate air flow condition 54 slpm 100 slpm 144 slpm continuous flow through 76.8i 1.8 68.6i 1.3 46.8i 1.3 both slots continuous flow through left 75.2f 1.7 67.8i 1.6 45.3f 1.8 slot only continuous flow through right 75.8& 1.8 68.2i 1.8 46.2f 1.9 slot only air oscillation frequency of 1 Hz 66.3f 1.8 62.1i~1.7 38.4f 1.7 air oscillation frequency of 28 Hz 66.8f 1.9 59.1 i 1.9 40.2f 1.8
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Diameters are also given for several frequencies of air oscillation. Fiber diameters are in microns, and the standard deviations of the measurements are included. The operating conditions for the experiments were as follows: Qp = 0.48cm3/min, Tp= 350 "C, and Ta= 321 "C.
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Figure 6. Effect of air oscillation frequency on fiber diameter for a gas flow of 144 slpm.
Figure 4 shows results for an air flow rate of 54 slpm (standard liters per minute). Figures 5 and 6 show that similar behavior occurs for flow rates of 100 and 144 slpm. For 100 slpm, the peak occurs at about 9 Hz and the plateau begins a t about 20 Hz. For 144 slpm, the peak occurs at about 15 Hz and the plateau begins at about 30 Hz. Because small fiber diameters occurred at low frequency (1 Hz) as well as in the plateau region, even lower frequencies were investigated. Figure 7 shows the results of operating in the range of 0.1-1.0 Hz at an air flow rate of 54 slpm. As expected, the diameter at 1.0 Hz matches the diameter for 1.0 Hz on Figure 4.
The fiber diameter almost linearly increases as the frequency is reduced to 0.1 Hz. At very low frequencies the fiber diameter becomes the same as the diameter for continuous flow (the dashed line in the figure). Figure 8 shows that similar behavior occurs for an air flow rate of 100 slpm. For the conditions of the experiments, there are two frequency regions where fiber diameters are minimized. The first region occurs at about 1 Hz, and the second region begins at about 25 Hz. In between these two regions, a t about 13 Hz, there occurs a peak diameter. A number of fibers were examined to see if there was any variation of fiber diameter along the fiber length (i.e., a thick-thin character). A qualitative, visual examination showed no variation. More significantly, quantitative measurements of fiber diameter along the fiber length showed no change in diameter as a function of position along the fiber. In ordinary melt blowing (with no air oscillation), the melt blown fibers move in a cone of volume below the melt blowing die. The cone's apex is a t the die, and the cone's diameter monotonically increases as the distance below the die increases. Photographic evidence of this cone is contained in a recent article by Rao and Shambaugh (1993). Similarly, a cone also can be visually observed during melt blowing with air oscillation. The diameter of this cone depends on the oscillation frequency. In the low-frequencyrange of oscillation (frequency x 5 Hz or less) the cone diameter is several times larger than the cone diameter in the highfrequency range (frequency = 25 Hz or more).
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 659 100
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As Rao and Shambaugh (1993) showed, in conventional melt blowing the fibers do not exhibit simple sinusoidal motion. However, there is regularity to the fibers' motion. For example, under fixed operating conditions the fibers have a constant maximum amplitude of vibration. Our visual observation of the cone of fibers during our experiments suggests that similar behavior occurs when there is air oscillation. Rao and Shambaugh's model shows that the resonant frequencies of melt blown fibers are in the range (for their operating conditions) of about 8-62 Hz. In our experiments, resonant frequencies were found in the range of 0-50 Hz. The overlap of these ranges suggests that Rao and Shambaugh's model may be of use in predicting resonant frequencies suitable for air oscillation techniques. Effects of Varying the Melt Blowing Conditions. As stated previously, we have added a fifth variable-air oscillation frequency-to the usual four major operating variables (polymer flow rate, polymer temperature, air flow rate, and air temperature). Figures 4-8 illustrate the effect of this fifth variable on fiber diameter. The effects of the other four variables-in the presence of oscillation-will now be illustrated. Figure 9 shows the effect of polymer flow rate on fiber diameter. Air flow rate Qa,polymer temperature Tp, and air temperature Taare constant for this figure. Data are shown for both continuous air flow and for air oscillation frequencies of 28 and 46 Hz. These frequencies were chosen because, as shown in Figures 4-6, these frequencies are in the plateau region where the effect of oscillation is significant. Observe that, for the entire range of polymer flow rates, there is always a reduction in fiber diameter when oscillation is used. Also, there is no significant difference between the results for 28 and 46 Hz. Figure 10 shows the effect of polymer temperature on fiber diameter. As with Figure 9, data are shown for both continuous air flow and for oscillation frequencies of 28 and 46 Hz. Similar to the situation in Figure 9, for the entire range of polymer temperature the effect of air oscillation is significant. Also, there is again little difference between the results a t the two selected oscillation frequencies. Figure 11shows the effect of air flow rate, and Figure 12 shows the effect of air temperature. Again, the results are as expected: for each figure there is a significant effect of oscillation for the entire range of the independent variable.
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Conclusions and Recommendations By oscillating the air jets in the melt blowing process, finer fibers can be produced than by using continuous air jets. Another way of interpreting this is that, for a given desired fiber diameter, less air is needed if oscillation is used. Since the cost of compressing, heating, and recycling air is a significant factor in melt blowing, air oscillation has distinct economic advantages. Though our work was done for the commonplace slot die, oscillation would also work with other types of dies.
660 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
For example, in the Schwarz die (Schwarz, 19831,each polymer orifice is surrounded by three or four air discharge vents. The air coming from these vents could be oscillated if the proper piping system was installed. Oscillation may also enhance the strength andor appearance of the melt blown mat. For example, by oscillating some of the fibers in the machine direction and some of the fibers in the cross direction, a wovenlike mat would be produced. This mat would be strong in the machine and cross directions, and the mat would visually appear more woven than an isotropic mat. Besides the use of oscillating jets in the melt blowing process, air oscillation may also reduce fiber diameters and improve the product in spunbonding and in conventional melt spinning.
Acknowledgment We wish to thank the following companies for their support: 3M, Conoco/Du Pont, Kuraray Limited (Japan), and Phillips Petroleum.
Literature Cited Kayser, J. C.; Shambaugh, R. L. The Manufacture of Continuous Polymeric Filaments by the Melt Blowing Process. Polym. Eng. Sci. 1990,30 (19),1237-1251. O'Neil, P. V. Advanced Engineering Mathematics; Wadsworth Publishing: Belmont, CA, 1983;p 125. Rao, R. S.; Shambaugh, R. L. Vibration and Stability in the Melt Blowing Process. Ind. Eng. Chem. Res. 1993,32,3100-3111. Schwarz, E. C. A. Apparatus and Process for Melt Blowing a Fiberforming Thermoplastic Polymer and Product Produced Thereby. US Patent 4,380,570, April 15, 1983. Shambaugh, R. L. A Macroscopic View of the Melt Blowing Process for Producing Microfibers.Ind.Eng. Chem. Res. 1988,27,23632372. Wente, V. A. Manufacture of Superfine Organic Fibers. Report PB111437, NRL-4364, April 15, 1954; US Department of Commerce, Office of Technical Services, Washington, DC. Wente, V. A. Superfine Thermoplastic Fibers. Ind. Eng. Chem. 1956,48,1342. Wu, T. T.;Shambaugh, R. L. Characterization of the Melt Blowing Process with Laser Doppler Velocimetry. Ind. Eng. Chem. Res. 1992,31,379-389.
Nomenclature
Received for review May 26, 1994 Revised manuscript received September 23, 1994 Accepted October 18, 1994@
d = final (in product mat) fiber diameter, pm Qa = air flow rate, Umin at 21 "C and 1 atm pressure Qp = polymer flow rate, cm3/min
T , = air temperature, "C T p = polymer temperature, "C Greek Letters = frequency of air oscillation, Hz
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Abstract published in Advance ACS Abstracts, January 15,
1995.
