Turbulent air flow field in slot-die melt blowing for manufacturing microfibrous nonwoven materials
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Melt blowing is an industrial approach for producing microfibrous nonwoven materials utilizing high-speed air to attenuate polymer melt. The melt-blowing air flow field which is widely believed to be turbulence determines the process of fiber formation. In this study, the turbulent air flow field in slot-die melt blowing was experimental measured by hot-wire anemometer. The fluctuations of air velocity and temperature, the mean velocity and mean temperature were measured and analyzed; moreover, the relationship between turbulent air flow field and fiber formation in melt blowing was discussed and predicted. In the last part of this paper, the coupling effect of air temperature and velocity was studied tentatively, results showed that air temperature not only had an enhanced effect on velocity, but contributed to the fluctuation of velocity. This work shows that the fluctuating characteristics of air velocity and temperature have dominant effect on fiber motion and the evenness of fiber diameter.
Melt blowing is one of industrial approaches for manufacturing nonwoven materials. Because of the fiber diameter produced by melt blowing is commonly in the range from about 1 μm to several micrometers, the melt-blown nonwoven materials have found a variety of advanced applications in areas of filtration, life science, medicine and industry [1, 2].
In the present work, our study still focused on discovering the characteristics of turbulent air flow field in slot-die melt blowing. Compared to the previous works [13, 14, 15, 16, 28], innovations of researching turbulent air flow field in this work were: (1) an advanced two-dimensional velocity probe instead of one-dimensional velocity probe was applied in hot-wire anemometer for measuring the fluctuation of air velocity, (2) the fluctuation of air temperature was originally measured, which has not been reported until now, (3) the coupling effect between air velocity and temperature was also analyzed by processing the data of measurements.
Experiments and measurements
Melt-blowing setups and conditions
The measurement of the air flow field in this study was taken on the single-orifice melt-blowing device; Fig. 1a shows the schematic of single-orifice melt blowing. The structure of this single-orifice slot die is shown in Fig. 1b, 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 orifice diameter for polymer melt was 0.42 mm. Figure 1c shows the real slot die used in this work. The details of this slot die can be found in Xie and Zeng . The coordinate system used is also shown in Fig. 1a, 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 transverse to the major axis of the nose piece and slots. The z direction is directed vertically downward.
Our previous work  online measured the fiber whipping motion in slot-die melt blowing for the gauge air pressure of 1.0 atm (All air pressures used in this work refer to gauge pressures.), in view of discovering the relationship of turbulent air flow field and fiber motion, therefore, an air pressure of 1.0 atm was chosen again for measuring the turbulent air flow field. It is worth noting that, although the air pressure of 1.0 atm was higher than those used in previous work [23, 24, 25, 26], it was still lower than the air pressure used in commercial manufacturing. For example, nonwoven fibers with diameter of 18 μm could be produced with air pressure of 1.0 atm in this melt-blowing device, whereas the fiber diameter of the commercial nonwoven products was about 1–5 μm. For air temperature, the mean air temperature can be measured by the thermocouples with high accuracy, in this study, we focused on the fluctuation of the air temperature; therefore, a low and a medium initial air temperatures of 50 and 100 °C were applied in consideration of the limitation of the measuring equipment (hot-wire anemometer).
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, i.e., the polymer flow was stopped during measurements. Figure 2b shows the relative positions of the hot-wire anemometer and the melt-blowing die.
Compared to one-dimensional velocity probe used in previous researchers [14, 15, 16, 28], a two-dimensional velocity probe was attempted for the velocity measurement in this work; Figs. 2c and 2d illustrate the structures of one-dimensional and two-dimensional velocity probe. The two-dimensional velocity probe is consisted of two systems of one-dimensional probes; each one-dimensional velocity probe has a metal wire with a 5-μm diameter and a 1.6-mm length suspended between two needle-shaped prongs. As shown in Fig. 2d, the two wires are placed in paralleled planes, the projection on y–z plane and x–z plane of the two wires is vertical and paralleled, respectively. Compared to one-dimensional velocity probe for measuring air velocity, two-dimensional probe has better stability, for the reason that the wire undergoes drastic and continual air flow during measurement, the junction of wire and the needle-shaped prongs or the wire itself probably become exfoliated or broken, resulting in incontinuity of measurement. However, if two-dimensional velocity probe is applied, the measurement will be suspended only in the case of the two wires are all disabled.
During the measurements of air velocity and temperature, the corresponding velocity and temperature probe was positioned with a one-dimensional traversing system as shown in Fig. 2b, which permitted up or down motion along z-axis in 2-mm increments. It was found that during the measurements, although two-dimensional velocity probe was applied, the wires were easily broken when it was very close to the die; therefore, the minimum distance below the die was set to z = 6 mm.
Calibrations including velocity calibration and temperature calibration were accomplished on calibration device at ambient temperature of 18 °C, before formal measurements.
Results and discussion
Fluctuation of velocity
Fluctuation of temperature
From Figs. 4 and 8, although air velocity and temperature have obviously different fluctuating characteristics in melt blowing, their characteristics of fluctuation play an important effect in fluctuating attenuation of the fiber. We suppose that the later research on fiber attenuation, fiber motion as well as the evenness of the nonwoven fibers in melt blowing should take the characteristics of fluctuating air velocity and temperature simultaneously into consideration.
Mean velocity and temperature profiles
Figure 11a shows the development of the mean air velocity profile as a function of the distance, z, along z-axis. It is shown that mean velocity decreases rapidly with increasing distance from the die until reaching the point of z = 50 mm. A maximum mean velocity of 183 m s−1 is obtained at the point of z = 6 mm, which is about 30 m s−1 higher than the maximum velocity measured by previous researchers [13, 14, 15, 16]. Although air velocity in the region of 0 mm < z < 6 mm is not shown in this work, it is no doubt that fiber undergoes strong attenuation effect by the high-speed air velocity in this region. However, as shown in Fig. 11a, the fiber velocity not increases rapidly but increases linearly and gradually, the measurement of the fiber velocity was described in our previous work , and the fiber was produced under the same air pressure using in this work. Moreover, in consideration of the fiber whipping paths shown in Fig. 7, we confirm that the turbulent air near the die with high-speed velocity results in inducing the lateral two-dimensional or three-dimensional whipping motion of the fiber, rather than rapidly increasing the fiber velocity along z direction.
Figure 11b shows the mean air temperature profiles along z-axis. The trend of the temperature decay is similar to the velocity decay shown in Fig. 11a. In view of the initial air temperature is 100 °C (air temperature at point of z = 0 is 100 °C), the air temperature decreases to be 50% of itself at the point of z = 13 mm. It is this temperature drop that is believed to be the driving force for crystallization of the polymer melt and fiber solidification in the melt-blowing process.
During melt-blowing process, both air velocity and temperature determine the attenuation of fiber, a lot of work have been done on optimizing the air flow field by designing new structure of the die [30, 31], these optimizations of air flow field were refer to that the new designed dies could provided air flow field with higher velocity and higher temperature, simultaneously. It is accessible that air with higher velocity can provide stronger attenuation of fiber and air with higher temperature can maintain molten status of fiber in longer time. However, Xie and Zeng  showed that higher air velocity and higher air temperature not always produced fibers with smaller diameter. In their work, a gauge air pressure of 1.0 atm created initial air velocities about 300 and 150 m s−1 for slot-die and swirl-die melt blowing; however, the fibers collected at 25-mm distance below the die have a mean diameter of 75.2 µm for the slot die, and 58.3 µm for the swirl die, for the reason that the type of turbulence also determines attenuating ratio of fiber. In addition, we think that the energy wasting should be considered in optimizing air flow field in melt blowing, in other description, the attenuation effect provided by the relative velocity of air and fiber occurs just in the region where the fiber is at the status of molten; otherwise, high air velocity has no contribution to fiber attenuation where fiber is solidified, i.e., velocity energy wasting appears. Similarly, temperature wasting appears where fiber attenuation effect disappears, not only this, the molten fiber tends to rebound under its internal viscoelastic force resulting in fiber diameter re-increasing. The phenomenon of fiber rebound has been discovered [33, 34], which is very harmful to melt-blowing productions.
Coupling effect of velocity and temperature
In melt blowing, air velocity and temperature exist simultaneously for fiber attenuation. The coupling effect between air flow field and the fiber is the essential mechanical mechanism for fiber attenuation. Besides this coupling effect, the coupling effect of velocity and temperature may exist. In this part, we have done a tentative research on this coupling effect, in particular, the effect of temperature on velocity was revealed by processing data of measurements.
Here Ecorr is the converted voltage; Tw is the constant temperature of the wires, i.e., 238 °C; T0 is the ambient temperature, 18 °C, and T is the temperature of the melt-blowing air, T at different points can be obtained from Fig. 9b.
Turbulent air flow field in slot-die melt blowing was experimentally studied by using hot-wire anemometer. The fluctuating characteristics of the velocity and temperature as well as mean velocity and temperature were obtained. The results showed that the characteristics of velocity and temperature fluctuations were obviously different, the velocity fluctuation was irregular and strong, and it contained some unexplored characteristics of the turbulence, such as a predicted steady vortex existed at the position near z = 14 mm. The fluctuation of temperature revealed a regular profile like sine or cosine curve, moreover, the frequency of the sine or cosine curve was changeless at different points along z-axis. In the last part of this study, the coupling effect of velocity and temperature was analyzed; the results showed that temperature has an enhanced effect on velocity. The fluctuation of temperature not only contributed to changing of viscosity of the fiber but also generated fluctuation of velocity. Fluctuation of both velocity and temperature could account for the reason why the evenness of fiber diameter produced by melt blowing was poor.
This work discovered that the characteristics of turbulent air flow field had great relationship with the fiber motion as well as fiber diameter evenness of melt-blowing products. Indicated that turbulent air flow field deserved to be further discovering in order to fully research on melt blowing.
This research was supported by the National Natural Science Foundation of China (Grant Nos. 11702113 and 51506075), Natural Science Foundation of Zhejiang Province (Grant Nos. LQ18E040001 and LQ18E030013) and Educational Commission of Zhejiang Province of China (Y201636479).
Compliance with ethical standards
Conflicts of interest
We declare that we have no conflict of interest.
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