Fine Filament Formation Behavior of Polymethylpentene and Polypropylene near Spinneret in Melt Blowing Process

Transcription

1 REGULAR CONTRIBUTED ARTICLES R. Ruamsuk, W. Takarada, T. Kikutani* Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo, Japan Fine Filament Formation Behavior of Polymethylpentene and Polypropylene near Spinneret in Melt Blowing Process Formation behavior of fine fibers of polymethylpentene (PMP) and polypropylene (PP) in the melt blowing process was investigated through the observation of the spin-line near the spinning nozzle using a high speed camera. Reduction of the polymer throughput rate and increase of the air flow rate were necessary to achieve fine diameter fibers, however these conditions generally cause the instability in the spinning process. Observation of the spin-line at the melt blowing die revealed the periodic accumulation of polymer flow near the spinning nozzle followed by the quick pulling down of the accumulated polymer by the air flow. This behavior caused the periodic fluctuation of fiber diameter as well as the intermittent breakage of the spin-line under extreme conditions. Because of high extrusion viscosity, PMP showed more stable spinning behavior than PP. Frequency of diameter fluctuation became higher with the increases of air flow rate and throughput rate, and the maximum frequency of about 60 Hz was observed for PP spinning with the throughput rate of 0.18 g/min. Diameter distribution of the fibers in the prepared web was also analyzed to compare with the spinning behavior. Fiber diameter distributions were narrow and symmetric under stable spinning conditions, whereas skewed diameter profiles with a maximum at low value and a long tail to the larger diameter region were observed under unstable conditions. Intermittent spin-line breakage caused flaws of \shot" and/or \fly", and the skewed fiber diameter distribution with the presence of a small amount of fiber of extremely large diameter was confirmed. 1 Introduction * Mail address: Takeshi Kikutani, Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, S8-32, O-okayama, Meguroku, Tokyo , Japan kikutani.t.aa@m.titech.ac.jp Melt blowing is a process for producing nonwoven fabrics with the utilization of high pressure hot air. Since the exposure of this process to public in 1950 (Wente, 1956), numerous researches on the melt blowing process have been conducted through both experimental and theoretical approaches. The process has been attracting more interests in commercial field recently due to the potential of producing very small diameter fibers with high productivity. Accordingly, the majority of recent research papers are focusing on the improvement of fabric properties through the reduction of fiber diameter to nanometer scale, and application of those fabrics for filtration and membrane separation (Ellison et al., 2007; Hammonds et al., 2014; Hassan et al., 2013; Uppal et al., 2013). There also are various studies on the spin-line behavior in the air flow, which is the key for controlling the attenuation of fibers in the melt blowing process (Chung and Kumar, 2013; Sinha- Ray et al., 2010; Tan et al., 2012), however, detailed understanding on the mechanism of fiber formation in this process is still incomplete. The experimental and numerical simulation studies for the influence of viscoelasticity and air flow rate on fiber diameter and its distribution were reported (Tan et al., 2010; Zhou et al., 2011). These works are based on one-dimensional slender-jet model which excludes the incorporation of whipping motion that usually occurs in melt blowing process. In order to clarify the fiber formation behavior in the melt blowing process, various techniques were employed to analyze the attenuation and motion of the spin-line in the melt blowing process. Shambaugh group firstly applied the high speed strobe photography with 100 flashes per second for determining \fiber vibrations" (Rao and Shambaugh, 1993). They further studied the vibration frequencies and amplitudes of spin-line using a single orifice die and applying the flash photography and laser Doppler velocimetry (Chhabra and Shambaugh, 1996). They also used high speed flash camera to perform online fiber diameter analysis with the size of down to 30 lm (Bansal and Shambaugh, 1998; Marla and Shambaugh, 2009). In 2007, they employed a swirl die, and investigated the frequency and amplitude of fiber vibration in the spin-line. In this case, spinning conditions of high throughput rate and low air flow rate were employed to gain high stability of the spin-line. Those conditions also lead to relatively large sizes of final fiber diameter (Beard et al., 2007). On the other hand, Bresee et al. (2003) utilized a high speed digital camera and pulsed laser to capture filament formation Intern. Polymer Processing XXXI (2016) 2 Ó Carl Hanser Verlag, Munich 217

2 behavior along the spin-line in the melt blowing process with a commercial scale multi-hole die. Fiber velocity and acceleration were obtained through the analysis of triply exposed spinline images of known time interval. They also analyzed fiber diameter the high speed camera images, however, only diameters larger than 60 lm could be observed. More recently, Xie and Zeng (2012; 2013) have observed the whipping motion of the melt blowing spin-line, which is similar to the instability occurring in the electrospinning process. They could show the three-dimensional path of the whipping motion of the spin-line, however they only employed relatively high throughput rate. Therefore the final fiber diameter could not be very small even though air flow rate was high. Throughout the above mentioned researches, the high speed camera was utilized mainly for capturing the motion of the spin-line in the turbulent air flow. In-situ observation of fiber diameter could be achieved only down to the range of a few tens of micrometers. In other words, in-situ observation for the spin-line attenuation behavior in the melt blowing process for producing fine diameter fibers with the final fiber diameter less than 10 lm has not been reported yet. Therefore, in this research, observation of the fiber formation behavior near the spinneret, where attenuation of the spin-line proceeds rapidly, was performed under the melt blowing conditions for producing fine diameter fibers. A particular attention was paid for the analysis of the periodic fluctuation of fiber formation behavior. 2 Experimental 2.1 Materials Polymethylpentene (PMP) with melt flow rate (MFR) of 3000 at 2608C, 5 kgf and polypropylene (PP) with MFR of 1550 at 230 8C, 2.16 kgf provided by Mitsui Chemicals, Inc., Tokyo, Japan, were used in this study. 2.2 Melt Blowing Equipment and Conditions A twin screw extruder (Asahi Engineering, Co., Ltd., Fukuoka, Japan) equipped with a metering pump and a melt blowing die was used to perform fiber spinning process. A schematic of the spinning die is shown in Fig. 1A, B. The polymer melt was extruded through a melt blowing die of 8 cm (3.15 in) long with 10 spinning nozzles. Diameter (D o in Fig. 1 and L/D of the nozzles were 0.12 mm and 12. The extrusion temperatures of 255 and 2508C were adopted for PMP and PP, respectively. The throughput rates per 10 holes for PMP were 0.06 and 0.50 g/min and those for PP were 0.06, 0.18 and 0.60 g/min. As a preliminary experiment, extrusion pressure was measured using a 0.06 mm diameter nozzle at the throughput rate of 0.18 g/min for comparison of the viscosity of PMP and PP. Extrusion pressure measured for PMP at the extrusion temperature of 2558C was 2 MPa while that for PP at 2508C was 0.2 MPa, indicating that PMP generally has higher viscosity than PP at the extrusion condition adopted in this experiment. Fig. 1. Schematic of the melt blowing die with ten nozzles, bottom view (, side view ( and experimental set-up with high speed camera (. Do is nozzle diameter, a is air-slot width, and S is setback 218 Intern. Polymer Processing XXXI (2016) 2

3 Regarding the air-blowing conditions, the width of air slot was 1.0 mm (a in Fig. 1 and the setback (S in Fig. 1 was 0 mm. In ordinary industrial melt blowing conditions, a few millimeters of setback is adopted so that the nozzle surface is not directly exposed to the outer air. In this study, it was set to 0 mm with the aim of observing the behavior of molten polymer right after its extrusion the nozzle. Temperature of the blowing air was 2908C for both polymers, and the air flow rates of 50, 100, 200 and 300 l/min were employed. Velocity of the blowing air was measured using a pitot tube (DT-8920, CEM Instruments, Shenzhen, PR at a position 5 mm below the spinning die. 2.3 In-situ Observation For the in-situ observation of the thinning behavior of extrudates near the spinning nozzle, a high-speed camera (Phantom V9.0, Vision Research, Inc.) was used at a frame rate of 4000 fps and resolution of pixels as shown in Fig. 1. The lenses used together with the high-speed camera were 105 mm f/2.8 (Nikon AF Micro, Nikon Corp., Tokyo, Japan) for a wide-angle view, and 200 mm f/4 (Nikon ED-IF AF Micro, Nikon Corp., Tokyo, Japan) for a high magnification view. The light source was a 450 W metal halide lamp. 2.4 Analysis of Fiber Diameter Distribution The melt blown fibers were collected using a wire-mesh set at the die to collector distance (DCD) of 10 cm. The image of collected fiber web was obtained using a scanning electron microscope (SM-200, Topcon Co., Ltd., Tokyo, Japan). For the analysis of fiber diameter distribution, diameters of 100 fibers on the horizontal line in the middle of at least 5 SEM images were measured using image processing software (Image J, NIH, Maryland, US. 3 Results and Discussion 3.1 Fiber Formation Behavior near Spinneret For the formation of fine diameter fibers, in the melt blowing process, reduction of throughput rate and increase of air flow rate are generally required, however these conditions tend to cause instability of the spinning process and breakage of the spin-line. When relatively high throughput rates of 0.50 g/min for PMP and 0.60 g/min for PP were adopted, which spinning line was stable at the air flow rates of 100, 200 and 300 l/min. As typical examples, thinning behaviors of PMP and PP near the spinneret captured using a high-speed camera are shown in Fig. 2. In the series of pictures with time-interval of 10 ms, no significant spin-line diameter fluctuation was observed. Captured images of spinning behavior for the PMP spinning with the reduced throughput rate of 0.06 g/min are summarized in Fig. 3. Spin-line was still stable at the air flow rate of 100 l/ min. When the air flow rate was increased to 200 l/min, spinning behavior became unstable, and significant diameter fluctuation with the period of around 160 ms, which corresponds Fig. 2. Series of pictures captured by high-speed camera for melt blowing process of PMP ( and PP ( at air flow rate 100 l/min. Throughput rates for PMP and PP are 0.5 and 0.6 g/min, respectively Fig. 3. Series of pictures captured by high-speed camera for melt blowing process of PMP with throughput rate of 0.06 g/min and different air flow rates of 100 (, 200 ( and 300 l/min ( to the frequency of around 6 Hz, was observed. After the further increase of the air flow rate to 300 l/min, unstable spinning behavior with intermittent breakage of the spin-line was observed. Period of diameter fluctuation and spin-line breakage was around 100 ms. Spinning behavior was more unstable in case of PP as shown in Fig. 4. When the throughput rate was reduced to 0.06 g/min, intermittent spin-line breakage was observed at the air flow rate of 100 l/min. Even after the further reduction of air flow rate to 50 l/min, intermittent breakage was still occurring. The period of spin-line breakage became longer with the reduction of the air flow rate and reached around 240 ms at the air flow rate of 50 l/min. In these cases, after the breakage of the spinline, a large amount of polymer was accumulated to form thick part near the spinneret before it was extended by the air flow. Intern. Polymer Processing XXXI (2016) 2 219

4 Materials Throughput rate Air flow rate l/min Fig. 4. Series of pictures captured by high-speed camera for melt blowing process of PP with throughput rate of 0.06 g/min and different air flow rates of 50 ( and 100 l/min ( Fig. 5. Series of pictures captured by high-speed camera for melt blowing process of PP with throughput rate of 0.18 g/min and different air flow rates of 100 ( and 300 l/min ( To discover the limit of continuous spinning condition for PP, throughput rate was increased to 0.18 g/min. The results are shown in Fig. 5. At the air flow rate of 100 l/min, continuous spinning was possible even though there still was a periodic fluctuation of fiber diameter. When the air flow rate was increased to 300 l/min, intermittent spin-line breakage with extremely short period of diameter fluctuation of about 18 ms was observed. In this case, only a small amount of cone shaped polymer was accumulate at the nozzle exit before it was stretched by the air flow to form thin fibers. Stability of the melt blowing process described above is summarized in Table 1, categorizing the spinning behavior into 1) stable and continuous formation of fibers, 2) unstable but continuous formation of fibers and 3) unstable and intermittent breakage of fibers. Frequency of the diameter fluctuation, which also corresponds to the frequency of intermittent breakage of the spin-line, is summarized in Fig. 6. Spinning was more stable for PMP in comparison with that for PP. This probably is mainly due to the higher melt viscosity of PMP than PP at the extrusion conditions adopted. In general, lower throughput rate and higher air flow rate lead to the unstable condition while the period of diameter fluctuation became shorter with Volume cm 3 /min Mass g/min PMP N/A O D X N/A O O O PP X X X X N/A D X X N/A O O O O: stable and continuous formation of fibers, D: unstable but continuous formation of fibers, X: unstable and intermittent breakage of fibers. Table 1. Summary of the fiber formation behavior in melt blowing experiments Fig. 6. Frequency of diameter fluctuation and intermittent spin-line breakage for melt blowing process of PMP and PP under various spinning conditions the increases of air flow rate and through-put rate. At similar spinning conditions, frequency of diameter fluctuation was higher for PP than PMP. Observation of the unstable spinning process suggested that there always an accumulation of molten polymer near the spinneret before it is pulled down by the air flow. It is followed by the breakage of the spin-line or continuous formation of extremely thin spin-line. Larger amount of accumulated volume may be needed for the occurrence of pulling down when the melt viscosity is higher and the air flow rate is lower. Time needed for periodic accumulation of maximum volume near the nozzle starting after sudden pulling down of the accumulated volume corresponds to the frequency of diameter fluctuation. In case of PP at throughput rate of 0.18 g/min, only a cone shaped portion of small volume was observed while at the 0.06 g/min, extremely large volume of thick fiber was formed below the spinneret. In other words, smaller volume of accu- 220 Intern. Polymer Processing XXXI (2016) 2

5 Fig. 7. Fiber diameter distributions in the PMP web prepared in the melt blowing process with throughput rate 0.06 g/min and different air flow rates of 100 (, 200 ( and 300 l/min ( mulation with higher throughput led to higher frequency whereas larger volume of accumulation with lower throughput led to lower frequency. 3.2 Fiber Diameter Distribution Fiber diameter distributions for the webs of PMP and PP prepared under various spinning conditions are shown in Figs. 7 and 8, respectively. In case of PMP at the throughput rate of 0.06 g/min, diameter distribution was narrow and symmetric with the peak at around 8 lm for the stable condition with the air flow rate of 100 l/min. When air flow rate was increased to 200 l/min, which corresponds to the condition of unstable but continuous fiber formation, peak value shifted to 3 lm whereas the shape of distribution was skewed with a long tail to higher D) Fig. 8. Fiber diameter distributions in the PP web prepared in the melt blowing process with throughput rate 0.6 g/min and air flow rate of 100 l/min (, with throughput rate 0.18 g/min and air flow rate of 100 l/min (, with throughput rate 0.18 g/min and air flow rate of 300 l/min (, with throughput rate 0.06 g/min and air flow rate of 100 l/min (D) Intern. Polymer Processing XXXI (2016) 2 221

6 diameter region. With the further increase of the air flow rate to 300 l/min, which corresponds to the condition of unstable and intermittent breakage of the spin-line, minimum diameter reduced to the range of 1 lm while the peak was at 5 lm with an extension of tail to the higher diameter region. At this condition, existence of a small amount of extremely large diameter fiber was confirmed. Results for PP showed similar tendency. When the spin-line was stable at 0.6 g/min and 100 l/min, diameter distribution was relatively narrow and symmetric. For the condition of continuous but unstable spin-line at 0.18 g/min and 100 l/min, wide and bimodal diameter distribution was observed. On the other hand, for the unstable spinning with intermittent spin-line breakage, i. e. for the conditions of 0.18 g/min, 300 l/min and 0.06 g/min, 100 l/min, diameter distribution showed a skewed shape with a peak at low value and long tail to the higher diameter region. 3.3 Estimation of Fiber Velocity Based on the concept of continuity theory for the steady state melt spinning process, i. e. mass throughput rate equals to the product of density, cross sectional area and fiber velocity, final fiber velocities were estimated both averaged diameter and averaged cross-sectional area, which is proportional to the average of the square of diameter, and compared with the measured air-velocity. The results are summarized in Table 2. It should be noted that the fiber velocity estimated the averaged cross-sectional area is lower than that the averaged diameter in principle, and the ratio between the two velocities can be a parameter indicating the degree of diameter distribution. In case of PMP, the ratios of the estimated fiber velocities to the maximum air velocity were around 0.2. In other words, fiber velocities were well below the maximum air velocity. This result appears to be reasonable because it is well known that the air velocity decreases quickly with the increase of distance Materials Throughput rate g/min Air flow rate l/min Air velocity at 5 mm spinneret m/min Average diameter lm the spinneret (Bansal and Shambaugh, 1998; Bresee and Ko, 2003; Marla and Shambaugh 2009; Tan et al., 2012; Xie and Zeng, 2012; 2013). On the other hand, in case of PP, the ratios were much higher and exceeded unity in some conditions even for the fiber velocity estimated the averaged crosssectional area was used. There are several possible reasons for such unusual results. For example, considerable fiber attenuation occurring in the region closer than 5 mm to the spinneret where air velocity can be higher than the measured value, underestimation of averaged diameter because of the significant diameter fluctuation and presence of small amount of extremely thick region. It was also reported by several researchers that the whipping motion of the spin-line caused by the air flow can induce additional attenuation of the filament in the melt blowing process (Sinha-Ray et al., 2010; Xie and Zeng, 2013; Hubsch et al., 2013). 3.4 Some Consideration on Instability The unstable but continuous fiber formation behavior with periodic fluctuation of filament diameter in the melt blowing process has a certain similarity with the draw resonance behavior which is observed in ordinary melt spinning process with high melt draw ratio. Melt draw ratio is defined as the draw ratio of molten polymer in the melt spinning process, i.e. ratio of the cross-sectional areas of spinning nozzle and prepared fiber (Petrie and Denn, 1976). It has been predicted theoretically that the draw resonance occurs in the isothermal melt spinning of Newtonian fluid when the melt draw ratio exceeds Tendency for the occurrence of instability at higher air flow rate and lower throughput rate in the melt blowing process both correspond to higher melt draw ratio. It should be noted, however, that the take-up velocity is constant in case of ordinary melt spinning process whereas final fiber velocity varies with time in case of melt blowing process. Another instability noted in this study is the filament breakup. From the view point of web formation, two types of flaws Average crosssectional area lm 2 Calculated fiber velocity average diameter m/min average crosssectional area m/min Ratio of calculated fiber velocity/air velocity average diameter a verage crosssectional area PMP PP Table 2. Comparison of air velocity and fiber velocity final fiber diameter 222 Intern. Polymer Processing XXXI (2016) 2

7 originated filament breakup were observed. One is the formation of polymer droplet which is called \shot". The other is the defect called \fly". We did not observe another typical flaw of melt blowing process, i. e. roping, throughout our experiment. \Shot" is thought to be formed with the effect of surface tension. When there is a cessation of spin-line and releasing of tensile stress, the molten polymer tends to shrink to form a large droplet because of the effect of surface tension. On the other hand, \fly" seems to be formed because of the strong cooling effect for thinned fibers. Accordingly, the fibers cannot be captured at the collector, however, still the break-up of the fiber at the beginning seems to be caused by the effect of surface tension for thin fibers, which is known as capillary breakup. In general \shot" and \fly" co-exist when spin-line breakage occurred. It was recognized that \shots" is the dominant flaws at the low throughput rate and high air flow rate, while \fly" is the dominant flaws at the small throughput rate and high air flow rate. Fiber diameter distribution could not be evaluated for PP web prepared at 0.18 g/min and 50 l/min because of the existence of too many shots on the web. 4 Conclusion Formation behavior of fine fibers of PMP and PP in the melt blowing process was investigated through the observation of the spin-line near the spinning nozzle. Reduction of the polymer throughput rate and increase of the air flow rate, which were necessary to achieve the fine diameter fibers, caused the instability in the spinning process. Periodic accumulation of polymer flow near the spinning nozzle followed by the quick pulling down of the accumulated polymer by the air flow was observed. This behavior caused the periodic fluctuation of fiber diameter and the intermittent breakage of the spin-line. Spinning behavior of PMP was more stable than PP presumably because of the higher viscosity of PMP. Frequency of diameter fluctuation increased with the increases of air flow rate and throughput rate. Fiber diameter distributions in the prepared web were narrow and symmetric under stable spinning conditions, whereas skewed diameter profiles with a maximum at low value and a long tail to the larger diameter region were observed under unstable conditions. Intermittent spin-line breakage caused flaws of \shot" and/or \fly", and the skewed fiber diameter distribution with the presence of a small amount of fiber of extremely large diameter was confirmed. References Bansal, V., Shambaugh, R. L., \On-line Determination of Diameter and Temperature during Melt Blowing of Polypropylene", Ind. Eng. Chem. Res., 37, (1998), DOI: /ie Beard, J. H., Shambaugh, R. L., Shambaugh, B. R. and Schmidtke, D. W., \On-line Measurement of Fiber Motion during Melt Blowing", Ind. Eng. Chem. 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Sci., 66, (2011), DOI: /j.ces Date received: August 16, 2015 Date accepted: November 11, 2015 Bibliography DOI / Intern. Polymer Processing XXXI (2016) 2; page ª Carl Hanser Verlag GmbH & Co. KG ISSN X Intern. Polymer Processing XXXI (2016) 2 223