Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol Vol: 2 Issue: 3

Structure and Air Permeability of Melt Blown Nanofiber Webs

Wanli Han1, Xinhou Wang1 and Gajanan S Bhat2*
1Department of Textile Engineering, College of Textiles, Donghua University, Shanghai 201620, People’s Republic of China
2Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200, USA
Corresponding author : Gajanan S Bhat
Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200, USA
E-mail: [email protected]
Received: March 28, 2013 Accepted: June 26, 2013 Published: July 05, 2013
Citation: Han W, Wang X, Bhat GS (2013) Structure and Air Permeability of Melt Blown Nanofiber Webs. J Nanomater Mol Nanotechnol 2:3. doi:10.4172/2324-8777.1000115

Abstract

Structure and Air Permeability of Melt Blown Nanofiber Webs

Melt blown submicron fibers have been produced using two different multi-holes dies from commercially available polymers on a pilot line. Mean fiber diameters (MFD) along with fiber diameter distribution were investigated. Our results clearly showed that the fiber diameter distribution was different from the one observed in a single hole melt blowing process. Besides, the web pore size has been studied using capillary flow porometry in terms of the relationships among the processing conditions, fiber diameter and permeability. These results reveal that the process air pressure during melt blowing significantly influences the fiber diameter and its distribution, and the permeability of the webs. Furthermore, the pore size and pore size distribution are strongly associated with fiber diameter. This research provides a general understanding of the nanofiber web structure in a commercial melt blowing process.
 

Keywords:

Keywords

Melt blowing; Nanofibers; Pore structure; Air permeability; Fiber diameter distribution

Introduction

Nonwovens are commonly defined as manufactured sheets or webs of randomly or sometimes directionally oriented fibers, which are entangled or adhered by fibers without requiring any downstream processing such as weaving or knitting [1]. One of the most significant developments of this technology is melt blowing, which involves applying a jet of hot air to an extruding polymer melt and drawing the polymer stream into microfibers. In general, the commercial melt blown products are composed of fibers with average diameters exceeding 1-2 µm. Meltblown fibers, due to their extremely high surface to weight ratio, have a wide range of applications such as in filtration, medical applications, tissue engineering and wound dressing [2-4].
There have been a few studies describing numerical simulation of the melt blowing process. Most of the researches on the melt blowing process are focused on the coat-hanger die for melt distribution [5,6], the air drawing of the polymer jet [7-11] and the performance of the jet slot dies [12,13]. The fundamental understanding of the melt blowing process is important, especially when one is trying to produce nanofibers. However, most of the models are based on many assumptions and the results do not match with the experimental observations. Whereas modeling has its merits, results from experimental research are important to understand this complex process. Lee and Wadsworth [14] have studied the relationships among the melt blowing process conditions, fabric structure and filtration efficiency. They observed that the filtration efficiency improved with decreasing fiber diameter. The structure of the melt blown webs was influenced not only by the fiber diameter but also the fiber diameter distribution and the pore size. It is useful to understand the controlling factors for pore size, which can inturn be used to engineer the porous structure of melt blown webs for specific applications. But, in their research, melt blown webs were composed of fibers with average diameters exceeding 5µm and the correlation between pore size and other relevant parameters such as pore size distribution were not discussed.
Lately, the focus is on reducing the fiber diameters under the commercial melt blowing process conditions. If melt blowing technology could be extended to nano-scale fiber sizes, unlike other technologies, melt blowing would provide a much easier, faster and economical method to produce nanofibers.
It is surprising that one image of the fibers was given by Wente [15] and showed the diameters to be less than 1 µm. It is well known that the melt blowing process is based on Wente’s original work and has been continuously studied in recent years. Wente [15] first described the process of melt blowing and introduced the construction of a melt blowing die composed of a series of orifices and slots. However, some details, such as fiber diameter distribution, air pressure, polymer throughout, etc were not reported.
To the best of our knowledge, there are very limited publications that report the nanofiber production by melt blowing using the multi-hole die under commercial process conditions. Only Ellison et al. [16] have studied the nano-scale melt blown fiber using a single orifice melt blowing apparatus and analyzed the fiber diameter and distribution. Meanwhile, they also observed the fiber breakup but did not study the melt blown fabric structure. However, the single hole melt blowing apparatus is significantly different from the multi-hole die because of the fiber movement during the melt blowing process. At this time, only a couple of papers [17,18] have reported that submicron fibers may be produced via melt blowing.
In this research, we studied the nano-scale melt blown fiber diameter and distribution with different multi-hole dies under commercially used processing conditions, and also evaluated the fabric structure, such as mean pore diameter and air permeability. This study provides a general understanding of the nanofiber structure of melt blown webs for commercial production.

Experimental

Materials and processing
The commercial polypropylene (PP) resin produced by ExxonMobil Chemical company was used for melt blowing. The melt flow rate (MFR) of the resin was 1550 g/10 min.
Melt blowing was performed on the 15 cm wide melt blowing pilot line at the University of Tennessee Nonwovens Research Laboratory (UTNRL), Knoxville, TN. A schematic diagram of the melt blowing process is shown in Figure 1. In this process, the polymer is fed to the extruder through the hopper. The motor-driven screw pushes the material through the extruder while five independently controlled heaters help melt the polymer and maintain the desired temperatures. A pressure transducer measured the melt pressure between zones 4 and 5, and the pressure was used to monitor and ensure that a constant polymer flow rate (throughput) was being supplied to the die while the samples were being collected.
Figure 1: Schematic of the melt blowing pilot line.
Two different meltblown nano dies were used in the melt blowing process: an AGR die with 14 orifices per cm and a Hills die with 24 orifices per cm. The heating temperatures of the five zones were 175, 190, 200, 230 and 230°C, respectively. The four sets of air pressures studied were 0.034 MPa, 0.055 MPa, 0.069MPa and 0.103 MPa.
Characterization
Meltblown webs produced using the two different melt blowing dies under various processing conditions were characterized. Samples were sectioned from meltblown fiber mats and were imaged using an ETEC Auto-scan scanning electron microscope at 12Kev after coating with a gold layer. At least 100 fiber diameters from more than 10 SEM images were measured using an image processing software (imageJ NIST). Averge fiber diameters and the diameter distributions are reported. The meltblown webs were cut into rectangle pieces for porometry measurement and the samples were tested individually. The pore diameter and distribution were measured by an SFP-1100- AEX capillary flow porometer (Porous Media Inc, Ithaca, NY). The pores of the samples are filled with a wetting liquid, the liquid is emptied by a pressurized gas to flow through the empty pores. The pore size and distribution are calculated by the software using equation 1 [19].
(1)
Where D is the pore diameter, γ is the surface tension of wetting liquid, θ is the contact angle of the wetting liquid and p is the differential pressure. Reported results are averages of three separate measurements for each samples. Air permeability was tested using the TEXTEST FX3000 according to ASTM standard D737-96 [20].

Results and Discussion

Fiber diameter and distribution
The average fiber diameters for melt blown webs produced from polypropylene using different levels of air pressures with both the dies are shown in Figure 2. Both experimental and theoretical studies have reported that mean fiber diameter decreases when the airflow rate increases [21,22], and our results are consistent with these earlier observations. The fiber diameter becomes smaller and more uniform with increasing air pressure for both the dies. During the melt blowing process, the airflow dominates many other processing variables affecting the fiber attenuation. The reason is that air speed exceeds fiber speed significantly and the drag force is increased, especially when the die-to-collector distance (DCD) is shorter.
Figure 2: melt blown fiber diameter and standard variance.
From Figure 2, it is clearly evident that the average fiber diameter decreases to nanometer scale with the multi-hole dies using commercially viable processing conditions. Especially for the AGR die, the average fiber diameter is under 1 um for all air pressures.
Figure 3a and 3b show the fiber diameter histograms for samples produced with different air pressures for the AGR and Hill dies, respectively. For both the dies, fiber diameter distribution is nearly symmetrical ignoring the tail at the large diameter end. It is obviously different from Bates’ research [16] where fiber diameter distributions are described by a log-normal distribution function regardless of average fiber diameter. It is reasonable to explain that the distribution of diameter is influenced by fiber contact and entanglement that is encountered in the multi-hole die in a melt blowing process. Bresee and Ko [23] obtained the pictures of fiber motion using high-speed imaging in a multi-hole melt blowing process, using the same equipment used here. Those images provided much information about the physical status of fiber and showed that the fiber entanglement and orientation changed with increasing distance from the die. Fiber entanglement and fiber orientation make the commercial multi-hole melt blowing process fundamentally different and substantially more complex than the single-hole melt blowing process. These complex variables change the fiber velocity and acceleration, which produce elongational forces, and then influence the fiber attenuation and resulting diameter distribution.
Figure 3: Fiber diameter distribution of samples produced at various air pressures.
Meanwhile, it can be seen from Figure 3 that the webs have a more uniform fiber diameter distribution at the higher air pressure. Also smaller average fiber diameters are consistently associated with smaller standard deviations with increasing air pressure.
It should be noted that there appears to be no further significant reduction in average fiber diameter beyond a certain air pressure. This asymptotic behavior is clearly seen for the AGR die. This phenomenon may be explained by two reasons. First, with the air pressure increasing, the air velocity becomes higher and will increase the cooling rate of the fibers. Due to this increased cooling rate, the effect of increasing air pressure reaches a point where the fiber will solidify and a further reduction in diameter is no longer possible. Second, as the fiber diameter becomes smaller and smaller, especially in the nanometer range, the polymer surface tension becomes dominant and can inhibit the fiber diameter attenuation.
Fibers commonly become entangled during processing into cohesive, tight fiber groups called bundles. Figure 4 shows that the bundles may be composed of a single fiber (entanglement of one) or a very large number of fibers. It seems that the Hills die generates more bundles. This is because the Hill’s die hole density is higher than that of the AGR die. When the number of fibers within the same area increases, the fiber entanglement increases leading to a higher number of fiber bundles. The effect of air pressure on fiber diameter and web structure is evident from SEM photographs shown in Figure 4. Some fiber breakup is observed in Hills_0.055MPa image, which is probably caused the Rayleigh-plateau instability (RPI) [16]. The underlying principle of RPI is based on a simple energy balance [24]. When fiber diameter reaches nanometer scale, the fiber has the area of a geometrical cylinder, which is obviously not at a minimum and can tend to become a sphere causing fiber break up.
Figure 4: SEM images of melt blown fibers.
Web structure
From SEM photographs shown in Figure 4, it is apparent that the melt-blown webs are comprised of randomly oriented fibers with diameters ranging from microns to sub-microns. The melt-blown webs with a high degree of fiber entanglement or a small average fiber diameter are expected to have smaller pore size with higher total porosity. The porous structure of the web plays an important role in improving performance in many applications, including filtration, wound dressing, membrane separation and scaffolds for tissue engineering [23-25]. Since important performance of many meltblown applications is closely associated with the porous structure of the webs, the porosity of the webs has been studied using the capillary flow porometer.
As shown in Figure 5, the web mean flow pore diameter is determined by the SFP-1100-AEX capillary flow porometry at the mean flow pressure, the point at which the wet curve and half dry curves intersect [26]. The differential pressure and gas flow rates through dry and wet samples are measured. In the dry sample, the flow rate increases with pressure increasing. In the case of wet sample, initially there is no flow because all the pores are filled with the liquid. At a certain pressure the gas empties the largest pore and gas flow starts through the wet sample. With further increase in pressure, smaller pores are emptied and the flow rate increases until all the pores are empty and the flow rate through the wet samples is the same as that through the dry samples. The half-dry curve in the Figure 5 is calculated from dry curve to yield fifty-percent of flow through dry sample at the same pressure. The dry and wet curves yield the bubble point, the mean flow pore diameter and pore size distribution of the tested samples [27].
Figure 5: Characterization of pore size and distribution measured by capillary flow porometry.
The porometry data for the melt blown webs are shown in Table 1, including thickness, pore size range, mean flow pore diameter and air permeability. The process air pressure influences the pore size distribution and the mean pore diameter. The mean pore diameter decreases as the air pressure increases. The data confirms that decreasing fiber diameter slightly decreases the average pore size. For example, when air pressure was increased from 0.034MPa to 0.103MPa, the average fiber diameter decreased by16.9% (from 0.77 um to 0.64 um) and 41.9% (from 1.05 um to 0.61 um) for the Hills die and the AGR die, respectively. Meanwhile, the mean pore diameter size decreased by 31.6% (from 14.8 um to 10.1 um) and 24.2% (from 13.4 um to 10.2 um) respectively for the two dies. In this study, Hill die had smaller orifice diameter and high L/D ratio that can generate smaller fiber diameter. But the pore size of the webs from the Hills die was similar to that from the AGR die in most of the cases.
Table 1: Porometry results of the melt blown webs.
Also, the Hill’s die had 24 holes per cm while AGR die had 14 holes per cm. The initial number of fibers from the die being different under the same melt blowing processing conditions the web structure is expected to be different. For the Hill’s die, the higher orifice density likely increases the number of bundles. Therefore, combination of finer fibers and coarser bundles can affect the pore size and pore size distribution.
Meanwhile, increasing the air pressure also influences the fiber entanglement, which can affect the web thickness. The observations of pore data have confirmed that for the webs produced from Hill’s die pore diameter range is wider than that of the AGR’s, and the thickness of the webs produced from the AGR die is higher than those from the Hill’s die. Because of the same mean pore diameter, the higher thickness means that the web is loftier and has more pore volume. According to the air permeability test, it shows that the value of air permeability of the webs from the AGR die is much higher than that from the Hill’s die, which is expected from a web of higher porosity, irrespective of pore size.

Conclusions

Submicron fibers have been produced by two different multihole dies under different processing conditions using a melt blown pilot line. The fiber diameter and distribution have been studied and the results demonstrate that the air pressure influences the fiber diameter significantly. The fiber diameter distribution conforms to the normal distribution. The nanofiber webs produced using different dies have similar mean pore size but different pore size distribution and air permeability. This is due to the difference in fiber movement and entanglement because of different number of holes in the dies. The air permeability of fabrics is influenced by pore size, pore size distribution and the thickness. This study has shown how the die geometry and some of the processing conditions affect the structure and properties of the nanofiber melt blown webs.

Acknowledgment

The authors are grateful to the financial support provided by the special excellent PhD international visit program by the Donghua University (No.102552011005). Also assistance from Stephen Sheriff, Josh Fogle and Sammy Akato in sample preparation, and support from UTNRL is acknowledged.

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