Melt Blown Nanofibers: Fiber Diameter Distributions and Onset of Fiber

Polymer 48 (2007) 3306e3316 www.elsevier.com/locate/polymer

Melt blown nanoﬁbers: Fiber diameter distributions and onset of ﬁber breakup

Christopher J. Ellison1, Alhad Phatak1, David W. Giles, Christopher W. Macosko, Frank S. Bates*

Department of Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue SE, Minneapolis, MN 55455, USA Received 4 December 2006; received in revised form 2 April 2007; accepted 3 April 2007 Available online 10 April 2007

Abstract

Poly(butylene terephthalate), polypropylene, and polystyrene nanofibers with average diameters less than 500 nm have been produced by a single orifice melt blowing apparatus using commercially viable processing conditions. This result is a major step towards closing the gap between melt blowing technology and electrospinning in terms of the ability to produce nano-scale fibers. Furthermore, analysis of fiber diameter distributions reveals they are well described by a log-normal distribution function regardless of average fiber diameter, indicating that the underlying fiber attenuation mechanisms are retained even when producing nanofibers. However, a comparison of the breadth of the distributions between mats with differing average fiber diameters indicates that the dependence of the breadth with average fiber diameter is not universal (i.e., it is material dependent). Finally, under certain processing conditions, we observe fiber breakup that we believe is driven by surface tension and these instabilities may represent the onset of an underlying fundamental limit to the process. Published by Elsevier Ltd.

Keywords: Nanoﬁbers; Melt blowing; Electrospinning

1. Introduction functionalization. In addition, the ultimate mat properties such as overall mat strength (related to individual fiber A ‘‘nonwoven’’ refers to a sheet or mat of fibers connected strength, average fiber length, and fiber entanglement density) together by physical entanglements, or contact adhesion be- and porosity play an equally important role in end-use appli- tween individual fibers, without any knitting or stitching. cations. Nonwoven fibers find use in a range of applications The nonwovens industry was worth $14 billion in 2004 [1] such as filtration, membrane separation, protective military and is expected to grow further due to new applications requir- clothing, biosensors, wound dressings, and scaffolds for tissue ing fibers with increasingly smaller sizes. Since the surface engineering [2e5]. area of a fiber scales linearly with the diameter and the volume Electrospinning, melt spinning, and melt blowing are the (and mass) scales as the square of the diameter, the specific most commonly used processes for nonwovens production. surface area varies inversely with diameter (w1/d ), leading Electrospinning involves applying a strong electric potential to high specific surface areas for small fibers. As an example, (w10 kV) to a polymer solution contained in a syringe to a gram of a mat containing 100 nm polymer fibers (of density force a jet of the solution onto a grounded screen located 1 g/cm3) has about 10 m2 of surface area that can be made a few centimeters away. Rapid evaporation of the solvent re- available for a wide variety of processes via surface sults in a mat of fine (10 nme1 mm) polymer fibers that are de- posited on the screen. In contrast, melt spinning is performed * Corresponding author. Tel.: þ1 612 624 0839; fax: þ1 612 626 1686. by extruding a polymer melt and drawing it down with a take- E-mail address: [email protected] (F.S. Bates). up wheel. Since the polymer solidifies during the drawing pro- 1 The authors contributed equally to this work. cess, this yields highly oriented chains resulting in strong

0032-3861/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.polymer.2007.04.005 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316 3307

fibers that can be produced at fast rates; however, the fibers are frequently whip across the air stream (a similar motion is ob- usually not smaller than 10 mm [2]. served in electrospinning, but is due to a fundamentally differ- During melt blowing, fibers are produced in a single step by ent mechanism [11,12]) after it exits the die and proceeds extruding a polymer melt through an orifice die and drawing towards the collection device [10,13]. This fiber motion pro- down the extrudate with a jet of hot air (typically at the duces a transient drag force from air flowing normal to and same temperature as the molten polymer). This environmen- down the axis of the fiber [14]. An additional level of exten- tally benign processing method was first developed in the 1950s sional force may be introduced when multiorifice dies are at the Naval Research Laboratory with the goal of making employed allowing adjacent fibers to participate in fibere sub-micron fibers to trap radioactive particles in the upper at- fiber interactions. mosphere [6]. Wente first described the construction of a melt Researchers have used high speed imaging techniques to blowing die composed of a series of orifices and slots [7]. perform inline investigations of fiber formation during melt Researchers at Exxon extended this basic design and first dem- blowing. Fiber diameter measurements revealed that most of onstrated the production of melt blown microfibers on a com- the fiber attenuation occurred within several centimeters of mercial scale by modifying sheet die technology [6,8]. Since the die exit and that the temperature of the fiber dropped to then, a number of companies such as Vose, 3M, Kimberlye near-ambient values over the same distance due to entrainment Clark, Cummins, and Johns Manville have used the technol- of ambient air into the hot air stream [15,16]. Additionally, ogy to produce commercial nonwoven products [6]. In general, laser Doppler velocimetry measurements showed that average these commercial products are composed of fibers with aver- fiber and air velocities approached the same value within age diameters exceeding 1e2 mm. several centimeters of the die exit [17,18]. This has also Melt blowing equipment is designed such that the air is been corroborated by modeling studies [15,19]. It is intuitive supplied in the form of two streams that form a v-slot (see that the attenuation during melt blowing occurs only between Fig. 1); other designs such as annular air jets have been the processing (Tp) and solidification (glass transition, Tg,or used only on a laboratory scale [9]. Commercially the v-slot crystallization, Tc) temperatures. This suggests a possible ap- design is used in the form of a long channel which encom- proach to achieve greater attenuation by holding the fibers in passes a single row of hundreds or thousands of orifices the active temperature window [Tg (or Tc) < T < Tp] for longer from which the fibers originate. The drag force exerted by periods of time (where T is the fiber temperature). Haynes and the air attenuates the melt extrudate into fibers, which are col- coworkers have suggested a design to implement this idea by lected a few feet away from the die producing a self-bonded combining melt blowing with entrainment of external heated mat [10]. The nature of the drag force is more complicated air; however, the diameters of the fibers obtained by this than the air stream simply acting in the axial direction along method were not dramatically smaller than those produced a taut fiber. The fiber is highly dynamic and is observed to without hot air entrainment [20]. Fig. 1 shows a schematic of the single orifice melt blowing die used in this study which is based on the design of a typical commercial melt blowing die. However, commercial melt blowing lines employ a multiorifice design composed of a linear bank of holes more than a meter in length with hole diameters w0.2e0.6 mm spaced at 10e20 holes/cm [6,7,21].In general, there are four basic processing parameters that can be varied e polymer and air temperatures (Tp and Ta)andmass flow rates (mp and ma). In principle, each process parameter can individually affect the average diameter and length of the fibers which are produced. Shambaugh [22] studied the existing industrial data from melt blowing processes and attemp- ted to provide a universal description for the variation of the fiber diameter in terms of a number of dimensionless groups. It was shown that the air-to-polymer mass flow rate mostly affected the resulting fiber size, but this parameter could not capture all the existing data from studies involving different materials, orifice diameters, processing conditions and die geometries. Milligan and Haynes employed a single-hole die to study the melt blowing of a series of polypropylenes and con- cluded that the ratio of air to polymer mass fluxes (G) provided a satisfactory description of the fiber size for a wide range of processing conditions [23,24].(G incorporates the cross-sectional areas of the die geometry that flow rates Fig. 1. Detailed schematic of the melt blowing die: (a) sectional and (b) end-on do not.) They further developed an empirical model for the views of the two pieces. dimensionless average fiber diameter in terms of relevant 3308 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316 dimensionless process variables, but noted that some of the processing conditions and the die orifice diameter are not model parameters are dependent on the polymer type (due to indicated. differing viscoelastic characteristics) and the particular melt Currently, electrospinning is the most popular process to blowing line employed (due to geometry, etc.). make fibers which have diameters in the 100 nm range A wide variety of polymers including polyethylene [7], [33,34], and as a result this process has begun to forge a niche polypropylene (PP) [15,23,25], poly(methyl methacrylate) in commercial production of ‘‘nanofibers’’ [35]. However, [7], poly(ethylene terephthalate) [25], poly(butylene tere- electrospinning is inherently slow due to the common require- phthalate) (PBT) [26], polyamides (e.g. nylon) [7,27], and ment of removing residual solvent from the nonwoven and is polystyrene (PS) [7] have been used for producing blown undesirable due to difficult solvent handling/recovery, slow fibers. This represents a set of both amorphous and semicrys- fiber production rates (on a per orifice basis because of the talline materials having a wide range of physical properties use of a polymer solution in contrast to melt blowing a polymer [28]. Thus, the compatibility of a polymer with this process melt), and the high voltages that are required [5,36]. While seems to be independent of the backbone chemical structure. multiorifice commercial electrospinning production lines alle- Naturally, the final properties of the melt blown fiber web viate some aspects of the slow nature of the process, the above (softness, toughness, solvent resistance, etc.) will depend on factors nevertheless lead to significantly higher costs com- the chemical nature of the polymer backbone; this is a separate pared to melt blowing. Furthermore, many polymers that issue. As long as the melt viscosity of the polymer is low have desirable physical and mechanical properties (e.g. poly- enough to facilitate significant attenuation of the extrudate, olefins, PBT, etc.) are hard to dissolve in common solvents it apparently can be melt blown. Even though the viscosity at room temperature, making them difficult to electrospin. can be reduced by increasing the processing temperature, the Even though electrospinning with polymer melts has been per- upper limit is prescribed by the degradation temperature of formed, the fibers obtained are relatively large (a few microm- the polymer. Of course, exposure of the polymer to elevated eters in diameter), presumably due to the high viscosity of temperatures may also be reduced by staging the temperature polymer melts [37e40]. profile of the melt processing equipment which feeds the melt If melt blowing technology could be extended to sub- blowing device. micron fiber sizes, it would provide a much easier, faster, Given that the size of a fiber primarily determines the prop- and cheaper alternative to electrospinning. Hence, a major fo- erties of the final nonwoven, it is surprising that size distribu- cus of melt blowing research should arguably be to extend the tions of melt blown fibers have rarely been documented technology to nanofibers due to the potential to penetrate new [22,24,27,29]. The nature of the fiber size distributions can markets and enhance current product offerings. In addition, provide valuable clues about the physics of the process. The a fundamental understanding of the limits of the melt blowing literature is also unclear about the smallest fibers that can be process is needed to determine the smallest fiber size that can produced by melt blowing. Most reviews say that even though be theoretically obtained. Modeling/simulation could provide isolated fibers w100 nm in diameter can be obtained by this some clues in this regard. However, all the simulations to process, average fiber diameters of 1e2 mm are produced date have been restricted to fiber diameters of a few microm- commercially [2,30,31]. However, there is a seemingly eters and have largely ignored surface tension effects which ‘‘under-cited’’ report of sub-micron fibers in a study by Wente; will likely become increasingly important as fiber size is in this work, fibers with average diameters as small as 500 nm reduced below 1 mm [15,19,41,42]. Existing studies correlate were made from a number of polymers, but fiber size distribu- fiber diameter with various processing and geometrical tions were not reported and only one image of the fibers was parameters [43,44], but these studies do not address the possi- displayed [7]. Unfortunately, air and polymer flow rates ble limits of this technology or seek to explore the potential were also not reported for any of the cases studied [7]. Others of extending it to applications that transcend the traditional have claimed to produce fibers with an average diameter of 1e2 mm fibers that have been produced for decades. w300 nm using a new die design composed of stacked plates In this paper, the fundamental lower limit of the average fi- which results in a row of orifices as small as 0.0125 mm in ber diameter in the melt blowing process is explored for two diameter, but present no data or images from such fibers commercial melt blowing materials (PBT and PP, semicrystal- [21]. While the approach of reducing the orifice size to reduce line) and for a low molecular weight (MW) PS sample (amor- overall fiber size is intuitive, such a small orifice is a challenge phous). The low MW PS sample is particularly interesting on an industrial melt blowing line due to the higher pressures because it has the lowest melt viscosity in this study. In addi- required to extrude the polymer (<8000 kPa is typically tion, this material has been selected as an avenue to begin to required to keep current commercial dies from fracturing explore the absolute limit of melt blowing capabilities of along the array of orifices or ‘‘unzippering’’ [21]) and the low melt viscosity materials. Importantly, solidification of higher probability of clogging the orifices with foreign mate- the molten fiber is possible due to a Tg which lies just above rial inherently present in production environments and poly- room temperature thus producing PS nonwoven fibers during mer feedstocks. During the writing of this manuscript, melt blowing. While the low MW bimodal PS may not have a third report [32] has emerged indicating that polypropylene excellent mechanical/thermal/chemical stability properties (unspecified viscosity, molecular weight, etc.) with sub-micron required for end-use nonwoven applications, it does provide average fiber diameters may be produced via melt blowing, but valuable information and raise some important questions about C.J. Ellison et al. / Polymer 48 (2007) 3306e3316 3309 what parameters may govern this process and ultimately what limitations may exist. For all three materials presented in this paper, the limiting diameter (at the highest possible air-to-polymer mass fluxes af- forded by our device) of each polymer was of primary interest. Fiber size distributions are characterized for fibers with average diameters in excess of 1 mm and compared to those which are several hundred nanometers. Equally important is the fact that these studies employ an experimental setup with die geometries and orifice diameters which are in the range of current commercial melt blowing processing equipment.

2. Experimental

2.1. Materials and characterization

PS (Sigma Aldrich; atactic bimodal PS, peaks at w1 and 90 kg/mol), PP (Exxon; isotactic PP 3746G, 1500 melt flow Fig. 2. Variation of complex viscosity with dynamic frequency from oscilla- rate [MFR] at 230 C) and PBT (Ticona; Celanex 2008, 250 tory shear measurements conducted at the temperatures used for melt blowing. MFR at 250 C) were used in this study (see Table 1 for physical property details). Molecular weight data were not attain- 2.2. Melt blowing able for PBT as it is insoluble in common solvents used in size exclusion chromatography such as chloroform, tetrahy- A Goetffert Rheo-Tester 1500 capillary rheometer was used to drofuran, toluene and trichlorobenzene. PS was dried under extrude polymer samples through melt blowing dies, which were vacuum at 140 C for a minimum of 12 h before use to remove composed of two parts (Fig. 1). Since the melt blowing die ex- residual solvent and monomer while PBT was dried at 120 C tended outside the barrel of the capillary rheometer, external heat- for 4 h in ambient air to remove absorbed water. PP was used ing was provided by band heaters (Watlow part #B3N1AP1) as received. Relevant molecular and physical parameters of wrapped around aluminum blocks that fit tightly around the die these polymers are shown in Table 1 (and Fig. 2) and were pieces shown in Fig. 1. A single-hole die with an orifice diameter characterized by a combination of size exclusion chromatogra- (do) of 0.2 mm, an air-gap distance (dg) of 1 mm, and a setback phy using refractive index and light scattering detection (Poly- distance (ds)of1.5mm(seeFig. 1) was employed. Each of the mer Laboratories PL-GPC 220, Wyatt Optilab, Wyatt Dawn), v-slot channels were 10 mm by 1 mm and they both fed into where absolute molecular weights were determined with the final air slot which was 10 mm long (indicated by ‘‘w’’ in appropriate standards, differential scanning calorimetry (TA Fig. 1b) by 1 mm. Polymer was fed at temperature Tp and mass Instruments Q1000), and dynamic mechanical spectroscopy flow rate mp between 0.035 and 0.35 g/min. The die orifice was (Rheometrics ARES, now TA Instruments) measurements. stepped down from 3 mm to the final orifice diameter as shown The glass transition (Tg) and melting transition (Tm) tempera- in Fig. 1. For the final section of the capillary, the capillary length tures were measured upon second heat (as onset for Tg and to diameter ratio was z10. Air flow rates (f ) between 4.5 and a endset for Tm) at a heating rate of 10 C/min following a tem- 10 standard cubic feet per minute (SCFM) were employed and perature quench at 10 C/min from well above Tg and Tm. G was calculated by a ratio of the air mass flux in the v-slots Crystallization temperatures (Tc) were measured as the onset and the polymer mass flux through the orifice. Heated air was temperature with a cooling rate of 10 C/min starting from supplied to the die at a temperature Ta (temperature at die exit), temperatures well above Tm. Melt viscosities of the polymers with a 6 kW Osram Sylvania threaded inline heater used in con- were measured by isothermal dynamic frequency sweep ex- junction with an open loop (manual) power control system. periments with 25 mm parallel plates. Measurements were Tp ¼ Ta for all conditions in this study. conducted between 0.1 and 100 rad/s in the linear viscoelastic Each melt blowing experiment was conducted using a few regions of the polymers, which were determined by dynamic grams of polymer and required approximately 2 h for instru- strain sweep experiments. ment assembly, heat up, melt blowing and cool down, making it ideal for materials testing as described in this paper. During Table 1 melt blowing, polymer was exposed to the processing temper- Physical properties of the polymers used for melt blowing ature for a maximum of 30 min in order to limit degradation. Polymer Mn (kg/mol) Mw (kg/mol) Tm ( C) Tc ( C) Tg ( C) For the highest processing temperature listed in Table 2 (where PS 2.1 48.8 ee61 the potential for degradation is highest), the Mn of PS remains PP 10.6 59.0 170 118 2 unchanged within error while Mw is reduced by 31% following PBT ea ea 240 190 35 processing. Similarly, the Mn of PP is reduced by 38% while a This polymer has a MFR of 250 at 250 C. the Mw of PP is reduced by 51% following processing. It is 3310 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316

Table 2 using a VCR high resolution ion beam sputtering system. Plat- Summary of the melt blowing experiments inum coated samples were imaged by a Hitachi S-4700 cold 1 Run I.D. Tp, Ta h*at1s mp fa G dav field emission gun SEM in the secondary electron mode, (C) (Pa s) (g/min) (SCFM) (mm) with accelerating voltages between 1.5 and 2.5 kV, emission PS-1 180 23 0.053 8 9 1.61 currents between 10 and 15 mA, working distances of PS-2 260 1.6 0.07 7.5 6.4 0.62 z12 mm and magnifications in the range of 300e10,000. PS-3 280 1.1 0.07 8 6.8 0.38 At least 250 fiber measurements from more than 20 SEM im- PP-1 180 35 0.35 6 0.5 1.23 ages were used in order to ensure reproducible statistics when PP-2 180 35 0.035 8 13.6 0.45 measuring fiber size distributions. Fiber diameters were mea- PP-3 220 15 0.035 8 13.6 0.30 sured by image processing software (ImageJ, NIST). Fiber PBT-1 265 137 0.35 4.5 0.4 1.22 diameters were measured from SEM images by drawing PBT-2 265 137 0.035 10 17 0.44 straight lines along the diagonals of an image (to ensure fibers were not counted twice) and measuring fibers that crossed the important to note that this melt blowing grade PP contains lines. To generate reliable statistics, fibers were imaged from a peroxide additive (added by Exxon) to control the polymer different parts of a 1 cm 1 cm sample area. In addition, rheology during processing and this contributes to the decrease the fibers lying outside the focal plane were not used in the in molecular weight. An analysis of PBT was not possible due statistical analysis. to its insolubility in available solvents used for SEC. The fiber diameter distributions were fit to log-normal func- Melt blown fibers were collected on a stainless steel screen tions using Origin data processing software (OriginLab Corp.). located 55 cm away from the die exit, which is in the range of The histograms were constructed using bin sizes that produced typical collection distances used in commercial melt blowing approximately 50 bins across the entire distribution. We found lines. At the collection location, the air jet possesses a substan- that this led to a good compromise between larger bin sizes tially lower average velocity, widened overall diameter (w5e which result in smoothing of the shape of the distribution 6 in at the collection point), and has cooled to temperatures and smaller bin sizes which do not capture the nature of the well below 50 C due to entrainment of ambient air. The col- distribution as the number of bins approach the sample size. lection distance was chosen such that the fibers had solidified Consequently, the choice of bin size can impact the R2 value by cooling below T or T . This ensures the production of non- g c that is reported in Table 3, which is representative of the good- wovens lacking fused fibers at contact points as sufficient ness of fit of the log-normal distribution function to the cooling has taken place to solidify the fibers. In addition, the experimental data. collection screen was placed on top of a duct 6 inches in diameter which was attached to the suction side of a small blower (Peerless Blowers model #d6AB). The blower produced an 3. Results and discussion average face velocity on the screen surface of approximately 24 ft/s which was greater than the average velocity of the 3.1. Average fiber diameter widened air jet at the collection location under all conditions employed. This collection scheme is provided for efficient col- Table 2 shows the average fiber diameters that have been lection of fibers produced by the die. The desired structure of produced for PS, PP, and PBT under specified melt blowing the nonwoven was verified by scanning electron microscopy conditions. The resultant fibers are consistent with the expec- (SEM) and the general feel of the fiber mat. tation of commercial materials (e.g. 1e2 mm) or substantially smaller (e.g. 0.3e0.6 mm) depending on the processing condi- 2.3. Fiber diameter characterization tions. These data show that it is possible to significantly decrease the average fiber diameter below 1 mm by modulating Samples (1 cm 1 cm) were sectioned from melt blown several different processing parameters. For example, at con- fiber mats and coated with a z2.5 nm thick platinum coating stant temperature, the average fiber diameter of PBT-1 is

Table 3 Average ﬁber diameters and log-normal distribution parameters 2 Run I.D. dav (mm) msMed[log(d )] Gaussian ﬁt to log (d ) R

xc d PS-1 1.61 0.20 0.07 0.20 0.20 0.07 0.98 PS-2 0.62 0.29 0.28 0.24 0.23 0.29 0.68 PS-3 0.38 0.48 0.24 0.47 0.47 0.28 0.69 PP-1 1.23 0.039 0.33 0.033 0.06 0.37 0.66 PP-2 0.45 0.42 0.25 0.41 0.41 0.25 0.85 PP-3 0.30 0.57 0.197 0.58 0.59 0.20 0.68 PBT-1 1.22 0.001 0.26 0.001 0.008 0.21 0.86 PBT-2 0.44 0.43 0.21 0.46 0.48 0.17 0.73 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316 3311 reduced significantly from 1.22 to 0.44 mm for PBT-2 by in- process. We estimate that the extensional strain rates are quite creasing the air to polymer mass flux ratio (G). The same trend high (w107 s1). Traditionally, extensional strain hardening is observed for PP in which fiber diameter reduces from has been attributed to chain entanglements or long chain 1.23 mm for PP-1 to 0.45 mm for PP-2. We have also observed branching [47e49]. (The molecular weight between entangle- [45] tunability of the fiber diameter in other experiments when ments (Me) for PS and PP are 13,000 and 5000 g/mol [50], re- changing air flow rate or polymer mass flow rate individually spectively, and the polymer MW must be at least 2e3 Me for consistent with other researchers [24,22]. Other process vari- entanglements to begin to make contributions to the flow char- ables being equal, fiber diameter decreases with higher air acteristics of polymers.) It is unclear if the PS and PP used in flow rate due to an increase in the drag on the fiber (and poten- this study exhibit significant extensional strain hardening at tial enhancement of the fiber whipping dynamics). Similarly, these strain rates. In addition, the bimodal MW distribution lowering polymer mass flow rate decreases fiber diameter of PS is a further complicating issue. A systematic study on because the same drag force from the air jet is acting on a model system which relates the viscoelastic material proper- less polymer mass. It is also noteworthy that the air to polymer ties to melt blowing behavior is required to further understand mass flux ratios for the smallest fibers in Table 2 are higher these issues. than any other values reported in the literature, yet the polymer It is surprising that the highest and lowest viscosities in flow rate and air pressure required (less than 70 psig) should Fig. 2 differ by more than two orders of magnitude, yet these be accessible by commercial melt blowing lines. materials can be easily melt blown into continuous defect free Similarly, an increase in Tp (Tp ¼ Ta at all conditions in this fibers. This is a testament to the high degree of process latitude study) from PS-1 to PS-2 results in a reduction of the average that melt blowing affords. Fig. 3 shows SEM micrographs of fiber diameter from 1.61 to 0.62 mm. The effect of the increase the typical fiber mats (largest and smallest average diameter in Tp clearly dominates the expectation of a slightly larger samples for PBT, PP and PS shown in Table 2) that are col- fiber size from a slightly increased polymer flow rate or lected and analyzed for average fiber diameter, as well as qual- slightly reduced air flow rate in PS-2 compared to PS-1. PS-2 itative features. The left column of images shows fibers with and PS-3 or PP-2 and PP-3 also show a significant reduction in average diameters in excess of 1 mm. When these samples fiber size due to an increase in Tp. This effect of processing are viewed at lower magnifications (150e350), a majority temperature is a result of two factors. First, there is an increase of the fibers may be identified to have contour lengths signif- in the active temperature window over which the fiber attenu- icantly exceeding 1 mm (as far as they can be traced in the ation may occur when Tp (and Ta) is increased. The attenuation field of view of several SEM images) and fiber ends are rarely process (attenuation requires the polymer be molten) continu- identified even upon analysis of more than 20 images. Based ously competes with cooling of the heated air jet due to on these facts, it is intuitive to expect that the average fiber entrainment of ambient air. Thus, higher processing tempera- is at least several orders of magnitude longer than 1 mm and tures allow for the polymer fiber to remain in the melt state this is consistent with other studies [13,10] employing high for longer periods of time and undergo additional attenuation speed photography which show continuous fibers (diameters before the polymer solidifies via sufficient crystallization or greater than 1 mm) which are at least several tens of centime- vitrification. ters in length. In addition, during melt blowing of the fiber Second, there is a substantial decrease in viscosity as the mats, short loose fibers escaping the mat (commonly referred temperature is increased. Fig. 2 shows the complex shear vis- to as ‘‘fly’’) are not observed. This is an indication that our cosity as a function of frequency (equivalent to the steady laboratory scale melt blowing device is producing long, defect shear viscosity as a function of steady shear rate by the free fibers of the size and quality that would be expected in CoxeMerz rule [46]) for PS, PP and PBT at the processing a commercial product. temperatures shown in Table 2. Note that at these temperatures The second column of Fig. 3 displays the fibers that have the viscosities exhibit little or no frequency dependence or been produced with average fiber diameters well below shear thinning. The viscosity of PS decreases by more than 1 mm. In all cases the fibers are observed to be essentially an order of magnitude when increasing Tp by 100 C and by defect free with contour lengths exceeding several hundred a factor of w3 upon increasing Tp by 40 C for PP. The poly- microns in length. This is also the case for individual fibers mer flow rates used in Table 2 correspond to apparent shear less than 0.1 mm. However, closer examination of Fig. 3b rates between w100 and 1000 s1 in the die capillary. Hence, and d reveals that there are a few isolated particles resembling the results shown in Fig. 2 represent the highest melt viscosity spheres intermingled amongst the fibers. These may be the for each polymer at the extrusion temperature. result of the onset of surface tension driven fiber breakup While these shear viscosity data give a general picture of and this will be discussed further in Section 3.3. We want to the rheology of the material at the processing temperature, emphasize that these particles are different from so called the more relevant viscoelastic and process parameters are the ‘‘shot’’ formation in melt blowing. ‘‘Shot’’ refers to larger extensional viscosity (and its temperature dependence), the particles of polymer (>several tens of microns in size) in extensional strain rate and the degree of extensional strain the fiber mat that have ill-defined shapes [10]. hardening. Extensional viscosity is not trivial to access di- While the fibers highlighted in Table 2 and Fig. 3 have been rectly in polymer melts at elevated temperatures and typical produced using single-hole melt blowing dies, we have suc- extensional strain rates present during the melt blowing cessfully melt blown fibers of similar materials with average 3312 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316

Fig. 3. Representative SEM images from (a) PS-1, (b) PS-3, (c) PP-1, (d) PP-3, (e) PBT-1, and (f) PBT-2 melt blowing runs. All black scale bars represent 2 mm.

diameters of w0.2e0.6 mm with multiorifice dies mimicking assumption that melt strength/elasticity, achieved via the those used in industry. Most importantly these results indicate presence of a substantial level of entanglements, is required that there is no fundamental restriction in attenuating a molten to prevent significant fiber breakup and avoid what might be polymer fiber down to several hundred nanometers in diameter called ‘‘melt spraying’’. via melt blowing; this reduces the gap between melt blowing In contrast, for electrospinning, entanglements often do and electrospinning technology in terms of the ability to pro- play a key role. It has been shown that chain entanglements duce nano-scale fibers. in the polymer solution are often required (thus solution The fact that lightly entangled polymer melts (PS and PP) concentrations must be above the chain overlap concentration with relatively low viscosities (especially in the case of PS) or c*) to produce defect free continuous fibers via electrospin- compared to most melt processed polymers can be melt blown ning [51e53]. This is not always the case if sufficient elastic- into w0.4 mm fibers (Table 2 and Fig. 3b) was unexpected and ity is built into the polymer solution by other means such as demonstrates how much fundamental knowledge is yet to be adding a third component that may interact with the primary learned about this process. It also suggests that entanglements polymer [54]. A PS sample such as that used in this study may not be a key requirement in producing defect free fibers would more than likely present a challenge for electrospinning several hundred nanometers in diameter. Furthermore, it may without such an additive. The present study suggests that sig- be possible that many materials suitable for melt blowing nificant elasticity of the melt may not be a primary require- have been overlooked due to the basic (and rational) ment for producing nanofibers by melt blowing. C.J. Ellison et al. / Polymer 48 (2007) 3306e3316 3313

3.2. Fiber diameter distributions curve with the arithmetic mean (m) of log(d ) being nearly equal to the median [m ¼0.42; med(log(d )) ¼0.41], and Fig. 4a shows a representative size distribution from a melt a standard deviation (s) of 0.25. These are characteristic fea- blown fiber mat; some of the obvious features are that it is tures of the log-normal distribution (symmetric when plotted asymmetric (skewed towards the left) and has a tail at the large versus log(d ) and med(log(d )) zm(log(d )) [55]. diameter end. An asymmetric distribution implies that as one In Fig. 4b, a normal (Gaussian) distribution profile with moves away from the peak of the distribution, the probability a mean (xc)of0.41 and a standard deviation (d) of 0.25 of observing the independent variable does not reduce linearly. (solid line in Fig. 4b) shows a good fit (with a reasonable R2 From a statistical perspective, the fiber diameter is an indepen- value) to the log(d ) data where the probability density func- dent variable and the probability of observing a particular fiber tion for a normal distribution is given by, size is distributed asymmetrically about the most probable " # fiber diameter (dp) (i.e., p(dp þ 3) s p(dp 3), where 3 is an 2 1 ðx xcÞ arbitrary number). Fig. 4b shows the distribution of Fig. 4a pðxÞ¼ pffiffiffiffiffiffi exp : ð1Þ d p 2d2 plotted versus the logarithm of the fiber diameter (log(d )) as 2 the independent variable. This is a reasonably symmetric Thus, the fiber diameter distribution for the melt blowing run PP-2 closely resembles a log-normal distribution. This analysis was repeated for all other melt blowing runs from Table 2 and the results are summarized in Table 3 (approximately 50 bins were used for the log(d ) histogram of each sample). In all cases described in this paper, the mean (m) and median of log(d ) are nearly equal, and a good fit is obtained for a Gaussian distribution with fitting parameters xc and d nearly equal to m and s. Thus, all the melt blown fiber mats can be said to follow log-normal distributions and, in fact, we have observed the melt blowing process to produce fibers which exhibit the features of the log-normal distribution in all cases we have investigated to date regardless of average fiber diameter. This is consistent with the only other study to measure fiber diameter distributions, although the reported average fiber diameters are in excess of 7 mm [27]. Others [32] have characterized the distribution for only a few samples, but have not provided any indication of the type of distribution. In the present study, the fact that the same type of fiber size distribution is being observed regardless of average diameter suggests that the basic fiber formation mechanism does not change substantially when producing nanofibers compared to those much larger in diameter. Fig. 5 compares the Gaussian fits to the fiber size distributions for the largest and smallest fibers from PS, PBT and PP (same samples as those shown in Fig. 3). It appears that there is no apparent dependence of the width of the distribution on average fiber diameter (width of PS increases, PBT is about the same and PP decreases in going to smaller fiber diameters). Surprisingly, the PS-1 sample distribution is the narrowest distribution we have been able to produce to date and it has been reproduced several times. It is unclear at this time what factors may determine the width of the distribution and more research is required in this area. In general, there is interest in controlling both the average fiber size and the width of the fiber size distribution as they control both the average pore size and the pore size distribution, which are important properties of nonwovens (e.g. in filtration applications). Fundamentally, it makes sense that this distribution is a manifestation of the Fig. 4. Fiber diameter distributions from the PP-2 melt blowing run on (a) dynamic aspects of the fiber as it whips across the air stream. linear and (b) logarithmic scales. The dotted line shows the best fit normal This whipping action produces a dynamic drag/extensional distribution to log(d ). force which in turn produces a variable fiber size and defines 3314 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316

the shape of the distribution. It is conceivable that the dynamics of this process will be impacted by the average ﬁber diameter, which in turn alters the nature of the drag force and the overall momentum of the ﬁber. More research is required to establish a direct relationship between these factors.

3.3. Onset of ﬁber breakup

SEM images taken from PP and PS fiber mats (shown in Fig. 3b and d) obtained at relatively high airepolymer flux ratios and processing temperatures show the presence of spherical particles dispersed amongst fibers. In Table 4 we characterize the extent of sphere formation by the ratio of the number of spheres to fibers (ns/nf) from a set of more than 200 fiber measurements. (Note that the count of fibers is based on the number of fiber segments observed in the images since the total length of fibers is not known and is difficult to determine quantitatively. Thus, ns/nf does not strictly represent a volumetric density of spherical particles. In addition, the determination of the relative level of spherical particles is only qualitative as it is possible that some particles are able to escape the mats during collection even though care is taken to produce a thick mat under all circumstances.) We believe that the origin of the spheres is a result of fiber breakup instabilities that are driven by surface tension. For a Newtonian liquid, these surface tension driven instabilities are termed Rayleigh instabilities [56,57]. A Rayleigh instability occurs when surface tension forces cause the development of necking of the fiber at locations of a characteristic frequency along the fiber. These necked regions grow and eventually pinch off the fiber, resulting in droplets of a characteristic size. However, for viscoelastic fluids such as those described in this paper, it is known [58e61] that the build up of extensional stress due to drawing of the fiber during processing can delay or even retard surface tension driven fiber breakup. Therefore extensional stress likely plays an important role in this phenomenon. We did not observe fiber breakup in any melt blowing experiments involving PBT. This may be due to higher melt viscosity and elasticity in the PBT melt compared to PP and PS. Higher processing temperatures and/or air-to-polymer flow rates should result in this phenomenon in PBT fibers as well. The fact that the droplets are often perfectly spherical indicates that the fiber breakup is taking place between the collec- tor and the die and then the spheres are ejected into the fiber mat. On average, any given sphere will have a higher velocity compared to fibers because fibers are connected to the die and

Table 4 Extent and size of droplet formation in PS and PP melt blown ﬁbers

Run I.D. dav (mm) ns/nf dsp (mm) PS-1 1.61 ee PS-2 0.62 0.05 0.97 PS-3 0.38 0.13 1.00 Fig. 5. Normalized Gaussian fits to log(d ) data for selected melt blowing runs. PP-1 1.23 ee The Gaussian fits are defined by xc and d in Table 3. PP-2 0.45 0.16 0.95 PP-3 0.30 0.32 0.99 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316 3315 undergo whipping, while spheres resulting from this instability converted into nanofibers with average diameters less than travel in a linear fashion directly towards the mat. Occasion- 500 nm. These results demonstrate that there is no fundamen- ally, a sphere appears to be attached to a particular fiber and tal restriction against producing nanofibers by this method and distorted from a spherical shape. Therefore, the distorted this study represents a major technical step in closing the gap shape might arise from spheres which access a portion of between electrospinning and melt blowing in terms of the size the jet with the highest velocity and temperature allowing of fibers that can be produced. them to remain molten as they approach the mat. Fiber diameter distributions were demonstrated to be well We have observed that the extent of fiber breakup is depen- described by log-normal functions regardless of the average dent on both processing temperature (supported by PS-2 and fiber diameter suggesting that the underlying mechanisms PS-3 data shown in Table 4), and polymer and air flow rates which produce the distributions remain unchanged, even dur- (supported by PP-1 and PP-2 data shown in Table 4) used during the production of nanofibers. Comparing the width/shape ing melt blowing. For the same polymer and air flow rate, an of the size distribution for nanofiber mats and mats with aver- increase in processing temperature results in more spheres; for age fiber diameters in excess of 1 mm reveals that the diameter the same processing temperature, an increase in air to polymer dependence of the distribution widths are nonuniversal (i.e., ratio results in a higher ns/nf. The average diameters of the depends on the material). Fibers made at the highest tempera- spheres (dsp) are also listed in Table 4 and are remarkably sim- tures and air flow rates (but with processing conditions acces- ilar regardless of the material or particle density in the mat. sible by commercial equipment) revealed the onset of what we This may be an indication that the instability occurs under believe are surface tension driven fiber breakup instabilities similar conditions (for materials with similar properties and leading to spherical particles dispersed among the fiber mat. individual fiber diameters sampling a similar portion of the po- This phenomenon may represent a fundamental limit on the pulation of fiber dynamics, etc.) for all the materials employed smallest fibers achievable by melt blowing for these materials in this paper. Finally, we have observed that it is possible to and processing conditions, but more research is required to induce complete fiber breakup in PP and PS (no fibers col- understand the controlling mechanisms. lected) by increasing temperature or reducing polymer flow- rate yet further compared to those cases described in Table 4. References To the best of the authors’ knowledge, this is the first report of such instabilities in melt blown fiber mats. It is important to [1] Nonwovens industry magazine e market introduction; 2004. note that similar instabilities are often observed in electrospin- [2] Grafe T, Graham K. Int Nonwovens J 2003;12(1):51e5. ning [52]. In the case of melt blowing, it is unclear if the [3] Huang Z-M, Zhang Y-Z, Kotaki M, Ramakrishna S. Compos Sci Technol 2003;63(15):2223e53. instabilities originate simply from the smallest fibers in the [4] Zhang Y, Lim CT, Ramakrishna S, Huang Z-MJ. Mater Sci Mater Med distribution under all circumstances or if their formation 2005;16(10):933e46. requires both a small fiber and a relatively extreme dynamic [5] Burger C, Hsiao BS, Chu B. Annu Rev Mater Res 2006;36:333e68. motion with regard to the fiber ‘‘whipping’’. Of relevance to [6] McCulloch JG. Int Nonwovens J 1999;8(1):139e49. e the former, one can identify long (several hundred microns [7] Wente VA. Ind Eng Chem Res 1956;48(8):1342 6. [8] Harding JW, Keller JB, Buntin RR. US Patent 3,825,380; 1974. in length) continuous sub 0.1 mm diameter fibers. However, [9] Uyttendaele MAJ, Shambaugh RL. Ind Eng Chem Res 1989;28(11): in general, the expectation is that the presence of a significant 1735e40. level of these instabilities would reduce the overall average [10] Bresee RR, Ko W-C. Int Nonwovens J 2003;12(2):21e8. fiber length and possibly the resulting mat strength. A con- [11] Fridrikh SV, Yu JH, Brenner MP, Rutledge GC. Phys Rev Lett certed simulation (using models which incorporate surface 2003;90(14):144502. [12] Fridrikh SV, Yu JH, Brenner MP, Rutledge GC. ACS Symp Ser tension effects and which capture behavior of fibers less 2006;918:36e55. than 1 mm) and experimental effort may be essential to further [13] Rao RS, Shambaugh RL. Ind Eng Chem Res 1993;32(12):3100e11. understand this behavior and to determine if it represents the [14] Ju YD, Shambaugh RL. Polym Eng Sci 1994;34(12):958e64. onset of a fundamental limit to this process. [15] Uyttendaele MAJ, Shambaugh RL. AIChE J 1990;36(2):175e86. e Finally, nanofibers with similar diameters (and other quali- [16] Yin H, Yan Z, Ko W-C, Bresee RR. Int Nonwovens J 2000;9(4):25 8. [17] Wu TT, Shambaugh RL. Ind Eng Chem Res 1992;31(1):379e89. tative features such as fiber breakup, etc.) to those described in [18] Yin H, Yan Z, Bresee RR. Int Nonwovens J 1999;8(1):130e8. this paper have also been produced using larger orifice dies [19] Marla VT, Shambaugh RL. Ind Eng Chem Res 2003;42(26):6993e7005. (do w 0.4 mm) [45]. [20] Haynes BD, McManus JL, Duellman JM, Clark DF. Roger Bradshaw Quincy I, US Patent 6,613,268; 2003. e 4. Conclusions [21] Ward GF. Filtr Sep 2001;39(9):42 3. [22] Shambaugh RL. Ind Eng Chem Res 1988;27(12):2363e72. [23] Milligan MW, Haynes BD. J Appl Polym Sci 1995;58(1):159e63. We show that a range of materials, both amorphous and [24] Haynes B, Milligan M. INDA J Nonwovens Res 1991;3(4):20e5. semicrystalline, with large variability in viscosity, solidifica- [25] Zhao R, Wadsworth LC. Polym Eng Sci 2003;43(2):463e9. tion temperature, and chemical/thermal stability are success- [26] Chen T, Li L, Huang X. J Appl Polym Sci 2005;97(4):1750e2. e fully melt blown into continuous fibers using a laboratory [27] Wang X, Ke Q. Polym Eng Sci 2006;46(1):1 7. [28] Brandrup J, Immergut EH, editors. Polymer handbook. New York, NY: scale single orifice melt blowing apparatus. This study in- Wiley; 1989. cluded standard/commercial (PBT, PP) and atypical (low [29] Milligan MW, Lu F, Buntin RR, Wadsworth LC. J Appl Polym Sci MW PS) melt blowing materials, all of which are easily 1992;44(2):279e88. 3316 C.J. Ellison et al. / Polymer 48 (2007) 3306e3316

[30] Gahan R, Zguris GC. Fifteenth annual battery conference on applications [46] Macosko CW. Rheology principles, measurements, and applications. and advances. Long Beach, CA; 2000. p. 145e9. New York, NY: Wiley-VCH, Inc.; 1994. [31] Lee Y, Wadsworth LC. Polym Eng Sci 1990;30(22):1413e9. [47] Marrucci G, Ianniruberto G. Macromolecules 2004;37(10):3934e42. [32] Podgorski A, Balazy A, Gradon L. Chem Eng Sci 2006;61(20):6804e15. [48] Nielsen JK, Rasmussen HK, Hassager O, McKinley GH. J Rheol 2006; [33] Reneker DH, Chun I. Nanotechnology 1996;7(3):216e23. 50(4):453e76. [34] Frenot A, Chronakis IS. Curr Opin Colloid Interface Sci 2003;8(1):64e75. [49] Hepperle J, Munstedt H. Rheol Acta 2006;45(5):717e27. [35] http://www.donaldson.com/en/filtermedia/products/nanofibers.html. [50] Fetters LJ, Lohse DJ, Graessley WW. J Polym Sci Part B Polym Phys [36] Ramakrishna S, Fujihara K, Teo W-E, Lim T-C, Ma Z. An introduction to 1999;37(10):1023e33. electrospinning and nanofibers. Singapore: World Scientific Publishing [51] McKee MG, Park T, Unal S, Yilgor I, Long TE. Polymer 2005;46(7): Co. Pvt. Ltd.; 2005. 2011e5. [37] Dalton PD, Klinkhammer K, Salber J, Klee D, Moller M. Biomacro- [52] Gupta P, Elkins C, Long TE, Wilkes GL. Polymer 2005;46(13):4799e810. molecules 2006;7(3):686e90. [53] Shenoy SL, Bates WD, Frisch HL, Wnek GE. Polymer 2005;46(10): [38] Lyons J, Li C, Ko F. Polymer 2004;45(22):7597e603. 3372e84. [39] Larrondo L, Manley RSJ. J Polym Sci Part B Polym Phys 1981;19(6): [54] Yu JH, Fridrikh SV, Rutledge GC. Polymer 2006;47(13):4789e97. 909e20. [55] Limpert E, Stahel WA, Abbt M. BioScience 2001;51(5):341e52. [40] Lyons J, Ko F. Polym News 2005;30(6):170e8. [56] Tomotika S. Proc R Soc London Ser A 1935;150(870):322e37. [41] Chen T, Huang X. Modell Simul Mater Sci Eng 2004;12(3):381e8. [57] Rayleigh L. Philos Mag 1892;34(207):145. [42] Chen T, Li L, Huang X. Modell Simul Mater Sci Eng 2005;13(4): [58] Funada T, Joseph DD. J Non-Newtonian Fluid Mech 2003;111(2e3): 575e84. 87e105. [43] Krutka HM, Shambaugh RL, Papavassiliou DV. Ind Eng Chem Res [59] Joseph DD, Beavers GS, Funada T. J Fluid Mech 2002;453:109e32. 2004;43(15):4199e210. [60] Bousfield DW, Keunings R, Marrucci G, Denn MM. J Non-Newtonian [44] Marla VT, Shambaugh RL. Ind Eng Chem Res 2004;43(11):2789e97. Fluid Mech 1986;21(1):79e97. [45] Phatak A. PhD thesis. University of Minnesota; 2006. [61] Chang HC, Demekhin EA, Kalaidin E. Phys Fluids 1999;11(7):1717e37.