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
A ‘‘nonwoven’’ refers to a sheet or mat of fibers connected
together by physical entanglements, or contact adhesion between individual fibers, without any knitting or stitching.
The nonwovens industry was worth $14 billion in 2004 [1]
and is expected to grow further due to new applications requiring fibers with increasingly smaller sizes. Since the surface
area of a fiber scales linearly with the diameter and the volume
(and mass) scales as the square of the diameter, the specific
surface area varies inversely with diameter (w1/d ), leading
to high specific surface areas for small fibers. As an example,
a gram of a mat containing 100 nm polymer fibers (of density
1 g/cm3) has about 10 m2 of surface area that can be made
available for a wide variety of processes via surface
* Corresponding author. Tel.: þ1 612 624 0839; fax: þ1 612 626 1686.
E-mail address: bates@cems.umn.edu (F.S. Bates).
1
The authors contributed equally to this work.
0032-3861/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.polymer.2007.04.005
functionalization. In addition, the ultimate mat properties
such as overall mat strength (related to individual fiber
strength, average fiber length, and fiber entanglement density)
and porosity play an equally important role in end-use applications. Nonwoven fibers find use in a range of applications
such as filtration, membrane separation, protective military
clothing, biosensors, wound dressings, and scaffolds for tissue
engineering [2e5].
Electrospinning, melt spinning, and melt blowing are the
most commonly used processes for nonwovens production.
Electrospinning involves applying a strong electric potential
(w10 kV) to a polymer solution contained in a syringe to
force a jet of the solution onto a grounded screen located
a few centimeters away. Rapid evaporation of the solvent results in a mat of fine (10 nme1 mm) polymer fibers that are deposited on the screen. In contrast, melt spinning is performed
by extruding a polymer melt and drawing it down with a takeup wheel. Since the polymer solidifies during the drawing process, this yields highly oriented chains resulting in strong
C.J. Ellison et al. / Polymer 48 (2007) 3306e3316
fibers that can be produced at fast rates; however, the fibers are
usually not smaller than 10 mm [2].
During melt blowing, fibers are produced in a single step by
extruding a polymer melt through an orifice die and drawing
down the extrudate with a jet of hot air (typically at the
same temperature as the molten polymer). This environmentally benign processing method was first developed in the 1950s
at the Naval Research Laboratory with the goal of making
sub-micron fibers to trap radioactive particles in the upper atmosphere [6]. Wente first described the construction of a melt
blowing die composed of a series of orifices and slots [7].
Researchers at Exxon extended this basic design and first demonstrated the production of melt blown microfibers on a commercial scale by modifying sheet die technology [6,8]. Since
then, a number of companies such as Vose, 3M, Kimberlye
Clark, Cummins, and Johns Manville have used the technology to produce commercial nonwoven products [6]. In general,
these commercial products are composed of fibers with average diameters exceeding 1e2 mm.
Melt blowing equipment is designed such that the air is
supplied in the form of two streams that form a v-slot (see
Fig. 1); other designs such as annular air jets have been
used only on a laboratory scale [9]. Commercially the v-slot
design is used in the form of a long channel which encompasses a single row of hundreds or thousands of orifices
from which the fibers originate. The drag force exerted by
the air attenuates the melt extrudate into fibers, which are collected a few feet away from the die producing a self-bonded
mat [10]. The nature of the drag force is more complicated
than the air stream simply acting in the axial direction along
a taut fiber. The fiber is highly dynamic and is observed to
Fig. 1. Detailed schematic of the melt blowing die: (a) sectional and (b) end-on
views of the two pieces.
3307
frequently whip across the air stream (a similar motion is observed in electrospinning, but is due to a fundamentally different mechanism [11,12]) after it exits the die and proceeds
towards the collection device [10,13]. This fiber motion produces a transient drag force from air flowing normal to and
down the axis of the fiber [14]. An additional level of extensional force may be introduced when multiorifice dies are
employed allowing adjacent fibers to participate in fibere
fiber interactions.
Researchers have used high speed imaging techniques to
perform inline investigations of fiber formation during melt
blowing. Fiber diameter measurements revealed that most of
the fiber attenuation occurred within several centimeters of
the die exit and that the temperature of the fiber dropped to
near-ambient values over the same distance due to entrainment
of ambient air into the hot air stream [15,16]. Additionally,
laser Doppler velocimetry measurements showed that average
fiber and air velocities approached the same value within
several centimeters of the die exit [17,18]. This has also
been corroborated by modeling studies [15,19]. It is intuitive
that the attenuation during melt blowing occurs only between
the processing (Tp) and solidification (glass transition, Tg, or
crystallization, Tc) temperatures. This suggests a possible approach to achieve greater attenuation by holding the fibers in
the active temperature window [Tg (or Tc) < T < Tp] for longer
periods of time (where T is the fiber temperature). Haynes and
coworkers have suggested a design to implement this idea by
combining melt blowing with entrainment of external heated
air; however, the diameters of the fibers obtained by this
method were not dramatically smaller than those produced
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) and mass
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 attempted 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 concluded 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
do not.) They further developed an empirical model for the
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
model parameters are dependent on the polymer type (due to
differing viscoelastic characteristics) and the particular melt
blowing line employed (due to geometry, etc.).
A wide variety of polymers including polyethylene [7],
polypropylene (PP) [15,23,25], poly(methyl methacrylate)
[7], poly(ethylene terephthalate) [25], poly(butylene terephthalate) (PBT) [26], polyamides (e.g. nylon) [7,27], and
polystyrene (PS) [7] have been used for producing blown
fibers. This represents a set of both amorphous and semicrystalline materials having a wide range of physical properties
[28]. Thus, the compatibility of a polymer with this process
seems to be independent of the backbone chemical structure.
Naturally, the final properties of the melt blown fiber web
(softness, toughness, solvent resistance, etc.) will depend on
the chemical nature of the polymer backbone; this is a separate
issue. As long as the melt viscosity of the polymer is low
enough to facilitate significant attenuation of the extrudate,
it apparently can be melt blown. Even though the viscosity
can be reduced by increasing the processing temperature, the
upper limit is prescribed by the degradation temperature of
the polymer. Of course, exposure of the polymer to elevated
temperatures may also be reduced by staging the temperature
profile of the melt processing equipment which feeds the melt
blowing device.
Given that the size of a fiber primarily determines the properties of the final nonwoven, it is surprising that size distributions of melt blown fibers have rarely been documented
[22,24,27,29]. The nature of the fiber size distributions can
provide valuable clues about the physics of the process. The
literature is also unclear about the smallest fibers that can be
produced by melt blowing. Most reviews say that even though
isolated fibers w100 nm in diameter can be obtained by this
process, average fiber diameters of 1e2 mm are produced
commercially [2,30,31]. However, there is a seemingly
‘‘under-cited’’ report of sub-micron fibers in a study by Wente;
in this work, fibers with average diameters as small as 500 nm
were made from a number of polymers, but fiber size distributions were not reported and only one image of the fibers was
displayed [7]. Unfortunately, air and polymer flow rates
were also not reported for any of the cases studied [7]. Others
have claimed to produce fibers with an average diameter of
w300 nm using a new die design composed of stacked plates
which results in a row of orifices as small as 0.0125 mm in
diameter, but present no data or images from such fibers
[21]. While the approach of reducing the orifice size to reduce
overall fiber size is intuitive, such a small orifice is a challenge
on an industrial melt blowing line due to the higher pressures
required to extrude the polymer (<8000 kPa is typically
required to keep current commercial dies from fracturing
along the array of orifices or ‘‘unzippering’’ [21]) and the
higher probability of clogging the orifices with foreign material inherently present in production environments and polymer feedstocks. During the writing of this manuscript,
a third report [32] has emerged indicating that polypropylene
(unspecified viscosity, molecular weight, etc.) with sub-micron
average fiber diameters may be produced via melt blowing, but
processing conditions and the die orifice diameter are not
indicated.
Currently, electrospinning is the most popular process to
make fibers which have diameters in the 100 nm range
[33,34], and as a result this process has begun to forge a niche
in commercial production of ‘‘nanofibers’’ [35]. However,
electrospinning is inherently slow due to the common requirement of removing residual solvent from the nonwoven and is
undesirable due to difficult solvent handling/recovery, slow
fiber production rates (on a per orifice basis because of the
use of a polymer solution in contrast to melt blowing a polymer
melt), and the high voltages that are required [5,36]. While
multiorifice commercial electrospinning production lines alleviate some aspects of the slow nature of the process, the above
factors nevertheless lead to significantly higher costs compared to melt blowing. Furthermore, many polymers that
have desirable physical and mechanical properties (e.g. polyolefins, PBT, etc.) are hard to dissolve in common solvents
at room temperature, making them difficult to electrospin.
Even though electrospinning with polymer melts has been performed, the fibers obtained are relatively large (a few micrometers in diameter), presumably due to the high viscosity of
polymer melts [37e40].
If melt blowing technology could be extended to submicron fiber sizes, it would provide a much easier, faster,
and cheaper alternative to electrospinning. Hence, a major focus of melt blowing research should arguably be to extend the
technology to nanofibers due to the potential to penetrate new
markets and enhance current product offerings. In addition,
a fundamental understanding of the limits of the melt blowing
process is needed to determine the smallest fiber size that can
be theoretically obtained. Modeling/simulation could provide
some clues in this regard. However, all the simulations to
date have been restricted to fiber diameters of a few micrometers and have largely ignored surface tension effects which
will likely become increasingly important as fiber size is
reduced below 1 mm [15,19,41,42]. Existing studies correlate
fiber diameter with various processing and geometrical
parameters [43,44], but these studies do not address the possible limits of this technology or seek to explore the potential
of extending it to applications that transcend the traditional
1e2 mm fibers that have been produced for decades.
In this paper, the fundamental lower limit of the average fiber diameter in the melt blowing process is explored for two
commercial melt blowing materials (PBT and PP, semicrystalline) and for a low molecular weight (MW) PS sample (amorphous). The low MW PS sample is particularly interesting
because it has the lowest melt viscosity in this study. In addition, this material has been selected as an avenue to begin to
explore the absolute limit of melt blowing capabilities of
low melt viscosity materials. Importantly, solidification of
the molten fiber is possible due to a Tg which lies just above
room temperature thus producing PS nonwoven fibers during
melt blowing. While the low MW bimodal PS may not have
excellent mechanical/thermal/chemical stability properties
required for end-use nonwoven applications, it does provide
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 afforded 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
rate [MFR] at 230 C) and PBT (Ticona; Celanex 2008, 250
MFR at 250 C) were used in this study (see Table 1 for physical property details). Molecular weight data were not attainable for PBT as it is insoluble in common solvents used in
size exclusion chromatography such as chloroform, tetrahydrofuran, toluene and trichlorobenzene. PS was dried under
vacuum at 140 C for a minimum of 12 h before use to remove
residual solvent and monomer while PBT was dried at 120 C
for 4 h in ambient air to remove absorbed water. PP was used
as received. Relevant molecular and physical parameters of
these polymers are shown in Table 1 (and Fig. 2) and were
characterized by a combination of size exclusion chromatography using refractive index and light scattering detection (Polymer Laboratories PL-GPC 220, Wyatt Optilab, Wyatt Dawn),
where absolute molecular weights were determined with
appropriate standards, differential scanning calorimetry (TA
Instruments Q1000), and dynamic mechanical spectroscopy
(Rheometrics ARES, now TA Instruments) measurements.
The glass transition (Tg) and melting transition (Tm) temperatures were measured upon second heat (as onset for Tg and
endset for Tm) at a heating rate of 10 C/min following a temperature quench at 10 C/min from well above Tg and Tm.
Crystallization temperatures (Tc) were measured as the onset
temperature with a cooling rate of 10 C/min starting from
temperatures well above Tm. Melt viscosities of the polymers
were measured by isothermal dynamic frequency sweep experiments with 25 mm parallel plates. Measurements were
conducted between 0.1 and 100 rad/s in the linear viscoelastic
regions of the polymers, which were determined by dynamic
strain sweep experiments.
Table 1
Physical properties of the polymers used for melt blowing
Polymer
Mn (kg/mol)
Mw (kg/mol)
Tm ( C)
Tc ( C)
Tg ( C)
PS
PP
PBT
2.1
10.6
ea
48.8
59.0
ea
e
170
240
e
118
190
61
2
35
a
This polymer has a MFR of 250 at 250 C.
Fig. 2. Variation of complex viscosity with dynamic frequency from oscillatory shear measurements conducted at the temperatures used for melt blowing.
2.2. Melt blowing
A Goetffert Rheo-Tester 1500 capillary rheometer was used to
extrude polymer samples through melt blowing dies, which were
composed of two parts (Fig. 1). Since the melt blowing die extended outside the barrel of the capillary rheometer, external heating was provided by band heaters (Watlow part #B3N1AP1)
wrapped around aluminum blocks that fit tightly around the die
pieces shown in Fig. 1. A single-hole die with an orifice diameter
(do) of 0.2 mm, an air-gap distance (dg) of 1 mm, and a setback
distance (ds) of 1.5 mm (see Fig. 1) was employed. Each of the
v-slot channels were 10 mm by 1 mm and they both fed into
the final air slot which was 10 mm long (indicated by ‘‘w’’ in
Fig. 1b) by 1 mm. Polymer was fed at temperature Tp and mass
flow rate mp between 0.035 and 0.35 g/min. The die orifice was
stepped down from 3 mm to the final orifice diameter as shown
in Fig. 1. For the final section of the capillary, the capillary length
to diameter ratio was z10. Air flow rates (fa) between 4.5 and
10 standard cubic feet per minute (SCFM) were employed and
G was calculated by a ratio of the air mass flux in the v-slots
and the polymer mass flux through the orifice. Heated air was
supplied to the die at a temperature Ta (temperature at die exit),
with a 6 kW Osram Sylvania threaded inline heater used in conjunction with an open loop (manual) power control system.
Tp ¼ Ta for all conditions in this study.
Each melt blowing experiment was conducted using a few
grams of polymer and required approximately 2 h for instrument assembly, heat up, melt blowing and cool down, making
it ideal for materials testing as described in this paper. During
melt blowing, polymer was exposed to the processing temperature for a maximum of 30 min in order to limit degradation.
For the highest processing temperature listed in Table 2 (where
the potential for degradation is highest), the Mn of PS remains
unchanged within error while Mw is reduced by 31% following
processing. Similarly, the Mn of PP is reduced by 38% while
the Mw of PP is reduced by 51% following processing. It is
C.J. Ellison et al. / Polymer 48 (2007) 3306e3316
3310
Table 2
Summary of the melt blowing experiments
h* at 1 s1
(Pa s)
fa
(SCFM)
Run I.D.
Tp, Ta
( C)
PS-1
PS-2
PS-3
180
260
280
23
1.6
1.1
0.053
0.07
0.07
8
7.5
8
PP-1
PP-2
PP-3
180
180
220
35
35
15
0.35
0.035
0.035
PBT-1
PBT-2
265
265
137
137
0.35
0.035
mp
(g/min)
G
dav
(mm)
9
6.4
6.8
1.61
0.62
0.38
6
8
8
0.5
13.6
13.6
1.23
0.45
0.30
4.5
10
0.4
17
1.22
0.44
important to note that this melt blowing grade PP contains
a peroxide additive (added by Exxon) to control the polymer
rheology during processing and this contributes to the decrease
in molecular weight. An analysis of PBT was not possible due
to its insolubility in available solvents used for SEC.
Melt blown fibers were collected on a stainless steel screen
located 55 cm away from the die exit, which is in the range of
typical collection distances used in commercial melt blowing
lines. At the collection location, the air jet possesses a substantially lower average velocity, widened overall diameter (w5e
6 in at the collection point), and has cooled to temperatures
well below 50 C due to entrainment of ambient air. The collection distance was chosen such that the fibers had solidified
by cooling below Tg or Tc. This ensures the production of nonwovens lacking fused fibers at contact points as sufficient
cooling has taken place to solidify the fibers. In addition, the
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
average face velocity on the screen surface of approximately
24 ft/s which was greater than the average velocity of the
widened air jet at the collection location under all conditions
employed. This collection scheme is provided for efficient collection of fibers produced by the die. The desired structure of
the nonwoven was verified by scanning electron microscopy
(SEM) and the general feel of the fiber mat.
2.3. Fiber diameter characterization
Samples (1 cm 1 cm) were sectioned from melt blown
fiber mats and coated with a z2.5 nm thick platinum coating
using a VCR high resolution ion beam sputtering system. Platinum coated samples were imaged by a Hitachi S-4700 cold
field emission gun SEM in the secondary electron mode,
with accelerating voltages between 1.5 and 2.5 kV, emission
currents between 10 and 15 mA, working distances of
z12 mm and magnifications in the range of 300e10,000.
At least 250 fiber measurements from more than 20 SEM images were used in order to ensure reproducible statistics when
measuring fiber size distributions. Fiber diameters were measured by image processing software (ImageJ, NIST). Fiber
diameters were measured from SEM images by drawing
straight lines along the diagonals of an image (to ensure fibers
were not counted twice) and measuring fibers that crossed the
lines. To generate reliable statistics, fibers were imaged from
different parts of a 1 cm 1 cm sample area. In addition,
the fibers lying outside the focal plane were not used in the
statistical analysis.
The fiber diameter distributions were fit to log-normal functions using Origin data processing software (OriginLab Corp.).
The histograms were constructed using bin sizes that produced
approximately 50 bins across the entire distribution. We found
that this led to a good compromise between larger bin sizes
which result in smoothing of the shape of the distribution
and smaller bin sizes which do not capture the nature of the
distribution as the number of bins approach the sample size.
Consequently, the choice of bin size can impact the R2 value
that is reported in Table 3, which is representative of the goodness of fit of the log-normal distribution function to the
experimental data.
3. Results and discussion
3.1. Average fiber diameter
Table 2 shows the average fiber diameters that have been
produced for PS, PP, and PBT under specified melt blowing
conditions. The resultant fibers are consistent with the expectation of commercial materials (e.g. 1e2 mm) or substantially
smaller (e.g. 0.3e0.6 mm) depending on the processing conditions. These data show that it is possible to significantly decrease the average fiber diameter below 1 mm by modulating
several different processing parameters. For example, at constant temperature, the average fiber diameter of PBT-1 is
Table 3
Average fiber diameters and log-normal distribution parameters
R2
Run I.D.
dav (mm)
m
s
Med[log(d )]
xc
d
PS-1
PS-2
PS-3
1.61
0.62
0.38
0.20
0.29
0.48
0.07
0.28
0.24
0.20
0.24
0.47
0.20
0.23
0.47
0.07
0.29
0.28
0.98
0.68
0.69
PP-1
PP-2
PP-3
1.23
0.45
0.30
0.039
0.42
0.57
0.33
0.25
0.197
0.033
0.41
0.58
0.06
0.41
0.59
0.37
0.25
0.20
0.66
0.85
0.68
PBT-1
PBT-2
1.22
0.44
0.001
0.43
0.26
0.21
0.001
0.46
0.008
0.48
0.21
0.17
0.86
0.73
Gaussian fit to log (d )
C.J. Ellison et al. / Polymer 48 (2007) 3306e3316
reduced significantly from 1.22 to 0.44 mm for PBT-2 by increasing the air to polymer mass flux ratio (G). The same trend
is observed for PP in which fiber diameter reduces from
1.23 mm for PP-1 to 0.45 mm for PP-2. We have also observed
[45] tunability of the fiber diameter in other experiments when
changing air flow rate or polymer mass flow rate individually
consistent with other researchers [24,22]. Other process variables being equal, fiber diameter decreases with higher air
flow rate due to an increase in the drag on the fiber (and potential enhancement of the fiber whipping dynamics). Similarly,
lowering polymer mass flow rate decreases fiber diameter
because the same drag force from the air jet is acting on
less polymer mass. It is also noteworthy that the air to polymer
mass flux ratios for the smallest fibers in Table 2 are higher
than any other values reported in the literature, yet the polymer
flow rate and air pressure required (less than 70 psig) should
be accessible by commercial melt blowing lines.
Similarly, an increase in Tp (Tp ¼ Ta at all conditions in this
study) from PS-1 to PS-2 results in a reduction of the average
fiber diameter from 1.61 to 0.62 mm. The effect of the increase
in Tp clearly dominates the expectation of a slightly larger
fiber size from a slightly increased polymer flow rate or
slightly reduced air flow rate in PS-2 compared to PS-1. PS-2
and PS-3 or PP-2 and PP-3 also show a significant reduction in
fiber size due to an increase in Tp. This effect of processing
temperature is a result of two factors. First, there is an increase
in the active temperature window over which the fiber attenuation may occur when Tp (and Ta) is increased. The attenuation
process (attenuation requires the polymer be molten) continuously competes with cooling of the heated air jet due to
entrainment of ambient air. Thus, higher processing temperatures allow for the polymer fiber to remain in the melt state
for longer periods of time and undergo additional attenuation
before the polymer solidifies via sufficient crystallization or
vitrification.
Second, there is a substantial decrease in viscosity as the
temperature is increased. Fig. 2 shows the complex shear viscosity as a function of frequency (equivalent to the steady
shear viscosity as a function of steady shear rate by the
CoxeMerz rule [46]) for PS, PP and PBT at the processing
temperatures shown in Table 2. Note that at these temperatures
the viscosities exhibit little or no frequency dependence or
shear thinning. The viscosity of PS decreases by more than
an order of magnitude when increasing Tp by 100 C and by
a factor of w3 upon increasing Tp by 40 C for PP. The polymer flow rates used in Table 2 correspond to apparent shear
rates between w100 and 1000 s1 in the die capillary. Hence,
the results shown in Fig. 2 represent the highest melt viscosity
for each polymer at the extrusion temperature.
While these shear viscosity data give a general picture of
the rheology of the material at the processing temperature,
the more relevant viscoelastic and process parameters are the
extensional viscosity (and its temperature dependence), the
extensional strain rate and the degree of extensional strain
hardening. Extensional viscosity is not trivial to access directly in polymer melts at elevated temperatures and typical
extensional strain rates present during the melt blowing
3311
process. We estimate that the extensional strain rates are quite
high (w107 s1). Traditionally, extensional strain hardening
has been attributed to chain entanglements or long chain
branching [47e49]. (The molecular weight between entanglements (Me) for PS and PP are 13,000 and 5000 g/mol [50], respectively, and the polymer MW must be at least 2e3 Me for
entanglements to begin to make contributions to the flow characteristics of polymers.) It is unclear if the PS and PP used in
this study exhibit significant extensional strain hardening at
these strain rates. In addition, the bimodal MW distribution
of PS is a further complicating issue. A systematic study on
a model system which relates the viscoelastic material properties to melt blowing behavior is required to further understand
these issues.
It is surprising that the highest and lowest viscosities in
Fig. 2 differ by more than two orders of magnitude, yet these
materials can be easily melt blown into continuous defect free
fibers. This is a testament to the high degree of process latitude
that melt blowing affords. Fig. 3 shows SEM micrographs of
the typical fiber mats (largest and smallest average diameter
samples for PBT, PP and PS shown in Table 2) that are collected and analyzed for average fiber diameter, as well as qualitative features. The left column of images shows fibers with
average diameters in excess of 1 mm. When these samples
are viewed at lower magnifications (150e350), a majority
of the fibers may be identified to have contour lengths significantly exceeding 1 mm (as far as they can be traced in the
field of view of several SEM images) and fiber ends are rarely
identified even upon analysis of more than 20 images. Based
on these facts, it is intuitive to expect that the average fiber
is at least several orders of magnitude longer than 1 mm and
this is consistent with other studies [13,10] employing high
speed photography which show continuous fibers (diameters
greater than 1 mm) which are at least several tens of centimeters in length. In addition, during melt blowing of the fiber
mats, short loose fibers escaping the mat (commonly referred
to as ‘‘fly’’) are not observed. This is an indication that our
laboratory scale melt blowing device is producing long, defect
free fibers of the size and quality that would be expected in
a commercial product.
The second column of Fig. 3 displays the fibers that have
been produced with average fiber diameters well below
1 mm. In all cases the fibers are observed to be essentially
defect free with contour lengths exceeding several hundred
microns in length. This is also the case for individual fibers
less than 0.1 mm. However, closer examination of Fig. 3b
and d reveals that there are a few isolated particles resembling
spheres intermingled amongst the fibers. These may be the
result of the onset of surface tension driven fiber breakup
and this will be discussed further in Section 3.3. We want to
emphasize that these particles are different from so called
‘‘shot’’ formation in melt blowing. ‘‘Shot’’ refers to larger
particles of polymer (>several tens of microns in size) in
the fiber mat that have ill-defined shapes [10].
While the fibers highlighted in Table 2 and Fig. 3 have been
produced using single-hole melt blowing dies, we have successfully 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
those used in industry. Most importantly these results indicate
that there is no fundamental restriction in attenuating a molten
polymer fiber down to several hundred nanometers in diameter
via melt blowing; this reduces the gap between melt blowing
and electrospinning technology in terms of the ability to produce nano-scale fibers.
The fact that lightly entangled polymer melts (PS and PP)
with relatively low viscosities (especially in the case of PS)
compared to most melt processed polymers can be melt blown
into w0.4 mm fibers (Table 2 and Fig. 3b) was unexpected and
demonstrates how much fundamental knowledge is yet to be
learned about this process. It also suggests that entanglements
may not be a key requirement in producing defect free fibers
several hundred nanometers in diameter. Furthermore, it may
be possible that many materials suitable for melt blowing
have been overlooked due to the basic (and rational)
assumption that melt strength/elasticity, achieved via the
presence of a substantial level of entanglements, is required
to prevent significant fiber breakup and avoid what might be
called ‘‘melt spraying’’.
In contrast, for electrospinning, entanglements often do
play a key role. It has been shown that chain entanglements
in the polymer solution are often required (thus solution
concentrations must be above the chain overlap concentration
or c*) to produce defect free continuous fibers via electrospinning [51e53]. This is not always the case if sufficient elasticity is built into the polymer solution by other means such as
adding a third component that may interact with the primary
polymer [54]. A PS sample such as that used in this study
would more than likely present a challenge for electrospinning
without such an additive. The present study suggests that significant elasticity of the melt may not be a primary requirement for producing nanofibers by melt blowing.
C.J. Ellison et al. / Polymer 48 (2007) 3306e3316
3.2. Fiber diameter distributions
Fig. 4a shows a representative size distribution from a melt
blown fiber mat; some of the obvious features are that it is
asymmetric (skewed towards the left) and has a tail at the large
diameter end. An asymmetric distribution implies that as one
moves away from the peak of the distribution, the probability
of observing the independent variable does not reduce linearly.
From a statistical perspective, the fiber diameter is an independent variable and the probability of observing a particular fiber
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
arbitrary number). Fig. 4b shows the distribution of Fig. 4a
plotted versus the logarithm of the fiber diameter (log(d )) as
the independent variable. This is a reasonably symmetric
Fig. 4. Fiber diameter distributions from the PP-2 melt blowing run on (a)
linear and (b) logarithmic scales. The dotted line shows the best fit normal
distribution to log(d ).
3313
curve with the arithmetic mean (m) of log(d ) being nearly
equal to the median [m ¼ 0.42; med(log(d )) ¼ 0.41], and
a standard deviation (s) of 0.25. These are characteristic features of the log-normal distribution (symmetric when plotted
versus log(d ) and med(log(d )) zm(log(d )) [55].
In Fig. 4b, a normal (Gaussian) distribution profile with
a mean (xc) of 0.41 and a standard deviation (d) of 0.25
(solid line in Fig. 4b) shows a good fit (with a reasonable R2
value) to the log(d ) data where the probability density function for a normal distribution is given by,
#
"
1
ðx xc Þ2
pðxÞ ¼ pffiffiffiffiffiffi exp :
2d2
d 2p
ð1Þ
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
dynamic aspects of the fiber as it whips across the air stream.
This whipping action produces a dynamic drag/extensional
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 collector 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
Fig. 5. Normalized Gaussian fits to log(d ) data for selected melt blowing runs.
The Gaussian fits are defined by xc and d in Table 3.
Run I.D.
dav (mm)
ns/nf
dsp (mm)
PS-1
PS-2
PS-3
1.61
0.62
0.38
e
0.05
0.13
e
0.97
1.00
PP-1
PP-2
PP-3
1.23
0.45
0.30
e
0.16
0.32
e
0.95
0.99
C.J. Ellison et al. / Polymer 48 (2007) 3306e3316
undergo whipping, while spheres resulting from this instability
travel in a linear fashion directly towards the mat. Occasionally, a sphere appears to be attached to a particular fiber and
distorted from a spherical shape. Therefore, the distorted
shape might arise from spheres which access a portion of
the jet with the highest velocity and temperature allowing
them to remain molten as they approach the mat.
We have observed that the extent of fiber breakup is dependent on both processing temperature (supported by PS-2 and
PS-3 data shown in Table 4), and polymer and air flow rates
(supported by PP-1 and PP-2 data shown in Table 4) used during melt blowing. For the same polymer and air flow rate, an
increase in processing temperature results in more spheres; for
the same processing temperature, an increase in air to polymer
ratio results in a higher ns/nf. The average diameters of the
spheres (dsp) are also listed in Table 4 and are remarkably similar regardless of the material or particle density in the mat.
This may be an indication that the instability occurs under
similar conditions (for materials with similar properties and
individual fiber diameters sampling a similar portion of the population of fiber dynamics, etc.) for all the materials employed
in this paper. Finally, we have observed that it is possible to
induce complete fiber breakup in PP and PS (no fibers collected) by increasing temperature or reducing polymer flowrate yet further compared to those cases described in Table 4.
To the best of the authors’ knowledge, this is the first report
of such instabilities in melt blown fiber mats. It is important to
note that similar instabilities are often observed in electrospinning [52]. In the case of melt blowing, it is unclear if the
instabilities originate simply from the smallest fibers in the
distribution under all circumstances or if their formation
requires both a small fiber and a relatively extreme dynamic
motion with regard to the fiber ‘‘whipping’’. Of relevance to
the former, one can identify long (several hundred microns
in length) continuous sub 0.1 mm diameter fibers. However,
in general, the expectation is that the presence of a significant
level of these instabilities would reduce the overall average
fiber length and possibly the resulting mat strength. A concerted simulation (using models which incorporate surface
tension effects and which capture behavior of fibers less
than 1 mm) and experimental effort may be essential to further
understand this behavior and to determine if it represents the
onset of a fundamental limit to this process.
Finally, nanofibers with similar diameters (and other qualitative features such as fiber breakup, etc.) to those described in
this paper have also been produced using larger orifice dies
(do w 0.4 mm) [45].
4. Conclusions
We show that a range of materials, both amorphous and
semicrystalline, with large variability in viscosity, solidification temperature, and chemical/thermal stability are successfully melt blown into continuous fibers using a laboratory
scale single orifice melt blowing apparatus. This study included standard/commercial (PBT, PP) and atypical (low
MW PS) melt blowing materials, all of which are easily
3315
converted into nanofibers with average diameters less than
500 nm. These results demonstrate that there is no fundamental restriction against producing nanofibers by this method and
this study represents a major technical step in closing the gap
between electrospinning and melt blowing in terms of the size
of fibers that can be produced.
Fiber diameter distributions were demonstrated to be well
described by log-normal functions regardless of the average
fiber diameter suggesting that the underlying mechanisms
which produce the distributions remain unchanged, even during the production of nanofibers. Comparing the width/shape
of the size distribution for nanofiber mats and mats with average fiber diameters in excess of 1 mm reveals that the diameter
dependence of the distribution widths are nonuniversal (i.e.,
depends on the material). Fibers made at the highest temperatures and air flow rates (but with processing conditions accessible by commercial equipment) revealed the onset of what we
believe are surface tension driven fiber breakup instabilities
leading to spherical particles dispersed among the fiber mat.
This phenomenon may represent a fundamental limit on the
smallest fibers achievable by melt blowing for these materials
and processing conditions, but more research is required to
understand the controlling mechanisms.
References
[1] Nonwovens industry magazine e market introduction; 2004.
[2] Grafe T, Graham K. Int Nonwovens J 2003;12(1):51e5.
[3] Huang Z-M, Zhang Y-Z, Kotaki M, Ramakrishna S. Compos Sci Technol
2003;63(15):2223e53.
[4] Zhang Y, Lim CT, Ramakrishna S, Huang Z-MJ. Mater Sci Mater Med
2005;16(10):933e46.
[5] Burger C, Hsiao BS, Chu B. Annu Rev Mater Res 2006;36:333e68.
[6] McCulloch JG. Int Nonwovens J 1999;8(1):139e49.
[7] Wente VA. Ind Eng Chem Res 1956;48(8):1342e6.
[8] Harding JW, Keller JB, Buntin RR. US Patent 3,825,380; 1974.
[9] Uyttendaele MAJ, Shambaugh RL. Ind Eng Chem Res 1989;28(11):
1735e40.
[10] Bresee RR, Ko W-C. Int Nonwovens J 2003;12(2):21e8.
[11] Fridrikh SV, Yu JH, Brenner MP, Rutledge GC. Phys Rev Lett
2003;90(14):144502.
[12] Fridrikh SV, Yu JH, Brenner MP, Rutledge GC. ACS Symp Ser
2006;918:36e55.
[13] Rao RS, Shambaugh RL. Ind Eng Chem Res 1993;32(12):3100e11.
[14] Ju YD, Shambaugh RL. Polym Eng Sci 1994;34(12):958e64.
[15] Uyttendaele MAJ, Shambaugh RL. AIChE J 1990;36(2):175e86.
[16] Yin H, Yan Z, Ko W-C, Bresee RR. Int Nonwovens J 2000;9(4):25e8.
[17] Wu TT, Shambaugh RL. Ind Eng Chem Res 1992;31(1):379e89.
[18] Yin H, Yan Z, Bresee RR. Int Nonwovens J 1999;8(1):130e8.
[19] Marla VT, Shambaugh RL. Ind Eng Chem Res 2003;42(26):6993e7005.
[20] Haynes BD, McManus JL, Duellman JM, Clark DF. Roger Bradshaw
Quincy I, US Patent 6,613,268; 2003.
[21] Ward GF. Filtr Sep 2001;39(9):42e3.
[22] Shambaugh RL. Ind Eng Chem Res 1988;27(12):2363e72.
[23] Milligan MW, Haynes BD. J Appl Polym Sci 1995;58(1):159e63.
[24] Haynes B, Milligan M. INDA J Nonwovens Res 1991;3(4):20e5.
[25] Zhao R, Wadsworth LC. Polym Eng Sci 2003;43(2):463e9.
[26] Chen T, Li L, Huang X. J Appl Polym Sci 2005;97(4):1750e2.
[27] Wang X, Ke Q. Polym Eng Sci 2006;46(1):1e7.
[28] Brandrup J, Immergut EH, editors. Polymer handbook. New York, NY:
Wiley; 1989.
[29] Milligan MW, Lu F, Buntin RR, Wadsworth LC. J Appl Polym Sci
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
and advances. Long Beach, CA; 2000. p. 145e9.
[31] Lee Y, Wadsworth LC. Polym Eng Sci 1990;30(22):1413e9.
[32] Podgorski A, Balazy A, Gradon L. Chem Eng Sci 2006;61(20):6804e15.
[33] Reneker DH, Chun I. Nanotechnology 1996;7(3):216e23.
[34] Frenot A, Chronakis IS. Curr Opin Colloid Interface Sci 2003;8(1):64e75.
[35] http://www.donaldson.com/en/filtermedia/products/nanofibers.html.
[36] Ramakrishna S, Fujihara K, Teo W-E, Lim T-C, Ma Z. An introduction to
electrospinning and nanofibers. Singapore: World Scientific Publishing
Co. Pvt. Ltd.; 2005.
[37] Dalton PD, Klinkhammer K, Salber J, Klee D, Moller M. Biomacromolecules 2006;7(3):686e90.
[38] Lyons J, Li C, Ko F. Polymer 2004;45(22):7597e603.
[39] Larrondo L, Manley RSJ. J Polym Sci Part B Polym Phys 1981;19(6):
909e20.
[40] Lyons J, Ko F. Polym News 2005;30(6):170e8.
[41] Chen T, Huang X. Modell Simul Mater Sci Eng 2004;12(3):381e8.
[42] Chen T, Li L, Huang X. Modell Simul Mater Sci Eng 2005;13(4):
575e84.
[43] Krutka HM, Shambaugh RL, Papavassiliou DV. Ind Eng Chem Res
2004;43(15):4199e210.
[44] Marla VT, Shambaugh RL. Ind Eng Chem Res 2004;43(11):2789e97.
[45] Phatak A. PhD thesis. University of Minnesota; 2006.
[46] Macosko CW. Rheology principles, measurements, and applications.
New York, NY: Wiley-VCH, Inc.; 1994.
[47] Marrucci G, Ianniruberto G. Macromolecules 2004;37(10):3934e42.
[48] Nielsen JK, Rasmussen HK, Hassager O, McKinley GH. J Rheol 2006;
50(4):453e76.
[49] Hepperle J, Munstedt H. Rheol Acta 2006;45(5):717e27.
[50] Fetters LJ, Lohse DJ, Graessley WW. J Polym Sci Part B Polym Phys
1999;37(10):1023e33.
[51] McKee MG, Park T, Unal S, Yilgor I, Long TE. Polymer 2005;46(7):
2011e5.
[52] Gupta P, Elkins C, Long TE, Wilkes GL. Polymer 2005;46(13):4799e810.
[53] Shenoy SL, Bates WD, Frisch HL, Wnek GE. Polymer 2005;46(10):
3372e84.
[54] Yu JH, Fridrikh SV, Rutledge GC. Polymer 2006;47(13):4789e97.
[55] Limpert E, Stahel WA, Abbt M. BioScience 2001;51(5):341e52.
[56] Tomotika S. Proc R Soc London Ser A 1935;150(870):322e37.
[57] Rayleigh L. Philos Mag 1892;34(207):145.
[58] Funada T, Joseph DD. J Non-Newtonian Fluid Mech 2003;111(2e3):
87e105.
[59] Joseph DD, Beavers GS, Funada T. J Fluid Mech 2002;453:109e32.
[60] Bousfield DW, Keunings R, Marrucci G, Denn MM. J Non-Newtonian
Fluid Mech 1986;21(1):79e97.
[61] Chang HC, Demekhin EA, Kalaidin E. Phys Fluids 1999;11(7):1717e37.