A Paper for Insight 2004
Austin, Texas, USA
Oct. 10-14, 2004
Stretching the Value of Melt Blown with Cellulose Microfiber and Elastic Resins
Rongguo Zhao
Biax Fiberfilm Corporation, Greenville, Wisconsin
Abstract
The melt blowing (MB) process has been popular in making fine fibered articles, such as
filter media, protective clothes, absorbent products and many others. Currently
Polypropylene (PP) is the most used resin. The advantages of MB process attributes to
its simplicity in converting a polymer resin to a variety of fibered products in a single
step. The products of cellulosic microfiber and elastic resins will provide many additional
values for special properties. This paper focuses on the process/property of products
made from wood pulps and elastic resins. The potential applications of these products
will also be discussed.
Introduction
Melt blowing (MB) process has been popular in making fine fibered articles, such
as filter media, protective clothes, absorbent products and many others.
Many
thermoplastics can be used in a MB technology, although polypropylene (PP),
polyethylene (PE), poly(ethylene terephthalate) (PET), poly(butylene terephthalate)
(PBT),
poly(cyclohexane
dimethylene
terephthalate)
(PCT),
co-polyesters
and
polyamides are among the preferred polymers.
The MB technologies today can be roughly classified into two categories: (1)
single-row-drilled-hole type die design, which is also well-know as Exxon design; (2)
multiple-row-concentric-nozzles type spinnerette design, which is also known as
Biax/Schwarz design. The fundamental descriptions of these technologies may be found
in the literatures [1-6].
According to Nonwovens Industry magazine [7], demand of nonwoven roll goods
in the U.S. is expected to increase 3.9% per year to $5 billion in 2007, which is driven by
key markets such as filters, protective apparel, and geotextiles. Approximately 15 % or
more of this demand is melt blown microfibers. Currently, PP holds about 90% of the
MB nonwovens market because of its low cost, ease of processing, good mechanical
properties, negligible shrinkage and chemical inertness.
The industry has been constantly exploring higher potentials of MB process from
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a variety of aspects, including using specialty polymers [8], developing unique fiber and
web structures [1, 9], bicomponent [10, 11], and microfiber composites.
In order to develop unique MB products with premium value, many factors need
to be considered, among which are the targeted end uses, the polymer properties, the
MB equipment characteristics and capabilities, and the environmental impacts.
Recently Calvin Woodings pointed out that the fibers used in nonwovens
industry being almost exclusively non-biodegradable trigged increasingly stronger
concern for the environment among consumers [12]. An "environmentally friendly"
product, such as coverstocks for disposable diapers, is strongly preferred by the
consumers, but does not exist in the market. Research and development efforts at Biax
Fiberfilm Corporation on microfiber cellulose nonwovens are knocking the door for
economical biodegradable disposable products. The process includes preparation of
cellulose solution, extrusion, microfiber spinning, web forming, regeneration, and posttreatments. In the first part of this paper, I will focus on the relationships of processing
conditions and property of products made from a cellulose solution.
Microfibered elastic nonwovens are also triggering significant interests in the
fields like medical fabric, protective clothing, and hygiene products. The second part of
this paper will briefly discuss the properties of elastic nonwoven webs made through the
Biax MB process.
1. Melt Blown Cellulosic nonwovens.
Cellulose from various woods and annual plants can be physically dissolved in Nmethylmorpholine oxide (NMMO) monohydrate under a carefully controlled procedure
[13-15]. The concentration of cellulose in this solution is normally in the range from 6%
to 15% and the rest of the solution is the NMMO hydrate. The amount of water used is
critical. Depending on the concentration of cellulose and the hydration number, the
processing temperature varies from 70ºC to 120ºC. However, the spinning solution over
120ºC is not recommended due to the potentially explosive reaction of the amine oxide
initiating at about 130ºC.
CH2-CH2
O
H2O2 /CO2
N-CH3
CH2-CH2
O
CH2-CH2
N
CH2-CH2
N- Methyl-Morpholine
N- Methyl-Morpholine-N-Oxide
2
O
CH3
1.1 Properties of cellulose/MNNO solution.
The cellulose NMMO solution exhibited a pseudoplastic behavior, i.e. under the
processing temperature, the solution dynamic viscosity decreases with the increase of
shear rate. At the same concentration of cellulose and hydrate number, the solution
made from cellulose with a higher degree of polymerization possesses higher dynamic
viscosity. The concentration of cellulose and hydration number are also affecting
parameters of solution dynamic viscosity. At a proper range, the dynamic viscosity
increases with the cellulose concentration. When this concentration is higher than a
critical number, the solution viscosity starts decreasing. Deformation rate has a great
impact on the behaviors of the solution. It behaves more like a viscous liquid at low
deformation rates and shows more elastic properties at high deformation rates.
Today, cellulosic fibers can be produced by spinning the wood pulp solution of
NMMO into an air gap before entering a coagulation bath. The process is known as dryjet-wet spinning with a solvent recovery of 99.6% in practice. This man-made fiber,
Lyocell, possesses excellent properties for many industrial, apparel, and medical
applications. However, this process is expensive for making fiber alone.
1. 2
Biax Cellulose Melt Blown Technology
A research Team at Biax Fiberfilm Corporation is exploring an economic way of
manufacturing cellulosic nonwovens by utilizing its patented MB technologies, which
features multiple rows of spinning nozzles (up to 200 nozzles per inch die) and
concentric air holes for high productivity, high efficiency and excellent product quality.
The Biax system can continuously spin under high melt pressure with a throughput
capacity 3~5 times higher than a conventional MB system. A special system is
particularly designed based on existing Biax MB technology [6] by taking the rheological
properties of the NMMO solution into consideration. The NMMO cellulose solution is in
solid form at room temperature and becomes a viscous fluid at an elevated temperature.
Its physical behaviors are very similar to a common thermoplastic resin. Therefore, it
may not be improper to keep the known terminology of melt blown for convenience.
1. 2. 1 Experimental Equipment and Process
The experimental MB system set-up is shown in Figure 1, which includes an
extruder, a screen changer, a metering pump, a spinning head, and a web forming
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system. Sensors, transducers, regulators and gauges were installed on the line to
monitor or control the parameters, such as temperature, pressure, speed of extruder, air
compressor, die assembly, collecting/take-up system and coagulant circulating system.
The spinning head consists of heated die body, a filter block, NMMO solution inlet, air
inlet and, coagulant inlet and a spinnerette of multiple-row-spinning nozzles. A collecting
drum with a take-up roller and a coagulating bath were located under the die assembly.
The coagulating solution is supplied to the jet heads by a high-pressure pump from the
reservoir, which is refilled with the circulating coagulant. The high velocity air jets
attenuate the viscous NMMO cellulose solution threads, which are turned into
microfibers by the coagulant jets through mass transfer. The air jets and the coagulant
jets work together to drag, regenerate and entangle the microfibers to form a selfbonded web.
1. 2. 2 Observations and discussions
Concentration of the cellulose-NMMO and the degree of polymerization (DP) of
cellulose significantly affect the rheological properties, which in part determines the
spinnability of the solution. At a certain concentration, solution of higher DP cellulose
exhibits higher viscosity and more elastic property. This would result in great difficulties
of fiber attenuation and web forming. A specially designed spinnerette is under
construction to handle high DP cellulose-NMMO solutions.
We have successfully made microfiber webs from a relatively low DP celluloseNMMO solution with our 15-inch pilot machine, which is equipped with a 4-row-nozzle
spinnerette. A typical process condition for a relatively low DP cellulose-NMMO solution
is presented in Table 1. Handling the cellulose-NMMO solution for extrusion starting with
a solid stage can be challenging. Due to its extremely hydrophilic nature, the ground
cellulose/NMMO solid solution tends to pickup moisture quickly and creates feeding
problems. A Continuous supply of a small quantity of Argon to the hopper bottom has
proven very helpful.
Regeneration of cellulose from its NMMO solution is one of the critical steps to
ensure desirable web and fiber properties. During the process, the attenuated filaments
interact with each other due to air turbulence. A water (non-solvent) spray to the fiber
stream will partially regenerate the filament surface, which significantly reduces the
interfusion of the filaments while touching each other. To study this effect, the coagulant
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jets in the spinnerette were turned off, when needed, with water spray nozzles installed
in both sides of the fiber stream.
Table 1. The processing conditions and size of the Cellulose Melt blown fibers
Polymer
Cellulose-NMMO solution (DP=330~420, 12.5%wt)
Extruder
T zone 1 =165F; T zone 2 =198F; T zone 3 =230F; T clamp = 230F
Screen changer
T Sc Ch = 230F
Metering Pump
T pump =230F
Die block
T melt =230F; P melt =650 psi; T air =250F; P air =14 psi
Water jet
P=150psi
DCD
29 ~ 51 cm
Fiber size
4 ~ 25 m
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1
2
3
4
5
8
6
1: extruder
2: screen changer
3: gear pump
4: air manifold
5: die block
6: spinnerette
7: air inlet
8: coagulant inlet
9: collector
10: hydro-jets
11: nip rollers
10
9
5
11
Figure 1. Experimental set-up for making microfiber cellulosic nonwovens
Figure 2 shows the effect of water spray on web structure. Since the attenuated
filaments (strands of cellulose-NMMO solution) are in a sticky state, without water spray
during MB, the filaments merge into one another at any contact points. Several fibers
may combine together to form big segments, as shown in Figure 2a. The resulting
products are stiff and brittle due to over bounding among fibers and incomplete
regeneration of the big fibers/segments.
Having water sprays in both sides of the MB filament stream at a proper location
of the spin line is proven essential for making valuable products. Spraying water on to
the web forming area on the collector surface helps the filaments to reduce bonding,
which results in softer products. Although the amount of combined big segments of
multiple fibers is reduced, the fused intersections remain, as shown in Figure 2b.
In a typical melt blown process, the fibers diameter reduces dramatically within
the first few centimeters from the spinnerette. It continues to decrease in the next few
centimeters with a much lower attenuation rate [1, 2]. The length of this attenuation
distance varies depending on the type of polymer, processing conditions, and
spinnerette settings [8]. Having this in mind, one will carefully apply water spray to a
location in the spin line where filament interactions are still not significant. At the
processing conditions listed in Table 1, the two-sided water spray is applied to the fiber
stream at 18 cm from the spinnerette. As shown in Figure 2c, the fibers are much
smaller and the merged segments of several fibers are considerably decreased.
Comparing Figure 2a, 2b, and 2c, one may reach a conclusion that water spray at a
proper location enhances fiber attenuation and prevents fibers from combining into big
fiber segments.
Figure 2d is a SEM microphotograph of cellulosic microfibers made from the
same equipment with coagulant jets instead of water spray. The coagulating jets have
the following major functions: (1) regenerating the cellulose from its NMMO solution into
Lyocell microfiber, (2) attenuating the filaments, and (3) entangling the microfibers to
form nonwoven web. These powerful jets minimize inter-fiber fusion and reduce the
formation of big fiber segments. The fiber size ranges from 4 m to 25 m with an
average of 14 m. The webs exhibit nice softness, good strength, and excellent
wetability.
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1.2.3
Potential Applications
The cellulose microfiber nonwovens have the following unique properties, (1)
based-on renewable natural recourses, such as woods and some annual plants, (2)
biodegradable, (3) high temperature endurance, (4) excellent strength, (5) antistatic, and
(6) to be colored readily. They will find various applications in a broad spectrum. The
following are some examples of these potential applications
Baby diapers
Battery separators
Feminine hygiene napkins
Vacuum cleaner bags
Adult continental products
Cosmic pads
Bandages
Cabin Air Filtration in Cars and Planes
Air filters
Food Packaging
Liquid filters
Apparels
Household wipers
Disposable underwear
Shop towels
1.2.4 Future Research Plans
Based on our previous research, a new prototype spinnerette has been
designed. It will be fabricated and assembled with other processing components to
develop a complete MB pilot line. By using this pilot line, at least three wood pulps with
degrees of polymerization (DP) of 400~1000 and different solutions will be thoroughly
investigated.
Great efforts of the present project will focus on productivity and efficiency. The
other issues include interactions among air, filaments, and coagulation solution, and
their effects on the properties of fibers and the nonwoven web. Specifically, we will (1)
identify the operation windows for given solution compositions, (2) improve the melt
blowing process, (3) establish the processing/property relationships (4) identify key
issues requiring resolution during the technology commercialization.
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a
b
c
d
Figure 2 Cellulose MB microfibers
(a) Without water spray during MB; (b) Spray at the point of web-forming;
(c) Spray onto the spin line; (d) water-jetting during MB
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2. Microfiber nonwovens with elastic resins
Although thermally treated and stretched PP nonwoven web can achieve one
dimensional elasticity, nonwovens from a elastic resin are attracting more interests for a
variety of applications, including hygiene, medical, apparel, and personal care products.
Via a bicomponent spunbond technology, elastic nonwoven fabrics have been produced
with an extreme low sheath/core ratio [16]. The very thin sheath layer of non-elastomer
is activated during fiber attenuating process and form a corrugated fiber surface after lay
down, which is important to achieve the elasticity of the core. This thin layer of nonelastomer covers the rubbery sheath to ensure a superior hand. Elastic MB nonwovens
are also under investigations by different researchers [17,18].
Thermoplastic polyurethane (TPU) elastomer is one of the popular resins to
make elastic fibers. Melt blown grade TPU is available in the market from a few
manufacturers. Other examples of resins available for MB process include polyetherester (PEE) elastomer, polyolefin elastomer and thermoplastic rubber.
Spinning these elastomers into fibers by a MB process is challenging in
comparison to PP melt blowing. Our experience taught us that drying is a critical step for
a successful TPU MB production. However, the selection of MB machine has a
significant impact on the web quality and their performances. Figure 3 shows the major
difference between a Biax design and a conventional design of MB technologies. Biax
MB technology features multiple rows of spinning nozzle with individual concentric air
jets to attenuate the fibers. It also tolerates high melt pressures at the spinnerette with a
wide operation window ranged from 300 PSI to 2000 PSI. A conventional MB technology
has a single row of spinning holes with impinging air streams from both sides of the die
tip to draw the fibers. The safe operation pressure of this process is about 300 PSI. This
will set the two technologies apart when trying to melt blow elastomers. Currently
available elastomers have melt flow rates mostly lower than 100 compared to common
MB grade resins that have melt flow rates higher than 300. In other words, one will
expect high melt pressure during melt blowing of these elastomers. Therefore, the
advantage of Biax MB technology is obvious over the other type.
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Figure 3, A comparison between two major MB technologies
The elasticity of the extruded filaments increases the filament turbulence during
MB process, which increases the tendency to produce merged fiber bundles. Figure 4
are the scanning electron microscopy images showing the PEE fiber bundles made from
a conventional MB process. Although fiber bundles or ropes are commonly observed for
MB PP and other semicrytalline polymers, under proper processing conditions, the fibers
are individual fibers in the bundles. In case of elastomer, fibers in a bundle are combined
and partially fused to each other, as shown in Figure 4b. As noted above, currently
available elastomers have relatively low melt flow rates. For a conventional MB line, high
temperature profiles are frequently used to reduce the high melt pressure at the die tip.
The heat from both melt and air keeps the attenuating and elastic filaments tacky for a
longer time, which causes a significant amount of fusion of entangled fibers.
With a Biax MB line, a lower temperature profile is normally employed and the
operation pressure at the spinnerette can be above 1500PSI. This allows the fiber
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surface cool enough before significant fiber entanglement occurs. The individual
concentric air jets apply friction forces on to filament surfaces parallel with the air
blowing direction, which permit filaments to travel longer distance without touching one
another. In addition, the spacing between Biax spinning nozzles is inherently bigger than
that in a conventional die. Therefore, the elastic web made from a Biax MB line exhibits
a structure similar to a normal PP MB web. The fused fiber bundles are significantly
reduced, as shown in Figure 5. This web structure allows the best fiber coverage,
enhances barrier properties, and ensures good elasticity in all directions, which are
critical for many end applications, such as protective apparel, medical and hygiene
products.
Conclusions:
With the increased environmental concerns on thermoplastic disposable
products, the nonwovens industry starts making efforts to develop fully biodegradable
products from biodegradable PLA, biodegradable polyester to biodegradable Lyocell.
Although Lyocell fibers are commercialized and the output increased lately, the cost of
this fiber is still too high for nonwoven disposable products. Making Lyocell nonwoven
roll goods from a one-step MB type process with high throughput and high efficiency will
open a door of hope for the industry to use this excellent fiber. Our R&D efforts in this
area are getting financial support from the Wisconsin Department of Commerce [19]. A
15-inch MB-type pilot line equipped with a multiple-row-spinning-hole spinnerette will be
built for extensive investigations focusing on operation windows,
process/structure/property relationships, and economic models of the process.
The nonwovens industry can also take good advantages of Biax MB technology
to produce elastic microfiber products. The concentric multiple-row-nozzle spinnerette
offers the industry high productivity, effectiveness, flexibility and energy saving. We have
made elastic microfiber webs from several elastomers, including TPU, PEE, EVA
polyolefin elastomer and thermoplastic rubber. Resulting products from a Biax MB line
showed superior web properties with higher productivity.
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a
b
Figure 4. Microphotograph of PEE fibers made from a conventional MB process
(a) fiber bundles, 200X; (b)cross-section of a fiber bundle, 700X
Figure 5. Microphotograph of elastic fibers made from a Biax MB process
(a) PEE elastomer, SEM, 500X, (b) Polyolefin elastomer, Optical, 100X
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