Ramaiah Kotra & Haoming Rong
Melt blowning is a process for producing fibrous webs or
articles directly from polymers or resins using high-velocity air or another
appropriate force to attenuate the filaments. The melt-blown process
is one of the newer and least developed nonwoven processes. This process is
unique because it is used almost exclusively to produce microfibers rather
than fibers the size of normal textile fibers. Melt-blown microfibers
generally have diameters in the range of 2 to 4
: m, although they may be as small as
0.1: m and as large as 10 to 15
: m. Differences between melt-blown
nonwoven fabrics and other nonwoven fabrics, such as degree of softness,
cover or opacity, and porosity can generally be traced to differences in
The basic technology to produce these microfibers was
first developed under U.S. government sponsorship in the early 1950s. The
Naval Research Laboratory initiated this work to produce microfilters for
the collection of radioactive particles in the upper atmosphere. The
significance of this work was recognized by an Exxon affiliate and a
development program was initiated in the middle 1960s. Five years later, a
patented prototype model successfully demonstrated the production of
microfibers. At present, Exxon has developed most of the licenses and/or
options to produce microfiber nonwoven and melt-blown equipment.
In the past 20 years there has been some activity outside
of the Exxon technology and patents obtained by companies such as 3M. The
company, 3M, has developed processes for making microfibers and blends of
microfibers with textile denier fibers that were apparently beyond Exxon
patents. Exxon has continued to aggressively support melt-blown R&D through
the years. The major portion of the Exxon-supported effort is now being
conducted at the University of Tennessee, Knoxville. Some other North
American major meltblown producers are Eastman Kodak, Hollingsworth and Vose,
Kimberly-Clark, 3M, Fleetguard Filter.
The most commonly accepted and current definition for the
melt-blown process is: a one-step process in which high-velocity air blows a
molten thermoplastic resin from an extruder die tip onto a conveyor or
takeup screen to form a fine fiberous and self-bonding web.
The melt-blown process is similar to the spunbond process
in that both convert resins to nonwoven fabrics in a single integrated
process. The schematic of the melt blowing process is shown in Figure 1. A
typical melt blowing process consists of the following elements: extruder,
metering pumps, die assembly, web formation, and winding.
The polymer pellets or granules are fed into the extruder
hopper. Gravity feed supplies pellets to the screw, which rotates within the
heated barrel. The pellets are conveyed forward along hot walls of the
barrel between the flights of the screw, as shown in Figure 2. As the
polymer moves along the barrel, it melts due to the heat and friction of
viscous flow and the mechanical action between the screw and barrel. The
screw is divided into feed, transition, and metering zones. The feed zone
preheats the polymer pellets in a deep screw channel and conveys them to the
transition zone. The transition zone has a decreasing depth channel in order
to compress and homogenize the melting polymer. The molten polymer is
discharged to the metering zone, which serves to generate maximum pressure
for extrusion. The pressure of molten polymer is highest at this point and
is controlled by the breaker plate with a screen pack placed near the screw
discharge, as shown in Figure 2. The screen pack and breaker plate also
filter out dirt and infused polymer lumps. The pressurized molten polymer is
then conveyed to themetering pump.
2.2. Metering Pump
The metering pump is a positive-displacement and
constant-volume device for uniform melt delivery to the die assembly. It
ensures consistent flow of clean polymer mix under process variations in
viscosity, pressure, and temperature. The metering pump also provides
polymer metering and the required process pressure. The metering pump
typically has two intermeshing and counter-rotating toothed gears. The
positive displacement is accomplished by filling each gear tooth with
polymer on the suction side of the pump and carrying the polymer around to
the pump discharge, as shown in Figure 3. The molten polymer from the gear
pump goes to the feed distribution system to provide uniform flow to the die
nosepiece in the die assembly (or fiber forming assembly).
2.3. Die Assembly
The die assembly is the most important element of the
melt blown process. It has three distinct components: polymer-feed
distribution, die nosepiece, and air manifolds.
The feed distribution in a melt-blown die is more
critical than in a film or sheeting die for two reasons. First, the
melt-blown die usually has no mechanical adjustments to compensate for
variations in polymer flow across the die width. Second, the process is
often operated in a temperature range where thermal breakdown of polymers
proceeds rapidly. The feed distribution is usually designed in such a way
that the polymer distribution is less dependent on the shear properties of
the polymer. This feature allows the melt blowing of widely different
polymeric materials with one distribution system. The feed distribution
balances both the flow and the residence time across the width of the die.
There are basically two types of feed distribution that have been employed
in the melt-blown die: T-type (tapered and untapered) and coat hanger type.
Presently, the coathanger type feed distribution is widely used because it
gives both even polymer flow and even residence time across the full width
of the die.
From the feed distribution channel the polymer melt goes
directly to the die nosepiece. The web uniformity hinges largely on the
design and fabrication of the nosepiece. Therefore, the die nosepiece in the
melt blowing process requires very tight tolerances, which have made their
fabrication very costly. The die nosepiece is a wide, hollow, and tapered
piece of metal having several hundred orifices or holes across the width.
The polymer melt is extruded from these holes to form filament strands which
are subsequently attenuated by hot air to form fine fibers. In a die's
nosepiece, smaller orifices are usually employed compared to those generally
used in either fiber spinning or spunbond processes. A typical die nosepiece
has approximately 0.4-mm diameter orifices spaced at 1 to 4 per millimeters
(25 to 100 per inch).
There are two types of die nosepiece used: capillary type
and drilled hole type. For the capillary type, the individual orifices are
actually slots that are milled into a flat surface and then matched with
identical slots milled into a mating surface. The two halves are then
matched and carefully aligned to form a row of openings or holes as shown in
Figure 4. By using the capillary type, the problems associated with precise
drilling of very small holes are avoided. In addition, the capillary tubes
can be precisely aligned so that the holes follow a straight line
accurately. The drilled-hole type has very small holes drilled by mechanical
drilling or electric discharge matching (EDM) in a single block of metal, as
shown in Figure 4.
During processing, the whole die assembly is heated
section-wise using external heaters to attain desired processing
temperatures. It is important to monitor the die temperatures closely in
order to produce uniform webs. Typical die temperatures range from 2l5oC
The air manifolds supply the high velocity hot air (also
called as primary air) through the slots on the top and bottom sides of the
die nosepiece, as shown in Figure 5. The high velocity air is generated
using an air compressor. The compressed air is passed through a heat
exchange unit such as an electrical or gas heated furnace, to heat the air
to desired processing temperatures. The exits from the top and bottom sides
of the die throughnarrow air gaps, as shown in Figure 5. Typical air
temperatures range from 230oC to 360oC at velocities
of 0.5 to 0.8 the speed of sound.
2.4. Web Formation
As soon as the molten polymer is extruded from the die
holes, high velocity hot air streams (exiting from the top and bottom sides
of the die nosepiece) attenuate the polymer streams to form microfibers. As
the hot air stream containing the microfibers progresses toward the
collector screen, it draws in a large amount of surrounding air (also called
secondary air) that cools and solidifies the fibers, as shown in Figure 5.
The solidified fibers subsequently get laid randomly onto the collecting
screen, forming a self-bonded nonwoven web. The fibers are generally laid
randomly (and also highly entangled) because of the turbulence in the air
stream, but there is a small bias in the machine direction due to some
directionality imparted by the moving collector. The collector speed and the
collector distance from the die nosepiece can be varied to produce a variety
of melt-blown webs. Usually, a vacuum is applied to the inside of the
collector screen to withdraw the hot air and enhance the fiber laying
The melt-blown web is usually wound onto a cardboard core
and processed further according to the end-use requirement. The combination
of fiber entanglement and fiber-to-fiber bonding generally produce enough
web cohesion so that the web can be readily used without further bonding.
However, additional bonding and finishing processes may further be applied
to these melt-blown webs.
Additional bonding, over the fiber adhesion and fiber
entanglement that occurs at lay down, is employed to alter web
characteristics. Thermal bonding is the most commonly used technique. The
bonding can be either overall (area bonding) or spot (pattern bonding).
Bonding is usually used to increase web strength and abrasion resistance. As
the bonding level increases, the web becomes stiffer and less fabric-like.
Although most nonwovens are considered finished when they
are rolled up at the end of the production line, many receive additional
chemical or physical treatment such as calendering, embossing, and flame
retardance. Some of these treatments can be applied during production, while
others must be applied in separate finishing operations.
3. Process Variables
Process variables can be classified into two categories:
operational or on-line variableS and off-line variables. The on line
variables include: polymer and its throughput, polymer/die and air
temperature, die-to-collector distance, and quench environments. The
off-line variables includes: hole size, die setback, air gap, air angle, web
collection type, and polymer/airdistribution. The following represent some
of the variables that must be controlled during melt blown production.
-Polymer characteristics: molecular weight, melt viscosity, melt strength
-Extruder conditions: temperature, shear, polymer degradation
-Die tip geometry: hole diameter, air gap, die tip position
-Hot air conditions: volume, temperature, velocity
-Polymer conditions: temperature, flow rate, shear rate
-die conditions: temperature profile, gas flow rate profile, polymer flow
-Ambient air conditions: temperature, lack of turbulance
-distance from the die to the forming drum or belt
Some efforts have been made to reduce the above variables to a few
4. Web Characteristics and Properties
The uniformity of the web is controlled by two important
parameters: uniform distribution of fiber in the air stream and proper
adjustment of the vacuum level under the forming wire or belt. Non-uniform
distribution of fiber in the air stream can result from poor die design and
from non-uniform ambient airflow into the air stream. The vacuum under the
forming media should be adjusted to pull the total air stream through the
media and lock the fibers in place. Generally, the closer the die is to the
forming drum or belt, the better the web uniformity.
Melt-blown webs usually have a wide range of product
characteristics. The main characteristics and properties of melt-blown webs
are as follows:
1.Random fiber orientation
2.Lower to moderate web strength, deriving strength from mechanical
entanglement and frictional forces
3.Generally high opacity (having a high cover factor)
4.Fiber diameter ranges from 0.5 to 30 :
m, but typically from 2-7 : m
5.Basis weight ranges from 8-350 g/m2 , but typically 20-200
6.Microfibers provide a high surface area for good insulation and
7.Fibers have a smooth surface texture and are circular in cross-section
8.Most melt-blown webs are layered or shingled in structure, the number
of layers increases with basis weight
The fiber length in a melt-blown web is variable; it can
be produced in the range from a few millimeters to several hundred
centimeters in length and usually exists over a broad range. The fiber
cross-section is also variable, ranging from circular to a flat
configuration and other variations.
Three of the major defects that occur in melt-blown
production are roping, shot, and fly. Roping is caused by uncontrolled
turbulence in the air-stream and by movement of fibers during and after
laydown. The defect is observed as a narrow, elongated, thick streak in the
web and resembles a slightly twisted "rope". Shot are small, spherical
particles of polymer formed during the blowing operation. Shot are generally
caused by excessively high temperatures or by too low a polymer molecular
weight. Fly is a defect that does not go directly into the web, but instead
contaminates the surrounding environment. Fly is composed of very short and
very fine microfibers not trapped on the drum or belt during laydown. This
can be caused by too violent blowing conditions.
The web structure in a melt-blown product is essentially
isotropic. This is not surprising since web formation is an air lay process.
This means that the fibers have a random distribution in terms of the
machine direction (MD) and cross direction (CD). As a result, the physical
properties will normally also be isotropic. If desired, the fiber
orientation in the web can be skewed by the use of selected processing
A study done by Malken et al. (2) showed that polymer
throughput of PP had a noticeable effect on the physical properties of
resultant webs. The mean fiber diameter, tensile strength, initials modulus,
stiffness and web density increase with increasing throughput. However, the
decrease in both breaking strain and the energy required to break indicates
the brittle nature of the web produced at higher throughput. Increasing
fiber diameter was attributed to die swell and change in polymer-to-air
ratio for a given airflow rate. Increase in airflow rate didn't result in
any significant change in average fiber diameter. The die orifice size had
only minimal effects on the average fiber diameter.
The fibers that result in melt-blown webs are usually
microfibers. The average fiber diameter can be controlled by the specific
resin employed and the processing conditions selected. A typical microfiber
can be as fine as 2 : m in diameter and
less for some special applications. With such fiber fineness, the number of
fibers per unit weight is greatly increased. Further, the amount of fiber
surface exposed is also substantially greater than that exposed in
conventional webs. As a consequence, these characteristics can have a
significant impact in a variety of product applications.
6. Polymer Type
The type of polymer or resin used will define the
elasticity, softness, wetability, dyeability, chemical resistance and other
related properties of formed fibers. One of the advantages of melt-blown
technology is to handle many different polymers as well as mixture of
polymers. Some polymers, which can be melt-blown, are listed below. However,
the list is not complete.
1.Polypropylene is easy to process and makes good web.
2.Polyethylene is more difficult to melt-blow into fine fibrous webs than
is polypropylene. Polyethylene is difficult to draw because of its melt
3.PBT processes easily and produces very soft, fine-fibered webs.
4.Nylon 6 is easy to process and makes good webs.
5.Nylon 11 melt-blows well into webs that have very unusual leather like
6.Polycarbonate produces very soft-fiber webs.
7.Poly (4-methyl pentene-1) blows well and produces very fluffy soft
8.Polystyrene produces an extremely soft, fluffy material with
essentially no shot defects.
The most widely used polymer that has a high MFR is
polypropylene. Polypropylene with its low viscosity has a low melting point
and is easy to draw into fibers. It comprises 70-80% of the total North
American production (1).
The feasibility of melt-blowing original and recycled PET
has also been studied (3). PET webs have a strong tendency to shrink,
depending on the airflow rate used. PET webs produced at high airflow rate
shrink more than those produced at low airflow rate because of their higher
level of molecular orientation. Heat-setting of melt-blown PET webs or,
alternatively, the use of PBT (poly-butylene terephalate) was suggested as a
possible means of producing thermally stable melt-blown PET nonwovens.
7. Major Process Modifications
In the case of melt-blown web, additional process
modifications can provide excellent flexibility in terms of designing new
and novel products. Composite formation has been used extensively to
supplement the limited physical strength of normal melt-blown webs and also
to provide enhancement of other properties. This feature has been very
effectively exploited in SMS structures based on a three-ply system
consisting of Spunbond/Melt-blown/Spunbond plies to give an enhanced
composite structure, supplementing the fiber and web properties of the melt
blown with the strength and toughness of the spunbond surface fabrics.
Lamination with a variety of other sheets and webs has also extended the
range of properties achievable with melt-blown systems.
The combination of melt-blown fibers with other fibers
has also been used effectively to design and enhance products. The most
notable success of this is the Coform structures produced by Kimberly-Clark
Corporation. These structures involve an intimate blend of melt blown fibers
and short woodpulp fibers. By varying the ratio of the two fiber feeds, a
broad range of products can be produced. Such Coform structures are often
combined with a spunbond fabric on one or both surfaces to provide
additional product versatility. Blends with other fiber types have also been
produced, such as 3M's Thinsulate blends of polypropylene (PP) or polyester
staple with melt-blown webs. In addition, the injection of solid materials,
especially super-absorbent resins, has provided another range of interesting
and useful products based on melt blown technology.
The melt-blown system is unique because the process
generates a fine fiber not available to the other nonwoven processes.
Micro-denier fiber (less than 0.1 denier per filament) is not really
available as a nonwoven fibrous raw material. Hence, the melt-blown process,
which can produce such a fiber, opens new vistas of products and
applications. At the present time, the following market segments are
successfully served by melt-blown products:
This market segment continues to be the largest single
application. The best known application is the surgical face mask filter
media. The applications include both liquid filtration and gaseous
filtration. Some of them are found in cartridge filters, clean room filters
The second largest meltblown market is in
medical/surgical applications. The major segments are disposable gown and
drape market and sterilization wrap segment.
Meltblown products are used in two types of sanitary
protection products - feminine sanitary napkin and disposable adult
incontinence absorbent products.
Melt blown materials in variety of physical forms are
designed to pick up oily materials. The best known application is the use of
sorbents to pick up oil from the surface of water, such as encountered in an
accidental oil spill.
The apparel applications of melt-blown products fall into
three market segments: thermal insulation, disposable industrial apparel and
substrate for synthetic leather. The thermal insulation applications takes
advantage of microvoids in the structure filled with quiescent air,
resulting in excellent thermal insulation.
The melt-blown process has a special feature: it can
handle almost any type of thermoplastic material. Thus, the task of
formulating a hot-melt adhesive to provide specific properties can be
greatly simplified by using the melt blown system to form the final uniform
Two major applications exist in the electronics
specialities market for melt blown webs. One is as the liner fabric in
computer floppy disks and the other as battery seperators and as insulation
Interesting applications in this segment are manufacture
of tents and elastomeric nonwoven fabrics which have the same appearance as
continuous filament spunbonded products.
9. Complexity of Melt Blown Technology
The melt-blown process is a complex one that involves
turbulence, which is poorly understood by the scientist even today.
Isolation of experimental factors is difficult because of highly variable
interaction. The multi-filament environments and factors such as humidity of
the processing room and quench air temperature wildly change boundary
The processing window for successfully melt-blowing
polymers is very limited. In order to produce webs with acceptable quality,
one has to be in the right range of process parameters and the range varies
between polymers. Unlike melt-spinning, there is almost virtually no control
over the individual filaments in melt-blowing. It is very difficult to
predict structure/property of melt-blown filaments since the isolation of
variables is very difficult in a multi-filament environment.
Melt-blown products are difficult to compare to other nonwoven products
because they are quite different in nature and function. Most nonwoven
fabrics are designed to function similarly to woven or knit fabrics and
generally can be replaced with such fabrics, although usually at a
performance and /or financial penalty. With limited exceptions such as some
wipes, melt-blown products are not designed to function as fabrics. They are
generally manufactured in sheet form but lack the physical strength of
conventional woven or nonwoven fabrics.
On the other hand, despite extensive research and development in this
area, there is a paucity of published research studies, mainly due to the
secretive and competitive nature of the work. However, there is considerable
patented literature available.
10. Melt-blown Vs Spunbond
The spunbond and melt-blown processes are somewhat
identical from an equipment and operator's point of view. The two major
differences between a typical melt-blown process and a spunbond process that
uses air attenuation are: (1) the temperature and volume of the air used to
attenuate the filaments and (2) the location where the filament draw or
attenuation force is applied.
A melt-blown process uses large amounts of high-temperature air to
attenuate the filaments. The air temperature is typically as high or higher
than the temperature of the polymer. In contrast, the spunbond process
generally uses a smaller volume of air close to ambient temperature to apply
the attenuation force.
In the melt-blown process, the draw or attenuation force
is applied at the die tip while the polymer is still in the molten state.
Application of the force at this point is ideal for forming microfibers but
does not allow for polymer orientation to build good physical properties. In
the spunbond process, this force is applied at some distance from the die or
spinneret, after the polymer has been cooled and solidified. Application of
the force at this point provides the conditions necessary for polymer
orientation and the resultant improved physical properties, but is not
conductive to forming microfibers.
11. Process Equipment
Although the melt-blown process is conceptually simple,
high-quality webs at commercial scale require precisely designed and
fabricated equipment. In a manner similar to spunbond technology, many
melt-blown web-manufacturing companies such as3M and Freudenberg have
developed proprietary technology. Most of the melt-blown processes in the
market now are based on the Exxon process. The process equipment layout,
which can be vertical or horizontal, is simpler and more compact than that
of spunbonding. A vertical layout is preferred when multiple dies are used,
but horizontal layout is preferred when a single die is used. The vertical
space requirement, usually a minimal of 20 ft., depends on the
die-to-collector distance. The horizontal space requirements depend on the
total width of the die and end product requirement. Usually three times the
vertical space is the minimal requirement for a horizontal space (1). In
summary, a schematic follows that shows components of a complete melt
12. Economics of Melt-blown Webs
The economics of melt-blown process is influenced by many
factors such as energy, capital investment, and production speed conversion.
With respect to energy, the melt-blown process requires more energy than
does the spunbond process. A typical melt blowing process consumes about 7-8
kWh/kg of polymer process, while a typical spunbond process consumes 2-3
kWh/kg. Melt-blown processing is more energy-intensive because of compressed
hot air is used for fiber attenuation. About 70% of total energy used for
hot air. This result in a high production cost. Typically, a 2.0-oz
PPspunbonded web cost US $0.12 to $0.24/yd2, while a melt-blown
equivalent is $0.32-$0.37/ yd2 (1).
Initial capital investment of a melt-blown line is much lower than that
of spunbond line. Typically, the later is 3-4 times higher than the former.
But the production speed of spunbonding is inherently faster than that of
13. The potential for meltblowing
The melt blown technique for making nonwoven products has
been forecast in recent years as one of the fastest-growing in the nonwovens
industry. With the current expansion and interest, it cannot be questioned
that meltblown is well on its way to becoming one of the major nonwoven
technologies. Technical developments are also on the horizon that will
increase the scope and utility of this technology. The application of
speciality polymer structures will no doubt offer new nonwoven materials
unobtainable by other competitive technologies. The considerable work to
modify the blowing step to something more akin to spraying is also going to
have an impact on this technology and the products derived from it. So a
strong and bright future be forecasted for this technology.
Malkan, S., Tappi Journal, Vol.78, No.6, pp185-190, 1995.
Malkan, S.R. and Wadsworth, L.C., IND JNR, No.2, pp21-23,1991.
Bhat, G.S., Zhang, Y., and Wadsworth, L.C., Processing of the Tappi
Nonwoven Conference, Macro Island, FL, May,
Vasanthakumar, N., Dissertation, Dimensional Stability of Melt-blown
Nonwovens. The University of Tennessee,
Additional Sources of Information:
Malkan, S. R., PH.D Dissertation, The University of Tennessee,
Vargas, E., Meltblown Technology Today, Miller Freeman Publications
Inc., San Francisco, CA, 1989.
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