Note: Descriptions are shown in the official language in which they were submitted.
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Description
ORIENTED MELT-BLOWN FIBERS, PROCESSES FOR MAKING SUCH
FIBERS, AND WEBS MADE FROM SUCH FIBERS
This application is divided out of parent application
Serial No. 584,310 filed on November 28th, 1988.
The parent application relates to a combination of a
nonwoven fabric comprising oriented microfibers and crimped
staple fibers. The present divisional application relates to a
nonwoven fabric comprising oriented microfibers and to a method
of preparing microfibers.
Technical Field
The invention is directed to melt-blown fibrous webs,
i.e., webs prepared by extruding molten fiber-forming material
through orifices in a die into a high-velocity gaseous stream
which impacts the extruded material and attenuates it into fibers,
often of microfiber size averaging on the order of 10 micrometers
or less.
Background Art
During the over twenty-year period that melt-blown
fibers have come into wide commercial use there has always been a
recognition that the tensile strength of melt-blown fibers was
low, e.g., lower than that of fibers prepared in conventional
melt-spinning processes (see the article "Melt-Blowing -- A One-
Step Web Process For New Nonwoven Products", by Robert R. Buntin
and Dwight D.Lohkcamp, Volume 56, No. 4, April 1973, Tappi,
page 75, paragraph bridging columns 2 and 3). At least as late
as 1981, the art generally doubted "that melt-blown webs, per se,
will ever possess the strengths associated with conventional
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nonwoven webs produced by melt-spinning in which fiber
attenuation occurs below the polymer melting point bringing about
crystalline orientation with resultant high fiber strength" (see
the paper "Technical Developments In The Melt-Blowing Process And
Its Applications In Absorbent Products" by Dr. W. John McCulloch
and Dr. Robert A. VanBrederode presented at Insight ''81,
copyright Marketing/Technology Service, Inc., of Kalamazoo, MI,
page 18, under the heading "Strength").
The low strength of melt-blown fibers limited
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the utility of the fibers, and as a result there have been
various attempts to combat this low strength. One such
effort is taught in Prentice, U.S. Pat. 3,704,198, where a
melt-blown web is "fuse-bonded," as by calendering or
5 point-bonding, at least a portion of the web. Although
web strength can be improved somewhat by calendering,
fiber strength is left unaffected, and overall strength is
still less than desired.
Other prior workers have suggested blending
10 high-strength bicomponent fibers into melt-blown fibers
prior to collection of the web, or lamination of the melt-
blown web to a high strength substrate such as a spunbond
web (see U.S. Pats. 4,041,203; 4,302,495; and 4,196,245).
Such steps add costs and dilute the microfiber nature of
the web, and are not satisfactory for many purposes.
McAmish et al, U.S. Pat. 4,622,259, is directed
to melt-blown fibrous webs especially suitable~for use as
medical fabrics and said to have improved strength. These
webs are prepared by introducing secondary air at high
20 velocity at a point near where fiber-forming material is
extruded from the melt-blowing die. As seen best in
Figure 2 of the patent, the secondary air is introduced
from each side of the stream of melt-blown fibers that
leaves the melt-blowing die, the secondary air being
introduced on paths generally perpendicular to the stream
of fibers. The secondary air merges with the primary air
that impacted on the fiber-forming material and formed the
fibers, and the secondary air is turned to travel more in
a direction parallel to the path of the fibers. The
30 merged primary and secondary air then carries the fibers
to a collector. The patent states that by the use of such
secondary air, fibers are formed that are longer than
those formed by a conventional melt-blowing process and
which exhibit less autogeneous bonding upon fiber
collection; with the latter property, the patent states it
has been noted that the individual fiber strength is
1 3 3 7 1 5n 60557-3528D
higher. Strength is indicated to be dependent on the degree of
molecular orientation and it is stated (column 9, lines 21-27)
that the
high velocity secondary air employed in the present
process is instrumental in increasing the time and
distance over which the fibers are attenuated.
The cooling effect of the secondary air enhances
the probability that the molecular orientation of
the fibers is not excessively relaxed on the
deceleration of the fibers as they are collected
on the screen.
Fabrics are formed from the collected web by embossing the webs
or adding a chemical binder to the web, and the fabrics are
reported to have higher strengths, e.g., a minimum grab tensile
strength to weight ratio greater than 0.8 N per gram per square
meter, and a minimum Elmendorf tear strength to weight ratio
greater than 0.04 N per gram per square meter.
Even if the fibrous webs of U. S. Patent 4,622,259
have increased strengths, those strengths are still less than
should ultimately be obtainable from the polymers used in the
webs. Fibers made from the same polymers as those of the webs
taught in U. S. Patent 4,622,259, but made by techniques other
than the melt-blown techniques of the patent, have greater
strengths than the strengths reported in the patents.
Disclosure of Invention
According to one aspect of the invention of the parent
application there is provided nonwoven fabric comprising
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oriented microfibers having an average diameter of about 10
micrometers or less and crimped staple fibers blended with the
microfibers to form a coherent handleable lofty resiliently
compressible web.
According to a further aspect of the invention of the
parent application there is provided nonwoven fabric comprising
oriented melt-blown fibers that have a crystalline orientation
function of at least about 0.5 and nonoriented randomly
entangled melt-blown fibers, the oriented and nonoriented
fibers being blended together as a coherent handleable web.
According to one aspect of the invention of the present
divisional application there is provided nonwoven fabric compris-
ing a bonded web of oriented melt-blown polyolefin fibers having
a minimum machine-direction grab tensile strength to weight
ratio greater than 1.5 Newton per gram per square meter, and
having a minimum machine-direction Elmendorf tear strength to
weight ratio greater than 0.1 Newton per gram per square meter.
According to a further aspect of the invention of the
present divisional application there is provided nonwoven fabric
comprising a bonded web of oriented melt-blown nylon fibers
having a minimum machine-direction grab tensile strength to
weight ratio greater than 2.5 Newton per gram per square meter,
and having a minimum machine-direction Elmendorf tear strength
to weight ratio greater than 0.25 Newton per gram per square
meter.
According to another aspect of the invention of the
present divisional application there is provided nonwoven fabric
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comprising a bonded web of oriented melt-blown polyethylene
terephthalate fibers having a minimum grab tensile strength to
weight ratio greater than 2.5 Newtons per gram per square meter,
and having a minimum Elmendorf tear strength to weight ratio
greater than 0.1 Newton per gram per square meter.
According to a further aspect of the invention of the
present divisional application there is provided a method for
preparing microfibers by extruding molten fiber-forming polymeric
material through orifices in a die into a high-velocity gaseous
stream, directing the fibers from the die exit into a tubular
chamber, and passing the fibers through the chamber together with
air blowing at a velocity sufficient to maintain the fibers under
tension and sufficient for the fibers to exit the chamber at a
velocity of at least about 4400 meters/minute.
The invention provides new melt-blown fibers and
fibrous webs of greatly improved strength, comparable for the
first time to the strength of fibers and webs prepared by
conventional melt-spinning processes such as spunbond fibers and
fibrous webs. The new melt-blown fibers have much greater
orientation and crystallinity than previous melt-blown fibers,
as a result of preparation by a new method which, in brief
summary, comprises extruding fiber-forming material through the
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--4--
.
orifices of a die into a high-velocity gaseous stream
where the extruded material is rapidly attenuated into
fibers; directing the attenuated fibers into a first open
end, i.e., the entrance end, of a tubular chamber disposed
S near the die and extending in a direction parallel to the
path of the attenuated fibers as they leave the die;
introducing air into the tubular chamber blowing along the
axis of the chamber at a velocity sufficient to maintain
the fibers under tension during travel through the
10 chamber; and collecting the fibers after they leave the
opposite, or exit end, of the tubular chamber.
Generally, the tubular chamber is a thin wide
box-like chamber (generally somewhat wider than the width
of the melt-blowing die). Air is generally brought to the
15 chamber at an angle to the path of the extruded fibers but
travels around a curved surface at the first open end of
the chamber. 8y the Coanda effect, the air turns around
the curved surface in a laminar, non-turbulent manner,
thereby assuming the path traveled by the extruded fibers
20 and merging with the primary air in which the fibers are
entrained. The fibers are drawn into the chamber in an
orderly compact stream and remain in that compact stream
through the complete chamber. Preferably, the tubular
chamber is flared outwardly around the circumference of
its exit end, which has been found to better provide
isotropic properties in the collected or finished web.
The orienting air generally has a cooling effect
on the fibers (the orienting air can be, but usually is
not heated, but is ambient air at a temperature less than
about 35C; in some circumstances, it may be useful to
cool the orienting air below ambient temperature before it
is introduced into the orienting chamber.) The cooling
effect is generally desirable since it accelerates cooling
and solidification of the fibers, whereupon the pulling
effect of the orienting air as it travels through the
orienting chamber provides a tension on the solidified
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fibers that tends to cause them to crystallize.
The significant increase in molecular orientation and
crystallinity of the fibers as taught herein over conventional
melt-blown fibers is illustrated by reference to Figures 4, 7,
8, 10 and 11, which show WAXS (wide-angle X-ray scattering)
photographs of fibers that, respectively, are oriented fibers
as taught herein (A photo) and are nonoriented conventional
fibers of the prior art (B photo). The ring-like nature of the
light areas in the B photos signifies that the pictured fibers
as taught herein are highly crystalline, and the interruption of
the rings means that there is significant crystalline orientation.
Brief Description of the Drawings
Figures 1 and 2 are a side view and a perspective view,
respectively, of two different apparatuses useful for carrying
out methods to prepare fabrics as taught herein.
Figures 3, 5 and 9 are plots of stress-strain curves
for fibers as taught herein (the "A" drawings) and comparative
fibers (the "B" drawings),
Figures 4, 7, 8, 10 and 11 are WAX photographs of
fibers as taught herein (the "A" photographs) and comparative
fibers (the "B" photographs); and
Figure 6 comprises scanning electron microscope photo-
graphs of a representative fibrous web as taught herein (6A) and
a comparative fibrous web (6B).
Detailed Description
A representative apparatus useful for preparing blown
fibers or a blown-fiber web as taught herein is shown schematic-
ally in Figure 1. Part of the apparatus, which forms the blown
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fibers, can be as described in Wente, Van A., "Superfine
Thermoplastic Fibers" in Industrial Engineering Chemistry,
Vol. 48, page 1342 et
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-
seq. (1956), or in Report No. 4364 of the Naval Research
Laboratories, published ~lay 25, 1954, entitled
"Manufacture of Superfine Organic Fibers," by
Wente, V. A.; Boone, C. D.; and Fluharty, E. L. This
5 portion of the illustrated apparatus comprises a die 10
which has a set of aligned side-by-side parallel die
orifices 11, one of which is seen in the sectional view
through the die. The orifices 11 open from the central
die cavity 12.
Fiber-forming material is introduced into the
die cavity 12 through an opening 13 from an extruder (not
illustrated). Orifices 15 disposed on either side of the
row of orifices 11 convey heated air at a very high
velocity. This air, called the primary air, impacts onto
15 the extruded fiber-forming material, and rapidly draws out
and attenuates the extruded material into a mass of
fibers.
From the melt-blowing die 10, the fibers travel
to a tubular orienting chamber 17. "Tubular" is used in
20 this specification to mean any axially elongated structure
having open ends at each axially opposed end, with walls
surrounding the axis. Generally, the chamber is a rather
thin, wide, box-like chamber, having a width somewhat
greater than the width of the die 10, and a height (18 in
25 Figure 1) sufficient for the orienting air to flow
smoothly through the chamber without undue loss of
velocity, and for fibrous material extruded from the die
to travel through the chamber without contacting the walls
of the chamber. Too large a height would require unduly
30 large volumes of air to maintain a tension-applying air
velocity. Good results have been obtained with a height
of about 10 millimeters or more, and we have found no need
for a height greater than about 25 millimeters.
Orienting or secondary air is introduced into
35 the orienting chamber through the orifices lg arranged
near the first open end of the chamber where fibers from
_7_ 1337150~ -
the die enter the chamber. Air is preferably introduced
from both sides of the chamber (i.e., from opposite sides
of the stream of fibers entering the chamber) around
curved surfaces 20, which may be called Coanda surfaces.
S The orienting air introduced into the chamber bends as it
travels around the Coanda surfaces and travels along the
longitudinal axis of the chamber. The travel of the air
is quite uniform and rapid and it draws into the chamber
in a uniform manner the fibers extruded from the melt-
blowing die 10. Whereas fibers exiting from a melt-blown
die typically oscillate in a rather wide pattern soon
after they leave the die, the fibers exiting from the
melt-blowing die in the present method tend to
pass uniformly in a surprising planar-like distribution
into the center of the chamber and travel lengthwise
through the chamber. After they exit the chamber, they
typically exhibit oscillating movement as represented by
the oscil ating line 21 and by the dotted lines 22 which
represent the general outlines of the stream of fibers.
As shown in Figure 1, the orienting chamber 17
is preferably flared at its exit end 23. This flaring has
been found to cause the fibers to assume a more randomized
or isotropic arrangement within the fiber stream. For
example, a collected web of fibers of the invention passed
25 through a chamber which does not have a flared exit tends
to have a machine-direction fiber pattern (i.e., more
fibers tend to be aligned in a direction parallel to the
direction of movement of the collector than are aligned
transverse to that direction). On the other hand, webs of
30 fibers collected from a chamber with a flared exit are
more closely balanced in machine and transverse
orientation. The flaring can occur both in its height and
width dimensions, i.e., in both the axis or plane of the
drawing and in the plane perpendicular to the page o~ the
35 drawings. More typically, the flaring occurs only in the
axis in the plane of the drawing, i.e., in the large-area
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sides or walls on opposite sides of the stream of fibers
passing through the chamber. Flaring at an angle (the
angle 0) between a broken line 25 parallel to the central
or longitudinal axis of the chamber and the flared side of
the chamber between about 4 and 7~ is believed ideal to
achieve smooth isotropic deposit of fibers. The length 24
of the portion of the chamber over which flaring occurs
(which may be called the randomizing portion of the
chamber) depends on the velocity of the orienting air and
the diameter of fibers being produced. At lower
velocities, and at smaller fiber diameters, shorter
lengths are used. Flaring lengths between 25 and 75
centimeters have proven useful.
The orienting air enters the orienting chamber
17 at a high velocity sufficient to hold the fibers under
tension as they travel lengthwise through the chamber.
Planar continuous travel through the chamber is an
indication that the fibers are under tension. The needed
velocity of the air, which is determined by the pressure
with which air is introduced into the orienting chamber
and the dimensions of the orifices or gaps 19, varies with
the kind of fiber-forming material being used and the
diameter of the fibers. For most situations, velocities
corresponding to pressures of about 70 psi (approximately
500 kPa) with a gap width for the orifice 19 (the
dimension 30 in Figure 1) of 0.005 inch (0.013 cm), have
been found optimum to assure adequate tension. However,
pressures as low as 20 to 30 psi (140 to 200 kPa) have
been used with some polymers, such as nylon 66, with the
stated gap width.
Surprisingly, the fibers can travel through the
chamber a long distance without contacting either the top
or bottom surface of the chamber. The chamber is
generally at least about 40 centimeters long (shorter
chambers can be used at lower production rates) and
preferably is at least 100 centimeters long to achieve
_ 9 1 337 1 50 J
desired orientation and desired mechanical properties in
the fibers. With shorter chamber lengths, faster air
velocities can be used to still achieve fiber orientation.
The entrance end of the chamber is generally within 5-10
centimeters of the die, and as previously indicated,
despite the turbulence conventionally present near the
exit of a melt-blowing die, the fibers are drawn into the
orienting chamber in an organized manner.
After exiting from the orienting chamber 17, the
solidified fibers are decelerating, and, in the course of
that deceleration, they are collected on the collector 26
as a web 27. The collector may take the form of a finely
perforated cylindrical screen or drum, or a moving belt.
Gas-withdrawal apparatus may be positioned behind the
collector to assist in deposition of fibers and removal of
gas.
The collected web of fibers can be removed from
the collector and wound in a storage roll, preferably with
a liner separating adjacent windings on the roll. At the
time of fiber collection and web formation, the fibers are
totally solidified and oriented. These two features tend
to cause the fibers to have a high modulus, and it is
difficult to make high-modulus fibers decelerate and
entangle to form a coherent web. Webs comprising only
oriented melt-blown fibers may not have the coherency of a
collected web of conventional melt-blown fibers. For that
reason, the collected web of fibers is often fed directly
to apparatus for forming an integral handleable web, e.g.,
by bonding the fibers together as by calendering the web
uniformly in areas or points (generally in an area of
about 5 to 40 percent), consolidating the web into a
coherent structure by, e.g., hydraulic entanglement,
ultrasonically bonding the web, adding a binder material
to the fibers in solution or molten form and solidifying
the binder material, adding a solvent to the web to
solvent-bond the fibers together, or preparing bicomponent
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fibers and subjecting the web to conditions so that one component
fuses, thereby fusing together ad~acent or intersecting fibers.
Also, the collected web may be deposited on another web, for
example, a web traveling over the collector; also a second web
may be applied over the uncovered surface of the collected web.
The collected web may be unattached to the carrier or cover web
or liner, or may be adhered to the web or liner as by heat-
bonding or solvent-bonding or by bonding with an added binder
material.
The blown fibers are preferably microfibers, averaging
less than about 10 micrometers in diameter. Fibers of that size
offer improved filtration efficiency and other beneficial
properties. Very small fibers, averaging less than 5 or even 1
micrometer in diameter, may be blown, but larger fibers, e.g.,
averaging 25 micrometers or more in diameter, may also be blown,
and are useful for certain purposes such as coarse filter webs.
There is provided an advantage in forming fibers of
small fiber size, and fibers produced as above are generally
smaller in diameter than fibers formed under the same melt-
blowing conditions as used for the present fibers but withoutuse of an orienting chamber as taught herein. Also, the fibers
have a narrow distribution of diameters. For example, in
preferred samples of the present webs, the diameter of three-
quarters or more of the fibers, ideally, 90 percent or more,
have tended to lie within a range of about 3 micrometers, in
contrast to a typically much larger spread of diameters in
conventional melt-blown fibers.
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The oriented melt-blown fibers are believed to be
continuous, which is apparently a fundamental distinction from
fibers formed inconventional melt-blowing processes, where the
fibers are typically said to be discontinuous. The fibers
generally travel through the orienting chamber without
interruption, and no evidence of fiber ends is found in the
collected web. For example, the collected webs are remarkably
free of shot (solidified globules of fiber-forming material such
as occur when a fiber breaks and the release of tension permits
the material to retract back into itself). Also, the fibers
show little if any thermal bonding between fibers.
Other fibers may be mixed into the present fibrous webs,
e.g., by feeding the other fibers into the stream of blown fibers
after it leaves the tubular chamber and before it reaches a
collector. U. S. Patent 4,118,531 teaches a process and
apparatus for introducing into a stream of melt-blown fibers
crimped staple fibers which increase the loft of the collected
web, and such process and apparatus are useful with the present
fibers. U. S. Patent 3,016,599 teaches such a process for
introducing uncrimped fibers. The additional fibers can have
the function of opening or loosening the web, of increasing the
porosity of the web, and of providing a gradation of fiber
diameters in the web.
Furthermore, added fibers can function to give the
collected web coherency. For example, fusible fibers, preferably
bicomponent fibers that have a component that fuses at a
temperature lower than the fusion temperature of the other
component, can be added and the fusible fibers can be fused at
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60557-3528D
points of fiber intersection to form a coherent web. Also, it
has been found that addition of crimped staple fibers to the
web, such as described in U. S. Patent 4,118,531, will produce a
coherent web. The crimped fibers intertwine with one another
and with the oriented fibers in such a way as to provide
coherency and integrity to the web.
Webs comprising a blend of crimped fibers and oriented
melt-blown fibers (e.g., comprising staple fibers in amounts up
to about 90 volume percent, with the amount preferably being
less than about 50 volume percent of the web) have a number of
other advantages, especially for use as thermal insulation.
First, the addition of crimped fibers makes the web more bulky
or lofty, which enhances insulating properties. Further, the
oriented melt-blown fibers tend to be of small diameter and to
have a narrow distribution of fiber diameters, both of which can
enhance the insulating quality of the web since they contribute
to a large surface area per volume-unit of material. Another
advantage is that the webs are softer and more drapable than webs
comprising nonoriented melt-blown microfibers, apparently
because of the absence of thermal bonding between the collected
fibers. At the same time, the webs are very durable because of
the high strength of the oriented fibers, and because the
oriented nature of the fiber makes it more resistant to high
temperatures, dry cleaning solvents, and the like. The latter
advantage is especially important with fibers of polyethylene
terephthalate, which tends to be amorphous in character when
made by conventional melt-blowing procedures. When subjected to
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higher temperatures the amorphous polyester polymer can
crystallize to a brittle form, which is less durable during use
of the fabric. But the oriented polyester fibers can be heated
without a similar degradation of their properties.
It has also been found that lighter-weight webs can
have equivalent insulating value as heavier webs made from non-
oriented melt-blown fibers. One reason is that the smaller
diameter of the fibers in the web, and the narrow distribution
of fiber diameters, causes a larger effective fiber surface area
in the present webs, and the larger surface area effectively
holds more air in place, as discussed in U. S. Patent 4,118,531.
Larger surface area per unit weight is also achieved because of
the absence of shot and "roping" (grouping of fibers such as
occurs in conventional melt-~lowing through entanglement or
thermal bonding).
Coherent webs may also be prepared by mixing oriented
melt-blown fibers with nonoriented melt-blown fibers. An
apparatus for preparing such a mixed web is shown in Figure 2
and comprises first and second melt-blowing dies 10a and 10b
having the structure of the die 10 shown in Figure 1, and an
orienting chamber 28 through which fibers extruded from the first
die 10a pass. The chamber 28 is like the chamber 17 shown in
Figure 1, except that the randomizing portion 29 at the end of
the orienting chamber has a different flaring than does the
randomizing portion 24 shown in Figure 1. In the apparatus of
Figure 2, the chamber flares rapidly to an enlarged height, and
then narrows slightly until it reaches the exit. While such a
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60557-3528D
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chamber provides an improved isotropic character to the web, the
more gradual flaring of the chamber shown in Figure 1 provides a
more isotropic character.
Polymer introduced into the second die lOb is extruded
through a set of orifices and formed into fibers in the same way
as fibers formed by the first die lOa, but the prepared fibers
are introduced directly into the stream of fibers leaving the
orienting chamber 28. The proportion of oriented to nonoriented
fibers can be varied greatly and the nature of the fibers (e.g.,
diameter, fiber composition, bicomponent nature) can be varied
as desired. Webs can be prepared that have a good isotropic
balance of properties, e.g., in which the cross-direction tensile
strength of the web is at least about three-fourths of the
machine-direction tensile strength of the web.
Some webs include particulate matter, which may be
introduced into the web in the manner disclosed in U. S. Patent
3,971,373, e.g., to provide enhanced filtration. The added
particles may or may not be bonded to the fibers, e.g., by
controlling process conditions during web formation or by later
heat treatments or molding operations. Also, the added
particulate matter can be a supersorbent material such as taught
in U. S. Patent 4,429,001.
The fibers may be formed from a wide variety of fiber-
forming materials. Representative polymers for forming melt-
blown fibers include polypropylene, polyethylene, polyethylene
terephthalate, and polyamide. Nylon 6 and nylon 66 are
especially useful materials because they form fibers of very high
strength.
1 337 1 50
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60557-3528D
.
The fibers may be made in bicomponent form, e.g., with
a first polymeric material extending longitudinally along the
fiber through a first cross-sectional area of the fiber and a
second polymeric material extending longitudinally through a
second portion of the cross-sectional area of the fiber. Dies
and processes for forming such fibers are taught in U. S. Patent
4,547,420. The fibers may be formed from a wide variety of
fiber-forming materials, with representative combinations of
components including: polyethylene terephthalate and poly-
propylene; polyethylene and polypropylene; polyethylene
terephthalate and linear polyamides such as nylon 6; polybutylene
and polypropylene; and polystyrene and polypropylene. Also,
different materials may be blended to serve as the fiber-forming
material of a single-component fiber or to serve as one component
of a bicomponent fiber.
The fibers and webs may be electrically charged to
enhance their filtration capabilities, as by introducing charges
into the fibers as they are formed, in the manner described in
U. S. Patent 4,215,682, or by charging the web after formation
in the manner described in U. S. Patent 3,571,679; see also
U. S. Patents 4,375,718, 4,588,537, and 4,592,815. Polyolefins,
and especially polypropylene, are desirably included as a
component in electrically charged fibers because they retain a
charged condition well.
The fibrous webs may include other ingredients in
addition to the microfibers. For example, fiber finishes may be
sprayed onto a web to improve the hand and feel of the web.
1 337 1 50
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Additives, such as dyes, pigments, fillers, surfactants,
abrasive particles, light stabilizers, fire retardants,
absorbents, medicaments, etc., may also be added to the webs by
introducing them to the fiber-forming liquid of the microfibers,
or by spraying them on the fibers as they are formed or after
the web has been collected.
A completed web may vary widely in thickness. For most
uses, webs have a thickness between about 0.05 and 5.0 centi-
meters. For some applications, two or more separately formed
webs may be assembled as one thicker sheet product.
The invention of both the present divisional applica-
tion and the parent application will be further described by
reference to the following illustrative examples.
Example 1
Using the apparatus of Figure 2, minus the second die
lOb, oriented microfibers were made from polypropylene resin
(Himont PF 442, supplied by Himont Corp., Wilmington, Delaware,
having a melt-flow index (MFI) of 800-1000). The die temperature
was 200C and the primary air temperature was 190C. The primary
air pressure was 10 psi (70 kPa), with gap width in the orifices
15 being between 0.015 and 0.018 inch (0.038 and 0.046 cm). The
polymer was extruded through the die orifices at a rate of about
0.009 pound per hour per orifice (89 g/hr/orifice).
From the die the fibers were drawn through a box-like
tubular orienting chamber as shown in Figure 2 having an interior
height of 0.5 inch (1.3 cm), an interior width of 24 inches (61
cm), and a length of 18 inches (46 cm). The randomizing or
- 15b - 1 337 1 50 60557-3528D
expansion portion 29 of the chamber was 24 inches (61 cm) long,
and as
- _ 1 337 1 50
-16-
illustrated in the drawing, was formed by portions of the
large-area walls defining the orienting chamber, which
flared at 90 to the portions of the walls defining the
main portion 28 of the chamber; the wall flared to a 6
inch (15.24 cm) height at the point of their connection to
the main portion of the chamber, and then narrowed to a 5
inch (12.7 cm) height over its 24 inch (61 cm) length.
Secondary air having a temperature of about 25C was blown
into the orienting chamber at a pressure of 70 psi (483
kPa) through orifices (like the orifices 19
shown in Figure 1) having a gap width of 0.005 inch
(0.013 cm).
The completed fibers exited the chamber at a
velocity of about 5644 meters/minute and were collected on
a screen-type collector spaced about 36 inches (91 cm)
from the die and moving at a rate of about 5 meters per
minute. The fibers ranged in diameter between 1.8 and
5.45 microns and had an average diameter of about 4
microns. The speed draw ratio for the fibers (the ratio of
exit velocity to initial extrusion velocity) was 11,288
and the dimeter draw ratio was 106.
The tensile strength of the fibers was measured
by testing a collected embossed web of the fibers
(embossed over about 34 percent of its area with 0.54-
square-millimeter-sized diamond-shaped spots) with an
Instron tensile testing machine. The test was ~erforme~3
using a gauge length, i.e., a separation of the jaws, of
as close to zero as possible, approximately 0.009
centimeter. Results are shown in Figure 3A. Stress is
plotted in dynes/cm2 x 107 on the ordinate and nominal
strain in per~ent on the abscissa (stress is plotted in
psi x 10 on the right-hand ordinate). Young's modulus
was 4.47 x 106 dynes/cm2, break stress was 4.99 x 107
dynes/cm and toughness (the area under the curve) was
2.69 x 10 ergs/cm . By using a very small spacing
between jaws of the tensile testing machine, the measured
~ 337 1 5~ -
-17- 60557-3528D
values reflect the values on average Eor individual
fibers, and avoid the eEfect of the embossing. The sample
tested was 2 centimeters wide and the crosshead rate was
2 cm/minute.
For comparative purposes, tests were also
performed on microfibers like those of this example, i.e.,
prepared from the same polypropylene resin and using the
same apparatus, except that they were not passed through
the orienting chamber. These comparative fibers ranged in
diameter between 3.64 and 12.73 microns in diameter, and
had a mean diameter of 6.65 microns. The stress-strain
curve is shown in Figure 3B. Young's modulus was 1.26 x
10 dynes/cm , break stress was 1.94 x 107 dynes/cm2, and
toughness was 8.30 x 108ergs/cm3. It can be seen that the5 more oriented microfibers produced by the present process
had higher values in these properties by
between 2S0 and over 300~ than the microfibers prepared in
the conventional process.
WAXS (wide angle x-ray scattering) photographs
were prepared for the oriented fibers and
the comparative unoriented fibers, and are pictured in
Figure 4A (the present fibers) and 4B (comparative
fibers) (as is well understood in preparation of WAXS
photographs of fibers, the photo is taken of a bundle of
fibers such as obtained by collecting such a bundle on a
rotating mandrel placed in the fiber stream exiting from
the orienting chamber, or by cutting fiber lengths from a
collected web and assembling the cut lengths into a
bundle). The crystalline orientation of the oriented
microfibers is readily apparent from the presence oE
rings, and the interruption of those rings in Figure 4A.
Crystalline axial orientation function
(orientation along the fiber axis) was also determined Eor
the fibers (using procedures as described
in Alexander, L.E., X-Ray Diffraction Methods in Polymer
Science, Chapter 4, published by R. E. Krieger Publishing
-18- 1 337 1 50 60557-3528D
Co., New York, 1979; see particularly, page 24l, Equation
4-21) and ound to be 0.65. This value would be very low,
at least approaching zero, for conventional melt-blown
fibers. A value of 0.5 shows the presence of significant
crystalline orientation, and preferred fibers
exhibit values of 0.8 or higher.
Example 2
Oriented nylon 6 microfibers were prepared using
apparatus generally like that of Example 1, except that
the main portion of the orienting chamber was 48 inches
(122 cm) long. The melt-blowing die had circular smooth-
surfaced orifices (2S/inch) having a 5:1 length-to-
diameter ratio. The die temperature was 270~-, the
lS primary air temperature and pressure were, respectively,
270C and lS psi (104 kPa), (0.020-inch [0.05 cm] gap
width), and the polymer throughput rate was 0.5 l~/hr/in
(89 g/hr/cm). The extruded fibers were oriented using air
in the orienting chamber at a pressure of 70 psi (483 kPa)
with a gap width of 0.005 inch (0.013 cm), and an
approximate air temperature of 25~C. The flared
randomizing portion of the orienting chamber was 24 inches
(61 cm) long. Fiber exit velocity was about 6250
meters/minute.
Scanning electron microscopy (SEM) of a
representative sample showed fiber diameters of 1.8 to
9.52 microns, with a calculated mean fiber diameter of 5.1
microns.
For comparison, an unoriented nylon 6 web was
prepared without use of the orienting chamber and with a
higher die temperature of 315~C chosen to produce Eibers
similar in diameter to those of the oriented fibers
(higher die temperature lowers the viscosity of
the extruded material, which tends to result in a lower
diameter of the prepared ibers; thereby the comparative
fibers can approach the size of the present fibers
1 3 3 7 1 5 0 60557-3528D
~- which as noted above, tend to be narrower in diameter than
conventionally prepared melt-blown Eibers). The fiber
diameter distribution was measured as 0.3 to 10.5 microns,
with a calculated mean fiber diameter of 3.1 microns.
The tensile strength of the prepared ~ibers was
measured as described in Example 1, and the res~ltant
stress-strain curves are shown in Figure 5A (the present fibers)
and SB (comparative unoriented fibers). Units
on the ordinate are in pounds/square inch and on the
abscissa are in percent.
Figure 6 presents SEM photographs of
representative webs prepared as described
above (6A) and of the comparative unoriented webs (6B) to
further illustrate the difference between them as to fiber
diameter. As will be seen, the comparative web includes
very small-diameter fibers, apparently produced as a
result of the great turbulence at the exit of a melt-
blowing die in the c~nventional melt-blowing process. A
muc,~ more uniform air flow occurs at the exit of the die
in the present process, and this appears to
contribute toward preparation of fibers that are more
uniform in diameter.
Figure 7 presents WAXS photos for the fibers of
the invention (7A) and the comparative Elbers (7B).
Example 3
Oriented microfibers of polyethylene
terephthalate (Eastman A150 from Eastman Chemical Co.)
were prepared using the apparatus and conditions of
Example 2, except that the die temperature was 315C, and
the primary air pressure and temperature were,
respectively, 20 psi ~138 kPa) and 315C. Fiber exit
velocity was about 6000 meters/minute. The distcibution
of fiber diameters measured by SEM was 3.18 to 7.73
microns, with a mean o~ 4.94 microns.
Unoriented microfibers were prepared for
60557-3528D
-20- 1 337 1 50
comparative purposes, using the same resin and operating
conditions except for a slightly higher die temperature
(335C) and the lack of the orienting chamber. The fiber
diameter distribution was 0.91 to 8.8 microns with a mean
of 3.81 microns.
Figure 8 shows the WAXS patterns photographed
for the oriented (Figure 8A) and comparative unoriented
fibers (Figure 8B). The increased crystalline orientation
of the oriented microfibers was readily apparent.
Examples 4-6
Oriented microfibers were prepared from three
different polypropylenes, having melt flow indices (MFI)
respectively of 400-600 (Example 4), 600-800 (Example 5),
and 800-1000 (Example 6). The apparatus of Example 2 was
used, with a die temperature of 185C, and a primary air
pressure and temperature of 200C and 20 psi (138 kPa),
respectively. Fiber exit velocity was about 9028
meters/minute. The 400-600-MFI microfibers prepared were
found by SEM to range in diameter between 3.8 and 6.7
microns, with a mean diameter of 4.9 microns.
The tensile strength of the prepared
800-1000-MFI microfibers was measured using an Instron
tester, and the stress-strain curves are shown in Figure
9A (the present fibers) and 9B (comparative
unoriented fibers).
Unoriented microfibers were prepared ~or
comparative purposes, using the same resins and operating
conditions except for use of higher die temperature and
the absence of an orienting chamber. The prepare-l
400-600-MFI fibers ranged from 4.S5 to 10 microns in
diameter, with a mean of 6.86 microns.
Example 7
Oriented microfibers were prepared from
polyethylene terephthalate (251C melting point,
-21- 1 337 1 5~
crystallizes at 65-70C) using the apparatus of Example 2,
with a die temperature oE 325C, primary air pressure and
temperature oE 325C and 20 psi (138 kPa), respectively,
and polymer throughput of 1 lb/hr/in (178 g/hr/cm).
S Fiber exit velocity was 4428 meters/minute. The fibers
prepared ranged in diameter between 2.86 and 9.05 microns,
with a mean diameter of 7.9 microns.
Comparative microfibers were also prepared,
using the same resins and operating conditions except for
a higher die temperature and the absence of an orienting
chamber. These fibers ranged in diameter between 3.18 and
14.55 microns and had an average diameter of 8.3 microns.
Examples 8-12
Webs were prepared on the apparatus of Example
2, except that the randomizing portion of the orienting
chamber was flared in the manner pictured in Figure I and
was 20 inches (51 cm) long. Only the two wide walls of
the chamber were flared, and the angle ~ of flaring was
6. Conditions were as described in Table I below. In
addition, comparative webs were prepared from the same
polymeric materials, but without passing the fibers
through an orienting chamber; conditions for the
comparative webs are also given in Table I (under the
label "C"). Additional examples (llX and 12X) were also
prepared using conditions like those described in Examples
11 and 12, except that the Elared randomizing portion of
the orienting chamber was 24 inches (61 centimeters) long.
The webs were embossed with star patterns (a central dot
and six line-shaped segments radiating Erom the dot), with
the embossing covering 15 percent of the area of the web,
and being prepared by passing the web under an embossing
roller at a rate of 18 Eeet per minute, and usiny
embossing temperatures as shown in Table I and a pressure
of 20 psi (138 kPa). Both the present webs and
the comparative webs were tested for grab tensile strength
-- 1 337 1 5~ -
-22-
and strip tensile strength (procedures described in ASTM D
1117 and D 1682) in both the machine direction (MD) -- the
direction the collector rotates -- and the transverse or
cross direction (TD), and results are given in Tables II
and III. Elmendorf tear strength (ASTM D 1424) was also
measured on some samples, and is reported in Table IV.
-23- 1 337 1 50
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-27- 1 3 37 1 50 60557 3528D
Table IV
_ 8C 9 9C 11 llC_
Avg. Tear Force
MD(g) 688 164 1916 680 880 1016
TD(g) 832 160 20841248 2160 1884
MD(N) 6.74 1.60 18.786.66 8.62 9.95
TD~N) 8.15 1.57 20.42 12.23 21.16 18.46
~asis Weight
g/m2 55 54 51 52 52 49
15 Avg. Tear Force
Per Unit of ~asis Weight
MD (N/g/m2~ 0.122 0.03 0.370.13 0.166 0.203
TD (N/g/m2) 0.148 0.029 0.4000.23 0.407 0.377
Example 13
As an illustration of a useful insulating web,
a web was made comprising 65 weight-percent
oriented melt-blown polypropylene microfibers made
according to Example 1 (see Table V below for the specific
conditions), and 35 weight-percent 6-denier crimped 1-1/4
inch (3.2 cm) polyethylene terephthalate staple fibers.
The web was prepared by picking the crimped staple fiber
with a lickerin roll (using apparatus as taught in U.S.
Pat. 4,118,531) and introducing the picked staple fibers
into the stream of oriented melt-blown fibers as the
latter exited Erom the orienting chamber. The diameter oE
the microfibers was measured by SEM and found to range
between 3 and 10 microns, with a mean diameter of 5.5
microns. The web had a very soft hand and draped readily
when supported on an upright support such as a bottle.
For comparison, a similar web (13C) was prepared
1 3 37 1 50 60557-3528D
-28-
comprising the same crimped staple polyethylene
terephthalate fibers and polypropylene microfibers
prepared like the microfibers in the present webs
except that they did not pass through an orienting
chamber.
Thermal insulating values were measured on the
two webs before and after 10 washes in a Maytag clothes
washer, and the results are given in Table VI.
Table V
Example No. 13 14 & 15 16
Die Temperature (C) 200 310 310
Primary ALr
Pressure (psi)20 25 25
(kPa)138 172 172
Temperature (C)200 310 310
Orienting Chamber
Pressure (psi)70 70 70
(kPa)483 483 483
Temperature (C)ambient ambientambient
Rate of Polymer
Extrusion
(lb/hr/in) 0.5
(g/hr/cm) 89 178 178
*
Trade-mark
-29- 1 337 1 50
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Example 14-lS
Insulating webs were prepared
which comprised 80 weight-percent oriented microfibers o~
polycyclohexane terephthalate (crystalline melting point
295C; Eastman Chemical Corp. 3879), made on apparatus as
described in Example 2 using conditions as described in
Table V, and 20 weight-percent 6-denier polyethylene
terephthalate crimped staple fiber lntroduced into the
1 stream of melt-blown oriented fibers in the manner
described for Example 13. Two different webs of excellent
drapability and soft hand were prepared having the basis
weight described below in Table VII. Thermal insulatiny
properties for the two webs are also given in Table VII.
Table VII
Example No. 14 15 . 16
Weight (g/m2) 133 106 150
Thickness (cm) 0.73 0.71
Insulating Ef~iciency (clo) 1.31 1.59
(clo/cm) 1. 79 2 . 241. 63
(clo-m2/kg) 9.8 15.0 13.9
After Washed 10 Times
Insulating Efficiency
% Retained 103.1 92.2 99.6
Thickness (% Retained)97 . 3 98.6
Example 16
An insulating web was made
comprising 65 weight-percent oriented melt-blown
polycyclohexane terephthalate micro~ibers (Eastman 3879)
and 35 weight-percent 6-denier polyethylene terephthalate
1 337 1 5 -31- 50557-3528D
-
crimped staple fibers. Conditions for manufacture of the
oriented melt-blown microfibers are as given in Table V,
and measured properties were as given in Table VII. rhe
web was of excellent drapability and soft hand.
Example 17 and 18
A first web (Example 17) was
prepared according to Example 1, except that two dies were
used as shown in Figure 2. For the die lOa, the die
temperature was 200C, the primary air temperature and
pres~ure were 200C and 15 psi (103 kPa), respectively,
and the orienting chamber air temperature and pressure
were ambient temperature and 70 psi (483 kPa),
respectively. Polymer throughput rate was O.S lb/hr/in
lS (89 g/hr/cm). The fibers leaving the orienting chamber
were mixed with non-oriented melt-blown polypropylene
fibers prepared in the die lOb. For die lOb, the die
temperature was 270C, and the primary air pressure and
temperature were 30 psi (206 kPa) and 270C, respectively.
The polymer throughput rate was 0.5 lb/hr/in (89 g/hr/cm).
As a comparison, another web
(Example 18) was prepared in the manner of Example 4,
which comprised only oriented melt-blown fibers. Both
the Example 17 and 18 webs were embossed at a rate of 18
feet per minute in a spot pattern (diamond-shaped spots
about 0.54 square millimeters in area and occupying about
34 percent of the total area of the web) using a
temperature of 275F (135C), and a pressure of 20 p~i
(138 kPa).
Both the Example 17 and 18 embossed webs were
measured on an Instron tester for tensile strength versus
strain in the machine direction, i.e., the direction of
mo~ement of the collector, and the cross direction, and
the results are reported below in Table VIII.
- 1 337 1 5 _32-
Table VIII
Example 17
MD CD
Stress
(psi) 1600 2400 2700 29S0 1600 23S0 26S0 2850
(kPa)11008 16S12 18576 20296 11008 16168 18232 19608
Strain % 6 12 18 24 6 12 18 24
Example 18
lS
MD CD
Stress
(psi) 2900 4000 4700 4S00 SS0 7S0 92S 107S
20(kPa)l99S2 27S20 32336 31023 3784 5160 6364 7396
Strain % 6 12 18 24 6 12 18 24
2S
