U.S. patent application number 10/728555 was filed with the patent office on 2004-06-17 for fibrous nonwoven webs.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Brostrom, Myles L., Brownlee, David C., Olson, David A., Percha, Pamela A., Thompson, Delton R. JR..
Application Number | 20040113309 10/728555 |
Document ID | / |
Family ID | 24879443 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040113309 |
Kind Code |
A1 |
Thompson, Delton R. JR. ; et
al. |
June 17, 2004 |
Fibrous nonwoven webs
Abstract
New fibrous nonwoven webs are taught that comprise a mass of
polyethylene terephthalate fibers that exhibit a double melting
peak on a DSC plot: one peak is representative of a first molecular
portion within the fiber that is in non-chain-extended crystalline
form, and the other peak is representative of a second molecular
portion within the fiber that is in chain-extended crystalline form
and has a melting point elevated over that of the
non-chain-extended crystalline form. Webs comprising fibers having
such a morphology have a unique combination of durability and
dimensional stability. The fibers are generally autogenously bonded
at points of fiber intersection.
Inventors: |
Thompson, Delton R. JR.;
(Woodbury, MN) ; Olson, David A.; (St. Paul,
MN) ; Brownlee, David C.; (St. Paul, MN) ;
Percha, Pamela A.; (Woodbury, MN) ; Brostrom, Myles
L.; (Stillwater, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
24879443 |
Appl. No.: |
10/728555 |
Filed: |
December 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10728555 |
Dec 5, 2003 |
|
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09716790 |
Nov 20, 2000 |
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6667254 |
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Current U.S.
Class: |
264/210.8 ;
264/235 |
Current CPC
Class: |
D01F 6/62 20130101; D04H
3/14 20130101; Y10S 428/903 20130101; Y10T 442/614 20150401; D01D
5/0985 20130101; Y10T 442/626 20150401; D04H 1/56 20130101; Y10T
442/619 20150401; Y10T 442/69 20150401; Y10T 442/641 20150401; Y10T
442/68 20150401; Y10T 428/2969 20150115; D04H 3/011 20130101; Y10T
428/2913 20150115; D04H 3/16 20130101 |
Class at
Publication: |
264/210.8 ;
264/235 |
International
Class: |
D01D 005/12 |
Claims
What is claimed is:
1. A method for preparing a nonwoven meltblown PET-fiber-based web
comprising a) extruding molten PET polymer having a temperature of
about 295 degrees C. or less through the orifices of a meltblowing
die into a high-velocity stream of air to produce a mass of PET
fibers, the stream of air having a manifold air temperature of
about 260 degrees C. or less and an air velocity of at least 100
meters per second sufficient to impart chain-extended
crystallization to the PET fibers; and b) collecting the prepared
PET fibers.
2. A method of claim 1 in which the PET fibers are prepared from
resin exhibiting an intrinsic viscosity of between about 0.45 and
0.75.
3. A method of claim 1 in which the prepared PET fibers exhibit a
double melting peak on a DSC plot which is representative of a
first molecular portion within the fiber that comprises a
non-chain-extended crystalline phase, and a second molecular
portion within the fiber that comprises a chain-extended
crystalline phase and melts at an elevated temperature over that of
the non-chain-extended crystalline phase.
4. A method of claim 1 in which other fibers or particles are
dispersed among the PET fibers before they are collected.
5. A method for preparing a nonwoven meltblown PET-fiber-based web
comprising a) heating PET polymer resin having an intrinsic
viscosity of between about 0.45 and 0.6 to a molten form, extruding
the molten PET polymer while at a temperature of about 285 degrees
C. or less through the orifices of a meltblowing die into a
high-velocity stream of air to thereby prepare a mass of meltblown
PET fibers having an average diameter of about 20 micrometers or
less, the stream of air having a temperature of less than about 270
degrees C. and an air velocity of at least 100 meters per second
sufficient to impart chain-extended crystallization to the PET
fibers; b) collecting the prepared PET fibers as a web; and c)
passing the collected web through an oven to anneal and
autogenously bond the PET fibers together at points of fiber
intersection.
6. A method of claim 5 in which the PET polymer has a temperature
of about 275 degrees C. or less when extruded through the orifices
of the meltblowing die.
7. A method of claim 5 in which the stream of air has an air
velocity of at least 150 meters per second.
8. A method of claim 5 which includes the further step of
introducing additional fibers into the stream of prepared PET
fibers before collecting the web of fibers.
9. A method of claim 8 in which the additional fibers comprise
staple fibers.
10. A method of claim 5 in which at least one additional polymeric
material is extruded through the orifices of the meltblowing die
together with the PET polymer to thereby prepare bicomponent
fibers.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 09/716,790, filed Nov. 20, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to fibrous nonwoven webs,
especially those that comprise polyethylene terephthalate
fibers.
BACKGROUND OF THE INVENTION
[0003] Direct formation of polymeric material into fibrous nonwoven
webs by processes such as meltblowing has many advantages; but the
strength properties of meltblown fibers can be less than desired.
The polymer chains in meltblown fibers are generally not oriented
sufficiently to provide a high level of strength properties to the
fibers; see Encyclopedia of polymer Science and Engineering, John
Wiley & Sons, Inc., 1987, Volume 10, page 240. Meltblown fibers
are typically prepared by extruding molten polymer through orifices
in a die into a stream of high-velocity air which rapidly and
greatly attenuates the extrudate to form generally small-diameter
fibers. Much of the extension of the extrudate occurs while the
polymer is above its melt temperature (T.sub.m), with the result
that the polymer molecules can relax some of the internal stresses
generated during attenuation of the extrudate, and hence, may not
achieve the rather high degree of orientation that can induce the
molecules to form an ordered crystalline state.
[0004] Meltblown polyethylene terephthalate (PET) fibers are
especially subject to the above tendencies. Collected PET meltblown
fibers exhibit almost a total lack of crystalline orientation,
because PET has a relatively high rate of relaxation, a relatively
low rate of crystallization, a relatively high melt temperature,
and a glass transition temperature (T.sub.g) well above room
temperature.
[0005] The lack of crystalline order weakens conventional meltblown
PET fibers, and it also makes the fibers dimensionally unstable
when exposed to elevated temperatures above their T.sub.g. Some
internal stresses--sometimes termed amorphous orientation, i.e., an
orientation insufficient to induce crystalline order--are produced
during attenuation of the meltblown extrudate and are frozen in due
to rapid quenching of the melt. Later heating of a nonwoven web of
the fibers can release the internal stresses and allow the polymer
chains to contract, whereupon the fibers shrink. Shrinkage at
elevated temperatures can approach 50% of the web's as-collected
dimensions. In addition to contraction of the PET molecules upon
exposure to elevated temperature, some crystallization of the
molecules occurs; but this crystallization of the generally
amorphous molecules actually embrittles and weakens the fibers.
[0006] The result is that while PET has a number of important
advantages--for example, it does not melt or degrade when exposed
to rather high temperatures such as 180 degrees C., has desired
flame retardancy as compared with polyolefins, and is of relatively
low cost--its use as a meltblown fiber has been limited.
[0007] Several attempts have been made to provide a more stable and
useful meltblown PET fiber. U.S. Pat. No. 5,958,322 teaches a
method for giving an already collected meltblown PET web increased
dimensional stability by annealing the web while it is held on a
tentering structure. While good dimensional stability is achieved
by this technique, the process requires an extra processing step
that adds expense; and greater improvement in morphology and
strength would be desirable. Japanese Kokai No. 3-45768 is another
teaching of heating a PET web or fabric under tension to increase
crystallinity, with similar deficiencies.
[0008] U.S. Pat. No. 4,988,560 teaches a technique for orienting
meltblown fibers, and achieves high-strength fibers. But the fibers
described in that patent require special steps to gather and hold
them into a coherent web, such as embossing the assembled fibers or
adding a binder material to the assembled fibers. U.S. Pat. No.
4,622,259 similarly discusses high-strength meltblown fibers that
require embossing or the like to consolidate assembled fibers into
a handleable and usable web.
[0009] Japanese Kokai 90663/1980 (as described in European Patent
No. 527,489, page 2, lines 36-51) teaches preparation of PET
fibrous webs by a meltblown process which, in combination, uses
high-pressure air blown through a narrow gap, PET polymers having
an intrinsic viscosity of 0.55 or higher, and extrusion at a
melt-viscosity higher than "assures good melt-blowing condition."
The process is said to provide PET meltblown fabric of good
properties, such as strength, hand and thermal resistance; but EP
527,489 states that the process is commercially impractical and
non-uniform, and that the fibers prepared lack adhesion with one
another, and instead scatter during collection.
[0010] EP 527 489 itself seeks to overcome the deficiencies of the
prior art by blending polyolefin into the PET polymer in an amount
of 2-25 weight-percent. The polyolefin is said to become dispersed
into the PET as discrete islands, resulting in a reduction in
melt-viscosity, which, together with use of low-pressure air, is
said to produce dimensionally stable meltblown fabrics.
[0011] U.S. Pat. No. 5,753,736 takes a different approach, using
certain nucleating agents in PET to prepare meltblown PET webs
having a combination of crystalline, amorphous and rigid amorphous
molecular portions said to achieve shrink-resistance.
[0012] None of the above techniques is known to have resulted in
actual, commercial, dimensionally stable meltblown fibrous PET
webs. Despite significant prior effort, the need for such webs
continues to be unsatisfied.
SUMMARY OF THE INVENTION
[0013] The present invention provides new nonwoven fibrous webs
having excellent strength, durability and dimensional stability in
comparison to conventional nonwoven webs. The fibers in these new
webs are preferably meltblown PET fibers, and are characterized by
a morphology that appears unique in such fibers. Specifically, the
new fibers of the invention exhibit a chain-extended crystalline
molecular portion (sometimes referred to as a strain-induced
crystalline (SIC) portion), a non-chain-extended (NCE) crystalline
molecular portion, and an amorphous portion. While not being bound
to theoretical explanations, it is believed that the chain-extended
crystalline portion in the new meltblown PET fibers of the
invention provides unique, desirable physical properties such as
strength and dimensional stability; and the amorphous portion in
these new fibers provides fiber-to-fiber bonding: an assembly of
the new fibers collected at the end of the meltblowing process may
be coherent and handleable, and it can be simply passed through an
oven to achieve further adhesion or bonding of fibers at points of
fiber intersection, thereby forming a strong coherent and
handleable web.
[0014] The unique morphology of the meltblown PET fibers of the
invention can be detected in unique characteristics, such as those
revealed by differential scanning calorimetry (DSC). A DSC plot for
PET fibers of the invention shows the presence of molecular
portions of different melting point, manifested as two
melting-point peaks on the DSC plot ("peak" means that portion of a
heating curve that is attributable to a single process, e.g.,
melting of a specific molecular portion of the fiber such as the
chain-extended portion; DSC plots of PET fibers of the invention
show two peaks, though the peaks may be sufficiently close to one
another that one peak is manifested as a shoulder on one of the
curve portions that define the other peak). One peak is understood
to be for the non-chain-extended portion (NCE), or less-ordered,
molecular fraction, and the other peak is understood to be for the
chain-extended, or SIC, molecular fraction. The latter peak occurs
at a higher temperature than the first peak, which is indicative of
the higher melting temperature of the chain-extended, or SIC,
fraction. We are not aware of any previous nonwoven web comprising
PET fibers that exhibit dual melting peaks on a DSC plot as
described, and such webs offer superior properties--e.g., combined
dimensional stability and toughness--as will be further explained
herein.
[0015] An amorphous molecular portion generally remains part of the
PET fiber, and can provide autogenous bonding (bonding without aid
of added binder material or embossing pressure) of fibers at points
of fiber intersection. This does not mean bonding at all points of
fiber intersection; the term bonding herein means sufficient
bonding (i.e., adhesion between fibers usually involving some
coalescence of polymeric material between contacting fibers but not
necessarily a significant flowing of material) to form a web that
coheres and can be lifted from a carrier web as a self-sustaining
mass. The degree of bonding depends on the particular conditions of
the process, such as distance from die to collector, processing
temperature of molten polymer, temperature of attenuating air, etc.
Further bonding beyond what may be achieved on the collector is
often desired, and can be simply obtained by passing the collected
web through an oven; calendering or embossing is not required but
may be used to achieve particular effects.
[0016] In brief summary, a new PET-based web of the invention
generally comprises a mass of PET fibers that a) exhibit dual
melting peaks on a DSC plot representative of a first molecular
portion within the fiber that is in a non-chain-extended (NCE)
crystalline form, and a second molecular portion within the fiber
that is in chain-extended crystalline form and has a melting point
elevated over that of the NCE crystalline form, and b) are
autogenously bondable.
[0017] For most uses of webs of the invention, the PET fibers
preferably are of microfiber size, i.e., have an actual average
diameter of 10 micrometers or less. However, larger fibers are
satisfactory for many uses. Most often, the effective fiber
diameter (EFD, measured by a technique that generally indicates a
larger size than actual diameter) is 20 micrometers or less.
[0018] Also, for most uses, the web preferably has a density of
less than 100 kilograms per cubic meter, though preferably more
than 2 kg/m.sup.3. The pressure drop through the web is preferably
at least 0.3 mm H.sub.2O pressure drop (as measured by passing a
stream of air through a 102.6-square-centimeter area at a face
velocity of 3.12 meters per minute), and more preferably at least
0.5 or 1 mm water. Such a pressure drop is characteristic of webs
that exhibit good sound insulation properties. Sound insulation
webs generally have a density of 50 kilograms per cubic meter or
less, and preferably of 25 kilograms per cubic meter or less, and
are preferably at least 1 or 2 centimeters thick.
[0019] Webs of the invention are generally used in an annealed
form, which provides increased stability. In contrast to prior-art
PET webs which have been annealed to achieve a degree of
dimensional stability, but which become embrittled and weakened by
random crystal growth during the annealing process, PET webs of the
invention retain good strength and durability after the annealing
process. Annealed webs of the invention also have enhanced bonding,
and these bonds are retained well upon heat-exposure.
[0020] Webs that combine excellent dimensional stability and
excellent strength, toughness and durability have been obtained.
For example, webs with a shrinkage of no more than about 2% when
exposed to a temperature of 160 degrees C. for 5 minutes have been
obtained. In general, webs that shrink less than 20% under such
conditions can be useful, though shrinkages of 5% or less are
especially useful. Also the webs of the invention retain excellent
strength, toughness and durability after annealing, even when
measured after a time of storage, e.g., one month at ambient
conditions. Webs of the invention comprising polyethylene
terephthalate fibers offer high strength, good modulus (e.g.,
stiffness) and good loft properties, low release of volatile
organic compounds upon heating, maintenance of physical properties
upon thermal and environmental exposure, relatively low
flammability, formability into micro-sized diameters, and lower
cost. With the heat-resistance achieved by the present invention,
meltblown PET webs of greatly increased utility are provided.
[0021] Webs of the invention are prepared by a new meltblowing
method. The new method comprises the steps of extruding molten PET
polymer through the orifices of a meltblowing die into a
high-velocity gaseous stream that attenuates the extruded polymer
into meltblown fibers, and collecting the prepared fibers, briefly
characterized in that the extruded molten PET polymer has a
processing temperature less than about 295 degrees C., and the
high-velocity gaseous stream has a temperature less than the molten
PET polymer and a velocity greater than about 100 meters per
second. Preferably, the PET polymer has an intrinsic viscosity of
about 0.60 or less.
[0022] In some methods of the invention, other fibers are dispersed
among the PET fibers before they are collected. For example,
crimped and/or uncrimped staple fibers may be dispersed among the
meltblown PET fibers to achieve a more lofty or a more resilient
web or to assist the web to be later molded and bonded in the
molded shape (webs of the invention can usually be molded without
presence of staple fibers).
[0023] While the invention is particularly applicable to
polyethylene terephthalate, it is also useful with other
semicrystalline polymeric materials, such as polyamides,
polyolefins, and other polyesters. Processes of the invention
better compensate against the effects of die swell (expansion of
the extrudate as it leaves the die orifice, meaning that there is
less relaxation of the polymer chains in the extrudate, and that
lesser relaxation, together with the strain imposed as the
extrudate solidifies in the high-velocity air, results in favorable
crystalline properties for these polymers also.
DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a mostly schematic diagram of apparatus useful for
practicing the invention.
[0025] FIGS. 2, 4, 6 and 8 are plots of differential scanning
calorimetry (DSC) for fibers in various of the examples described
later in this specification (a particular form of DSC, known as
Modulated DSC.TM., using an instrument supplied by TA Instruments,
Inc of New Castle, Del., was conducted, and provides additional
information): FIGS. 2 and 4 are the DSC plot for fiber in the web
of Example 31; FIG. 2 is a plot for the fiber before annealing, and
FIG. 4 is a plot for the fiber after annealing; FIG. 6 is the DSC
plot for fiber in the web of Example 10; and FIG. 8 is the DSC plot
for fiber in the web of Example 22.
[0026] FIGS. 3, 5, 7 and 9 are WAXS diagrams for the fiber for
which a DSC plot is pictured, respectively, in FIGS. 2, 4, 6 and
8.
[0027] FIGS. 10a and 10b are scanning electron micrographs, at
2500.times. and 7500.times., respectively, for a web of Example
30.
[0028] FIGS. 11a and 11b are atomic force micrographs of fiber of
the invention, before etching (FIG. 11a) and after etching (FIG.
11b).
[0029] FIG. 12 is a plot of sound insulation values for a web of
Example 37.
DETAILED DESCRIPTION
[0030] A representative apparatus useful for preparing meltblown
fibers or a meltblown fibrous web of the invention is shown
schematically in FIG. 1. Part of the apparatus, which forms the
blown fibers, can be as described in Wente, Van A., "Superfine
Thermoplastic Fibers" in Industrial Engineering Chemistry, Vol. 48,
page 1342 et seq. (1956), or in Report No. 4364 of the Naval
Research Laboratories, published May 25, 1954, entitled
"Manufacture of Superfine Organic Fibers," by Wente, V. A.; Boone,
C. D.; and Fluharty, E. L. This 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 from an extruder 13. An elongated (perpendicular
to the page) opening or slot 15 disposed on either side of the row
of orifices 11 conveys heated air at a very high velocity. This
air, called the primary air, impacts onto the extruded
fiber-forming material, and rapidly draws out and attenuates the
extruded material into a mass of fibers.
[0031] From the meltblowing die 10, the fibers travel in a stream
16 to a collector 18. As the meltblown fibers in the stream 16
approach the collector 18, they decelerate. In the course of that
deceleration the fibers are collected on the moving collector as a
web 19. 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, e.g., the air in which the
fibers are carried in the stream 16.
[0032] Although the collected web may be coherent and handleable
upon collection, the web is usually transported from the collector
18 to an oven where the web is heated to cause the fibers to
further bond together at points of fiber intersection. Because of
the presence of a substantial amorphous portion in the fibers of
the web, including in exterior portions of the fibers, the fibers
soften and adhere to achieve interfiber bonding. But because of the
crystalline character of the fibers, especially the chain-extended
crystalline structure, the webs show little shrinkage during the
bonding operation. Also, the heat of the oven further anneals the
fibers, increasing the crystalline content of the fibers, and
enhancing their dimensional stability.
[0033] In general, dimensionally stable webs of the invention are
achieved by controlling a number of the parameters of the
meltblowing process. Two such parameters are the temperature of the
polymer in the meltblowing die, i.e., the temperature of the molten
polymer in the extruder 13 and die cavity 12, and the temperature
of the gas, generally air, blown through the slots 15 onto the
polymer extrudate. By heating the polymer in the extruder and die
cavity to a temperature lower than conventionally used in
meltblowing, and thereby lowering the temperature of the polymer as
it exits the die orifices 11, the frost line (the point at which
the molten extrudate freezes or solidifies, i.e., changes from a
molten condition to a solid condition) is brought closer to the
die. The result is that during attenuation of the extrudate into
fibers the polymer chains tend to be straightened and oriented and
to retain a substantial portion of that orientation. A portion of
the straightened and oriented polymer chains are still amorphous
("amorphous orientation," in which the orientation is not
sufficient to induce formation of a crystalline structure). But
another portion of the polymer chains experiences sufficient
stress, the "critical stress," to align the polymer chains
sufficiently to facilitate a chain-extended crystalline structure.
This chain-extended crystalline structure, also called
strain-induced crystallization, contributes to the unique
properties of meltblown fibers of the invention.
[0034] In addition to a chain-extended crystalline structure,
fibers in webs of the invention generally also include some
non-chain-extended (NCE) crystalline structure. This NCE
crystalline structure may be initiated during original attenuation
of the fibers and is increased during annealing of collected webs
by crystallization of amorphous and amorphous oriented polymer
chains.
[0035] Crystallization of an amorphous or semi-crystalline material
upon heating is termed "cold crystallization." A typical amorphous
or partially crystalline PET material lacking significant
orientation cold crystallizes at approximately 125.degree. C. when
it is heated. Dimensionally stable fibers of this invention that
have been annealed after collection by exposure to temperatures
higher than 125 degrees C. lose this cold-crystallization peak.
Before annealing, the as-collected fibers generally do exhibit a
cold-crystallization peak, but they are nevertheless quite
dimensionally stable because of the presence of chain-extended
crystalline structure.
[0036] Formation of the stated morphology is enhanced by lowering
(with respect to conventional meltblowing operations) the
temperature of the primary air blown through the slots 15, because
air of lower temperature helps lower the temperature of the
extrudate. Also, because crystallization is an exothermic event,
blowing air of lower temperature onto the fibers helps remove the
generated exothermic heat and assists the process of
crystallization.
[0037] Preferably, the temperature of the polymer in the die cavity
is held to a temperature less than about 35 degrees C. higher than
the melting point of the polymer. For PET this generally means a
temperature of about 295 degrees C. or less. Lower temperatures,
such as 285 degrees C. or lower, are generally better; preferably
the temperature is no more than about 20 degrees C. higher than the
melting temperature, i.e., for PET is about 275 degrees C. or less
(generally the melting point of the non-chain-extended crystalline
structure of PET is regarded as the melting point of PET). The
temperature of the primary air or other gas is generally less than
that of the polymer in the die cavity, typically about 15 degrees
C. less than the temperature of the polymer in the die cavity.
[0038] A different parameter useful in achieving dimensionally
stable webs of the invention is the velocity of the primary air
blown from the slots 15. The higher the velocity of that air, the
greater the force applied to the extrudate, which tends to orient
the polymer chains within the extrudate. Higher velocity is
achieved by increasing the pressure in the supply leading to the
slots 15, thus increasing the volume of air or other gas blown
through the slots 15. Through analysis of exemplary processes of
the invention, we have found that the primary air (or other gas)
preferably has a velocity of at least 100 meters per second, and
more preferably at least 150 meters per second. This velocity in
feet/sec is determined by the following equation where Q is the
SCFM of air flow used, P is the pressure in psi at the die exit and
is assumed to have a value of 0 psi, t is the air temperature 1 Air
velocity = [ Q ( P + 1.03529 10 5 1.03529 10 5 ) ( 295.1 k t + 273
k ) ] 1 / 60 1 / a
[0039] in degrees F., and a is the combined area of the slots 15 in
square feet.
[0040] For SI units (where distances are in meters, so velocity is
in meters/second, area is in square meters and Q is in SCMM;
pressure is in pascals, and temperature is in degrees C.), the
equation is:
[0041] Another parameter that can be controlled to achieve
dimensionally stable webs as well as a small effective fiber
diameter is the molecular weight of the polymer, as manifested by
the intrinsic viscosity of the polymer. PET polymers of a common
molecular weight and intrinsic viscosity, including, for example,
at least intrinsic viscosities of about 0.6-0.75, are useful in the
invention. But best results in achieving microfiber-size fibers
have been achieved with lower-intrinsic-viscosity polymers, e.g.,
about 0.50 intrinsic viscosity. The lower intrinsic viscosity
allows the extrudate to be drawn to a narrow diameter. While a
lower intrinsic viscosity tends to lower the forces acting within
an extrudate to straighten polymer chains, sufficient
chain-straightening does occur at selected polymer temperatures and
primary air velocities for strain-induced crystallization to occur.
However, best results in SIC and dimensional stability have
presently been obtained with PET polymers of greater than about
0.45 intrinsic viscosity.
[0042] When PET meltblown fibers prepared in the manner described
herein are subjected to differential scanning calorimetry (DSC;
conditions for the measurements are stated in Examples 1-17), a
dual-melting-peak plot is obtained such as the solid-line plots
shown in FIGS. 2 and 4, which are plots for the web of Example 31
below; FIG. 2 is a plot for the unannealed web of Example 31, and
FIG. 4 is a plot for the web after it was annealed for 5 minutes at
160 degrees C. As seen in both plots, there is a first endothermic
peak 30, which is typically seen at 250-260 degrees C. under the
described measuring conditions and which is associated with the
melting of the polymeric molecular portions crystallized in a
non-chain-extended (NCE) configuration. There is a second
endothermic peak, or higher melting shoulder 40, which is
associated with polymeric molecular portions 2 Air velocity = [ Q (
P + 14.7 14.7 ) ( 530 t + 460 ) ] 1 / 60 1 / a
[0043] crystallized in chain-extended or strain-induced (SIC)
configuration. The chain-extended crystalline polymeric molecular
portions associated with the second peak 40 have a higher melting
point than the polymeric portions associated with the peak 30; the
higher melting point is typically seen in the temperature range 260
to 280 degrees C.
[0044] In addition to the described chain-extended and
non-chain-extended crystalline portions, the described PET fibers
of the invention also include an amorphous component, which is
revealed during DSC and other analysis, and which is also
distinguished in that it is available for autogenous bonding of the
PET fibers at points of fiber intersection. As discussed above,
collected webs of fibers of the invention are sufficiently coherent
that they can be removed from a collector as a handleable, integral
structure. Further, when a collected web of the invention is heated
in an oven to a temperature greater than T.sub.g, but less than
T.sub.m, portions of the fiber soften and adhere at points of fiber
intersection. Generally a temperature above the
cold-crystallization temperature (125 degrees C. for PET) is used;
a comparison of FIGS. 2 and 4 shows that the cold-crystallization
peak 50 revealed in the unannealed web (FIG. 2) has been removed by
the annealing/bonding operation (FIG. 4), indicating that further
crystallization and ordering of molecules has occurred. Such
crystallization limits remelting of bond points during later
heat-exposure of the annealed and bonded web. A deflection in the
DSC plot typically appears slightly above the annealing
temperature, and is seen at point 60 in FIG. 4. Higher
annealing/bonding temperatures, such as 160 degrees C., are
desired, because they accomplish annealing/bonding in a shorter
time. The bonding does not require embossing pressure, though webs
of the invention may be embossed or calendered to enhance bonding
or to give the web a desired configuration or other properties.
[0045] A significant portion of the amorphous content is present at
the exterior circumference of the prepared fibers. The surface of
the extruded filaments cool or quench faster and may experience a
different stress pattern from the central portion of the filament,
which may lead to formation of amorphous content at the surface.
Whatever the reason, amorphous content can be revealed by the
bonding that occurs in webs of the invention. FIG. 10a is a
scanning electron micrograph of the annealed web prepared in
Example 30, at 2500.times., showing a bond site 70 between
intersecting fibers, and FIG. 10b shows the same bond site at
7500.times..
[0046] Amorphous content at the surface of the fibers is also shown
by analyses such as atomic force microscopy (AFM). FIG. 11a is an
AFM of a portion of fiber of the invention, and FIG. 11b is an AFM
of the fiber after it has been etched with sodium hydroxide. As
seen in FIG. 11a, before etching, the surface of the fiber is
relatively smooth and glass-like, indicating amorphous content. But
after etching with sodium hydroxide, which preferentially etches
the exterior amorphous PET material, the surface is striated as
shown in FIG. 11b, presumably showing the crystalline structure.
The presence of the circumferential layer of amorphous polymeric
PET material is advantageous to bonding of webs of the
invention.
[0047] Polyethylene terephthalate is a greatly preferred polymer
for use in the invention, but other polymers or materials can be
blended with PET by using appropriate control of other parameters
such as melt temperature and viscosity and primary air velocity.
Also, by using techniques taught for example in U.S. Pat. No.
6,057,256, webs of the invention can incorporate bicomponent fibers
in which PET or a related polymer is one component (extending
longitudinally along the fiber through a first cross-sectional area
of the fiber) and one or more other polymers are other components
(extending longitudinally along the fiber through one or more other
cross-sectional areas of the fiber; the term "bicomponent" herein
includes fibers having two or more components). Process parameters
should be controlled to develop crystallization in the PET
component manifested as the noted dual-melting-peak DSC plot.
[0048] Other fibers may be mixed into a fibrous web of the
invention, e.g., by feeding the other fibers into the stream of
blown fibers before it reaches a collector. U.S. Pat. No. 4,118,531
teaches a process and apparatus for introducing crimped staple
fibers into a stream of meltblown fibers to increase the loft of
the collected web, and such process and apparatus are useful with
fibers of the present invention. U.S. Pat. No. 3,016,599 teaches
such a process for introducing uncrimped fibers. The additional
fibers can have many functions: opening or loosening the web,
increasing the porosity of the web, providing a gradation of fiber
diameters in the web, increasing compression-resistance or
resilience, etc.
[0049] Also, the added fibers can function to give the collected
web added 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
points of fiber intersection to form a coherent web useful to
provide enhanced web moldability (see U.S. Pat. No. 5,841,081).
Also, addition of crimped staple fibers to the meltblown fiber
stream can produce a coherent web, with the crimped fibers
intertwining with one another and with the oriented fibers.
[0050] Some webs of the invention include particulate matter, which
may be introduced into the web in the manner disclosed in U.S. Pat.
No. 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. Pat.
No. 4,429,001. In addition, additives may be incorporated into the
PET fibers such as dyes, pigments or flame-retardant agents.
[0051] In another variation, fiber streams from two or more
meltblowing dies are merged; see FIG. 1 of U.S. Pat. No. 4,429,001
and FIG. 2 of U.S. Pat. No. 4,988,560. The streams may each
comprise PET fibers of the present invention, or the second (or
additional) stream(s) may comprise a different fiber, including a
conventional meltblown PET fiber.
[0052] Webs of the invention are especially useful as insulation,
e.g., acoustic or thermal insulation. Webs comprising a blend of
crimped fibers and oriented melt-blown PET fibers as described
herein (e.g., comprising staple fibers in amounts up to about 90
weight percent, with the amount preferably being less than about 50
weight percent of the web) are especially useful as insulation. The
addition of crimped fibers makes the web more bulky or lofty, which
enhances insulating properties. Insulating webs of the invention
are preferably 1 or 2 centimeters or more thick, though webs as
thin as 5 millimeters in thickness have been used for insulating
purposes. The oriented melt-blown PET fibers described herein
desirably have a small diameter, which also enhances the insulating
quality of the web by contributing to a large surface area per
volume-unit of material. The combination of bulk and small diameter
gives good insulating properties.
[0053] Because of their dimensional stability under thermal stress,
webs of the invention are particularly suited for lining chambers
such as automobile engine compartments or small and large appliance
housings, for example, air-conditioners, dishwashers,
refrigerators, etc. The webs also have increased tensile strength
and durability because of the SIC of the PET meltblown fibers, and
the webs have good flexural strength. Their durability enhances
their utility in insulation, providing, for example, increased
resistance to wear and launderability. Other illustrative uses for
webs of the invention are as acoustical dampers, filters and
battery separators.
EXAMPLES 1-17
[0054] A series of meltblown, nonwoven, fibrous PET webs were
prepared from PET having an intrinsic viscosity of 0.60 (3M PET
resin 651000) using a meltblowing die generally as illustrated in
FIG. 1. The array of orifices at the die tip was 10 inches ( 25.4
centimeters) wide, with 0.015-inch-diameter (0.381 mm) orifices
aligned in a row and spaced on 0.040-inch (1.02 mm) centers. The
forward edge of the tip of the die that defines the slot 15 (the
point 23 in FIG. 1) was 0.049 inch (1.25 mm) in advance (further
downstream) of the tip (24) that defines the orifice 11 (this is
called a negative setback). The combined width of the slots 15 (the
dimension 21 in FIG. 1) was set at 0.069 inch (1.75 mm), and the
slots were 16 inches (40.6 centimeters) long, i.e., they extended
three inches past the end of the row of orifices 11 on each side of
the die. The collector was spaced 18 inches (about 46 centimeters)
from the meltblowing die.
[0055] The temperature of the PET polymer in the extruder, and the
temperature and pressure of the air passing through the air knife
(slot 15), the primary air, were varied as shown in Table 1. Air
velocity was calculated by the above-stated equation. The
throughput rate of the polymer was held constant at about 1
pound/inch/hour (about 180 g/cm/hour), and the collecting surface
was moved at a rate so as to produce a web of about 260
grams/square meter.
[0056] The shrinkage of as-extruded webs prepared in the examples
was measured by marking a 10-inch-by-10-inch square area
(25.4-centimeter-by-25.4-centimeter) on each sample, and placing
the samples individually into an oven heated to 160 degrees C.,
where they were subjected to unrestrained heating for five minutes.
The samples were removed, allowed to cool, and re-measured for
dimensional changes. Results for shrinkage in both the machine
direction (the direction the collector was moving during collection
of the sample web) and cross direction were determined and
averaged.
[0057] The average effective fiber diameter can be estimated by
measuring the pressure drop of air passing through the major face
of the web and across the web as outlined in the ASTM F 778-88 test
method, except using a face area of 102.6 square centimeters, and a
face velocity of 3.12 meters per minute. As used herein, the term
"average effective fiber diameter" means that fiber diameter
calculated according to the method set forth in Davies, C. N., "The
Separation of Airborne Dust and Particles," Institution of
Mechanical Engineers, London, Proceedings 1B, 1952. Actual average
fiber diameters were also measured for some of the examples from
scanning electron micrographs.
[0058] Web thickness for each example was measured in accordance
with ASTM D5736 using a pressure plate force of 0.002 pound per
square inch (13.8 pascal).
[0059] Results are reported in Table 1.
[0060] A differential scanning calorimetry plot, attached as FIG.
6, was generated for a representative fiber web of Example 10 using
a Modulated DSC system (Model 2920 supplied by TA Instruments Inc,
New Castle, Del.), and using a heating rate of 4 degrees C./minute,
a perturbation amplitude of plus-or-minus 0.636 degrees C. and a
period of 60 seconds. A WAXS diagram for fibers of Example 10,
attached as FIG. 7, was collected by use of a Bruker
microdiffractometer, copper K.sub..alpha. radiation, and Hi-STAR 2D
position sensitive detector registry of the scattered radiation
(supplied by Bruker AXS, Inc, Madison, Wis.). The diffractometer
was fitted with a 300 micron collimator and graphite incident beam
monochromator. The X-ray generator consisted of a rotating anode
source using a copper target operated at settings of 50 kV and 100
mA. Data were collected using a transmission geometry for 60
minutes with the detector centered at 0 degrees (2.theta.) at a
sample to detector distance of 6.0 cm. Samples were mounted so as
to place the fiber direction in the vertical. The 2D detector data
were corrected for detector sensitivity and spatial irregularities
using the Bruker GADDS data analysis software.
1 TABLE 1 Average Extruder Die Air Die Air Air Measured Temp. Temp.
Pressure Velocity % EFD Diameter (C.) (C.) psi (kPa) (m/sec)
Shrinkage (microns) (microns) 1 260 246 8 (55) 173 6 30.6 2 260 245
10 (69) 199 4 31.0 3 260 246 12 (83) 222 3 30.0 4 260 245 23 (159)
331 0 30.2 5 273 258 6 (41) 144 3 18.1 10.2 6 273 258 8 (55) 173 1
21.1 12.0 7 273 258 10 (69) 204 1 21.0 8 273 258 12 (83) 227 1 21.4
9 273 258 14 (97) 249 1 22.9 10 273 258 16 (110) 271 1 23.2 11 273
259 23 (159) 335 0 26.3 12 286 271 8 (55) 182 1 11.7 13 286 271 10
(69) 201 0 9.1 14 286 270 12 (83) 226 0 9.4 15 286 271 14 (97) 263
1 9.0 16 286 271 16 (110) 272 1 10.2 17 286 271 23 (159) 343 3
14.1
EXAMPLES 18-22
[0061] A different set of examples was prepared generally by the
process described in Examples 1-17 except that the rate of polymer
extruded was increased from 1 pound/inch/hour to 3 pounds/inch/hour
(about 540 grams per centimeter per hour). Results are reported in
Table 2. A DSC plot for representative fibers of Example 22 is
pictured in FIG. 8, and a WAXS diagram for the fibers is pictured
in FIG. 9.
2TABLE 2 Average Extruder Die Air Die Air Air Measured Temp. Temp.
Pressure Velocity % EFD Diameter Ex. (C.) (C.) psi (KPa) (m/sec)
Shrinkage (microns) (microns) 18 260 245 16 (110) 271 6 23.4 19 260
246 23 (159) 331 3 22.8 20 273 257 14 (97) 250 3 21.7 10.23 21 273
257 16 (110) 275 2 21.0 22 273 258 23 (159) 338 0 22.5
EXAMPLES 23-36
[0062] A different set of examples was prepared generally by the
process described in Examples 1-17 except that polyethylene
terephthalates of different molecular weight, or intrinsic
viscosity, were used. Specifically, the PET used in Examples 23-31
had an intrinsic viscosity of 0.5, and the PET used in Examples
32-36 had an intrinsic viscosity of 0.45. Results are reported in
Table 3. A DSC plot for a representative fiber of Example 31, as
collected, is pictured in FIG. 2 and a WAXS diagram for that fiber
is pictured in FIG. 3. A DSC plot for a representative fiber after
the collected web was annealed at 160.degree. C. for 5 minutes is
pictured in FIG. 4, and a WAXS diagram for that fiber is pictured
in FIG. 5. Note that the post-annealed fibers in FIG. 4 retain the
dual melting peak. The WAXS diagram also indicates that after
annealing, the SIC portion is retained and crystallinity has
increased.
[0063] Atomic force micrographs of unannealed fibers like those
made according to Example 31 were prepared using a scanning probe
microscope (SPM supplied by Digital Instruments (Santa Barbara,
Calif.) "Dimension 5000"). The fibers were imaged in tapping-mode
AFM mode (TM-AFM) using silicon probes (OMCL-AC160TS, Olympus,
Japan). Some fibers were then etched in an unstirred 30% NaOH
solution for 5 hours, then rinsed copiously with de-ionized water.
The fibers were air dried before imaging. The images (2.88
micrometer by 1.44 micrometer) were scanned in the longitudinal
direction on the fiber (images perpendicular to the fiber direction
(not shown) were also captured to confirm the directionality of the
structures in the fiber direction). FIG. 11a shows the fiber before
etching and FIG. 11b shows the fiber after etching.
3TABLE 3 Average Extruder Die Air Die Air Air Measured Temp. Temp.
Pressure PET Velocity % EFD Diameter Ex. (C.) (C.) (psi) IV (m/sec)
Shrinkage (micr ns) (microns) 23 260 246 8 0.50 164 4 15.4 9.2 24
260 245 10 0.50 193 4 18.6 25 260 245 12 0.50 210 3 17.9 26 260 245
14 0.50 231 3 19.1 27 260 245 16 0.50 261 3 18.6 28 260 246 23 0.50
333 1 22.0 29 273 259 12 0.50 220 8 9.0 6.3 30 273 259 14 0.50 239
1 8.4 6.5 31 273 259 16 0.50 262 1 9.1 32 260 245 10 0.45 202 9
12.5 33 260 244 12 0.45 211 6 13.5 34 260 245 14 0.45 234 6 14.6 35
260 246 16 0.45 255 8 13.5 36 260 246 23 0.45 334 3 16.3
EXAMPLE 37
[0064] A nonwoven fibrous web was prepared using two meltblowing
dies vertically aligned one over the other and spaced 9 inches (23
cm) apart. The dies were angled 45 degrees to the centerline
separating the two dies, so that the fiber streams from each die
converged and merged in front of the dies. Both meltblowing dies
were configured as described in Examples 1-17 with the exception
that the width of the slots 15 (the dimension 21 in FIG. 1) was set
at 0.060 inch (1.52 mm) and the die tip to air slot negative
setback was 0.049 inch (1.25 mm). PET meltblown fibers were
prepared on the first die from PET resin of 0.52 intrinsic
viscosity extruded at a rate of 1.0 pounds/inch/hour. The
processing temperature for the PET polymer was 273.degree. C. The
temperature of the attenuating air pressing through the slot 15 was
255.degree. C. The air pressure was set at 11 psi (76 kilopascal).
Meltblown polyethylene fibers were prepared on the second die at a
throughput rate of 0.4 pounds/inch/hour from polyethylene resin
6806 available from The Dow Chemical Company. The processing
temperature for the polythylene resin was set at 265.degree. C. The
temperature of the attenuating air passing through the slot 15 was
230.degree. C. The air pressure was set at 3 psi (21
kilopascal).
[0065] A web comprising 71 weight-percent PET fibers and 29
weight-percent PE fibers was collected at a rate that produced a
basis weight of about 377 grams/square meter. The collector was
spaced 26 inches (66 cm) from the plane defined by the two
meltblowing die tips.
[0066] The prepared web was thermally bonded and annealed by
heating in an oven at 160.degree. C. for 5 minutes. The web was
tested for sound absorption using an impedance tube as described in
the ASTM E-1050 test method. The test was replicated once and the
average results are reported in Table 4 and depicted in FIG. 12.
The effective fiber diameter of the webs of Example 37 was about 13
micrometers, the webs had an average bulk density of about 14.6
kilograms per cubic meter, and the webs showed a pressure drop of
about 1.2 mm water (based on tests of 6 sample webs) under the
previously stated measurement conditions.
4TABLE 4 Frequency 160 200 250 315 400 500 630 800 (hz) %
Absorption 2.10 5.95 6.70 9.15 12.65 17.50 22.90 30.90 Coefficient
Frequency 1000 1250 1600 2000 2500 3150 4000 5000 6300 (hz) %
Absorption 39.95 50.40 62.70 75.00 85.40 92.75 94.90 92.35 89.70
Coefficient
EXAMPLES 38-40
[0067] A series of webs of the invention were prepared from PET
having an intrinsic viscosity of 0.50 using a meltblown die as
described in Examples 1-17. The processing temperature for the PET
polymer was set to 273.degree. C. and the temperature of the air
passing through the slot 15 was set to 258.degree. C. The collector
was set as described in Examples 1-17 to produce a web of about 260
grams/square meter. The webs were annealed at 160 degrees C. for 5
minutes and then measured for tensile properties using tests as
described in ASTM D 5034 (maximum load, in pounds-force) in the
machine direction and using an Instron Tensile Tester (Model 4302)
at a separation rate of 12 inches/minute (30.48 cm/minute). The jaw
gap was set to 0.25 inches (0.64 cm) and the sample width was 1.0
inch (2.54 cm). The test was based on 5 samples and the averaged
results are reported in Table 5.
5TABLE 5 Air Pressure Drop m.d. Die Air @ 3.12 m/min Web Basis Max.
Load Pressure Face Velocity Weight EFD Avg. lb/in Ex. (psi) (kPa)
(mm H2O) (g/m2) (microns) (N/m) 38 12 (83) 3.30 255 9.0 120
(21,000) 39 14 (97) 3.14 230 8.4 128 (22,400) 40 16 (110) 3.26 299
9.1 128 (22,400)
EXAMPLES 41-42
[0068] Two nonwoven, fibrous composite webs comprising meltblown
fibers and staple fibers were prepared as described in U.S. Pat.
No. 4,118,531 (Hauser). The webs included meltblown PET microfibers
prepared from PET resin having an intrinsic viscosity of 0.52 and
using a meltblowing die generally as illustrated in FIG. 1 and as
described generally in Examples 1-17, but of a 47-inch (119
centimeter) width and a combined width of the slots 15 (the
dimension 21) of 0.059 inch (1.5 mm). The webs also included a
mixture of staple crimped bulking fibers and bicomponent
thermobonding staple fibers (Example 41) or just bicomponent
thermobonding staple fibers (Example 42). More specifically,
Example 41 comprised 63% PET meltblown fibers, 18.5% 6-denier
polyester staple fiber (Type 295 of 1.5-inch length available from
KoSa), and 18.5% bicomponent thermobonding fiber (Type T-257
available from KoSa). The basis weight of the collected combination
web was 280 grams per square meter. Example 42 comprised 80% PET
meltblown fibers, and 20% bicomponent 1.5-inch long thermobonding
fiber (Type T-252 available from KoSa). The basis weight of the
collected combination web was 275 grams per square meter.
[0069] The collected webs were thermally bonded and annealed by
heating in a conveyor oven at 160 degrees C. for one minute, after
which the webs were sewn between pieces of rip-stop nylon to
prepare twenty-two inch square samples. The webs were washed or
laundered in a front-loading washer (UNIMAC Model UF50) with a
water temperature of 170.degree. C. and then dried. This was
repeated for 50 cycles. Prior to laundering, the thermal insulation
value was measured in clo as described in ASTM D1518 and measured
again after the 50.sup.th washing cycle. The web thickness was
measured before and after the 50 laundering cycles as described in
ASTM D5736 using a pressure plate force of 0.002 pound per square
inch. The percent thickness loss is reported. Both exemplary webs
passed the visual test for durability and the results are set forth
in Table 6.
6 TABLE 6 CLO CLO Before 50 After 50 Wash % Thickness Example Wash
Cycles Cycles Loss 42 2.6 1.7 45 43 2.2 1.3 29
* * * * *