U.S. patent application number 14/520369 was filed with the patent office on 2015-04-23 for melt-spun polypropylene fine-grade nanofibrous web.
The applicant listed for this patent is E I DU PONT DE NEMOURS AND COMPANY. Invention is credited to Thomas Patrick Daly, Zachary R. Dilworth, TAO HUANG.
Application Number | 20150111456 14/520369 |
Document ID | / |
Family ID | 51842933 |
Filed Date | 2015-04-23 |
United States Patent
Application |
20150111456 |
Kind Code |
A1 |
HUANG; TAO ; et al. |
April 23, 2015 |
MELT-SPUN POLYPROPYLENE FINE-GRADE NANOFIBROUS WEB
Abstract
The present invention is directed toward a to fine-grade
stand-alone nanoweb and nanofibrous membrane comprising a nanofiber
network with a number average nanofiber diameter less than 200 nm
and the mean flow pore size less than 1000 nm that yield the
selective barrier medium with a superior balance of flow versus
barrier properties.
Inventors: |
HUANG; TAO; (Downingtown,
PA) ; Daly; Thomas Patrick; (Aston, PA) ;
Dilworth; Zachary R.; (Wilmington, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I DU PONT DE NEMOURS AND COMPANY |
Wilmington |
DE |
US |
|
|
Family ID: |
51842933 |
Appl. No.: |
14/520369 |
Filed: |
October 22, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61893961 |
Oct 22, 2013 |
|
|
|
Current U.S.
Class: |
442/351 |
Current CPC
Class: |
D04H 1/724 20130101;
D10B 2505/04 20130101; D04H 1/72 20130101; D04H 1/4291 20130101;
Y10T 442/626 20150401; D01F 6/06 20130101; D10B 2321/022 20130101;
D04H 3/007 20130101; D04H 1/736 20130101; D04H 3/016 20130101; D04H
1/4382 20130101; D10B 2401/10 20130101; D01D 5/14 20130101; D01D
5/18 20130101 |
Class at
Publication: |
442/351 |
International
Class: |
D04H 1/4291 20060101
D04H001/4291; D04H 1/724 20060101 D04H001/724 |
Claims
1. A melt-spun polypropylene fine-grade nanofibrous web comprising
a nanofiber network with a number average nanofiber diameter of
less than about 200 nm and the mean flow pore size of less than
about 1000 nm.
2. The nanofibrous web of claim 1, wherein the nanofiber network
has a fiber diameter both mean and median of less than about 200 nm
and the individual nanofibers have a fiber diameter in the range of
a minimum of about 10 nm to a maximum of about 1000 nm.
3. The nanofibrous web of claim 1, wherein the nanofibrous web has:
(a) less than about 5% Mw reduction of the nanofibrous web as
compared to the polymer used for making the nanofibrous web; (b)
essentially the same thermal weight loss as compared to the polymer
used for making the nanofibrous web as measured by TGA; and (c)
higher crystallinity of the nanofibrous web as compared to the
polymer used for making the nanofibrous web.
Description
[0001] This application claims the benefit of priority of U.S.
Provisional Application No. 61/893,961 filed Oct. 22, 2013, which
is incorporated herein by reference in it's entirety.
FIELD OF THE INVENTION
[0002] This invention relates to melt-spun polypropylene fine-grade
nanofibrous web comprising a nanofiber network with a number
average nanofiber diameter less than 200 nm and the mean flow pore
size less than 1000 nm.
BACKGROUND
[0003] The increased surface to volume ratio afforded by nanofibers
has significant influences on a broad range of applications. In
particular, in filter performance, which is based on producing the
highest flow rate while trapping and retaining the finest particles
without blocking the filter, nanofibers have improved interception
and inertial impaction efficiencies and result in slip flow at the
fiber surface, affording better performance at a given pressure
drop. Consequently, nanofibers as a coating layer on substrate or
laminated with a substrate are currently incorporated into filters
in air, liquid and automotive applications.
[0004] Polymer nanofibers can be produced from solution-based
electrospinning or electroblowing, however they have very high
processing cost, limited throughputs and low productivity. Melt
blowing nanofiber processes that randomly lay down fibers do not
provide adequate uniformity at sufficiently high throughputs for
most end use applications. The resulting nanofibers are often laid
on substrate layer of coarse fiber nonwoven or microfiber nonwoven
to construct multiple layers. A problem with melt-blown
polypropylene nanofibers or small microfibers, exposed on the top
of the web, they are very fragile and are easily crushed by normal
handling or contact with some object. Also, the multilayer nature
of such webs increases their thickness and weight, and also
introduces some complexity in manufacture. Centrifugal spun
nanofiber process has demonstrated the lower manufacturing cost in
massive nanoweb production.
[0005] U.S. Pat. No. 8,277,711 B2 to DuPont discloses a nozzle-less
centrifugal melt spin process through rotational thin film
fibrillation. The nanofibers with number average diameter less than
about 500 nm have been claimed and shown in the examples spun from
polypropylene and polyethylene resins. In practice, the operation
window is very narrow for making the uniform nanofibers due to the
requirement of uniform and smooth thin film flow on the inner
surface of the spinning disk, which requires the right rheological
properties of polymer and the right combination of the temperature,
the rotating speed and melt feeding rate. Otherwise, there would
not have a uniform and smooth thin film flow on the inner surface
of the spinning disk. As results, the instability of the thin film
flow and variation of the thickness in the thin film will cause the
formation of larger fibers mixed with the nanofibers.
[0006] The nanofibers made from the process of U.S. Pat. No.
8,277,711 B2 can be laid on a belt collector to form uniform web
media using the process of WO 2013/096672, in which the complicate
air flow management needs to be implemented. Otherwise, the uniform
web cannot be laid down because of the swirling and the twisting of
fiber stream due to the "tornado"-like effect under the high speed
rotating disk. U.S. Pat. No. 8,231,378 B2 to University of Texas
(later the FibeRio Technology Corporation) discloses a centrifugal
nanofiber spinning from rotating spinnerets with nozzles), such as,
syringes, micro-mesh pores or non-syinge gaps with a typical
openings of diameter sizes of 0.01-0.80 mm. The microfibers with
the number average diameter of one micron or larger and the
nanofibers have been shown. The nanofiber with number average
diameter less than about 300 nm has been disclosed. In general, the
centrifugal spinning through nozzles has much less throughput due
to the capillary flow through the nozzle orifices and the melt die
swell at the nozzle exit. For the current state of the art, only
very low basis weight of thin layer nanofibers can be deposited on
scrim when the polypropylene nanofiber is spun from a melt. The
polypropylene about 600nm has been reported with the mixture of
fibers with defects, especially the powders and and the"spatters".
The PP web has very low strength and is difficult to handle without
scrim due to the thermal degeradation.
[0007] What is needed is the improvement of centrifugal melt spun
nanofiber process to make to make a fine-grade nanofibrous web.
SUMMARY OF THE INVENTION
[0008] The present invention is directed toward a melt-spun
polypropylene fine-grade nanofibrous web comprising a nanofiber
network with a number average nanofiber diameter of less than about
200 nm and the mean flow pore size of less than about 1000 nm.
BRIEF DESCRIPTION OF THE FIGURES
[0009] FIG. 1A is a low magnification SEM image and FIG. 1B is a
high magnification SEM image of the web structure in the present
invention.
[0010] FIG. 2 is an illustration of the apparatus using a spinning
disk based on the process of U.S. Pat. No. 8,277,711 B2 with the
improvements according to the present invention.
[0011] FIG. 3 is an illustration of the spin disk with the
stand-alone web collector for improving process of U.S. Pat. No.
8,277,711 B2 according to the present invention.
[0012] FIG. 4A is the graphical form of the number average of
nanofiber diameter distribution of Example 1 in the present
invention. FIG. 4B is the tabular form of the number average of
nanofiber diameter distribution of Example 1 in the present
invention.
[0013] FIG. 5 is the pore size distribution of Example 1 in the
present invention.
[0014] FIG. 6 is the thermogravimetric analysis (TGA) data of web
sample of Example 1 and the polymer resins pellets used in making
Example 1.
[0015] FIG. 7 is the molecular weight (Mw) data of web of Example 1
and the polymer resins pellets used in making Example 1 measured by
using high temperature size exclusion chromatography (SEC).
[0016] FIG. 8 is the Differential Scanning calorimeter (DSC)
thermal analysis data of web sample of Example 1 and the polymer
resins pellets used in making Example 1.
[0017] FIGS. 9A and 9B show the SEM images of Comparative Example 1
at 250.times. and 10,000.times. magnification, respectively.
[0018] FIG. 10 is the pore size distribution of Comparative Example
1.
[0019] FIGS. 11A and 11B show the SEM images of Comparative Example
2 at 250.times. and 10,000.times. magnification, respectively.
[0020] FIG. 12 is the pore size distribution of Comparative Example
2.
DETAILED DESCRIPTION
Definitions
[0021] The term "web" as used herein refers to layer of a network
of fibers commonly made into a nonwoven.
[0022] The term "nonwoven" as used herein refers to a web of a
multitude of essentially randomly oriented fibers where no overall
repeating structure can be discerned by the naked eye in the
arrangement of fibers. The fibers can be bonded to each other, or
can be unbounded and entangled to impart strength and integrity to
the web. The fibers can be staple fibers or continuous fibers, and
can comprise a single material or a multitude of materials, either
as a combination of different fibers or as a combination of similar
fibers each comprising of different materials.
[0023] The term "nanofibrous web" as used herein refers to a web
constructed predominantly of nanofibers. "Predominantly" means that
greater than 50% of the fibers in the web are nanofibers.
[0024] The term "nanofibers" as used herein refers to fibers having
a number average diameter less than about 1000 nm. In the case of
non-round cross-sectional nanofibers, the term "diameter" as used
herein refers to the greatest cross-sectional dimension.
[0025] The term "microfibers" as used herein refers to fibers
having a number average diameter from about 1.0 .mu.m to about 3.0
.mu.m
[0026] The term "coarse fibers" as used herein refers to fibers
having a number average diameter greater than about 3.0 .mu.m.
[0027] The term "coarse-grade nanofibrous web" as used herein
refers to the nanofibrous web having the mean flow pore size
greater than about 5.0 .mu.m.
[0028] The term "intermediate-grade nanofibrous web" as used herein
refers to the nanofibrous web having the mean flow pore size
greater than about 1.0 .mu.m and smaller than 5.0 .mu.m.
[0029] The term "fine-grade nanofibrous web" as used herein refers
to the nanofibrous web having the mean flow pore size smaller than
about 1.0 .mu.m.
[0030] The term "stand-alone" as used herein refers to the
nanofibrous web is a single layer, self-contained and without any
substrate.
[0031] The term "centrifugal spinning process" as used herein
refers to any process in which fibers are formed by ejection from a
rotating member.
[0032] The term "rotating member" as used herein refers to a
spinning device that propels or distributes a material from which
fibrils or fibers are formed by centrifugal force, whether or not
another means such as air is used to aid in such propulsion.
[0033] The term "concave" as used herein refers to an inner surface
of a rotating member that can be curved in cross-section, such as
hemispherical, have the cross-section of an ellipse, a hyperbola, a
parabola or a frustoconical, or the like.
[0034] The term "spin disk" as used herein refers to a rotating
member that has a disk shape with a concave, frustoconical or flat
open inner surface.
[0035] The term "fibril" as used herein refers to an elongated
structure that may be formed as a precursor to fine fibers that
form when the fibrils are attenuated. Fibrils are formed at a
discharge point of the rotating member. The discharge point may be
an edge, serrations or an orifice through which fluid is extruded
to form fibers.
[0036] The term "nozzle-free" as used herein refers to the fibril
or fibers that are not from a nozzle-type spinning orifices,
including nozzles on a rotating member.
[0037] The term "charged" as used herein refers to an object in the
process that has a net electric charge, positive or negative
polarity, relative to uncharged objects or those objects with no
net electric charge.
[0038] The term "spinning fluid" as used herein refers to a
thermoplastic polymer in either melt or solution form that is able
to flow and be formed into fibers.
[0039] The term "discharge point" as used herein refers to a
location on a spinning member from which fibrils or fibers are
ejected. The discharge point may, for example, be an edge, or an
orifice through which fibrils are extruded.
[0040] The term "serration" as used herein refers to a saw-like
appearance or a row of sharp or tooth-like projections. A serrated
cutting edge has many small points of contact with the material
being cut.
[0041] The term "tornado-like" as used herein refers to a violently
rotating column of fibers that is in contact with both the surface
of the collector and a cumulonimbus cloud of the swirling fiber
bundles.
[0042] The term "essentially" as used herein refers to that if a
parameter is held "essentially" at a certain value, then changes in
the numerical value that describes the parameter away from that
value that do not affect the functioning of the invention are to be
considered within the scope of the description of the
parameter.
[0043] The present invention is directed toward to melt-spun
polypropylene fine-grade stand-alone nanoweb and nanofibrous
membrane comprising a nanofiber network with a number average
nanofiber diameter around or less than 200 nm and the mean flow
pore size less than 1000 nm, the SEM images as shown as in FIG. 1,
the number average of nanofiber diameter distribution as shown as
in FIGS. 4A and 4B and the pore size distribution as shown as in
FIG. 5.
[0044] In principle, the nonwoven web can be made using the
centrifugal melt spinning process as disclosed in U.S. Pat. No.
8,277,711 B2. The nanofiber formation is through uniform thin film
fibrillation. The melt flow spread on the inner surface of the spin
disk to form a thin film. The film fibrillation occurs at the edge
of spinning disk and forms thin threads. These thin threads are
further stretched into fibers by centrifugal force. For a given
polymer, nanofibers are formed from a uniform stable thin film
fibrillation in U.S. Pat. No. 8,277,711 B2. The operation
parameters of fiber spinning are temperatures, melt feeding rate
and disk rotating speed. In practice of U.S. Pat. No. 8,277,711 B2,
the fully pure nanofibers can only be made from the uniform and
smooth thin film flow on the inner surface of the spinning disk,
which requires the right rheological properties of polymer and the
right combination of the temperature, the rotating speed and melt
feeding rate. However, the surface of the rotating polymer thin
film on the inner surface on the open-end spinning disk would be
cooling down due to reaction with the cold air brought in by the
high speed rotating. In practice, the heating to the spinning disk
would be to the higher temperature in order to have the right melt
viscosity and the uniform thin film flow. Therefore, there was a
potential thermal degradation if the temperature was set too high.
The present invention is about to address this problem. A thermal
shield on top of the spinning disk is designed to minimize the
reduction of the surface temperature of the rotating polymer thin
film. With the thermal shield on top of the spinning disk will
lower the disk heating temperature to minimize or to eliminate the
thermal degradation.
[0045] Considering FIG. 2 for spinning disk 205 mounted on a high
speed rotating hollow shaft 200, fibers 210 are shown exiting the
discharge points at the edge of the spinning disk. A protecting
shield 206 with the same diameter as the spinning disk is mounted
on top of the spinning disk as a thermal protecting shield for melt
spinning in order to prevent the heating lost to the inner surface
of the spinning disk; as an air protecting shield for solution
spinning to prevent the rapidly solvent evaporation from the thin
film flow on the inner surface of the spinning disk.
[0046] The protecting shield is placed to contact to the serrations
on the edge of the rotating disk to form an enclosed serrations.
The enclosed serrations on the edge of the rotating disk suppress
the instability of the thin film flow and variation of the
thickness at the edge of the spinning disk.
[0047] A stationary shield 208 for the spinning disk is mounted on
a stationary shaft through the rotating hollow shaft at the bottom
of the spinning disk to protect the thermal lost, and to prevent
the swirling and the twisting of fiber stream due to the
"tornado"-like effect under the high speed rotating disk for the
uniform web laydown.
[0048] A stretching zone surrounding the edge of the rotating disk
is indicated in the dash line rectangle area. The stretching zone
temperature is established by the gentle air comes from the
combination of three heating air streams. One is from the gentle
heating air 202 above the spinning disk; another is from a stream
of gentle heating air 209 coming from a stationary hot air tube
within the rotating hollow shaft 200, through the gap between the
bottom of the spinning disk and the stationary shield to reach the
stretching zone; the other gentle heating air is a downward flow
201. The stretching zone temperature is designed and implemented to
keep the threads in molten state to maximize the stretching or
elongation by the centrifugal force. The stretching zone diameter
is about 1.5 times the diameter of the spin disk. The stretching
zone temperature is the key element to make the nanofibers. For
polypropylene in Example, the stretching zone temperature is
optimized around 180.degree. C. by the gentle heating air for the
better nanofiber spinning and for the fibers to take electrostatic
charging as an option.
[0049] The nanofibers are deposited on the surface of a horizontal
scrim belt collector or a vertical tubular scrim belt collector
using the web laying process of WO 2013/096672, then a roll of the
web is wind-up as a stand-alone web roll off from the collection
belt. Typically, fibers do not flow in a controlled fashion towards
the collector and do not deposit evenly on the collector. The
improved process of WO 2013/096672 with the stationary shield under
the spinning disk is used in the present invention. The stationary
shield prevents the "tornado"-like affect under the high speed
rotating disk, therefore, the swirling and the twisting of fiber
stream are eliminated in the present invention. A charged ring 203
is optional with needle assembly or a ring saw with sharp teeth is
mounted on the top of stretching zone air heating ring for applying
the electrostatic charge to fibrils and fibers 210 being ejected
from a spinning disk.
[0050] Considering FIG. 3 for the fibers laydown on a belt
collector to form nanofibrous web, 301 is the spin pack shown in
FIG. 2. The nanofibrous web 300 is laid on a vacuum box web laydown
collector 310 may be placed under the whole spin pack. The
collector may have a perforate surface. Vacuum is applied to
collector with the highest vacuum strength at the corners and the
edges of the collector and the vacuum strength gradually reduce
moving away from the corners and the edges of the collector to the
center of the collector where the vacuum strength is zero. The
fibers were collected on a circling belt 302, driven by 303, with
304 as a tension adjusting roll, 305 is a supporting roll for the
stand-alone nanofibrous web, and the stand-alone web is sent
through a pair of nips 306 and onto a wind-up roll, 307, and is
taken up.
[0051] The present invention is directed toward a melt-spun
polypropylene fine-grade nanofibrous web comprising a nanofiber
network with a number average nanofiber diameter of less than about
200 nm and the mean flow pore size of less than about 1000 nm.
[0052] The nanofiber network has a fiber diameter both mean and
median of less than about 200 nm and the individual nanofibers have
a fiber diameter in the range of a minimum of about 10 nm to a
maximum of about 1000 nm.
[0053] The nanofibrous web has: (a) less than about 5% Mw reduction
of the nanofibrous web as compared to the polymer used for making
the nanofibrous web, (b) essentially the same thermal weight loss
as compared to the polymer used for making the nanofibrous web as
measured by TGA, and (c) higher crystallinity of the nanofibrous
web as compared to the polymer used for making the nanofibrous
web.
Test Methods
[0054] High-Speed Video Image: In order to visualize the filming
and fiber spinning, high-speed video image has been used for
observing the spinning of poly(ethylene oxide) (PEO) in water
solutions. Weight percent solutions ranging between 0% and 12% of
300,000 Mw PEO, purchased from Sigma-Aldrich, were prepared in
deionized water. A Harvard apparatus PHD2000 Infusion syringe pump
was used to control the flow rate of solution to a rotating
geometry spinning between 1,000 and 30,000 RPM. Flow rates examined
range between 0.01 to 50.00 mL/min. Two Photon FASTCAM SA5 model
1300K-M3 high speed video cameras with Canon 100 mm Macro lenses
were used to capture the images included in this case with one
camera positioned parallel and one camera positioned perpendicular
to the spinning geometry. The camera and lens settings were chosen
to maximize clarity at 7,000 fps, shudder speeds ranging between
0.37 and 4.64 ps, and apertures between f2.8 and f32.
[0055] Thermal analysis: In order to study the thermal degradation
and crystallinity, thermal analysis was conducted using TA
Instruments a Q2000 series differential scanning calorimeter (DSC)
and a Q500 series thermo gravimetric analyzer (TGA). DSC samples
underwent a standard heating, cooling, re-heating cycle from room
temperature to 250.degree. C. at 10.degree. C./min under nitrogen.
TGA samples underwent a standard ramp heat from room temperature to
900.degree. C. at 10.degree. C./min under nitrogen. TA Instruments
Universal Analysis 2000 was used to analyze thermal data.
[0056] The percent crystallinity of samples was determined using
the accepted value for the enthalpy of fusion for 100% crystalline
polypropylene equaling 207 J/g. (REFERENCE: A van der Wal, J. J
Mulder, R. J Gaymans. Fracture of polypropylene: The effect of
crystallinity. Polymer, Volume 39, Issue 22, October 1998, Pages
5477-5481)
[0057] Measurement of Molecular Weight: Molecular weight for
polyolefin resins was measured by using high temperature size
exclusion chromatography (SEC). This method includes the use of
multi-angle light scattering and viscosity detectors in
trichlorobenzene (TCB) at 150.degree. C. The instruments used
include a Polymer Laboratories PL220 liquid chromatograph
instrument, with solvent delivery and autoinjector, and a Wyatt
Technologies Dawn HELEOS multi-angle light scattering detector
(MALS). The Polymer Laboratories SEC includes an internal
differential viscometer and differential refractometer. Four
Polymer Laboratories mixed B SEC columns were used for the
separations. The sample injection volume was 200 microliters with a
flow rate of 0.5 mL/min. The sample compartment, columns, internal
detectors, transfer line, and Wyatt MALS were held at a controlled
temperature between 150 and 160.degree. C. depending on the
polymer. After the solution passes through the columns within the
Polymer Laboratories SEC, the flow was directed out of the
instrument and through a heated transfer line to the Wyatt MALS
before being returned back to the Polymer Laboratories SEC. The
data recovered from the instrumentation was analyzed using Wyatt
Technologies Astra software. The concentration was calculated using
a do/dc of 0.092 for polyolefin in TCB. Molecular weights were
calculated from the light scattering intensities rather than
elution time, and are not relative to standards. In order to ensure
instrument performance and accuracy, available NIST polyethylene
standards are periodically analyzed.
[0058] Measurement of Web Strength: Tensile strength and elongation
of nanofibrous web samples were measured using an INSTRON tensile
tester model 1122, according to ASTM D5035-11, "Standard Test
Method for Breaking Force and Elongation of Textile Fabrics (Strip
Method)" with modified sample dimensions and strain rate. Gauge
length of each sample is 2 inches with 0.5 in. width. Crosshead
speed is 1 inch/min (a constant strain rate of 50% min.sup.-1).
Samples are tested in the "Machine Direction" (MD) as well as in
the "Transverse Direction" (TD). A minimum of 3 specimens are
tested to obtain the mean value for tensile strength or
elongation.
[0059] SEM: Scanning Electron Microscope (SEM) image was used
dominantly in nanofiber characterization because it delivers superb
image clarity at high magnification and has become the industry
standard for measuring nanofiber diameter. The differences of
nanofiber morphology in high magnification SEM images with
.times.5,000 or .times.10,000 of nanofibrous webs produced from
different nanofiber processes are difficult to be distinguished
beside the fiber diameter. In order to reveal the fiber morphology
in different levels of details, the SEM images were taken at
.times.25, .times.100, .times.250, .times.500, .times.1,000,
.times.2,500, .times.5,000 and .times.10,000.
[0060] Mean Flow Pore Size was measured according to ASTM E
1294-89, "Standard Test Method for Pore Size Characteristics of
Membrane Filters Using Automated Liquid Porosimeter." Individual
samples of different size (8, 20 or 30 mm diameter) were wetted
with the low surface tension fluid as described above and placed in
a holder, and a differential pressure of air was applied and the
fluid removed from the sample. The differential pressure at which
wet flow is equal to one-half the dry flow (flow without wetting
solvent) is used to calculate the mean flow pore size using
supplied software. Mean flow pore size was reported in .mu.m.
[0061] Bubble Point was measured according to ASTM F316, "Standard
Test Methods for Pore Size Characteristics of Membrane Filters by
Bubble Point and Mean Flow Pore Test." Individual samples (8, 20 or
30 mm diameter) were wetted with the low surface tension fluid as
described above. After placing the sample in the holder,
differential pressure (air) is applied and the fluid was removed
from the sample. The bubble point was the first open pore after the
compressed air pressure is applied to the sample sheet and is
calculated using vendor supplied software.
[0062] Pore Size Uniformity Index: The uniformity index (UI) for
the pore size is defined as the ratio of the difference in bubble
point diameter and the minimum pore size to the difference in the
bubble point and mean flow pore.
UI pore = BP - Min BP - MFP ##EQU00001##
[0063] The closer this ratio is to the value of 2, and then the
pore distribution is a Gaussian distribution. If the Uniformity
Index is very much larger than 2, the nanofibrous structure is
dominated by pores whose diameters are much bigger than the mean
flow pore. If the Uniformity Index (UI) much lower than 2, then the
more structure is dominated by pores which have pore diameters
lower than the mean flow pore diameter. There will still be a
significant number of large pores in the tail end of the
distribution.
EXAMPLES
Example 1
[0064] Continuous fibers were made by a spin disk with the enclosed
serrations and the stationary shield using an apparatus as
illustrated in FIG. 3, from a polypropylene (PP) homopolymer,
Metocene MF650Y from LyondellBasell. It has Mw=75,381 g/mol, melt
flow rate=1800 g/10 min (230.degree. C./2.16 kg), and zero shear
viscosity of 9.07 Pa.S at 200.degree. C. The temperature of the
spinning melt from the melt transfer line was set to 240.degree. C.
The temperature of spin disk edge was about 200.degree. C. The
stretching zone heating air was set at 250.degree. C. The
stretching zone air through the gap between the disk and the
stationary shield was set at 200.degree. C. with the air flow rate
of 50 SCFH. The downward shaping air was set at 150.degree. C. The
shaping air flow was set at 50 SCFH. The rotation speed of the spin
disk was set to a constant 10,000 rpm.
[0065] The fiber size was measured from an image using scanning
electron microscopy (SEM) as shown as in FIG. 1 and the
distribution of the number average diameter of the nanofibers is
shown in FIG. 5. Example 1 has a fiber diameter mean and median for
the total fibers measured of 217.31 nm and 193.85 nm from total
counts of 973 individual nanofibers in the range of the minimum of
64.12 nm to the maximum of 872.47 nm, respectively. The PMI
measurement result shows that the nanofibrous web has a mean flow
pore (MFP)=504.1 nm, MO=465.6 nm, Min=197.7 nm and Max (BP)=3442.2
nm. 51 MFP-M0|=38 nm, UI=1.104.
[0066] FIG. 6 shows the almost identical TGA measurement of the
nanofibrous web of Example 1 and the polymer resin pellets used in
making the web. FIG. 7 shows the macromolecules weight measurement
of the nanofibrous webs of Example 1 and the polymer resin pellets
used in making the web. There is small reduction of macromolecules
weight in the nanofibrous webs of Example 1 comparing to the
polymer resin pellets used in making the web. FIG. 8 shows the
crystallinity of the nanofibrous web is higher than the polymer
resin used for making nanofibers from the DSC measurement. Overall,
the measurements show that the thermal degradation has been reduced
to minimum.
Comparative Example 1
[0067] Continuous fibers were made by an open-end spin disk using
the process of U.S. Pat. No. 8,277,711 B2, from the same
polypropylene (PP) homopolymer used in Example 1. A PRISM extruder
with a gear pump was used to deliver the polymer melt to the
rotating spin disk through melt transfer line. The temperature of
the spinning melt from the melt transfer line was set to
200.degree. C. The temperature of spin disk edge was to be about
240.degree. C. The stretching zone heating air was set at
200.degree. C. The downward shaping air was set at 150.degree. C.
The shaping air flow was set at 15.0 SCFM. The rotation speed of
the spin disk was set to a constant 10,000 rpm.
[0068] The fiber size was measured from an image using scanning
electron microscopy (SEM) as shown as in FIGS. 9A and 9B.
Comparative Example 1 has a fiber diameter mean and median for the
total fibers measured of 685 nm and 433 nm from total counts of 583
individual nanofibers in the range of the minimum of 126 nm to the
maximum of 8460 nm.
Comparative Example 2
[0069] Continuous fibers were made by an open-end spin disk using
the process of U.S. Pat. No. 8,277,711 B2, from the same
polypropylene (PP) homopolymer used in Example 1. The temperature
of the spinning melt from the melt transfer line to the rotating
spin disk was set to 200.degree. C. The temperature of spin disk
edge was about 200.degree. C. The stretching zone heating air was
set at 180.degree. C. The downward shaping air was set at
150.degree. C.
[0070] The shaping air flow was set at 50.0 SCFH. The rotation
speed of the spin disk was set to a constant 10,000 rpm.
[0071] The fiber size was measured from an image using scanning
electron microscopy (SEM) as shown as in FIGS. 11A and 11B.
Comparative Example 2 has a fiber diameter mean and median for the
total fibers measured of 935 nm and 670 nm from total counts of 431
individual fibers in the range of the minimum of 172 nm to the
maximum of 17,052 nm. There are about 83.88% nanofibers, 14.92% of
microfibers and 1.2% coarse fibers.
* * * * *