U.S. patent number RE35,206 [Application Number 08/177,218] was granted by the patent office on 1996-04-16 for post-treatment of nonwoven webs.
This patent grant is currently assigned to The University of Tennessee Research Corporation. Invention is credited to Charles B. Hassenboehler, Jr., Larry C. Wadsworth.
United States Patent |
RE35,206 |
Hassenboehler, Jr. , et
al. |
April 16, 1996 |
Post-treatment of nonwoven webs
Abstract
A method for post-treating a precursor nonwoven web including
consolidating the web laterally and thereby reducing the maximum
pore size measure of the web. The resultant nonwoven web is also
disclosed as is utilization of the product web as a filter.
Inventors: |
Hassenboehler, Jr.; Charles B.
(Knoxville, TN), Wadsworth; Larry C. (Knoxville, TN) |
Assignee: |
The University of Tennessee
Research Corporation (Knoxville, TN)
|
Family
ID: |
26873044 |
Appl.
No.: |
08/177,218 |
Filed: |
January 4, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
858182 |
Mar 26, 1992 |
05244482 |
Sep 14, 1993 |
|
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Current U.S.
Class: |
55/528;
55/DIG.39; 55/DIG.5 |
Current CPC
Class: |
B01D
39/08 (20130101); B01D 39/163 (20130101); B32B
5/02 (20130101); B32B 5/26 (20130101); B32B
27/12 (20130101); D04H 13/00 (20130101); D06C
3/00 (20130101); D04H 3/02 (20130101); B01D
2239/0241 (20130101); B01D 2239/04 (20130101); B01D
2239/0492 (20130101); B01D 2239/0622 (20130101); B01D
2239/0627 (20130101); B01D 2239/065 (20130101); B01D
2239/1216 (20130101); B01D 2239/1233 (20130101); B01D
2239/1291 (20130101) |
Current International
Class: |
B01D
39/16 (20060101); B01D 39/08 (20060101); B32B
27/12 (20060101); B32B 5/02 (20060101); B32B
5/26 (20060101); B32B 5/22 (20060101); D06C
3/00 (20060101); D04H 13/00 (20060101); D04H
1/54 (20060101); B01D 039/16 () |
Field of
Search: |
;55/527,528,DIG.5,DIG.39 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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59-145013 |
|
Aug 1984 |
|
JP |
|
1347176 |
|
Feb 1974 |
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GB |
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Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Weiser & Associates
Claims
What is claimed is:
1. A .Iadd.planar .Iaddend.nonwoven web made from a .Iadd.planar
.Iaddend.precursor nonwoven web comprising randomly deposited
nonelastomeric thermoplastic fibers and being uniaxially drawn and
heat set, wherein the uniaxial heating and drawing are carried out
continuously by passing the precursor web into an oven .Iadd.at a
temperature in the range of 250.degree. and 425.degree. F. and
between the softening temperature and the melting point of the
thermoplastic fibers, thereby heat setting it .Iaddend.at a first
linear velocity and withdrawing the precursor web from the oven at
a second linear velocity and wherein the .[.ratio of the.]. second
velocity .[.to.]. .Iadd.exceeds .Iaddend.the first velocity
.[.ranges from about 1.1:1 to 2:1.]., whereby a majority of the
fibers are .[.consolidated and.]. aligned generally in the
direction of draw and a minority of segments of fibers are disposed
crosswise or transversely to the direction of draw, whereby the web
is .Iadd.consolidated in the cross-direction, longitudinally
elongated and heat set with respect to the precursor and
.Iaddend.characterized by a maximum pore size of less than 80%
.Iadd.of .Iaddend.that of the precursor web and has an elasticity
in the cross direction of at least 70% recovery from a 50%
elongation in the cross direction.
2. The nonwoven web of claim 1 wherein the .[.web is composed of
nonelastomeric thermoplastic meltblown.]. fibers .Iadd.are
meltblown, .Iaddend.randomly bonded and .[.having.]. .Iadd.have
.Iaddend.an average diameter of 0.5 to 8 microns.
3. The nonwoven web of claim 2 wherein the web has a mean flow pore
size of between 3 to 40 microns.
4. The nonwoven web of claim 1 wherein the fibers are at least
partially coated with a surfactant for increasing the water
wettability of the fabric.
5. A laminate comprising:
(a) the web of claim 1.[.;.]. and
(b) a polymeric substrate bonded to said web.
6. The .[.nonwoven web.]. .Iadd.laminate .Iaddend.of claim 5
wherein the substrate is an elastomeric web.
7. The .[.nonwoven web.]. .Iadd.laminate .Iaddend.of claim 5
wherein the substrate is a film.
8. The .[.nonwoven web.]. .Iadd.laminate .Iaddend.of claim 5
wherein the substrate is a textile that limits the cross direction
extensibility of the laminate to less than 100% strain.
9. A .Iadd.planar .Iaddend.nonwoven web made from a .Iadd.planar
precursor .Iaddend.nonwoven web, said precursor comprising a
lamination of individual nonwoven webs having nonelastomeric
.Iadd.thermoplastic .Iaddend.fibers and being uniaxially drawn and
heat set, wherein the uniaxial heating and drawing are carried out
continuously by passing the precursor web into a oven .Iadd.at a
temperature in the range of 250.degree. and 425.degree. F. and
between the softening temperature and the melting point of the
thermoplastic fibers, thereby heat setting it .Iaddend.at a first
linear velocity and withdrawing the precursor web from the oven at
a second linear velocity and wherein the .[.ratio of the.]. second
velocity .[.to.]. .Iadd.exceeds .Iaddend.the first velocity
.[.ranges from about 1.1:1 to 2:1.]., whereby a majority of the
fibers are .[.consolidated and .]. aligned generally in the
direction of draw and a minority of segments of fibers are disposed
crosswise or transversely to the direction of draw, whereby the web
is .Iadd.consolidated in the cross-direction, longitudinally
elongated and heat set with respect to the precursor and
.Iaddend.characterized by a maximum pore size of less than 80% that
of the precursor web and has an elasticity in the cross direction
defined by at least 70% recovery from a 50% elongation in the cross
direction.
10. A longitudinally extending .Iadd.planar .Iaddend.nonwoven web
made from a precursor .Iadd.planar .Iaddend.nonwoven web having
nonelastomeric, thermoplastic fibers characterized in that the
majority of the fibers .[.have been consolidated in a direction
transverse the length of the web and are.]. .Iadd.are
.Iaddend.disposed generally in the longitudinal direction of the
web, .Iadd.thereby consolidating the web in a direction transverse
to the length of the web is longitudinally elongated and heat set
with respect to the precursor, .Iaddend.wherein the consolidation
is carried out by passing the passing the precursor web into an
oven .Iadd.at a temperature in the range of 250.degree. and
425.degree. F. and between the softening temperature and the
melting point of the thermoplastic fibers, thereby heat setting it,
the passing being performed .Iaddend.at a first linear velocity and
withdrawing the precursor web from the oven at a second linear
velocity and wherein the .[.ratio of the.]. second velocity
.[.to.]. .Iadd.exceeds .Iaddend.the first velocity .[.ranges from
about 1.1:1 to 2:1.]., whereby the maximum pore size of the web
represents less than 70% of the maximum pore size of the precursor
web and, a minority of the fibers in the web .[.extent.].
.Iadd.extend .Iaddend.transverse the longitudinal direction of the
web and a portion of which are bonded to the longitudinal fibers,
.[.disposed.]. said web further being characterized as having a
level of elasticity defined by at least 70% recovery from a 50%
elongation in a direction crosswise of the longitudinal direction
of the web.
11. A filter comprising meltblown web having nonelastomeric
thermoplastic fibers having an average diameter of 0.5 to 8
microns, the majority of which are generally aligned in one
direction, wherein the fibers are aligned by passing a .Iadd.planar
.Iaddend.precursor web into an oven .Iadd.at a temperature in the
range of 250.degree. and 425.degree. F. and between the softening
temperature and the melting point of the thermoplastic fibers,
thereby heat setting it, the passing being performed .Iaddend.at a
first linear velocity and withdrawing the .[.precursor.]. web from
the oven at a second linear velocity and wherein the .[.ratio of
the.]. second velocity .[.to.]. .Iadd.exceeds .Iaddend.the first
velocity .[.ranges from about 1.1:1 to 2:1.]., said web .Iadd.being
planar and made from a planar precursor nonwoven web of randomly
deposited nonelastomeric thermoplastic fibers, consolidated in the
cross direction, longitudinally elongated and heat set with respect
to the precursor, thereby reducing the average pore size and the
pore size distribution in the web, and .Iaddend.having a mean flow
pore size of 2 to 50 microns, a pore size distribution spread of 4
to 100 microns, and an elastic recovery of at least 40% from a 100%
elongation in a direction normal to the direction of the aligned
fibers.
12. The filter of claim 11 wherein the filter having a basis weight
of less than 1.5 oz/sq. yd. is capable of capturing at least 50% of
particles larger than or equal to 1 micron measured at an aerosol
velocity of 10 centimeters/sec.
13. A face mask comprising the filter of claim 11.
14. The filter as defined in claim 11 wherein the fibers contain an
electrostatic charge.
15. The filter as defined in claim 11 wherein the web has an
electrostatic charge applied thereto.
16. A method for post treating a .Iadd.planar .Iaddend.nonwoven web
which comprises:
(a) selecting a .Iadd.planar .Iaddend.precursor nonwoven web made
of randomly collected nonelastomeric thermoplastic fibers having a
crystallinity of at least 30%, said web having a maximum processing
draw ratio at break of less than 4 at a temperature at least
10.degree. F. less than the melting point of the precursor
thermoplastic and at a strain rate of at least 2500%/min, and a
maximum pore size measure of from 4 to 250 microns based on ASTM
F316-86;
(b) heating the precursor web at a temperature between the
softening temperature and melting temperature of the
thermoplastic,
(c) drawing .Iadd.and heat-setting .Iaddend.the heated web under
tension in a substantially longitudinal direction to cause the web
to .Iadd.be longitudinally elongated and heat set with respect to
its precursor and .Iaddend.consolidate laterally .Iadd.without
disrupting the planar integrity of the web .Iaddend.and thereby
reduce the maximum pore size measure of the web by at least 20%,
wherein the heating and drawing steps are carried out continuously
by passing the precursor web into an oven .Iadd.at a temperature in
the range of 250.degree. and 425.degree. F. .Iaddend.at a first
linear velocity and withdrawing the precursor web from the oven at
a second linear velocity.[., the ratio of.]. .Iadd.and wherein
.Iaddend.the second velocity .[.to.]. .Iadd.exceeds the
.Iaddend.first velocity .[.ranges from about 1.1:1 to 2:1.].;
and
(d) cooling the web or permitting the web to cool.Iadd., whereby
there is obtained the nonwoven web which is consolidated laterally,
elastic in the cross direction, longitudinally elongated and heat
set. .Iaddend.
17. The .[.process as.]. .Iadd.method of .Iaddend.claim 16 wherein
the drawing step is sufficient to provide the web with a means flow
pore size measure at least 20% smaller than the mean flow pore size
of the precursor web.
18. The .[.process.]. .Iadd.method .Iaddend.of claim 16 wherein the
drawing step is sufficient to provide the web a packing density at
least 20% greater than that of the precursor web.
19. The method of claim 16 wherein the cooling step is carried out
prior to release of the drawing tension and cools the web to a
temperature below the softening temperature of the
thermoplastic.
20. The method of claim 16 wherein the nonwoven precursor web is a
meltblown web having an average fiber diameter of 0.5 to 8 microns
and having an elongation at break of less than
.Badd..[.30%.]..Baddend. .Iadd.40% .Iaddend.based on ASTM
D5035-90.
21. The method of claim 20 wherein the precursor nonwoven web has a
breaking elongation less than .Badd..[.40%.Baddend. .Iadd.30%
.Iaddend.based on ASTM D 5035-90.
22. The method of claim 16 where in the thermoplastic is a
polyolefin selected from the group consisting of polypropylene,
polyethylene, and copolymers thereof, and the heating step is
carried out at a temperature of between 190 to 350 degrees
Fahrenheit.
23. The method of claim 22 wherein the meltblown precursor web has
randomly distribute throughout fiber-fiber bonding.
24. The method of claim 16 wherein the thermoplastic is selected
from polyesters, polyamides, cellulose triacetate, cellulose
diacetate, .[.poly-4-methylpenetene-1.].
.Iadd.poly-4-methylpentene-1.Iaddend., polyphenylene sulfide,
liquid crystal polymers and fluoropolymers.
25. The method of claim 16 wherein the precursor nonwoven web is a
spun-bond web having fibers of from 7 to 50 micron average diameter
and having spaced apart bonds distributed through the web.
26. The method of claim 16 and further comprising the step of
passing the draw web over a bar or roller having a surface to
impart transverse compression forces inwardly of the width of the
web.
27. The method of claim 16 wherein the .[.first.]. velocity
.Iadd.of passing the precursor web into the oven .Iaddend.is
controlled by passing the precursor web through the nip of first
counter rotating rollers prior to the heating step, and the
.[.second.]. velocity .Iadd.of withdrawing the web from the oven
.Iaddend.is controlled by passing the consolidated web through the
nip of second counter rotating rollers after the heating step, and
wherein the consolidated web is permitted to cool to a temperature
below the softening point of the thermoplastic prior to passing
through the nip of the second counter rotating roller.
28. The method of claim 16 wherein the precursor nonwoven web is
made of a laminate of at least two distinct nonwoven webs, each web
made of randomly collected nonelastomeric thermoplastic fibers and
each web has distributed throughout fiber-fiber bonding.
29. The method of claim 28 wherein the precursor web is a composite
comprising a spunbound layer/a meltblown layer/and a spunbound
layer, wherein the layers are thermally bonded together at spaced
apart locations.
30. The method of claim 16 wherein the precursor web has a width of
6 to 160 inches and a thickness of 2 to 100 mils and wherein the
drawn web has a width of less than 80% of the precursor web and
thickness of 2 to 10 mils and wherein the thickness ratio of the
drawn web and precursor web ranges from 1:1 to 1.5:1. .Iadd.31. The
method of claim 16 wherein the planar integrity of the web is
maintained during drawing by developing sufficient stresses in the
longitudinal direction to reorient the fibers and by developing
sufficient compressive stresses to laterally consolidate the
fibers, thereby reducing the average pore size and pore size
distribution of the web. .Iaddend. .Iadd.32. The nonwoven web of
claim 1 wherein the crystallinity of the fibers is high enough to
provide a room temperature breaking elongation of less than 40%.
.Iaddend. .Iadd.33. The nonwoven web of claim 9 wherein the
crystallinity of the fibers is high enough to provide a room
temperature breaking elongation of less than 40%. .Iaddend.
.Iadd.34. The nonwoven web of claim 1 wherein the pore size
distribution has been reduced by at least 20% as compared to that
of the precursor nonwoven web. .Iaddend. .Iadd.35. The nonwoven web
of claim 9 wherein the pore size distribution has been reduced by
at least 20% as compared to that of the precursor nonwoven web.
.Iaddend. .Iadd.36. The process of claim 16 wherein the pore size
distribution is reduced by at
least 20% as compared to that of the precursor web. .Iaddend.
.Iadd.37. The nonwoven web of claim 1 wherein the reduction of the
maximum pore size is by at least 20% and the pore size distribution
is reduced by at lest 20%. .Iaddend. .Iadd.38. The nonwoven web of
claim 1 wherein the crystallinity of the fibers is in the range of
30 to 70%. .Iaddend. .Iadd.39. The nonwoven web of claim 1 wherein
the porosity of the precursor web is in the range of 50 to 95%.
.Iaddend. .Iadd.40. The nonwoven web of claim 1 wherein the fibers
are randomly fiber-to-fiber bonded. .Iaddend. .Iadd.41. The
nonwoven web of claim 40 wherein the area of the web bonding ranges
from 5 to about 25%. .Iaddend. .Iadd.42. The nonwoven web of claim
1 wherein the fibers are meltblown fibers. .Iaddend. .Iadd.43. The
nonwoven web of claim 1 wherein the fibers are spunbound fibers.
.Iaddend. .Iadd.44. The nonwoven web of claim 1 wherein the
composite web is a spunbound/meltblown/spunbound web. .Iaddend.
.Iadd.45. The nonwoven web of claim 40 wherein the meltblown fibers
have an average
diameter of 0.5 to 8 microns. .Iaddend. .Iadd.46. The nonwoven web
of claim 1 wherein the drawing of the thermoplastic fibers is
performed while in the rubbery state and the temperature is in the
range of 250.degree. to
350.degree. F. .Iaddend. .Iadd.47. The nonwoven web of claim 9
wherein the drawing of the thermoplastic fibers is performed while
in the rubbery state and the temperature is in the range of
250.degree. to 350.degree. F. .Iaddend. .Iadd.48. The nonwoven web
of claim 10 wherein the passing is performed while in the rubbery
state and the temperature is in the range
of 250.degree. to 350.degree. F. .Iaddend. .Iadd.49. The method of
claim 16 wherein the drawing of the web is performed while the
fibers of the web are in the rubbery state and the temperature is
in the range of 250.degree. to 350.degree. F. .Iaddend. .Iadd.50.
The nonwoven web of claim 1 wherein the upper temperature in the
range is within 15.degree. F. below the melting point of the
thermoplastic fibers. .Iaddend. .Iadd.51. The nonwoven web of claim
9 wherein the upper temperature in the range is within 15.degree.
F. below the melting point of the thermoplastic fibers. .Iaddend.
.Iadd.52. The nonwoven web of claim 10 wherein the upper
temperature in the range is within 15.degree. F. below the melting
point of the thermoplastic fibers. .Iaddend. .Iadd.53. The method
of claim 16 wherein the upper temperature in the range is within
15.degree. F. below
the melting point of the thermoplastic. .Iaddend. .Iadd.54. The
nonwoven web of claim 1 wherein the ratio of the difference in
breaking elongation in the cross direction between the nonwoven
consolidated web and the precursor to the cross direction breaking
elongation of the precursor is at least 1.87. .Iaddend. .Iadd.55.
The nonwoven web of claim 9 wherein the ratio of the difference in
breaking elongation in the cross direction between the nonwoven
consolidated web and the precursor to the cross direction breaking
elongation of the precursor is at least 1.87. .Iaddend. .Iadd.56.
The nonwoven web of claim 10 wherein the ratio of the difference in
breaking elongation in the cross direction between the nonwoven
consolidated web and the precursor to the cross direction breaking
elongation of the precursor is at least 1.87. .Iaddend. .Iadd.57.
The method of claim 16 wherein the ratio of the difference in
breeaking elongation in the cross direction between the nonwoven
consolidated web and the precursor to the cross direction breaking
elongation of the
precursor is at least 1.87. .Iaddend. .Iadd.58. As a product, a
segment of the nonwoven consolidated web of claim 1 which has two
pairs of sides and a ratio of a/c of at least 1.3, wherein "a" is
the width of the web entering the oven and "c" is its width exiting
the oven. .Iaddend. .Iadd.59. As a product, a segment of the
nonwoven consolidated web of claim 9 which has two pairs of sides
and a ratio of a/c of at least 1.3, wherein "a" is the width of the
web entering the oven and "c" is its width exiting the oven.
.Iaddend. .Iadd.60. As a product, a segment of the nonwoven
consolidated web of claim 10 which has two pairs of sides and a
ratio of a/c of at least 1.3, wherein "a" is the width of the web
entering the oven and "c" is its width exiting the oven. .Iaddend.
.Iadd.61. As a product, a segment of the nonwoven consolidated web
of claim 16 which has two pairs of sides and a ratio of a/c of at
least 1.3, wherein "a" is the width of the web entering the oven
and "c" is its width exiting the oven.
.Iaddend. .Iadd.62. The nonwoven web of claim 1 which is
longitudinally elongated by at least 5% and heat set with respect
to the length of its precursor. .Iaddend. .Iadd.63. As a product, a
segment of the nonwoven consolidated web of claim 1 which has two
pairs of sides and is longitudinally elongated by at least 5% and
heat set with respect to its precursor. .Iaddend. .Iadd.64. The
nonwoven web of claim 1 which is longitudinally elongated from by
at least 10% and heat set with respect to
the length of its precursor. .Iaddend. .Iadd.65. The nonwoven web
of claim 16 which is longitudinally elongated from by at least 10%
and heat set with respect to the length of its precursor. .Iaddend.
.Iadd.66. The nonwoven web of claim 1, the nonwoven precursor of
which has not been necked by drawing at ambient temperature.
.Iaddend. .Iadd.67. The nonwoven web of claim 9, the nonwoven
precursor of which has not been necked by drawing at ambient
temperature. .Iaddend. .Iadd.68. The nonwoven web of claim 10, the
nonwoven precursor of which has not been necked by drawing at
ambient temperature. .Iaddend. .Iadd.69. The filter of claim 11,
the nonwoven precursor web of which has not been necked by drawing
at ambient temperature. .Iaddend. .Iadd.70. The method of claim 16
wherein the precursor nonwoven web has not been necked by drawing
at ambient temperature. .Iaddend. .Iadd.71. The nonwoven web of
claim 1, the nonwoven precursor web of which has a room temperature
elongation (strain) at break between 2 and 40 percent, based on
test method ASTMD 1117-77. .Iaddend.
.Iadd.72. The nonwoven web of claim 9, the nonwoven precursor web
of which has a room temperature elongation (strain) at break
between 2 and 40 percent, based on test method ASTMD 1117-77.
.Iaddend. .Iadd.73. The nonwoven web of claim 10, the nonwoven
precursor web of which has a room temperature elongation (strain)
at break between 2 and 40 percent, based on test method ASTMD
1117-77. .Iaddend. .Iadd.74. The filter of claim 11, the nonwoven
precursor web of which has a room temperature elongation (strain)
at break between 2 and 40 percent, based on test method ASTMD
1117-77. .Iaddend. .Iadd.75. The method of claim 16 wherein the
nonwoven precursor nonwoven web has a room temperature elongation
(strain) at break between 2 and 40 percent, based on test method
ASTMD 1117-77. .Iaddend.
Description
FIELD OF THE INVENTION
This invention relates generally to nonwoven webs which have been
post-treated to reduce the pore size in the web. In one aspect, the
invention relates to post-treatment of meltblown webs to improve
the web's properties for a variety or uses. In another aspect, the
invention relates to the post-treatment of spun-bond webs for the
same purpose. In still another aspect of the invention, nonwoven
webs are firstly drawn under thermal conditions and secondly
mechanically compacted to efficiently alter the geometric
arrangement of the fibers making up the web resulting in web having
reduced measures of pore size.
BACKGROUND OF THE INVENTION
Meltblowing is a process for manufacturing nonwoven products by
extruding molten thermoplastic resins through fine capillary holes
(orifices) and blowing hot air on each side of the extruded
filaments to attenuate and draw down the filaments. The filaments
are collected on a screen or other suitable collection device as a
random entangled nonwoven web. The web may be withdrawn and further
processed into consumer goods such as mats, fabrics, webbing,
filters, battery separators, and the like. Also, the consumer goods
may be produced in line with the meltblowing line.
As indicated above, the present invention relates to the
post-treatment or nonwoven webs to alter the filament spacing and
structure or the webs. It should be observed that the terms
"filaments" or "fibers" are used interchangeably herein, although
"fibers" in nonwovens generally refers to discontinuous strands and
"filaments" as continuous strands. The present invention
contemplate webs with continuous filaments and/or discontinuous
fibers.
Since the development or the meltblowing process by the Naval
Research Laboratory in 1951 (published in 1954 by the U.S.
Department of Commerce in an article entitled "MANUFACTURE OF
SUPERFINE ORGANIC FIBERS"), there has been a considerable effort by
several companies operating in the industry to find new uses for
the nonwoven product having micro-sized fibers. Because of the
random, geometric assembly or structure of the fibers, and
relatively small fiber size. the fibers have received extensive use
as filters.
In the formation process for most random laid or unordered fibrous
webs, the pore size that develops is inversely related to the
square of the fiber diameter. The spunbonded process is
distinguished from meltblowing by self-bonding and non uniform draw
down (plastic deformation) of filaments forming the web. Thus
meltblown webs have a relatively broad distribution of fiber
diameters. Typical nonwoven webs produced by meltblowing have fiber
diameters of 0.5 to 20 microns, preferably 0.5 to 8 microns, making
them suitable for filtering out 5 micron particles at 80 percent
efficiency or greater. It is known that filtration can be improved
by practicing the web formation process to produce smaller and
smaller diameter fibers while controlling other formation
parameters such as porosity and thickness. As noted above, this
results in smaller pore size thereby improving the efficiency of
particle removal in filtration. By operating the meltblowing
process under extreme conditions, the fiber size can be produced in
the order of 0.1 to 5 microns. The process, however, has the
following disadvantages: low production rates, high energy usage.
In order to improve the properties of the nonwoven web, efforts
have been made to post-treat the webs by a variety of processes.
Such efforts have included post calendering the web to improve, the
tensile strength or the web, post electrification as disclosed in
U.S. Pat. No. 4,592,815 to improve filtration performance or the
web, to name but two of such efforts. It is significant to note
that none of these prior art techniques have been directed
specifically at planar consolidation to reduce the size of the
pores in the web.
Calendering of nonwovens flattens fibers and consolidates the web
in a direction normal to the plane of the web and reduces the
thickness. This, however, leads to reduction in permeability which
is an important property to conserve for many filtration purposes.
U.S. Pat. No. 4,048,364 discloses a process for drawing the
meltblown web in the machine direction (MD) to produce a ten-fold
increase in the tensile strength of the post-drawn web. It is
significant to note, however, that the precursor web required in
this invention contains relatively coarse fibers (10 to about 40
microns average fiber diameter) and polymer of low crystallinity.
Low crystallinity generally means about 22% or less. The extensive
drawing of the web reduces the diameter of the fibers in the
machine direction to an average diameter of 1 to 8 microns at draw
ratios ranging from 2:1 to 10:1 and preferably 5:1 to 7:1. The main
purpose of the process is to increase the molecular orientation to
enhance the strength of the greatly drawn fibers. Precursor webs of
very high post processing draw ratio capability are required in
order to prevent rupture of fibers in the web drawing process.
Tests have shown that the stretching of a precursor web having hot
(e.g., 10.degree. F. less than the melting point of the precursor
web) drawing capabilities from about 5:1 to 10:1 does not alter the
measure of pore size of the web. This is probably due to the fact
that the high and easy drawability of the fibers prevents the
development of sufficient, compressive forces to bend the stout
fibers in the web and physically reduce its pore dimensions and
measures of pore size distribution in general.
SUMMARY OF THE INVENTION
It has surprisingly been discovered that by selecting a nonwoven
web with certain properties and post-drawing the web under certain
conditions, the fibers making up the web are restructured to
provide the web with reduced pore sizes, and a narrower pore size
distribution. Such post-treated webs have unique measures of pore
size, directional absorption and elastic recovery properties which
make them ideally suited for a variety of end use applications such
as filters, vacuum cleaner bags, protective apparel, face masks,
diapers or sanitary napkin parts, wound dressings, respirators,
wipes, chemical reservoirs, wicks, and surgical drapes.
The method of the present invention involves subjecting a bonded
thermoplastic nonwoven web having a relatively low tensile
extensibility during post processing (as reflected by a low draw
ratio at break) to primary drawing under an elevated temperature.
This uni-directional drawing in the MD laterally consolidates the
web to a great extent thereby reducing both the average pore size
of the web and narrowing the pore size distribution. The resultant
web exhibits improved uniformity in pore size and high lateral
elasticity characteristic of "stretch fabric" having approximately
120% elongation to break.
In an alternate embodiment, the web being drawn may be passed into
supplemental mechanical compacting means to induce and/or refine
the primary web consolidation.
Although the present invention is described and exemplified in
connection with meltblown and spunbond webs, it is to be understood
that it has application with other nonwovens such as
hydro-entangled, needled webs, and laminated combinations of these
and with other web forms such as air laid, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of apparatus for producing meltblown
webs.
FIG. 2 is a perspective view of apparatus for the practice of the
present invention.
FIG. 3 is a perspective view of an alternate embodiment of an
apparatus for the practice of the invention illustrating the drawn
web passing over a torus surface for variably imparting compaction
forces to the consolidating web.
FIG. 4 is an enlarged plan view of a tiny planar segment of a
meltblown web illustrating the random nature of a precursor web
useable in the present invention.
FIG. 5 is an idealized plan view representation of the fibers of a
precursor web facilitating the analysis of the mechanisms involved
in the present invention.
FIG. 6 is a view similar to FIG. 5 after the web had been
drawn.
FIG. 7 presents two curves illustrating the pore size distribution
of a web before and after drawing.
FIG. 8 is a plot illustrating that precursor meltblown webs
(circles) having average fiber diameter less than eight microns
(sample data from Tables I and II) are increasingly densified by
the post-drawing (squares).
FIG. 9 is a plot illustrating that precursor meltblown webs
(circles) having fiber diameter greater than about eight microns
show negligible improvement in particle filtration efficiency after
post drawing (squares) .
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As indicated above, the present invention relates to the
post-treatment of a precursor nonwoven web to reconstitute or
restructure the fibers of the web and reduce the measures of pore
size. The term "nonwoven" as used herein means randomly laid fibers
or filaments to form a web wherein some of the fibers are bonded by
fiber-to-fiber fusion or fiber entanglement, or thermal bonds as by
point bonding. The term "pore size" means a quantification of the
physical dimensions of channels oriented in a generally normal
direction to the plane of the web. The pore size values recited
herein are based on standard test method ASTM F 316-86.
The present invention described with specific reference to the
preferred webs will be meltblown webs; it is to be emphasized,
however, that the method and product produced thereby includes
other nonwoven webs, specifically spunbond, hydro-entangled,
needled webs and laminated combinations of these. Also the web
produced according to the present invention used in combination
with other webs or substrates such as webs from elastomeric
polymers, microporous films or stretch limiting materials post
laminated to limit the CD extensibility to less than 100% provide
additional performance properties for added utility.
Meltblowing is a well known process which generally utilizes
equipment depicted in the schematic of FIG. 1. The process is
carried out by introducing a thermoplastic resin into an extruder
10 where the polymer is heated, melted, and extruded through a die
11 to form a plurality or side-by-side filaments 12 while
converging layers of hot air (discharging from slots 13 on each
side of the row or filaments) contact the filaments and through
drag forces stretch and attenuate the filaments 12 to a
micron-size. The fibers 12 are collected onto a collector such as a
rotating screen 15 forming a nonwoven web 17 which may be withdrawn
on a take-up roller for later processing. The collator 15 may
include a vacuum screen wherein a vacuum, through line 18, is drawn
by a vacuum pump 19.
The hot air (primary jet air) is introduced into opposite sides or
the die through line 14. Although not indicated on the drawing,
secondary air which is aspirated into the primary air/fibrous
stream serves to cool the filaments discharging from the die
11.
The process and apparatus described above forms no part of the
present invention; however, variables used in the process
(including the type of resin employed, the mount and temperature or
primary air and polymer melt, and the spacing of the collector 15
from the die discharge) will have a significant effect on the
precursor web properties.
Briefly, the process in one embodiment of the present invention
comprises the steps or (a) selecting a thermoplastic nonwoven
precursor web with substantial fiber bonding and having relatively
low processing extensibility and (b) passing the nonwoven web
through a heated zone to increase the temperature of the web to its
softening temperature while drawing the web in the machine
direction (MD) thereby greatly plastically bending the cross
direction (CD) fibers in the web which consolidates the web in the
CD reducing the maximum pore size of the precursor web by at least
20 percent, and, more significantly, reducing the pore size
distribution by at least 20%. As described in detail below, the
precursor web must have certain properties to enhance
consolidation.
Apparatus for carrying out a preferred process is illustrated
schematically in FIG. 2 wherein the precursor web 17 is unwound
from roll 20 and red through the nip counter-rotating feed rollers
22, through oven 23, and finally through the nip or
counter-rotating rollers 24. The oven 23 is maintained at a
temperature to the precursor web 17 to a temperature between its
softening point and the melting point or the polymers in the web.
Preferably the web is heated to a temperature within 15.degree. F.
of its melting point. The rotating rollers 24 are driven at a speed
in excess of the rotating reed rollers 22 so that the output
velocity (V2) of the web is in excess of the feed velocity (V1) for
the draw ratio which is a function of the velocity ratio V2/V1. The
initial drawing or the web 17 under thermal conditions causes web
to contract within the oven 23 from its feed width 17a as
illustrated by web section 17b in FIG. 2. This contraction is due
primarily to the plastic bending deformation by planar compression
of generally CD fibers of the web thereby reducing the measures of
pore size or the web. It is important to note that the high MD
tensile forces developed at low MD strain during drawing, together
with the network nature or the fiber-fiber bonds in the web
augments the generation of enough compressive stress to easily bend
most CD fiber segments 27 and compact the web in the CD as shown in
FIG. 6. Since fiber bending rigidity is related to the fourth power
of the fiber diameter, only webs having small average fiber
diameters can be consolidated by the available stresses with the
associated reduction in pore size measures. Average fiber diameter
for meltblown webs arc preferably less than about 9 microns, and
less than about 50 microns for spunbonded webs.
The lateral contraction which results in pore size reduction is not
accompanied by significant average fiber diameter reduction of MD
fibers. Continued web stretching beyond that necessary for web pore
size reduction may cause fiber diameter reductions. The web is
contracted to a minimum width 17c as the web 17 exits the oven 23
or as the web 17 passes the nip of rollers 24. It is preferred but
not essential to cool or permit the web to cool between the exit of
the oven 23 and the nip of the rollers 24 thereby controlling the
heat set or annealing in the restructured fibers under stress.
As the web 17 cools to between 130.degree. and 90.degree. C. (for
PP), the web can be electrostatically charged to impart durable
enhanced filtration efficiency to the web products. (The nip of the
rollers 24 and that of rollers 22 preferably are parallel so that
the tensile force applied by rollers 24 and the resistance applied
by rollers 22 are uni-directional [e.g., uniaxial]).
To further control or narrow the distribution of pore sizes,
supplementary or alternative web-width compaction means can be
added between 17a and 17c as schematically illustrated in FIG. 3.
FIG. 3 shows one alternate web processing embodiment in which the
web 30 passes into a supplementary or alternative web compacting
device consisting of a (tilted) section of torus 25. The
consolidation interval of the web 17 and the torus bar 25 are
heated in an oven or heated to provide the proper temperatures for
drawing and consolidating the web. The web enters the outboard
surface (of diameter D) or the torus at width dimension 17d and
exits near the inboard surface of the torus which has a lesser
width dimension 17e. The converging surface of the path around the
torus applies transverse compressive forces in the plane of the web
of entry width 17d. The added compressive forces overcome the
bending resistance of inefficiently deformed large CD fiber
segments or shot imperfections remaining in the web 17 following
primary consolidation (if used). This improves the uniformity in
pore sizes. The heating and stretching of the apparatus in FIG. 2
(gross drawing) and FIG. 3 (secondary drawing) can be carried out
in series. The primary heating-drawing step imparts gross
consolidation while the secondary torus consolidator refines the
processing. The maximum compressive strain imparted to the web by
traversing about 180.degree. around the torus surface is given by
(D-d)/D, where D is the outboard or entry perimeter related to the
entry width 17d and d is the inboard or web exit perimeter of the
torus 25. The magnitude of the supplementary consolidation can be
adjusted by adjusting the two diameters of the torus 25 compacting
device or "c-roll" shown in FIG. 3. If the c-roll is made straight
(VIZ. radii=.infin.), then no lateral compaction occurs and the
roll solely increases the anneal time and maintains the thickness
of the web. The torus surface can be fixed or can be a rotatable
curved flexible bar. A fixed torus 25 with an air bearing between
the surface and the web allows high lateral compressive strain and
low friction for additional MD draw. It should be noted that
revolving "Bowed rolls" are only used in textile applications to
remove wrinkles from a moving textile fabric by laterally
stretching the fabric as the textile proceeds around a surface of
diverging width.
The important parameters of the precursor web 17 and the process
condition, along with the unique properties of the web produced by
the process are described in detail below.
Precursor Web: A nonelastomeric nonwoven precursor sot web is
selected based on its dimensions, and its hot processing tensile
properties (VIZ., elongation-at-break). In general, the breaking
draw ratio of the web during hot processing should be less than
about 4.0 and greater than about 1.4 evaluated while hot drawing at
a strain rate greater than 2500%/min and temperature greater than
the softening point but at least 10 degrees F. less than the
polymer melting temperature. This is an important indicator of
precursor molecular orientation state for achieving sufficient
stresses for CD fiber buckling and bending to cause reduction of
the measures of pore size distribution of the web by the process
of-the present invention. The room temperature elongation (strain)
at break should be between 2 and 40 percent, preferably between 5
and 20 percent, based on test method ASTM D 1117-77 using the
Instron tensile testing machine. Note that the precursor webs
disclosed in U.S. Pat. No. 4,048,364 are totally unsatisfactory for
use in the present invention because such precursor webs are
characterized as having at least 50%, preferably at least 70%,
standardized elongation before break, preferable max processing
draw ratio greater than 5. Webs made up of low modulus, low
crystalline (less than 22%), exhibit too much elongation at low
tension in the heating and drawing step to permit development of
the necessary stresses. The webs useful in the process of U.S. Pat.
No. 4,048,364 have far greater maximum draw ratio than 4 under the
hot draw condition described above. It is estimated that these draw
ratios are greater than 5.
Compressive stresses which buckle and bend CD fibers in the present
invention are given by a sine function of the fiber tensile stress
and the angles (see FIGS. 4 and 5) involved become smaller as MD
processing draw ratio increases, so compressive forces diminish
with draw ratio. In addition, the distribution of filament
diameters in the above precursor web is an order of magnitude
larger than those of the present invention and thus the bending
rigidity of CD fibers is very much higher while compression
stresses are relatively small during processing. Elastomeric
polymer webs (e.g., elastomers having rubber-like properties of an
elastomer or rubber; that is, having the ability to stretch at
least twice their original length and retract at room temperature)
cannot be used in the present invention.
The precursor nonwoven web may be made from many of the
thermoplastics capable of being melt blown, provided the polymer
selected develops filaments of sufficiently high tensile processing
modulus to permit the development of high lateral compression
forces on the web. The thermoplastic resins useable in the
production of nonwovens includes the nonelastomeric polyolefins
such as polyethylene, polypropylene including high density
polyethylene, ethylene copolymers (including EVA and EMA copolymers
with high tensile moduli), nylon, polyamides, polyesters,
polystyrene, poly-4-methylpentene-1, polymethylmethacrylate,
polytrifluorochlorethylene, polyurethanes, polycarbonates,
silicones, polyphenylene sulfide.
The crystallinity of the precursor web preferably should be
sufficiently high to provide a room temperature breaking elongation
less than 40%. Meltblown webs useable in the present invention
should break at a strain of less than 40 percent in accordance with
ASTM test method D 5035-90. The crystallinity in the range of 30 to
70 percent is preferred. In general, the proper high modulus and
state of molecular orientation of the precursor is best reflected
by a maximum or breaking draw ratio of the web during post treating
of less than about 4.0.
In the post-treatment process, the thickness of the web should
preferably be at least 2 mils and up to about 200 mils. The width
of the web, or course, can vary within wide limits, with 5 to 150
inches being preferred. The average fiber diameter of the precursor
meltblown web will preferably range from 0.5 to 8 microns, with 2
to 6 microns being preferred in order to provide the proper range
of processing tensile stiffness for PP web. The porosity of the
precursor web will normally be in the range of 50 to 95 percent.
Calendered precursor webs approach 50%.
Other properties of the web, which while not critical, are
important include a low occurrence of large shot or excessive
ropiness.
Another important feature of the precursor web is that it includes
at least some fiber-to-fiber bonding which is typical in meltblown
webs. The bonding can be achieved by inherent fiber-to-fiber
fusion, or by point bonding, calendering, or by fiber entanglement.
The properties of the selected polymer can be controlled to a
degree by operation of the meltblowing process. Some of these
control variables are disclosed under the experiments below.
Process Conditions: As indicated above, the primary purpose of the
process of the present invention is to consolidate the web in the
cross direction to reduce the average pore size and the pore size
distribution in the web. Consolidation of the web in the
cross-direction is to be distinguished from consolidation resulting
from calendering since consolidation to reduce thickness as in
calendering flattens the fibers and closes flow channels, thus
decreasing the permeability of the web to a greater extent compared
to web draw consolidation.
The random nature of low stretch meltblown webs with the attendant
thermal bonding and/or filament entanglement enable the development
of MD stresses to reorient fibers and create sufficient compressive
stresses to laterally consolidate or squeeze them together thus
reducing the size of voids there between during uniaxial drawing.
This results in narrowing of the web width without disrupting the
planar integrity of the web and produces a product of unique
properties. During the post-drawing process, the modulus that is in
effect while the filament segments are being drawn depends on
processing time-temperature effects. Maximum consolidation in the
CD is achieved at a trial and error modulus at which the
compressive stresses overcome to the largest extent the critical
buckling stresses for the population of CD segments in the web.
This is illustrated in the post-drawing process preferably carried
out at a temperature where the polymer is in the rubbery state.
This is best illustrated with reference to FIGS. 4, 5 and 6 which
depict, respectively, the random disposition of nonwoven fiber, an
idealized representation of unconsolidated nonwoven fibers, and an
idealized representation of consolidated nonwoven fibers. The
random disposition of the filaments forming a thin planar layer of
the meltblown web is represented in FIG. 4 wherein longitudinal
fibers 26 extend generally in the MD, transverse fibers 27 extended
in the CD, and intermediate segments of fibers 28 extend at an
angle with respect to the MD and CD.
For purposes of analysis, this planar disposition may be
represented by representative cells illustrated in FIG. 5. In the
idealized representation or model in FIG. 5, the fibers 26, 27, and
28 are shown interconnected or bonded as a loose network at
junctions 29 or the fibers. Again, it is to be emphasized that the
bonds are fuse bonded during the meltblown process, or by fiber
entanglement, or by thermal point calendering techniques. When the
web structure shown in FIG. 5 is subjected to tension in the MD,
the intermediate fibers 28 are easily aligned in the MD thus
reducing pore dimensions whereas the CD fibers 27 tend to resist
compression or the cell in which it is associated and may buckle
and bend as illustrated in FIG. 6. The result is that the lateral
consolidation or the precursor web in accordance with the present
invention leaves pore space throughout the web layer which depends
on the extent to which CD fibers are buckled. Fiber having a high
slenderness ratio or length by diameter buckle easier. Ideally, the
compressive force on element 27 in FIG. 6 is 2Tsin(theta) where T
is the tensile force in elements 28 and .theta. is the angle
between element 28 and the MD. Without the bonding at junctions 29,
the webs would easily rupture without generating lateral (CD)
compression as in a carded web. Although actual webs do not include
only the idealized structure as depicted in FIGS. 4 and 5, there is
sufficient bonding and stresses developed in the select precursor
web to provide the reduced porosity following the thermal drawing
process as in FIGS. 6 and 7. Note that the buckled CD fibers 27 act
as spacers limiting the residual porosity and pore dimensions
developed by the resultant compression forces due to the MD tensile
drawing force. To supplement the compression or large diameter
fibers and shot, external mechanical means can be incorporated to
further compress the hot drawn web near 17c in order to augment the
CD fiber bending and buckling beyond that obtained by hot drawing
alone. One such apparatus embodiment is illustrated in FIG. 3
described above in which the mostly drawn web is subjected to
transverse compression forces because the web is tracking the
converging surface or the torus.
The post-drawn web withdrawn from the oven and preferably heat set
exhibits two surprising and highly useful properties: (1) the pore
space and all measures or pore size distribution have been reduced,
and (2) the web exhibits remarkable elasticity in the CD. These two
properties will be discussed in detail later.
The post-drawing process conditions and precursor properties for
achieving the web with the improved properties described above are
as follows:
______________________________________ BROAD PREFERRED BEST RANGE
RANGE MODE ______________________________________ Draw ratio, V2/V1
1.05-3.00 1.10-2.00 1.2-1.80 Temperature, .degree.F. 165-425
250-350 275-300 (PP) V1, Feed Speed, 10-400 25-200 35-60 F/M MAX
pore size, .mu.M 5-250 10-150 20-50 Crystallinity, % 30-95 30-80
35-60 Thickness, mils 2-200 2-100 6-20 Avg. Fiber Dia. .mu.M 0.5-50
0.5-8 1.7-6 Strain rate, per 10-500 20-200 30-60 min Hot processing
1.3-4 1.7-3.5 2-3 breaking draw ratio, V2/V1 Reduction in pore
20-85 25-75 35-70 size (MAX, MFP, and range), % Elastic recovery
50-99 70-99 80-95 from 50% strain, % Liquid absorption 1.2-6 1.76-5
2-4 aspect ratio ______________________________________
It should be Observed that the geometric minimum MD strain for
complete lateral consolidation of an idealized web in FIG. 5 is 42
percent or DR=1.42. However, in the most preferred embodiment the
invention contemplates draw ratios in excess of about 1.42 since
higher draw ratios will enhance the reduction in porosity, which is
limited by the spacer effects of partially buckled CD fibers.
OPERATION
The selection of the resin and meltblowing operating conditions,
precursor webs having the necessary properties may be obtained
based upon the above description.
Although the precursor webs made up of any of the thermoplastic
polymers used in meltblowing (provided they possess the necessary
properties) may be used, the following polypropylene precursor
meltblown web has produced excellent results in experiments carried
out at the University of Tennessee.
______________________________________ PP Grade (Exxon Grade)
PD-3495 G MFR 800 Thickness 13 mil Width 14 inches Basis Weight 1.5
oz/yd.sup.2 Porosity 87% Crystallinity 50% Web elongation at break
10% ______________________________________
As illustrated in FIG. 2, the precursor web 17 in a generally flat
disposition is processed according to the present invention by
passing the flat web 17 in an oven 23 at a temperature between the
softening and melting temperature of the polymer (e.g., for PP,
about 310 degrees F.). The line speed and draw ratio are selected
to impart the desired lateral consolidation of the web expressed as
a ratio of the web width entering to web 17 width exiting the oven
(.[.c/a.]. .Iadd.a/c .Iaddend.in FIG. 2). The .[.c/a.]. .Iadd.a/c
.Iaddend.values should be from 1.3 to 4, preferably from 1.5 to 3,
and most preferably 2 to 2.5. Web thickness entering the oven may
range from 2 mils to 100 mils and those exiting may range from
between 2 and 150 mils, indicating that the thickness may under
certain conditions increase. Draw ratios of 1.05 to 3.00,
preferably from 1.10 to 2.00, and most preferably 1.2 to 1.8 may be
used to achieve satisfactory consolidation. Line speeds (V2) can
range from 10 to 400 fpm. As mentioned above, webs capable of hot
processing breaking draw ratios greater than about 4 are
unsuitable.
It is preferred that the consolidated and annealed web leaving the
oven be cooled, either by ambient temperature or supplemental air
to impart a set to the fibers in the deformed condition. The
consolidated heat set web can be rolled up for later conversion to
end use products.
The web consolidation restructures the fibers of the web by
aligning more of the fibers in the MD. The fiber bonding transforms
tensile stress into CD consolidation in the manner described above,
thereby reducing all of the web's measures of pore size
distribution. These measures of pore size distribution of the-web
are the maximum pore size (MAX), the mean flow pore size (MFP), and
the minimum pore size (MIN) as measured by a Coulter Porometer,
described below in connection with Experiments. The Coulter
Porometer produces a characteristic distribution--size plot for
each web where pore size plotted against percent differential flow
through the web. FIG. 7 compares the characteristic curve for a
precursor web (Plot 31), and the characteristic plot for the
consolidated web (Plot 32). A comparison of Plot 31 (precursor web)
and Plot 32 (consolidated web) illustrates the dramatic effect of
consolidation. As can be seen in FIG. 7, the pore size distribution
ranged from about 13 to about 40 microns (a range or spread of 27
microns), and the mean flow pore size was about 20 microns.
In the consolidated web (Plot 32), pore size distribution ranged
from 6 to 17.5 microns (a spread of only 11.5 microns). with the
mean flow pore size of 9.4 microns. The web consolidation according
to the present invention thus reduced the spread of the pore size
distribution from 25 to 11.5 microns and the average pore size from
about 20 (Plot 31) to about 9 (Plot 32). The maximum pore size (BP)
was reduced from 38.7 to 17.5 microns. The consolidated web
exhibited excellent "stretch fabric" elasticity in the CD and
tested extremely well as a filter.
EXPERIMENTS
Definitions: In order to better understand the terms used herein,
particularly in the Experiments described below, the following
definitions consistent with the accepted technical definitions in
the industry, are submitted.
Web Pore Space (porosity)--the ratio of the volume or air or void
contained within the boundary or a material to the total volume
expressed as a percentage. Packing density equals 1 minus
porosity.
Coulter Porometer--a semiautomated instrument using a liquid
displacement technique to measure the pore size measures and
distributions of a sample according to ASTM F 316-86.
Web Pore Size Distribution--the distribution of pore sizes between
the maximum and the minimum pore size as determined by ASTM F
316-86 on the Coulter II Porometer. (The maximum pore size [or
bubble point] measure is distinguished in that it strongly relates
to permeability, pressure drop, and filtration efficiency
performance properties for the entire family of meltblown webs we
studied.)
ASTM 316-86 Measures of Pore Size Distribution--MAX is the
standardized measure of the diameter of the largest pore channels
in the distribution of pore sizes supporting flow through the web.
MFP is the measure of the median (or mean) pore channel diameter
for the pores supporting the total flow. MIN is the minimum pore
size measured for the web.
Polymer Crystallinity--the relative fraction of highly ordered
molecular structure regions compared to the poorly ordered
amorphous regions. Crystallinity is determined by X-ray or DSC
analysis.
Polymer Birefringence--is a property which is usually observed in
optical microscopes when a material is anisotropic, that is when
its refractive index is directional. Fibers having molecular chains
of higher axial directionality have higher birefringence and
relatively low tensile elongation at break.
Softening Temperature--is a thermal property of a polymer
characterized by a temperature at which the material becomes
sticky, viscous, or elastic (soft) prior to melting and looses its
room temperature modulus (and can undergo plastic elongation)
leading to maximum molecular orientation and breakage.
Average Fiber Diameter--a measure of the mean fiber diameter or the
fibers in the web obtained from individual measures of the fibers
diameters in focus on a scanning electron micrograph or the subject
fibrous web--about 100 fibers are measured. Finer fibers generally
arise from greater draw-down in meltblowing and have higher
birefringence.
Web Elongation at Break--for a crystalline polymer is strain rate
and temperature dependent. The elongation at break primarily
measures the extent of a plastic deformation process beginning at
the initial state and terminating at the final well ordered state
of molecular orientation (MO) of the polymer. Precursor webs having
fibers of high crystallinity and order have low elongation to break
(from R. J. Samuels, Structured Polymer Properties, John Whiley
& Sons, 1973). For the meltblown webs, evaluating the precursor
MO state by breaking elongation is best accomplished at high
temperatures instead of at standardized ASTM D 5035-90 room
temperature test because of the wide range in fiber diameters, MO
state and bonding in meltblown webs. The meltblown precursor webs
were characterized by the magnitude of the breaking draw ratio
attained while hot drawing at a strain rate at least 25 min--1 (or
2500%/min) and temperature at least 10.degree. F. below the melting
temperature of the precursor thermoplastic polymer (Hot breaking
draw ratio).
Web Tensile Modulus--is the measure of the force required to
produce a small extension (or compression). A highly inextensible
material will usually have a large modulus.
Web Elasticity--that property of a body by virtue of which it tends
to recover its original size and shape after deformation. Elastic
recovery from elongation is given by (stretched length-recovered
length)/(stretched length-original length). The recovery from an
initial elongation is stated, such as, from a 100% CD strain.
Materials and Equipment: All the samples used in the experiments
were prepared using a meltblowing line at The University of
Tennessee. The process conditions to produce a desired sample for
evaluation were controlled as follows:
(a) the level of hot-drawability, as related to birefringence and
tensile modulus during processing is a function of fiber-diameter
and was controlled by varying the primary air levels in the line
from 70 to 95%,
(b) the level of bonding in the web was controlled by adjusting the
air to polymer ratio, the die to collector distance, the air
temperature, the melt temperature and collector vacuum. Tenacity
and the elongation-at-break was used to qualify the bonding
strength for the samples.
The slenderness ratio of fiber segments subjected to compression as
well as the magnitude the bending forces developed by drawing are
ultimately related to the above.
The post-drawing on the precursor web was done in experimental
apparatus similar to that illustrated in FIGS. 2 and 3. The rollers
were provided with speed controls.
The polymer used in all of the tests was polypropylene (PP). The PP
precursor web samples used in the tests are described in TABLE
I.
TABLE I
__________________________________________________________________________
% Packing Ave. Fiber Break Pore Sz. Measures, .mu.m Sample Air
Density Diam. .mu.m Elong. Max MFP Min Break D.R.
__________________________________________________________________________
A 90 0.095 3.2 7.4 19.3 15.4 11.1 2.2 B 90 0.110 3.9 6.3 17.9 14.3
10.5 2.5 C 85 0.085 4.0 17.4 28.3 16.6 10.7 2.5 D 80 0.129 5.5 6.6
38.8 20.1 13.8 3.0 E 70 0.145 8.5 3.0 20.8 14.4 10.9 3.5 F 70 0.163
9.9 4.1 40.5 24.2 16.5 3.7 G 70 0.172 8.8 5.7 33.0 20.6 13.7 3.8 H
60 0.168 18.5 2.7 117.0 68.0 25.0 6.0
__________________________________________________________________________
Filtration Measurement: The filtration efficiency was based on dry
particulate capture efficiency test using nominal 1.0 micrometer
mono dispersed latex particles al an aerosol flow rate of 10
centimeters per second (ASTM 1215).
The filtration experiments were carried out as described above.
Each webs "filtration" performance is expressed as particle capture
efficiency (filtration efficiency) based on the following formula:
##EQU1##
Web Measurements: Fiber diameters were measured by SEM photographs
of the specimens.
Maximum, mean flow pore size, minimum, and pore size distribution
spread in terms of the maximum and minimum, was based on a Coulter
Porometer according to ASTM F 316-86.
Pore Space: Measurements were based on weights of dry specimens and
the weight of the specimen wetted out with a liquid of known
density. Planar densification is evidenced by the increase in
packing density (PD) measure of the web given by the ratio or dry
web weight to the weight of the void-free web. Porosity of the web
or pore space is given by one minus the packing density.
The tests for measuring elasticity of the consolidated web were as
follows: Measured the percentage to which specimen recovered its
original (CD) length immediately following a given % (CD)
elongation, for example, sample A recovered 92% of its original
length following a 100% CD elongation. Another test on the
consolidated webs included directional absorption of liquids.
Surfactants for improving the water wettability of the fibers were
applied to PP webs prior to aqueous absorption tests. The
surfactants were nonionic and other types such as nonionic
polyoxyethylenated tertoctylphenol, anionic ammonium lauryl
sulfate, and cationic sulfobetaines. Directional absorption was
characterized by the aspect ratio of the absorption pattern
produced when a ML of liquid was applied to a point on the specimen
supported on a horizontal surface. For a variety of meltblown and
spunbonded specimens, absorption aspect ratios ranged from 1.7 to
about 5. The test results carried out on the webs consolidated at a
DR of 2 are presented in TABLE II. Table III gives the filtration
efficiency values for meltblown webs variously consolidated st draw
ratios of 1.0 (undrawn precursor web), 1.5 (precursor drawn 50%),
2.0 (these data also plotted in FIG. 9), and 2.5.
TABLE II ______________________________________ Properties of DR =
2.0, Elastic % of precursor web Oven recovery Pore Size Temp. from
strain of Packing Measures, .mu.m Sample .degree.C. 50% 100%
Density Max. MFP Min. ______________________________________ A 150
95 92 214 50 46 42 B 155 93 Break 250 44 39 39 C 150 95 90 302 49
60 65 D 150 95 90 163 38 48 51 E 150 87 Break 124 155 124 118 F 150
Break Break 101 73 76 78 G 150 85 Break 95 113 103 108 H 150 Break
Break 99 128 115 -- ______________________________________
The Table II data and properties of webs consolidated at DR=2
reveal that the pore sizes of Samples A through D were reduced by
38 to 65% and the packing density for the same samples were
increased from 163 to 302%.
TABLE III ______________________________________ Filtration
Efficiency, % Sample DR = 1 DR = 1.5 DR = 2.0 DR = 2.5
______________________________________ A 43 74 77 Break B 45 86 89
80 C 32 64 66 81 D 21 35 54 60 E 14 13 22 18 F 16 20 19 29 G 14 13
18 18 H 6 4 4 3 ______________________________________
In Table I, the maximum hot draw ratio is the magnitude of the
breaking draw ratio during hot processing and solely defines the
molecular orientation present in the filaments of the precursor
melt blown webs. Web of PP having a maximum DR greater than about
3.5 are not consolidated according to the present invention.
Compare pore measures in Table I and the changes produced at a DR.
of 2.0 in Table II. The data on FIG. 9 indicates that filtration
efficiencies improve significantly for fiber diameters less than 8
microns, particularly less than 6 microns, the preferred and most
preferred fiber sizes. Note that these small fiber sizes further
distinguish over U.S. Pat. No. 4,048,364.
FIG. 8 is a plot of packing density (PD) versus average fiber
diameter for the precursor and processed webs. FIG, 8 indicates
that web densification or consolidation initiates in meltblown
precursor webs having average fiber diameters less than about 8
.mu.m for meltblown polypropylene webs. MB webs from precursors
having fiber diameters greater than about 8 microns experience
little to no alteration in packing density (or other performance
properties) according to the method of the present invention. Other
measures of web performance such as filtration efficiency, air
permeability, and maximum pore size (see Table I, II and Table III)
show similar response to web average fiber diameter as shown in
FIG. 9 for filtration efficiency. In the experiments, these
properties were generally maximized by post treating at draw ratio
between about 1.5 and 2.0 for the precursors.
ALTERNATIVE EMBODIMENTS
Spunbond Webs: As indicated above, the principles embodied in the
present invention have application with nonwoven webs others than
meltblown webs. For example, for spunbond webs which are
characterized as having average filament diameters or 7 to 50
microns and elongation to break less than about 200% according to
ASTM Test D 5035-90. The spunbond webs are prepared by melt
spinning a multiplicity of filaments molecularly oriented by
plastic deformation draw-down and depositing the same on a moving
collector to form a random collection of uniform filaments arranged
similar to that depicted in FIG. 4. The deposited filaments are
then bonded by mechanical entangling, needling, hot calendering or
otherwise thermal bonding at a plurality or points to impart
integrity and strength to the spunbond material. It should be noted
that bonding such as thermal or mechanical bonding is normally
necessary since the filaments are not typically fused or
sufficiently entangled upon being laid or deposited on the
collector. For spunbonded precursors, the bonding must be strong
(such as high temperature point bonding) in order to locally
elongate, buckle, and bend the filament segments without spoiling
the web integrity (see FIGS. 5 and 6) because the drawn filaments
have relatively high tenacity and modulus. In point bonding, the
bond points and bonding pattern generally are as follows: The area
of heated bonding points are 5 to 25% of the roll area and the
shape of the raised points can be diamond shaped or a number of
other shapes and point distribution.
The consolidation or the spunbond (SB) web in accordance with the
present invention occurs as follows: Hot drawing the SB web creates
reduction in the measures or pore size and creates CD elasticity
because the tensile forces generate sufficient compressive forces
to plastically buckle and bend C segments of the filaments for
inventive reduction of pore measures. The elasticity in the CD
direction is a result or elastic recovery from bending of the well
bonded network of strong filaments arranged similar to that
idealized in FIG. 6.
An example of the spunbond process was as follows: Spunbonded web
was 1 meter wide, 1 oz/sq. yd. produced from 35 MFR PP on a
Reicofil machine bonded between 90.degree. and 140.degree. C. at
the University of Tennessee. Oven temperature 315.degree. F., draw
ratio 1.20 output velocity (V2) 50 FPM.
Electrostatic Charged Webs: Another variation contemplated by the
present invention is to apply an electrostatic charge to the
consolidated web to improve its filtration performance. The
charging in the production of electrets can be applied by a variety
of techniques described in the patent literature. See for example
U.S. Pat. No. 4,592,815, the disclosure of which is incorporated
herein by reference. It is anticipated that the higher packing
density of fiber in the hot consolidated webs results in an
unusually high effectiveness of electron implantation in the web.
As an example of the effect of charging consolidated samples on web
Filtration Efficiency (FE), a 1.0 oz/sq.yd. precursor meltblown
ample had an FE of 30%, the FE after only consolidating this web at
a DR of 1.5 was 79%, and finally the FE after charging this
consolidated web was 99.8%.
Since meltblown webs and spunbonded webs are relatively isotropic,
the invention process can also be carried out by hot drawing in the
CD as a continuous process (such as on a tenter frame at negative
or minimal MD tension) or on a "by piece" process.
Laminate and Composite Webs: As mentioned above, the precursor web
may comprise a composite of the following combinations: meltblown
web/meltblown web (different webs), meltblown web/other nonwoven
web (e.g., spunbond, hydroentangled, etc.) also, webs of
thermoplastic/nonthermoplastics combinations make useful
precursors. These composite precursors can be made by techniques
well known in the art. The composite may also include more than two
layers. The meltblown web of the composite will have the properties
described above.
One particularly useful composite precursor is the
spunbond/meltblown/spunbond (SMS) structure.
The meltblown web should have the properties described above for
meltblown webs. The spunbond webs may be the same or different but
should have the properties described above for spunbond webs. The
SMS composite precursor may be made by conventional methods, well
known in the art.
The spunbond webs add strength and abrasion resistance to the
structure thus increasing the application of the webs consolidated
by the process of the present invention, particularly in the areas
of surgical gowns, drapes, health care packaging, etc. The
consolidated composite is characterized by:
a) good elasticity in the CD;
(b) good strength; and
(c) improved filtration efficiency.
It has also been observed that hot or cold CD stretching following
consolidation by MD stretching (as described above) produces an
open reticulated fabric having exceptional web uniformity and high
porosity for an open structure. Hot stretching in the CD at large
draw ratios (e.g., about 1.4) produces a netting structure that has
applications such a high porosity HVAC filters and containers.
The following experiments demonstrate the effect of drawing an SMS
precursor web in accordance with the process of the present
invention. The SMS web was thermally point bonded and had the
following composition:
______________________________________ Web Thickness, Basis Wt,
Composition Mils oz/yd.sup.2 ______________________________________
S Spunbond PP 3 0.3 M Meltblown PP 9 1.7 S Spunbond PP 3 0.3
______________________________________
The precursor web was processed at a draw ratio or 1.9 through a
315 degree Fahrenheit oven at 21 fpm. The drawn web was permitted
to cool to room temperature while under the applied MD tension.
Cyclic load-extension tests in the CD were carried out. TABLE IV
presents the results.
TABLE IV ______________________________________ Peak Load CD
Extension (Grams) Recovery, % Sample Stretch 1st Cycle 5th Cycle
1st Cycle 5th Cycle ______________________________________ SMS 50%
95 90 82 73 100% 410 380 60 46 200% 1540 1440 37 32
______________________________________
The elasticity of the drawn SMS fabric makes the fabric
particularly useful in surgical gowns requiting relatively high
strength, stretchability and barrier properties.
The same consolidated SMS fabric was tested for filtration
efficiency. The filtration tests were carried out on the SMS fabric
without consolidation and the SMS fabric after consolidation. The
drawn or consolidated SMS web exhibited a filtration efficiency of
80.8% whereas the precursor SMS web exhibited a filtration
efficiency of only 67.7%.
SUMMARY
As demonstrated by the experimental data herein, the method of the
present invention produces a nonwoven fabric that posses unique and
useful properties that lends the fabric to application in a variety
of fields. The properties of reduced pore size and pore size
distribution makes the web ideally suited for filtration and
absorption. The property of CD elasticity increases the web utility
in filtration (e.g., surgical masks where conformance to the face
contours is important) and other uses such as flexible gowns or
diapers and hygiene products.
* * * * *