U.S. patent application number 10/664708 was filed with the patent office on 2004-06-17 for extremely high liquid barrier fabrics.
Invention is credited to Bryner, Michael Allen.
Application Number | 20040116028 10/664708 |
Document ID | / |
Family ID | 32030675 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040116028 |
Kind Code |
A1 |
Bryner, Michael Allen |
June 17, 2004 |
Extremely high liquid barrier fabrics
Abstract
One embodiment of the present invention is a nonwoven fabric
comprising a support web and a fibrous barrier web, having a
hydrohead of at least about 145 cm and a Frazier permeability of at
least about 0.3 m.sup.3/m.sup.2-min.
Inventors: |
Bryner, Michael Allen;
(Midlothian, VA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
32030675 |
Appl. No.: |
10/664708 |
Filed: |
September 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60411422 |
Sep 17, 2002 |
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Current U.S.
Class: |
442/381 ;
442/382; 442/79; 442/82 |
Current CPC
Class: |
Y10T 442/614 20150401;
Y10T 442/2189 20150401; Y10T 442/659 20150401; Y10T 442/68
20150401; Y10T 442/608 20150401; Y10T 442/681 20150401; B32B 5/022
20130101; B32B 5/26 20130101; Y10T 442/626 20150401; A61F 13/51462
20130101; Y10T 442/60 20150401; D04H 1/56 20130101; B32B 2255/02
20130101; B32B 2307/724 20130101; D04H 1/559 20130101; Y10T 442/66
20150401; B32B 2255/26 20130101; Y10T 442/2164 20150401; Y10T
442/621 20150401; A41D 31/102 20190201; B32B 2307/73 20130101 |
Class at
Publication: |
442/381 ;
442/079; 442/082; 442/382 |
International
Class: |
B32B 003/00; B32B
005/02; B32B 009/00; B32B 027/04; B32B 027/12; D04H 001/00; D04H
003/00; D04H 005/00; D04H 013/00; B32B 005/26 |
Claims
We claim:
1. A nonwoven fabric comprising a support web and a fibrous barrier
web, having a hydrohead of at least about 145 cm and a Frazier
permeability of at least about 0.3 m.sup.3/m.sup.2-min.
2. A nonwoven fabric comprising at least one support web and a
hydrophobic barrier web with fibers having diameters of less than
2.0 micrometers, a hydrohead of at least about 145 cm and a Frazier
permeability of at least about 0.3 m.sup.3/m.sup.2-min.
3. The nonwoven fabric of claims 1 or 2, wherein said barrier web
fibers have diameters of less than 1.0 micrometer.
4. The nonwoven fabric of claims 1 or 2, wherein said barrier web
fibers have diameters of less than 0.5 micrometer.
5. The nonwoven fabric of claim 3, wherein the barrier layer basis
weight is no more than 15 g/m.sup.2.
6. The nonwoven fabric of claim 4, wherein the barrier layer has a
basis weight of no more than 3 g/m.sup.2.
7. The nonwoven fabric of claims 1 or 2, wherein said barrier web
comprises nanofibers of hydrophobic polymer or copolymer.
8. The nonwoven fabric of claim 7, wherein said hydrophobic polymer
or copolymer is a polyolefin, a partially fluorinated polymer or a
perfluorinated polymer.
9. The nonwoven fabric of claim 8, wherein said hydrophobic polymer
or copolymer has repeating units derived from ethylene, propylene,
butenes, hexenes, octenes, styrenes, 4-methylpentene-1 and
combinations thereof.
10. The nonwoven fabric of claims 1 or 2, wherein said barrier web
is rendered hydrophobic by coating with a hydrophobic coating.
11. The nonwoven fabric of claim 10, wherein said hydrophobic
coating is a fluorocarbon coating material.
12. The nonwoven fabric of claims 1 or 2, wherein the barrier web
has a maximum pore size between fibers of no more than about 23
micrometers.
13. The nonwoven fabric of claims 1 or 2, wherein the barrier web
has a solids fraction of no less than about 0.03.
14. A nonwoven barrier fabric comprising a fibrous barrier web,
said fabric having a hydrohead of at least about 145 cm and a
Frazier permeability of at least about 0.3 m.sup.3/m.sup.2-min and
having a relationship between barrier web basis weight, and fabric
hydrohead and Frazier permeability described by the formula: 18 Bwt
( g / m 2 ) 4000 c ( 1 - 2.3 c ) f Frazier Hydrohead k ( c )
,wherein .rho..sub.f, is the density of the barrier fibers,
kg/m.sup.3c is the solids volume fraction of the barrier web,
k(c)=3.58.multidot.c.sup.2-1.32.multidot.c+1.77, Frazier is in
units of m.sup.3/m.sup.2-min, and Hydrohead is in units of
centimeters of water column.
15. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of FF/mSB, wherein FF is a barrier web.
16. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of FF/SB, wherein FF is a barrier web.
17. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of mSB/FF/mSB, wherein FF is a barrier
web.
18. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of FF/MB/SB, wherein FF is a barrier
web.
19. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of SB/MB/FF/MB/SB, wherein FF is a barrier
web.
20. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of FF/MB/mSB, wherein FF is a barrier
web.
21. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of mSB/MB/FF/MB/mSB, wherein FF is a barrier
web.
22. A nonwoven fabric according to one of claims 1, 2 or 14,
comprising a structure of SB/MB/FF/SB, wherein FF is a barrier
web.
23. A nonwoven fabric of claims 1 or 2, wherein said support web
comprises fibers having diameters less than about 20 times the
barrier web fiber diameters.
24. The nonwoven fabric of claim 23, wherein said support web
fibers have diameters less than about 13 micrometers.
Description
[0001] This invention relates to nonwoven fibrous structures and
more particularly to breathable fabrics and sheet structures formed
by fibers which are held together without weaving or knitting.
[0002] Nonwoven fibrous structures have existed for many years and
today there are a number of different nonwoven technologies in
commercial use. To illustrate the breadth of nonwoven technologies,
paper is probably one of the earliest developed nonwoven fibrous
structures. Nonwoven technologies continue to be developed by those
seeking new applications and competitive advantages. One broad
market area that has proven to be highly desirable because of its
large volume and economics is the protective apparel market. This
market comprises protection from hazardous chemicals such as in
chemical spill clean up, from liquids such as blood in the medical
field and from dry particulates or other hazards such as painting
or asbestos removal.
[0003] It is known that for a garment to be comfortable, it must
accommodate the body's physiological need for thermal regulation.
In warm environments, heat energy must be expelled from the body.
This is done principally by a combination of direct thermal
conduction of heat away from the body through the fabric and air
layers at the skin surface, convection of heat away from the body
by flowing air, and by the cooling effects of evaporation of sweat
from the surface of the skin. Clothing which appreciably inhibits
heat transfer can cause heat and moisture buildup and this can
result in discomfort due to warm, sticky, clammy and or sweaty
sensations. In the extreme case, for example, where protective
clothing prevents adequate thermal regulation during activity in a
warm and humid environment, such clothing limitations not only lead
to discomfort, but can result in life-threatening heat stress. For
this reason, frequently, clothing limitations impose limitations on
activity to avoid the consequences of heat stress.
[0004] Studies have shown that the most comfortable garments with
the least restrictions on physical activity in warm, humid
environments, are those most able to breathe through mechanisms of
air exchange with the environment. (Bernard, T. E., N. W. Gonzales,
N. L. Carroll, M. A. Bryner and J. P. Zeigler. "Sustained work rate
for five clothing ensembles and the relationship to air
permeability and moisture vapor transmission rate." American
Industrial Hygiene Conference, Toronto, June 1999; N. W. Gonzales,
"Maximum Sustainable Work for Five Protective Clothing Ensembles
and the Effects of Moisture Vapor Transmission Rates and Air
Permeability" Master's Thesis, College of Public Health, University
of South Florida, December 1998).
[0005] Physical activity flexes fabric and garment. If a fabric has
low enough resistance to air flow, this, in turn, produces a
pumping action which pushes and pulls air back and forth through
the fabric. By this mechanism, the exchange of warm moisture laden
air within the garment with ambient air provides a significant
cooling effect. Tests of protective garments made of the same cut,
but with widely differing air flow resistance under warm humid
conditions (32.degree. C., 60% RH), have shown that the garments
made of fabrics with the least air flow resistance repeatedly
allowed subjects to achieve higher levels of activity without
incurring heat stress. Conversely, garments made of fabrics with
the highest air flow resistance limited the physical activity of
the same subjects to the lowest levels to avoid heat stress.
Garments made of fabrics having intermediate air flow resistance
allowed subjects to achieve intermediate levels of activity without
heat stress. The intermediate activity levels correlated very well
with the air flow resistance of the fabric.
[0006] Clearly, under conditions where the body must transfer heat
and moisture to maintain comfort or avoid heat stress, it is
desirable to for garments to be made with fabrics having low air
flow resistance.
[0007] Clothing provides protection from hazards in the
environment. The degree of protection clothing imparts is dependent
upon the effectiveness of the barrier characteristics of the
clothing. Where the function of the barrier is to keep
environmental particulates or fluids from penetrating a garment to
reach the wearer, barrier is easily correlated with fabric pore
size. The most effective barriers generally have the smallest pore
size.
[0008] Unfortunately, smaller pore size also generally results in
higher air flow resistance. In the studies cited above, the
garments with the highest barrier properties had the lowest airflow
permeability and vise versa. So the ability to provide effective
barrier protection in clothing and the ability to provide low air
flow resistance, i.e., high air flow permeability, in the same
garment are inversely related.
[0009] Hydrostatic head or "hydrohead" (AATCC TM 127-194) is a
convenient measure of the ability of a fabric to prevent water
penetration. It is presented as the pressure, in centimeters of
water column (cmwc) required to force liquid water through a
hydrophobic fabric. It is known that hydrohead depends inversely on
pore size. Lower pore size produces higher hydrohead and higher
pore size produces lower hydrohead.
[0010] Fabric air flow permeability is commonly measured using the
Frazier measurement (ASTM D737). In this measurement, a pressure
difference of 124.5 N/m.sup.2 (0.5 inches of water column) is
applied to a suitably clamped fabric sample and the resultant air
flow rate is measured as Frazier permeability or more simply as
"Frazier". Herein, Frazier permeability is reported in units of
m.sup.3/m.sup.2-min. High Frazier, corresponds to high air flow
permeability and low air flow resistance while low Frazier
corresponds to low air flow permeability and high air flow
resistance.
[0011] Microporous films have been used in barrier materials to
achieve extremely high hydrostatic head liquid barrier properties,
but at the expense of breathability, such that their Frazier
permeabilities are unacceptably low, rendering fabrics containing
such films uncomfortable for the wearer.
[0012] Currently, most melt-spun fibers have diameters on the order
of several tens of micrometers, whereas melt-blown fibers are known
to have fiber diameters on the order of from about 1 to 10
micrometers. Recently, many researchers have made efforts to
decrease fiber sizes in order to obtain different benefits, as
compared to conventional fibers.
[0013] Advances have been made in providing both high hydrohead
properties and high Frazier properties in the same fabric. For
example, U.S. Pat. No. 5,885,909 discloses low or sub-denier
nonwoven fibrous structures which demonstrate an unusual
combination of high Frazier permeability and high hydrostatic head
liquid barrier properties.
[0014] More recently, efforts have centered around obtaining fiber
diameters in the `nanofiber` range, i.e. with diameters on the
order of less than about 0.5 micrometers (500 nm). However,
production of such small fibers has presented many problems
including low throughput, poor efficiency in spinning and
difficulties in fiber collection.
[0015] Conventionally, nanofibers have been produced by the
technique of electrospinning, as described in "Electrostatic
Spinning of Acrylic Microfibers", P. K. Baumgarten, Journal of
Colloid and Interface Science, Vol. 36, No. 1, May, 1971. According
to the electrospinning process, an electric potential is applied to
a drop of a polymer in solution hanging from a metal tube, for
example a syringe needle, which results in elongation of the drop
of the solution to form very fine fibers which are directed to a
grounded collector. Fibers with diameters in the range of 0.05 to
1.1 micrometers (50 to 1100 nm) are reported. An example of a
suitable electrospinning apparatus for forming the
nanofiber-containing fabrics of the present invention is disclosed
in U.S. Pat. No. 4,127,706, incorporated herein by reference.
[0016] The vast majority of investigations into nanofiber
production reported in the prior art literature have been directed
to formation of essentially hydrophilic polymer nanofibers, such as
polyamide, polyurethane and the like. While some investigators have
suggested that nanofibers could be produced from hydrophobic
polymers, few actual examples of such hydrophobic nanofibers are
disclosed in the literature. U.S. Pat. No. 4,127,706 discloses
production of porous fluoropolymer fibrous sheet, suggesting the
production of PTFE fibers with diameters in the range of 0.1 to 10
micrometers, but exemplifying only fibers with diameters of 0.5
micrometer and above.
SUMMARY OF THE INVENTION
[0017] One embodiment of the present invention is a nonwoven fabric
comprising a support web and a fibrous barrier web, having a
hydrohead of at least about 145 cm and a Frazier permeability of at
least about 0.3 m.sup.3/m.sup.2-min.
[0018] Another embodiment of the present invention is a hydrophobic
nonwoven fabric comprising at least one support web and a barrier
web with fibers having diameters of less than 2.0 micrometers, a
hydrohead of at least about 145 cm and a Frazier permeability of at
least about 0.3 m.sup.3/m.sup.2-min.
[0019] Another embodiment of the present invention is a nonwoven
fabric comprising a fibrous barrier web, said fabric having a
hydrohead of at least about 145 cm and a Frazier permeability of at
least about 0.3 m.sup.3/m.sup.2-min and having a relationship
between barrier web basis weight, and fabric hydrohead and Frazier
permeability described by the formula: 1 Bwt ( g / m 2 ) 4000 c ( 1
- 2.3 c ) f Frazier Hydrohead k ( c )
[0020] wherein .rho..sub.f, is the density of the barrier fibers,
kg/m3, c is the solids volume fraction of the barrier web,
k(c)=3.58.multidot.c.su- p.2-1.32.multidot.c+1.77, Frazier is in
units of m.sup.3/m.sup.2-min, and hydrohead is in units of
centimeters of water column.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a log/log plot of barrier properties of various
prior art fabrics.
[0022] FIG. 2 is a reproduction of FIG. 1 with a plot of the line
of Equation 10 laid thereon.
[0023] FIG. 3 is reproduction of FIG. 1 with a plot of data from
Equation 14 wherein basis weight is maintained as a constant, and
fiber size is reduced.
[0024] FIG. 4 is a plot of basis weight v. liquid barrier at
constant air permeability (Frazier).
[0025] FIG. 5 is a reproduction of FIG. 3 with a plot of data from
Equation 14 wherein air permeability (Frazier) is maintained as a
constant, and fiber size is reduced.
[0026] FIG. 6 is an illustration of the structure of the nonwoven
fabrics of the present invention presenting a barrier to the
advance of a liquid surface.
[0027] FIG. 7 is a graphical presentation illustrating the
relationship of Equation 16 wherein achievable hydrohead as a
fraction of potential hydrohead is dependent upon D.sub.fS/D.sub.fL
and GPD.times.Bwt.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Unless otherwise specified, references to fiber diameters
herein are intended to be directed to the number average fiber
diameter of the fibers.
[0029] FIG. 1 illustrates the inverse relationship between air
permeability and hydrohead for three sets of data. The first set is
taken from U.S. Pat. No. 5,585,909, the second presents data
measured on samples of melt-blown nonwoven fabric, the third
presents data measured on three commercial nonwoven products: K-C
Ultra.RTM. unreinforced surgical gown, available from Kimberly
Clark Health Care, Roswell, Ga.; Trimax.RTM. unreinforced surgical
gown and DuPont Sontara.RTM. Optima.RTM. unreinforced surgical
gown, both available from Allegiance Health Care, Mc Gaw Park,
Ill.
[0030] It is of note in FIG. 1, that commercial nonwoven products
have air permeabilities in the range of woven fabrics. By way of
reference, a tightly woven polyester fabric (basis weight 95
g/m.sup.2) used in the testing described above had a Frazier value
of about 0.5 m.sup.3/m.sup.2-min, while, ASTM D737-96 lists the
Frazier values for a sampling of several woven fabrics in the range
of 2.5 to 66 m.sup.3/m.sup.2-min.
[0031] FIG. 1 shows that nonwoven barrier fabrics have a hydrohead
typically lower than about 100 centimeters of water column. The
forcing pressure difference, .DELTA.P, across the fabric can be
related to the equivalent capillary radius, R of the largest pore
water will penetrate, using the Washburn equation: 2 P = - 2 R Cos
. ( Equation 1 )
[0032] Here .sigma. is the surface tension of water (0.072 N/m) and
.theta. is the wetting angle, i.e., the angle of intersection of
the fluid surface with the solid surface. For .DELTA.P in units of
centimeters of water column and R in micrometers and assuming an
ideally nonwetted surface (.theta.=180.degree.), 3 P ( cmwc ) =
1468 R ( microns ) . ( Equation 2 )
[0033] From which it is concluded that hydrohead lower than about
100 cmwc in FIG. 1 corresponds to largest pores of radius,
R.gtoreq.15 micrometers.
[0034] The Washburn relationship shows that to create better liquid
barriers which can withstand higher fluid pressures, fabric pore
size must be reduced. Better liquid barrier fabric would be of
benefit in many applications including protective apparel. For
example, in response to concern about contamination with
blood-borne pathogens, ASTM F1670 specifies that an acceptable
fabric must prevent penetration of synthetic blood (.sigma.=0.042
N/m versus 0.072 N/m for water) at a pressure of 13800 N/m.sup.2
(141 cmwc). From Equation 1, for a fabric to pass this test
(wetting angle .theta.=180.degree.), the maximum fabric pore radius
must be less than about 6 micrometers.
[0035] Microporous films with pores radii typically less than 1
micrometer satisfy this criterion. Such films can be effective
liquid barriers, but they are very impermeable to air flow as well.
Typical microporous film air permeabilities, e.g., in the range of
Frazier<0.008 m.sup.3/m.sup.2-min, are too low to provide
effective air exchange in a protective garment. This often leads to
heat buildup and discomfort. In the extreme, it can even impair or
limit work performance.
[0036] Fibrous porous media are inherently more permeable than
microporous films and a good choice for protective fabrics, but the
relationship of FIG. 1 shows, in general, that significant
increases in barrier function resulting from reduced pore size will
also significantly reduce air permeability.
[0037] To understand the requirements for a nonwoven fibrous fabric
to have both high liquid barrier and high air permeability, it is
useful to construct an analytical model of the fabric structure.
Hydrohead as a measure of liquid barrier is related to pore size as
discussed above, and pore size is determined by structural
characteristics of the fabric, including fiber size and void
fraction. Fabric air permeability is also determined by fundamental
structural characteristics, including fiber size, void fraction and
basis weight.
[0038] Pore Size:
[0039] The size of the pore space between fibers in a random fiber
web, is proportional to fiber diameter, D.sub.f, as a determinant
of the number of fibers which can occupy a space. It is inversely
proportional to the solids volume fraction, c, which is the ratio
of web volume occupied by fibers to the total web volume (i.e.,
(1--void fraction)). For metal fiber filters, Goeminne, et al,
("The Geometrical and Filtration Characteristics of Metal Fiber
Filters--A Comparative Study", Filtration and Separation, Vol. 11,
No. 4, pp 350-355 (1974)) report that the maximum pore diameter,
D.sub..rho. is described by: 4 Dp = D f c . ( Equation 3 )
[0040] An independent analysis of the stochastic structure of ideal
random fibrous webs for this work gives: 5 Dp = 3 D f 8 c . (
Equation 4 )
[0041] Equation 4 predicts slightly larger maximum pore size than
Equation 3. Combining Equation 4 with Equation 2 provides a
conservative estimate of random web hydrohead in terms of fiber
size and solids fraction as: 6 P ( cmwc ) = 2493 c D f ( microns )
. ( Equation 5 )
[0042] Equation 5 is used for the results below.
[0043] Air Permeability:
[0044] Davies has presented a careful and well attested correlation
of flow rate, pressure drop, fiber size and solids fraction on pads
made of a wide variety of fibrous materials. (Davies, C. N., "The
Separation of Airborne Dust and Particles," The Institution of
Mechanical Engineers Proceedings (B), Nos. 1-12, Vol 1B, p 185,
1952-53) In terms of definitions above, this correlation gives
volumetric flow rate, Q, per unit flow area, A, as: 7 Q A = P D f 2
h f ( c ) , ( Equation 6 )
[0045] where,
.function.(c)=64.multidot.c.sup.1.5.multidot.(1+56.multidot.c.sup.3)
(Equation 7)
[0046] Here .DELTA.P is the pressure drop across the fibrous pad of
thickness, h, and .eta. is the viscosity of the flowing fluid. The
Davies correlation is valid for 0.006<c<0.3 when the flow
around fibers in the medium is laminar.
[0047] The thickness of the fibrous medium is related to the basis
weight (Bwt) of the medium, the fiber density, .rho..sub.f, and the
solids fraction as follows: 8 h = Bwt f c . ( Equation 8 )
[0048] Combining Equations 6 and 8, gives: 9 Q A = f P D f 2 c Bwt
f ( c ) . ( Equation 9 )
[0049] Taking hydrohead to be the forcing pressure, .DELTA.P, of
Equation 5, the relationship between hydrohead and fiber size of
Equation 5 can be combined with the above relationship between Q/A
and fiber size to give 10 Q A = 6.2 .times. 10 6 f P c 3 Bwt
Hydrohead 2 f ( c ) . ( Equation 10 )
[0050] If the flow forcing pressure difference, .DELTA.P, of
Equation 10 is set equal to 124.5 N/m.sup.2, and consistent units
are used, Q/A is calculated directly as Frazier in units of cubic
meter per square meter per minute (m.sup.3/m.sup.2-min). FIG. 2
shows that for typical polypropylene fabrics, Bwt=34 grams/m.sup.2,
c=0.1, and .rho..sub.f,=920 kg/m.sup.3, the model of Equation 10
reasonably fits the data of FIG. 1 accounting for the general
trend.
[0051] Two further refining effects must be taken into account.
First, thermal bonding (which is almost always necessary in the
production of nonwoven fabrics) at bond points which comprise a
bonded area fraction, f.sub.ba, will reduce Q/A by the factor
(1-f.sub.ba), hence, 11 Q A = f P D f 2 c Bwt f ( c ) ( 1 - f ba )
. ( Equation 11 )
[0052] Second, for the fabrics with fiber sizes less than about 5
micrometers, air flow is known to "slip" past the fibers without
encountering the full effects of viscous drag. The slip effect
increases as fiber size decreases. The effect is to increase flow
at a give pressure drop over that predicted by Equation 10.
Chmielewski and Goren ("Aerolsol Filtration With Slip Flow",
Environmental Science and Technology, Vol. 6, No. 13, p 1101, 1972)
have presented a correction factor for the case of slip flow
through fibrous fabrics. The correction factor, here defined as
S(c, N.sub.kn) varies with solids volume fraction, c, and with the
Knudsen Number, N.sub.kn, defined as 12 N kn = 2.48 D f , (
Equation 12 )
[0053] where .lambda. is the mean free path for collisions between
air molecules. Here, .lambda. is taken to be 0.065 micrometers. For
this work, the graphical presentation of Chmielewski and Goren was
fit very well empirically with the function 13 S ( c , N kn ) = 1 +
( 1.662 + 19.66 c - 47.027 c 2 ) N kn 1 + .9489 N kn . ( Equation
13 )
[0054] The slip correction is incorporated in the flow model which
then becomes: 14 Q A = f P D f 2 c Bwt f ( c ) ( 1 - f ba ) S ( c ,
N kn ) . ( Equation 14 )
[0055] As above, if the forcing pressure drop across the fabric,
.DELTA.P, is 124.5 N/m.sup.2 (12.7 mm of water column), and .eta.
is the viscosity of air at room temperature, and if consistent
units are used, then Q/A is the Frazier permeability, here denoted
in units of m.sup.3/m.sup.2-min.
[0056] The present inventor has determined that the model for
hydrohead, Equation 5, and the model for flow, Equation 14, can be
used together to define the requirements for functionally superior
liquid barrier fabrics. If the fabric is a multi-layer fabric, the
model can be used for each layer to determine the properties of
each, then the individual layer properties can be combined to
determine composite sheet properties. For example, in a layered
fabric, hydrohead is taken to be the maximum hydrohead of any layer
in the fabric. Air permeability is obtained from the relationship:
15 1 Frazier TotalFabric = Layer = 1 j 1 Frazier Layer - j . (
Equation 15 )
[0057] Models:
[0058] Model 1: Constant Basis Weight as Fiber Size is Decreased to
Increase Liquid Barrier.
[0059] For the case of a polypropylene fabric where Bwt=33.9
g/m.sup.2, f.sub.ba=0, c=0.1, and .rho..sub.f=920 kg/m.sup.3, the
model provides the results of Table 1, and FIG. 3.
1TABLE 1 Fiber Diameter Hydrohead Frazier Permeability
(micrometers) (cmwc) (m.sup.3/m.sup.2-min) 2.0 125 2.58 1.5 166
1.54 1.0 249 0.75 0.7 356 0.41 0.5 499 0.23 0.3 831 0.10
[0060] The results of Model 1 illustrate the detrimental decrease
of fabric permeability when liquid barrier is increased by
decreasing fiber size alone, without a decrease in basis
weight.
[0061] Model 2: Constant Air Permeability (Frazier) as Fiber Size
is Decreased to Increase Liquid Barrier.
[0062] For the case of a polypropylene fabric where Frazier=10
m.sup.3/m.sup.2-min, f.sub.ba=0, c=0.1, and .rho..sub.f=920
kg/m.sup.3, the model provides the results of Table 2, and FIGS. 4
and 5.
2 TABLE 2 Hydrohead Basis Weight (cmwc) (g/m.sup.2) 125 8.761 166
5.213 249 2.553 356 1.386 499 0.788 831 0.339
[0063] From FIGS. 4 and 5, it is clearly seen that the basis weight
of the barrier layer must be decreased dramatically to maintain air
permeability as liquid barrier in terms of hydrohead is
increased.
[0064] Problem of High Barrier and Thin Barrier Web:
[0065] In the extreme case of high permeability or high liquid
barrier or both, the mechanical strength of the barrier layer can
pose a practical limit the barrier level achieved. FIG. 6 shows a
liquid interface advancing against a fibrous barrier layer. The
barrier layer consists of a layer of small fibers with
characteristically small pores supported by a layer of large fibers
with characteristically larger pores. The pressure required to
force a nonwetting fluid through the small pores of the barrier
layer is given by Equation 5. This pressure force is distributed
across all the small fibers of the barrier layer. Hence, the
loading of a representative small fiber, e.g., Fiber AB, is readily
obtained as a force per unit length. The span over which the small
fiber must carry the pressure load is determined by the pore size
of the support layer as given by Equation 4. If the span is too
great, the tension in the small fibers can exceed the strength of
the fibers, causing them to break.
[0066] In this case, the hydrohead is limited by a relationship
between the strength of the barrier fiber, and the basis weight of
the barrier fiber layer, which determine the strength of the
barrier layer, and the pore size of the support layer which
determines the force load on the barrier fibers. A relationship can
be developed between the maximum force load a barrier fiber can
sustain just before breaking and the force loading that the barrier
fiber would have to sustain to achieve maximum hydrohead. If it is
assumed that the barrier layer fiber loading and geometry are
microscopic equivalents to the uniform loading of a macroscopic
cable strung between two supports and if it is assumed that a small
barrier fiber deflects a distance equivalent to one large support
layer fiber diameter before it breaks, the analysis of this
relationship as a cable problem gives: 16 Hydrohead act Hydrohead
max = 613 D fS D fL GPD Bwt ( g / m 2 ) 1 + 0.0867 ( D fL / D fS )
2 c 2 , ( Equation 16 )
[0067] (Higdon, A., Stiles, W. B., Engineering Mechanics Statics
and Dynamics, Vector Edition, Prentice-Hall, 1962).
[0068] Here, Hydrohead.sub.act is the hydrohead actually achieved.
Hydrohead.sub.max is the maximum hydrohead the barrier layer can
achieve, given by Equation 5. D.sub.fS and D.sub.fL are the
diameters of the small barrier layer fibers and the large support
fibers respectively. GPD is the tensile strength of the barrier
layer fibers in grams per denier. Bwt is the basis weight of the
barrier layer. The solids volume fraction is c.
[0069] Model 3: Illustration of the Problem of Low Barrier Layer
Strength When Barrier Layer Basis Weight is Reduced to Maintain Air
Permeability
[0070] If the barrier layer of Model 2 consisting of polypropylene
(.rho..sub.f=920 kg/m.sup.3) fibers of diameter D.sub.fS=0.6
micrometers (Frazier=10 m.sup.3/m.sup.2-min, c=0.1, GPD=1 gram per
denier, and Bwt=1 g/m.sup.2) is laminated to a support layer with
fibers of diameter D.sub.fL=12 micrometers, then Equation 16 gives:
17 Hydrohead act Hydrohead max = 0.52 . ( Equation 17 )
[0071] The maximum potential hydrohead for the barrier layer as
obtained from Equation 5 is 415 cmwc, but at a basis weight of 1
g/m.sup.2, the layer is strong enough to withstand only about half
of that pressure before collapsing. The maximum hydrohead could be
realized by doubling the basis weight of the barrier layer, but
doubling the basis weight would reduce the air permeability of the
composite fabric by half. There would be an economic penalty as
well for the higher basis weight.
[0072] An alternative solution is to reduce the pore size of the
support layer by reducing the support layer fiber size. Per FIG. 7,
the Hydrohead.sub.act/Hydrohead.sub.max curve for GPD.times.Bwt=1,
reaches unity when the ratio D.sub.fS/D.sub.fL=0.075. So the
maximum hydrohead possible can be realized if the support layer
fiber diameter is reduced to about 8 micrometers. If the basis
weight of such a support layer is less than about 9 grams/m.sup.2,
per Equation 7, the Frazier air permeability, is still about 10
m.sup.3/m.sup.2-min.
[0073] The model relationships presented here permit the rational
design of fabrics for various balances of barrier and air
permeability. Clearly, the underlying physics allow only certain
balances of properties to exist. Once a realizable balance is
specified, choices can be made as to how to create a given
balance.
[0074] For example, since permeability depends upon the square of
fiber diameter, choosing the largest fiber size consistent with
achieving a desired barrier might be preferred as a means of
achieving the highest permeability. Hydrohead can be increased by
calendering the fabric to increase solids fraction (Equation 5).
The dependence of hydrohead and Frazier on solids fractions is such
that calendering the barrier layer to increase the solids fraction
will increase barrier more than it decreases Frazier. If smaller
fiber size is selected for barrier as the product basis, basis
weight can be adjusted within bounds to achieve the desired air
permeability. Other such tradeoffs can be assessed based on
economics and the practicalities of fabric processing.
[0075] The model presented is based on the geometry of a random
fiber web made of rigid, straight, continuous fibers, of which a
glass fiber mat is a good example. This is perhaps the simplest,
most open and ideal web geometry. Certainly many deviations from
this ideal exist in practice. A common deviation is due to
non-random fiber deposition associated with fiber bunching or
clumping. As discussed by Davies (cited above), the resulting
structure acts as if it is made of fibers of an effective fiber
size somewhat larger than the actual fiber size.
[0076] Changes in fiber properties which affect how fibers pack in
three dimensions, such as fiber shape, stiffness, crimp, etc, will
result in structural deviations from the ideal. Also, the
fluid-fiber wetting characteristics reflected in surface tension,
.sigma., and the wetting angle, .theta., may vary. In most cases
this would reduce the maximum achievable hydrohead as per Equations
4 and 5. Hence, there can be other specific property balances and
these are implicit in the scatter of the data in FIGS. 1 and 2 and
the data set forth in Tables 4 and 5 below. In principle the model
can be refined for specific cases. Nevertheless, the analysis of
the ideal structure serves well as a benchmark and a guide.
[0077] The present invention is a nonwoven fabric comprising a
support web and a barrier web, having a hydrohead of at least about
145 cm and a Frazier permeability of at least about 0.3
m.sup.3/m.sup.2-min. The nonwoven barrier sheet can be hydrophobic,
said hydrophobicity being derived from either coating a hydrophilic
sheet with a hydrophobic coating material, such as a fluorocarbon-
or silicone-based coating material, or by forming the sheet from
hydrophobic polymers or copolymers, such as polyolefins, including
but not limited to those having repeating units derived from
ethylene, propylene, butenes, hexenes, octenes, styrenes,
4-methylpentene-1 and combinations thereof, and partially
fluorinated or perfluorinated polymers or copolymers, including but
not limited to ethylene/tetrafluoroethylene (E/TFE),
ethylene/chlorotrifluoroethylene, polyvinylidene fluoride (PVDF),
fluorinated ethylene/propylene (FEP), a copolymer of
tetrafluoroethylene and a perfluoro(alkyl vinyl ether) (PFA), and
the like.
[0078] The diameter of the barrier web fibers is usually less than
about 2 micrometers, more usually less than about 1 micrometer and
can even be in the "nanofiber" range, having diameters of less than
about 0.5 micrometer, wherein the diameter is the number average
fiber size.
[0079] The fibers in the support webs for the barrier webs are
usually less than 20 times, more usually less than 15 times and
most usually less than 10 times the diameter of the corresponding
barrier web fibers. For example, the support web fibers can have
diameters greater than about 13 micrometers, which roughly
corresponds to the diameter of conventional spunbond fibers, about
12 micrometers or less, which roughly corresponds to the diameter
of micro-denier spunbond fibers, or about 5 micrometers or less,
which roughly corresponds to the diameter of melt blown fibers.
[0080] The support web can be any fabric which is configured to
provide suitable support to the very fine fiber web. Among suitable
support webs are conventional spunbond and melt-blown webs,
micro-denier spunbond webs such as disclosed in U.S. Pat. No.
5,885,909, and various combinations of such different conventional
nonwoven webs with one or more of the very fine fiber webs.
[0081] It is also possible to provide a hydrophobic nonwoven sheet
containing nanofibers according to the present invention by
depositing a nonwoven web of conventional hydrophilic polymer
nanofibers onto a collecting/supporting web and coating the web's
nanofibers with a hydrophobic coating material, such as a
fluorocarbon coating material. When the coating material is applied
in an extremely thin layer, little if any change in the air
permeability properties of the underlying web is caused, for
example as described in co-pending U.S. provisional application No.
60/391,864, filed 26 Jun. 2002.
[0082] In order to minimize air flow resistance and maximize
flexibility of the nonwoven fabrics of the invention, the support
layer basis weight can be less than 17 g/m.sup.2, or less than 14
g/m.sup.2, or less than 11 g/m.sup.2, or less than 7 g/m.sup.2, or
less than 3 g/m.sup.2, or even less than 1 g/m.sup.2.
[0083] The nonwoven fabrics of the present invention have
hydroheads of at least about 145 cmwc and Fraziers of at least
about 0.3 m.sup.3/m.sup.2-min, or at least about 1
m.sup.3/m.sup.2-min, or at least about 3 m.sup.3/m.sup.2-min, or at
least about 5 m.sup.3/m.sup.2-min, or even at least about 10
m.sup.3/m.sup.2-min. The hydroheads of the inventive fabrics can be
greater than or equal to 150 cmwc, greater than or equal to 200
cmwc, greater than or equal to 300 cmwc, or even greater than or
equal to 400 cmwc.
[0084] The nonwoven fabrics of the present invention have maximum
pore size between fibers, as measured by bubble point (ASTM E128),
of less than about 23 micrometers, or less than about 20
micrometers, or less than about 15 micrometers, or even less than
about 12 micrometers.
EXAMPLES
[0085] Sample fabrics were made by dissolving various polymers in
suitable solvents which were then fed into an electrospinning
apparatus, such as that described in U.S. Pat. No. 4,127,706,
incorporated herein by reference. The fine fibers formed were
deposited onto a melt blown fabric support layer to form a barrier
layer of the fine fibers, and mechanical strength was imparted to
the samples by sandwiching the fine fiber/melt blown layers between
layers of spunbond polyester fibers, to form a four-layer laminate
of spunbond/melt blown/fine fiber/spunbond configuration.
[0086] Fine fibers were spun from two different hydrophobic
polymers: Kraton.TM. D1134x, a styrene-butadiene copolymer
(specific gravity=0.94), available from Kraton.TM. Polymers of
Houston, Tex.; and Kynar.TM. 761, a polyvinylidene fluoride polymer
(specific gravity=1.76), available from Atofina Chemicals, Inc. of
Philadelphia, Pa. Kraton.TM. fine fibers were spun from solutions
of 9wt. % polymer in a mixed solvent of 88/12 wt %
tetrahydrofuran/dimethyl acetamide (THF/DMAC) and Kynar.TM. fine
fibers were spun from solutions of 14-15 wt. % polymer in
acetone.
[0087] A Sage.TM. Model 362 syringe pump by Orion was used to pump
solution through a standard syringe with a blunt 27 gauge needle.
High voltage was supplied to the needle by inserting the needle
through an insulated aluminum foil strip connected to a Spellman
SL300 negative high voltage power supply. To assure reliable
syringe pump operation in a high voltage environment it was
necessary to isolate the pump electrically and then to ground the
power supply reference lead, the metal case, and the support jack
of the syringe pump.
[0088] The deposition target was a brass disk 89 mm in diameter by
64 mm thick with a fully radiused edge. The disk was mounted on an
electrically insulating stand, for example, make of Lexan.RTM.,
such that it was suspended about 4 mm in front of the stand and
connected via a screw through the stand to ground. A spunbond
shroud (18 g/m.sup.2 Remay polyester) covered the face of the disk
and stand to keep fibers from accumulating on the back side of the
disk. A 76 mm diameter circle was cut out of the shroud over the
face of the disk to expose the target area. A circular portion of
melt blown substrate was mounted in the target area. For Kraton.TM.
spins, uniform deposition of fibers was aided by insulating the
target area with a polymeric film.
[0089] In one Kynar.TM. case (Example 12), a 15.times.15 cm fabric
was made by depositing fibers directly onto stainless steel
cylinder 48 mm in diameter by 148 mm long. The melt blown layer was
wrapped around the cylinder and the two layers were cut and peeled
away to form the core laminate.
[0090] Fabric properties were measured on 25 mm diameter circular
areas of each fabric.
[0091] Air permeability and Bubble Point were measured on a Porous
Media, Inc. Capillary Flow Porometer, according to the principles
of ASTM F778 and ASTM F316-03, respectively, and are reported as
Frazier Permeability in units of m.sup.3/m.sup.2-min and pore size
in micrometers, respectively.
[0092] Hydrohead measurements were run on an Aspull Mk III
Hydrostatic Head tester per method AATC TM 127, modified by using
aluminum plates and an O-ring seal to hold the small fabric
samples. Hydrohead was recorded at the first water penetration and
is reported in centimeters of water column (cmwc).
[0093] Fine fiber loading was measured gravimetrically by the mass
difference of the sample before and after fine fiber deposition,
and is reported as an average over the surface area of the sample
(total grams fine fibers deposited/sample area).
Control Examples
[0094] Three control examples were made of spunbond/melt
blown/spunbond construction, wherein the spunbond layers were 18
g/m.sup.2 polyester (polyethylene terephthalate) and the melt blown
layers were 18 g/m.sup.2 bicomponent 65 wt. % polyester/35 wt. %
polyethylene fibers made according to the description of WO
01/09425 A1, incorporated herein by reference. The control fabrics
were prepared in the same way as the exemplary fabrics, except that
no fine fiber layer was deposited.
3TABLE 3 Frazier Hydrohead Control Example # (m.sup.3/m.sup.2-min)
(cmwc) 1 31.4 29 2 6.7 57 3 7.7 56
[0095] A number of electrospinning runs were conducted in order to
determine the most effective combination of polymer, solvents, and
concentrations, as well as uniform deposition and handling
techniques, to make the fine fiber barrier layers of the present
invention. Data from the best combination of electrospinning
parameters and collection techniques determined is set forth
below.
EXAMPLES 1-9
[0096] Kraton.TM. D1133.times.copolymer was dissolved in a mixed
solvent of 88 wt. % tetrahydrofuran/12 wt. % dimethyl acetamide at
a polymer concentration of 9 wt. %, and electrospun at -18 KV at a
rate of 0.5 ml/hr. Fine fibers were deposited onto samples of 18
g/m.sup.2 bicomponent melt blown fabric described in the Control
Examples at a collection distance of approximately 22 cm. The fine
fiber layer was then covered with a layer of spunbonded polyester,
removed from the sample target. The layer of melt blown collection
fabric was also covered with a layer of spunbonded polyester and
all four layers were consolidated into a laminate. The barrier
properties of the Examples were measured and are reported below in
Table 4.
[0097] The fine fibers collected were measured by scanning electron
microscopy and found to have diameters in a general range of
between about 0.1 to 1.8 micrometers, with the average fiber
diameters believed to be less than about 1 micrometer.
4TABLE 4 Fiber load Frazier Hydrohead Bubblepoint Example #
(g/m.sup.2) (m.sup.3/m.sup.2-min) (cmwc) (micrometers) 1 5* 0.4 222
-- 2 2.5 1.6 79 -- 3 11 .24 37 -- 4 4.8 0.6 86 -- 5 1.5 3.2 26 -- 6
2.6 1.0 92 -- 7 6.0 0.7 118 -- 8 1.5 3.8 105 -- 9 12.8 0.3 128 21.6
*Estimate based on measured average and visual assessment of mass
distribution.
[0098] In some cases the fine fibers were observed to shrink and
crack upon drying. While the reasons for the data inconsistencies
are not fully understood, it is believed that the relative humidity
of the air during sample formation and collection drastically
affects the spinning process and ultimately the barrier properties
of the styrene-butadiene/THF/DMAC system, which makes it difficult
to obtain uniform fiber deposition across the surface of the
collection webs and barrier properties.
EXAMPLES 10-19
[0099] Kynar.TM. polymer was dissolved in acetone solvent at a
polymer concentration of 15 wt. %, and electrospun at -20 KV at a
rate of 5 ml/hr. Fine fibers were deposited onto samples of 18
g/m.sup.2 bicomponent melt blown fabric described in the Control
Examples at a collection distances of approximately 22-30 cm. The
fine fiber layer was then covered with a layer of spunbonded
polyester, removed from the sample target. The layer of melt blown
collection fabric was also covered with a layer of spunbonded
polyester and all four layers were consolidated into a laminate.
The barrier properties of the Examples were measured and are
reported below in Table 5.
[0100] The fine fibers collected were measured by scanning electron
microscopy and found to have diameters in a general range of
between about 0.14 to 2.8 micrometers, with the average fiber
diameters believed to be less than about 1 micrometer.
5TABLE 5 Fiber load Frazier Hydrohead Bubblepoint Example #
(g/m.sup.2) (m.sup.3/m.sup.2-min) (cmwc) (micrometers) 10 13.6 11.2
131 11.3 11 22 3.1 115 10.9 12 16.5 1.0 278 10.7 13 " 2.1 347 -- 14
51.1 0.7 399 9.4 15 22.7 1.6 345 11.8 16 26 1.0 368 8.0 17 15.4 1.5
322 8.7 18 23.9 0.7 332 4.3 19 20.0 0.8 279 5.8
[0101] Example 13 was a portion of the fabric sample of Example 12
which was calendered using a metal roll on a metal plate with a
linear pressure estimated to be about 2-4 kg/cm.
[0102] Overall, the barrier fabrics containing the Kynar.TM. fine
fibers exhibited much greater hydrohead values than those of either
the Control Samples or Examples 1-9. It is believed that the more
hydrophobic nature of the polyvinylidene fluoride polymer in
Examples 10-19, relative to the styrene-butadiene polymer of
Examples 1-9, is a major reason for the improved hydrohead values.
However, those of skill in the art will recognize that the
hydrohead of the styrene-butadiene polymer fabrics of Examples 1-9
could be enhanced by treatment with a water-repellant chemical
finish, such as a fluorochemical finish, without appreciable
detriment to the Frazier permeability of the fabrics.
[0103] Further, it is important to note that in almost all cases,
the hydrohead measurements of the fine fiber-containing exemplary
fabrics of the present invention exceed those of the Control
Examples, which are essentially spunbond/melt blown/spunbond fabric
construction. This demonstrates that the presence of a fine fiber
layer, especially wherein the fine fibers comprise fibers of less
than about 2 micrometers in diameter, or even less than about 1
micron in diameter, can greatly enhance liquid barrier properties
of a fabric.
[0104] The laminate fabric configurations, Fine Fiber barrier
layer/spunbond support layer (FF/SB) and spunbond support
layer/Fine Fiber barrier layer/spunbond support layer (SB/FF/SB)
are viable configurations for achieving higher barrier with air
permeability below about Frazier=m.sup.3/m.sup.2-min. Typical
spunbond fiber diameter size is 10 micrometers and above.
[0105] Suitable support layers must have pore sizes scaled to the
mechanical strength of the barrier layer. The weaker the barrier
layer, the smaller the support layer pore size must be for adequate
support. Smaller pores sizes, in turn, require smaller fiber
diameter sizes. Hence, as barrier layer basis weight is reduced to
facilitate high air permeability, suitable support layers must have
fiber diameter sizes smaller than typical spunbond fiber sizes.
[0106] Such smaller fibers could be micro-denier spunbond (mSB), as
discussed in U.S. Pat. No. 5,885,909, e.g., 6<D.sub.f<10
micrometers which are strong enough to meet the mechanical strength
requirements for the fabric as a whole. Micro-denier spunbond
support would give rise to two fabric configurations: FF/mSB and
mSB/FF/mSB.
[0107] Non-self-supporting support layers with fiber diameters in
the range of 1<D.sub.f<10 micrometers can be made by the melt
blowing process. Typically these fibers are not strong
(0.3<GPD<0.6). They are used to provide barrier properties
with a support layer of spunbond fibers to provide strength. If
melt blown fibers are used to support the Fine Fiber barrier layer,
the melt blown fiber layer still requires a support layer to
maintain over all fabric mechanical strength. A spunbond fiber
layer is well suited to be the support layer.
[0108] This gives rise to the laminate fabric configurations:
FF/MB/SB, SB/MB/FF/MB/SB, FF/MB/mSB and mSB/MB/FF/MB/mSB.
[0109] There can be asymmetrical combinations of these layer types,
e.g., SB/FF/MB/SB, which could have asymmetrical barrier
performance, which might provide unusual but useful function to
fabrics of such constructions. For example, if the liquid challenge
is from the SB/FF side, the barrier will be high and equal to the
maximum barrier capability of the FF layer. If the liquid challenge
is from the SB/MB side, the spunbond layer will not provide
adequate support for the FF layer which will break at some
hydrohead lower than FF layer capability.
* * * * *