Note: Descriptions are shown in the official language in which they were submitted.
~90$~7
NONWOVEN FABRIC WITH IMPROVED ABRASION RESISTANCE
Field of the Invention
The present invention relates to improved nonwoven fabrics
made of microfiber web~, characterized by high ~urfa~e
abrasion resi6tance, and especially suitable for u6e as
medical fabric6.
Backaround of the Invention
The pregent invention i~ directed to nonwoven fabrics and
particularly to medical fabrics. The term l'medical
fabric~l, as u6ed herein, refers to a fabric which may be
used in 6urgical drapes, surgical gowns, instrument wraps,
or the like. Such medical fabrics have certain required
properties to insure that they will perform properly for
the intended u6e. These properties include strength, the
capability of resisting water or other liquid penetration,
often referred to a~ ~trike-through re6istance,
breathability, 60ftnes6, drape, 6terilizability, and
bacterial barrier properties.
The use of microfiber webs in application6 where barrier
propertie~ are desired is known in the prior art.
Microfiber~ are f~bers having a diameter of from le6s than
1 micron to about 10 microns. Microfiber webs are often
referred to as melt-blown webs as they are u~ually made by
a melt blowing proce6s. It is generally recognized that
the use of relatively ~all diameter fiber~ in a fabric
Btructure hould allow the achievement of high repellency
or filtration properties without undue compromise of
breathability. Microflber web fabric~ made heretofore,
and intended for u6e as medical fabrics, have been
composites of microfiber webs laminated or otherwise
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bonded to spunbonded thermoplastic fiber webfi, or films,
or other reinforcing webs which provide the requisite
stren~th.
Another important property for both nonwoven fabrics and
~edical ~abrics i~ abra~ion resistance. Resistance to
surface abra6ion affects not only the performance of a
fabric but may also affect the aesthetics of a fabric.
For example, linting of broken ~urface fibers is
particularly undesirable in medical fabrics. In addition,
surface abrasion can affect the strike-through resistance
and bacterial barrier properties of a medical fabric.
Linting, a~ well as pilling or clumping of surface fibers
i8 also unacceptable for many wipe applications. An outer
lS layer of a spunbonded fiber web, film or other reinforcing
web has been used to develop surface abrasion resistance
in melt-blown fiber products.
U.S. Patent 4,041,203 discloses a nonwoven fabric made by
combining microfiber webs and spunbonded webs to produce a
medical fabric having good drape, breathability, water
repellency, and surface abrasion resis~ance.
U.S. Patent 4,196,245 discloses combinations o~ melt-blown
~- 25 or microfine fibers with apectured films or with apertured
films and spunbonded fabrics. Again, the apertured film
and the spunbonded fabric are the components in the
finished, nonwoven fabric which provide the strength and
surface stability to the fabric.
U.K. Patent Application 2,132,939 discloses a melt-blown
fabric laminate suitable as a medical fab~ic, comprising a
melt-blown microfiber web welded at localized points ts a
nonwoven reinforcing web of dificontinuous fibers, such as
an air laid or wet laid web of staple fibers.
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While ~he above-mentioned fabrics have the potential to
achieve a better balance of repellency and breathability
compared to other prior art technologies not using
microfiber~, the addition of surface reinforcing layer6 of
S relatively large diameter fibers limit6 their advantages.
U.S. Patent No. 4,436,780 to Hotchki~s et al. describe6 a
melt-blown wipe with low linting, reduced ~reaking and
improved absorbensy, comprising a middle layer of
melt-blown fiber~ and on either ~ide thereof, a ~punbonded
layer.
In order to improve surface abrasion re~istance and reduce
lint of melt-blown web~ generally, it i6 al~o known to
compact the web to a high degree, or add or increase the
`_- 15 level of binder. Copending Canadian Patent, se~. No. 515,440-5 filedon August 6, 1986 provides a medical fabric f~om an unreinforced web or webs
of microfine f~s. The fabric i6 unreinforced in that it
need not be laminated or bonded to another type of web or
film to provide adequate strength to be used in medical
applications. The fabric also achieves a balance of
repellency, strength, breathability and other aesthetics
superior to prior art fabrics. However, as described in
the application, in order to render the fabric especially
effective for use in applications requiring high abra6ion
~, resistance, a ~mall amount of chemical binder may be
~~ applied to the surface of the fabric.
U.K. Patent 2,104,562 discloses surface
heating of a melt-blown fabric to give it an anti-linting
fini~h. It is also generally known to u6e a level of heat
and compaction, e.g., embos6ing, of a microfiber web to
improve abrasion re6i6tance.
The above fabric~ which have reinforcing web~ have to be
a66embled using two or more web forming technologie6.
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re~ulting in increased proces6 complexity and c06t.
Furthermore, the bonding of relatively conventional
fibrous webs to the microfiber~, the compaction or the
addition of binder to a microfiber web can result in stiff
fabrics, e6pecially where high strength is desired.
Brief SummarY of the Invention
The pre6ent invention provid~s a melt-blown microfiber
e~bos6ed web with improved wet and dry ~urface abra6ion
resiRtance of greater than 15 cycle6 to pill. The
abrasion resistance i~ achieved without the u~e of
additional binder and doe6 not sacrifice the drape or hand
of the material.
According to the pre6ent invention, surface abrasion
reaistance is achieved with the addition of a surface
veneer of melt-blown fibers having an average fiber
diameter of greater than 8 microns, and in which 75% of
the fibers have a fiber diameter of at least 7 microns.
The surface veneer may be bonded to a melt-blown core web,
~uch as that described i:n th~ aforelTentioned copending ~pplication,
by heat embos6ing or other methods. The bondinq of the
veneer to the core web and heat embossing of the core web
may be achieved in one processing step. In addition, when
the core web and veneer web are produced in one fabric
making step using multiple die6, the veneer may be
produced atop the core web, with high initial autogenous
bonding, eliminating the need to bond the ~eneer to the
core web.
By eliminating the need for additional binder, the present
invention provides a method for making melt-blown
microfiber web without the additional proce66ing steps of
adding binder and dEying and/or curing the bindec. Also,
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potential heat damage during binder curing or drying which
may adversely affect the drape and hand of a fabric i8
eliminated. Stiffening of the fabric through the use of
binder solution i~ also eliminated, thereby permitting
adjustment of proces6ing condition6 of the core web to
maximize otheL propertie6.
In addition, the u~e of a surface veneer of melt-blown
fibers pcovides a ~abric with a combination of drape and
surface abrasion re~istance which cannot be achieved with
the addition of binder materials. ~he u~e of melt-blown
fibers tO form the surface veneer alco provide6 economic
advantage6 and minimizes the technoloqies necessary to
produce the fabric.
Thu6, the present inven~ion provides an improved
melt-blown or microfiber fabric with improved surface
abrasion resistance but without binder, which may be used
as a medical fabric or wipe or in other applications where
high surface abrasion resistance is required. In a
preferred embodiment, the fabric of the present invention
compri6es an unreinforced, melt-blown, microfiber fabcic
with improved surface abrasion resi6tance, e.g., greater
than 15 cycles to pill, suitable for use as a medical
fabric, ~aid fabric having a minimum gtab tensile strength
to weight ratio greater than 0.8 N per gram per square
meter, and a minimum Elmendorf tear strength to weight
ratio greater than 0.04 N per gram per ~quare meter. In a
most preferred embodiment of the present invention, the
embossed unreinforced fabrics described above have a wet
abrasion resistance of at least 30 cycles to pill, and a
dry abra~ion resistance of at least 40 cycle~ to pill.
The6e properties are achieved while also obtaining the
properties of repellency, air permeability and e~pecially
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drapability -that are desired for -the use of the fabric in
medical applications.
~ccording to a still further broad aspect of the present
invention there is provided an improved unreinforced melt-
blown microfiber fabric having improved surface abrasion
resistance, the fabric comprises at least one unreinforced
thermoplastic melt-blown microfiber core web having an
average of length of greater than 10 cm, and an average
diameter not exceeding 7 microns. The core web also has a
minimum grab tensile s-trength to weight ratio greater than
0.8N per gram per square meter and a minimum Elmendorf tear
strength to weight ratio greater than 0.04N per gram per
square meter. The core web has a basis weight in the range
of 14 grams per square meter to 85 grams per square meter,
and at least one unreinforced surface veneer web on the core
web. The veneer web is formed of melt-blown thermoplastic
fibers having an average fiber diameter of greater than 8
microns in which 75% of the fibers have a diameter of at
least 7 microns, a wet and dry surface abrasion resistance
of greater than 15 cycles to pill and a basis weight in the
range of 3 grams per square meter to 10 grams per square
meter. At least one veneer web is directly contiguous to
the at least one core web.
According to a still further broad aspect of the present
invention there is provided a method of producing a melt-
blown microfiber fabric having improved abrasion resistance.
The method comprises forming at least one core web of thermo-
plastic melt-blown microfibers having a minimum grab tensile
strength to weight ratio greater than 0.8N per gram per
square meter, a minimum Elmendorf tear strength to weight
ratio greater than 0.04N per gram per square meter, and a
basis weight in the range of 14 grams per square meter to 85
grams per square meter. The method also comprises forming
at least one unreinforced surface veneer web of melt-blown
thermoplastic fibers on the core web. The veneer web has a
high initial autoyenous bonding and an average fiber
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diame-~er greater than 8 microns, in which 75~ of the
fibers have a fiber diameter of at least 7 microns
The veneer web has a basis weight in the range of 3
grams per square meter to 10 grams per square meter
and a wet and dry surface abrasion resistance greater
than 15cycles to pill. At least one veneer web is
directly contiguous to the at least one core web.
Brief Description of the Drawings
Figure 1 is an isometric view of the melt~blowing
process.
Figure 2 is a cross-sectional view of the placement of
the die and the placement of the secondaxy air source.
Figure 3 is a detailed fragmentary view of the extrusion
die illustrating negative set back.
Figure 4 is a detailed fragmentary view of the extrusion
die illustrating positive set back.
Detailed Description of the Invention
In its broadest aspect, the present invention comprises
providing a surface veneer of melt-blown fibers to a
melt-blown microfiber web, said surface veneer having an
average fiber diameter greater than 8 mlcrons in which
at least 75% of the fibers have a diameter of at least 7
microns. For most fabric applications the surface veneer
will be laminated to the remainder of web, e.g., by
emboss bonding, or combined by other known methods. Thus,
the surface veener may be formed separately from the
remainder of the web and thermally bonded thereto,
preferably at discrete intermittent bond regions.
Alternatively, the veneer may be formed with high initial
autogenous bonding atop the remainder of the web
eliminating the need to bond the veneer to the remainder
of the web, though thermal embossing the fabric may be
preferred. The fabrics of the present invention exhibit
improved wet and dry surface abrasion resistance and are
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especially applicable for use as wipes or medical
fabrics.
In its broadest aspects, the process of the present
invention may be carried out on conventional melt-
blowing equipment which has been modified to providehigh velocity secondary air, such as that shown in
the aforementioned co-pending application and shown
in Figure 1. In the apparatus shown, a thermoplastic
resin in the form of pellets or granules, is fed into
a hopper 10. The pellets are then introduced into the
extruder 11 in which the temperature is controlled
through multiple heating zones to raise the temperature
of the resin above its melting point. The extruder
is driven by a motor 12 which moves the resin through
the heating zones of the extrudex and into the die 13.
The die 13 may also have multiple heating zones.
- As shown in Figure 2, the resin passes from the extruder
into a heater chamber 29 which is between the upper and
lower die plates 30 and 31. The upper and lower die
plates are heated by heaters 20 to raise the temperature
of die and the resin in the chamber 29 to the desired
level. The resin is then forced through a plurality
of minute orifices 17 in the face of the die.
Conventionally, there are about 12 orifices per centimeter
of width o the die.
An inert hot gas, usually air, is forced into the
die through lines 14 into gas chamber 19. The heated
gas, known as primary air, then flows to gas slots
32 and 33 which are located in either side of the
resin orifices 17. The hot gas attenuates the resin
into fibers as the resin passes out of the orifices 17.
The width of the slot 32 or 33 is referred to as
the air gap. The fibers
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are directed by the hot gas onto a web forming foraminous
conveyor or receiver 22 to form a mat or web 26. It is
usual to employ a vacuum box 23 attached to a suitable
vacuum line 24 to assi6t in the collection of the fibers.
The conveyor 22 i6 driven around rollers 25 so as to form
a web continuously.
The outlets of the orifices 17 and ~he gas $10ts 32 and 33
may be in the same plane or may be offset. Fig. 3 shows
the orifice 17 terminating inward of the face of the die
and the slots 32 and 33. Thi~ arrangement iB ceferred to
as negative ~etback. The setback dimension is shown by
the space between the arrows in Fig. 3. Positive ~etback
is illustrated in Fig. 4. The outlet of the orifice 17
-` 15 terminates outward of the face of the die and the slots 32
and 33. The setback dimen6ion is shown by the space
between t~e arrows in Fig. 4. A negative setback is
preferred in the present process as it allows greater
flexibility in setting the air gap without adversely
affecting the quality of the web produced.
The fabrics of the present invention comprise at least one
surface veneer and a core web. Preferably, the fabric
comprises a core web and surface veneers on both surfaces
of the core web. As used herein, veneer means a web of
-- fibers having a basis weight no greater than 50% of the
total weight of the fabric. Preferably, the basis weight
of the veneer web is about 25% of the weight of the total
fabric, and most preferably, between about 15% to 25% of
the total weight of the fabric. The veneer web(s) may be
formed separately from the core web and then combined
therewith in a face-to-face relationship. ~hen using this
method, each veneer web must have a basis weight of about
6g~m2 to facilitate handling of the web to combine it
with the core web. ~lternatively, the core and veneer
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webs may be formed atop one another, e.g., by depositing
the core web fibers atop the veneer web disposed on the
conveyor 22 and acting as the receiver for the fibers of
the core web. In this preferred method of the present
invention, a veneer web of about 3g/m2 may be deposited
on the conveyor and focm the receiver for the core web
and/or a veneer web of about 3g/m may be deposited on
the core web acting as a receiver. Alternatively, the
fiber of the veneer webs may be deposited on both surfaces
lo of t~e core web in ~eparate web forming step~. Thereafter
the core web and veneer web(s) may be laminated, e.g., by
heat embossing. When depo~iting the veneer web(s) on the
core web, if the veneer web(s) is formed undec conditions
which provide high initial interfiber oc autogenous
bonding, including high die temperature, no secondary air
and a short forming distance, (as described more fully
below) it may not be necessary to laminate the veneer
web(s) to the core as, e.g., by heat embossing, nor to
emboss the veneer. The core web may be embossed or
unembossed prior to the deposition of the fibers of the
veneer web thereon. The embossed fabric laminates of the
present invention exhibit a wet surface abrasion
resistance of at least 30 cycles to pill and a dry surface
abrasion resistance of at least 40 cycles to pill.
2S
As stated hereinbelow, it is possible to form the
fabric of the present invention according to the above
m~thod6 with only one melt-blown die by increasing the
polymer throughput and reducing the primary air to form
the veneer web(s). In a most preferred method of making
the fabrics of the present invention. multiple dies are
used.
In its mo6t preferred aspect the present invention
comprise6 an improved unreinforced melt-blown microfiber
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fabric for use as a medical fabric. said fabric having a
minimum grab ten6ile 6trength to weight ratio of at least
0.~ N per gram per 6quare meter and a minimum Elmendorf
tear ~trength to weight ratio of at lea6t 0.04 N per gra~
per 6quare ~eter. The invention will now be further
de~cribed in relation to thi6 preferred embodiment.
The cequirement6 for medical grade fabric~ are quite
demanding. The fabric must have sufficient strength to
lo resist tearing or ~ulling apart during normal u6e, for
instance, in an operating room environment. Thi6 is
especially true for fabric6 that are to be used for
operating room apparel, such as surgical gowng, or scrub
suits, or for surgical drapes. One measure of the
~trength of a nonwoven fabric i8 the grab tensile
strength. The grab tensile strength is generally ~he load
necessary to pull apart or break a 10 cm wide sample of
the fabric.
The te6t for grab ten6ile strength of nonwoven ~abrics is
described in ASTM D1117. Nonwoven medical fabrics must
also be resistant to tearing. The tearing strength or
re~istance is generally ~easured by the Elmendorf Tear
Test a~ de~cribed in ASTM D1117. While the grab tensile
strengths, measured in the weakest, normally cross machine
direction, of the least strong commercially used medical
fabric6 are in the Lange of 45 newton~ (N) with tear
strengths in the weakest direction of approximately 2N, at
the6e strength level6, fabric failure can occur and it is
generally desired to achieve higher strength level~. Grab
tensile ~trength levels of approximately 65 N and above
and tear re6istance levels of approximately 6N and above
would allow a particular medical fabric to be u ed in a
wider range of application~. The preferred fabrics of the
pre6ent invention have a high strength to weight ratio,
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such that at desirable weights, both geab tensile and tear
strengths higher than the above values can be achieved,
and generally have basis weights in the range of 14 to 85
g/m2 ~
Medical fabrics must also be repellent to fluids including
blood, that are commonly encountered in hospital operating
rooms. Since these fluids offer a convenient vehicle for
microorganisms to be transported from one location to
another, repellency is a critical functional attribute of
~edical fabrics. A measure of repellency that i
influenced primarily by the pore structure of a fabric is
the "hydrostatic head" test, AATCC 127-1977. The
hydrostatic head test measures the pressure, in units of
height of a column of water, necessary to penetrate a
given sample of fabric. Since the ultimate resistance of
a given fabric to liquid penetration is governed by the
pore ~tructure of the fabric, the hydrostatic head test is
an effective method to assess the inherent repellent
attribute~ of a medical fabric. Nonwoven medical fabrics
which do not include impermeable film6 or microfiber webs
genecally possess hydrostatic head values between 20 to
30 cm of water. It is generally recognized that these
values are not optimu~ for gowns and drapes, especially
for those situations in which the risk of infection is
high. Values of 40 cm or greater are desirable.
Unfortunately, prior art disposable fabrics which possess
high hydrostatic head values are associated with low
breathability or relatively low strength. The fabrics of
the present invention can attain a high level of fluid
repellency.
The breathability of medical fabrics is also a desirable
property. This is especially true if the fabrics are to
be u~ed for wearing apparel. The breathability of fabrics
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is related to both the rate of moicture vapor transmission
(MVTR) and air permeability. Since most fibrous webs used
for medical fabrics possess reasonably high levels of MVTR,
the measurement of air permeability i6 an aperopriate
discriminating quantitative te~t of breathability.
Generally the more open the structure of a fabric, the
higher its air permeability. Thus, highly compacted,
dense webs with very zmall pore structures result in
lo fabrics with poor air permeability and are consequently
perceived to have poor breathability. An increase in the
weight of a given fabric would also decrea6e ies air
permeability. A measure of air permeability i5 the
Prazier air porosity test, ASTM D737. Medical garments
made of fabrics with Frazier air porosity below 8 cubic
meters per minute per square meter of fabric would tend to
be uncomfortable when worn for any length of time. The
fabric of the present invention achieve good breathability
without sacrifice of repellency or strength.
Medical fabrics must also have good drapability, which may
be measured by various methods including the Cusick drape
test. In the Cusick drape test, a circular fabric sample
is held concentrically between horizontal discs which are
smaller than the fabric sample. The fabric i~ allowed to
drape into folds around the lower of the discs. The
shadow of the drap~d sample is projected onto an annular
ring of paper of the same size as the unsupported portion
of the fabric sample. The outline of the shadow i6 traced
onto the annular ring of paper. The mass of the annular
ring of paper is determined. The paper is then cut along
the trace of the shadow, and the mass of the inner portion
of the ring which represents the shadow is determined.
The drape coefficient is the mass of the inner ring
divided by the ma~s of the annular ring times 100. The
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lower the drape coefficient, the more drapable the
fabric. The fabric6 of the present invention demonstrate
high drapability when measured by this method.
~rapability correlates well with softness and flexibility.
s
In addition to the above characteristics, medical grade
fabrics must have anti-6tatic properties and fire
retardancy. The fabrics should also po~ses~ good
resistance to abrasion, and not shed small fibrous
particles, generally referred to as lint.
In addition to the characteristics mentioned above, the
preferred fabric of the present invention differs from
peior art melt-blown webs in that the average leng~h of
the individual fibers in the web is greater than the
average length of the fibers in prior art webs. The
average fiber length in the core webs is greater than
10 cm, preferably greater than 20 cm and most preferably
in the range of 25 to 50 cm. Also, the average diameter
of the fibers in the core web should be no greater than
7 microns. The dictribution of the fiber diameters is
such that at least 80t of the fibers have a diameter no
greater ~han 7 micron6 and preferably at least 90% of the
fibers have a diameter no greater than 7 microns.
In the description of the present invention the term "web"
refer6 to the unbonded web formed by the melt blowing
process. The term "fabcic" refers to the web after it i6
bonded by heat embossing or other means.
The preferred fabric of the present invention comprises an
unreinforced melt-blown embo6sed fabric having a core web
of average fiber length greater than 10 centimeters and in
which at least 80% of the fibers have a diame~er of 7
micron~ or les6, and a surface veneer provided on one or
both 6urface6 of the core web, said surface veneers having
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an average fiber diameter of greater than 8 micron~, and
in which 75% of the fiber6 have a fiber diameter of at
lea6t 7 micron6.
In the process of ~aking ~hi6 preferred fabric of the
pre6ent invention, the fibers of the core web are
contacted by high velocity 6econdary air immediately after
the fiber6 exit the die. The fibers of the surface veneer
may or may not be contacted by high velocity secondary
air. The secondary air i6 a~bient air at room temperature
or at outside air temperature. If desired, the secondary
air can be chilled. The 6econdary air i~ forced under
pre~sure from an appropriate 60urce through feed line6 15
and into di6tributor 16 located on each 6ide of the die.
The di~tributor6 are generally as long as the face of the
die. The distributors have an angled face 35 with an
opening Z7 adjacent the die face. The velocity of the
secondary air can be controlled by increasing the pressure
in feed line 15 or by the use of a baffle 28. The baffla
would re6trict the size of the opening 27, theceby
increasing the velocity of air exiting the di6tribution
box, at constant volume.
The pre6ent nonwoven fabric differs from prior art
microfiber-containing fabric6 in the utilization of the
melt-blowing proce66 to produce a 6urface veneer of fibers
with characteri6tics which differ from the chacacteristics
of the microfibers of the core web and which re6ult in a
fabric with high strength to weight ratios, good surface
abra6ion resi6tance and drape if the fiber6 are formed
into a core web and surface veneer and thermally bonded as
de6cribed herein.
In the practice of prior art melt-blown technology for
fabric related application6, it i6 typical to produce
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microfibers which cange in average diame~er from about 1
to 10 microns. While in a given web, there may be a range
of fiber diameter~, it is often necessary to keep the
diameters of these fibers low in order to fully exploit
the advan~ages of microfiber structures as good filtration
media. Thus, it is usual to produce webs or batts with
average fiber diameters of le6s than 5 microns or at times
even less than 2 microns. In such prior art proces~es, it
is also typical for such fibers to be of average length6
between 5 tO 10 centimeters (cm). As discu~sed in the
review of the prior art fabrics, the webs formed from such
fibers have very low strength and abrasion resistance.
The tensile strength and abrasion resistance of such a web
is primarily due to the bonding that occurs between fibers
as they are deposited on the forming conveyor. Some
degree of interfiber surface bonding can occur because in
the conventional practice of melt-blown technology, the
fibers may be deposited on the forming conveyor in a state
in which the fibers are not completely solid. Their
semi-molten surfaces can then fuse together at crossover
points. This bond formation is sometimes referred to a~
autogenous bonding. The higher the level of autogenous
bonding, the hiqher the integrity of the web. However, if
autogenous bonding of the thermoplastic fibers is
exces6ively high, the webs become stiff, harsh and quite
brittle. The strength of such unembossed webs is
furthermore not adequate for practical applications such
a~ medical fabrics. Thermal bonding of these webs can
generally improve strength and abrasion resi6tance.
However, as di~cu6sed in previous section6, without
introduction of surface reinforcing elements or binder, it
has heretofore not been possible to produce melt-blown
microdenier fabric~ with high surface abrasion resistance,
particularly for use as surgical gowns, scrub apparel and
drapes.
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In forming the core webs of thi6 p~eferred fabric of the
present invention, fibers are produced which are longer
than fiber6 of the prior art. Fiber lengths were
determined u~ing rectangular-shaped wire form6. The6e
forms had 6pan length6 ranging from 5 to 50 cm in 5 cm
increment6. Strip6 of double-faced adhe~ive tape were
applied to the wire to provide adhe6ive sites to collect
fibers from the fiber stream. Fiber lengths were
determined by firs~ pas~ing each wire form quickly through
the fiber stream, perpendicular to the direction of flow,
and at a di6tance closer to the location of the forming
conveyo~ than to the melt blowing die. Average fiber
lengths were then approximated on the basis of the number
of individual fibers spanning the wire forms at ~uccessive
span lengths. If a 6ub6tantial portion of the fibers are
longer than 10 cm, such that the average fiber length is
at least greater than 10 cm and preferably greater than
20 cm, the webs, thus formed, can result in embossed
fabrics with good strength, while maintaining other
de6ired features of a medical fabric. Fabrics with highly
desirable properties are produced when average fiber
length~ are in the range of 25 to 50 cm. In order to
maintain the potential of microdenier fibers to resist
liquid penetration, it is necessary to keep the diameters
of the fibers low. In order to develop high repellency,
it i6 necessary for the average diameter of the fiber6 of
the pre~ene core web to be no greater than 7 mic~ons. At
least 80% of the fibers 6hould have diameter~ no greater
than 7 micron6. Preferably, at least 90% of the fiber~
should have diameters no greater than 7 micron6. A narrow
distribution of fiber diameters enhance6 the potential for
achieving the unique balance of propertie6 of thi~
invention. While it is possible to produce fdbrics with
average fiber diameters greater than 7 micron6 and obtain
high 6~rength, the ultimate repellency of such a fabric
JSU-61
~05~ 7
would be compromi6ed, and it would then not be fea6ible to
produce low weight fabrics with high repellency.
When the melt-blown fibrou~ core web is foImed in such a
manner that autogenous bonding is very low and the webs
have little or no integrity, the fabrics that re~ult upon
thermal embossing the~e webs are much stronger and possegs
better aesthetics than fabric~ made of web~ with high
initial strength. That i8, the weakest unembo~6ed webs,
with fiber dimension6 a~ described above, form the
~trongest embossed fabrics. The higher the level of
initial interîiber bonding, the stiffe~ and more brittle
the re~ulting fabric, leading to poor grab and tear
strengths. As autogenous bonding is reduced, the
resulting fabric develops not only good strength but
becomes softer and more drapable after thermal embossing.
Becau6e of the relatively low levels of web integrity, it
is useful to determine the strength of the unembossed web
by the strip tensile strength method, which uses a
2.54 cm-wide 6ample and grip facings which are also a
minimum 2.54 cm wide (ASTM D1117). In prior art
melt-blown fabrics the machine direction (MD) strip
tensile strength of the autogenously bonded web is
generally greater than 30~ and frequently up to 70% or
more of the strip tensile 6trength of the bonded fabric.
That is, the potential contribution of autogenous bonding
to the 6trength of the embossed fabric is quite high. In
the fabric of the pre6ent invention the autogenous bonding
of the core web contributes le6s than 30%, and preferably
less than 10%, of the strip tensile strength of the bonded
fabric.
For example, a Nylon 6 melt-blown web with a weight of
approximately 50 g/m2 made under prior art conditions
may pos6e~s a ~trip tensile strength in the machine
JSU-61
~L',''P9~)51~7
-18-
direction of between 10 to 20 N. In this prefeered
fabric of the invention, it i~ necessary to keep the strip
tensile strength of the unembossed core web below 10 N and
preferably below 5 N to achieve the full benefit6 of the
invention. In other word6, when long fibers are produced
and collected, in such a way that initial interfiber
bonding i8 low, the individual fibers are stronger, and
there i8 greater exploitation of the inherent strength of
the fiber~ themselve~.
While it iz necessary to produce the fibers of the core
web in such a way that initial interfiber bonding i6 low
and 80~ of the fibers have a fiber diameter of no more
that 7 microns, such webs when embossed do not exhibit
high surface abra6ion resi~tance, and a chemical binder i8
often added to the surface of such fabrics to increa6e
surface abrasion resistance. The addition of binder
negatively impacts the drape of the fabric, therefore the
amount of binder added mu6t be kept to a minimum, and, in
Z0 practice, the amount of binder which can be added while
maintaining adequate drape gives only satisfactory, but
not high, abrasion resistance.
In the fabric of the present invention, the use of binder
2S and its negative impact on drape is avoided by providing
the core web with a surface veneer of microfibers
on one or both 6urfaces of the core web. The fibers of
the 6urface veneer have an average fiber diameter of
greater than 8 microns and 75% of the fibers have a fiber
diameter of at least 7 microns. In addition, in a
preferred embodiment, the surface veneer is formed with
high initial interfiber bonding.
In summary, this preferred fabric of the present
invention, in contra6t to conventional melt-blown webs of
JSU-61
9(~7
the prior ar~, is characterized by a core web of high
average fiber length, low interfiber bonding, 6tronger
individual fibers and low fiber diameters in a rslatively
narrow distribution range to provide high resistance to
fluid penetration, and at least one surface veneer of
higher fiber diameters and, preferably, high interfiber
bonding.
The method of producing the desired core web and ~urface
13 veneer characteristi~ of this ~referred fabric o~ the
invention i8 ba~ed on the control of the key process
variables and their interactions to achieve the desired
fiber, web, and fabric properties. The6e process
variables include extru6ion temperature6, primary air flow
and temperature, 6econdary air flow, and forming length
(distance from die to receiver). The influence of these
variables on the key desired web and veneer properties i6
de~cribed below.
For both the core web and 6urface veneer, individual fiber
6trength can be enhanced 6ignificantly if the die melt
temperature, for instance, can be maintained at levels
generally 10 to 35C below temperatures recommended for
prior art processe6. Generally, in the present proces6
Z5 the die melt temperature i6 no qreater than about 75C
above the melting point of the polymer.
In forming the core web, the velocity and temperature of
the primary air, and the velocity and temperature of the
secondaey air must be adjusted to achieve optimum fiber
strength at zero span length for a given polymer. The
high velocity secondary air employed in the p~esent
process i6 in6trumental in increa~ing the time and ehe
distance over which the fibers of the core web are
attenuated adding ~o fiber strength. The u6e of 6econdary
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~90~7
-20-
air in the process of producing the surface veneer fibers
is not esRential, and secondary air is preferably omitted
in forming the preferred surface veneer with high initial
interfiber bonding.
The fiber length achievable in the core web and surface
veneer is influenced by the primary and secondary air
velocitie~, the level of degradation of the polymer and,
of critical importance, air flow uniformity. It is
important to maintain a high degree of air and fiber flow
uniformity, avoiding large amplitude turbulence, vortices,
streaks, and other flow irregularities. Introduction of
high velocity secondary air may serve to control ehe
air/fiber ~tream, by cooling and maintaining molecular
orientation of the fibers 80 that stronger fibers are
produced that are more resistant to possible breakage
caused by non-uniform air flow.
In order to deposit the fibers of the core web on the
forming conveyor as a web with low strip tensile strength,
the forming air and forming distance are clearly
important. In the present proces6, the forming distance
is generally between 20 and 50 centimeters. First, in
order for the core web to have minimal interfiber bonding,
the fiber6 must arrive at the forming conveyor in a
relatively solid state, free of 6urface tackines6. To
allow the fibers time to solidify, it is possible to set
the forming conveyor or receiver farther away from the
die. However, at excessively long distances, i.e.,
greater than 50 cm., it is difficult to maintain good
uniformity of the air/fiber stream and "roping" may
occur. Roping is a phenomenon by which individual fibers
get entangled with one another in the air stream to form
coarse fiber bundles. Excessive roping diminishes the
capacity of the resultant fabric to resi~t fluid
JSU-61
~J~0517
penetration, and al80 lead6 to poor ae6thetic attributes.
A primary air flow of high uniformity enhances the
oppor~unity to achieve good fiber attenuation and
relatively long di~tance forming without roping.
The primary air volume iB al80 an important factor.
Sufficient air volume must be used, at a given polymer
flow rate and forming length, to maintain good fiber
6eparation in the air/fiber stream, in order to ~inimize
the extent of roping.
The u6e of the secondary air 6ystem also i6 important in
achieving low interfiber bonding in the core web without
roping. A6 noted previously, the high velocity 6econdary
air is effective in improving the uniformity of the
air/fiber 6tream. Thu6, it enhances the potential to
increase the forming length without cau6ing undesirable
roping. Furthermore, since the 6econdary air is
maintained at ambient temperature, or lower if desired, it
can serve also to cool and solidify the fibers in a
6horter time, thu6 obviating the need for det~imentally
large forming length6. For the 6econdary air system to
have an influence on flow uniformity and cooling, and the
rate of deceleration of the fibers, it~ velocity should be
high enough that its flow i6 not completely overwhelmed by
the prima~y air flow. In the pre6ent proce66, a 6econdary
air velocity of 30 m/sec to 200 m/6ec or higher i6
e~fective in providing the desired air flow
characteristics. Obviously, there are variou approaches
and co~binations of primary and 6econdary air flows,
tempera~ure6, and forming lengths that can be used to
achieve low interiber bonding in the unembossed core
web. The ~pecific proce~s
parameters depend on the polymer being used, the design of
JSU-61
17
the die and its air systems, the production rate, and the
desired product properties.
The unembossed core web or layer6 of unembo6sed core webs
mu~t be bonded to form this preferred fabric of the
pre~ent invention. It has been determined to be
advantageous to use thermal bonding techniques. In a most
preferred method of the present invention, the core web or
webs are ~hermally bonded and the veneer thermally bonded
and laminated to the core web in one thermal embossing
step. ~ither ultrasonic or mechanical embossing roll
6yfitems using heat and pressure may be used. For the
present invention, it is preferred to use a mechanical
embossing system for point bonding using an engraved roll
on one side and a solid smooth roll on the other side of
the fabric. In order to avoid ~'pinholes~ in the fabric,
it has also been found desirable to set a small gap, of
the order of 0.01 to 0.02 mm, between the top and bottom
rolls. For the intended use of the fabrics which can be
produced by this invention, the total embossed area must
be in the range of 5 to 30% of the total fabric surface,
and preferably should be in the range of 10-20~. In the
examples given to illustrate the invention, the embossed
area is 18%. The embossing pattern is 0.76 mm x 0.76 mm
diamond pattern with 31 diamond6 per square centimeter of
roll surface. The particular embossing pattern employed
i8 not critical and any pattern bonding between 5 and 30
of the fabric surface may be used.
The principles of this invention apply to any of the
commercially available resins, ~uch as polypropylene,
polyethylene, polyamides, polyester or any polymer or
polymer blends capable of being melt-blown. It has been
found particularly advantageous to use polyamides, and
particularly Nylon 6 ~polycaprolactam), in ordec to obtain
JSU-61
~ ,905~'7
6uperior aesthetic6, low cusceptibili~y to degradation due
to cobalt irradiation, excellent balance of properties,
and overall ease of proc~s~ing.
A6 stated previously, the preferred fabrics of the pre6ent
invention have a basis weight of fro~ 14 to B5 grams per
&quare meter. The surface veneers when geparately formed,
have a ba~is weight of from about 6 gram~ per square
meter, and when co-formed, a basis weight of from about 3
grams per ~quare meter. Ba~is weight6 of the surface
veneers are generally no greater than 10 to 15 grams per
6guare meter, as higher veneer base weight~ may require
lower core web basis weights to achieve the de~ired
overall ba6is weight of the fabric. The fabrics have a
minimum grab tensile strength to weight ratio greater than
0.8 N per gram per square meter, a minimum Elmendorf tear
strength to weight ratio gceater than 0.04 N per gram per
square meter and wet and dry surface abrasion resistance
of greater than 15 cycles to pill. For disposable medical
fabrics where high 6trength and abrasion resistance are
required, the preferred fabrics have basis weights no
greater than 60 grams per square meter, a minimum grab
tensile strength of not less than 65 N, a minimum
Elmendorf tear strength not less than 6 N, and dry surface
abrasion resistance of at least 40 cycles to pill and a
wet surface abrasion resistance of at least 30 cycles to
pill.
It is to be understood that the fiber~, webs or fabrics
produced according to thi~ invention can be combined in
various ways, and with other fiber~, webs, or fab~ics
posse~6ing different characteri6tics to for~ products with
specifically tailored properties.
The examples which follow are intended to clarify further
JSU-61
~905~7
-24-
the present invention. and are in no way intended to 6erve
a6 the limits of the content or cope of thi6 invention.
ExamPle 1
In the following example, web6 1, 2 and 3 were produced
under the condition6 6et forth in Table I below.
TABLE I
PROCESS CONDITIONS USED TO PRODUCE
MæLT-BLowN NYLON WEBS
Webs
Proces6 Conditions 1 2 3
Extruder Temperature - Feed C 260 232 260
Extruder Temperature - Exit C 275 275 300
Screen/Mixer Temperature C 275 275 2a7
Die Temperature C 287 265 300
Primary Air Temperature C 2B7 287 335
Primary Air Velocity m~sec290 255 221
Polymer Rate g/min-hole 1 0.14 0.14 0.28
Die Air Gap mm1.14 1.14 1.14
Die Setback - Negative mml.02 1.02 1.02
Secondary Air Velocity m/6ec 30 30 30
8a6is Weight g~m2 52 44 6
Average Fiber Diam~ter microns 3.6 4.1 9.8
Web 1 was produced under condition~ similar to tho6e ~et
fo~th in the aforementioned co~ending application for optimizing
both barrier and 6trength propertie6 in the f~nal fabric.
Web 2 was produced under modified conditions to produce a
fabric with enhan~ed fabric strength. but with a ~light
108s of barrier properties, achieved by lowering the die
temperature and the primary air velocity relative to web 1
JSU-61
~,
. ~ , ....
o~
-25-
conditions. Web 3 was produced by increa6ing the polymer
throughput rate and further decrea~ing primary air
velocity to produce a fiber layer having an average fiber
diameter of 9.8 micron6 and in which 80% of the fiberg
have a fiber diameter greater ehan 7 micron6.
Additionally the die temperature was raised to increase
the initial interfiber bonding of Web 3. Table II lists
the physical propertie6 of embossed fabrics made from webs
1, 2 and 3. Table III ~ets forth the processing
conditions for producing the embossed fabric6 who6e
physical characteristic6 are li6ted on Table II.
JSU-~l
1~0517
-Z6-
TA~LE II
DE5CRIPTION AND PHYSICAL PROPERTY CHARACTERISTICS
OF THERMALLY-EMBOSSED MELT-BLOWN NYLON
Fabric~
Characteri~tics 4 5 6 _ _ 7
Composition - Layer 1 Web 1 Web 2 Web 3Web 3
- Layer 2 - -Web 2 Web 2
- Layer 3 - - - Web 3
Total ~asis Weight (g~m2) 52 44 50 56
Grab Tensile Strength
to Weight Ratio
(N/g-m 2) MD 2.06 2.77 2.55 2.48
CD1.531.94 1.95 1.90
Hydro~tatic Pres~ure
(cm of water) 49 36 39 39
Abrasion Re~istance (cycles)
Side 1 Dry - to pill 15 15 40 50
- to fail 100 100 100 100
Wet - to pill 15 15 30 35
- to fail 100 100 100 100
Side 2 Dry - to pill 15 15 15 50
- to fail 100 100 100 100
Wet - to pill 15 15 15 35
- to fail 100 100 100 100
JSU-61
0517
-27-
TA81,E III
PROCESS CONDITIONS FOR THERMAL EMBOSSING
OF M~LT-BLOWN NYLON
Fabrics
~Process Conditions 4 5 6 7
Percent Embossed Area (~) 18 18 18 18
10 Oil Temperature (C)
Top Embos~ed Roll 126 122121 121
Bottom Smooth Roll 126 122122 L22
Nip Pressure Between Rolls (N~cm)685 685685 685
Web Speed (m/min) 15 9 9 9
As noted in Table II, Fabric 5 shows superior grab ten6ile
strength than Fabric 4, but decrea6ed barrier properties
as reflected in the hydrostatic pressure. The abrasion
re6istance remains the same. Fabrics 6 and 7 illustrate
ehe improved abrasion resistance achieved with the use Or
~urface veneers of web 3. Fabrics 6 and 7 show an
increasing fall off of normalized grab tensile strengths
due to the incorporation of the veneer layer(s) of web 3
which, while it adds to the weight of the fabric, it does
not contribute a6 much grab tensile per unit weight as web
2. Veneer layers of web 3 add slightly to the hydrostatic
head of Fabric~ 6 and 7, but add remarkable surface
abrasion resistance.
The dry surface abrasion resi6tance was ~easured as
follows. A ~ample of th~ ~abric to be ts3eed wa~ placed
atop a ~oam pad on a boteom testing plate. A 7~6 cm by
12.7 cm ~ample of a 6tandard Lytron ~inishQd abrading
eloth waa added to a top plate and placed in contact'with
the fabric test sample, with the machine direction of the
* Reg. TM
JSU-61
:
.. ~ . .. . ..
1;.'905i17
-28-
fabric te6t sample aligned with the machine directisn
(length~ of the Lytron finished cloth. A 1.1 Kg weight
was placed atop the top plate and the bottom plate rotated
at a fixed 6peed of 1.25 revolution6 per minute, each
rotation of the plate being recorded as one cycle. The
fabric test sample wa~ inspected under magnification after
each of the first five cycle6, and at five cycle intervals
thereafter. The number of cycles to pill was recorded, as
well as the number of cycles to create a hole in the
fabric test sample. Pilling is defined as the breaking
off of fibers which start to form clumps or beads. Four
samples of the fabcic were tested and the average number
of cycles to pill and to fabric failure was repoLted.
The wet 6urface abra6ion re6istance was measured under a
similar testing procedure, with the following
modifications; the fabric test sample, fastened to the
bottom plate was wetted with 5 drops of purified water,
and only a 0.2 Kg weight wa6 placed atop the top plate.
Example 2
In the following example webs 9, 9, 10, and 11 were
produced under conditions set forth in Table IV below.
JSU-61
~'~90517
-29-
TABLE IV
PROCESS CONDITIONS USED TO PRODUCE
MELT-BLOWN NYLON BASE WEBS
Extruder Temperature - Feed C 246 232 232 260
Extruder Temperature - Exit C 274 274 274 301
Web~
Process Conditions 8 9 lo 11
Screen/Mixer Temperature C 274274 274 301
Die Te~perature C 274 265265 301
Primary Air Temperature C 309285 285 331
Primary Air Velocity mJ~ec299 252191 299
Polymer Rate g/min-hole 1 0.140.140.28 0.28
Die Air Gap mm1.141.141.14 1.14
Die Setback - Negative mm1.021.021.02 1.02
Secondary Air Velocity m/6ec 30 30 30 0
Basi6 Weight g/m2 52 42 6 6
Average Fiber Diameter microns 8.2 8.8
The proces6 conaitions for webs 8, 9, 10 and 11 fall
within the proce66 conditions set forth in aforementioned
25 copending applica~ion. - . Web 8 was produced under condition6
for optimizing both strength and barr~er proper~ie~ in the
final fabric. Web 9 was produced under modified conditions
to produce a fabric with enhanced fabeic strengeh with a
slight 1086 in barrier properties, by loweeing the die
te~perature and primary air velocity relative to web B
condition~. Web 10 was produced by increa~ing the polymer
throughout rate and further decreasing the primary air
velocity to produce a fiber layer having an average fiber
diameter of approximately 9 micron6, and in which ~0% of
the fiber6 have a fiber diameter greater than 7 microns.
JSU-61
, ., . ~.,
1~90517
-30-
The die temperature remained the same for web~ 9 and 10.
Web 11 was produced under conditions substantially similar
to tho6e for producing web 3 but with no 6econdary air ~o
as to increa~e initial interfiber bonding. The die
temperatuIe for the production of web 11 wa6 al60
increa6ed over that u6ed to produce web 10 to increa~e
initial interfiber bonding.
Table V, below, lists the phy6ical characteristic6 of
embocsed fabrics made from webs 8, 9, 10 and 11 under the
conditions set forth in Table III. Fabric 13 comprise
Pabric 12 with 3 g/m of Primacor 4990* a 80/20
copolymer of ethylene and acrylic acid. manufactured by
the Dow Chemical Company, added to each side of the fabric.
TABLE V
pESCRIPTION AND PHYSICAL PROPERTY CHARACTERISTICS
OF THERMALLY-EMBOSSED MELT-BLOWN NYLON
Fabric~
Characteristics 12 13 14 15
Composition - Layer 1 Web 8 Binder Web L0 Web 11
- Layer 2 - Web 8 ~eb 9 Web 9
- Layer 3 - 3inder Web 10 Web 11
Total Ba~is Weight (g/m2) 52 58 54 54
Grab Tensile Strength ~N)
MD 94.1 103 94.0 108
CD 71.7 71.9 58.9 69.1
Hydrostatic Pres6ure
(cm of water)41 38 37 38
* Reg. TM
JSU-61
1~,90~17
TABLE V
(Continued)
DESCRIPTION AND PHYSICAL PROPERTY CHARACTERISTICS
5OF THERMALLY-EMBOSSED MELT-BLOWN NYLON
Fabr iC5
Characteri~tics 12 13 14 15
10 Abra6ion Resi~tance (cycles)
Side 1 Dry - to pill 5 15 40 45
- to fail 100 100 100 100
Wet - to pill 5 15 30 40
- to fail 100 100 100 100
Cusick Drape (%) 46 65 45 44
TABLE VI
PROCESS CONDITIONS FOR THERMAL EMBOSSING
OF MELT-8LOWN NYLON WEBS
Fabrics
Process Conditions 12 14 15
Percent Embo6sed Area (~) 18 18 18
Oil Temperature (C)
Top Embossed Roll 104 106 93
Bottom Smooth Roll 97 99 95
Nip Pressure Between Rolls (N/cm) 685 685 685
30 Web Speed (m/min) 9 9 9
JSU-61
1~9~)517
-32-
As shown in Table V, Fabric 13 shows an increase in
surface abrasion resistance with a large increase in
Cusick Drape. Further increase in binder level add-on
will contribute to abrasion resi6tance but will continue
to negatively impact the drape.
Fabric 14 exhibits far greater ~urface abrasion resistance
than Fabric 13 with no attendant loss in drape. Fabcic ~5
exhibits an~even greater improvement in surface abrasion
resistance over that shown by Fabric 14. The increase i8
believed to be due to the increase in initial interfiber
bondinq of web 11.
Thus, it is apparent that there has been provided, in
accordance with the invention, a new, unreinforced,
melt-blown, microfiber fabric having enhanced surface
abrasion resistance that satisfies the objects, aims and
advantages set forth above. While the invention has been
described in conjunction with specific embodiment~
thereof, it is evident that many alternatives,
modifications and variations will be apparent to those
skilled in the art in light of the above description.
Accordingly, it is intended to embrace all such
alternatives, modifications and variations that fall
within the spirit and broad scope of the appended claims.
JSU-61
