Note: Descriptions are shown in the official language in which they were submitted.
CA 02641972 2008-10-17
LYOCELL FIBERS AND PROCESS
FOR THEIR PREPARATION
This application claims priority from Provisional Applications Serial Nos-
60/023,909 and 60/024,462, both filed August 23, 1996.
The present invention is directed to lyocell fibers having novel characteris-
tics and to the method for their preparation. It is also directed to yarns
produced from
the fibers, and to woven and nonwoven fabrics containing the fibers. In
particular, the
method involves first dissolving cellulose in an amine oxide to form a dope.
Latent fi-
bers are then produced either by extrusion of the dope through small apertures
into an
air stream which draws the latent filaments of cellulose solution or by
centrifugally ex-
pelling the dope through small apertures. The fibers are then formed by
regenerating the
spun latent fibers in a liquid nonsolvent. Either process is amenable to the
production of
self bonded nonwoven fabrics.
BACKGROUND OF THE INVENTION
For over a century strong fibers of regenerated cellulose have been pro-
duced by the viscose and cuprammonium processes. The latter process was first
pat-
ented in 1890 and the viscose process two years later. In the viscose process
cellulose is
first steeped in a mercerizing strength caustic soda solution to form an
alkali cellulose.
This is reacted with carbon disulfide to form cellulose xanthate which is then
dissolved
in dilute caustic soda solution. After filtration and deaeration the xanthate
solution is
extruded from submerged spinnerets into a regenerating bath of sulfuric acid,
sodium
sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting
so-called
viscose rayon is presently used in textiles and was formerly widely used as
reinforcing in
rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammoniacal copper oxide. This
property formed the basis for production of cuprammonium rayon. The cellulose
solu-
tion is forced through submerged spinnerets into a solution of 5% caustic soda
or dilute
sulfuric acid to form the fibers. After decoppering and washing the resulting
fibers have
great wet strength. Cuprammonium rayon is available in fibers of very low
deniers and
is used almost exclusively in textiles.
More recently other cellulose solvents have been explored. One such sol-
vent is based on a solution of nitrogen tetroxide in dimethyl formamide. While
much re-
search was done, no commercial process has resulted for forming regenerated
cellulose
fibers using this solvent-
CA 02641972 2008-10-17
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The usefulness of tertiary amine-N oxides as cellulose solvents has been
known for a considerable time. Graenacher, in U.S. Patent No. 2,179,181,
discloses a
group of amine oxide materials suitable as solvents. However, the inventor was
only
able to form solutions with low concentrations of cellulose and solvent
recovery pre-
sented a major problem. Johnson, in U.S. Patent No. 3,447,939, describes the
use of
anhydrous N-methyimorpholine-N-oxide (NMMO} and other amine N-oxides as sol-
vents for cellulose and many other natural and synthetic polymers. Again the
solutions
were of relatively low solids content. In his later U.S. Patent No. 3,508,941,
Johnson
proposed mixing in solution a wide variety of natural and synthetic polymers
to form in-
timate blends with cellulose. A nonsolvent for cellulose such as
dimethylsulfoxide was
added to reduce dope viscosity. The polymer solution was spun directly into
cold
methanol but the resulting filaments were of relatively low strength.
However, beginning in 1979 a series of patents were issued to preparation
of regenerated cellulose fibers using various amine oxides as solvents. In
particular, N-
methylmorpholine-N-oxide with about 12% water present proved to be a
particularly
useful solvent. The cellulose was dissolved in the solvent under heated
conditions, usu-
ally in the range of 90 C to 130 C, and extruded from a multiplicity of fine
apertured
spinnerets into air. The filaments of cellulose dope are continuously
mechanically drawn
in air by a factor in the range of about three to ten times to cause molecular
orientation.
They are then led into a nonsolvent, usually water, to regenerate the
cellulose. Other
regeneration solvents, such as lower aliphatic alcohols, have also been
suggested. Ex-
amples of the process are detailed in McCorsley and McCorsley et al. U.S.
Patents Nos.
4,142,913; 4,144,080; 4,211,574; 4,246,221, and 4,416,698 and others. Jurkovic
et al.,
in U.S. Patent No 5,252,284 and Michels et al., in U.S. Patent 5,417,909 deal
especially
with the geometry of extrusion nozzles for spinning cellulose dissolved in
NMMO.
Brandner et al, in U.S. Patent 4,426,228, is exemplary of a considerable
number of pat-
ents that disclose the use of various compounds to act as stabilizers in order
to prevent
cellulose and/or solvent degradation in the heated NMMO solution. Franks et
al., in
U.S. Patent Nos. 4,145,532 and 4,196,282, deal with the difficulties of
dissolving cellu-
lose in amine oxide solvents and of achieving higher concentrations of
cellulose.
Cellulose textile fibers spun from NMMO solution are referred to as
lyocell fibers. Lyocell is an accepted generic term for a fiber composed of
cellulose pre-
cipitated from an organic solution in which no substitution of hydroxyl groups
takes
place and no chemical intermediates are formed. One lyocell product produced
by
Courtaulds, Ltd. is presently commercially available as Tencel' fiber. These
fibers are
available in 0.9-2.7 denier weights and heavier. Denier is the weight in grams
of 9000
meters of a fiber. Because of their fineness, yarns made from them produce
fabrics hav-
ing extremely pleasing hands.
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One limitation of the lyocell fibers made presently is a function of their ge-
ometry. They are continuously formed and typically have quite uniform,
generally circu-
lar or oval cross sections, lack crimp as spun, and have relatively smooth,
glossy
surfaces. This makes them less than ideal as staple fibers since it is
difficult to achieve
uniform separation in the carding process and can result in non-uniform
blending and un-
even yarn. In part to correct the problem of straight fibers, man made staple
fibers are
almost always crimped in a secondary process prior to being chopped to length.
Exam-
ples of crimping can be seen in U.S. Patent Nos. 5,591,388 or 5,601,765 to
Sellars et al.
where the fiber tow is compressed in a stuffier box and heated with dry steam.
It might
also be noted that fibers having a continuously uniform cross section and
glossy surface
produce yarns tending to have a "plastic" appearance. Yarns made from
thermoplastic
polymers frequently must have delustering agents, such as titanium dioxide,
added prior
to spinning. Wilkes et al., in U.S. Patent 5,458,835, teach the manufacture of
viscose
rayon fibers having cruciform and other cross sections. U. S. Patent No.
5,417,909 to
Michels et al. discloses the use of profiled spinnerets to produce lyocell
fibers having
non-circular cross sections but the present inventors are not aware of any
commercial
use of this method.
Kaneko et al. in U.S. Patent 3,833,438 teach preparation of self bonded
cellulose nonwoven materials made by the cuprammonium rayon process. Self
bonded
lyocell nonwoven webs have not been described to the best of the present
inventors'
knowledge.
Low denier fibers from synthetic polymers have been produced by a num-
ber of extrusion processes. Three of these are relevant to the present
invention. One is
generally termed "melt blowing". The molten polymers are extruded through a
series of
small diameter orifices into an air stream flowing generally parallel to the
extruded fi-
bers. This draws or stretches the fibers as they cool. The stretching serves
two pur-
poses. It causes some degree of longitudinal molecular orientation and reduces
the
ultimate fiber diameter. A somewhat similar process is called "spunbonding"
where the
fiber is extruded into a tube and stretched by an air flow through the tube
caused by a
vacuum at the distal end. In general, spunbonded fibers are continuous while
melt
blown fibers are more usually in discrete shorter lengths. The other process,
termed
"centrifugal spinning", differs in that the molten polymer is expelled from
apertures in
the sidewalls of a rapidly spinning drum. The fibers are drawn somewhat by air
resis-
tance as the drum rotates. However, there is not usually a strong air stream
present as
in meltblowing. All three processes may be used to make nonwoven fabric
materials.
There is an extensive patent and general technical literature on the processes
since they
have been commercially important for many years. Exemplary patents to
meltblowing
are Weber et al., U.S. Patent No. 3,959,421, and Milligan et al., U.S. Patent
No.
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5,075,068. The Weber et al. patent uses a water spray in the gas stream to
rapidly cool
the fibers. A somewhat related process is described in PCT Publication WO
91/18682
which is directed to a method for coating paper by modified meltblowing.
Coating ma-
terials suggested are aqueous liquids such as "an aqueous solution of starch,
carboxy-
methylcellulose, polyvinyl alcohol, latex, a suspension of bacterial
cellulose, or any
aqueous material, solution or emulsion". However, this process actually
atomizes the
extruded material rather than forms it into latent fibers. Zikeli et al., in
U.S. Patent
Nos. 5,589,125 and 5,607,639, direct a stream of air transversely across
strands of ex-
truded lyocell dope as they leave the spinnerets. This air stream serves only
to cool and
does not act to stretch the filaments.
Centrifugal spinning is exemplified in U.S. Patents Nos. 5,242,633 and
5,326,241 to Rook et al. Okada et al., in U.S. Patent No. 4,440,700 describe a
centrifu-
gal spinning process for thermoplastic materials. As the material is ejected
the fibers are
caught on an annular form surrounding the spinning head and moved downward by
a
curtain of flowing cooling liquid. Included among the list of polymers suited
to the
process are polyvinyl alcohol and polyacrylonitrile. In the case of these two
materials
they are spun "wet"; i.e., in solution, and a "coagulation bath" is
substituted for the cur-
tain of cooling liquid.
With the exception of the Kaneko et al. patent noted above, processes
analogous to melt blowing, spunbonding and centrifugal spinning have never
been used
with cellulosic materials since cellulose itself is basically infusible.
Extremely fine fibers, termed "microdenier fibers" generally are regarded
as those having a denier of 1.0 or less. Meltblown fibers produced from
various syn-
thetic polymers, such as polypropylene, nylons, or polyesters are available
with diame-
ters as low as 0.4 gm (approximately 0.001 denier). However, the strength or
"tenacity" of most of these fibers tends to be low and their generally poor
water absorb-
ency is a negative factor when they are used in fabrics for clothing.
Microdenier cellu-
lose fibers, as low as 0.5 denier, have been produced before the present only
by the
viscose process.
The present process produces a new lyocell fiber that overcomes many of
the limitations of the fibers produced from synthetic polymers, rayons, and
the presently
available lyocell fibers. It allows formation of fibers of low denier and with
a distribu-
tion of deniers. At the same time each fiber has a pebbled surface, a cross
section of
varying shape and diameter along its length, and significant natural crimp.
All of these
are desirable characteristics that are found in most natural fibers but are
missing in
lyocell fibers produced commercially to the present.
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SUMMARY OF THE INVENTION
The present invention is directed to a process for production of regener-
ated cellulose fibers and webs and to the fibers and webs so produced. The
terms "cel-
lulose" and "regenerated cellulose" as used here should be construed
sufficiently broadly
to encompass blends of cellulose with other natural and synthetic polymers,
mutually
soluble in a spinning solvent, in which cellulose is the principal component
by weight. In
particular it is directed to low denier fibers produced from cellulose
solutions in amine
N-oxides by processes analogous to melt blowing or centrifugal spinning. Where
the
terms "melt blowing", "spunbonding", and "centrifugal spinning" are used it
will be un-
derstood that these refer to processes that are similar or analogous to the
processes used
for production of thermoplastic fibers, even though the cellulose is in
solution and the
spinning temperature is only moderately elevated. The term "continuously
drawn" refers
to the present commercial process for manufacture of lvocell fibers where they
are me-
chanically pulled, first through an air gap to cause elongation and molecular
orientation
then through the regeneration bath.
The processes involve dissolving a cellulosic raw material in an amine ox-
ide, preferably N-methylmorpholine-N-oxide (NMMO) with some water present.
This
dope, or cellulose solution in NMMO, can be made by known technology; e.g., as
is dis-
cussed in any of the McCorsley or Franks et at. patents aforenoted. In the
present proc-
ess, the dope is then transferred at somewhat elevated temperature to the
spinning
apparatus by a pump or extruder at about 90 C to 130 C. Ultimately the dope is
di-
rected through a multiplicity of small orifices into air. In the case of melt
blowing, the
extruded threads of cellulose dope are picked up by a turbulent gas stream
flowing in a
generally parallel direction to the path of the filaments. As the cellulose
solution is
ejected through the orifices the liquid strands or latent filaments are drawn
(or signifi-
cantly decreased in diameter and increased in length) during their continued
trajectory
after leaving the orifices. The turbulence induces a natural crimp and some
variability in
ultimate fiber diameter both between fibers and along the length of individual
fibers.
This is in marked contrast to continuously drawn fibers where diameters are
uniform and
crimp is lacking or must be introduced as a post spinning process. The crimp
is irregular
and will have a peak to peak amplitude greater than about one fiber diameter
and a pe-
riod greater than about five fiber diameters.
Spunbonding can be regarded as a species of meltblowing in that the fibers
are picked up and drawn in an airstream without being mechanically pulled. In
the con-
3 5 text of the present invention meltblowing and spunbonding should be
regarded as func-
tional equivalents.
Where the fibers are produced by centrifugal spinning, the dope strands
are expelled through small orifices into air and are drawn by the inertia
imparted by the
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spinning head. The filaments are then directed into a regenerating solution or
a regener-
ating solution is sprayed onto the filaments. Regenerating solutions are
nonsolvents
such as water, lower aliphatic alcohols, or mixtures of these. The NMMO used
as the
solvent can then be recovered from the regenerating bath for reuse.
Turbulence and oscillation in the air around the latent fiber strands is be-
lieved to be responsible for their unique geometry when made either by the
melt blowing
or centrifugal spinning process.
Filaments having an average size as low as 0.1 denier or even less can be
readily formed. Denier can be controlled by a number of factors including but
not lim-
ited to orifice diameter, gas stream speed, spinning head speed, and dope
viscosity.
Dope viscosity is, in turn, largely a factor of cellulose D.P. and
concentration. Fiber
length can be similarly controlled by design and velocity of the air stream
surrounding
the extrusion orifices. Continuous fibers or relatively short staple fibers
can be produced
depending on spinning conditions. Equipment can be readily modified to form
individual
fibers or to lay them into a mat of nonwoven cellulosic fabric. In the latter
case the mat
may be formed and become self bonded prior to regeneration of the cellulose.
The fi-
bers are then recovered from the regenerating medium, further washed, bleached
if nec-
essary, dried, and handled conventionally from that point in the process.
Gloss or luster of the fibers is considerably lower than continuously drawn
lyocell fiber lacking a delusterant so they do not have a "plastic"
appearance. This is be-
lieved to be due to their unique "pebbled" surface apparent in high
magnification
micrographs.
By properly controlling spinning conditions the fibers can be formed with
variable cross sectional shape and a relatively narrow distribution of fiber
diameters.
Some variation in diameter and cross sectional configuration will typically
occur along
the length of individual fibers and between fibers. The fibers are unique for
regenerated
cellulose and similar in morphology to many natural fibers.
Fibers produced by either the melt blowing or centrifugal spinning proc-
esses possess a natural crimp quite unlike that imparted by a stuffier box.
Crimp im-
parted by a stuffier box is relatively regular, has a relatively low amplitude
usually less
than one fiber diameter, and short peak-to-peak period normally not more than
two or
three fiber diameters. That of the present fibers has an irregular amplitude
greater than
one fiber diameter, usually much greater, and an irregular period exceeding
about five
fiber diameters, a characteristic of fibers having a curly or wavy appearance.
Properties of the fibers of the present invention are well matched for card-
ing and spinning in conventional textile manufacturing processes. The fibers,
while hav-
ing many of the attributes of natural fibers, can be produced in microdenier
diameters
CA 02641972 2012-02-27
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unavailable in nature. it is possible to directly produce self bonded webs or
tightly wound multi-ply
yarns.
A particular advantage of the present invention is the ability to form blends
of
cellulose with what might otherwise be considered as incompatible polymeric
materials. The amine
oxides are extremely powerful solvents and can dissolve many other polymers
beside cellulose. It is
thus possible to form blends of cellulose with materials such as lignin,
nylons, polyethylene oxides,
polypropylene oxides, poly(acrylonitrile), poly(vinylpyrrolidone),
poly(acrylic acid), starches,
poly(vinyl alcohol), polyesters, polyketones, casein, cellulose acetate,
amylose, amylopectins,
cationic starches, and many others. Each of these materials in homogeneous
blends with cellulose
can produce fibers having new and unique properties.
According to one embodiment of the invention, a lyocell nonwoven fabric is
provided
in which the fabric is formed of lyocell fibers characterized by greater
variability in the cross
sectional diameters in cross sectional configurations along the fiber lengths
and from fiber to fiber,
in comparison to variability in cross sectional diameters and cross sectional
configurations along the
fiber lengths, and from fiber to fiber, of continuously mechanically drawn
lyocell fibers. The
lyocell fibers forming the fabrics may be characterized by a pebbled surface.
The fabric may further
have an irregular crimp. The crimp may have an amplitude greater than about
one fiber diameter
and a period greater than about five fiber diameters. The fibers that form the
fabric may be
continuous. The fibers that form the product may be melt blown, continuous
centrifugal spun, or
continuous spunbond. The fibers may be self bonded. The lyocell fibers may be
hydroentangled.
The lyocell fibers may be adhered to each other. The lyocell fibers may have
an average denier less
that 0.45.
In one embodiment of the invention, a method is provided for forming low
denier
regenerated cellulose fibers or cellulose blend fibers from solution in an
amine oxide-water medium
by processes analogous to melt blowing, spunbonding, or centrifugal spinning.
Another embodiment provides low denier cellulose fibers having advantageous
geometry and surface characteristics for forming into yarns.
Yet another embodiment provides fibers having natural crimp and low luster.
Yet another embodiment provides regenerated cellulose fibers having many
properties
similar or superior to natural fibers.
In yet another embodiment, a method is provided for forming fibers of the
above
types by a process in which all production chemicals can be readily recovered
and reused.
Yet another embodiment provides self bonded nonwoven lyocell fabrics.
These and many other embodiments will become readily apparent to those skilled
in
the art upon reading the following detailed description in conjunction with
referral to the drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the steps used in practice of the present
process.
FIG. 2 is a partially cut away perspective representation of typical
centrifugal
spinning equipment used with the invention.
FIGS. 3 is a partially cut away perspective representation of melt blowing
equipment adapted for use with the present invention.
FIG. 4 is a cross sectional view of a typical extrusion head that might be
used with
the above melt blowing apparatus.
FIGS. 5 and 6 are scanning electron micrographs of a commercially available
lyocell
fiber at 1 00X and 10,000x magnification respectively.
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FIGS. 7 and 8 are scanning electron micrographs of a lyocell fiber pro-
duced by centrifugal spinning at 200X and 10,000x magnification respectively.
FIGS. 9 and 10 are scanning electron micrographs at 2,000X showing
cross sections along a single centrifugally spun fiber.
S FIGS. 1 I and 12 are scanning electron micrographs of a melt blown lyocell
fiber at I00X and 10,000x-magnification respectively.
FIG. 13 is a drawing illustrating production of a self bonded nonwoven
lyocell fabric using a melt blowing process.
FIG. 14 is a similar drawing illustrating production of a self bonded non-
woven lyocell fabric using a centrifugal spinning process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The type of cellulosic raw material used with the present invention is not
critical. It may be bleached or unbleached wood pulp which can be made by
various
processes of which kraft, prehydrolyzed kraft, or sulfite would be exemplary.
Many
other cellulosic raw materials, such as purified cotton linters, are equally
suitable. Prior
to dissolving in the amine oxide solvent the cellulose, if sheeted, is
normally shredded
into a fine fluff to promote ready solution.
The solution of the cellulose can be made in a known manner; e.g., as
taught in McCorsley U. S. Patent No. 4,246,221. Here the cellulose is wet in a
non-
solvent mixture of about 40% NMMO and 60% water. The ratio of cellulose to wet
NMMO is about 1:5.1 by weight. The mixture is mixed in a double arm sigma
blade
mixer for about 1.3 hours under vacuum at about 120 C until sufficient water
has been
distilled off to leave about 12-14% based on NMMO so that a cellulose solution
is
formed. The resulting dope contains approximately 30% cellulose.
Alternatively,
NMMO of appropriate water content may be used initially to obviate the need
for the
vacuum distillation. This is a convenient way to prepare spinning dopes in the
labora-
tory where commercially available NMMO of about 40-60% concentration can be
mixed
with laboratory reagent NMMO having only about 3% water to produce a cellulose
sol-
vent having 7-15% water. Moisture normally present in the cellulose should be
ac-
counted for in adjusting necessary water present in the solvent. Reference
might be
made to articles by Chanzy, H. and A. Peguy, Journal of Polymer Science,
Polymer
Physics Ed. 18: 1137-1144 (1980) and Navard, P. and J. M. Haudin British
Polymer
Journal, p 174, Dec. 1980 for laboratory preparation of cellulose dopes in
NMMO-
water solvents.
Reference to FIG. 1 will show a block diagram of the present process. As
was noted, preparation of the cellulose dopes in aqueous NMMO is conventional.
What
is not conventional is the way these dopes are spun. The cellulose solution is
forced
CA 02641972 2008-10-17
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from extrusion orifices into a turbulent air stream rather than directly into
a regeneration
bath as is the case with viscose or cuprammonium rayon. Only later are the
latent fila-
ments regenerated. However, the present process also differs from the
conventional
processes for forming lyocell fibers since the dope is not continuously drawn
linearly
downward as unbroken threads through an air gap and into the regenerating
bath.
FIG. 2 is illustrative of a centrifugal spinning process. The heated cellu-
lose dope 1 is directed into a heated generally hollow cylinder or drum 2 with
a closed
base and a multiplicity of small apertures 4 in the sidewalls 6. As the
cylinder rotates,
dope is forced out horizontally through the apertures as thin strands 8. As
these strands
meet resistance from the surrounding air they are drawn or stretched by a
large factor.
The amount of stretch will depend on readily controllable factors such as
cylinder rota-
tional speed, orifice size, and dope viscosity. The dope strands either fall
by gravity or
are gently forced downward by an air flow into a non-solvent 10 held in a
basin 12
where they are coagulated into individual oriented fibers having lengths from
about I to
25 cm. Alternatively, the dope strands 8 can be either partially or completely
regener-
ated by a water spray from a ring*of spray nozzles 16 fed by a source of
regenerating so-
lution 18. Also, as will be described later, they can be formed into a
nonwoven fabric
prior to or during regeneration. Water is the preferred coagulating non-
solvent although
ethanol or water-ethanol mixtures are also useful. From this point the fibers
are col-
lected and may be washed to remove any residual NMMO, bleached as might be
neces-
sary, and dried. Example 2 that will follow gives specific details of
laboratory
centrifugally spun fiber preparation.
FIGS. 3 and 4 show details of a typical melt blowing process. As seen in
FIG. 3, a supply of dope, not shown, is directed to an extruder 32 which
forces the cel-
lulose solution to an orifice head 34 having a multiplicity of orifices 36.
Air or another
gas is supplied through lines 38 and surrounds and transports extruded
solution strands
40. A bath or tank 42 contains a regenerating solution 44 in which the strands
are re-
generated from solution in the solvent to cellulose fibers. Alternatively, the
latent fibers
can be showered with a water spray to regenerate or partially regenerate them.
The
amount of draw or stretch will depend on readily controllable factors such as
orifice
size, dope viscosity, cellulose concentration in the dope, and air speed and
nozzle
configuration.
FIG. 4 shows a typical extrusion orifice. The orifice plate 20 is bored with
a multiplicity of orifices 36. It is held to the body of the extrusion head 22
by a series of
cap screws 18, An internal member 24 forms the extrusion ports 26 for the
cellulose so-
lution. It is embraced by air passages 28 that surround the extruded solution
filaments
causing them to be drawn and to assist in their transport to the regenerating
medium.
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Example 3 that follows will give specific details of laboratory scale fiber
preparation by melt
blowing.
FIGS. 5-12 are drawings depicting the scanning electron micrographs
originally filed as FIGS. 5-12. The scanning electron micrographs shown in
FIGS. 5-6 are of
lyocell fibers made by the conventional continuously drawn process. It is
noteworthy that
these are of quite uniform diameter and are essentially straight. The surface
seen at 10,000X
magnification in FIG. 6 is remarkably smooth.
FIGS. 7-10 are of fibers made by a centrifugal spinning process of the present
invention. The fibers seen in FIG. 7 have a range of diameters and tend to be
somewhat curly
giving them a natural crimp. This natural crimp is quite unlike the regular
sinuous
configuration obtained in a stuffer box. Both amplitude and period are
irregular and are at
least several fiber diameters in height and length. Most of the fibers are
somewhat flattened
and some show a significant amount of twist. Fiber diameter varies between
extremes of
about 1.5 pm and 20 pm (<0. 1 - 3. 1 denier), with most of the fibers closely
grouped around
a 12 pm diameter average (c. 1 denier).
FIG. 8 shows the fibers of FIG. 7 at 10,000x magnification. The surface is
uniformly pebbly in appearance, quite unlike the commercially available
fibers. This results
in lower gloss and improved spinning characteristics.
FIGS. 9 and 10 are scanning micrographs of fiber cross sections taken about 5
mm apart on a single centrifugally spun fiber. The variation in cross section
and diameter
along the fiber is dramatically shown. This variation is characteristic of
both the centrifugally
spun and melt blown fiber.
FIGS. 11 and 12 are low and high magnification scanning micrographs of melt
blown fiber. Fiber diameter, while still variable, is less so than the
centrifugally spun fiber.
However, crimp of these samples is significantly greater. The micrograph at
10,000X of FIG.
12 shows a pebbly suffice remarkably like that of the centrifugally spun
fiber.
The overall morphology of fibers from both processes is highly advantageous
for forming fine tight yams since many of the features resemble those of
natural fibers. This is
believed to be unique for the lyocell fibers of the present invention.
FIG. 13 shows one method for making a self bonded lyocell nonwoven material
using a modified melt blowing process. A cellulose dope 50 is fed to extruder
52 and from
there to the extrusion head 54. An air supply 56 acts at the extrusion
orifices to draw the dope
strands 58 as they descend from the extrusion head. Process parameters are
preferably chosen
so that the resulting fibers will be continuous rather than random shorter
lengths. The fibers
fall onto an endless moving foraminous belt 60 supported and driven by rollers
62, 64. Here
they form a latent nonwoven fabric mat 66. A top roller, not shown, may be
used to press the
CA 02641972 2010-11-15
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fibers into tight contact and ensure bonding at the crossover points. As mat
66 proceeds along its
path while still supported.
CA 02641972 2008-10-17
-I on belt 60, a spray of regenerating solution 68 is directed downward by
sprayers 70.
The regenerated product 72 is then removed from the end of the belt where it
may be
further processed; e.g., by further washing, bleaching, and drying.
FIG 14 is an alternative process for forming a self bonded nonwoven web
using centrifugal spinning. A cellulose dope 80 is fed into a rapidly rotating
drum 82
having a multiplicity of orifices 84 in the sidewalls. Latent fibers 86 are
expelled
through orifices 84 and drawn, or lenghtened, by air resistance and the
inertia imparted
by the rotating drum. They impinge on the inner sidewalls of a receiver
surface 88 con-
centrically located around the drum. The receiver may optionally have a
frustroconical
lower portion 90. A curtain or spray of regenerating solution 92 flows
downward from
ring 94 around the walls of receiver 88 to partially coagulate the cellulose
mat impinged
on the sidewalls of the receiver. Ring 94 may be located as shown or moved to
a lower
position if more time is needed for the latent fibers to self bond into a
nonwoven web.
The partially coagulated nonwoven web 96 is continuously mechanically pulled
from the
lower part 90 of the receiver into a coagulating bath 98 in container 100. As
the web
moves along its path it is collapsed from a cylindrical configuration into a
planar two ply
nonwoven structure. The web is held within the bath as it moves under rollers
102, 104.
A takeout roller 1 06 removes the now fully coagulated two ply web 108 from
the bath.
Any or all of rollers 100, 102, or 104 may be driven. The web 108 is then
continuously
directed into a wash and/or bleaching operation, not shown, following which it
is dried .
for storage. It may be split and opened into a single ply nonwoven or
maintained as a
two ply material as desired.
Example I
Cellulose Dope Preparation
- The cellulose pulp used in this and the following examples was a standard
bleached kraft southern softwood market pulp, Grade NB 416, available from
Weyer-
haeuser Company, New Bern, North Carolina. It has an alpha cellulose content
of about
88-89% and a D.P. of about 1200. Prior to use, the sheeted wood pulp was run
through
a Huffer to break it down into essentially individual fibers and small fiber
clumps. Into a
250 mL three necked glass flask was charged 5.3 g of fluffed cellulose, 66.2 g
of 97%
NMMO, 24.5 g of 50% NMMO, and 0.05 g propyl gallate. The flask was immersed in
an oil bath at 120 C, a stirrer inserted, and stirring continued for about 0.5
hr. A readily
flowable dope resulted that was directly suitable for spinning.
CA 02641972 2008-10-17
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Example 2
Fiber Preparation by Centrifugal Spinning
The spinning device used was a modified "cotton candy" type, similar to
that shown in U.S. Patent No. 5,447,423 to Fuisz et al. The rotor, preheated
to 120 C
was 89 mm in diameter and revolved at 2800 rpm. The number of orifices could
be
varied -between I and 84 by -blocking off orifices. Eight orifices 700 gm in
diameter
were used for the following trial. Cellulose dope, also at 120 C, was poured
onto the
center of the spinning rotor. The thin strands of dope that emerged were
allowed to fall
by gravity into room temperature water contained in the basin surrounding the
rotor.
Here they were regenerated. While occasional fibers would bond to each other
most re-
mained individualized and were several centimeters in length.
In addition to the process just described, very similar microdenier fibers
were also successfully made from bleached and unbleached kraft pulps, sulfite
pulp, mi-
crocrystalline cellulose, and blends of cellulose with up to 30% corn starch
or
poly(acrylic acid).
Diameter (or denier) of the fibers could be reliably controlled by several
means. Higher dope viscosities tended to form heavier fibers. Dope viscosity
could, in
turn, be controlled by means including cellulose solids content or degree of
polymeriza-
tion of the cellulose. Smaller spinning orifice size or higher drum rotational
speed pro-
duces smaller diameter fibers. Fibers having diameters from about 5-20 gm (0.2-
3.1
denier) were reproducibly made. Heavier fibers in the 20-50 gm diameter range
(3.1-19.5 denier) could also be easily formed. Fiber length varies between
about 0.5-25
cm and depended considerably on the geometry and operational parameters of the
system.
Example 3
Fiber Preparation by Melt Blowing
The dope as prepared in Example I was maintained at 120 C and fed to an
apparatus originally developed for forming melt blown synthetic polymers.
Overall ori-
fice length was about 50 mm with a diameter of 635 pm which tapered to 400 gm
at the
discharge end. After a transit distance in air of about 20 cm in the turbulent
air blast the
fibers dropped into a water bath where they were regenerated. Regenerated
fiber length
varied. Some short fibers were formed but most were several centimeters to
tens of
centimeters in length. Variation of extrusion parameters enabled continuous
fibers to be
formed. Quite surprisingly, the cross section of many of the fibers was not
uniform
along the fiber length. This feature is expected to be especially advantageous
in spinning
tight yarns using the microdenier material of the invention since the fibers
more closely
resemble natural fibers in overall morphology.
CA 02641972 2008-10-17
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In a variation of the above process, the fibers were allowed to impinge on
a traveling stainless steel mesh belt before they were directed into the
regeneration bath.
A well bonded nonwoven mat was formed.
It will be understood that the lyocell nonwoven fabrics need not be self
bonded. They may be only partially self bonded or not self bonded at all. In
these cases
they may be bonded by any of the well known methods including but not limited
to hy-
droentangling, the use of adhesive binders such as starch or various polymer
emulsions
or some combination of these methods.
Example 4
Use of Microcrystalline Cellulose Furnish to Prepare Melt Blown Lyocell
The process of Example I was repeated using a microcrystalline furnish
rather than wood pulp in order to increase solids content of the dope. The
product used
was Avicel'* Type PH-101 microcrystalline cellulose available from FMC Corp.,
New-
ark, Delaware. Dopes were made using 15 g and 28.5 g of the microcrystalline
cellulose
(dry weight) with 66.2 g of 97% NMMO, 24.5 g of 50% NMMO and 0.05g propyl gal-
late. The procedure was otherwise as described in Example 1. The resulting
dopes con-
tained respectively about 14% and 24% cellulose. These were meltblown as
described
in Example 3. The resulting fiber was morphologically essentially identical to
that of
Examples 2 and 3.
It will be understood that fiber denier is dependent on many controllable
factors. Among these are solution solids content, solution pressure and
temperature at
the extruder head, orifice diameter, air pressure, and other variables well
known to those
skilled in meltblowing and centrifugal spinning technology. Lyocell fibers
having an av-
erage 0.5 denier or even lower may be consistently produced by either the melt
blowing
or centrifugal spinning processes. A 0.5 denier fiber corresponds to an
average diameter
(estimated on the basis of equivalent circular cross sectional area) of about
7-8 gm.
The fibers of the present invention were studied by x-ray analysis to deter-
mine degree of crystallinity and crystallite type. Comparisons were also made
with some
other cellulosic fibers as shown in the following table. Data for the
micro.denier fibers
are taken from the centrifugally spun material of Example 2.
CA 02641972 2008-10-17
-14-
Table I
Crystalline Properties of Different Cellulose Fibers
Microdenier Cellulose Generic
Fibers of Present Invention Lyocell Tencei Cotton
Crystallinity Index 67% 65% 70% 85%
Crystallite Cellulose II Cellulose 11 Cellulose II Cellulose I
Some difficulty was encountered in measuring tensile strength of the
individual fi-
bers so the numbers given in the following table for tenacity are estimates.
Again, the
rnicrodenier fibers of the present invention are compared with a number of
other fibers.
Table 2
Fiber Physical Property Measurements
Centrifugally
Fibers Cotton So. Pine Rayon') Silk Spun Lyocell Tencel
Typical Length, cm 4 0.5 40 >104 5-25 Variable
Typical Diam., gm 20 40 16 10 5 12
Tenacity, g/d 2.5-3.0 --- 0.7-3.2 2.8-5.2 2.1 4.5-5.0
Viscose process
The centrufugally spun lyocell with an average diameter of about 5 .tm corre-
sponds to fibers of about 0.25 denier.
The pebbled surface of the fibers of the present invention result in a desir-
able lower gloss without the need for any internal delustering agents. While
gloss or
luster is a difficult property to measure the following test will be exemplary
of the differ-
ences between a fiber sample made by the method of Example 2 and a commercial
lyocell fiber. Small wet formed handsheets were made from the respective
fibers and
light reflectance was determined. Reflectance of the Example 2 material was
5.4%
while that of the commercial fiber was 16.9%.
The inventors have herein described the best present mode of practicing
their invention. It will be evident to others skilled in the art that many
variations that
have not been exemplified should be included within the broad scope of the
invention.
