Note: Descriptions are shown in the official language in which they were submitted.
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Method for producing regenerated cellulosic fibers
Field of the invention
The current disclosure relates to innovations in the field of the production,
use and
application of man-made cellulosic fibers. Particularly the current disclosure
relates to
processes for the production of regenerated cellulosic fibers which are
produced
according to a cold-alkali process, the thus produced fibers and their use.
Description of the Related Art
Man-made cellulosic fibers are manufactured fibers that are based on
cellulosic
matter as a source material.
In the context of the current disclosure the term "cellulose" denotes an
organic
compound derived from plant cell walls or synthetically produced. Cellulose is
a
polysaccharide and is unbranched. Typically, cellulose comprises several
hundred to
ten thousand 13-D-glucose molecules (13-1,4-glycosidic bound) or cellobiose
units,
respectively. The cellulose molecules that are used by plants to produce
cellulose
fibers are also used in technical processes to produce regenerated cellulose.
The term "regenerated cellulose" denotes a class of materials manufactured by
the
conversion of natural or recycled cellulose to a soluble cellulosic derivative
or a
directly dissolved cellulose solution and subsequent regeneration, forming
shaped
bodies, such as fibers (e.g., rayon), films or foils (e.g., cellophane) or
bulk solids (e.g.
beads, powders or pellets).
The term "fibers", as it is used herein, denotes continuous filaments as well
as cut
staple fibers of any desired length.
Cellulosic fibers can be used to produce woven, knitted or non-woven
structures
(including fabrics) comprising the cellulosic fibers. Woven fabrics comprise
textile
planar fabrics made from at least two crossed thread systems, which can be
referred
to as warp- and weft-yarns. By contrast, the yarn in knitted fabrics follows a
meandering path (a course), forming symmetric loops (also called bights)
symmetrically above and below the mean path of the yarn.
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The term "non-woven fabric" denotes fabrics that are neither woven nor
knitted. Non-
woven fabrics can be in the form of a fabric comprising randomly oriented
fibers
and/or cut yarns of finite length. Non-woven fabrics can also comprise endless
yarns,
e.g. produced by a melt-blown-process.
Viscose fibers are regenerated cellulosic fibers, which are manufactured by
means of
a wet spinning method which is called the viscose-method. The starting raw
material
of the viscose-method is cellulose which is usually provided on the basis of
wood.
From this starting raw material a highly pure cellulose in form of chemical
pulp is
obtained. Additionally or as an alternative other cellulosic materials, such
as bamboo,
cotton linters, recycled cellulosic materials, reed, etc., or mixtures of such
materials
can be used as a starting raw material. In subsequent process stages, the pulp
is first
treated with caustic soda (NaOH), whereby alkali cellulose is formed. In a
subsequent
conversion of said alkali cellulose with carbon disulfide, cellulose-
xanthogenate is
formed. From this, by further supplying NaOH, the viscose-spinning solution is
generated which is pumped through holes of shower-like spinning nozzles into a
coagulation bath (also referred to as spin bath). There, one viscose-filament
per
spinning nozzle hole is generated by coagulation. To coagulate the spinning
solution,
an acidic coagulation bath is used. The thus generated viscose-filaments are
subsequently post processed. The post processing usually comprises several
washing- and stretching steps and the filaments are cut to viscose-staple
fibers.
Several other post-processing steps, such as crimping, bleaching and/or
finishing
("soft finish") can be performed on the uncut and/or the cut fibers. In the
context of
this document, the term õviscose process" denotes such a xanthogenate process.
The term "Lyocell", as used herein, denotes a regenerated fiber type
comprising
cellulose, which is manufactured according to a direct solvent method. The
cellulose
for the lyocell-method is extracted from the raw material containing the
cellulose. The
thus obtained pulp may subsequently be dissolved in a suitable organic solvent
under
dehydration without chemical modification. In large-scale industrial
implementation N-
methylmorpholine-N-oxide (NMMO) is currently used a solvent, nonetheless it is
known that other solvents, such as ionic liquids, can also be used for the
process.
The solution is then filtered and, for the production of fibers, subsequently
extruded
through spinning nozzles into an air gap where they are drawn and coagulated
by
means of a moist airstream and then are fed into a coagulation bath containing
an
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aqueous NMMO-solution. Subsequently the fibers can be further processed, e.g.
washed, bleached, finished, crimped, cut to staple fibers, etc.
It is known that lyocell fibers may exhibit a tendency to fibrillate when
subjected to
mechanical stress in wet state. Fibrillation means that the fiber structure
breaks down
in longitudinal direction. Because of mechanical abrasion in the wet state,
fine fibrils
become partially detached from the fiber giving a hairy appearance to the
fabric
containing this fiber. This phenomenon takes place during wet fabric
processing
steps like dyeing or scouring as well as during laundering of garments. The
surface of
the fabric may get an aesthetically undesirable appearance. The surface of the
fabric
becomes matted as the fibrils entangle with each other and where fibrillation
occurs,
the fabric has a lighter color due to spectral reflection from the surface of
the fibrils.
The fibrillation commonly occurs on the high points of the fabric and lines of
whiteness can appear where fibrillation occurs on creases. Fibrillation of a
fabric can
occur whenever the fabric is subjected to wet abrasion. Longer times and
higher
temperatures during wet treatment processes such as dyeing processes tend to
produce greater degrees of fibrillation_
EP0538977A1 discloses a method for treatment of lyocell fibers with agents
having
functional groups reactive with cellulose to reduce or inhibit its tendency to
fibrillate. A
chemical reagent having two to six cellulose reactive functional groups a so
called
crosslinking agent is applied to the fiber. It is then exposed to conditions
that cause
the agent to react with the cellulose in the fiber causing the cellulose
molecules to be
attached together more strongly than can occur with the natural hydrogen bonds
which normally bind the molecules in the fiber together. Hydrogen bonds can be
broken by wetting with water and hence fibrillation can occur. The bonds
formed with
the crosslinking agent cannot be broken by exposure to water and hence the
fiber
does not fibrillate. It has however been found that such methods of treatment
may
have negative impacts on the mechanical properties of the fiber such as its
tenacity
and extensibility.
Another well-known process for the manufacturing of regenerated cellulose
fibers is
the carbamate-method, which is similar to the viscose-process but uses urea
instead
of carbon disulfide. Still another process, which is called modal-process, is
a modified
viscose-process for the production of higher quality fibers. For these
processes, also
an acidic coagulation bath is used.
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Further, processes for manufacturing of cellulosic products are known that can
use
an alkaline spin bath comprising a salt. To prepare the spinning solution,
cellulose is
dissolved in an aqueous inorganic alkaline medium comprising zinc oxide (ZnO)
under controlled low temperatures. Such processes are herein generally denoted
as
"cold-alkali process".
W02018/169479 discloses an example of a fiber produced by a cold-alkali
process.
The method comprises: providing a spinning dope comprising a solution of
cellulose
and an additive in an alkaline solvent, in which solvent cellulose is present
at a
concentration of from about 5 to 12 percent per weight by weight and the
additive is
present in the range of from 0.1 ¨ 10 percent per weight calculated on the
cellulose;
contacting the cellulose spinning dope with an aqueous coagulation bath fluid
having
a pH value above 7 and comprising a salt; forming a regenerated cellulosic
fiber
composition; and stretching and washing the fiber composition in one or more
washing and stretching baths.
EP3231901A1 discloses a similar process, wherein a spin dope is prepared by
dissolving cellulose in an aqueous NaOH solution. The spin bath comprises a
coagulation liquid comprising an aqueous sodium salt solution.
EP3231899A1 discloses a method for preparing a spin dope by direct dissolution
of
cellulose in cold alkali.
W02020171767A1 discloses a process for forming a fiber tow involving a wet
spinning procedure comprising the steps of: dissolving cellulose pulp in an
alkaline
aqueous solvent to form a cellulose spin dope composition, spinning the
cellulose
spin dope composition in a coagulation bath having a pH of more than 7.0,
preferably
a pH of at least 10, to produce a fiber tow, and passing the produced fiber
tow
through a sequence of consecutive stretching and washing steps in which the
formed
fiber tow is washed with a washing liquid by a counter-current flow washing
procedure.
Still, many aspects of the cold-alkali process are not fully understood and it
would be
desirable to find further means to improve the fiber quality and increase the
efficiency
of the production processes.
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Summary
The present disclosure describes methods and apparatus for producing
regenerated
fibers that are produced according to a cold-alkali process. It has been found
that the
properties of cold-alkali fibers, although they are being directly spun into a
5 coagulation bath, are very different compared to viscose or modal fibers
that use
comparable spinnerets. Nonetheless, cold-alkali fibers also differ a lot from
lyocell
fibers. Further to this, cold-alkali fibers show a strong tendency to
fibrillate. This
creates a need for new processing methods that can be used in connection with
the
cold-alkali process to improve fiber properties.
In a first aspect the present disclosure relates to a method for producing
regenerated
cellulosic fibers, the method comprising extruding a spinning solution into a
coagulation bath which contains a salt and preferably an alkali to produce the
fibers,
the spinning solution comprising cellulose dissolved in an aqueous solvent
comprising NaOH and ZnO, the coagulation bath having a pH-value of at least
seven,
wherein the method further comprises a continuous process of applying to the
fibers
in a never-dried state a crosslinking agent with two or more reactive groups
and
heating the fibers to a curing temperature while maintaining the never-dried
condition
to produce a reaction between the crosslinking agent and the cellulose of the
fibers.
The wet abrasion properties and the fibrillation tendency of a cold alkali
fiber is
reduced by crosslinking. It has been found that according to this protocol a
crosslinking can be effectuated on the cold-alkali fibers while mostly
maintaining the
mechanical properties of the fiber, in particular the tensile strengths and
the
elongation of the fibers can be retained. Further, an embrittling effect can
be avoided.
Embrittlement of the cold alkali fiber means that the elongation of the fiber
becomes
so low that processing of the fibers into yarn is difficult or even
impossible.
The term "crosslinking agent" as it is used herein, denotes a chemical reagent
whose
molecules contain a plurality of ¨ i.e. at least 2 and preferably up to 6 ¨
functional
groups capable of reacting with the hydroxyl groups in cellulose to form
crosslinks.
The molecules of the crosslinking agent may preferably belong to a substance
class
that can be selected from the group comprising triazines, pyrimidines,
acrylamides,
methacrylamides / haloacrylamides, vinyl sulfone precursors, vinyl sulfones,
epoxides, aldehydes, acetals, resins (e.g. methylol derivatives), carboxylic
acids,
isocyanates, thioisocyanates, aziridines / sulfonylaziridines, sulfates,
thiosulfates,
organosilanes, acrylates, vinyl ketones, inorganic acids, acrylate esters,
haloacetyls
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haloheterocycles, 2-step processes or mixed x-linkers (such as (poly)acrylic
acids
and phosphinates; periodate/amine, e.g. melamine; mixed x-linker substance
classes, such as triazine-Fvinyl sulfone, etc.).
The functional groups reactive with cellulose may be any of those known in the
art.
Numerous examples of such groups are, for example, given in the article
entitled
"Dyes, Reactive" in Kirk-Othmer, Encyclopaedia of Chemical Technology, 3rd
edition,
Volume 8 (1979, Wiley-Interscience) at pages 374-392. Dyes described therein
contain a chromophore system attached directly or indirectly to a unit which
carries
one or more functional groups reactive with the material to be dyed. The
chemical
reagents utilised as a crosslinking agent differ from reactive dyes in that
they do not
contain a chromophore and so are substantially colourless. Treatment with such
reagents in the absence of dyes therefore does not substantially alter the
colour of
the solvent-spun cellulose fiber. Accordingly, the treated fiber is suitable
for dyeing in
any manner known for cellulose fibers, yarns or fabrics. Nonetheless,
crosslinking
agents can also comprise a chromophore system, in which case they also act as
a
dyestuff.
Preferred examples for functional groups are reactive halogen atoms attached
to a
polyazine ring, for example fluorine, chlorine or bromine atoms attached to a
pyridazine, pyrimidine or sym-triazine ring. Other examples of such functional
groups
include vinyl sulphones and precursors thereof. Each functional group in the
reagent
may be of the same or a different type.
The crosslinking agent is preferably applied to the fiber in an aqueous
system, more
preferably in the form of an aqueous solution. The crosslinking agent may
contain
one or more solubilising groups to enhance its solubility in water. A
solubilising group
may be an ionic species, for example a sulphonic acid group, or a nonionic
species,
for example an oligomeric poly(ethylene glycol) or poly(propylene glycol)
chain.
Nonionic species generally have less effect on the essential dyeing
characteristics of
the cellulose fiber than ionic species and may be preferred for this reason.
The
solubilising group may be attached to the chemical reagent by a labile bond,
for
example a bond which is susceptible to hydrolysis after the chemical reagent
has
reacted with the cellulose fiber.
The person skilled in the art and having knowledge of the teachings disclosed
herein
is able to choose a suitable salt for use in the coagulation bath. The salt
facilitates a
coagulation of the spinning solution and preferably can be present in the
coagulation
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bath in a ratio ranging from 10 percent per weight to 30 percent per weight.
Preferably, the salt is a sodium salt, e.g. sodium carbonate or sodium
sulfate. Further
suitable salts can be chosen by taking into account the Hofmeister series
(also known
as the lyotropic series), which classifies ions in order of their
precipitation capacities.
The salt should, for one thing, allow for a quick coagulation and secondly, it
should
facilitate recovery and recycling of the compounds. Alternative, but less
preferred
coagulation sodium salts include sodium salts wherein the counter ion is a
carboxylate (e.g. formate, acetate, propionate, butyrate or benzoate), an
aliphatic or
aromatic sulfonate (e.g. benzenesulfonate, toluenesulfonate, or
nnethanesulfonate),
an aliphatic or aromatic phosphonate ion or mixtures thereof. Preferably, the
anionic
counter ion has a dense electric charge, placing it in the beginning of the
Hofmeister
series. Anionic counter ions having a dense electric charge are characterized
as
strongly "salting out" proteins, due to their ability to increase surface
tension and
organize water molecules in solvation shells around them. Further, the
coagulation
sodium salt is preferably a sodium salt precipitating as a hydrate. Preferably
the
molar ratio of water to sodium salt in the precipitated hydrate is at least
4:1.
Surprisingly it was found that the regenerated fibers that are produced
according to a
cold-alkali process may be crosslinked without the need to fully wash the
fibers after
the coagulation bath. According to the prior art the fibers first have to be
fully washed
to remove all residues coming from the coagulation bath.
According to a preferred embodiment, the crosslinking agent is applied to the
fibers
after the fibers have been partially washed, preferably to a pH of between 10
and 12.
Thereby, the inherent alkaline condition of the fibers can be utilized for
crosslinking.
The functional groups reactive with cellulose in the chemical reagents used in
the
present invention may react most rapidly with cellulose under alkaline
conditions and
reagents containing such groups may be preferred. Although crosslinking would
also
be possible with the freshly spun fibers before washing, partial washing of
the fibers
is preferred to facilitate alkaline recycling. Examples of functional groups
that are
preferably applied under alkaline conditions are halogenated polyazine rings.
According to another embodiment the crosslinking agent is applied on the fiber
after
the fiber has been essentially fully washed, preferably to a pH of between 5
and 11,
wherein the crosslinking agent is applied in combination with an inorganic
alkali.
According to a further embodiment, the crosslinking agent can be applied to
the
never dried fibers by directing the never dried fibers through an application
bath
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comprising the crosslinking agent. This is an efficient way to apply the
crosslinking
agent while the fiber is still uncut and conveyed in form of a fiber tow. In
addition
alkali can be added to the crosslinking agent in the same application system
or in
later system to increase the pH and effect a better crosslinker fixation to
the cellulose.
The heating of the fibers to a curing temperature while maintaining the never
dried
condition can preferably comprise one or more steps independently selected
from:
- heating a fluid comprising the crosslinking agent to a temperature above
the
curing temperature before applying the crosslinking agent to the fiber,
- steaming the fiber, preferably with low-pressure steam, after applying
the
crosslinking agent to the fiber.
In another embodiment the crosslinking agent can be a reactive dyestuff having
two
or more reactive groups. This allows for a combined crosslinking and dying.
According to another approach, the crosslinking agent and a monolinking
dyestuff
can be applied to the fibers. The crosslinking of the fibers can improve the
dying
process.
In another embodiment, a monolinking dyestuff can be added to the crosslinking
agent. This allows for crosslinking and dying in a single process step.
According to another embodiment, a water-soluble polymeric alcohol can either
be
incorporated into the fiber before the application of the crosslinking agent
or can be
applied as a component in the crosslinking agent. This can increase the dye
affinity of
the fiber.
In a second aspect, the present disclosure relates to a processing facility
for
producing regenerated cellulosic fibers comprising a spinneret for extruding a
spinning solution into a coagulation bath which contains a salt and preferably
an
alkali to produce the fibers, the spinning solution comprising cellulose
dissolved in an
aqueous solvent comprising NaOH and ZnO, the coagulation bath having a pH-
value
of at least seven, wherein the processing facility comprises a crosslinking
facility in
which a crosslinking agent with two or more reactive groups is applied to the
fibers in
a never-dried state, and wherein the crosslinking facility comprises a curing
facility for
heating the fibers to a curing temperature while maintaining the never-dried
condition
to produce a reaction between the crosslinking agent and the cellulose of the
fibers.
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The processing facility allows for the industrial implementation and scale-up
of the
methods disclosed herein.
In a preferred embodiment the crosslinking facility comprises an application
bath in
which the crosslinking agent is provided and applied to the never dried
fibers.
According to another embodiment, the processing facility further comprises a
washing facility upstream from the crosslinking facility.
In a third aspect, the present disclosure relates to regenerated cellulosic
fibers
produced in a processing facility as described herein and/or produced by a
method
as described herein. The fibers can meet enhanced quality standards, both in
view of
requirements for further processing steps as well as in terms of properties of
intermediate- and end products comprising the fiber.
In a preferred embodiment the fiber comprises the crosslinking agent in an
amount of
0,5 % per weight to 5 % per weight, preferably between 1 % per weight and 3%
per
weight, based on the weight of the dry fiber. On the one hand the amount of
crosslinking agent bond to the cellulose of the fiber should be high enough to
substantially reduce fibrillation tendencies, on the other hand too much of
the
crosslinking agent could lead to embrittlement of the fibers. The optimal
amount also
depends on the Molecular weight of the crosslinking molecule.
In another aspect the present disclosure relates to a product, particularly a
consumer
product or an intermediate product, comprising the regenerated cellulosic
fibers as
disclosed herein. Preferably, the product can be selected from a list
comprising
yarns, fabrics, textiles, home textiles, garments, nonwovens, hygiene
products,
upholstery, technical applications, such as filter material, paper.
Brief Description of the Drawings
Hereinafter, exemplary embodiments of the invention are described with
reference to
the drawings, wherein
Fig. 1 is a schematic and exemplified representation of a
fiber production
process according to the present disclosure focusing on the spinning
dope preparation,
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Fig. 2 is a schematic and exemplified representation of
processing facility
according to the present disclosure focusing on the post-processing of
the spun fibers and
Fig. 3 is a schematic representation of a part of the fiber-tow processing
5 facility according to another embodiment.
Detailed Description of the Drawings
Fig. 1 shows a flowchart representing an exemplary fiber production process
according to the present disclosure. The diagram is a simplified
representation and
shows the process in a schematized manner.
10 The process can be sectioned into the following basic steps, which are
denoted in
with roman numbers in Fig. 1:
I. Supplying the raw material
For the process according to the present disclosure a broad range of possible
cellulosic raw materials can be used. Generally the intrinsic viscosity and
the degree
of polymerization of the cellulose used as a raw material is lower than it is
common
for the viscose- or lyocell-process. For example dissolving pulp (kraft or
sulphite) with
an intrinsic viscosity (measured in Cuen, according to SCAN-CM 15:99) of about
200
mL/g to 700 mL/g (degree of polymerization DP of 500 to 1900), preferably
between
about 250 and about 400 mL/g (DP 01 600 to 950) can be used. Further,
recycling
pulp or cotton linters (preferably having the same DP as stated above) can be
used.
The recycling pulp can, for example, be derived from waste paper, recycled
viscose
textile material, recycled modal textile material, recycled lyocell textile
material an/or
recycled cotton fiber textile material. Blends of pulps of different origin,
such as
blends of virgin wood pulp with recycling pulp, are possible and may be even
desirable.
In Fig. 1 an example of a staple of dissolving pulp 1 is depicted as the raw
material
II. Pretreatment of the raw material
The cellulosic raw material can be subjected to a pretreatment, wherein the
degree of
polymerization is adjusted to a desired DP to adjust the viscosity of the
spinning dope
to a value that allows for filtering and spinning. The pretreatment can
comprise
subjecting the raw material to an acidic pulp treatment, wherein the DP-value
is
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mainly influenced by the duration of the pretreatment and the concentration of
the
acid. In other cases the pretreatment can be omitted, if the DP-value is
already at the
desired value. For example, pulp derived from cellulosic regenerated fibers
can have
a DP that allows for a direct dissolution without a pretreatment.
In a more specific example, an acidic pulp treatment with 1-10 percent per
weight
sulfuric acid at 50 C to 95 C for a duration from 5min to 2h can be used as a
pretreatment. As the profitability of the process is reduced by a long
duration of this
treatment step, it is generally preferable to minimize the duration of the
pretreatment
as far as possible. The person skilled in the art, who is aware of the
teachings of this
disclosure, is able to find suitable parameters and optimize them without
undue
burden.
The pretreatment further comprises washing the cellulosic material with water
and
pressing to reduce moisture content, e.g. to about 50 percent per weight of
the
cellulosic material.
In Fig. 1 a source for a pretreatment chemical 2, e.g. sulfuric acid, and a
pretreatment
vessel 3 are exemplarily depicted. After the pretreatment in pretreatment
vessel 3 the
cellulosic material can be squeezed and washed to reduce the amount of acid
that is
transported to the next step.
III. Preparation of the spinning dope
To prepare the spinning dope (also called spinning solution), the wet and
pretreated
pulp is first cooled to about 0 C (while freezing of the pulp should be
avoided), and an
aqueous solvent comprising NaOH and ZnO is prepared. Preferably the solvent is
adjusted to provide a spinning solution comprising 5 to 10 percent per weight
NaOH
and 0.8 to 3 percent per weight ZnO. The solvent is cooled down to a process
temperature, which preferably lies between -5 C and -10 C.
The pulp and the solvent are blended to dissolve the cellulose in the solvent.
To
improve the processability, the preparation of the spinning dope comprises a
mixing
step followed by a homogenization step. During the mixing step the blend is
mixed
with a high shear stress, which can be done in a high-shear mixer. This high
shear
stress mixing is preferably only performed for a rather short period of time,
for
example the mixing can be done for 1 ¨2 minutes. In the following
homogenization
step the blend is agitated with a lower shear intensity. The homogenization
step can
last longer than the mixing step, for example about 5 minutes.
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During both the mixing and the homogenization step the temperature of the
mixture is
controlled, especially cooled. Preferably the temperature is kept below 0 C.
The
process temperature should never exceed 5 C, as the solution could then
thicken
and be irrecoverably lost.
The so prepared spinning solution is then filtered and de-aerated. For
example, the
spinning dope can be filtered at least twice via a KK filter (Kolben-Korb-
Filter, Lenzing
Technik) with a mesh size of 15 micrometer.
For the de-aeration the spinning solution is exposed to reduced pressure. This
step is
per-se known from the viscose process. Other techniques for filtering and de-
aerating
the dope that can be used are known to the person skilled in the art.
The prepared spinning dope should be free of voids, have a homogenous
consistency and a proper viscosity that allows for an extrusion in the
spinneret used
in the following extrusion step.
In a preferred embodiment the ballfall-viscosity of the spinning dope should
be in the
range of about 30 to 200 s. The ballfall-viscosity can be measured according
to DIN
53015-2019. The viscosity of the spinning dope can be adjusted by several
different
means. For example, the viscosity can be adjusted by altering the DP-value of
the
cellulose, by changing the composition of the solvent and/or the concentration
of the
cellulose in the spin dope. For example, the concentration of the cellulose
can be in
the range of about 4 percent per weight to about 12 percent per weight,
particularly in
the range of about 5 percent per weight to about 8 percent per weight
preferably
about 6 percent to about 7 percent per weight.
The specific parameters of the mixing, homogenization and filtering steps can
be
found by the person skilled in the art, who is aware of the current
disclosure, by
routine work and experiments.
In Fig. 1 a chemical repository 4 for the storage of the ingredients of the
solvent, a
solvent cooling device 5 for the cooling of at least parts of the solvent, a
pulp cooling
device 6, a mixing vessel 7 and a de-aerating filter 8 are exemplarily
depicted. The
mixing vessel 7 is provided with a cooling jacket 9.
IV. Extrusion into the coagulation bath
The spinning dope can be extruded through a nozzle directly into a coagulation
bath.
In case additives are added to the spinning dope, the dope can be homogenized
via
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a static mixer to incorporate additives. Before the extrusion step, the dope
can
preferably be tempered to spinning temperature, for example to a temperature
in the
range of from 5 C to 30 C. For fiber production, a straightforward approach
could be
to use as the extrusion nozzle a spinneret comprising, for example, up to 150
cups
with a diameter of 12.5 to 16 mm, comprising up to 3000 holes with a diameter
of
about 40 to 75 micrometer, which corresponds to dimensions as they are known
per
se and commonly used in connection with the viscose spinning process.
Nonetheless, it was surprisingly found that in connection with the cold-alkali
process
broader diameters can improve process stability and facilitate the coagulation
and
stretching of the fibers. According to the present disclosure it is therefore
suggested
to use a spinneret comprising holes with a diameter of about 80-120 pm,
preferably
between 90 and 110 pm. For example, in an industrial scale production plant
one
spinneret could comprise up to 150 cups with a diameter of 12,5 to 16 mm,
comprising about 600 to 1400 holes with a diameter of about 80-120 pm,
preferably
between 90 and 110 pm. The relatively thick diameter of the spinning holes
causes
different course of coagulation, i.e. that the freshly extruded fibers first
only coagulate
at the outer surface, while the middle of the fiber stays in a liquid state
for a longer
time. This allows for a higher stretching and the stretching conditions can be
uphold
in a more stable way.
The coagulation bath comprises an alkali, preferably NaOH, and a salt,
preferably
sodium carbonate, Na2CO3, or sodium sulfate, Na2SO4.
As an example, the coagulation bath can comprise from 10 percent per weight to
30
percent per weight Na2CO3 or Na2SO4 and from 0 to 3 percent per weight NaOH,
preferably from 0.1 to 3 % and still more preferred from 0.2 to 0.7 percent
per weight
NaOH. In a specific example the coagulation bath can comprise about 22 percent
per
weight Na2CO3 and about 0.5 percent per weight Na0H. The temperature of the
coagulation bath can, for example, be adjusted to between 10 C and 30 C, and
preferably be tempered at about 20 C.
The optimal distance, that the freshly extruded fiber travels through the
coagulation
bath (i.e. the coagulation bath distance) depends, inter alia, on the
extrusion speed,
the pull-off speed, the composition and consistency of the spinning dope, the
composition of the coagulation bath and the temperature. Without being
restricted to
these values, that under most parameter conditions the optimal coagulation
bath
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distance may be found within a range from about 10 cm to about 100 cm.
Preferred
values for the coagulation bath distance range from about 15 cm to about 60
cm.
The fiber tow is drawn out of the coagulation bath to a transporting section,
which can
comprise several godets and/or guides that transport the fiber tow through a
series of
post-processing stages. The pull-off force that is exerted on the freshly
extruded
fibers can be regulated by the extrusion speed and the speed of the first
transporting
unit (or godet), which preferably can be positioned outside of the coagulation
bath.
Due to the pull-off force, which is exerted on the freshly extruded fibers by
the first
transporting unit, the fibers get stretched already inside the coagulation
bath. Further
stretching steps can be during the following post processing of the fibers.
In Fig. 1 a coagulation bath 10 comprising a coagulation liquid 11, a
spinneret 12 and
a first godet 13 are exemplarily depicted. The spinneret 12 extrudes a number
of
fibers 14 (corresponding to the number of holes of the spinneret 12) into the
coagulation liquid 11. The freshly extruded fibers 14 are gathered together
into a fiber
tow 15 by the first godet 13. By adjusting the extrusion speed at the
spinneret 12 and
the speed of the godet 13 the amount of stretching, that is done directly
after
extrusion within the coagulation bath 10 can be set. Although an inclined
angle of the
spinneret 12 (and the freshly extruded fibers 14) is shown in Fig. 1, the
skilled
practitioner, who is aware of the current teaching, is able to apply other
spinning
configurations that are per se known in the field, e.g. from viscose
production.
V. Post-processing of the fiber tow
As it is used throughout this disclosure, the term "post-processing"
encompasses all
processing steps that are performed on the extruded fibers after they have
been
withdrawn from the coagulation bath. Post-processing steps can be applied to
the
fiber tow while it is transported on the transporting unit. Additionally, the
fiber tow can
be cut in a cutting apparatus and further post-processing steps can be
performed on
the cut fibers.
In Fig. 1 the post-processing is only schematically represented by the
respective
reference sign V.
Post-processing of the fibers can comprise, but are not restricted to, any
combination
of one or more of the following steps:
- washing of the fiber tow and/or the cut fibers,
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- pressing the fiber tow and/or the cut fibers to reduce the amount of
liquid
therein,
- neutralizing the fiber tow and/or the cut fibers with an acidic liquid,
- bleaching the fiber tow and/or the cut fibers,
5 -
crosslinking the fiber tow and/or the cut fibers by applying a crosslinking
agent
on the fibers and curing it,
- applying a finishing agent ("soft finish") to the fibers of the fiber tow
and/or the
cut fibers,
- drying the fiber tow and/or the cut fibers.
10 Immediately after the fibers in the fiber tow have been withdrawn from
the
coagulation bath, they already have been stretched to a certain extent,
preferably to
about 20-30 % but have not reached their final elongation (and final cellulose
specific
diameter).
In a different approach, several successive stretching steps during the post-
15 processing can be implemented. For example a counter current flow
washing can be
implemented in the post processing, wherein the fibers in the fiber tow are
being
incrementally stretched during and/or in-between the several washing steps
until they
have reached their final extension.
According to another approach, the fiber tow can be led into a conditioning
bath
comprising from 10 percent per weight to 30 percent per weight a salt that
facilitates
a further coagulation of the spinning solution, the conditioning bath
preferably being
fluidly separated from any downstream washing facilities, and stretched to
essentially
the final cellulose specific diameter of the fibers and oriented to
essentially their final
state within the conditioning bath. The conditioning bath can comprise a
coagulation
liquid that is similar or identical to the coagulation bath liquid. The
coagulation speed
in the conditioning bath can be adjusted by the temperature of the liquid
therein,
which preferably can be controlled independently from the coagulation bath.
Following the second bath, the fiber tow can be washed in a downstream washing
line, where no additional stretching is applied to the fiber.
The term "fluidly separated", as it is used herein, denotes systems that are
either
associated to completely separated circulation systems, or that are connected
via an
installation that significantly changes the properties of the liquid, e.g. by
adding
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16
substances to and/or removing substances from the liquid or by concentrating
or
diluting the liquid.
The salt in the conditioning bath can preferably be identical to the salt that
is used in
the coagulation bath, or it can be chosen according to the same requirements
as the
salt in the coagulation bath that are outlined above.
As the case may be (and according to the technical requirements), other post-
processing steps can be arranged in the processing line according to any
technically
useful configuration.
Fig. 2 is a schematic block-diagram showing an exemplary configuration of a
post-
processing facility for treating a fiber-tow which is produced according to
the current
disclosure, e.g. by the facility depicted in Fig. 1.
Fibers 14 are extruded by a spinneret 12 into a coagulation liquid 11 within a
coagulation bath 10 and gathered together into a fiber tow 15 by the first
godet 13
(similar to Fig. 1). From the first godet 13 the fiber tow is directed to a
second godet
18. Between the first godet 13 an the second godet 18 the fiber tow 15 is
diverted via
a rotating or static guide 16, e.g. a roller or a bar", and submerged into a
conditioning
bath 17 containing a coagulation liquid 11'. The coagulation liquid can be
identical or
similar to the coagulation liquid 11 in the coagulation bath 10. Preferably
the
coagulation liquid 11 in the coagulation bath 10 and the coagulation liquid
11' in the
conditioning bath 17 are circulated in a common fluid cycle. Preferably the
temperature of the coagulation liquid 11' in the conditioning bath 17 can be
controlled
independently from the temperature of the coagulation liquid 11 in the
coagulation
bath 10. Generally a higher temperature is preferred for the coagulation
liquid 11' in
the conditioning bath 17. For example, the temperature of the coagulation
liquid 11 in
the coagulation bath 10 can be adjusted to a value between about 10 C and
about
20 C and the temperature of the coagulation liquid 11' in the conditioning
bath 17 can
be adjusted to a value between about 20 C and about 40 C.
Between the first godet 13 and the second godet 18 and essentially within the
conditioning bath 17 the fibers in the fiber tow are stretched to essentially
their final
cellulose specific diameter and oriented to essentially their final state.
The term "stetched to essentially their final cellulose specific diameter", as
it is used
herein, is to be interpreted to that effect that downstream of this stretching
step no
further stretching steps are preformed on the fiber tow, i.e. the diameter of
the fiber is
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17
held essentially constant until the fibers are either cut (after which a small
amount of
relaxation is unavoidable and sometimes even intended) or dried (where the
diameter
of the fibers as it would be actually measured is reduced due to the loss of
liquid,
generally without any change of the stretch of the fibers).
The term "cellulose specific diameter", as it is used herein, denotes a
diameter in a
virtually washed and dried state, i.e. only comprising the dry cellulose. One
example
of a cellulose specific diameter which is used in connection with fibers is
the fiber
titer, which is defined as the weight of the cellulosic contents of the fiber
per unit of
length.
The term "oriented to essentially their final state", as it is used herein, is
to be
interpreted to that effect that the molecular orientation of the cellulose in
the fibers is
not actively changed in downstream processing steps, i.e. remains constant,
apart
from minor changes that may occur naturally or are a (generally unwanted) side
effect of other downstream post-processing steps.
It was surprisingly found that stretching the fibers to their final cellulose
specific
diameter and state within the conditioning bath allows for an economic and
controllable production of fibers having adequate properties that allow, for
example, a
spinning of the fibers to yarn.
In Fig. 2 only one conditioning bath is shown. Nonetheless it would be
possible to
install more than one conditioning bath, for example two successive
conditioning
bathes or a series of consecutive conditioning bathes. Preferably the
conditioning
baths share the same fluid circuit with the coagulation bath and have an
essentially
identical or at least similar content of salt and/or alkali. The temperature
of the
conditioning baths can either be the same or controlled independently, as the
case
may be. Depending on the configuration, the fibers can, for example, be
stretched in
a cascading style, i.e. consecutive conditioning baths have an increasing
stretching
rate. The fibers could also be stretched to essentially their final state in
an upstream
conditioning bath (or several upstream conditioning bathes) and than be
further
coagulated and "fixed" within one (or more) downstream conditioning bath(s)
with
constant speed and stretch. The person skilled in the art and having knowledge
of the
teachings disclosed herein is able to optimize the number of conditioning
baths, their
temperatures and extension rated by routine tests and experiments without
deviating
from the scope of the current disclosure. The fiber parameters, such as
tensile
strength, elongation, crystallinity etc., can so be optimized in a methodical
manner.
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18
From the second godet 18 the fiber tow 15 is directed to a washing line 19
which can
comprise several washing steps which are exemplarily depicted in Fig. 2 as
washing
steps 20 and 20'. As the case may be, the washing line 19 can also comprise
only
one washing step 20 or any number of washing steps exceeding two. Further, any
washing techniques for washing fiber tows, that are known per se in the art,
can be
used for in the washing line 19.
The transporting means for the fiber tow, such as rollers and godets or the
like, in the
washing line are operated at a constant speed so that the tension is kept
essentially
constant and no further stretching of the fibers in the fiber tow occurs. This
also
keeps the orientation of the fibers essentially at the state they were when
leaving the
second godet 18 after the stretching within the conditioning bath.
After the washing line 19 the fiber tow 15 is directed to a cutter 21, which
cuts the
fiber tow into staple fibers 22. During the washing steps 20 the consistency
of the
fibers has sufficiently settled so that the fibers essentially keep their
cellulose specific
diameter, elongation and orientation even if they are cut in wet state.
Therefore, it is
not necessary to dry the fiber tow 15 before cutting, which can reduce costs
and
allows for the implementation of more efficient post-processing steps.
In the lower part of Fig. 2 an exemplary post-processing facility for the cut
staple
fibers is shown. The cut staple fibers are transported (or fall) from the
cutter 21 to a
fleece-forming device 23 having a basin 24 filled with a liquid, e.g. water,
and a
conveyer belt 25. The conveyer belt 25 is permeable to liquid and a current is
maintained in the basin that transports the fibers that are suspended in the
liquid of
the basin to the conveyer belt 25, where they are collected and form a non-
woven
fiber layer 26 on the top surface of the conveyer belt 25. The surface of the
conveyor
belt is tilted and transports the newly formed non-woven fiber layer 26 out of
the liquid
and to further transport equipment (which is, for reasons of conciseness, not
shown
in Fig. 2). The freshly cut staple fibers 22 should be regularly distributed
across the
width of the fleece-forming device 23 so that the non-woven fiber layer 26 has
a
uniform width and consistency.
After leaving the fleece-forming device 23, the non-woven fiber layer 26 is
squeezed
in a first pressing device 27a to remove some of the liquid in the non-woven
fiber
layer 26. Several further pressing devices 27b to 27f can be arranged
downstream
between several processing steps. Especially the first pressing device 27a,
but also
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the other pressing devices, create a natural crimp on the fibers in the non-
woven fiber
layer which is preferable for many fiber applications.
The post-processing that is performed on the non-woven fiber layer 26, as it
is shown
in Fig. 2, comprises a first washing facility 31, a crosslinking facility 30,
a second
washing facility 31', a neutralizer 32, a finishing facility 33, a dryer 34
and a baling
press 35. The crosslinking facility 30 comprises a first crosslinker
application 28, a
second crosslinker application 28' and a curing facility 29. Between the
different
stages the pressing devices 27a-27f are arranged to squeeze processing liquids
out
of the non-woven fiber layer 26.
The crosslinking facility 30 is shown in Fig. 2 with two crosslinker
applications 28 and
28'. Before entering the crosslinking facility 30 the fibers in the fiber
layer 26 are
squeezed by the first pressing device 27a, washed in the first washing
facility 31 and
squeezed again in the second pressing device 27b. This washing step reduces
the
amount of chemicals that are transported into the crosslinking facility 30 and
improves recycling of the process fluids. In the first crosslinker application
28 the non-
woven fiber layer 26 is impregnated with an aqueous solution comprising a
first
crosslinking agent. The solution is pressed out in the pressing device 27c and
in the
second crosslinker application 28' another chemical reagent can be applied to
the
fibers to improve the crosslinking reaction For example, an alkali can be
applied in
the second crosslinker application 28'. Depending on the properties of the
fibers after
the first washing facility 31 only one crosslinker application 28 could be
sufficient,
depending on the residues of alkali in the fibers.
The fiber layer 26 is again pressed in pressing device 27d and then fed into
the
curing facility 29, in which the fibers are heated to a curing temperature,
which
facilitates the reaction between the crosslinking agent and the cellulose of
the fibers.
The heating can be done by application of steam at a desired temperature.
After the crosslinking facility 30 the remaining crosslinking agent and alkali
is washed
out in the second washing facility 31'.
In the neutralizer 32 the fibers that may still contain residues of alkali may
be
neutralized with an acidic liquid. The acidic liquid may be selected from a
list
comprising diluted acetic acid, lactic acid, sulphuric acid or the like,
without being
restricted to this list. It is to be noted that depending on the previous
treatment steps
and on other parameters of the fibers this neutralization is not always
necessary.
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If appropriate, a further washing step (not shown in Fig. 2) can be
implemented after
the neutralizer 32 to remove residues of chemicals that can still be present
in the
fibers. The used water of this (and any other) washing step can be forwarded
to any
upstream water-consuming devices, such as the washing steps 20, 20' of the
5 washing line 19 and/or the cutter 21. In this way a a countercurrent
washing system
can be implemented.
In the finishing facility 33 a finishing agent or soft finish can be applied
to the fibers.
After dewatering the non-woven fiber layer 26 in the last pressing device 27g,
the
non-woven fiber layer 26 is fed into the dryer 34. Before drying, the non-
woven fiber
10 layer 26 can be loosend in an opener (not shown), which opens the
structure of the
fiber layer 26 to improve the drying efficiency in the following dryer 33 and
also to
improve the further processing of the finished staple fibers.
After the dryer, the staple fibers are pressed to bales in the baling press
35.
The post-processing facilities listed above can be implemented in any
technically
15 reasonable and useful order, and the person skilled in the art, being
aware of the
current teachings, is able to implement numerous configurations without
deviating
from the current disclosure. The post-processing can also comprise other
processing
steps known in the art, such as, for example, a bleaching step or a dying
step.
A dying step can for example be combined with the crosslinking step, either by
using
20 dyestuffs that also act as crosslinking agents or by using dyestuffs
that show
improved staining properties after the application of a crosslinking agent.
In Fig. 2 the post-processing steps of crosslinking, neutralizing, finishing
and drying
are done on the never-dried fibers in the non-woven fiber layer 26, i.e. after
cutting of
the never-dried fibers. Nonetheless, some or even all of these steps can also
be
applied on the fibers before cutting, i.e. on the still uncut fiber-tow. Such
embodiments are well within the scope of the present disclosure and the
skilled
practitioner, who is aware of the teachings herein, is able to implement such
configurations.
In another embodiment, the treatment of the fibers, i.e. all the washing,
crosslinking,
curing, neutralizing, finishing, etc. steps can be done on the uncut fiber tow
and also
the fibers can be dried while still in the fiber tow. In this case a crimp can
be applied
on the fibers, for example by stuffer-box-crimping, and the fibers can then be
supplied
as tow for stretch breaking applications or cut to staple fibers and pressed
into bales.
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Fig. 3 shows the washing line 19, 19' of an alternative processing facility,
wherein the
crosslinking step is done on the still uncut fiber tow 15. The crosslinking
facility 30 of
Fig. 3 comprises a crosslinker application 28, which has the form of an
application
bath into which the fiber tow is submerged, and a curing facility 29, which
heats the
fiber-tow 25 after application of the crosslinking agent to improve the
reaction
between the crosslinking agent and the cellulose of the fibers in the fiber
tow. The
fiber tow 15 is led to the crosslinking facility 30 after having been directed
through a
first washing line 19, which performs at least one washing step 20 on the
fiber tow.
After the first washing line 19 the fibers in the fiber tow 15 have only been
partially
washed, for example to a pH of between 10 and 12, i.e. the fibers still
contain an
amount of alkali that is adjusted to improve the effect of the crosslinking
agent.
After applying and curing the crosslinking agent, the fiber tow is led to a
second
washing line 19', wherein the remaining chemicals, for example the excess
crosslinking agent and the remaining alkali, are further washed out of the
fiber tow
15. Any further processing steps can be arranged in a technically feasible way
upstream or downstream of the washing and crosslinking line shown in Fig. 3.
Execution examples
In the following paragraphs several examples for putting the teachings of the
current
disclosure into practice are disclosed.
EXAMPLE 1
The fibers can be produced according to the cold-alkali process disclosed
herein, for
example with a titer of 1.3 dtex. After extrusion and stretching of the fibers
the fibers
are partially washed to a pH of about 11 and then passed through an aqueous
bath
containing 1,3,5-triacryloyl ¨hexahydro-1,3,5-hexahydrotriazine (TAHT). This
bath is
maintained at steady state (TAHT ca. 15g/1) and at a temperature of ca. 50 C
by
addition of solid TAHT to the circulating liquor using an in-line high shear
mixer/pump.
The fibers are squeezed in a nip before being exposed to saturated steam for a
treatment time that allows for curing the crosslinking agent. The fibers are
then
washed and dried.
EXAMPLE 2
The fibers can be produced according to the cold-alkali process disclosed
herein, for
example with a titer of 1.3 dtex. After extrusion and stretching of the fibers
the fibers
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are essentially fully washed within the fiber tow. The never dried and uncut
fibers are
passed through an aqueous bath containing 1,3,5-triacryloyl-hexahydro-1,3,5-
hexahydrotriazine (TAHT) and trisodium phosphate (TSP). This bath is
maintained at
steady state (TAHT ca. 15g/1 and TSP ca. 15g/1) at a temperature of ca. 50 C
by
addition of solid TAHT and TSP to the circulating liquor using an in-line high
shear
mixer/pump. The fibers are squeezed in a nip before being exposed to saturated
steam for a treatment time that allows for curing the crosslinking agents. The
fibers
are then washed and dried.
EXAMPLE 3
Fibers can be produced according to the cold-alkali process disclosed herein,
for
example with a titer of 1.3 dtex. After extrusion and stretching of the fibers
the fibers
are essentially fully washed within the fiber tow. The washed fiber tow is
passed into
a first impregnation bath into which 8.3 weight% of p-[(4,6-dichloro-1,3,5-
triazin-
2-yl)amino]benzenesulfonic acid, sodium salt (SDTB) on cellulose was dosed
into an
aqueous circulation system at a temperature of 35 C to give a concentration of
10-
40g/1 of an aqueous paste-like suspension. The fiber tow is then pressed to a
moisture content of ca. 200%, and then passed through a second impregnation
bath
at 10 C into which 1.3% of NaOH and 1.1% of Na2CO3 on weight cellulose are
dosed
into an aqueous circulation system.
The fibers are then pressed to a moisture content of ca. 200%, heated to 100 C
in a
steaming chamber for 8.5 minutes and then washed thoroughly by adding acidic
water (pH 3.5) and then by water until they are free of excess chemicals and
then
dried.
Reference signs:
dissolving pulp 1
source for a pretreatment chemical 2
pretreatment vessel 3
chemical repository 4
solvent cooling device 5
pulp cooling device 6
mixing vessel 7
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de-aerating filter 8
cooling jacket 9
coagulation bath 10
coagulation liquid 11
spinneret 12
first godet 13
fibers 14
fiber tow 15
guide 16
conditioning bath 17
second godet 18
washing line 19
washing step 20
cutter 21
staple fibers 22
fleece-forming device 23
basin 24
conveyer belt 25
non-woven fiber layer 26
pressing device 27
crosslinker application 28
curing facility 29
crosslinking facility 30
washing facility 31
neutralizer 32
finishing facility 33
dryer 34
baling press 35
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