The invention pertains to a process for spinning fibers or filaments
from a spinnable solution using a centrifuge of which the wall has one or more
spinning orifices, in which process the spinning solution is jetted from the centrifuge
into a coagulant inside a jacket.
Such a process is known. In Japanese laid open patent application
JP 27021/79 it is described how an optically anisotropic spinning solution of a
para-aramid, e.g., poly(paraphenylene terephthalamide), is spun with the aid of
a centrifuge. Four examples serve to explain how the solution is introduced into
a centrifuge having 25 or 50 spinning orifices of 0.08 or 0.1 mm in diameter and
extruded through the spinning orifices at a rotational speed in the range of 70
to 1000 revolutions per minute (rpm). The solution then ends up in a coagulant
flowing downward at 2 or 5 cm distance from the centrifuge. The coagulated fibers
are collected batchwise and washed for 24 hours. The properties of the resulting
fibers are such as will give them a certain commercial value.
Such a process has a low productive capacity and high times of passage,
int. al., because the fibers are processed batchwise.
One way of increasing the productive capacity consists in raising
the centrifuge's rotational speed. However, doing so has other highly disadvantageous
effects, which accounts for the comparatively low rotational speeds in the examples
of the aforementioned patent application. The maximum rotational speed at which
fibers of fair quality can actually be spun using the above-described technique
is of the order of 1000 rpm. Rotational speeds in excess of this recommended value
produce an unacceptable number of fiber breaks. Moreover, aerosol is formed between
the centrifuge and the coagulant flowing along the jacket. Such conditions produce
poor and irregular fiber properties (tobacco-like appearance) as well as a dangerous
and contaminated working environment due to the aerosol often containing a strong
Fiber properties have to satisfy ever higher demands. In a conventional
wet spinning process, such as described in US 4,320,081, the resulting fibers have
properties substantially superior to those of the fibers obtained by the process
according to the aforementioned Japanese patent application (higher strength and
modulus). A conventional wet spinning process employs a large number of spinning
orifices per spinneret (say, 1000), so the productive capacity is high also. However,
because of the comparatively low winding speed (some hundreds of meters per minute),
which is comparable to the productive capacity per spinning orifice, and the process's
high susceptibility to foreign substances in the spinning solution (requiring thorough
filtration and shutting down of the process when one or more of the spinning orifices
has clogged up), this process also produces an expensive product. Especially when
it is to be processed into pulp, which is used, e.g., as friction and packing material,
such a fiber is really too expensive.
In other words, what is wanted is a process having a higher productive
capacity than the existing wet spinning processes and by means of which fibers
can be made which are less expensive and possess comparable or superior properties
for a particular purpose, such as pulp. Preferably, it should be possible to spin
less pure spinning solutions and spinning solutions made of already somewhat coagulated
polymers by means of such a process.
These objectives are attained using the process according to the invention,
by a process for spinning fibers or filaments from a spinnable solution using a
centrifuge of which the wall has one or more spinning orifices and in which process
the spinning solution is jetted from the centrifuge into a coagulant inside a jacket,
characterized in that the angular velocity of the centrifuge multiplied by the
inner radius of the jacket is higher than 20 m/s.
Preferably, the inner radius of the jacket is at least 35%, more preferably
at least 50% wider than the radius of the outer circumference of the centrifuge
and does not exceed 350% or, more preferably, 200%.
It was found that this makes it possible to substantially increase
the rotational speed of the centrifuge, even to 5000 rpm or higher per minute.
Further, the process according to the invention allows larger draw ratios and the
average fiber length can be set arbitrarily, so that the production of endless
filaments also becomes possible.
The formation of aerosol (when using liquid coagulants) has reduced
significantly, probably because the fibers hardly disturb the coagulant surface
as they are laid.
It should be noted that Korean patent specification KR 9208999 discloses
a process for manufacturing staple fibers of polyaramid in which liquid-crystalline
prepolymers are fed to a rotary apparatus and then extruded as a dispersion through
the spinning orifices in the wall of the apparatus. In other words, this is not
a case of a spinnable solution of a prepared polymer. The prepolymers end up in
a polymerization promoting medium flowing downwards along the wall of a vessel.
The diameter of the vessel is 1.1 to 5.0 times that of the rotary apparatus. The
process is hard to control because it requires not only good fiber spinning, coagulation,
and discharge, but also a proper polymerization process and the satisfactory conclusion
thereof. Moreover, the staple fibers obtained have a low tensile strength and a
structure that is more critical to fibrillate.
It has proven possible to enhance the fiber properties and the productive
capacity of the process not only by centrifugally spinning a spinnable solution
with the angular velocity of the centrifuge multiplied by the inner diameter of
the jacket exceeding 20 m/s, but also by selecting a proportionally large jacket
The product of the angular velocity of the centrifuge (in rad/s) and
the inner radius of the jacket (in m) will be referred to as "take-off speed" (in
Preferably, the take-off speed is higher than 40 m/s, or even higher
than 60 m/s and lower than 600 m/s, more preferably lower than 400 m/s.
Within the framework of this invention, the term "spinnable solution"
is used to denote solutions of a polymer that can be converted into man-made fibers
or filaments by extrusion and subsequent solidification. Preferably, the spinnable
solutions are made by dissolving a prepared polymer in a suitable solvent.
In addition to the solutions of polymers mentioned in JP27021/79,
the term "spinnable solution" comprises, int. al., solutions of meta-aramid, cellulose,
and cellulose derivatives.
Preferably, the spinnable solution exhibits optical anisotropy. Solutions
are considered to be anisotropic if birefringence is observed in a condition of
rest. Generally speaking, this holds for measurements carried out at room temperature.
However, within the framework of the present invention solutions which can be processed
at temperatures below room temperature and which display anisotropy at said lower
temperature are considered anisotropic also. Preference is given to solutions that
are anisotropic at room temperature.
Visual determination of the isotropy or anisotropy is performed with
the aid of a polarization microscope (Leitz Orthoplan-Pol (100x)). To this end
about 100 mg of the solution to be defined is arranged between two slides and placed
on a Mettler FP 82 hot-stage plate, after which the heating is switched on and
the specimen heated at a rate of about 50C/min. In the transition from
anisotropic to isotropic, i.e., from colored to black, the temperature is read
off at virtual black.
With a strength greater than 13 cN/dtex, of even greater than 20 cN/dtex,
an elongation of 2-5%, and a modulus of 40-50 GPa, fibers of poly(paraphenylene
terephthalamide) spun at take-off speeds of higher than 20 m/s are comparable
with fibers spun by means of a conventional wet spinning process. Moreover, they
were found to be highly suitable for making pulp, even more suitable in fact than
fibers obtained by means of a conventional wet spinning process (see Examples,
especially Table 3).
It is also observed - perhaps unnecessarily - that the invention also
has the aforementioned advantages at low rotational speeds, although in that case
the productive capacity will be low also.
Surprisingly, it has been found that because of the combination of
reduced fiber breaks (or even no fiber breaks at all) and the increased productive
capacity now available, the fibers which "fall" from the bottom of the jacket at
the same time as the coagulant can be joined together to form a sliver. The two
parameters, i.e., a sufficient number of fibers and a sufficient fiber length,
play a major part in the cohesion of such a sliver. If because of a high productive
capacity (sufficient fibers) and reduced fiber breaks or no breaks at all (long
fibers) the sliver has sufficient cohesion, it can be neutralized, washed, dried,
and cut in a continuous process.
One example of a product that can be manufactured directly from said
sliver is cigarette filters. By spinning a solution of cellulose acetate into a
nitrogen atmosphere (in this case the coagulant is a gas), the solvent evaporates,
resulting in a solidified sliver which can be made directly into cigarette filters.
Holding good irrespective of the end product (textiles, composites,
packings, brake shoes, and the like) is that the difference between the inner radius
of the jacket and the outer radius of the centrifuge (the so-called airgap) preferably
is more than 7 cm.
Centrifuges having a diameter of more than 20 cm and less than 60
cm are highly suited to be used in the process according to the invention. Such
a centrifuge is large enough to guarantee good productive capacity, yet small
enough to keep the construction of the spinning machine simple.
The rotational speed of the centrifuge preferably is in the range
of 1000 to 5000 rpm. As was stated earlier, a rotational speed of less than 1000
rpm makes for a too low productive capacity. Good fibers can still be made at rotational
speeds exceeding 5000 rpm. However, at such speeds the process is less easy to
control, and the centrifuge is subjected to high mechanical load.
In addition, the centrifuge is preferably provided with means (such
as a so-called viscous seal) which permit the spinning solution to be supplied
under pressure. This makes it possible to enforce a spinning solution throughput,
which will improve the controllability of the process, especially of the draw ratio.
It will also make for improved safety, since the spinning solution, which often
contains strong acid, can only exit through the spinning orifices, where it is
collected by the jacket and discharged in the usual manner.
The number of spinning orifices is not essential in itself and can
be selected on the basis of common considerations (sufficient space between the
spinning orifices, risk of filament or fiber sticking, productive capacity). In
the process according to the invention, the number will generally be in the range
of 40 to 1000, but a number of, say, 10000 is not ruled out (especially for centrifuges
with a large diameter).
The diameter of the spinning orifices plays an important part in the
centrifugal spinning process according to the invention. As this diameter increases,
the risk of clogging as a result of foreign substances in the spinning solution
is reduced, so that less thorough filtration is required. Moreover, when the diameter
is larger, it is possible to spin a spinning solution made wholly or in part of
polymer which is already somewhat coagulated, for instance residual products of
the spinning process.
As was stated earlier, pulp made of fibers produced by the process
according to the invention has favorable properties. This is evident, int. al.,
from the high strength of products made of this pulp. Surprisingly, it has been
found that these properties can be enhanced still further by increasing the diameter
of the spinning orifices. It is for these reasons that the diameter of the spinning
orifice or spinning orifices preferably exceeds 30 µm. Optimum results are obtained
when the diameter is greater than 120 µm and smaller than 500 µm.
The properties of pulp made in this way are superior to those of pulp
made of fibers produced by a conventional wet spinning process, and the pulp is
also much less expensive. The reason for the superior properties is not fully known,
but it is a fact that fibers made by the process according to the invention have
a number of features not previously observed. For instance, it has been found
that the fibers have a number of elongated and/or spherical voids (with a diameter
usually in the range of about 30 - 40 % of the fiber diameter and a volume fraction
relative to the total fiber volume ranging from, e.g., 0,1 - 0,2). In addition,
contrary to what the person skilled in art would expect, the polymer structure
at and beneath the fiber surface is essentially the same as the polymer structure
in the fiber core, and the fiber diameter range (linear density range) is wider
with a larger spinning orifice diameter. A larger average linear density, higher
than 2 dtex and preferably higher than 4 dtex, was also found to have a favorable
effect on the pulp properties in the case of fibers made by a process according
to the invention.
It should be noted that fibers having a linear density smaller than
2 dtex are by no means excluded from the scope of the invention since these finer
fibers are very suitable for, e.g., textile purposes.
The invention will be further illustrated below with reference to
an embodiment depicted in the figure and a number of examples. The figure shows
a schematic cross-section of a construction suitable for use in the process according
to the invention, but, needless to say, the invention is not restricted to such
A centrifuge 1 having a diameter of 30 cm is connected to a feed pipe
2 for the spinning solution. At the point where the centrifuge 1 changes over to
the feed pipe 2 there is a seal 3 (a so-called viscous seal). The centrifuge 1
is made of stainless steel and is double-walled in order to keep the spinnerets
9 (which are made of a 70/30 Au/Pt alloy) at a particular temperature by having
a hot liquid flow around them. A number of spinnerets 9 is spaced out evenly across
the circumference of the centrifuge. Each spinneret 9 has several spinning orifices.
The spinning orifices are made up of a conical section (inflow) and a cylindrical
section (outflow), and the ratio of the overall height of the spinning orifice
to the diameter of the cylindrical section is 1.5. Provided around the centrifuge
1 is a jacket 4 with an inner diameter of 50 cm. The jacket 4 is made of polyvinyl
chloride (PVC) and has an annular channel 5 at the top. Connected to this annular
channel are feed pipes 6 through which the coagulant can be supplied. If there
is a supply of coagulant, it will fill up the annular channel 5. The coagulant
cannot leave the annular channel 5 except through the orifice 7, which is also
annular. Depending on the width of the orifice 7 and the quantity of coagulant
supplied, a curtain or film 8 will form on the jacket 4. After extrusion through
the spinnerets 9 the fibers or filaments end up in the coagulant. The coagulant
ensures that the fibers or filaments reach the solid state and also sees to their
discharge. At the open bottom of the jacket 4 is placed a slanting receptacle 10.
The receptacle 10 is tapered, and at the end the water from the receptacle 10 flows
to a drain. The sliver, which has become somewhat narrower because of this tapering,
is passed to the washing plant.
Example 1 - Fibers of pure polymer
a) Preparation of the pure polymer
As specified in the procedure disclosed in Example 6 of US 4,308,374,
poly(para-phenylene terephthalamide) (PPTD) was prepared using a mixture of N-methyl
pyrrolidone and calcium chloride. After neutralization, washing, and drying a polymer
was obtained which had an inherent viscosity of 5.4.
b) Preparation of a spinning solution of the pure polymer
The solvent used was sulfuric acid in a concentration of 99.8%. The
solution was prepared as specified in Example 3 of US 4,320,081. The final PPTD
content of the spinning solution was 19.4%. The spinning solution exhibited optical
c) Centrifugal spinning of the spinning solution
The spinning solution was spun in the set-up described above. The
selected coagulant was water having a temperature of 15°C and a volume throughput
of 3000 l/hour. The outer diameter of the centrifuge being 30 cm and the inner
diameter of the jacket being 50 cm, the so-called airgap was 10 cm. The inner
radius of the jacket was 67% wider than the outer radius of the centrifuge. The
number of spinning orifices was 48. The sliver was discharged, neutralized, washed,
and wound in a continuous process under all of the aforementioned conditions.
The other parameters (Rotation = rotational speed, Dorf = diameter
of the spinning orifices, Press = excess pressure in the centrifuge, Through =
mass throughput of the spinning solution, Draw = draw ratio of fibers or filaments)
are listed in Table 1. In addition, it should be noted that in this example the
excess pressure in the centrifuge is a so-called output parameter, which is independent
of the rotational speed and the throughput set.
Example 2 - fibers made from spinning process residuals
a) Preparation of a spinning solution of spinning process residuals
330 g of coarsely ground spinning process residuals were fed to an
IKA duplex kneader in two portions at an interval of about 5 minutes. There was
kneading in vacuo at 87°C for half an hour, after which 18.4 g of
sulfuric acid (99.8%) were added. Then there was another half hour of kneading,
after which all of the spinning solution was melted. The calculated aramid content
b) Centrifugal spinning of a spinning solution
A spinning solution prepared in accordance with a) was spun in the
set-up described above, except that an open centrifuge was employed. The temperature
of the coagulant was 13°C, the number of spinning orifices was 300. The other parameters
are listed in Table 1, experiment no. 15.
Example 3 - fibers having a high filament count
The spinning solution of Example 2 was spun under the conditions specified
for said example, except that the number of spinning orifices was 72. The other
parameters are listed in Table 1, experiment no. 16.
Example 4 - fibers having a low filament count
The spinning solution of Example 1 was spun under the conditions specified
for said example, except that the number of spinning orifices was 144. The other
parameters are listed in Table 1, experiment no. 17. After being spun, the fibers
of this example were dried with an apron drier at a temperature of 90°C for 3
minutes to a moisture content of 8%.
Example 5 - fibers spun at high throughput
The spinning solution of Example 1 was spun under the conditions specified
for said example, except that the number of spinning orifices was 576. The coagulant
consisted of water containing 17.2 % sulfuric acid and the inner diameter of the
jacket was 60 cm (i.e., 100% wider than the outer radius of the centrifuge). The
other parameters are listed in Table 1, experiment no. 18.
Example 6 - fibers spun at high rotation
The spinning solution of Example 1 was spun under the conditions specified
for said example, except that the number of spinning orifices was 60. The other
parameters are listed in Table 1, experiment no. 19. The term 'Draw' in Table 1
is used to denote the calculated (by dividing the take-off speed by the speed of
the solution in the spinning orifice) draw ratio.
Take-off sp. m/s
The filament strength of Examples 5, 12, 14, and 19 was measured in
accordance with ASTM/DIN D2256-90 giving 13.75, 15.24, 14.20, and 20.00 cN/dtex
Example 7 - Processing of the sliver into pulp
The slivers obtained according to Examples 1, 2, 3, 4 and 5 and four
samples of fibers obtained via a conventional wet spinning process (experiment
nos. v1 - v4) after being neutralized and washed were passed to a cutter (Neumag
NMC 150) and cut up into pieces of 6 mm in length. The pieces were fibrillated
in a refiner and pulped. Both the pulp and a gasket made of said pulp have exceptionally
favorable properties, cf. Tables 2 and 3, respectively. (SR = Schopper-Riegler
number, SSA = specific surface area, AL = average fiber length, WL = weighed fiber
length, GP = gas permeability, Ql = gasket strength in longitudinal direction of
the fibers, Qw = gasket strength in transverse direction to the fibers, Sieve =
sieve fraction, Wet dens. = wet density. Note: measuring techniques with regard
to pulp properties have not been standardized yet. Where possible, the measuring
methods employed derive from the paper industry (TAPPI standards)).
Wet dens. ml
When determining the suitability of pulp as raw material for gasket
or friction material, the Qw and sieve fraction parameters are especially important.
Qw is normative as to the strength of such materials, because it is always lower
than Ql. The sieve fraction is a direct measure of the pulp's particle retaining
capacity, so providing an indirect indication of the cohesion of the material in
the finished product (packing, brake shoe, etc.). The tables show very clearly
that the pulp quality improves with increasing take-off speed. At high take-off
speeds this quality even surpasses that of pulp made of fibers from a conventional
wet spinning process.