Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR MANUFACTURING FILAMENTS FROM AN OPTICALLY
ANISOTROPIC SPINNING SOLUTION
The invention pertains to a process for manufacturing filaments from an
optically anisotropic spinning solution in which the spinning solution is
extruded through spinning orifices grouped in at least one spinning section
and in which the extrudates are passed through an inert gas and a
coagulation bath in succession.
Such a process is known from Japanese laid-open No. 1986-239012,
which describes a method of spinning filaments from poly(para-phenylene-
terephthalamide) (PPTA) where the filaments are spun through spinning
orifices grouped to form a rectangle. The ratio of the rectangle's long side
length to its short side length has to be at least 4. In the coagulation bath
a
hole, also rectangular, is provided beneath the spinning section. Since both
the spinning section and the hole in the coagulation bath are rectangular,
the bundle of filaments is rectangular also. As a result of this rectangular
shape of the bundle, very few vortexes are created in the coagulant, a
portion of which is discharged from the coagulation bath together with the
filaments. This leads to a substantial reduction of filamentation in the
coagulation bath (where the filaments are not yet fully coagulated) and
makes it possible to increase the spinning speed.
In the examples of said Japanese patent specification filaments of good
strength are made. This strength is to be attributed first of all to the
coagulant's low concentrations of sulphuric acid (0 and 10%) and the
' wideness on average of the spacing of the spinning orifices (the so-called
pitch). The low acid concentration, which can only be maintained by
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treating the coagulant and replenishing it, and the large pitch, which makes
it necessary to employ a large apparatus in relation to the number of
produced filaments, make the described process into an expensive one
with a very large waste stream.
Furthermore, at high spinning speeds there will have to be a
subatmospheric pressure beneath the coagulation bath, this in order to
further accelerate the speed of the coagulant and so reduce the tension in
the' filaments.
If in the process according to the Japanese patent specification the pitch of
the' spinning orifices is reduced in order to increase their number {and
hence the number of filaments) per unit of area, the filaments in the
coagulation bath will stick together at the spinning speeds mentioned,
rendering the end product unsuitable for use in the envisaged high-grade
applications (e.g., woven fabrics or composite reinforcement).
An object of the invention is the provision of a process enabling the high-
speed spinning 0300 m/min) of a plurality of filaments having good to very
good physical properties. According to the invention, in the process as
described, the spacing of the spinning orifices (referred to as the pitch) has
a ratio to the width of the spinning section of more than 0.15 and less than
0.7, and the width of the spinning section is less than 5 mm.
Preferably, that ratio (which is easily calculated by dividing the pitch, in
millimeters, by the width of the spinning section, also in millimeters) is in
the
range of 0.20 to 0.55, the spinning section has a width in the range of 1.5
to f~ mm, and the pitch is in the range of 0.3 to 0.7 mm. Also, the spinning
section is preferably rectangular.
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Surprisingly, it was found that using this process makes it possible to
manufacture filaments having good physical properties at a small pitch (and
hence a large number of filaments per unit of area) at a comparatively high
acid concentration in the coagulation bath, resulting in an economical
process with a small waste stream. As can be seen from the example, the
number of stickings occurring during the process (from filaments making
contact before there has been sufficient coagulation of the outer shell) is
low.
The process according to the invention makes it possible to use a
comparatively compact spinning apparatus or to equip existing spinning
apparatus with spinneret plates with a higher number of spinning orifices.
For instance, the production of an existing spinning apparatus can be
increased from 1000 to 2000 or 3000 filaments per spinning position.
The favourable results are probably attributable to the low resistance
experienced by the coagulant as it flows to the core of the filament bundle
(alternatively, this may be referred to as high filament bundle permeability).
The resistance depends on the route to be travelled, i.e., half of the width
of the filament bundle, and the space between the various filaments (the
pitch).
Preferably, the spinning orifices are grouped in more than one spinning
section. The separate sections can then be positioned vis-a-vis one
another such as to ensure the least possible hindrance of the coagulant's
approaching flow and the fullest possible avoidance of disturbing the
coagulation bath.
Also, the separate spinning sections preferably are positioned such that the
maximum space between the outermost fibres is relatively small at the
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moment of extrusion from the spinning orifices of the different spinning
sections, so that the convergence to, say, a guide may be low.
One highly effective way of positioning the rectangular spinning sections
takes the form of the spinning sections being distributed equidistantly over
a circle, with the longitudinal direction of each of the spinning sections
coinciding with a radius. Such positioning hinders the approaching flow of
the coagulant hardly (if at all) and gives a low convergence for each of the
filament bundles.
To further reduce convergence in the filament bundle or filament bundles it
is preferred to provide the bottom of the coagulation bath per spinning
section with a rectangular opening which has a greater length than the
spinning section and is somewhat narrower in width. In that case neither
the length nor the width of the opening in the bottom of the coagulation
bath will give rise to filament bundle convergence, and the filaments are
prevented from being pressed together or suffering damage from scraping
along the edge of the opening.
The physical properties of the filaments obtained by the process according
to the invention can be enhanced still further by selecting a range for the
distance travelled by the threadlike extrudates through the gaseous inert
medium (the air gap) of more than 0.5 mm and less than 8 mm.
When very small air gaps are employed (say, smaller than 2 mm), there is
a risk of the coagulant, which will always display some motion under the
influence of the filament bundle (vibrations, small waves, etc.), making
contact with the spinneret plate. When this happens, the process may be
disturbed to such a degree as will require it to be stopped. Hence, if very
small air gaps are to be used, it is of the essence to have the calmest
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possible coagulation bath surface. Surprisingly, it was found that the extent
to which the coagulation bath surface is in motion is highly dependent on
the geometry of the coagulation bath's bottom. If use is made of more than
two spinning sections and a corresponding number of discharge openings
5 in the bottom of the coagulation bath, the extent to which there is motion
at
the coagulant surface can be reduced substantially by introducing
variations in height in or on the bottom. A very simple and effective
embodiment of this is the one where the edges of adjacent openings are at
different heights ("on different levels"). A possible explanation of this
phenomenon is given below.
At the edges of the discharge openings the liquid which is entrained by the
outgoing filament bundle is stopped or scraped off. Because of inertia, the
liquid retains (part of) its speed and flows parallel to the bottom in the
direction of the adjacent discharge opening. However, coagulant flow
approaches also from the direction of this adjacent discharge opening,
resulting in the collision of streams flowing in opposite directions. The
liquid
is pushed up as a result, and the coagulation bath surface rises above this
stagnation point. Obviously, the damming up of the coagulant constitutes a
significant restriction when selecting the air gap; after all, the coagulant
has
to be prevented from making contact with the spinneret plate.
When the aforementioned streams come together at different levels, the
disclosed damming up does not arise. On the contrary, because the speed
of one of the streams (i.e., the one flowing from the lowest edge) already
has a component going in the direction of the liquid surFace, there is
extinction and the liquid surface remains calm.
" When the coagulation bath has a depth of more than 10 mm and less than
20 mm (preferably less than 15 mm), on the one hand the filaments
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encounter only slight resistance in the bath and the use of coagulant is low,
and on the other hand the residence time in the coagulation bath is long
enough to achieve the required coagulation.
It should be noted that EP 172 001 discloses a process for spinning aramid
yarns in which use is made of rectangular spinning sections of narrow width
and a small pitch. However, this process is substantially different from the
process according to the invention, since the coagulant is not contained in
a bath but supplied in the form of a waterfall. Because of the strong current
in the waterfall and the small number of rows of filaments, the resistance
encountered by the coagulant in the filament bundle does not play an
essential part.
The process according to EP 172 001 involves very high coagulant
consumption. Moreover, in the examples only water (0% sulphuric acid) is
employed. As a result, the (very large) stream of coagulant has to be
subjected to extensive post-treatment and/or neutralisation.
It should also be noted that in Japanese laid-open No. 1985-065110 a
process is described which uses a spinneret plate having twenty spinning
sections each with fifty spinning orifices. The pitch is 1.5 mm, giving a
small number of filaments per unit of area.
The coagulant used in the process in question is water containing 0% or
10% of sulphuric acid, so this process is likewise attended with a large
waste stream.
It is noted that FR-A-1 102 056 (filing date June 16, 1947) discloses a very
small spinneret with a large number of spinning orifices. Such spinnerets
can only be used in true wet spinning processes, i.e., those spinning
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processes which do not comprise an air gap (for instance, viscose
spinning) and wherein the extruded filaments are immediately contacted
with the coagulant and coagulated. True wet spinning processes therefore
are not confronted with filament sticking and problems occurring at the free
surface of the coagulant. Further, in said publication it is prescribed that
if
the spinning orifices are grouped in spinning sections, the width of the
groups should not exceed two orifices, whereas the invention allows
greater widths.
EP-A-0 168 879 pertains to a process involving the use of two or more
separate, spaced spinning sections. The sections according to EP-A-0 168
879 are rather large and filaments obtained with this process leave much to
be desired in terms of mechanical properties and yarn regularity, especially
if the process is carried out at high speed.
Within the framework of the invention the term pitch is used to indicate the
average distance between the spinning orifice centres of adjacent spinning
orifices.
The invention will be further illustrated below with reference to an example
and figures. Needless to say, the invention is illustrated but not limited by
this example.
Figure 1 shows a bottom view of a spinneret according to the invention
provided with eight rectangular spinning sections.
Figure 2 shows two of the eight spinning sections of the spinneret
according to Figure 1 in greater detail.
' Figure 3 shows a bottom view of a spinneret serving as comparative
example.
Figure 4 shows one of the spinning sections of the spinneret according to
Figure 3 in greater detail.
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EXAMPLE
In an analogous manner to the procedure described in Example 6 or US
4,308,374 poly(para-phenyiene terephthalamide) was prepared using a
mixture of N-methyl pyrrolidone and calcium chloride. After neutralisation,
washing, and drying a polymer having an inherent viscosity of 5.4 was
obtained.
The polymer was dissolved in sulphuric acid of 99.8% concentration in the
manner described in Example 3 of US 4,320,081. The thus prepared
spinning solution had a polymer concentration of 19.4%.
Tlhe spinning solution was spun using different spinnerets.
A first circular spinneret 1, depicted in Figures 1 and 2, having an outer
diameter of 57 mm (in the Table this spinneret is indicated with the code
S1) was provided with eight rectangular spinning sections 2 (2.58 mm wide,
indicated with 3 in Figure 1, and 9 mm long) each having 125 spinning
orifices 4. The spinning orifices 4 had a diameter of 65 Nm and a distance
of one to the other (pitch) 5 of 0.5 mm (the ratio of the pitch 5 to the width
3
of the spinning section 2 thus was 0.2).
A second circular spinneret 6, depicted in Figures 3 and 4 (in the Table this
spinneret is indicated with the code S2), serving as a comparative example,
had an outer diameter of 57 mm and was provided with four spinning
sections 8 (having a constant width 7 of 9.5 mm) each following the curve
of the circumference of the circular spinneret and each comprising 250
spinning orifices. The spinning orifices had a diameter of 65 Nm and a
distance of one to the other 9 of 1.0 mm (the ratio of the pitch 9 to the
width
7 of the spinning section 8 thus was 0.11 ).
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The spinning solution was spun through an air gap, as indicated in the
Table. The same-level or flat bottom of the coagulation bath (having a
depth of 10 mm) was provided with eight and four openings, respectively
(S1: rectangular 2.0 mm x 15 mm; S2: circular with a diameter of 5 mm)
each positioned directly beneath a spinning section.
The coagulant was made up of water having a sulphuric acid concentration
of 20% and a temperature of 10°C. The spinning speeds and the draw
ratios are indicated in the Table. The physical properties were determined
in accordance with ASTM D885.
The term fluffs is used to indicate various irregularities (resulting from
breaks, filament lapping around rolls, etc.) in the manufactured yarn.
The de~reeof stickin~was evaluatedvis~tally.1irsdic~tes that- there
little or no sticking (less than 1 % of the filaments subject to sticking), 5
indicates a very strong degree of sticking (over 25% of the filaments
subject to sticking).
25
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TABLE
Spinning rate (m/min) 300 400 300 400 300
5 Draw ratio 7.1 9.5 7.1 9.5 7.1
Spinneret S1 Tenacity Fluffs Sticking
(mN/tex): per 15 min): (-):
10 Air gap: 3 mm 2218 2162 0 1 1
4 mm 2179 2143 1 0 1
6 mm 2181 2177 0 0 1
8 mm 2158 2032 2 1 1
Spinneret S2
Air gap: 8 mm 1912 1879 5 40 4
8 mm 1864 1873 1 34 4
8 mm 1902 1955 5 33 4
8 mm 1921 1953 4 6 4
The filaments manufactured using S1 have significantly higher tenacity
than those made using S2. Also, the number of stickings is far lower.
Furthermore, in view of the available space, the number of spinning
sections in a spinneret such as S1 can be increased to, say, 12 or 16,
whereas S2 provides no such opportunity.
A third circular spinneret (S3; this spinneret, unless specified otherwise,
corresponds to S1) having an outer diameter of 75 mm was provided with '
eight rectangular spinning sections (2.58 mm wide and 18 mm long) each
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having 250 spinning orifices, giving 2000 filaments in all. The spinning
orifices had a diameter of 65 Nm and were spaced 0.5 mm apart.
Spinneret S3 was used to spin the spinning solution described above
(under conditions which, unless specified otherwise, correspond to those
disclosed above) employing an air gap of 6 mm and a spinning speed of
300 mlmin. The resulting yarn had a tenacity of 2202 mN/tex. The number
of fluffs per 15 minutes was 4, and there was no sticking.
