Background art A spinneret of the type defined above is known from EP-A-0,756, 025. That document discloses a spinneret with several flat perforated plates of metal which each have several holes for the spinning of filaments. The perforated plates in that case have been fitted on all sides in a frame section of stainless steel. Preferably, the frame section of EP-A-0,756, 025 has a flange projecting outwards on the end opposite the perforated plates.

EP-A-0,700, 456 proposes a spinning head for spinning cellulose filaments from a solution of cellulose in a solvent, the spinning head having a frame element and a plate with holes supported thereby, through which holes the solution passes and forms filaments. The holes in a first area of the plate lying closest to the frame element have a larger diameter at their narrowest point than in another area of the plate which is further removed from the frame element.

These two prior documents describe spinnerets which contain spinning plates or nozzle plates with very large numbers of holes, but which nevertheless are comparatively easy to make and, because of their high pressure stability, should guarantee good spinning behaviour even in the case of high-viscosity spinning solutions.

With increasing numbers of holes for spinning the filaments and the increases in throughput associated therewith, however, other problems may occur. As is known, prior to spinning, the cellulosic starting material is dissolved in an appropriate solvent at elevated temperature, generally at about 70 to 130°C. Within the framework of the present invention, the preferred solvent is a tertiary amine N-oxide and, optionally, water. The solution of cellulose in the tertiary amine N-oxide and, optionally, water is extruded in the hot state with the aid of a spinneret and is formed (shaped) in the extrusion process. The solution shaped in this way is passed to an aqueous coagulation bath, where the tertiary amine N-oxide is extracted from the shaped solution and the cellulose precipitates in the shaped state. Between the spinneret and the coagulation bath cooling and drawing of the shaped solution generally takes place in a so-called air gap. The cooling can take place with only the aid of the ambient air, but generally cooling takes place with the aid of an additional blown-in gaseous medium, in most cases with blown-in air.

The greater the number of filaments being spun at the same time and the higher the spinning rate, the more crucial it is that the cooling process within the air gap takes place quickly and efficiently, in order to achieve good spinning uniformity and hence a good quality of the filaments obtained. Especially when there is a blowing in of a gaseous medium (e. g. air) at right angles (transversely) to the direction of travel of the shaped solution, it has to be guaranteed that even the filaments which are on the opposite side of the shaped solution from the side where the gaseous medium is blown in will still have the blown-in gaseous medium flowing well around them. Of course it is possible, and has also been put into practice, to increase the gas flow, i. e. the volume of gaseous medium blown in per unit of time. However, besides the increased cost due to the apparatus connected with such a measure, the practice often also generates a non-uniform spinning pattern, e. g. as a result of turbulence, and this has a negative effect on the quality of the filaments obtained.

Disclosure of the invention The present invention seeks to provide a spinneret for spinning cellulosic filaments from a cellulose solution in a solvent, which spinneret, especially at high throughput and high speed, ensures a good uniformity of the filaments and at least reduces problems associated with the prior art spinnerets.

Surprisingly, this object is met by a spinneret of the type described in the opening paragraph, which is characterised in that the nozzle plates located in the frame have the shape of a quadrilateral of which the respective opposite sides are of the same length and parallel to one another and of which the contiguous sides form an angle different from 90°.

Preferably, the frame and the nozzle plates are made of metal, for example of stainless steel.

The spinneret and the nozzle plates or perforated plates located therein thus have a quadrilateral shape, with the nozzle plates forming quadrilaterals which do not possess only right angles. The term quadrilateral is used in the broad sense here, that is to say it of course also includes geometric structures which have one or more (up to four) "rounded"corners. In such cases the two contiguous sides which are connected to one another by means of such a rounding should notionally simply be extended linearly until at their point of intersection they once again form a"sharp"corner, the angle of which can be determined in the usual manner. Quadrilaterals where the opposite sides are of the same length and parallel are called parallelograms in geometry. Further properties of parallelograms are that the angles opposite to one another are equal in size and that the diagonals bisect one another.

This definition covers a whole range of quadrilateral geometric structures, for example also rectangles and squares. The two last-mentioned figures, however, have only right angles and their diagonals are of the same length. What is remarkable about the spinneret of the present invention, however, is that the nozzle plates have the shape of quadrilaterals or parallelograms which do not have only right angles, that is to say they have two or preferably four angles which differ from 90°. In the terminology of the present invention, by"contiguous"sides in a parallelogram are meant sides which come together to form an angle."Opposite"sides, on the other hand, do not come together and thus do not form an angle together.

Although the spinneret defined above already sufficiently satisfies the object of the invention, it is preferred that the quadrilaterals constituting the frame and the nozzle plates located in this frame have the shape of elongate parallelograms, with the contiguous sides of the parallelograms having different lengths, the individual nozzle plates being located in the frame with their respective longer sides parallel to one another and with their respective shorter sides parallel to the longer sides of the frame.

Such an embodiment is for example provided by means of a rectangular frame of which the length is greater than its width. The respective opposite sides of the frame are of the same length and parallel, the contiguous sides, however, are of a different length. Within the frame the individual nozzle plates, for which likewise the respective contiguous sides are of different lengths, may be so arranged that the nozzle plates are parallel to one another and parallel to the respective longer sides. Relative to the frame surrounding them, the nozzle plates in this preferred embodiment are positioned such that they are aligned with the respective shorter sides parallel to the longer sides of the frame. In this case it is especially desirable if the shorter sides of the adjacent individual nozzle plates are aligned with one another.

Of course, the nozzle plates have holes for the spinning of filaments. These holes are preferably arranged in the nozzle plates in such a way that they form at least one row which runs parallel to at least one of the sides of the quadrilaterals constituting the nozzle plates. It is even more preferred if the holes for the spinning of filaments are arranged in the nozzle plates in such a way that they form a plurality of rows which run parallel to one another and parallel to the respective shorter sides of the quadrilaterals constituting the individual nozzle plates. Advantageously, the holes are positioned in the nozzle plate in such a way that they form rows parallel to one another, which run parallel to the shorter and longer sides of the quadrilaterals constituting the nozzle plates. To be able to distinguish more readily between them, the rows of spinning holes which run parallel to the shorter sides of the nozzle plates can be called"rows", while the rows of spinning holes which run parallel to the longer sides of the nozzle plates can be called"columns". Arranged in rows and columns and at the same time parallel to the boundary sides of the nozzle plates, the spinning holes thus form a matrix structure.

An especially favourable arrangement of the spinning holes within the individual nozzle plates and at the same time of the nozzle plates within the frame in which they are located is obtained when the individual nozzle plates are located in the frame in such a way that an imaginary line which is drawn perpendicular to one of the longer sides of the frame and which runs through a corner hole of a nozzle plate, that is to say the hole positioned furthest into one of the corners of the quadrilateral constituting this nozzle plate, likewise runs through a corner hole of the nozzle plate adjacent to it. The corner holes are those spinning holes closest to the acute angles formed by the contiguous sides of the quadrilateral forming the nozzle plate. Thus, in one embodiment of the present invention an imaginary line perpendicular to the longer side of the frame in which the nozzle plates are located could be drawn in such a way that it runs through the outermost hole of the first row of spinning holes in a nozzle plate A. When the line is continued, then in this example it also runs through the outermost hole of the last row of spinning holes of the nozzle plate B positioned immediately adjacent to the nozzle plate A and aligned parallel to its longer sides.

The nozzle plates thus are at an incline vis-a-vis the shorter sides of the frame in which they have been arranged. It is preferred that the longer sides of the individual nozzle plates are tilted towards the shorter sides of the frame by from about 2° to about 8°, for instance about 4°, away from the perpendicular. Because of this tilted arrangement a very even flow around the filaments in the air gap is achieved. This also holds true for the filaments which are on the opposite side to the blowing-in side, i. e. furthest removed therefrom.

The invention also provides a spinneret for spinning cellulosic filaments from a cellulose solution in a solvent, the spinneret having a plurality of quadrilateral nozzle plates each of which has a plurality of holes for the spinning of filaments, the nozzle plates being located in a frame surrounding them on all sides, characterised in that the holes for the spinning of filaments have each been so arranged within the nozzle plates that they form one or more rows which are tilted towards the sides constituting the nozzle plates.

In this case the nozzle plates may be aligned straight throughout, i. e. with a tilt of 0° towards the sides of the frame in which they are located. The holes for the spinning of filaments form one or more rows within the nozzle plates. These rows, or an imaginary line drawn through these rows, in this case show (s) a tilt towards the sides of the nozzle plate, preferably of about 2° to about 8°, for example of about 4°, away from the perpendicular.

When the holes are arranged in several rows parallel to one another, then they form a hole pattern which may have the shape of a quadrilateral of which the respective opposite sides are of the same length and parallel and of which the contiguous sides form an angle which differs from 90°. In this case the holes themselves thus can be arranged in such a way that they form a hole pattern within the nozzle plates in which they have been placed which takes on the shape of a parallelogram, as already described. In this case the parallelogram is formed by the imaginary connection of the respective outer rows and columns of the holes for the spinning of filaments. Such a spinneret has the advantage that it can be obtained also by modification of an already existing spinneret where the nozzle plates and the rows of holes are not tilted towards the frame in which they have been placed. This can for example be done by means of a simple sealing of holes already present, for instance by welding them shut, in such a way that the remaining holes in the nozzle plates form rows corresponding with the required tilt towards the sides forming the nozzle plates.

The number of nozzle plates located within the frame ordinarily is not subject to any restrictions. However, for the spinnerets of the invention it is preferred when up to 100, preferably 30 to 60, nozzle plates are located within a frame.

There is as little restriction with respect to the number of holes in the nozzle plates. As s rule, however, it is preferred when the individual nozzle plates in the case of the spinnerets claimed have from 10 to 1000, preferably from 20 to 300, more preferably from 30 to 90, holes for the spinning of filaments.

The invention furthermore includes a nozzle block which contains a eatable top housing, a screen packing, a breaker (distributor) plate, and a spinneret according to the invention. Advantageously, the nozzle block is designed to be supplied by only one spinning pump, i. e. the supply of the cellulose solution to the nozzle block takes place with a single pump. Each nozzle plate within the spinneret in that case corresponds to one thread or multifilament composed of the number of filaments resulting from the number of spinning holes in this nozzle plate.

As a rule, the spinning mass (dope) is filtered before it is conveyed to the spinning block. In the filtering process candle filters, for example metal wool filters with a fineness between 5 and 100 um, have proved useful. The preparation of cellulosic dopes in appropriate solvents, e. g. tertiary amine N-oxide and, optionally, water, is known to the skilled person and is described for instance in WO 98/06754 and the literature cited therein, so that it does not need any further elucidation here.

The eatable top housing of the nozzle block can be heated, for example by means of low-pressure steam heating.

Before the dope reaches the spinneret, it is advantageously led through a screen packing, which may for instance be made up of a braided fabric of metal with a fineness between 15 and 40 urn. This screen packing lies directly on a breaker plate, which is followed by the actual spinneret, which consists of the above-described frame and the nozzle plates. The nozzle plates have desirably been welded into the frame. The nozzle block is, for example, made of stainless high-grade steel.

The eatable top housing of the nozzle block serves to provide even distribution of the dope over the entire length and width of the spinneret. In this process the dope may be carried to the centre of the top housing, for instance via a flexible metal tube. The volume of the top housing is preferably kept small, because the dope at elevated temperatures and longer residence times has a tendency towards decomposition reactions. On the other hand, the residence time must be long enough to keep the dope at a constant temperature over the entire length and width. In this way it is ensured that the dope stream is very uniform. Every hole in the nozzle plate thus receives the same amount of cellulose solution arid the resulting filaments or threads have very high uniformity, which becomes noticeable especially with regard to dye uptake or uniformity in dye absorption (affinity).

The skilled person is in a position to determine the dimensions of the top housing through simple experiments and corresponding rheological calculations.

Underneath the top housing there is the breaker plate with the wire gauze lying thereon.

The wire gauze or screen packing serves for a final filtration before the spinneret and protects the relatively fine spinning holes in the nozzle plates from dirt contamination.

The holes for the spinning of filaments preferably have a diameter from 30 to 200 m, more preferably from 60 to 130 lim. Furthermore, the flow-pressure drop caused by the wire gauze serves to increase the dope uniformity as regards pressure, temperature and homogeneity over the length and width of the entire spinneret.

The breaker plate likewise serves to make the dope uniform as regards pressure, temperature and homogeneity over the length and width of the entire spinneret as well as to support the wire gauze.

In a preferred embodiment, the breaker plate is made of a highly thermally conductive material. Unlike in the case of the commonly used breaker or support plates, the temperature of the dope can be made uniform even at right angles (transversely) to the direction of flow and thus across all spinning positions when highly thermally conductive materials are used. It is preferred in that case to make use of materials for the breaker plate of which the specific thermal conductivity is above about 50 W/ (m*K), preferably above about 80 W/(m*K). Examples of such materials are silicon carbide (about 100 W/ (m*K)), certain copper alloys, e. g. CuZn 38 Sn 1 (about 135 W/ (m*K)), or else even aluminium alloys, such as AIMg 1 Si 0.5 (about 200 W/ (m*K)).

As was stated earlier, the nozzle plates are generally welded individually into the frame.

The nozzle plates of the spinneret according to the invention preferably are flat and have a thickness in that case of from 1 to 3 mm, preferably about 1.5 to 2 mm, and are designed for pressures above about 60 bar.

Because of the geometrically favourable construction of the spinneret according to the invention as well as of the nozzle block which contains this spinneret, it is possible to produce in a very economical manner a large number of cellulosic multifilaments with at the same time good quality and process stability. This applies especially for spinning rates of the filaments of more than about 500 m/min, preferably more than 800 m/min.

In principle, there is no restriction on the attainable spinning rates. Even at rates of 1,500 to 2,000 m/min filaments of very good quality are still obtained.

Description of the drawings The invention is further described with reference to the accompanying drawings in which Figure 1 is a schematic figure which shows a nozzle block containing an embodiment of the spinneret according to the invention in cross-section, and Figure 2 is a schematic figure which shows an embodiment of the spinneret according to the invention in plan view from above.

Fig. 1 shows a nozzle block with an inlet 1 for the dope. The dope is supplied to the centre of a eatable top part 2 (top housing) of the spinning block. Connected to the top housing 2 is a wire gauze 3, which is situated on a breaker (distributor) plate 4.

Quadrilateral nozzle plates 5 are placed in a eatable nozzle frame 7 and are separated from one another by lands 6. These lands 6 at the same time serve as reinforcement for the breaker plate 4.

In Fig. 2 a spinneret containing the nozzle frame 7 and the nozzle plates 5 is shown in top view. Furthermore, rows 8 of holes for the spinning of filaments and columns 9 of these spinneret holes are shown. It can be seen that the rows 8 of these holes run in each case parallel to the shorter sides of the nozzle plates 5 while the columns of the holes for the spinning of filaments run in each case parallel to the longer sides of the nozzle plates 5. The columns 9 and rows 8 of the holes for the spinning of filaments form an elongate parallelogram. In the embodiment of the spinneret according to the invention represented by Fig. 2, the nozzle plates and the holes therein are not so arranged-as especially preferred-that an imaginary line drawn perpendicular to one of the respective longer sides of the nozzle frame 7 and running through a corner hole of a nozzle plate 5 likewise runs through a corner hole of the adjacent nozzle plate 5.

The arrangement of the holes for the spinning of filaments nevertheless makes possible an excellent approaching flow to all filaments during the spinning process.