WO1996031267A1

WO1996031267A1 - Filter pack intended for filtering a liquid polymer, and a spinning head comprising such a pack

Info

Publication number: WO1996031267A1
Application number: PCT/NL1996/000139
Authority: WO
Inventors: Remmelt Bakker
Original assignee: Stork Veco B.V.
Priority date: 1995-04-03
Filing date: 1996-04-01
Publication date: 1996-10-10
Also published as: AU5125696A; NL1000015C2

Abstract

Filter pack intended for the filtration of a liquid polymer, in particular prior to fibres being spun from said melt, which comprises a filter disc made of a metal perforated plate (1), which plate (1) comprises essentially conical sieve holes (3) having a hole dimension between 1 and 100 νm and more than 100 holes (3) per linear cm, while the thickness of the perforated plate (1) is at least 50 νm, the perforated plate (1) during filtration of a liquid polymer being oriented in such a way that the hole dimension increases in the flow direction of the liquid polymer. Moreover, a spinning head for spinning polymer fibres is described which comprises such a filter pack.

Description

Filter pack intended for filtering a liquid polymer, and a spinning head comprising such a pack.

The present invention relates to a filter pack intended for the filtration of a liquid polymer, in particular prior to fibres being spun from said melt, at least comprising a number of stacked filter and supporting discs. The term liquid polymer refers to a melt or solution of one or more polymers.

Such a filter pack is known from NL-A-79 05055. The disclosed filter pack comprises alternately filter discs and supporting discs, flow of a liquid to be filtered being able to take place between the core and the outer shell of the filter pack. To this end, the supporting filters are in each case successively closed at an opposite side, i.e. the inner circumference and outer circumference, respectively, so that a liquid to be filtered flows, via a supporting disc, essentially parallel to the plane of said disc and then through an adjacent filter disc. Thereafter the liquid can flow into a subsequent supporting disc and then flow away parallel to the plane of said supporting disc. The supporting discs consist of such a coarse supporting gauze that they do not act as a filter for the liquid in question and can be perfused by the liquid in the direction essentially parallel to the plane of the supporting gauze.

The filter discs disclosed are made, for example, from filter gauze which acts as a filter for the liquid in question. A particular application of such a filter pack is the filtration of a polymer melt prior to said polymer melt being processed by spinning for the purpose of manufacturing fibres.

Said filter pack has a number of drawbacks. In order to keep the pressure drop across the filter pack sufficiently small it is necessary for a relatively large number of filter discs, alternating with supporting discs, to be employed, which increases the dimensions of the pack, makes the pack expensive and complicates cleaning. Furthermore, filter discs made of metal gauze become plugged relatively rapidly by the filtered impurities. Additionally, setting the mesh number by means of such filter packs, in particular if these are to be suitable for filtering very small particles, cannot be done accurately. In other words, it is not possible to set an accurate threshold of the dimensions of the particles to be kept back by the filter.

Filter packs made of a coarse supporting gauze, with a number of layers of metal gauze thereon, serving as a filter disc, are likewise known, yet have the same drawbacks. The object of the present invention is to provide a filter pack which does not have the above entioned drawbacks, and to this end is characterized in that the filter pack comprises a filter disc made of a metal perforated plate, which plate comprises essentially conical sieve holes having a hole dimension between 1 and 100 μm and more than 100 holes per linear cm, while the thickness of the perforated plate is at least 50 μm, the perforated plate during filtration of a liquid polymer being oriented in such a way that the hole dimension increases in the flow direction of the liquid polymer.

The metal perforated plate according to the invention has a larger number of holes per linear centi¬ metre than materials from the prior art. The present holes are also characterized in that they are dimensioned accurately. Together with the structure, deformable only with great difficulty, of the perforated plate, said accurate holes ensure absolute filtration, which is not possible, with these hole dimensions, with any other material from the prior art. By suitable choice of the hole dimension an absolute threshold can be set for the filtration. This means that all particles larger than the hole dimension are intercepted controllably in the perforated plate, which has a very positive effect on fibre quality. At the same time, with said absolute upper limit the risk of the spin plate becoming blocked is drastically reduced, which is of great benefit to the service life of a spinning position.

With the aid of the filter pack according to the invention, the entire filter pack comprising the large number of filter discs can be replaced by just one filter disc, made of a metal perforated plate, with a supporting disc.

Owing to the large number of holes per linear cm and the associated hole dimension, together with the conical shape of the holes and the thickness of the perforated plate, a very low initial pressure drop over the pack is obtained during use, and the service life of such a filter is very long. The pressure drop will increase only very gradually as a filter cake builds up on the surface of the perforated plate. Any small lumps present in the liquid polymer, such as, for example, gel particles in a polymer melt, will in many cases, owing to the flow of the polymer around said particles, dissolve or break off. Owing to the conical shape of the sieve holes it is furthermore virtually impossible for material to nestle in the perforated plate and plugging is avoided.

Perforated plates having conical sieve holes and the fabrication thereof are known per se and have been disclosed, for example, in EP-A-0 038 104 and

EP-A-0 049 022 in the name of STORK SCREENS B.V. Apart from said overgrowth method, the perforated plates employed with the present invention may also be fabricated in other ways such as, for example, etching, laser engraving etc. It will be clear that the sieve hole dimension is chosen in accordance with the material to be filtered off, which may be present in a liquid polymer to be filtered.

Preferably, the perforated plate comprises at least 400 holes per linear cm and more preferably at least 600 holes per linear cm.

Advantageously, the open area of the sieve is as large as possible in order to keep the pressure drop across the sieve as small as possible and to maximize service life. The open area depends not only on the number of holes per linear cm and the hole dimension, but also on the arrangement of the holes. A close-packed arrangement, that is the so-called hexagonal arrangement, is preferred in this case. In a particular embodiment, the ratio between the thickness of the perforated plate and the difference between the largest and the smallest diameter of the sieve holes is at most 20, preferably at most 16 and most preferably between 5 and 13.

The supporting disc can be fabricated from a variety of sieve materials, but advantageously comprises a metal gauze. Said supporting disc serves to support the filter disc, but if used in a spinning head also has the function of preventing the distributor plate from closing the orifices in a subsequent metal plate.

The supporting disc need not per se be of disc-type character, since a number of wires, which may or may not be stretched in parallel, can fulfil the same, suitable function.

The metal gauze may, for example, comprise woven, knitted or welded metal gauze.

Advantageously, the filter disc is made of nickel. The invention further provides a spinning head for spinning polymer fibres, at least comprising a polymer infeed, a filter pack and spinnerets, said spinning head being characterized in that the filter pack is a filter pack according to the invention.

By the use of the filter pack according to the invention in a spinning head it is possible for fibres to be manufactured with a greater strength than similar fibres according to the prior art, and as a result it becomes possible to use fibres having a smaller diameter. The filter pack according to the invention provides not only better filtration, but the polymer molecule chains are also oriented. Owing to the very small hole dimensions in the perforated plate and the conical shape thereof, the polymer chains during filtration are oriented in the flow direction and will, in this oriented manner, pass through the spinnerets and thus be spun while oriented.

In this context it is beneficial for the filter pack to be situated near the spinnerets, in order to prevent possible reorientation of the polymer chains after filtration. The invention will hereinafter be explained in more detail with reference to the accompanying drawing, in which:

Figure 1 shows a schematic cross-section of a perforated plate in a filter pack according to the invention;

Figure 2 shows an embodiment of a filter pack according to the invention;

Figure 3 shows an embodiment of a filter pack according to the invention and also a distributor plate; Figure 4 shows a simplified representation of a spinning head with a filter pack according to the invention; Figure 5 shows a graph which displays the difference in pressure drop increase of a filter pack according to the invention and a filter pack according to the prior art; and Figure 6 shows a graph which displays the difference in pressure drop increase of a filter pack according to the invention and a filter pack according to the prior art.

Figure 1 schematically shows a cross-section of a nickel perforated plate 1 with dams 2 and sieve holes 3. Flow takes place, towards the sieve holes in the direction of the arrow A, of polymer melt to be filtered, and 4 schematically indicates a portion of a filter cake consisting of impurities.

This perforated plate has 600 holes per linear cm and has a thickness of 75 μm. The ratio between the thickness D of the perforated plate and the diameter difference (dl - d2) in this case is 11. The sieve holes are arranged hexagonally, as a result of which the pressure drop across the filter is very low. The conical sieve holes have the advantage that no nestling of material such as impurities in the sieve material is possible, and the sieve material can be cleaned very simply, by reversing the flow or rinsing with a cleaning agent. Figure 2 shows a filter pack according to the invention, comprising a perforated plate 1, as shown in Figure 1, and a supporting disc which in this case is constructed in the form of a metal gauze 5, both the perforated plate 1 and the supporting disc being clamped by means of a clamping ring 6.

The shape of the filter pack according to the invention is not particularly restricted and may have any shape desired, depending always on the intended use, but generally its shape will be circular.

Figure 3 shows an embodiment of the filter pack 7 according to the invention in combination with a distributor plate 8 comprising flow ducts 9, which serves to improve the flow towards subsequent spinnerets. The entire assembly of filter pack 7 and distributor plate 8 is indicated by reference numeral 10.

Figure 4 shows a simplified cross-section of a spinning head 11 for spinning fibres 12, said spinning head 11 comprising an assembly 10 including a filter pack 7 according to the invention. The spinnerets are indicated by reference numeral 13.

As a result of the perforated plate according to the invention being used, firstly, a complicated filter pack according to the prior art is avoided. Filter performance is improved. A longer service life is achieved. And the polymer chains are oriented in the flow direction, so that these also reach the spinnerets 13 in an oriented state, as a result of which fibres 12 can be spun which comprise oriented polymer molecular chains, which affords higher-quality fibres.

Figure 5 shows a graph including two curves (pack 1 and pack 2) , the time in weeks being plotted along the x- axis, whereas the pressure drop across the pack is plotted along the y-axis. Pack 1 is a pack according to the prior art, comprising a wire gauze pack having a diameter of 50 mm with the following layers of steel wire gauze: 20/60/100/200/60 holes per linear cm, with a smallest aperture of 20 μm. Pack 2 is a pack according to the invention with a diameter of 50 mm and is a combination of a steel wire gauze with 8 holes per linear cm and an electroformed nickel perforated plate with 250 conical holes per linear cm, a minimum hole diameter of 20 μm and a thickness of 150 μm. The ratio between the thickness and the hole diameter difference was 7.5. In this example, Nylon 66 was filtered at a temperature of 285°C. It is evident that the service life of a filter pack according to the invention is very considerably longer than in the case of a filter pack according to the prior art. Finally, Figure 6 shows a graph identical to Figure

5, but for the purpose of comparing packs 3 and 4, again involving the filtration of Nylon 66 at 285°C. Pack 3 is a steel wire pack having a diameter of 50 mm, with six different layers of intermediate gauze with a smallest aperture of 8 μm. Pack 4 likewise has a diameter of 50 mm and is a combination of an electroformed nickel perforated plate with 480 holes per linear cm, a minimum hole diameter of 5 μm and a thickness of 150 μm, and a steel wire screen with 8 holes per linear cm. The ratio between the thickness and the hole diameter difference was 11.5. The longer service life of pack 4 according to the invention is evident.

Claims

1. Filter pack intended for the filtration of a liquid polymer, in particular prior to fibres being spun from said melt, at least comprising a number of stacked filter and supporting discs, characterized in that the filter pack (7) comprises a filter disc made of a metal perforated plate (1), which plate (1) comprises essentially conical sieve holes (3) having a hole dimension between 1 and 100 μm and more than 100 holes (3) per linear cm, while the thickness of the perforated plate (1) is at least 50 μm, the perforated plate (1) during filtration of a liquid polymer being oriented in such a way that the hole dimension increases in the flow direction of the liquid polymer.

2. Filter pack according to claim 1, characterized in that the perforated plate (1) contains at least 400 holes per linear cm.

3. Filter pack according to claim 2, characterized in that the perforated plate (1) contains at least 600 holes per linear cm.

4. Filter pack according to one or more of the preceding claims, characterized in that the ratio between the thickness of the perforated plate (1) and the differ¬ ence between the largest and the smallest diameter of the holes (3) is at most 20.

5. Filter pack according to one or more of the preceding claims, characterized in that the supporting disc comprises a metal gauze (5) .

6. Spinning head for spinning polymer fibres, at least comprising a polymer infeed, a filter pack and spinnerets, characterized in that the filter pack is a filter pack (7) according to one or more of the preceding claims.

7. Spinning head according to claim 6, characterized in that the filter pack (7) is situated near the spinnerets (13).