Filter for molten polymer filtration

Abstract

The filter for gel shearing and particle filtration of molten polymer has a first layer of metal fibers of an average equivalent diameter between 8 and 65 m. The cross-section of the metal fibers has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides. The metal fibers have an average length of at least 6 mm. The metal fibers are bonded to each other by metal bonds; where the metal of the metal fibers of the first layer is the bonding agent forming the metal bonds. The filter has a second layer of metal fibers. The average equivalent diameter of the metal fibers of the second layer is smaller than the average equivalent diameter of the metal fibers of the first layer.

Claims

1. A filter for gel shearing and particle filtration of molten polymer; comprising a first layer of metal fibers of an average equivalent diameter between 8 and 65 m; wherein the metal fibers of the first layer have a cross-section, wherein the cross section has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides; and wherein the metal fibers of the first layer have an average length of at least 6mm; wherein the metal fibers of the first layer are bonded to each other by means of metal bonds; whereby the metal of the metal fibers of the first layer is the bonding agent forming the metal bonds; and a second layer of metal fibers; wherein the average equivalent diameter of the metal fibers of the second layer is smaller than the average equivalent diameter of the metal fibers of the first layer, wherein the metal fibers of the second layer of metal fibers have a hexagonal cross-sectional shape.

2. The filter as in claim 1, wherein the first layer of metal fibers and the second layer of metal fibers are positioned one on top of the other without a physicochemical bonding between the metal fibers of the first layer of metal fibers and the metal fibers of the second layer of metal fibers.

3. The filter as in claim 1, wherein the first layer of metal fibers and the second layer of metal fibers are bonded to each other by means of metal bonds; wherein the metal of the metal fibers of the first layer and of the second layer is the bonding agent forming the metal bonds.

4. The filter as in claim 3, wherein the metal bonds are sinter bonds or are welded bonds.

5. The filter as in claim 1, wherein the filter circumference is surrounded by a clamping element sealing the sides of the filter and clamping the second layer of metal fibers onto the first layer of metal fibers.

6. The filter as in claim 1, wherein the metal fibers of the first layer of metal fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 25% of the equivalent fiber diameter.

7. The filter as in claim 1, wherein the filter comprises a metal wire mesh.

8. The filter as claim 1, wherein the metal fibers of the second layer of metal fibers have a cross-section, wherein the cross section has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides.

9. The filter as in claim 1, wherein the porosity of the first layer of metal fibers is between 50% and 80%.

10. The filter as in claim 1, wherein the second layer of metal fibers comprises at least two sub-layers, wherein the metal fibers of the at least two sub-layers differ in average equivalent diameter; wherein a sub-layer closest to the first layer of metal fibers comprises metal fibers of higher average equivalent diameter than a sub-layer further away from the first layer of metal fibers.

11. A spin pack filter comprising a filter as in claim 1.

12. A leaf disk filter comprising a filter as in claim 1.

13. The leaf disk filter as in claim 12, wherein the first layer of metal fibers and the second layer of metal fibers of the filter are bonded to each other by means of metal bonds; wherein the metal of the metal fibers of the first layer and of the metal fibers of the second layer is the bonding agent forming the metal bonds.

14. A method for filtering molten polymer in polymer extrusion, comprising the steps of: using a filter for filtering molten polymer, wherein the filter comprises: a first layer of metal fibers of an average equivalent diameter between 8 and 65 m; wherein the metal fibers of the first layer have a cross-section, wherein the cross section has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides; and wherein the metal fibers of the first layer have an average length of at least 6 mm; wherein the metal fibers of the first layer are bonded to each other by means of metal bonds; whereby the metal of the metal fibers of the first layer is the bonding agent forming the metal bonds; a second layer of metal fibers; wherein the average equivalent diameter of the metal fibers of the second layer is smaller than the average equivalent diameter of the metal fibers of the first layer; wherein the metal fibers of the second layer of metal fibers have a hexagonal cross sectional shape, and wherein in the method, gels are broken and particles are removed from the molten polymer.

15. A method for filtering molten polymer in polymer extrusion, wherein a spin pack is used as in claim 11 for breaking gels and for removing particles from the molten polymer via filtration.

16. A method for filtering molten polymer in polymer extrusion, wherein a leaf disk filter is used as in claim 12 for breaking gels and for removing particles from the molten polymer via filtration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1-6 show examples of fiber cross sections of metal fibers that can be used for the first layer of metal fibers of the filter.

(2) FIG. 7 shows an exemplary set up for fiber machining to manufacture metal fibers of an average length of at least 6 mm and that have a cross-section that has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides.

(3) FIG. 8 shows a filter according to the invention that has a circular shape.

(4) FIG. 9 shows a filter according to the invention that has a half moon shape.

(5) FIG. 10 shows the cross section of the filter of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIGS. 1-6 show examples of fiber cross sections of metal fiberse.g. stainless steel fibersthat can be used for the first layer of metal fibers of the filter. The fiber cross sections are having two neighboring straight lined sides 110, 120 with an included angle of less than 90 and one or more irregularly shaped curved sides 130. Metal fibers having such cross-sections can also be used in the second layer of metal fibers. Such fibers can be made according to the method described in WO2014/048738A2.Such fibers for the first layer (and for embodiments of the second layer)fibers that have a cross-section that has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sidescan be made according to the method comprising the steps of: fixing on a lathe a metal piece (or work piece), for instance an ingot, from which the metal fibers will be cut; mounting a tool on a tool holder and sliding the tool holder with a feed rate along the axis of the lathe; imposing a vibration upon the tool thereby cutting metal fibers from the metal piece; measuring the rotational speed of the lathe and using the measurement signal in order to dynamically synchronize (preferably steer and synchronize) the vibration frequency of the tool with the rotational speed of the lathe by means of an electronic control circuit.

(7) The vibration of the tool can be obtained by means of a piezomotor, the frequency of which is controlled. This method results in metal fibers with a cross-section having two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides. This follows from the way the tool cuts fibers out of the work piece. A previous cut formed two straight lines, during cutting the fiber, the first will be deformed in an irregularly shaped curve the second stays straight and forms an included angle of less than 90 with a newly formed straight edge. The latter is formed by the cutting action on the cutting plane of the knife. The one or more irregularly shaped curved sides are formed by upsetting/bulging of a side not in contact with the cutting tool during the cutting process, by the compressive forces in the material being cut.

(8) This way, metal fibers can be made that have a low standard deviation between fibers of the equivalent fiber diameter. Fibers of discrete length are produced by exiting the cutting tool each vibration cycle out of the tool. This way of working has the benefit that fibers with low variation in length can be produced.

(9) Preferably, a ball bearing, and more preferably a pre-tensioned ball bearing, is used to slide the tool holder along the axis of the lathe. This feature further ensures low variation between fibers of the equivalent diameter of the fibers.

(10) Alternatively, the sliding of the tool holder along the axis of the lathe is realized by means of a direct drive by means of a linear motor, meaning that no reduction of motor speed nor clutch is required. Such a method contributes to the production of metal fibers with low variation.

(11) Preferably, the tool holder set up and/or tool mounting is such that displacement of the tool due to bending of the tool holder during fiber cutting is less than 5 m, preferably less than 2 m. This feature improves the uniformity of the equivalent diameter of the fibers that are cut. More preferably, the tool holder and/or the tool is supported in order to minimize or prevent bending of the tool holder due to the cutting forces. Preferably, the tool holder and/or the tool is supported by a mechanical support, preferably the mechanical support is connected to the block onto which the tool holder is mounted. The tool and/or the tool holder can e.g. vibrate in a bush. This way, it is possible to obtain metal fibers with even lower variation between fibers of the equivalent fiber diameter.

(12) FIG. 7 shows an exemplary set up for fiber machining to manufacture metal fibers of an average length of at least 6 mm and that have a cross-section that has two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides. Block 710 will slide with a constant speed along the axis of the lathe (not shown in the figure). The sliding movement can be provided via a pre-tensioned ball bearing.

(13) A housing 715 is fixed to the block 710. The housing 715 comprises a piezomotor 720.

(14) The vibration frequency of a few thousand Hertz is synchronized via electronic means (using an appropriate controller) with the revolving speed of the lathe, via measurement of the revolving speed of the lathe. A tool holder 730 is connected via a connection 740 to the piezomotor, hence the tool holder 730 will vibrate in the bush 745 thanks to the action of the piezomotor. A chisel (cutting tool) 750 is fixed by means of a clamp 760 and a bolt 770 onto tool holder 730. A supporting piece 780 which is fixed to the block 710 is supporting the tip of the chisel 750 as it is supporting the tool holder 730 under the position of the tip of the chisel 750.

(15) The dimensions of the cross section of the metal fibers can be determined via image analysis.

(16) The filter of the invention can be provided in different shapes. FIG. 8 shows a filter 80 according to the invention that has a circular surface shape with diameter D1, e.g. 80 mm. FIG. 9 shows a filter 90 according to the invention that has a half moon shape, e.g. with dimensions H 80 mm and B 40 mm. Other shapes for the filter are possible, e.g. the shapes that are known and in use for polymer filtration.

(17) FIG. 10 shows the cross section of the filter of FIG. 8. The filter comprises a first layer 12 of metal fibers. The metal fibers of the first layer of metal fibers have a cross-section having two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides. The metal fibers of the first layer of metal fibers have an average length of at least 6 mm, e.g. of 8 mm. The first layer 12 has been built up by superimposing a number of webs of such metal fibers, e.g. five webs 13. It is clear however that the first layer 12 can be made by using one web or by using any other number of webs superimposed on top of each other. The metal fibers of the first layer 12 are bonded to each other by metal bonds; e.g. by means of sintering, although welding is an alternative technique that can be used, e.g. capacitive discharge welding (CDW). The filter comprises a second layer of metal fibers 15. The average equivalent diameter of the metal fibers of the second layer 15 is smaller than the average equivalent diameter of the metal fibers of the first layer 12. In the example, the second layer 15 comprises two sub-layers 16, 17. The metal fibers of the two sub-layers differ in average equivalent diameter 16, 17. The sub-layer 16 closest to the first layer 12 of metal fibers comprises metal fibers of higher average equivalent diameter than the sub-layer 17 further away from the first layer of metal fibers.

(18) The filter comprises a metal wire mesh 18. Although not necessarily, the first layer 12, the second layer 15 and the metal wire mesh 18 can be bonded to each other by metal bonds; e.g. by means of sintering, although welding is an alternative technique that can be used to create metal bonds, e.g. capacitive discharge welding (CDW).

(19) The filter circumference can be surrounded by a metal clamping element 19 (e.g. out of aluminum or aluminum alloy) sealing the sides of the filter and clamping the second layer 15 of metal fibers onto the first layer 12 of metal fibers.

(20) In order to make filters with the size as in FIG. 8 or 9 a porous panel has been made of size 1.5 m by 1 m out of which a multiple number of filters as shown in FIGS. 8 and 9 can be made by means of punching out the correct size of the required filter.

(21) A first layer of 3000 g/m.sup.2 of stainless steel fibers of average equivalent diameter of 35 m with a cross-section having two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides, with an average length of 8 mm and with a standard deviation between fibers of the equivalent fiber diameter of 18.1% of the equivalent fiber diameter is provided. The first layer can e.g. be built up by superimposing 5 webs of 600 g/m.sup.2 each. The webs have been made by means of a dry-laid nonwoven production process wherein panels of 1.2 m by 1.5 m have been made. It is also possible to manufacture rolls of web. The panels are put on top of each other to build the first layer of stainless steel fibers. As an alternative to dry-laid nonwovens, wet laid webs can be used, or any other technology to make a stainless steel fiber nonwoven web.

(22) In the first layer, instead of fibers of 35 m equivalent diameter, fibers of other equivalent fiber diameters can be used, e.g. 22 m, 12 m or 8 m; e.g. in AISI 316 steel grade.

(23) A second layer of stainless steel fibers is provided. The second layer comprises two sub-layers.

(24) The sub-layer that will be closest to the first layer of stainless steel fibers comprises 450 g/m.sup.2 of stainless steel fibers of 22 m equivalent diameter and the sub-layer that will be positioned further away from the first layer of stainless steel fibers comprises 900 g/m.sup.2 stainless steel fiber of 12 m equivalent diameter. Both sub-layers comprise bundle drawn stainless steel fibers and thus fibers of hexagonal cross section.

(25) Each of the sub-layers has been made by means of carding, wherein panels of 1.2 m by 1.5 m have been made. It is also possible to manufacture rolls of web. The panels for the sub-layers have been superimposed in the correct order on the first layer.

(26) A woven stainless steel wire mesh, a K-mesh, has been provided and put on top of the second layer. This way, a porous panel is built up.

(27) After putting all the layers on top of each other, the porous panel was bonded by means of sintering in a sinter oven in order to obtain a panel of size 1.5 m by 1 m according to the invention. Alternatively the panel can be bonded by means of capacitive discharge welding, welding the stainless steel fibers to each other and to the woven wire mesh at cross over contacting points.

(28) The obtained porous paneland also the filters punched out of ithad a thickness of 1.75 mm, a weight of 5650 g/m.sup.2, a porosity of 59.8%, an air permeability of 42.4 litre/(dm.sup.2*min) as measured at a differential pressure of 200 Pa and according to ISO4022; and a bubble point pressure of 2240 Pa, as measured according to ASTM E128-61.Tests have shown that the filters provided excellent shearing results.

(29) As an alternative to make the porous panel via superimposing and sintering panels of a certain size, e.g. 1.5 m by 1 m; it is also possible to unwind metal fiber web layers from rolls, and superimpose themtogether with the an appropriate mesh layer if requiredin order to make a porous panel that can be sintered.

(30) Alternatively, it is also possible to unwind metal fiber web layers from rolls, and superimpose themtogether with the appropriate mesh layer if requiredin order to make the porous panel. If such porous panel is made in continuous length, continuous sintering or welding (e.g. capacity discharge welding) is possible in order to bond the superimposed layer. After bonding, the porous panel can be cut to a size to enable its transport, e.g. to a panel size of e.g. 1.5 m by 1 m.

(31) Another example of a filter according to the invention comprises a first layer of 675 g/m.sup.2 of stainless steel fibers of average equivalent diameter of 8 m with a cross-section having two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides and a length of 10 mm. The filter comprises a second layer of stainless steel fibers, comprising a sub-layer of 300 g/m.sup.2 of stainless steel fibers with hexagonal cross section (made via bundled drawing) of average equivalent diameter 8 m; a sub-layer of 150 g/m.sup.2 of stainless steel fibers with hexagonal cross section (made via bundled drawing) of average equivalent diameter 6.5 m; and a sub-layer of 300 g/m.sup.2 of stainless steel fibers with hexagonal cross section (made via bundled drawing) of average equivalent diameter 4 m

(32) The filter can comprise a steel wire mesh. The first layer of stainless steel fibers, the second layer of stainless steel fibers and the meshif presentare bonded by means of sintering. Such a filter composition is especially suited for gel shearing and filtration of molten polymers using leaf disk in polymer film extrusion.

(33) Another example of a filter according to the invention comprises a first layer of stainless steel fibers of 1200 g/m.sup.2, comprising a first sub-layer of 900 g/m.sup.2 of stainless steel fibers of average equivalent diameter of 22 m with a cross-section having two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides and an average fiber length of 10 mm; and a second sub-layer of 300 g/m.sup.2 of stainless steel fibers of average equivalent diameter of 12 m with a cross-section having two neighboring straight sides with an included angle of less than 90 and one or more irregularly shaped curved sides and an average fiber length of 10 mm. The filter comprises a second layer of stainless steel fibers, comprising a sub-layer of 300 g/m.sup.2 of stainless steel fibers with hexagonal cross section (made via bundled drawing) of average equivalent diameter 8 m.

(34) The filter can comprise a steel wire mesh. The first layer of stainless steel fibers, the second layer of stainless steel fibers and the meshif presentare bonded by means of sintering. Such a filter composition is especially suited for a leaf disk filter for gel shearing and filtration of molten polymers in polymer film extrusion.

(35) Filters according to the invention that do not have a metal bond between the first layer of metal fibers and the second layer of metal fibers can be manufactured as well. The first layer of metal fibers can be made, e.g. via superimposing a number of webs, and sintering the first layer of metal fibers. The second layer of metal fibers can be made, e.g. via superimposing a number of webs, and sintering the second layer of metal fibers. Out of the first layer of metal fibers, and out of the second layer of metal fibers, the size is cut or punched required to make the filter. The cut or punched parts are put on top of each other. This assembly can e.g. be sintered to create metal bonds between the metal fibers of the first metal fiber layer and the metal fibers of the second metal fiber layer. One or more meshes can be added in this assembly, e.g. via sintering. As an alternative to sintering, metal bonds can be formed via welding, e.g. capacitive discharge welding.

(36) When no metal bonds are created between the metal fibers of the first metal fiber layer and the metal fibers of the second metal fiber layer, the different layers of the filter (the first layer of metal fibers, the second layer of metal fibers andif presentone or more meshes) can be combined in the filter by means of a metal clamping element surrounding the filter circumference, clamping the first layer and second layer in order to seal the sides of the filter.