WO2000029196A1

WO2000029196A1 - Filter element and use thereof

Info

Publication number: WO2000029196A1
Application number: PCT/US1999/025920
Authority: WO
Inventors: John C. Hoagland; Aristoteles Karagiannis
Original assignee: Solutia Inc
Current assignee: Solutia Inc
Priority date: 1998-11-13
Filing date: 1999-11-05
Publication date: 2000-05-25
Anticipated expiration: 2001-05-13

Abstract

Filter elements for use in removing contaminants from molten thermoplastic melt comprising a rigid planar member having a plurality of ridges arranged perpendicular to a direction of processing over the ridges which gradually increase in height in such direction of processing. The invention also provides a filter assembly wherein a plurality of filter elements in the face of a disc are mounted around a perforated hollow cylindrical tube, the elements abuttingly engage each other to define radial (with respect to the tube) filtering passages between adjacent side-faces of successive reusable toroidal filter elements, contaminating particles are trapped in the decreasing space of the radial filtering passages of abutting filter elements as melt flows through the radial filtering passageways over the ridges and through the troughs of abutting elements.

Description

TITLE

FILTER ELEMENT AND USE THEREOF

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to the filtration of thermoplastic melt and more particularly to a reusable filter element for filtration of molten thermoplastic melt to remove unwanted matter from the melt. The invention also relates to apparatus containing such element and a method for the filtering of thermoplastic melt.

Related Background Art

Thermoplastic polymer in a viscous, molten state is often filtered under pressure to remove contaminants before shaping into article form. Such contaminants include gel particles, solid impurities from the polymer itself, additives compounded into the melt and burnt stock from local overheating and the like. Typical prior art filter elements comprise screens interposed in the path of the melt. The apertures of the screens chosen depend on the size of contaminant being removed. The screen should block contaminants and prevent passage through the screen to provide contaminant-free melt downstream of the filter, yet certain types of immiscible matter in the melt are often not removed with conventional screen filters even though the openings are smaller than the minimal size of the immiscible matter.

For example, when filtering polyvinyl butyral ("PVB") melt using commercially fabricated screens, globules of foreign matter can nevertheless be observed in the final sheet. These immiscible thermoplastic globules likely have different rheology (e.g., polymerized to a different degree) from the surrounding bulk material. It is believed that these thermoplastic globules under pressure of the flowing melt may deform into long slender shapes which are expelled through the screen opening and then viscoelastically relax downstream taking on the original swollen shape in the filtered bulk material. An approach to deal with this has been to filter the melt a second time through an identical screen and this does appear to reduce globules in the final bulk material. This is probably because repeat stretching of the globules either breaks them into smaller particles or greatly reduces the probability of passing through a second filtering. However, this double filtering method is costly in industrial settings.

To achieve very fine filtration (e.g., 0.02 mm screen openings) requires very dense filter weaves that are expensive to fabricate and rapidly plug requiring costly downtime for changing. Another disadvantage of present melt filtering using filter screens is the lack of long-term durability and availability of the screen for reuse. Screen burst pressure is reached relatively rapidly as back pressure builds from relatively rapid gradual clogging of the openings. Once removed from the filter it is usually more economical to scrap the used filter element than laboriously remove contaminants from the openings to make it fit for reuse.

SUMMARY OF THE INVENTION

The present invention overcomes shortcomings of the prior art and provides a reusable filter element for filtering molten thermoplastic melt which is functional after many successive cleanings and over long periods without appreciable progressive loss of filtering efficiency. Cleaning is rapid and simple between cycles of use. Filtering can be carried out under significant pressure while retaining large quantities of contaminants before cleaning compared with conventional prior art filters.

In particular, the invention provides a reusable filter element comprising a rigid planar member with top and bottom faces, the top face having a plurality of ridges arranged perpendicularly to a direction of processing over the ridges and spaced from each other to form troughs therebetween, the ridges gradually increasing in height in the direction of processing. The bottom face includes ridges similar to those in the top face.

The invention further provides a reusable filter structure comprising a filter element adjacent a bearing surface, the filter element comprising a plurality of ridges arranged perpendicularly to a direction of processing over the ridges with immediately adjacent ridges forming troughs therebetween, the plurality of ridges gradually increasing in height in the direction of processing. In a preferred embodiment, the bottom and top faces of the filter element are provided with such ridges and troughs.

Further provided is a filtering column for removing contaminating particles from molten thermoplastic melt comprising a perforated hollow cylindrical tube, a plurality of reusable rigid toroidal filter elements removably mounted on the tube and abuttingly engaging each other thereby defining radial filtering passages between adjacent side-faces of successive reusable toroidal filter elements effective to separate contaminating particles from the molten thermoplastic melt flowing through the radial filtering passages, each reusable toroidal filter element having a top face and a bottom face, the top face having a plurality of ridges arranged perpendicularly to a direction of processing over the ridges and adjacent ridges forming troughs therebetween, the plurality of ridges gradually increasing in height toward the exit end of the radial filtering passages, whereby the contaminated particles are trapped in the decreasing space of the radial filtering passages of abutting filter elements as the melt flows through the radial filtering passageways over the ridges and through the troughs of abutting elements. In a preferred embodiment the bottom face of the filter element is configured identically to the top face.

The invention also provides a method of removing contaminated particles from thermoplastic melt comprising forcing molten liquid thermoplastic melt containing contaminated particles radially through a filtering passage of decreasing height defined by opposing faces of toroidal filter elements, each toroidal filter element comprising a rigid member with a top face and a bottom face, the top face having a plurality of ridges arranged perpendicularly to a direction of processing over the ridges and adjacent ridges forming troughs therebetween, the plurality of ridges gradually increasing in height toward the exit end of the radial filtering passages, whereby the contaminated particles are trapped in the decreasing space of the radial filtering passages of abutting filter elements as the melt flows through the radial filtering passageways over the ridges and through the troughs of abutting elements.

BRIEF DESCRIPTION OF THE DRAWINGS

In describing the overall invention, reference will be made to the accompanying drawings, wherein:

FIG. 1 is a plan view of a filter element embodiment of the invention;

FIG. 2 is an enlarged, partial, perspective view of the filter element of FIG. 1;

FIG. 3 is an enlarged sectional view along 3-3 of FIG. i;

FIG. 4 is a plan view of a filter structure containing plural filter elements;

FIG. 5 is a central sectional view of a filter assembly containing the plural filter elements of FIG. 1; and

FIG. 6 is an enlarged sectional view along 6-6 of FIG. DETAILED DESCRIPTION OF THE INVENTION

The drawings do not imply any limitation of the scope of the invention and minor variations on the location and detailed design of features herein described are considered within the purview of this invention.

Referring to the drawings, FIGS. 1-3 illustrate a reusable filter element 1 comprising rigid planar member 2 formed of metal such as steel or the like having top face 4 and bottom face 6. Plural ridges 8 are formed in top face 4 and are circularly arranged perpendicular to direction A of processing over ridges 8 and through troughs 10 formed by immediately adjacent ridges 8. As illustrated in FIGS. 2 and 3 ridges 8 gradually increase in height in the direction of processing A, the height in the vertical direction of ridge 8b in FIG. 3 being greater than that of ridge 8a for a purpose to be described. Bottom face 6 in FIG. 3 may optionally preferably have identical ridges and troughs to those in top face 4. The embodiment of FIG. 4 differs from FIG. 1 in that rigid planar member 14 comprises a plurality (eight shown) of circularly arranged filter elements 1, each of such plurality being between a flat bearing surface 16 on each side.

Ridges 8 are preferably perpendicular to but may be at any desirable angle to a radial line 9 through the filter element as shown in FIG. 3. The ridges on the opposite side face of the element may be at the same angle or slanted in the opposite direction. As shown particularly in FIG. 3 troughs 10 are between adjacent ridges. The height of the ridges from the upstream 12b (FIG. 3) side to the downstream side 12a of the filter element preferably increase uniformly. Space lla between adjacent ridges is from about 0.002 inches (0.00254 cm) to about 0.200 inches (0.508 cm) and the height of each ridge lib relative to radial line 9 is about 0.001 inch (0.00254 cm) to about 1 inch (2.54 cm). Width lie (FIG. 3) in the direction of processing is about 0.001 inch (0.00254 cm) to about 1.0 inch (2.54 cm). Each face of each filter element has about 1 to about 1000 troughs. The varying depth lid of each trough is from about 0.001 (0.00254 cm) to about 0.200 inches (0.508 cm). Though the cross section of a trough may vary, it is preferably as shown in FIG. 3 with a flat bottom and outwardly divergent sides, though a curvilinear shape (e.g., U-shaped ) could be used as well.

The ratio of height lib to width lie of each ridge is typically preferably from about 0.5 to about 200. Alternatively to the circular peripheral contour shown in the drawings, a filter element may have a shape selected from the group consisting of rectangular, oblate, toroidal, and partially toroidal (i.e., a fraction of a toroidal) .

The FIG. 4 embodiment is torodial having plural (eight) filter elements 1, with a triangular raised bearing surface 16 between an adjacent pair of elements. Bearing surface 16 shape is selected from the group consisting of triangular, circular, square, and the like. The ridges and troughs of each element 1 in FIG. 4 are shown identical to those in FIGS. 1-3 and, for brevity, are not here further described.

FIG. 5 illustrates filter assembly 17 comprising housing 18 having inlet 19 communicating with an upstream source of melt supply (not shown) . Perforated 28 hollow cylindrical tube 22 of assembly 17 supports plural toroidal filter discs 24 along its longitudinal axis. Discs 24 are as shown in FIG. 4 and collectively define the filter. Discs 24 via bore 25 surround and are removably mounted on tube 22 and abuttingly engage each other to define radial (with respect to the axis of tube 22) filter passages 20 (FIG. 6) between side faces of an adjacent pair of filter elements. Contaminating particles from the melt are trapped in passages 20 as melt flows through the passages to downstream outlet 32. Cone shaped cap 30 on the end of tube 22 hold the discs 24 in place. In a preferred embodiment each disc has a through-hole drilled through its thickness to accept an alignment pin during assembly thereby positioning bearing surfaces 16 of abutting discs 24 opposite one another.

Hollow cylindrical tube 22 has about 2 to 10 perforations per linear inch along its axis. The length of tube 22 is set by the number of discs 24 it supports.

In the illustrated embodiment melt flow through tube 22 in direction A (FIG. 1) is from the hollow inside through perforations 28 and radial filter passageways 20 between adjacent filter elements into chamber 29 and eventually through outlet 32. The opposite path may also be used by appropriately redefining the passages in discs 24, i.e., outlet 32 becomes the inlet and inlet 19 becomes the outlet.

As illustrated in FIG. 6, each filter element 1 (FIG. 1) comprises plural ridges 8 on its top and bottom faces which are arranged perpendicular to a direction A of processing molten polymer melt being filtered, i.e., over successive ridges and through successive troughs 10 therebetween, the plurality of ridges of a filter element 1 gradually increasing in height in the A direction. A pair of abuttingly engaged filter elements define radially extending filter passage 20 between adjacent side-faces of successive filter elements. This arrangement effectively separates contaminating particles from melt flowing through the passages 20. For example, contaminated particles in the form of globules, depending on size, are unable to pass through succeeding troughs without passing through the narrow gap 21 defined by opposite ridges 8 of abuttingly engaged filter elements.

The ridges and troughs on the faces of the filter elements are fabricated by known techniques including machining the surfaces to created the ridges and troughs. Alternatively short metal dowels can be set in holes drilled in the face of the filter element to create ridges and associated troughs.

The filter of the invention provides many advantages in removing foreign materials, such as sand or foreign particles, from molten thermoplastic. The radial filter passages which progressively decrease in open area from inlet to outlet trap coarse particles first, followed by the finer particles of lesser size. Clean melt flows around contaminated particles trapped in a trough. Filtering action is efficient to reduced pressure drop across the filter because coarse particles are first separated before they can block the downstream fine section of a passage. Melt flow continues around particles trapped in a trough or between opposite land surfaces. Moreover, the globule- holding capacity of the filter is substantially increased since the large particles are separated first and held away from the filter outlet rather than being concentrated along the outlet surface of the filter body as the case in most screen-like conventional filters. Still further, a filter constructed in accordance with the foregoing features may be more effectively cleaned by back-flushing. Another advantage is the higher resistance to rupture the reusable toroidal filter discs have compared to the apertured-screen filter. A further advantage is that by adjusting gap space 11a and length lie the filter elements can accommodate thermoplastic melts of different heat sensitivity. For example, for a heat sensitive melt such as PVB, pressure drop (and therefore frictional increase in melt temperature) can be selectively set by increasing gap space 11a and decreasing land length lie over that chosen for a less heat sensitive material able to withstand greater pressure drop and associated heat buildup.

The process of the invention removes contaminating particles from thermoplastic melt by steps comprising forcing (e.g., by pressure generated by an upstream screw extruder (not shown) ) molten liquid thermoplastic melt containing contaminated particles radially through a filtering passage of decreasing height defined by opposing faces of toroidal filter elements of the type described above and shown in the drawings. The contaminated particles are trapped in the decreasing space of the radial filtering passages of abutting filter elements as the melt flows through the radial filtering passageways over the ridges and through the troughs of abutting elements.

The troughs along the radial filtering passageways permit the melt to relax (i.e., stress relieve) after flowing through the previous tight gap restriction created between the lands at the tops of opposite ridges. As the melt flows through the radial filtering passages, each gap is narrower than the last, with the final one defining the size that may be used to characterize the filtration rating of the filter. Thus, filter elements of the invention subject the melt flow to multiple filtrations as melt passes through the series of gaps decreasing in size toward the outlet. Each successive gap increases the probability that foreign matter, such as globules, will be either retained or broken into smaller globules before passing through the gap, which also reduces the drag and cross- sectional area on which the melt flow has to act.

The overall capacity of the stacked filter elements for removing and retaining foreign material from the melt being filtered, is substantially increased by the troughs as compared to conventional disc construction wherein the removed foreign particles are retained directly on the screen which ultimately completely blocks the filter.

After continued operation the troughs and gaps between adjacent ridges eventually accumulate sufficient unwanted contaminants to create excessive pressure drop requiring removal of the unit from service for cleaning. The filter elements of the invention are cleaned by any conventional cleaning method such as backwashing, high temperature pyrolysis followed by solvent cleaning, and flushing at high pressure. In the case of high temperature pyrolysis, the elements are removed from the filter unit and placed in a high temperature oven to be "burned out". After pyrolyzing, the residual carbon is brushed away with a soft brush and light solvent to provide a clean filter. This contrasts with prior art metal screens which are cut off the support structure and discarded. The cleaned filter elements of the invention are reused many times before wear and damage cause replacement.

Other variations and modifications which will be obvious to those skilled in the art can be made in the foregoing example without departing from the spirit or scope of the invention.

Claims

What is claimed is:

1. A reusable filter element comprising a rigid planar member with a top face and a bottom face, said top face having a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height in said direction of processing.

2. The reusable filter element of claim 1, wherein said bottom face has a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height in said direction of processing.

3. The reusable filter element of claims 1 or 2 , wherein the space between adjacent ridges is from about 0.002 to about 0.200 inches.

4. The reusable filter element of claims 1 or 2, wherein the height of each ridge relative to a radial axis through the thickness of said element is from about 0.001 to about 1.0 inch.

5. The reusable filter element of claims 1 or 2, wherein the width of each ridge in said direction of processing is from about 0.001 to about 0.250 inches.

6. The reusable filter element of claims 1 or 2, wherein each said face has about 1 to about 1000 troughs.

7. The reusable filter element of claims 1 or 2, wherein the depth of each trough is from about 0.001 to about 0.200 inches.

8. The reusable filter element of claims 1 or 2, wherein each trough's cross-section is formed with a flat bottom and outwardly divergent sides or a curvilinear shape.

9. The reusable filter element of claims 1 or 2, wherein the ratio of height to width of each ridge is from about 0.5 to about 200.

10. The reusable filter element of claims 1 or 2, wherein said reusable filter element has a shape selected from the group consisting of rectangular, oblate, toroidal, and partially toroidal.

11. A reusable filter structure comprising a filter element adjacent a bearing surface, said filter element comprising a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height in said direction of processing.

12. The reusable filter structure of claim 11, wherein said bottom face of said filter element comprises a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height in said direction of processing.

13. The reusable filter structure of claims 11 or 12, wherein the space between adjacent ridges is from about 0.002 to about 0.200 inches.

14. The reusable filter structμre of claims 11 or 12 , wherein the height of each ridge relative to a radial axis through the thickness of said element is from about 0.001 to about 1.0 inch.

15. The reusable filter structure of claims 11 or 12, wherein the width of each ridge in said direction of processing is from about 0.001 to about 0.250 inches.

16. The reusable filter structure of claims 11 or 12, wherein each said face has about 1 to about 1000 troughs.

17. The reusable filter structure of claims 11 or 12, wherein the depth of each trough is from about 0.001 to about 0.200 inches.

18. The reusable filter structure of claims 11 or 12, wherein each trough's cross-section is formed with a flat bottom and outwardly divergent sides or a curvilinear shape.

19. The reusable filter structure of claims 11 or 12, wherein the ratio of height to width of each ridge is from about 0.5 to about 200.

20. The reusable filter structure of claims 11 or 12, wherein said reusable filter element has a shape selected from the group consisting of rectangular, oblate, toroidal, and partially toroidal.

21. The reusable filter structure of claims 11 or 12, wherein said bearing surface has a shape selected from the group consisting of triangular, circular or square.

22. A reusable filter structure comprises a plurality of filter elements, each separated by a bearing surface, each filter element comprising a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height in said direction of processing.

23. The reusable filter structure of claim 22, wherein said bottom face of each of said filter element comprises a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height in said direction of processing.

24. The reusable filter structure of claims 22 or 23, wherein the space between adjacent ridges is from about 0.002 to about 0.200 inches.

25. The reusable filter structure of claims 22 or 23, wherein the height of each ridge relative to a radial axis through the thickness of said element is from about 0.001 to about 1.0 inch.

26. The reusable filter structure of claims 22 or 23, wherein the width of each ridge in said direction of processing is from about 0.001 to about 0.250 inches.

27. The reusable filter structure of claims 22 or 23, wherein each said face has about 1 to about 1000 troughs.

28. The reusable filter structure of claims 22 or 23, wherein the depth of each trough is from about 0.001 to about 0.200 inches.

29. The reusable filter structure of claims 22 or 23, wherein each trough's cross-section is formed with a flat bottom and outwardly divergent sides or a curvilinear shape.

30. The reusable filter structure of claims 22 or 23, wherein the ratio of height to width of each ridge is from about 0.5 to about 200.

31. The reusable filter structure of claims 22 or 23, wherein said reusable filter element has a shape selected from the group consisting of rectangular, oblate, toroidal, and partially toroidal.

32. The reusable filter structure of claims 22 or 23, wherein said bearing surface has a shape selected from the group consisting of triangular, circular or square.

33. A filtering column for removing contaminating particles from molten thermoplastic melt comprising a perforated hollow cylindrical tube, a plurality of reusable toroidal filter elements removably mounted around said tube abuttingly engaged with each other thereby defining radial filtering passages between adjacent side-faces of successive reusable toroidal filter elements effective to separate contaminating particles from said molten thermoplastic melt flowing through said radial filtering passages, each reusable toroidal filter element comprising a rigid member with a top face and a bottom face, said top face having a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height toward the exit end of said radial filtering passages, whereby said contaminated particles are trapped in the decreasing space of said radial filtering passages of abutting filter elements as the melt flows through said radial filtering passageways over the ridges and through the troughs of abutting elements.

34. The filtering column of claim 33, wherein said bottom face has a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of raised ridges gradually increasing in height toward the exit end of said radial filtering passages.

35. The filtering column of claims 33 or 34, wherein the space between adjacent raised ridges is from about 0.002 to about 0.200 inches.

36. The filtering column of claims 33 or 34, wherein the height of each raised ridge relative to a radial axis through the thickness of said element is from about 0.001 to about 1.0 inch.

37. The filtering column of claims 33 or 34, wherein the width of each raised ridge in said direction of processing is from about 0.001 to about 0.250 inches.

38. The filtering column of claim 33 or 34, wherein each face has about 1 to about 1000 troughs.

39. The filtering column of claims 33 or 34, wherein the depth of each trough is from about 0.001 to about 0.200 inches.

40. The filtering column of claims 33 or 34, wherein each trough cross-section is flat bottom and outwardly divergent sides or a curvilinear shape.

41. The filtering column of claims 33 or 34, wherein ratio of height to width of each ridge is from about 0.5 to about 200.

42. A method of removing contaminated particles from thermoplastic melt comprising forcing molten liquid thermoplastic melt containing contaminated particles radially through a filtering passage of decreasing height defined by opposing faces of toroidal filter elements, each toroidal filter element comprising a rigid member with a top face and a bottom face, said top face having a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of ridges gradually increasing in height toward the exit end of said radial filtering passages, whereby said contaminated particles are trapped in the decreasing space of said radial filtering passages of abutting ilter elements as the melt flows through said radial filtering passageways over the ridges and through the troughs of abutting elements.

43. The method of claim 42, wherein said bottom face has a plurality of ridges arranged perpendicularly to a direction of processing over said ridges and adjacent ridges forming troughs therebetween, said plurality of raised ridges gradually increasing in height toward the exit end of said radial filtering passages.

44. The method of claims 42 or 43, wherein the space between adjacent ridges is from about 0.002 to about 0.200 inches.

45. The method of claims 42 or 43, wherein the height of each raised ridge relative to a radial axis through the thickness of said structure is from about 0.001 to about 1.0 inch.

46. The method of claims 42 or 43, wherein the width of each ridge is said direction of processing is from about 0.001 to about 0.250 inches.

47. The method of claim 42 or 43, wherein each face has about 1 to about 1000 troughs.

48. The method of claims 42 or 43, wherein the depth of each trough is from about 0.001 to about 0.200 inches.

49. The method of claims 42 or 43, wherein each trough in cross-section is formed with a flat bottom and outwardly divergent sides or a curvilinear shape.

50. The method of claims 42 or 43, wherein the ratio of height to width of each ridge is from about 0.5 to about 200.