Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles
Reexamination Certificate
1999-10-06
2001-10-02
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
C264S628000, C264S630000, C264S122000, C264S125000, C264SDIG004
Reexamination Certificate
active
06296794
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to filtering apparatus and more particularly to compact porous monolithic filter bodies useful, for example, for the efficient purification of drinking water.
Many filter designs for the removal of unwanted impurities from drinking water supplies are known. These include the increasingly common water carafe filters, commercially sold as replaceable filter cartridges for drinking water carafes at relatively low cost. These filters are reasonably effective for the removal of unwanted tastes, odors, and harmful pollutants from drinking water.
Replaceable filter cartridges of these types are typically packed-bed units containing carbon pellets for the removal of adsorbable/catalyzable constituents such as chlorine and organics. Also included are ion exchange resins for the removal of metal ions such as lead, and in some cases bacteriostatic agents such as silver to inhibit the growth of harmful bacteria.
Although gaining in acceptance, filters designed for use in replaceable cartridge filtration systems suffer from a number of disadvantages. Packed bed systems typically develop preferential flowpaths within the bed, resulting in less than optimal use of the available adsorbents and exchange media and correspondingly decreased purifying effectiveness and service life. Packed beds also tend to release fine adsorbent particles into the treated water during initial use.
Also limiting the utility of these systems is water filtration rate. In general, gravity-assisted water flow rates through these filters are not high. For this reason alone, the use of more tightly packed, higher-efficiency filtration media is not commercially acceptable.
Fluid filtration elements comprising plugged honeycomb filters are well known in the art. U.S. Pat. No. 4,329,162 and published European patent application EP 0745416, for example, describe the use of such filter elements for the removal of particulates from vapor, water and other fluid streams. One disadvantage of these filter elements, however, is cost. The porous ceramic honeycombs used in these filter designs to support carbon adsorbents and provide wall filtration are relatively expensive. Further, manufacturing processes for infiltrating the ceramics with adsorbents and/or other active filtering materials are somewhat elaborate and time-consuming, as are present methods for plugging of the honeycombs to define effective filtration flowpaths through the honeycomb walls.
A number of attempts to decrease the cost and/or increase the efficiency of these honeycomb structures have been made. U.S. Pat. No. 5,006,432, for example, describes a method of making a plugged honeycomb (or “z-flow”) filter by first machining a block of open-pore plastic foam to form a filter preform, then infiltrating the preform with a ceramic slurry, and finally heating the filled form to oxidize the foam and sinter the ceramic into a porous honeycomb filter. U.S. Pat. No. 3,788,486 describes pressed or machined porous z-flow filter of similar design for use in filtering molten thermoplastics, while U.S. Pat. No. 2,819,209 describes a pressed fluoropolymer filter for caustic or acidic liquid filtration.
Honeycomb structures for other applications have also been formed by pressing. Thus U.S. Pat. Nos. 2,734,843 and 3,226,460 describe pressing processes and apparatus for the continuous production of lengths of honeycomb core material for the fabrication of reinforced composite structures. These structures are not, however, designed for liquid filtration.
Unfortunately, none of the various alternative methods for making elements of honeycomb structure for use as fluid filters have proven adaptable for the economic commercial production of filters for drinking water purification in large volumes. One particular problem not solved by prior art methods arises from the nature of some of the materials used for the manufacture of these elements. In the case of preforms for activated carbon filters, for example, carbon precursors including phenolic resins or other carbon-yielding organic species can be present which impart a sticky quality to the pressing batch. It is difficult to form thin-walled honeycomb structures from batches containing such precursors, because the thin-walled structures are quite susceptible to damage in the course of separation of the batch material from conventional metal or other forming surfaces.
SUMMARY OF THE INVENTION
The present invention provides an economical yet effective pressing process for the production of preforms for porous filter bodies of the type useful for fluid filters. Manufacture can be effected at sufficiently low cost to provide an economic filter element in a system such as a replaceable water purification filter cartridge.
The preforms are pressed from mixtures of powders with appropriate binders, and optionally precursors for supplemental filter constituents such as activated carbon. Prior to pressing the mixtures are blended and worked into plasticized powder batches having compositions which can be converted after pressing to porous filter materials, using suitable post-forming treatments such as polymerization or sintering heat treatments.
The pressed preforms provided in accordance with the invention are for filters of z-flow filter design. Such filters generally comprise an entrance face, a discharge face, a plurality of elongated inlet channels opening on the entrance face and extending into the body toward the discharge face, and a plurality of elongated outlet channels, offset from the inlet channels, opening on the discharge face and extending into the body toward the entrance face.
In the preforms of the invention, the discharge channels are parallel with but separated from the inlet channels by thin channel walls of the plasticized powder batch material, the inlet channels extending past or overlapping the discharge channels over most of the lengths of each. Further, in the typical preform design, each inlet channel is closed at the discharge face by an end segment of plasticized batch material continuously connecting with the channel walls for the inlet channels, while each discharge channel is similarly closed at the inlet face of the preform by a segment of batch material similarly integrated with the adjacent wall structure.
An alternative preform structure is a pressed preform comprising a structural element or sub-unit for a z-flow filter which includes a first channel array of channels closed at a first face of the structure and a second array of channels opening on both faces, i.e. completely traversing the preform shape. Two of these structural elements may then be combined to provide a completed z-flow filter or filter preform comprising two sets of oppositely closed channels identical to the typical preform design.
The method of making a channeled honeycomb filter preform in accordance with the invention comprises the initial step of introducing a charge of the plasticized powder batch material into a suitable pressing chamber. The pressing chamber includes a sidewall member and at least first and second opposing pressing elements for forming channels in the batch charge. The pressing elements include first and second base plates supporting, respectively, first and second arrays of channel-forming members attached to and extending generally perpendicularly from the base plates, each array extending toward the other when the chamber is open.
To press the preform into the desired configuration, the sidewall member and opposing first and second pressing elements are brought into contact with each other and with the charge of batch material, and the pressing elements are advanced toward each other to close the pressing chamber. Within the closed chamber, the channel-forming members of the first pressing element extend from the first base plate through the batch charge toward the second base plate, being separated from the second plate by a first interval, while the channel-forming members of the second pressing element extend from the second base plate towar
Day J. Paul
Hickman David L.
Rajnik Lawrence S.
Corning Incorporated
Sterre Kees van der
Vargot Mathieu D.
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