Abstract:
A filter support sheet extruded from a fluoroplastic material includes a base section having spaced-apart ribs extending outwardly from at least one surface thereof. Segments between spaced-apart ribs provide flow channels and apertures are punched through the sheet in a predetermined pattern unaffected by the structure of the support sheet. A method includes the steps of: (1) extruding a thermoplastic, fluoroplastic material to form of a sheet; (2) passing the sheet through a nip region provided by opposed rollers; at least one having an outer surface with counter-sunk grooves. Counter-sunk grooves in one roller are aligned with an outer surface or counter-sunk grooves of the other roller in the nip region to form a ribbed sheet having ribs upstanding from at least one surface of said sheet; (3) setting the ribbed sheet and (4) punching apertures through the ribbed sheet in a predetermined pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This international application claims the benefit under 35 U.S.C. §119(e) of U.S. Patent Application Ser. No. 62/033,683, filed Aug. 6, 2014, entitled RIBBED AND APERTURED FLUOROPLASTIC SUPPORT SHEET FOR A FILTER SUBSTRATE AND METHOD OF MAKING SAME, the disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a ribbed and apertured support sheet for a filter substrate extruded from a thermoplastic, fluoroplastic material and to a method of making same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Filter members employing filter media supported on each side by an embossed and apertured support sheet are known in the art. These filter members are commonly employed in cartridge filters for filtering particulate material in a variety of cleaning operations. 
         [0004]    Filter members for use in cleaning semiconductor components preferably include support sheets made of a fluoroplastic material and most preferably such sheets are apertured to permit filtrate to pass through the sheets into underlying filter media. However, it has been difficult to extrude, emboss, and aperture a thermoplastic fluoroplastic material into a bi-planar sheet by conventional means. 
         [0005]    On prior art approach for forming embossed and apertured webs from a fluoroplastic material is disclosed in Cullen et al. U.S. Pat. No. 5,776,343. The method disclosed in this patent has a number of deficiencies and has not been entirely satisfactory in form the desired pattern of apertures in the sheet. 
         [0006]    The fluoroplastic, pleated and apertured sheet disclosed in the Cullen &#39;343 patent is formed by embossing an extruded sheet between a smooth surfaced roll and an opposed roll having projections extending therefrom and completely surrounded by proximal, flat surfaces. This embossing step provided two, interrelated functions. First, it pleated the polymeric sheet film in a specific pattern dictated by the pattern of the projections on the embossing roll opposed to the smooth surface roll. Second it established the location of apertures to be formed in the sheet by a subsequent orientation step. To further explain, the projections on the embossing roll either slit the sheet or provided thin, weakened regions. Upon biaxially stretching the sheet the slits or weakened regions were at least partially opened or perforated to form apertures. Thus, it should be apparent that the embossing operation was responsible both for the embossed, pleated pattern in the sheet and the arrangement of apertures formed therein. The method disclosed in the &#39;343 patent, while producing a pleated, apertured web, did not produce the most clearly formed apertures. Moreover, the pattern of apertures formed in the sheet was dictated by the pattern of raised projections in one of the embossing rolls, which also dictated the pattern of pleats formed in the sheet. Thus, the process disclosed in the &#39;343 patent did not provide the desired degree of flexibility of establishing a predetermined pattern of apertures independent of establish a three dimension configuration in the sheet. 
         [0007]    U.S. Pat. No. 8,563,114, assigned to Astenjohnson, Inc, generally discloses and embossed and apertured sheet but is not directed to a high purity, fluoroplastic support sheet employed as part of a filter member. In the process disclosed in the &#39;114 patent slits are cut into a substrate, which can be either paper or a polymer, and then regions between the slits are raised above the plane to form a multitude of bridges or overpasses between the slit regions. The apertures run parallel to the sheet plane; not in the Z-direction through the plane. A Z-direction force imposed by the filtrate on the substrate disclosed in the &#39;114 will tend to flatten out the substrate and close the apertures or slits. In addition, the method employed in the &#39;114 patent requires the use of a preexisting film and does not suggest the forming of a patterned sheet formed or cast from a molten fluoroplastic material. 
         [0008]    Attempts were made to form desired, ribbed and apertured support sheets in accordance with the teachings in Hovis et al. U.S. Pat. No. 5,207,962, but that process had only limited commercial success and was unable to be used to provide a patterned, rib structure with ribs being continuous in the machine direction of formation. 
         [0009]    Other prior art disclosing embossed and apertured webs are the following: 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 i. 
                 U.S. Pat. No. 4,798,604 
                 Carter 
               
               
                 ii. 
                 EP 0 509012 
                 Carter 
               
               
                 iii. 
                 U.S. Pat. No. 8,241,543 
                 O&#39;Donnell et al. 
               
               
                 iv. 
                 U.S. Pat. No. 2008/0206529 
                 Ueminami et al. 
               
               
                 v. 
                 U.S. Pat. No. 8,575,417 
                 Oetjen et al 
               
               
                 vi. 
                 U.S. Pat. No. 2002/0146540 
                 Johnston et al. 
               
               
                 vii. 
                 U.S. Pat. No. 7,736,688 
                 Oetjen et al 
               
               
                   
               
             
          
         
       
     
         [0010]    All of the prior art documents disclosed herein are fully incorporated by reference herein. 
         [0011]    However, none of the above patents disclose or suggest a 3-dimensional patterned (ribbed) and apertured, fluoroplastic support sheet of a filter member wherein a three-dimensional ribbed pattern is formed in the sheet prior to forming the apertures therein and wherein the apertures are formed in a predetermined pattern that is unaffected by the 3-dimensional pattern of ribs in the fabric. However, this invention relates to this latter described structure of a filter support sheet and its method of manufacture. 
       SUMMARY OF THE INVENTION 
       [0012]    A support sheet for a filter substrate is extruded from a thermoplastic, fluoroplastic material and includes a planar, continuous base section having opposed outer surfaces and spaced-apart ribs extending outwardly from said at least one outer surface thereof. The ribs each include an outer distal end and an inner proximal end; said inner proximal end being continuous with an outer surface of said base section. Spaced-apart ribs cooperate with an outer surface from which they extend to provide outer surface segments between adjacent ribs providing continuous flow channels bound by said adjacent spaced-apart ribs for fluid containing material to be filtered. The support sheet includes apertures punched through the entire thickness thereof in a predetermined pattern unaffected by the structure of the support sheet, said predetermined pattern including apertures extending through ribs and outer surface segments between ribs, said ribbed and apertured sheet directing fluid containing material to be filtered along channels and through apertures into said filter substrate. 
         [0013]    In one preferred embodiment, the support sheet for the filter substrate of claim includes spaced-apart ribs extending outwardly from both outer surfaces of the continuous base section of the sheet. 
         [0014]    In a preferred embodiment, the plurality of channels between adjacent ribs extends linearly along the support sheet. 
         [0015]    In a preferred embodiment, the support is a cast, unoriented structure. 
         [0016]    A method of this invention for forming support sheet for a filter structure includes the steps of: (1) extruding a substrate of a thermoplastic, fluoroplastic material through an extrusion die in the form of a continuous sheet and prior to setting said sheet; (2) passing the sheet through a nip region provided by opposed rollers having outer surfaces, said outer surfaces being free of projections extending outwardly therefrom, at least one of said outer surface including counter-sunk grooves therein, said counter-sunk grooves in one of said rollers being aligned with an outer surface or counter-sunk grooves of said other of said rollers in the nip region, whereby said continuous sheet is formed into a ribbed sheet having a planar, continuous base section and upstanding ribs extending outwardly from at least one outer surface of said base sheet; (3) setting said ribbed sheet; and thereafter, (4) punching apertures through the entire thickness of said ribbed sheet in a predetermined pattern. 
         [0017]    In a preferred method the ribbed sheet is a cast sheet that is unoriented in either the machine-direction or cross-machine-direction thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0018]    The invention may be more fully understood with reference to the accompanying drawings, wherein: 
           [0019]      FIG. 1  is schematic, isometric view showing a three-layer filter member including a filter substrate sandwiched between two support sheets; 
           [0020]      FIGS. 2A-2C  are isometric views showing different embodiments of support sheets of this invention at an intermediate stage of formation; with ribs extruded therewith in different arrangements/patterns, but prior to such support sheets being apertured; 
           [0021]      FIG. 3A  is a plan view of the support sheet of this invention showing the arrangement of ribs and apertures in the structure without stretching the sheet; 
           [0022]      FIG. 3B  is a plan view of the support sheet of this invention similar to  FIG. 3 , but after the sheet has been stretched in the machine direction of formation to elongate the apertures. 
           [0023]      FIGS. 4A-4C  show cooperating pattern rolls with different arrangements of pockets to vary the rib structure in the support sheet for forming the embodiments of support sheets illustrated in  FIGS. 2A-2C  respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Reference throughout this application including the claims to the apertures being punched through the entire thickness thereof “in a predetermined pattern unaffected by the structure of the support sheet” means that the support sheet is not embossed or otherwise provided with slits or weakened regions that are intended to constitute the regions in which apertures are created or formed. The predetermined pattern of apertures in this invention may very well be set, selected or determined by the structure of the ribbed sheet, but the structure of the ribbed sheet is not responsible for providing or establishing the predetermined pattern of apertures. In the preferred embodiment the predetermined pattern of apertures is provided by a predetermined pattern of punch members in a punch press used to create the apertures in the ribbed sheet. 
         [0025]    Reference throughout this application to “filter media” or “filter substrate” being a component of the filter member of this invention can be of any desired configuration or composition for carrying out the desired filtering operation. For example, and not by way of limitation, the filter media can be either a single layer or a multilayer structure and can be formed from melt blown layers, spun bond layers, other non-woven layers and combinations thereof. 
         [0026]    Referring to  FIG. 1 , filter support members  10  in accordance with this invention support an internal filter substrate  12  to form a composite filter member  14 . The filter support members  10  can be of identical constructions; formed from an extruded sheet of a thermoplastic, extrudable fluoroplastic material, most preferably a perfluoroalkoxy (PFA) polymer sold under the designation Noeflon by Daikin. However, other fluoroplastics may be usable in this invention, e.g., ethylenechlorotrifluorethylene (ECTFE); ethylenetetrafluoroethylene (ETFE) of polyvinylidene fluoride (PVDF). 
         [0027]    It has been recognized that a fluoroplastic material such as PFA is highly desirable for use in filters intended to clean semiconductor components and in other environments where extreme cleanliness is required and the possibility of contamination is minimized. Moreover the support members  10  need to be designed to both direct fluids to be filtered along their surfaces and also for directing the fluids through the structure into the underlying filter substrate  12  to remove undesired particulates from the filtrate. 
         [0028]    Although the filter support members depicted in  FIG. 1  are designated generically as “10” it should be understood that these members can be of on any of the arrangements/configurations described hereinafter, including but not limited to the arrangements illustrated in  FIGS. 2A, 2B and 2C , as well as an arrangement of skewed ribs on opposed sides of the continuous base section of the support member. Moreover, as shown in  FIG. 1 , the support members  10  include ribs  22  disposed outwardly from only one side of said member. As illustrated the ribs  22  face outwardly of the composite filter member  14 . However, if desired the support members  10  can be reversed to position the spaced-apart ribs in engagement with the internal filter substrate  12 . The orientation of the ribs  22  relative to the internal filter substrate  12  is a matter of customer preference. 
         [0029]    Referring to  FIG. 2A , the support member  10  in accordance with one embodiment of this invention is shown in an intermediate stage of formation; after being passed through a nip provided by opposed rolls  30 ,  32 ; one roll, e.g.,  30  having axially spaced-apart pockets  34  extending circumferentially around its periphery and cooperating with a smooth-surfaced, opposing roll, e.g.,  32  ( FIG. 4A ). Thereafter the support member  10  is set to be processed further. At this stage of formation the support member  10  is a ribbed substrate including a continuous base section  16  having opposed outer surfaces  18 ,  20 , and spaced-apart ribs  22  extending outwardly from outer surface  20 , whereby outer surface segments  24  between adjacent spaced-apart ribs  22  provide continuous fluid directing or flow channels for fluids containing particulates to be filtered therefrom. 
         [0030]    Referring to  FIG. 2B , in an alternative embodiment of the support member  10 A, spaced-apart ribs  22 A also can be provided projecting outwardly from outer surface  18  of the base section  16 ; either in vertical alignment with ribs  22 , as illustrated in  FIG. 2B , or transversely offset with respect to such ribs  22 , as illustrated in  FIG. 3C . It should be noted that the arrangement of ribs  22 A illustrated in  FIGS. 2B and 2C  are formed with the use of the pattern rolls  30 A,  32 A and  30 B,  32 B, respectively, and as will be described in greater detail hereinafter. 
         [0031]    Other arrangement of ribs can be provided. For example, and not by way of limitation, ribs  22  and/or  22 A can be formed in a skewed pattern across the sheet as the sheet is passed through the nip of a specially designed set of pattern rolls (not shown). The formation of pockets or grooves in pattern rolls to provide skewed ribs should be easily accomplished by a person skilled in the art; based on the disclosure herein. 
         [0032]    Referring again to  FIG. 2A , each of said ribs  22  include an upper distal end  26  and an inner, proximal end  28 , and each rib  22  is a continuous extension from base section  16 . As can be seen in  FIG. 2B and 2C , the ribs  22 A have the same arrangement of distal and proximal ends  26 A,  28 A as the distal and proximal ends  26 ,  28  of the ribs  22 . 
         [0033]    It should be noted that the ribbed filter support members  10 ,  10 A and  10 B, as well as other ribbed support members within the scope of this invention, are not pleated structures having projections including underlying recesses complimentary in shape to the projections. In fact, the ribbed structures  10 ,  10 A and  10 B, as well as other ribbed structures in accordance with this invention all have a continuous, planar base sections. Also the ribbed filter support members  10 ,  10 A and  10 B, as well as other ribbed support members within the scope of this invention, do not include any predetermined pattern of weakened regions or slits in the originally formed sheet to affect the location or pattern of apertures that subsequently is formed therein. 
         [0034]    Referring to  FIG. 3A , after formation of the ribs  22  and/or  22 A in the extruded substrate, the substrate is set; preferably without either longitudinal or transverse stretching. This cast substrate can then be wound into a roll and subsequently unwound and directed through a punch press to form apertures  28  through the Z-direction thereof in a desired, predetermined pattern ( FIG. 3A ). Alternatively, the sheet, after being set, can be directed in a continuous operation through a punch press to form the predetermined pattern of apertures  28  therein. This predetermined pattern can provide apertures  28  extending both through channel sections, e.g.,  24 , of the substrate and ribbed sections, e.g.,  22  of the substrate, as desired. 
         [0035]    Referring to  FIG. 3B , after aperturing, the filter support members can be stretched in the machine direction, as indicated by the double-headed arrow  40 , to elongate the apertures  28  for providing greater open area for passage of the fluid to be filtered by the filter media or substrate  12 . This greater open area is evident by comparing  FIG. 3B  with the non-elongated filter support member shown in  FIG. 3A . 
         [0036]    As noted earlier, the three-dimensional configuration of the ribbed substrates of this invention does not affect the actual formation of the apertures  28  therein. The predetermined pattern of apertures  28  is determined by the pattern of punches in a punch press or similar device. This latter pattern may be selected based on the arrangement of ribs  22  and/or  22 A in the substrate but there are no weakened regions or slits formed in the ribbed and cast sheet to dictate or predetermine the pattern of apertures formed therein. 
         [0037]    Referring to  FIG. 4A , the substrate for the filter support member  10  shown in  FIG. 2A  is extruded in a molten state from a conventional extruder into a substantially planar sheet that is passed through the nip of opposed rolls illustrated schematically and in fragmentary view at  30 ,  32  to form the ribs  22  projecting outwardly from planar base section  16  thereof. 
         [0038]    The roll  30  is a patterned roll with counter sunk grooves  34  circumferentially continuous about the periphery of the roll and spaced-apart from each other along the axial extent of the roll. However, if desired it is within the scope of this invention to form the grooves or pockets discontinuously about the circumference and/or arranged at an acute angle, e.g., skewed to the rotational axis of the roll. 
         [0039]      FIG. 4A  shows the arrangement of cooperating rolls  30 ,  32  when ribs, e.g.,  22  are only intended to be formed on one side, e.g., side  20  of the planar base  16 . As is apparent in  FIG. 4A  the roll  32  opposed to roll  30  has a continuous peripheral surface  34 A free of pockets, projections or other three dimensional patterns. 
         [0040]    Referring to  FIG. 4B , an arrangement of cooperating rolls  30 A,  32 A is depicted for forming ribs  22  and  22 A in alignment with each other and extending from opposed surfaces  20 ,  18 , respectively, of the support member  10 A as shown in  FIG. 2B . As can be seen in  FIG. 4B , opposed rolls  30 A,  32 A have circumferential, countersunk grooves  34 ,  36  in axial alignment with each other to form the ribs  22 ,  22 A, in the support member  10 A shown in  FIG. 2B . 
         [0041]    The arrangement of the counter sunk grooves  34  can be selected independently of the arrangement of the counter sunk grooves  36 . For example, as shown in  FIG. 4B  the grooves  34 ,  36  can be in alignment with each other about the complete circumference of the rolls  30 A,  32 A, in which case the raised ribs  22 ,  22 A on opposed surfaces of the substrate will be in alignment with each other, as illustrated in  FIG. 2B . 
         [0042]    Alternatively, the grooves  36  in roll  32 B can be out of axial alignment with the grooves  34  in rolls  30 B as shown in  FIG. 4C  to provide ribs  22  that are out of alignment with ribs  22 A. 
         [0043]    Thus, the counter-sunk grooves  34 ,  36  in the roll pairs  30 A,  32 A ( FIG. 4B ) and  30 B,  32 B ( FIG. 4C ), can be directly over each other, offset with respect to each other and/or skewed relative to each other, depending on the desired flow paths to be established in one or both surfaces of the support members, e.g.,  10 ,  10 A,  10 B of this invention. 
         [0044]    In preferred, non-limiting embodiments of this invention a support member  10 , includes 4-60 ribs per inch on one side; more preferably 10-30 ribs per inch and most preferably 20 ribs per inch. The spacing of the ribs may be asymmetrical if so desired. These ribs desirably can be formed in the machine direction of the support members  10 ,  10 A, and  10 B. The depth of each of the ribs  22 , from the distal end  26  thereof to the proximal end  28  contiguous with the outer surface  20  of planar base section  16 , is in the range of 2 to 40 mils; more preferably in the range of 2 to 10 mils and most preferably about 3 mils. The depth of the ribs can be varied over the structure, i.e., they may be asymmetrical if so desired. If desired, the opposed outer surface  18  can have the same arrangement, spacing and dimensions of ribs as extend from outer surface  20 . 
         [0045]    The important feature in the method of this invention is that there are no sharp projections extending from any of the pattern forming rolls that could provide weakened or slit regions in the extruded substrate or that are intended to pleat the substrate. 
         [0046]    Although the filter support members of this invention preferably are non-pleated, cast (non-oriented) sheets it is within the broad scope of the invention to subsequently pleat the support members after they have been provided with ribs and apertures therein. Also, as described in connection with  FIG. 3B , it is within the scope of the invention to stretch or orient the support members after they have been apertured 
         [0047]    Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention defined in the appended claims.