BATTERY SEPARATOR AND METHOD OF MAKING

Abstract: A battery separator (10) for use in flooded cell type lead acid batteries comprising a backweb of a porous, acid resistant, embossable material with a plurality of major ribs (14) and submini-ribs extending from at least one planar surface (12) of the backweb. The ribs (14) are located across the width of the backweb and extend in a direction substantially parallel to the longitudinal edges (16, 18) of the backweb. Each major rib (14) overlies at least one submini-rib (15) and is an embossed corrugated structure comprised of alternating ridges and furrows (22, 23). The ridges and furrows (22, 23) are in non-parallel alignment to the longitudinal dimension of the separator, and preferably perpendicular thereto. The major ribs are formed on a battery separator (10) backweb having a plurality of submini-ribs (15) extending from at least one planar surface (12) thereof by passing the backweb through the nip formed by a pair of opposed embossing rollers and embossing the backweb in the area of at least one submini-rib (15).

Description: BATTERY SEPARATOR AND METHOD OF MAKING TECHNICAL FIELD This invention relates to a battery separator for use in flooded cell type lead acid batteries and to a method for making such separators. BACKGROUND ART In a flooded cell type lead acid battery the positive and negative electrodes or "plates" are separated by a battery separator. The battery separator typically has "ribs" or protrusions extending from at least one planar face of the separator. Such ribs are formed in one of several ways: the ribs can be formed integrally with the backweb of the separator; the ribs can be subsequently applied to the backweb as a bead of the same or different material as the backweb; or the ribs can be formed by embossing the backweb. The ribs function to provide proper spacing between the plates and to provide a space wherein free electrolyte resides. The battery separator currently used by most flooded cell type lead acid battery manufacturers is of the microporous polyethylene type. This type of separator has a composition consisting essentially of an ultra high molecular weight polyethylene, a filler (typically amorphous silica) , a plasticizer (typically a processing oil) , and certain minor ingredients such as an antioxidant, lubricant and carbon black. Microporous polyethylene separator material is commercially manufactured by passing the ingredients through a heated extruder, passing the extrudate generated by the extruder through a die and into the nip formed by two heated calender rolls to form a continuous web, extracting a substantial amount of the processing oil from the web by use of a solvent, drying the extracted web, slitting the web into lanes of predetermined width, and winding the lanes into rolls. Such separators and a method of manufacturing them are described in U.S. Patent No. 3,351,495. Microporous polyethylene separators typically have a configuration comprising a backweb having a predetermined thickness, and a plurality of parallel ribs spaced apart a predetermined distance and extending outwardly from one planar surface of the backweb. The ribs extend continuously in a longitudinal direction parallel to the edges of the separator material . A partial perspective view of such a prior art separator is illustrated in FIG. 7. The thickness of the backweb and height and spacing of the ribs is specified to the separator manufacturer by the battery manufacturer; the specifications are designed to maximize certain battery characteristics desired by the battery manufacturer. It is also known to form "mini-ribs" between such "major" ribs to add stiffness to separator webs having thinner backwebs. Generally, such mini-ribs have a lower height than the major ribs and are spaced closer together. The height of such mini-ribs typically varies between about 0.006 inch and about 0.009 inch. The spacing of such mini-ribs varies between about 0.060 inch and about 0.250 inch. Such ribs (both major and mini) are formed during manufacture of the microporous polyethylene separator by providing that one of the two heated calender rolls forming the nip through which the extrudate from the extruder is fed is engraved with grooves so that the ribs are formed as an integral part of the separator web. There are many different specifications required by battery manufacturers relative to rib size and rib spacing. In manufacturing separator material to meet customer requirements, almost every change in rib size and spacing requires that the separator manufacturer shut down its manufacturing line in order to remove the engraved roll that had been in use to fill the prior order and to insert a differently configured engraved roll capable of producing the rib size and spacing required for the new order to be filled. Manufacturing time is lost during such shut-down and extra scrap material is generated during start-up of the line. In addition, integrally formed ribs in the polyethylene type separator undergoes extraction along with the backweb and, because it has relatively more volume than a portion of the backweb occupying the same planar surface area, generally the ribs retain more processing oil than the backweb, thereby raising the overall electrical resistance of the separator. In commonly owned copending U.S. patent application Serial No. 08/646,764, filed May 8, 1996, there is disclosed a battery separator having a longitudinal dimension, a width dimension perpendicular to said longitudinal dimension, upper and lower planar faces, and a plurality of ribs (at least three) projecting from at least one planar face, said ribs extending in a direction substantially parallel to the longitudinal dimension of the separator, each of the ribs being formed of a plurality of individual projecting embossments forming a corrugated structure comprised of alternating ridges and furrows. The ribs may extend from one or both planar faces of the separator. Where the ribs extend from both planar faces, adjacent projecting embossments (ridges) on one planar face are separated by an indentation (furrow) which forms a projecting embossment (ridge) on the other planar face of the separator. Where ribs extend from both planar faces of the separator, the ribs projecting from one planar surface may have a height equal to or different from the height of the ribs extending from the other planar surface. While the separator described in Serial No. 08/646,764 performs very well where the rib height does not exceed about 0.030 inch, it has been found that with rib heights in excess of about 0.030 inch compression resistance of the ribs becomes less satisfactory. By "compression resistance" is meant resistance to a co pressive force applied to the tops of the ribs. It is an object of this invention to provide a separator having ribs formed of a plurality of individual projecting embossments forming a corrugated structure comprised of alternating ridges and furrows which has improved compression resistance. DISCLOSURE OF THE INVENTION This invention relates to a battery separator having a longitudinal dimension, a width dimension perpendicular to said longitudinal dimension, upper and lower planar faces, a plurality of submini-ribs extending from at least one of said planar faces, and a plurality of individual projecting embossments forming a corrugated structure comprised of alternating ridges and furrows extending through at least some of the submini-ribs. The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial perspective view of the upper planar surface of the separator material of this invention. FIG. 2 is an enlarged partial edge view in cross-section of the separator taken along line 2 - 2 of FIG. 1. FIG. 3 is an enlarged partial edge view in cross-section of a separator rib taken along line 3 - 3 of FIG. 1. FIG. 4 is an enlarged partial top plan view of a single rib of this invention. FIG. 5 is a frontal view of two embossing rolls employing a plurality of embossing wheels. FIG. 6 is a side view of two adjacent embossing wheels. FIG. 7 is a perspective view of a portion of a prior art separator showing the backweb and integrally formed ribs extending from the backweb. FIGS. 8 through 11 are partial top plan views of the upper planar surface of the separator illustrating several alternative configurations for the rib configuration. FIG. 12 is a perspective view of an embossing tooth having a rounded wedge shape. FIG. 13 is a perspective view of an embossing tooth having a conical shape. FIG. 14, is a perspective view of an embossing tooth having a hemispherical shape. FIG. 15 is a perspective view of an embossing tooth having the shape of a truncated cone. BEST MODES FOR CARRYING OUT THE INVENTION FIG. 1 is a partial perspective top view of the separator 10 of this invention. Projecting from the upper planar surface 12 of separator 10 are a plurality of major ribs 14a through 14j . The major ribs 14 are disposed in the longitudinal direction of the separator 10, substantially parallel to longitudinal edges 16 and 18 thereof. Also projecting from the upper planar surface 12 of separator 10 are a larger plurality of substantially evenly spaced "submini-ribs" 15. These ribs are called "sub-mini ribs" because they are shorter and spaced closer together than prior art mini-ribs. Submini-ribs 15 are also disposed in the longitudinal direction of the separator 10, substantially parallel to longitudinal edges 16 and 18 thereof. Major ribs 14 are formed by embossing separator 10 in the area of one or more submini-ribs 15. The major ribs 14 illustrated in FIG. 1 do not show the presence of submini ribs 15 therein for the sake of clarifying the drawing. The relationship of the submini-ribs 15 and major ribs 14 are shown in FIGS. 2-4. FIG. 2 is a partial edge view in cross-section of separator 10 taken along Line 2 - 2 of FIG. 1. Major rib 14 projects above upper planar surface 12 of backweb 20 of separator 10 and above adjacent submini-rib 15a. Submini-rib 15b is shown located on outer surfaces 17 and 19 (shown in FIG. 4) of the embossed rib. A partial edge view in cross-section of major rib 14 is illustrated in FIG. 3. As can be seen, the exterior of major rib 14 is a corrugated structure comprised of alternating ridges and furrows 22a and 23a, 22b and 23b, 22c and 23c, 22d and 23d, etc., respectively. Likewise, the interior of major rib 14 is a corrugated structure comprised of alternating ridges and furrows 24a and 25a, 24b and 25b, 24c and 25c, 24d and 25d, etc., respectively. A ridge on one planar surface forms the furrow on the other planar surface, and vice versa. For example, the underside of ridge 22a of the exterior surface of major rib 14 forms the furrow 25a of the interior surface of major rib 14. Submini-rib 15b is shown located on outer surfaces 17 and 19 of the major rib 14. Adjacent submini-rib 15c is shown in the background. FIG. 4 is an enlarged, partial top view of a major rib 14. As can be seen, ridges 22 and furrows 23 of major rib 14 are perpendicular to the longitudinal side walls 26 and 28 of major rib 14 and thus also perpendicular to the longitudinally disposed side edges 16 and 18 of separator 10. Major rib 14 is shown as having been embossed in an area encompassing two submini- ribs 15d and 15e. While two submini-ribs 15d and 15e are shown, the major rib 14 can be formed through a single submini-rib or through more than two submini-ribs by appropriate selection of the width of the major rib 14, the spacing of the submini-ribs, or a combination of both. The length dimension of the ridges 22 and 24, i.e., the length of the ridges 22 and 24 from one side wall 26 to the other side wall 28 of major rib 14, is selected in accordance with the desired width of the rib. This dimension will generally be between about 0.020 and about 0.100 inch. The frequency of the ridges, i.e., the number of ridges per unit of rib length, will preferably be between about 5 and about 25 ridges per inch. The height of the ridges 22 and 24 above the respective planar surfaces 12 and 13 of the backweb 20 is selected in accordance with the desired height of the major ribs 14. This dimension will generally be between about 0.01 to about 0.10 inch. The submini-ribs 15 are particularly useful in providing improved compression resistance when the desired height of major ribs 14 is greater than about 0.030 inch. The distance between adjacent major ribs 14 will generally be between about 0.25 to about 1.0 inch. The height of the submini-ribs above the upper planar surface of the separator will generally be between about 0.003 inch and about 0.009 inch. Selection of an appropriate height for the submini-ribs 15 will depend upon the height of the major ribs 14 and the desired compression resistance. The submini-ribs 15 will generally be evenly spaced, between about 0.025 inch and about 0.050 inch, across the width of the separator. The width of the separator of this invention can be any width used by battery manufacturers; generally this width will range between about 2 to about 12 inches with side edges 16 and 18 being parallel to each other. The thickness of backweb 20 of the separator 10 will typically range between about 0.002 to about 0.025 inch. The cross-section of the corrugated structure forming the rib of this invention is illustrated in FIGS. 1-4 as being triangular, with each individual embossed projection thus being wedge shaped. Each wedge shaped embossment is comprised of a sloping leading wall 17, a sloping trailing wall 19 and vertical side walls 26 and 28, as best seen in FIG. 4. However, other similar wedge shapes may be used, including but not limited to a rounded wedge shape where the ridge is slightly rounded instead of angular, or a flattened wedge shape where the ridge is either formed flat during embossing or the ridge flattened by subsequent pressing. One of the advantages obtained in using the separator of the present invention is that, since the ribs are a corrugated structure formed of adjacent ridges and furrows, the separator provides a substantially barrier free environment for flow of electrolyte and any gases released during charging and discharging since the ribs contact the plates only in the ridge area. Prior art ribs (see, for example, FIG. 7) , being solid, are in contact with the positive plate in the entire upper surface area of the rib which imposes a barrier to electrolyte and/or gas flow. A further advantage obtained in using the separator of the present invention is in reducing or eliminating rib bending and/or flattening. In assembling a battery the number of alternating enveloped (with separator material) and unenveloped plates required to form a cell are stacked together, compressed and inserted into a cell compartment of a battery case. The compressive forces to which the separators are subjected can cause undesirable rib bending and/or flattening with conventional rib construction, especially at higher "overall" dimensions, i.e., rib heights. The rib configuration of the present invention reduces or eliminates such rib bending and/or flattening. The use of submini-ribs to provide higher major rib height without loss of compression resistance also allows separators to be made with thinner backwebs which means a savings in the amount of material required to make a specified square footage of separator product. Another advantage to the submini-ribbed separator product is that longer production runs between tooling changes can be made compared to manufacturing separator product with major ribs being formed on the production line. Also, solvent extraction and drying of submini-ribbed product is easier and faster than product having major ribs formed thereon during manufacture. Although the separator of the invention is illustrated as having major ribs 14 extending from only one planar surface 12 of the backweb 20, the invention is intended to include a separator where the major ribs are formed on both sides. Where the major ribs 14 extend from both planar surfaces of separator 10 the height above of the ribs 14 above planar surfaces 12 and 13, respectively, of backweb 20, may be the same or the height of the ribs on one side may be greater or lesser than the height of the ribs on the other side. The submini-ribs 15 may also extend from both planar surfaces of the separator and may have the same or different heights. In addition, adjacent ridges of a major rib 14 may vary in height. FIGS. 5 and 6 illustrate an apparatus suitable for making the separators of this invention. FIG. 5 is a front view of two embossing rolls 30 and 32 having a plurality of embossing wheels 31a-31g and 33a-33g, respectively, located thereon. Each of embossing wheels 31a-31g on embossing roll 30 are aligned with embossing wheels 33a-33g, respectively, on embossing roll 32. Each of the embossing wheels 31 and 33 are, essentially, specially shaped gear wheels with a plurality of gear teeth 32 and 33, respectively, projecting therefrom, as best seen in FIG. 6. The phrase "embossing wheels" ' is intended to include both separate gear wheels located on an embossing roll or axle (such as by press fitting) or an embossing roll having multiple rows of teeth machined therein. Embossing rolls 30 and/or 32 are rotated by suitable drive means, not shown. It is preferred to have only one of embossing rolls 30 or 32 driven by external drive means with the other roll being rotated by the driven embossing roll. The gear teeth 32 and 34 of aligned embossing wheels 31 and 33 intermesh and emboss major ribs 14 and 15 into flat separator material 20 when it is passed through the nip between rotating embossing rolls 30 and 32 in the direction indicated by the arrow in FIG. 6. Where the height of the ribs extending from both planar faces are the same, the teeth 32 and 34 are egual in height. Where the height of the ribs one side is different than the height of the ribs on the other side, the height of gear teeth 32 and 34 are, correspondingly, different in height. Where it is desired to have ribs extending from only one surface of the separator, one of embossing wheels 31 or 33 may be a "male" roll having gear teeth as illustrated with the other embossing wheel being a "female" roll having grooves therein with dimensions corresponding to the dimensions of the gear teeth extending from the male roll. Where it is desired to have a rib where adjacent ridges have different heights above the adjacent planar surface of the separator backweb this may be achieved by using as the first embossing roll a male with gear teeth of varying height and as the second embossing roll a smooth roll formed of rubber or other deformable material . For ease of illustration gear teeth 32 and 33 are shown as having sharp apexes to form sharp ridges in the product illustrated in FIGS. 1-4. However, it is preferred to round the apexes of teeth 32 and 33 and other sharp edges of the teeth in order to minimize stress cracking of the separator substrate. Such a rounded tooth 132 is shown in FIG. 12. In addition, instead of having essentially wedge-shaped teeth, teeth of other configurations may be used to form the major rib embossments. For example, FIG. 13 shows a gear tooth 232 having a conical shape. FIG. 14 shows a gear tooth 332 having a hemispherical shape. FIG. 15 shows a gear tooth 432 having the shape of a truncated cone. The formation of the ribs of the present invention involves plastic deformation of the backweb material at the location where the embossing takes place. Plastic deformation indicates that the material was loaded beyond its yield point which, by definition, means that it has experienced plastic flow. It has been shown that oxidation resistance is improved in the area of plastic deformation due to oil being driven to the surface upon collapse of micropores. FIG. 7 is a perspective view of a prior art separator 40 having a plurality of ribs 42 extending from the upper planar face 44 of backweb 46. Ribs 42 are integral with backweb 46 and are formed during the calendering of extrudate, as described above in the discussion of the background material. Although the invention has been described relative to forming ribs in a microporous polyethylene separator, as this is the principal type of separator material currently used by flooded cell type lead acid battery manufacturers, any separator material which is porous, acid resistant and capable of being permanently embossed may be used. These materials may be generally characterized as filled or unfilled films and nonwoven webs of thermoplastic or thermoset polymers. Suitable thermoplastic polymers include polymers and copolymers of ethylene, propylene, butylene, vinyl chloride and styrene. Suitable thermoset compositions include phenolics, ethylene/propylene/diene, isoprene, butadiene, styrene and similar ther osetting polymers. The ribs have been illustrated in the preferred embodiment disclosed herein as having ridges and furrows that are perpendicularly aligned with the longitudinal dimension of the separator. However, the alignment of the ridges and furrows may be such as to form an angle to the longitudinal dimension of the separator, such angle being less than 180 degrees, and preferably less than about 160 degrees, to the longitudinal dimension, but greater than 0 degrees and preferably greater than about 20 degrees. In addition, the alignment of the ridges and furrows of some ribs to the longitudinal dimension of the separator may vary from the alignment of the ridges and furrows of other ribs. Examples of such alternative configurations are illustrated in FIGS. 8-11. FIG. 8 illustrates a separator 100 in which the major ribs 114 have embossments in adjacent major ribs 114a, 114b and 114c whose ridges 122 and 124, and corresponding furrows, are alternatingly angled with respect to each other. FIG. 9 illustrates a separator 200 in which the ridges of major rib 214 form a chevron pattern made up of ridge elements 222 and 224. FIG. 10 illustrates a separator 300 in which the ridges of major rib 314 form a tractor tread pattern made up of ridge elements 322 and 324. FIG. 11 illustrates a separator 400 in which the ridges of major rib 414 form a continuous sinusoidal (zig-zag) pattern made up of ridge elements 422 and 424. The continuous sinusoidal pattern of the ridges could be rounded ("S" shaped) instead of sharp as illustrated in FIG. 11. A primary advantage of the present invention is that it allows battery separator manufacturers to continuously produce a uniform sheet of microporous material without interrupting the manufacturing process to change engraved calender rolls. Such sheet material would then be supplied to the battery manufacturer in rolls of appropriate width and the battery manufacturer would apply the major ribs in accordance with this invention. It is contemplated that such ribs would be applied just prior to the plate enveloping operation. The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims: WE CLAIM : 1. A battery separator comprising: a backweb of porous, acid resistant, embossable material, said backweb having longitudinal side edges, a width dimension perpendicular to said longitudinal side edges and upper and lower planar surfaces; said separator having a plurality of submini-ribs projecting from at least one planar surface of said backweb, said submini- ribs extending substantially parallel to the longitudinal side edges of said backweb; said separator having a plurality of major ribs, each major rib being embossed into at least one of said submini-ribs, each of said major ribs being a corrugated structure comprised of alternating ridges and furrows, said ridges and furrows being in non-parallel alignment with said longitudinal side edges of said backweb. 2. The battery separator of claim 1 wherein said backweb is microporous polyethylene. 3. The battery separator of claim 1 wherein said ridges and furrows are substantially perpendicular to the longitudinal edges of said backweb. 4. The battery separator of claim 3 wherein the frequency of said ridges is between about 5 and about 25 per inch. 5. The battery separator of claim 1 wherein said ridges and furrows are at an angle to the longitudinal dimension of said backweb of between about 20 degrees and about 160 degrees. 6. The battery separator of claim 5 wherein the ridges and furrows of at least some of the ribs are at a different angle to the longitudinal dimension of said backweb than that of the ridges and furrows of immediately adjacent ribs. 7. The battery separator of claim 6 wherein the ridges and furrows of a first set of ribs consisting of every other rib is at an angle to the longitudinal dimension of said backweb of between about 20 degrees and less than about 90 degrees and the angle of the ridges and furrows of a second set of ribs consisting of ribs immediately adjacent said first set of ribs is at an angle to the longitudinal dimension of said backweb of between about 160 degrees and greater than about 90 degrees. 8. The battery separator of claim 1 wherein said ridges and furrows of at least some of the ribs are in a chevron pattern. 9. The battery separator of claim 1 wherein said ridges and furrows of at least some of the ribs are in a tractor tread pattern. 10. The battery separator of claim 1 wherein said ridges and furrows of at least some of the ribs are in a continuous sinusoidal pattern. 11. The battery separator of claim 1 wherein said submini-ribs are substantially evenly spaced across the width of said separator. 12. The battery separator of claim 1 wherein the width of the major ribs and the spacing of the submini-ribs are such as to cause each of said major rib to be embossed into at least two adjacent submini-ribs. 13. The battery separator of claim 12 wherein each said major rib is embossed into three adjacent submini-ribs. 14. The battery separator of claim 1 wherein said submini-ribs have a height above the backweb of between about 0.003 inch and about 0.009 inch. 15. The battery separator of claim 1 wherein said submini-ribs have a height above the backweb of between about 0.003 inch and about 0.009 inch. 16. The battery separator of claim 1 wherein said submini-ribs are substantially evenly spaced across the width of said backweb. 17. The battery separator of claim 1 wherein said submini-ribs are spaced apart by a distance of between about 0.025 inch and about 0.050 inch. 18. The battery separator of claim 1 wherein said backweb has a thickness of between about 0.002 inch and about 0.008 inch. 19. A method of making a battery separator comprising: forming a plurality of submini-ribs on a substantially flat web of a porous, acid resistant, embossable separator material, said submini-ribs being substantially parallel to the longitudinal edges of said web; embossing said web in a plurality of narrow lanes, each of said lanes overlying at least one submini-rib, by subjecting said web to plastic deformation at a plurality of embossment sites within said lanes to form in each of said lanes a corrugated structure extending from at least one planar face of said web, said corrugated structure having alternating ridges and furrows that are non- parallel to said longitudinal edges of said web. 20. The method of claim 19 wherein said embossing is effected by passing the web into the nip formed by pairs of opposed embossing wheels positioned in the locations of said narrow lanes. 21. The method of claim 19 wherein said separator material is microporous polyethylene. 22. The method of claim 19 wherein each of said lanes overlies at least two submini-ribs. 23. The method of claim 22 wherein each of said lanes overlies three submini-ribs.

Inventors: ALEXANDER FRANCIS E, WEERTS DANIEL E, YOUNG JAMES, ALEXANDER, FRANCIS, E., WEERTS, DANIEL, E., YOUNG, JAMES
Applicants: ALEXANDER FRANCIS E, AMTEK RESEARCH INTERNATIONAL L, WEERTS DANIEL E, YOUNG JAMES, ALEXANDER, FRANCIS, E., AMTEK RESEARCH INTERNATIONAL LLC, WEERTS, DANIEL, E., YOUNG, JAMES
Application year: 1998
Main classification: H01M2
IPC codes: B29C59, B29D16, H01M2, H01M10