BATTERY SEPARATOR FOR LEAD-ACID TYPE BATTERIES

Abstract: Battery separators are provided for maintenance-free lead-acid batteries. From 10 to 90 percent by weight of the fiber content of the battery separator is comprised of a non-metallic acicular mineral, wollastonite, that is substantially less expensive than the microglass fibers used heretofore yet provides comparable, and even superior results with respect to electrolyte absorption and retention characteristics of the battery separator material. The microglass fibers used heretofore also are used in the battery separator of the present invention together with a minor amount, 5-15 percent, of synthetic binder fiber.

Description: Description Battery Separator for Lead-Acid Type Batteries Technical Field The present invention relates to battery separators and is more particularly concerned with a new and improved battery separator material especially designed for use in maintenance-free automotive batteries of the lead-acid type. Heretofore, battery separators for use in lead-acid storage batteries have consisted of thin fibrous sheets, such as papers, of electrically insulating porous mate¬ rial adapted for prolonged immersion in the acidic elec¬ trolyte between adjacent plates of the battery. More recently maintenance-free batteries have employed non- cellulosic, nonwoven fibrous webs as the " electrolyte absorbing and retaining structure. The nonwoven materials used in maintenance-free batteries have been comprised predominately of glass and/or synthetic fibers. The electrolyte absorbing and retaining separators of such batteries typically absorb the liquid acid electrolyte and distribute it throughout the separator to such an extent that substantially all of the electrolyte is absorbed within the pores of the separator and only a thin film of electrolyte is present on the plates of the battery. The highly retentive and porous flexible separators are typically placed between the plates of opposite polarity which are capable of being stacked and confined within a container exhibiting a variety of configurations and shapes. Such separators retain the electrolyte in intimate proximity to the plate regardless of the position of the cells. In the maintenance-free battery it is important to control the amount of electrolyte placed in the cell. There must be enough so that the hydrogen and sulphate ions together with water are sufficient to support the electrochemical reaction, but there should be little or no excess or free electrolyte. Substantially all of the electrolyte should be retained within the interstices of the separator structure. Thus, the electrolyte content of the battery is in a more or less starved condition. It is important to maintain a starved electrolyte con¬ dition in order to maximize or enhance the recombination at the negative plate of oxygen formed at the positive plate. In this way gas evolution is reduced and improved electrical performance is obtained over a prolonged period. A glass separator material made from microfiber diameter, short staple, glass fibers for use in a starved electrolyte system is described in GB Patent 1,364,283. In that patent the sheets of fiberglass material have a high surface area with correspondingly small fiber di¬ ameter and are capable of retaining the electrolyte OMP within the separator itself. In order to obtain max¬ imum electrolyte retention, the separator uses micro- fine fiber filaments with high surface area per unit of weight. The diameter of the fibers used in these highly flexible materials is ih the range of 0.2 to 10 microns and they have a surface area of approximately 0.1-20 square meters per gram of silica. This very high surface area, together with the action by the sulfuric acid of the electrolyte on the glass, is said to result in a separator having a very high volume retentivity of electrolyte per unit volume of separator. Examples of synthetic fiber battery separators may be found in GB Patent Applications 2,017,184A and 2,020,086A. In the former application, the separators consisted of 70 percent synthetic fiber and 30 percent wood fiber; while in the latter application, a mixture of polyolefin fibers of different coarseness are used, either alone or together with an inert filler. In accordance with the present invention it has been found that superior acid retention can be achieved together with comparable electrical properties in a more economical manner. This is accomplished by form¬ ing a battery separator from a noncellulosic nonwoven fibrous web material having an acicular mineral mate- rial as a significant component of the fibrous web. The separator material also contains icrofine glass fibers, preferably in amounts almost equal to the amounts of the acicular mineral material and utilizes substantially less than 25 percent by weight of a synthetic binder fiber. Advantageously this material exhibits equivalent or superior performance character- istics relative to prior separators used in mainten¬ ance-free batteries yet is of significantly lower cost. Other advantages and features will be in part ob¬ vious and in part pointed out more in detail hereinafter. These and related advantages are obtained by provid- ing an electrolyte absorbing and retaining battery sep¬ arator for lead-acid batteries of the starved electro¬ lyte type wherein the separator is a noncellulosic non¬ woven fibrous web material comprised of (1) 10-90 percent by weight of an acicular mineral material ex- hibiting a specific gravity substantially greater than 2.46 g/cc, (2) electrolyte resistant microfine fibers, such as glass fibers, having an average fiber diameter of less than 10 microns, and (3) less than 25 percent by weight of a synthetic binder fiber. The nonwoven web material exhibits a void volume greater than about 85 percent, an electrolyte absorption of at least about 0.06 g/sq. cm. and an acid retention of at least about 35 percent of said absorption. A better understanding of the invention will be obtained from the following detailed description wherein the article of manufacture possesses the features, properties and relation of elements described and exem- plified herein. Description of a Preferred Embodiment As mentioned, battery separators utilized hereto¬ fore in maintenance-free lead-acid batteries have been made almost entirely from microglass or synthetic fibers In accordance with the present invention from 10 to 90 percent by weight of the fiber content of the battery separator is comprised of a nonmetallic acicular mineral material that is substantially less expensive than the microglass fibers yet provides comparable, and even superior, results with respect to the electrolyte ab¬ sorption and retention characteristics of the battery separator material. The microglass fibers used hereto¬ fore also are used in the battery separator of the present invention together with a minor amount of sny- thetic binder fiber. In view of the acidic environment of the battery separator during use, it is preferred that the sheet material be noncellulosic and not subject to attack by that environment. Consequently, one of the pre¬ ferred major fib'er components of the sheet material is the inorganic fiber used heretofore. Although glass fibers are preferred, other fibers that are re¬ sistant to attack by the acid used as the electrolyte, e.g., sulfuric acid, also may be employed. For example, typical inorganic fibrous materials such as quartz, acid resistant ceramic fibers, and the like may be used, OMPI ' with the preferred fibers being borosilicate glass fibers. Generally, the inorganic fibers utilized are of ex¬ tremely fine diameter, that is, they exhibit a diameter of less than 15 microns and preferably have a diameter on the order of 0.1-10 microns with best results obtaine in the range of 0.5-6.0 microns. Further, since the fibrous web material is produced by a wet papermaking process, it is preferable that the fibers be of a paper- making length and capable of being readily dispersable in water to form a dilute fiber slurry or furnish usable in a papermaking operation. As will be appreciated fibers of different diameter and length may be combined within a single web. aterial and combinations of dif- ferent fibers also may be used with good results. As mentioned it is a feature of the present inven¬ tion that up to about 90 percent by weight of the total fiber content of the sheet material is an acicular mineral material exhibiting a specific gravity substan- tially greater than that of the borosilicate glass nor¬ mally used in the manufacture of battery separators. Such glass typically has a specific gravity of 2.46 g/cc. The preferred mineral is the naturally occuring material known as wollastonite. This material is a nonmetallic calcium metasilicate that typically exhibits a very fine diameter size and a length to diameter ratio in the range of from 3:1 to 20:1. It has an acicular crystallin appearance, that is, it is of needle shape, long, slender and usually pointed at its ends. It has a specific gravity of about 2.9 g/cc, well above the 2.46 value for glass. It exhibits a fiber diameter range of 0.1 to 5.5 microns and an average fiber length in the range of 0.3 to 45 microns with a surface area of approximately 6.3 m-/g . The preferred acicular material has a length to diameter ratio at the upper end of the aforementioned range, i.e., in the range of about 10:1 to 20:1 and may be used as mined and separated or may be treated with surface modifying material to enhance its water dispersibility. One of the benefits of this material is that it can be obtained in a relatively pure form as a white, nonmetallic, highly uniform crystalline material. It exhibits excellent dielectric proper¬ ties and exceptional resistance to moisture absorp¬ tion as well as resistance to attack by acids such as those used as the electrolyte in lead-acid batteries. A commercially available wollastonite found to give good results is the material sold under the trademark "Nyad" by Interpace Corp. The attrition-milled grade of this material having a typical aspect ratio of 15:1 to 20:1 is preferred. As mentioned, the acicular calcium metasilicate is used in amounts of from about 10 to 90 percent by weight of the total fiber content and preferably is employed OMPI within the range of 40 to 60 percent by weight with the preferred composition being approximately 50 per¬ cent. As can be appreciated many factors must be considered in determining the appropriate amount of 5 the acicular mineral material. From a commercial standpoint it has been determined that the more eco¬ nomically desirable calcium metasilicate can be used without sacrifice in the necessary physical properties of the battery separator at levels of at least 40 to 10 60 percent by weight. When levels greater than 60 percent are used, satisfactory results are obtained but the performance level of the battery separator drops as the amount of glass fiber is reduced and re¬ placed with the crystalline calcium metasilicate. I*-* Below 40 percent, the economic advantage of the meta¬ silicate over the microglass fibers is lessened with¬ out a corresponding improvement in the absorption characteristics of the separator material. In a similar manner, it is necessary to consider 20 the properties of the separator material at various binder levels. Since the binder fiber provides the web material with certain strength characteristics but at the same time tends to reduce the electrolyte absorption character of the battery separator material, 25 it is necessary to determine the optimum amount of binder fiber and thereby balance the degree of absorp¬ tion loss against the amount of strength necessary for the sheet material. Additionally, as will be appre¬ ciated, the hydrophobic nature of the binder fiber tends to reduce the wettability of the fibrous sheet material and correspondingly decrease the absorption characteristics of that material. Therefore, it is preferred that the lowest possible amount of syn¬ thetic binder fiber be used in order to achieve the desired strength characteristics. In this connection it has been found that less than 25 percent binder should be used in the fiber furnish. Although the all-glass sheets used heretofore as battery separators have incorporated binders that were applied as dilute solutions after the web material was formed, it is preferred, in accordance with the present invention, to provide the binder in the form of fibers which constitute significantly less than 25 percent of the total fiber content of the sheet material. The binder fibers typically constitute up to about 15 percent of the total fiber content and as little as about 2 percent by weight with preferred amounts being in the range of about 5 to 10 percent by weight. As mentioned, the binder used for the battery separator web material of the present invention is preferably in the form of binder fibers that can be disbursed with the glass fibers prior to deposition on a paper-forming wire. In this way it is possible to achieve excellent random distribution of the fi¬ bers throughout the sheet material. The binder fi¬ bers found particularly advantageous in this connec- -^URE * OMPI _ W tion are the polyolefin fibers of high molecular weight and low melt index. These fibers are described in greater detail in British patent specification No. 1,386,983. As mentioned in that patent, the essential characteristics of the polyolefin fibers which distin¬ guishes them from conventional fibers is their surface area of greater than one square meter per gram and their gross morphology, that is, their microfibrillar structure similar to wood pulp, comprising fibrils which in turn are made up of microfibrils. In general, the fibrous polyolefin fibers are such that the polymer cannot be processed into smooth rod-like fibers by the conventional melt spinning technique. These high molecular weight polymeric materials have a melt index of less than about 0.5 and are not adaptable to conven¬ tional processing equipment due to their poor flow- ability characteristics under pressure. These materials preferably have a melt index below 0.1 and an average molecular weight greater than 800,000. In general, the polyolefin material should have a viscosity average molecular weight of at least 40,000 and preferably greater than 500,000. The binder fibers may be formed under conditions of sheer stress in an apparatus such as a disc refiner. The resultant fibers have a typical size and shape com¬ parable to the size and shape of wood fibers and are commonly referred to as "synthetic wood pulp." They ' — O have an average length of about 1 millimeter, although variations in the manner of their manufacture can re¬ sult in lengths up to 4 millimeters and more. Of course, shorter fiber lengths are also produced with the lower limit of the fiber length being about 0.025 millimeters, with fibers of the 0.1 to 0.2 millimeters being more commonly observed as the shortest fibers. Most of the fibers have a length of about 0.2 to 3 millimeters. These materials have a structure which comprises me- chanically inter-entangled bundles of fibrils and macro- fibrils, the macrofibrils generally having a width in the range of 1 to 20 microns. In the case of polyethy¬ lene, polypropylene and combinations thereof, the polymeric material exhibits an average molecular weight between 500,000 and 20,000,000 and a surface area In excess of 1 square meter per gram up to 100 square meters per gram and generally greater than 25 square meters per gram. Other synthetic thermoplastic fibers that could be used for the manufacture of the separator accord¬ ing to the present invention include polyamides such as nylon 6.6 and nylon 6 and polyesters such as poly¬ ethylene terephthalate as well as the polyolefins mentioned hereinbefore, including co-polymers and mixtures thereof. The preferred fibers have a diameter of 5 to 30 microns and a length of 1 to 2 microns, al¬ though the individual fibrils are present in various sizes and various specific surfaces, the shape and size distribution not being unlike that of refined wood pulp. The sheet material of the present invention gener¬ ally is made in accordance with conventional papermaking techniques and preferably takes the form of a nonwoven structure wherein the binder fibers inter-entangle with the nonbinder fibers and provide sufficient structural integrity through simple physical interaction to permit handling of the web material despite a binder fiber content as low as 2-5 percent. As is well known, all of the fibers are mixed and thoroughly disbursed in an aqueous medium, frequently at reduced pH levels by means of a paper mill beater or other mixing devices. The resultant mixture of fiber furnish is then conveyed to the headbox of a papermaking machine where typically it is further diluted and fed onto the continuous fiber accumulating paper-forming wj:re, such as a Fourdrinier wire. Where pH control is required for dispersion of the inorganic or glass fibers, this can be achieved either during the mixing operation or as the fiber furnish is fed to the headbox of the papermaking machine. Generally, when handling inorganic fibers, the pH con¬ trol is significant and is adjusted to a neutral of acidic condition prior to the slurry being fed to the headbox. Although conventional Fourdrinier, cylinder type or other commercially available papermaking machines may be employed, the battery separator web material of the present invention is most desirably formed in a paper¬ making machine utilizing an inclined wire since more dilute dispersions may be used and greater uniformity with sheet structure can be achieved. In such inclined wire papermaking machines the inorganic fiber dispersion is generally maintained at a concentration of about 1 percent by weight and preferably at about 0.1 to 0.3 percent by weight. Higher concentrations or consisten- cies may, of course, be applied on cylinder machines and conventional Fourdrinier machines so long as the resul¬ tant nonwoven web material will exhibit the requisite physical characteristics adapting it for use as a bat¬ tery separator. A typical .example of the inclined wire papermaking machine can be found in U.S. patent No. 2,045,095 issued to F. H. Osborne on June 23, 1936. Nonwoven web materials formed on such a machine gener¬ ally exhibit a desirable three-dimensional network or configuration with only slight orientation in the machine direction. The fibrous web material formed in accordance with the present invention is typically dried in a conven¬ tional manner and subsequently subjected to temperatures of about 265 β F and higher so that the binder fibers approach and preferably exceed their fusion temperature thereby imparting greater strength characteristics with¬ out interfering with the porosity of the web material. y y wipo As will be appreciated, the melting point of the binder fiber will permit the web material to be dried imme¬ diately after formation without disadvantageously melt¬ ing or causing binder buildup on the drier cans of the papermaking machine. The fibrilliform structure of the binder facilitates rapid melting or fusing and at the same time promotes binder adherence to a greater number of individual glass and wollastonite fibers. Thus, as the sheet material is subjected to the elevated fusion temperature for a brief period of time, the binder particles fuse and flow onto the adjacent fibers in a substantially complete fashion so as to eliminate the fiber structure of the binder. The binder material forms an extremely thin coating, predominately at the cross-over points of the remaining fibers resulting in an effective fiber diameter of only slightly greater than the diameter of the original fibers themselves. As will be appreciated, some small globs of binder will be present at the cross-over or connecting points of the individual inorganic fibers, but in most in¬ stances, even these melted and resolidified portions are no larger than the diameter of the fibers consti¬ tuting the bulk of the sheet material. As will be appreciated, this morphology change of the binder assists in retaining the porosity of the fibrous web material while substantially enhancing its tensile strength. The resultant nonwoven separator web material of the present invention provides improved electrolyte absorption and retentivity which can be attributed at least in part to the high specific gravity of the acicular mineral material relative to the specific gravity of borosilicate glass normally used in the manufacture of nonwoven battery separators. As mentioned, the specific gravity of the acicular crystal¬ line material is about 2.9 g/cc and the average surface area of that material is about 6.3 m 2 /g as contrasted with the surface area of the borosilicate glass of about 3.1 m /g. These characteristics result in a substantially smaller average pore size in the web material and a sub¬ stantially larger number of pores thereby providing a greater surface area within the fibrous web material. These characteristics, in turn, contribute to the im¬ proved absorption characteristics of the web material. There is also a relationship between the void volume of the material and the specific gravity of the fibers. This can be found in the definition of void volume as set forth in the following equation: where V is void volume expressed as a percentage, W is OMPI the basis weight of the sheet material, T is the thickne and S is the specific gravity of the fibers. In accorda with the present invention, . the utilization of a materia which exhibits a substantially higher specific gravity 5 results in a higher void volume and therefore improved absorption characteristics. The void volume, in reality is a statement of the relationship between the apparent density of the material and the actual density of the fibers used to make the material expressed as a percenta 10. In this connection, the void volume of the web material is well above the 85 percent level and is typically at or about 90 percent. The absorption level of the nonwoven web material produced in accordance with the present invention on a 15 weight basis is approximately ten fold and more that of the material itself in a dry condition. Absorption leve of about 0.06 g/sq. cm. are typical with the level seldo dropping below 0.045 g/sq. cm. and extending to and above 0.08 g/sq.cm. 0 In order to determine the absorption characteristics of the nonwoven web materials the following 'test procedur is utilized as a screening test. In this procedure, a small sample of the battery separator material of known dimensional size is weighed in its dry condition and then is allowed to absorb a standarized sulfuric acid so¬ lution for a period of 24 hours. The standard acid solution is a 43 percent by weight sulfuric acid so¬ lution having a specific gravity of 1.335. Although absorption is essentially instantaneous, the 24 hour absorption period is used to allow for any deterioration or breakdown of the material in the acid and provide a stabilizing effect. After the 24 hour acid absorption period, the saturated sample of battery separator material is weighed to determine the amount of acid absorbed by the nonwoven web. As mentioned the sheet material of the present invention absorbs about ten times and more its own weight. To determine the degree of electrolyte retention, the saturated sample obtained as mentioned above is then lightly blotted on one side with a single dry piece of a battery separator made from glass fibers only and the blotted sample is reweighed. The retention values may be reported as the amount of acid remaining in the sheet material together with the amount originally absorbed, or as a percentage of the amount originally absorbed. In accordance with the present invention, it is preferred that the acid retention level as determined by this test procedure be at least 35 to 40 percent and preferably 50 percent or more of the absorption level at saturation. It is also desirable that the acid be absorbed with¬ in the sheet material as rapidly as possible and that the acid be capable of saturating the entire separator. As will be appreciated, the high surface area of the battery separator material of the present invention per¬ mits extremely rapid saturation of the nonwoven material by the acid electrolyte. Additionally, the very small pore size provides for improved retention of the absorbed liquid, thereby providing low resistance to the flow of the electrolyte while at the same time providing improved or enhanced inhibition to the passage or migration of fine active materials formed at one electrode and flow¬ ing toward the opposite electrode during use. Further the electrical resistivity .of the material is very low. A typical average valve is about 1.2 milli-ohms/sq. cm. with the maximum value being 3.0 milli-ohms/sq. cm. and many readings being less than the indicated typical average" value. The following specific examples are given in order that the effectiveness of the present invention may be more fully understood. These examples are set forth for the purpose of illustration only and are not intended in any way to limit the practice of the invention. Unless otherwise specified all parts are given by weight. O EXAMPLE ONE Using a commercial papermaking machine, a nonwoven battery separator material was made in accordance with the present invention. About 141 pounds of wet synthetic pulp polyolefin fibers (56 1/4 -pounds on a dry weight basis) sold under the trademark "Pulpex E" were added to a beater containing 1800 gallons of water heated to a temperature of 80° F. The fiber dispersion was brushed for 20 minutes and 62 pounds of Code 106 glass fibers having a fiber diameter of approximately 0.5 - 0.75 microns were added to the beater together with 62 pounds of 1/4 inch long, chopped 6 micron diameter glass rovings CGrade DE) and 195 pounds of the acicular calcium metasilicate known as wollastonite (attrition-milled grade, sold under the trademark "Nyad-G") . The slurry was defibered for 8 minutes and an additional 600 gallons of water were added thereto. The fiber furnish was mixed further and fed to the headbox of an inclined wire papermaking machine where it was deposited on the paper-forming wire at a sufficient concentration 'to provide a fibrous web having a basis weight of about 80 g/n2.The machine was run at a speed of about 120 feet per minute and the nonwoven material that was produced was dried on can driers heated to a "&υREΛ OMPI IP te perature of about 300°F. - 325°F. The resultant web material exhibited a basis weight of 81.42 g/π , a thickness of 11.0 mils, and a void volume of 90 percent. A mercury intrusion analysis was conducted both on this material and on a commercial all- glass fiber battery separator material hereinafter re¬ ferred to as the "control material". The analysis showe that the material of the present invention exhibited a larger total pore area and total pore volume and a smalle average pore diameter than the control material. The total pore volume and total pore area were 3.0728 cc/g and 1.5504 mr/g, respectively, for the web material of the present invention as compared with values of 2.9468 cc/g and 0.8354 m /g for the control material. The average pore diameters for the respective materials were measured as 7.9278 microns and 14.1101 microns. The web material produced in accordance with this example was also tested for electrolyte absorption and retention using the screening test described heretofore. Circular samples of the web material were cut to a size having a diameter of 6.85 cm. Each sample was weighed and placed into a solution of 43 percent by weight sulfuric acid having a specific gravity of 1.335 g/cc. It was visually observed that the samples exhibited very rapid wettability. The samples were covered and allowed to remain in the acid for 24 hours. Each sample was removed from the acid, allowed to drain for 5 - 10 seconds and then weighed on a preweighed dish to determine the amount of electrolyte absorbed. The average absorption value for four samples of the mate¬ rial of this example was 0.0531 g/sq. cm. The saturated samples were lightly blotted by placing one side on an all-glass commercial grade separator material such as the control material for 5 - 10 seconds. The blotted samples were reweighed to determine the percent of electrolyte retention. The average retention value was found to be 61 per¬ cent. The retention value for commercial separators using all-glass fibers is between about 56 and 60 percent. EXAMPLE TWO Battery separator material was made in the form of continuous webs using the procedure and apparatus of Example One. The fiber composition was changed by reducing the snythetic binder fiber to a 5 percent level and slightly adjusting the remaining components. The properties of the resultant material from this example are compared with those of the material from Example One and the control material in Table I TABLE I Fiber Example Example Content (%) Control 1 2 Calcium metasilicate _ 52 50 Synthetic pulp (Pulpex E) - 15 5 Code 106 glass fiber - 16.5 22.5 DE glass rovings - 16.5 22.5 (1/4" length) Basis Weight (g/sq. m) 60 81 95 Thickness (mils) 11.8 11 12.7 Void Volume (%) ' 91 90 90 Avg. Tensile (g/25 mm) 700 1188 1070 Absorption (g/sq. cm) .0728 .0531 .0742 Retention (%) 60 61 70 EXAMPLE THREE Hand sheets were prepared from a fiber furnish Identical to that used in Example Two. The fiber slurry was prepared in substantially the same manner as in Example One. After mixing the entire furnish, hand sheet were made and the properties thereof measured. The aver- , age values for the physical properties of the hand sheets are set forth in Table II, which also includes, for comparative purposes, the data associated with an all- glass fiber control material. EXAMPLE FOUR The procedure of Example Three was repeated except that the amount of synthetic pulp fiber was increased to 10 percent and the amount of glass fiber was reduced so that the fiber furnish contained 20 percent of each type of glass fiber. Hand sheets were prepared as in Example Three and the physical data relative thereto is set forth in Table II. EXAMPLE FIVE The procedure of Example Three was repeated again except that the amount of synthetic fiber, was increased to 10 percent and the amount of acicular calcium meta¬ silicate was reduced to 45 percent. Hand sheets were prepared and the physical data relating to this material is set forth in Table II. TABLE I I Fiber Content (%) Control Ex . 3 Ex . 4 Ex . 5 Calcium metasilicate 50 50 45 Synthetic pulp 5 10 10 Fiber Content (- Control Ex. ' 3 Ex. " 4 Ex. 5 Code 106 glass 22.5 20 22.5 DE glass rovings 1/4" 22.5 20 22.5 Basis Weight (g/m2) 61 ' 72 71 76 Thickness (mils) 12.05 10.2 9.5 9.9 Absorption (g/sq. cm) 0.0610 0.0800 0.0612 0.07 (corrected to 10 mils) Retention (%) 57 59 56 56 EXAMPLE SIX In this example, the procedure of Example Three was followed but the amount of acicular calcium metasilicate fiber was varied from 10 percent to 90 percent using 10- percent synthetic pulp fiber in all instances. Hand sheets were prepared from the different fiber furnishes and the formulations and physical properties relative thereto are set forth in Table III. TABLE III Fiber Content (%) Calcium metasilicate 10.0 25.0 75.0 90.0 Synthetic fiber 10.0 10.0 10.0 10.0 (Pulpex E) Fiber Content (%) Code 106 glass 40.0 32.5 7.5 0 DE glass 40.0 32.5 7.5 0 Basis ' Weight (g/m^) 56 63 100 Thickness (mils) 9.68 9.98 9.45 10.18 Absorption (g/sq. cm) 0.0650 0.0659 0.0506 0.0506 (corrected to 10 mils) Retention ( ) 48 52 49 60 As will be apparent to persons skilled in the art, various modifications, adaptations and variations of the foregoing specific disclosure can be made without depart¬ ing from the teachings of the present invention.

Claims: Claims 1. An electrolyte absorbing and retaining battery sep¬ arator for lead-acid batteries of the starved electro¬ lyte type comprising a nonwoven fibrous web material comprising up to 90 percent by weight of an acicular mineral material exhibiting a specific gravity sub¬ stantially greater than 2.4 g/cc, an inorganic electrolyte-resistant fibrous material and less than 25 percent by weight of a synthetic binder, said nonwoven web material having a void volume greater than about 85 percent, an acid absorption of at least about 0.04 g/sq. cm. and an acid retention of at least about 35 percent of said absorption. 2. The battery separator of claim 1 wherein the acicular mineral material is a nonmetallic, highly uniform, crystalline material having a length to diameter ratio of about 3:1 to 20:1 and the inorganic fibrous mate¬ rial includes glass fibers having an average fiber diameter of 0.1 - 10 microns. 3. The battery separator of claim 1 wherein the fibrous web material is noncellulosic and comprises 10-90 percent by weight of the ' acicular mineral material, glass fibers having an average fiber diameter of less than about 10 microns and about 2-15 percent by weight of a synthetic binder consisting essentially of thermoplastic synthetic pulp, the web material ex¬ hibiting a smaller pore size and larger pore surface area than all-glass fiber separator material 4. The battery separator of claims 1, 2 or 3 wherein the acicular mineral material is naturally occuring calcium metasilicate having an average fiber diameter of 0.1- 5.5 microns, a length of 3-45 microns and a length to diameter ratio of from 3:1 to 20:1. 5. The battery separator of claim 1 wherein the acicular mineral material is calcium metasilicate having a length to diameter ratio of 10:1 to 20:1 and a specific gravity of 2.9 g/cc; the inorganic fibrous material includes glass fibers having an average fiber diameter of 0.5- 6.0 microns; the synthetic binder consists essentially of thermoplastic synthetic pulp in amounts of 5-15 percent by weight and the web material exhibits an electrolyte absorption of about 0.06 g/sq. cm. and more and a retention level of 50 percent and more of the absorption. 6. The battery separator of claim 1 wherein the fibrous web material comprises 40-60 percent by weight of wollastonite having a length to diameter ratio of 15:1 to 20:1 and a specific gravity of 2.9 g/cc; 55 percent by weight and less of glass fibers having an average fiber diameter of 0.5-6.0 microns; the syn¬ thetic binder consists essentially of thermoplastic polyolefin synthetic pulp in amounts of 5-10 per¬ cent by weight, and the web material exhibits a smaller pore " size and larger pore surface area than -fURlA T OMPI K WIPO all-glass fiber separator material and an electrolyte absorption of at least about 0.06 g/sq. cm. and a re¬ tention level of 50 percent and more of the absorption. 7. The battery separator of claim 1 wherein the acicular mineral material is a natrually occuring material, the synthetic binder consists essentially of thermo¬ plastic synthetic pulp In amounts of 2-15 percent by weight and the web material exhibits a smaller pore size and larger pore surface area than all-glass fiber separator material. 8. The battery separator of claim 1 wherein the fibrous web material comprises 10-90 percent by weight of the acicular mineral material, the synthetic binder consists essentially of thermoplastic polyolefin synthetic pulp in amounts of 5-15 percent by weight and the web material exhibits a void volume of about 90 percent. The battery separator of claim 1 wherein the fibrous σ web material is noncellulosic and comprises 10-90 percent by weight of the highly uniform, crystal¬ line wollastonite having an average fiber diameter of 0.1-5.5 microns and a length to diameter ratio of 15:1 to 20:1, the synthetic binder is a thermo¬ plastic polyolefin synthetic pulp in amounts of 2- 15 percent by weight, and the web material exhibits an electrolyte absorption of about 0.06 g/sq. cm. ξTTR OM ty• > wi and a retention level of 50 percent and more of the absorption.

Inventors: BERNIER R
Applicants: BERNIER R, DEXTER CORP
Application year: 1980
Main classification: H01M2
IPC codes: H01M2