Battery separators

Abstract

A battery separator is described that comprises a nonwoven, pre-strengthened by needling and calibrated with a calender, which is final-strengthened with an acid-resistant binder. The battery separators have a uniform thickness, are not hairy, and can be used in a versatile manner, in particular also for tubular pockets.

Description

[0001] The invention relates to battery separators, methods for producing them, and their use in batteries, lead accumulators, etc.

[0002] Batteries, in particular wet cell batteries, have been known for a long time. They contain alternately arranged positive and negative battery electrodes, between which separators are positioned in order to prevent physical contact. These battery separators are made from chemically resistant material; in addition to adequate mechanical strength and chemical resistance, they should also have adequate permeability for the electrolytes or electrolyte ions, i.e. an appropriate porosity.

[0003] The separators are produced in the form of foils, membranes, laminates, fabrics, but also in the form of so-called nonwovens. Numerous patents and scientific publications describe such materials and attempts to improve the chemical resistance, in particular, of such substances and to enhance the efficiency and operating life of the batteries with such battery separators.

[0004] U.S. Pat. No. 5,935,884, for example, describes the production of a battery separator based on a wet-laid nylon staple fiber nonwoven. In this process, polyamide-6,6-staple fibers are processed together with polyamide-6 binder fibers on a paper machine in a wet process into a nonwoven. Using an infrared radiator, the nonwoven is then partially bonded and subsequently dried. The nonwoven with the partially bonded fibers is then passed over heated calendaring rollers.

[0005] The production process is rather complicated and requires, in particular for the nonwoven production, numerous chemicals for the wet process, which is a disadvantage especially because of environmental concerns.

[0006] WO 98/58111 describes a laminate usable as a battery separator. This laminate is a product consisting of two layers, i.e. a layer of spun fibers and a layer of so-called melt-blown fibers. The two layers are strengthened using a copolymerization reaction with vinyl monomers. This manufacturing method is also rather complicated, since on the one hand two very different layers must be combined, and on the other hand a graft polymerization takes place that must be controlled very precisely.

[0007] EP 0 109 619 B1 describes battery separators that have been produced on the basis of fine and rough fiber sections. This manufacturing method is also rather complicated, especially since a complicated stretching mechanism for producing the different diameters must be controlled.

[0008] U.S. Pat. No. 6,120,939 also describes battery separators with a construction based on a nonwoven made of ultra-fine fibers. This process also uses the so-called melt-blow process, which is also complicated and produces fibers with irregular diameters.

[0009] Also known are battery separators based on nonwovens of polyester filaments, for example the commercially available products known under the trade name “Type 021” by Johns Manville Sales GmbH, Bad Homburg, Germany. These are battery separators used in the form of so-called tubular pockets. The tubular pockets are filled with electrode material and placed into the electrolyte of the battery.

[0010] The polyester nonwovens are endless filament nonwovens that have been pre-strengthened using a stamping calender and are final-strengthened using an acrylate binder.

[0011] The tubular pockets are then produced in a separate work step. Two nonwoven webs or nonwovens are placed on top of each other and sewn to each other at regular intervals. Then the tubular shape (for example round, square, or rectangular) is created by inserting heated rods.

[0012] Because of their strength and structure, the nonwovens are well suited for use as battery separators, but have the disadvantage that their thickness cannot be adjusted as desired and also have a reduced usable surface as a result of the stamping. In the area of the stamped points, the nonwoven fabric is much denser, i.e. there is no porosity there. These battery separators also may have an undesired hairiness.

[0013] It is indeed possible to calender without stamping, resulting in a thin thickness of the nonwoven and also an acceptable porosity; however, the nonwovens produced in this way have a distinctly greater surface hairiness and leave much to be desired in terms of delamination strength.

[0014] Although an entire series of processes for producing battery separators, especially based on nonwovens, is known, a need still exists for improved battery separators and improved processes for their production.

[0015] It is the objective of this invention to make available battery separators that can be produced according to a simple and reproducible process, have good mechanical strength, and a good chemical resistance, and have satisfactory permeability for the electrolytes, which can be produced with the most constant thickness possible, which permit the production of high-capacity batteries, and which, most of all, do not have a hairy surface as this is often the case with battery separators based on fibers, and which also can be used in a varied manner.

[0016] This objective is realized with a battery separator comprising a nonwoven pre-strengthened by needling and calibrated using a calender, which has been finally-strengthened with an acid-resistant binder.

[0017] The nonwoven is preferably a polyester nonwoven, advantageously a nonwoven of manganese-free polyester fibers. The nonwoven preferably consists of endless fibers; in particular, the nonwoven exists as a spun-bonded nonwoven.

[0018] It is advantageous if the fabric weight of the nonwoven, including the binder, is 80 to 180 g/m2, in particular 100 to 150 g/m2.

[0019] The nonwoven is preferably needled with a stitch density of 30 to 60 stitches/cm3.

[0020] The acid-resistant binder is preferably an acrylate binder. Very suitable are, for example, aqueous dispersions of a thermoplastic acrylic polymer based on methyl methacrylate, as distributed, for example, by Polymer Latex GmbH und Co. KG, D-45764 Marl under the name of Plextol M 630. The nonwoven preferably contains 10 to 25 weight % of acrylate binder (dry weight of binder). The nonwoven preferably was calibrated previously with a calender with a temperature of 170 to 200° C.

[0021] In a particularly advantageous version of the invention the separator is manufactured in the form of a tubular pocket.

[0022] Another object of the invention is a process for producing battery separators, which is characterized in that an endless filament nonwoven is produced by spinning polymers, especially polyester material, such as polyethylene terephthalate, the nonwoven is pre-strengthened using needles, the nonwoven, pre-strengthened in this way, is calibrated using a calender and is then final-strengthened with an acid-resistant binder.

[0023] The calibration is preferably performed with a calender with a temperature from 170 to 200° C. It is advantageous if the needling is performed using a stitch density of 40 to 60 stitches per cm2. An especially suitable acid-resistant binder is a cross-linking acrylate binder.

[0024] The battery separators according to the invention can be used especially advantageously in lead accumulators.

[0025] The battery separators according to the invention can be produced as follows.

[0026] First a nonwoven, preferably of polyester filaments, is produced, as described, for example, in DE-OS 24 60 755 (so-called spunbonds). Preferably polyethylene terephthalate is used as polyester; however, co-polymers also can be used.

[0027] In the spunbond process, filaments are continuously deposited onto a moving band. In order to achieve good uniformity, as many deposition rows (spinning beams) as possible are used when the nonwoven is produced.

[0028] It is especially advantageous that polymers, especially polyethylene terephthalate, which contain no or only negligible amounts of metals, are used to produce the filaments. The metals, present in the form of chemical compounds, get into the polymer or polyester since metal-containing catalysts are used during transesterification or polycondensation. Since metals, for example manganese, count among the so-called battery toxins, it is important to use raw materials in which these metals are not contained in the catalysts. It is especially advantageous to use polyethylene terephthalate, which does not contain manganese compounds.

[0029] The resulting nonwovens are then pre-strengthened mechanically by needling. For this purpose, 36-38 G. CB needles are used, preferably 38 G. CB needles. The nonwoven, after having been pre-strengthened in this way, is then calibrated using a smoothing calender. With the aid of the calender, the desired thickness of the nonwoven is achieved. The thickness per se can be changed within wide ranges and depends on the requirements of the future use. In general, the thickness set by calendering is, however, below 0.8 mm, particularly 0.5 to 0.7 mm. It is advantageous if the temperature of the calender is at least 170° C., preferably at least 180° C., preferably 180° to 200° C.

[0030] After calendering, the acid-resistant binder is added. The binder generally is applied by passing the nonwoven through a binder-containing bath. Especially suitable for this purpose are so-called immersion foulards. After leaving the bath, the excess is squeezed off. Suitable binder systems are acid-resistant, so-called “thermoset” binders, especially cross-linking acrylate binders like the already mentioned dispersions.

[0031] The binder application must be sufficient to ensure the required strengths and stiffness. On the other hand, the binder content should however not be too high, since otherwise the porosity of the separator is reduced, and the production costs will increase unnecessarily. In general, a binder application of 5 to 25 weight % was found to be very suitable. A person with average skill in this field also will be able to easily determine the most suitable amount of binder with simple pre-tests.

[0032] The binder application is strengthened with heat, for example in an oven, where the binder dries. The actual strengthening process also may be preceded with a separate drying process.

[0033] After the strengthening is complete, the nonwoven can be fabricated in an actually known manner, for example into flat pieces used between smooth accumulators or, preferably, into tubular pockets. This is done in a separate work step. Hereby two nonwoven webs or nonwovens are placed on top of each other and sewn to each other at regular intervals. The tubular shape (for example round, square, or rectangular) is then created by introducing heated rods.

[0034] It was especially surprising that according to the invention it is possible to make available battery separators that can be produced in a simple and uncomplicated manner, have a uniform thickness, and, in particular, have sufficient porosity.

[0035] It was especially surprising that it is possible to use the invention to produce battery separators on nonwoven basis which no longer have hairiness, or only have it to a minor and negligible extent.

[0036] The mechanical strength of the battery separators is excellent, the chemical resistance, especially against acids, such as sulfuric acids or saline solutions, is excellent. The delamination resistance is excellent.

[0037] The invention is explained in more detail using the following example:

EXAMPLE

[0038] An endless filament nonwoven is produced from polyethylene terephthalate. The nonwovens are produced according to the known spunbond process by deposition onto a moving band. By using nine spinning beams, nonwovens with a fabric weight, including binder, of 147 g/m2 are obtained.

[0039] Without any further strengthening, needling with a needling density of 36 stitches/cm2 is performed. The subsequent calibration of the nonwoven thickness is done with a calender at 200° C. The binder application of 16.6% is performed using an immersion foulard. The used binder is a commercial acrylate binder.

[0040] The resulting needled and final-strengthened nonwoven is fabricated in an actually known manner into a battery separator. 1 TABLE 1 Type New nonwoven 021/150 fabric Fabric weight [g/cm2] 149 147 Thickness [mm] 0.67 0.62 Maximum tensile strength (N/5 cm] [1/q] 504/358 573/324 Max. tensile strength stretching [%] 24.7/23.3 27.2/32.0 Tear resistance strength [N] (1/q) 157/172 202/228 Delamination [N/5 cm] 4.8 32.4 Air permeability [1/m2 sec] 1085 1153

Claims

1. Battery separator, comprising a nonwoven pre-strengthened by needling and calibrated with a calender, which is final-strengthened with an acid-resistant binder.

2. Batter separator according to claim 1, characterized in that the nonwoven is a polyester nonwoven.

3. Battery separator according to claim 2, characterized in that the nonwoven consists of manganese-free polyester fibers.

4. Battery separator according to at least one of claims 1 to 3, characterized in that the nonwoven consists of endless fibers.

5. Battery separator according to claim 4, characterized in that the nonwoven is a spun-bonded nonwoven.

6. Battery separator according to at least one of claims 1 to 5, characterized in that the fabric weight of the nonwoven, including the binder, is 80 to 180 g/m2.

7. Battery separator according to claim 6, characterized in that the fabric weight, including binder, is 100 to 150 g/m2.

8. Battery separator according to at least one of claims 1 to 7, characterized in that the nonwoven is needled with a stitch density of 30 to 60 stitches per cm2.

9. Battery separator according to at least one of claims 1 to 8, characterized in that the nonwoven is final-strengthened with an acrylate binder.

10. Battery separator according to claim 9, characterized in that the nonwoven contains 5 to 25 weight acrylate binder.

11. Battery separator according to at least one of claims 1 to 10, characterized in that the nonwoven has been calibrated with a calender with a temperature of 170 to 200° C.

12. Battery separator according to at least one claims 1 to 11, characterized in that the battery separator is fabricated in the form of a tubular pocket.

13. Method for producing battery separators, characterized in that an endless filament nonwoven is produced by spinning polymers, the nonwoven is pre-strengthened using needles, the nonwoven, pre-strengthened in this way, is calibrated using a calender and is then final-strengthened with an acid-resistant binder.

14. Method for producing battery separators according to claim 13, characterized in that an endless filament nonwoven is produced by spinning polyester material, the nonwoven is pre-strengthened using needles, the nonwoven, pre-strengthened in this way, is calibrated using a calender and is then final-strengthened with an acid-resistant binder.

15. Method according to claim 14 for producing battery separators, characterized in that an endless filament nonwoven is produced by spinning polyethylene terephthalate, the nonwoven is pre-strengthened using needles, the nonwoven, pre-strengthened in this way, is calibrated using a calender and is then final-strengthened with an acid-resistant binder.

16. Method according to one of claims 13-15, characterized in that the calibration is performed using a calender with a temperature of 170 to 200° C.

17. Method according to at least one of claims 13 to 16, characterized in that the needling is performed with a stitch density of 30 to 60 stitches per cm2.

18. Method according to at least one of claims 13 to 17, characterized in that a cross-linking acrylate binder is used as acid-resistant binder.

19. Use of the battery separators according to at least one of claims 1 to 12 or produced by a method according to at least one of claims 13 to 18 in lead accumulators.