JPH04339B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a porous sheet for use as a separator in a sulfuric acid storage battery. In the production of batteries, it is necessary to insert a separator between the positive and negative electrodes in order to maintain a certain uniform space between the battery plates and prevent contact between the battery plates. The separator is formed of a porous sheet material that can be obtained by various methods and made of various materials. Each separator has a number of spaced vertical ribs (spacers) on the surface adjacent to the positive electrode plate for shedding solid particles that are dislodged from the battery plate during operation and for dissipating generated battery gases. One form of existing separator is made from absorbent paper impregnated with a thermoset phenolic resin. Another form of existing separator is made from microporous polyethylene consisting of a mixture of high molecular weight polyethylene and silica or sintered. Made of extruded sheet of PVC. It has been proposed to produce separators from nonwoven sheets by pressing and heating layers of polyolefin fibers without the use of binders. All of the aforementioned methods have the disadvantage of resulting in significantly thicker separators and with excessive expense or disadvantage in manufacturing the necessary ribs or protrusions on the separators. In the case of polyvinyl chloride separators, the spacers consist of the same material that forms the porous sheet and are obtained directly by extrusion during the manufacture of the separator. In contrast, in separators made from cellulosic material, the spacers are made from a plastic material that is bonded to the support by adhesive or by deposition or by melt extrusion. Attaching the spacer to a separately manufactured support, whether done by heat welding during extrusion or by gluing, inherently serves to increase the resistance of the separator as it reduces the microporous surface. The fact is that the adhesive or cement spreads to the surrounding areas of the deposited area, while in the case of thermal welding during extrusion, the development of overpressure localized on the just-formed spacer increases the cross-section and this cross-section The increase in the active surface of the separator is reduced, and
Stresses arise due to the shrinkage experienced by the spacers due to the cooling effect, which can lead to loss of smoothness of the separators, resulting in a prejudice against their use in the production of accumulators. When extruding spacers directly onto paper, the technique is further complicated each time the size or distance between the spacers is varied. In the case of direct extrusion, this latter can only be carried out by changing the extruder head and therefore by changing the position of the extrusion hole, thus increasing the economic burden and loss of time. invite The object of the present invention is to provide a new method for producing porous sheets based on synthetic thermoplastic polymers suitable for use as separators in sulfuric acid storage batteries. The method according to the invention for producing the porous sheet described above consists of the following sequential operations. (a) 3 to 40% by weight of cellulose fibers, 97 to 60% by weight of at least one thermoplastic polymer, fibrils having a surface area of at least 1 m ² /g and uniformly distributed in the fiber mixture; At least 0.1% of the dispersed fiber mixture
(b) melting the fibrils in the absence of pressure to a temperature equal to or higher than the melting temperature of the polymer; The process of heating for a sufficient period of time. Fibrils include low or high density polyethylene, polypropylene consisting essentially of isotactic macromolecules, ethylene/propylene random or block copolymers, poly-4-methyl-1-pentene, polyamide, polyester, polyacrylonitrile, polyurethane, polycarbonate. , vinyl resins such as polyvinyl chloride or polyvinyl acetate, ethylene/vinyl acetate copolymers, acrylic resins or polyethers. For example, fibrils are described in British Patent No. 891945,
1262531, 1355912, 1355913, 1392667, 1471097
and US Pat. No. 3,770,856, 3,750,383 and 3,808,691. The fibrils have a length of 0.2 to 50 mm, a diameter of 1 to 500 μ and a surface area of 1 to 50 m ² /g. Preferably, the fibrils have a length of 1 to 3 mm, more preferably 1.4 to 2.2 mm. The fibrils may contain minerals such as kaolin, talc, plaster, titanium dioxide or silicates during their manufacture. The mineral loading can be up to 70% of the total fibril weight, but preferably no more than 50-55% by weight. Preferably, the cellulose fibers consist of bleached softwood pulp. The production of sheets of fibrous material is preferably carried out by papermaking techniques starting from a suspension of a mixture of cellulose fibers and fibrils in water or other inert liquid medium. Preference is given to using flat-bed or rotary machines that work continuously or in batches. Preferably, the starting aqueous suspension contains 0.7 to 1.5% by weight of total fiber material. The wetting agent is preferably added to the suspension in an amount of 0.1 to 5% by weight, based on the weight of the dry fibrous material. Wetting agents can increase fiber absorption and are almost completely retained in the final sheet. In some cases, depending on the wetting agent used, it may be desirable to incorporate antifoaming agents into the sheet to prevent foam formation in the electrolytic bath of the cell in which the separator is ultimately used. Defoamers need to be non-ionic and are suitable for fiber mixtures.
It can be present in the sheet in a proportion of 0.01 to 1% by weight. Defoamers can be incorporated into the sheet using the same techniques as wetting agents. Prior to forming into a sheet, the fiber mixture can be refined by techniques used in the paper industry for processing cellulosic fibers. A high degree of scouring of the initial fiber mixture makes it possible to obtain small sized pores in the final product. Fiber mixtures range from 15 to
It is preferable to refine to a freeness of 55°SR (Schoppel-Riegler). Values of 15-30°SR are even better as they result in separators with very fine pores and relatively low electrical resistance. If the sheet is made from fibrils of different polymers, the heating should be at a temperature at least equal to the melting temperature of the polymer with the lowest melting temperature.
Preferably, the heating is at a temperature of at least 5°C, more preferably 20-40°C above the melting temperature of the polymer. Heating takes place without the application of substantial pressure, but if, for example, the heating means is equipped with a pair of rollers through which the sheet passes, the pressure required to bring the sheet into contact with the heating means is not a problem. . However, the pressure on the sheet is 0.01Kg/cm ²
shall not exceed. Heating can be accomplished by passing the sheet through an oven or across a number of infrared heat lamps or over the surface of a heated roller. After heating, the sheet is cooled and
Spaced vertical ribs (spacers) required on its surface
After treatment, it can be used as a separator in lead-acid batteries. As mentioned above, the fibrils in the present invention have a high specific surface area and therefore occupy a large volume in the sheet. When the fibrils melt during heat treatment, their volume is reduced to very small drops, and corresponding voids are created within the sheet. The formation of voids in this way causes porosity, and eventually porosity occurs due to the melting of the fibrils, resulting in a porous sheet. In particular, when fibrils of high-density polyethylene are used in the production of the porous sheet of the present invention, the formation of spacers on the sheet involves applying filaments of low-density polyethylene with a specific melt index value to the surface of the sheet. , can be advantageously achieved by heat welding under certain conditions. Thus, the separator for a sulfuric acid storage battery with a spacer is preferably manufactured by the following process. (a') at least one of 3-40% by weight cellulose fibers, 97-60% by weight high-density polyethylene;
producing a sheet of fibrous material comprising fibrils having a surface area of m ² /g and a mixture of nonionic wetting agents homogeneously distributed in the fiber mixture and at least 0.1% of the fiber mixture; (b′) (c) heating said sheet to a temperature equal to or higher than the melting temperature of said polyethylene for a time sufficient to melt the fibrils without applying pressure; (c) having a diameter of 0.2 to 3 mm and melting; Step of depositing filament-shaped spacers made of low-density polyethylene with an index of 0.3 to 10 on the sheet, (d) depositing the spacers arranged on the sheet at 120 to 180°C
(e) At the above temperature, the spacer is compressed onto the sheet by running an idler roller over the spacer. Steps (a') and (b') can be carried out in the same manner and method as steps (a) and (b). After heating step (b'), the sheet can be cooled and used in step (c). However, the heating step (b') can be omitted and the sheet can be sent directly from step (a') to step (c). In this case, the heating step (b') necessary for melting the polyethylene fibrils can be carried out in combination with the heating step (d). The filaments acting as spacers can be obtained by conventional polyethylene extrusion methods.
Preferably, such filaments consist of polyethylene having an MI of 2 to 8. The filament may also contain inorganic substances such as kaolin, carbon black, and talc. Preferably, the spacer filaments contain micropores which, as surprisingly found by the inventors, limit the increase in resistance of the separator due to the reduction of the active surface, but also favor the smoothness of the support. shows the quality structure. In such cases, the filament can be manufactured by extruding polyethylene with blowing agents by techniques well known to those skilled in the art. The temperature selection in step (d) depends on the operating speed of the extruder, the MI (melt index) of the polyethylene that is the material for the spacer, the amount of blowing agent contained in the polyethylene, and the amount of blowing agent added to the spacer by the pressure roller in step (e). This is done based on the pressure applied. This temperature is generally between 120 and 180<0>C, but preferably between 125 and 160<0>C when the polyethylene forming the spacer has an MI of 2 to 8. The step (e) of pressing the spacer onto the support is preferably carried out by applying a pressure of 25 to 250 g per cm of width of the support to the spacer. This pressure can be a pressure determined by the weight of the roller itself, which rests on the support, or it can be a pressure determined by the weight of the roller itself, which is placed on the support, or the support with the spacers is pressed against the surface of the press roller and a heated surface carrying the support with the spacers. It can be obtained by passing it through a hole having an appropriate gap formed by. Apparatus suitable for carrying out the method of the invention is capable of achieving in any suitable manner the application and adhesion of the spacers onto the sheet or support coming from step (c), and the support on which the spaced filaments are arranged. It is characterized by having a movable heating surface carrying the body and idler rollers that can rotate on the support. The idler roller (presser) must be able to run over the spacers present on the support, depositing the spacers on the support and at the same time controlling the overall thickness of the separator. In the drawing, B is a heated cylinder, P
is an idle roller (presser) and C is a roller that supports the presser roller. To achieve the objectives of the invention during operation of the device,
A prefabricated sheet or support d is conveyed from coil A through transmission rollers R to heating cylinder B. The spacer filament f is fed onto the sheet or support d by a multi-bladed pulley D. The heating roller drags the support having the spacer filament with it while rotating, and the presser roller P is rotated over the spacer, so that the spacer is adhered to the support by the weight of the presser. In the drawing, h indicates a continuous sheet of ready-to-use separator emerging from the device. Roller C acts as a support for the press roller and does not come into contact with either the heating roller or the support or the filaments arranged on the support. Preferable conditions for implementing this method are as follows. - Composition of the fiber mixture of the sheet before melting of the high density polyethylene fibrils present in the sheet: 60-80
% by weight of unfilled fibrils of high-density polyethylene, 40-20% by weight of cellulose fibers; - melt index (MI) of low-density polyethylene of the spacer 3.5 - 1-10% by weight of filler present in the spacer %
Carbon black - Residence time of spacer on heating roller 0.5~15
Sec. The following examples further illustrate the invention, but do not limit it. In the following examples, the porosity of the obtained sheet is expressed by Bendtzen porosity. This is obtained by a test method used in the UK and elsewhere for pulp, paper, board, cartons, etc., according to which the porosity of the packaging material is determined by the effective pressure of 150 mm of water. It is given as the amount of air passing through 10 cm ² per minute. Example 1 To make fibers, 25 kg of cellulose paste consisting of 30% by weight cellulose sheet and 70% sulphate of softwood cellulose are introduced into a paper beater. Fibrils have a melt index of 30 and a melting temperature of
It is prepared from high density polyethylene at 135° C. and a density of 0.96 g/cm ² by the method described in the aforementioned British Patent No. 1392667. The fibrils obtained have a length of 1.4-2.2 mm, an average apparent diameter of 19 μ and a surface area of 5.7 m ² /g,
Contains about 30% by weight of kaolin powder with an average particle size of about 5μ based on the fibrils. 75 Kg of these fibrils are introduced into a paper beater with cellulose fibers. The fiber mixture was refined in a conical mill to 30° SR and then treated with 750 g of a softener or surfactant (TyPol 610 nonionic surfactant from Shell) and 250 g of a neutral surfactant (from Lamberi de Varese). Transfer to a tank containing Synthol H14). The resulting mixture, after being diluted with water to a suspension with a fiber content of 10 g/min, is sent to a continuous sheet-forming machine and sheeted at a speed of 4 m/min. 3
The properties of the sheet after drying to % humidity are as follows: Weight 148g/m ² Thickness 433μ Specific volume 2.93cm ³ /g Benzene porosity relative to air 1040cm ³ /min Then, the sheet is subjected to pressure Without 1.2
Pass through the oven at 165°C at a speed of m/min. The residence time of the sheet in the oven is 45 seconds. The sheet treated in this way has the following properties. Weight 150g/ ^m2 Thickness 400μ Transverse tensile strength 3.4Kg/cm Longitudinal tensile strength 2.6Kg/cm Transverse elongation at failure 2.6% Longitudinal elongation at failure 4.3% Transverse failure length 2190m Longitudinal failure Length 1600m Specific volume 2.51cm ³ /g Benzene porosity relative to air 2600cm ³ /min Total pore volume 65.7% Average pore diameter 10.6μ Maximum pore diameter 12.2μ Electrical resistance at 20℃ in sulfuric acid with specific gravity 1.250
5Ω/dm ²・mm Example 2 Same cellulose as Example 1, 25Kg and length 1.4~2.2
mm, average apparent diameter 20μ and surface area approximately 5m ² /g
using an aqueous mixture of 75Kg of polyethylene fibrils without mineral fillers. Polyethylene is the same as in Example 1. The fiber mixture was scoured to 25°SR, then 760°
g of T-Ball 610, fiber content 10g/
Dilute until it becomes a suspension. By the method of Example 1, a sheet is obtained which, after drying, has the following properties: Weight 187g/m ² Thickness 420μ Specific volume 2.75cm ³ /g Benzene porosity relative to air 840cm ³ /min The sheet was placed on the surface of a cylinder at a temperature of 165°C.
Feed at a speed of m/min and with little pressure. Hold the sheet at this temperature for 10 seconds. The obtained sheet has the following properties. Weight 176.4g/ ^m2 Thickness 418μ Transverse tensile strength 1.62Kg/cm Longitudinal tensile strength 2.78Kg/cm Transverse elongation at break 3.3% Longitudinal elongation at break 1.8% Specific volume 2.37cm ³ /g Air Benzene porosity 2000cm ³ /min Total pore volume 63% Average pore diameter 8.5μ Maximum pore diameter 10.5μ Electrical resistance at 20℃ in sulfuric acid with specific gravity 1.250
The properties of the 10Ω/dm ² ·mm sheet were measured by the following method. The total pore volume is calculated using the formula below. Pore volume % = V _A − V _T / V _A = 100 (where V _A = volume of the sheet specimen under examination calculated from the geometric dimensions, V _T = calculated by the formula V _T = 50 − T ₂ the effective volume of the sample (i.e. derived from the pore volume),
Here, 50 is the volume of the tare pine tress where the sample was placed.
cm ³ , T ₂ is the volume cm ³ of n-propanol filling the pine tress at 20°C. The pore size is calculated by the following formula. Pore diameter (μ) = 40.8 × surface tension of n-propanol (dyne × cm) / air pressure (cm of water) A sample with a diameter of 41.3 mm was fixed between two cylinders, the bottom of the lower cylinder was closed, and the air jet was supply. The upper cylinder has a bottom made of porous sample and is supplied with about 4 cm ³ of n-propanol. The maximum pore diameter is calculated starting from the air pressure (cm of water) that constitutes the three columns of air escaping from the sample via n-propanol. The average diameter is calculated with sufficient air pressure to form an air column across the surface of the sample. Electrical resistance measurements are made at 20°C. The change in the Wheatstone bridge equilibrium caused by the porous separator is measured. The measuring cell is kept at a constant temperature. porous separator with sulfuric acid (specific gravity =
1.28, 25°C) and measurements are taken after 20 minutes of immersion. Mechanical Properties: Example 3 According to Standard Methods Used in Paper Characterization An aqueous suspension of sulfated softwood cellulose with a fiber concentration of 25 g/ is introduced into a conical scouring machine where the cellulose is scoured to 35° SR. . This suspension then contains 30% by weight of kaolin incorporated as a filler to give an average ponderal
High-density polyethylene fibrils (MI=30, melting temperature 135°C, density 0.96) with length 1.6 mm, average (apparent) diameter 19 microns and surface area 3.5 m ² /g
g/cc) was added in an amount corresponding to 200 parts by weight of fibrils per 100% by weight of cellulose. This mixture was then transferred to a skimming blade scouring machine and operated until complete homogenization. The suspension was then diluted with water to a fiber content of 8 g/
7g of T-pol PG (Shell) per 100g of fiber
A nonionic surfactant (manufactured by Com-pany) was added thereto, and a sheet-like fiber support was produced using this on a flat table continuous sheet forming machine. During the manufacture of the support, 100
Approximately 0.6 parts by weight of VP-1144/IT per part of fiber by weight
(defoamer from Bayer Company) was added. The obtained support consisted of a sheet having a specific gravity of 150 g/cm ² . The support was passed substantially in the absence of pressure over a steam-heated cylinder heated to 175°C to melt the polyethylene fibers contained in the sheet and form a support 150 mm long and 386 microns thick. . Using this support and the apparatus shown in the drawings, a series of spacer-equipped storage battery separators were manufactured. For this purpose, as a spacer to be applied to the support, 100 parts by weight of low-density polyethylene (density = 0.918, MI = 3.5, melting temperature = 110 °C), 2.5 p.
bw Kabot CB-9PE.732 (mixture of 50p.bw carbon black and 50p.bw polyethylene wax,
CABOT Co.), 5p.bw of kaolin and 2p.b.
w's Therma 2002/L (antifoaming agent, manufactured by SARMA Co.)
A filament with a diameter of 1.2 mm was used which was extruded separately starting from a mixture consisting of: The heating cylinder B was maintained at 125° C. and the pressure on the spacer (corresponding in this case to the weight of the presser roller p) reached 100 g/cm. The feeding speed or advancement of the support onto roller B is 1.5 m/
It was hot in minutes. The spacer-equipped carrier that comes out of the device is
Dimensions 15 x 12.8 cm and overall thickness 1.35 mm (support +
spacers) were cut into sheets. Interesting features identified in the final product are shown in the table below. Example 4 The same procedure as in Example 1 was repeated, except that the polyethylene fibrils were free of filler and the fibrillated polyethylene had an MI=2.3. The characteristics of the separator are shown in the table. Example 5 The prepared support has a thickness of 415 microns,
The polyethylene used for the spacer contains only 25% by weight of carbon black, contains no foaming agent, and the bonding pressure for the spacer is 66g/
The method of Example 1 was repeated except that cm was reached. The characteristics of the spacer and the electrical resistance of the separator and support alone are shown in the table. Example 6 The method of Example 3 was repeated except that a bonding pressure on the spacer equal to 100 g/cm was used. The electrical resistance of the separator and spacer is shown in the table below. Example 7 The same procedure as Example 3 was repeated except using a bonding pressure of 200 g/cm. 【table】

[Brief explanation of drawings]

The drawing shows an apparatus suitable for carrying out the method of the invention, comprising a cylindrical heating surface and a set of accessory elements that can be used during operation. A... Coil, B... Heating cylinder, C... Support roller, D... Multi-blade pulley, P... Presser roller, R... Transmission roller, d... Sheet, f... Spacer filament, h... Separator.

Claims (1)

[Scope of Claims] 1. A method for producing a porous sheet suitable for use as a separator for sulfuric acid storage batteries, the method comprising the following steps. (a) 3 to 40% by weight of cellulose fibers, 97 to 60% by weight of at least one thermoplastic polymer having a surface area of at least 1 m ² /g of fibrils and uniformly dispersed in the fiber mixture; producing a sheet of fibrous material containing 0.1% of a mixture of non-ionic wetting agents; (b) subjecting said sheet to a temperature equal to or higher than the melting temperature of the polymer to form fibrils without applying pressure; The process of heating for a sufficient time to melt. 2. The method according to claim 1, wherein the fibrils have a length of 1 to 3 mm. 3. The method according to claim 1, wherein the fiber mixture has a beating degree of 15 to 55° SR.