Abstract:
A fusible, structurally stabilized battery separator is disclosed. The separator is formed by extruding a cylindrical parison of a polymer film and quenching the film on both sides with a low temperature fluid stream prior to processing the film to impart microporosity. Most preferably, the film includes at least a polyethylene layer and polypropylene layer.

Description:
TECHNICAL FIELD 
     The present invention relates to battery separators and particularly to battery separators exhibiting an extended high electrical resistance profile over temperatures to 180° C. or more. 
     BACKGROUND 
     Batteries separators may be prepared by various techniques, for example, by way of extraction, or by way of a multi-step annealing/stretching process. This latter process was discovered by the Celanese Plastics Company of Summit, New Jersey in the early 1970&#39;s. A crystalline polymer, such as polypropylene is first extruded into a film under conditions which enhance stress in the molten polymer. It is desirable to anneal the film in an untensioned or low tensioned state to perfect the necessary crystalline structure. The precursor thus prepared is elongated in the machine direction to introduce a network of slit-like voids. The deformation process may be used to control the pore size and pore size distribution as well as the overall porosity. See Bierenbaum, H. S., Isaacson, R. B., Druin, M. L., and Plovan, S. G., Microporous Polymeric Films, I &amp; EC PRODUCT RESEARCH AND DEVELOPMENT, Vol. 13, pp. 2-9, March, 1974. 
     As battery design requirements became more sophisticated, the characteristics of battery separators over various temperature ranges has also been refined. Fusible separators with suitable “shut down” characteristics are particularly desirable as is apparent from the most recent literature. 
     U.S. Pat. No. 4,650,730 to Lundquist et al. discloses a multi-ply polymeric sheet useful as a battery separator. Typically, the sheet includes a first layer in the form of a microporous sheet (unfilled) and a second, filled microporous sheet. The microporous component sheets are produced by an extraction process, then laminated together to form the structure, which will become non-porous at elevated temperatures. Note column 11, example 1. The claims specify a thickness of less than 10 mils per layer, various pore sizes and filler loadings. See also U.S. Pat. No. 4,731,304 to Lundquist et al. 
     U.S. Pat. No. 5,281,491 to Rein et al. is directed to a multi-ply unfilled sheet product reported to be useful as a “shut-down” battery separator. The product is formed by co-extrusion (blown film) followed by extraction. See Columns 6-8; examples 1-6. It is noted in Column 10 that uniaxial stretching can be used to impart porosity. 
     U.S. Pat. No. 5,240,655 to Troffkin et al. describes yet another possible process for making a multi-ply battery separator. The process therein described includes a first co-extrusion step, followed by cold (liquid nitrogen) stretching, followed by warm stretching, followed by annealing. 
     Japanese Patent Application Nos. 98394 and 98395 of Kurauchi et al. teach a porous film. Both documents refer to co-extrusion as a fabrication possibility, however, note that lamination of films is the preferred option, followed by heat treatment and two-step stretching to impart porosity. 
     U.S. Pat. No. 5,667,911 to Yu et al. teaches a process for making seamless, cross-piled battery separators. The method described involves extruding a tubular film, collapsing the film, annealing, cold stretching, hot stretching and heat setting to produce microporous membranes. The membranes are then spirally slit and subsequently laminated. 
     U.S. Pat. No. 5,565,281 to Yu et al. teaches a process not unlike the &#39;911 patent as applied to making a thin, bi-layer shutdown battery separator of high puncture strength. Particular parameters appear in the specification and claims. See also, U.S. Pat. No. 5,691,077 directed to making a thin tri-layer membrane including two outer polypropylene membranes sandwiching a microporous polyethylene membrane. Note Table 8, column 9. 
     U.K. Publication No. 2, 298,817 discloses a porous film prepared by forming a non-porous laminate, stretching the laminate to impart porosity, followed by heat treatment. See p. 9 and following. The laminate may be prepared initially by co-extrusion as set forth in example 1, p. 13 and following. A similar process to prepare A/B/A tri-layer films is described in Kokai 8-250097. Note working examples. See also European Publication No. 0 794 583 at p. 5, lines 48 and following. Note FIG.  1 ( c ) thereof. 
     Additional tri-layer membranes are disclosed in Japanese Patent Application Nos. 8-266398 and 8-293612 and Kokai 10-154499. 
     As can be seen from the foregoing, there has been continuous refinement of battery separator preparation, particularly in connection with multi-layer separators. Early developments involve making a plurality of porous structures followed by lamination to complete fabrication. Thereafter non-porous films, separately prepared, were laminated together and the multi-layer non-porous structures were further processed to impart porosity to the product. Most recently multiple layers are co-extruded into a single non-porous structure which is processed into a porous structure thereby minimizing the number of processing steps needed to make the product. 
     So also, it is desirable to improve the thermal characteristics of battery separators, particularly with respect to “shut-down” properties desirable in the high performance market. Ideally, a membrane designed for service where a thermal shut-down is desired should radically increase its impedance at a first temperature threshold of 120-130° C. or so and continue exhibiting increased impedance as long as it is possible, up to the crystalline melting point of the polymer or beyond at high rates of temperature increase. Some of the more recent literature in the field recognize such desirable characteristics. 
     Asahi Chemical Industry Co., Ltd. Discloses in Laid-Open Application No. 3-203160 a temperature resistant battery separator with a maximum impedance at least about 10° C. higher than the temperature at which the impedance of the separator initially rises to 10 times its value at room temperature (R25). The porous membranes are prepared by way of extraction from high molecular weight polyethylene and maximum impedance temperatures at scan rates of 2° C. per minute are reported to be up to about 25° C. higher than the temperature at which impedance initially begins to rise. 
     U.S. Pat. No. 5,480,745 to Nishiyama et al., discloses co-extruded porous bi-layer films, where one layer is polypropylene and one layer is a mixture of polyethylene and polypropylene. The membranes are reported to exhibit a rise in impedance at about 130° C. and a decay in impedance at about 170° C. 
     It has been found in accordance with the present invention that membranes with enhanced resistance performance against temperature are prepared by rapidly quenching a molten film prior to imparting porosity to the separator. 
     SUMMARY OF THE INVENTION 
     There is provided in accordance with the present invention a battery separator formed of a microporous polyolefinic membrane generally capable of maintaining an electrical resistance greater than about 10,000 ohms-square centimeter at a temperature of at least about 185° C. as measured at a scan rate of 60° C. per minute. Typically the membrane is capable of maintaining an electrical resistance of greater than about 10,000 ohms-square centimeter at a temperature of at least about 185° C. at a scan rate of 2° C. per minute; while, preferably, membranes in accordance with the invention are capable of maintaining an electrical resistance greater than about 10,000 ohms-square centimeter at temperatures from about 130° C. to at least 185° C. as measured at a scan rate of either 60° C. per minute or 2° C. per minute. Most preferably the foregoing high resistance is maintained to 195° C. or more, such as 200° C. or more at scan rates of 2° C. per minute or 60° C. per minute. 
     The separator in accordance with the invention may be made from a variety of polymers including high density polyethylene, isotactic polypropylene or combinations thereof. Other polypropylenes and polyethylenes such as ultra high molecular weight polyethylenes may be employed. In the most preferred embodiments, multi-layer membranes are employed having at least one polypropylene layer and at least one polyethylene layer. Tri-layer membranes are particularly preferred. 
     In another aspect of the invention, there is provided a method of making a battery separator including the steps of: extruding a polymer film in a molten state in the form of a cylindrical parison having first and second surfaces; applying a low temperature fluid stream to both the first and second surfaces of the cylindrical parison, the low temperature fluid stream being operative to quench the molten polymer film such that it is in a substantially solidified state; followed by imparting microporosity to said polymer film to make the battery separator. Most preferably the process includes co-extruding a polymer film with at least one polyethylene layer and at least one polypropylene layer. 
     Following quenching of the polymer film in accordance with the inventive process, microporosity is most preferably imparted to the film by annealing the film to enhance its crystalline structure, elongating the film and heat setting it to provide dimensional stability. The step of elongating the film preferably includes elongating the film by at least about 20% at a low temperature (15-35° C.) followed by further elongating the film at an elevated temperature (110° C.-130° C.) by at least about 100%. Relaxation upon heat setting is typically about 8% to about 15%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail below in connection with numerous examples and various drawings. In the drawings: 
     FIG. 1 is a schematic diagram illustrating an extrusion apparatus useful for manufacturing membranes in accordance with the present invention; 
     FIG. 2 is a detail of the apparatus of FIG. 1 showing the operation of the quench of both sides of the cylindrical polymer film parison; 
     FIG. 3 is another more detailed schematic diagram illustrating operation of the inventive process. 
     FIGS. 4A and 4B are schematic diagrams of a test cell used to measure electrical resistance of a microporous battery separator as a function of temperature; 
     FIG. 5 is a plot of electrical resistance versus temperature for various commercially available membranes at a scan (heating) rate of 60° C. per minute; and 
     FIG. 6 is a plot of electrical resistance versus temperature for membranes manufactured in accordance with the present invention as well as two other commercially available membranes. 
    
    
     DETAILED DESCRIPTION 
     The invention is described in detail below with reference to numerous examples which are provided for purposes of illustration only. The spirit and scope of the present invention is defined in the appended claims. 
     The preferred process by which the inventive separators are made broadly comprises the following steps: extruding a polymer film to form a sheet; annealing the sheet to enhance the crystal structure and stretching the annealed sheet. The following references, as well as the reference cited therein, illustrate the state of the art of the preferred process and are incorporated herein by reference: U.S. Pat. No. 5,565,281 of Yu et al.; U.S. Pat. No. 5,691,077 of Yu; and U.S. Pat. No. 5,667,911 of Yu et al. Knowledge of these methods being readily available to those of skill in the art, the inventive process shall be described by explaining differences between the prior art and the inventive method for making battery separators in accordance with the invention. 
     Broadly, process improvements which produce films in accordance with the present invention involve utilizing an apparatus which applies a quenching fluid, such as air, to both sides of the polymer sheet as illustrated in connection with FIGS. 1 and 2. 
     There is shown in FIG. 1 a schematic representation of an extrusion apparatus capable of co-extruding a multi-layer cylindrical parison. An extrusion apparatus  10  includes generally a first extruder  12  and a second extruder  14  connected to a die assembly  16 . Assembly  16  defines a of die orifice indicated at  18  as well as an outer air quench ring  20  and an inner quench ring apparatus  22 , as shown in FIGS. 1 and 2. Cooling air is supplied by way of a fan  24  to die assembly  16  and inner quench ring apparatus  22 . Another fan  26  supplies cooling air to the outer quench ring  20 . A plurality of valves  28 ,  30  control air flow; while polymer flow can be controlled by shut off valves  32  and  34 . 
     In operation, polymer pellets  36 ,  38  are melted in extruders  12 ,  14  and fed through valves  32 ,  34  to die assembly  16 . One may choose to feed only one polymer to make it monolayer film if so desired by utilizing only one extruder, or employ more than two extruders to make multi-layer separators. In each case, a suitable die is configured to extrude one layer or co-extrude multiple layers. 
     Fully molten polymers are fed to die assembly  16  and extruded through orifice  18 . Upon exit, cylindrical film parison  40  is molten as it meets the cooling air streams on both sides of the film, as is better illustrated in connection with FIG.  2 . 
     In FIG. 2, like parts are indicated with the same numerals as in FIG.  1 . As cylindrical film  40  exits orifice  18  it is molten and there is applied about its entire outer periphery an outer quenching air stream indicated at  42  and about its entire inner periphery another quenching air stream indicated at  44 . Both air streams  42 ,  44 , are low temperature air streams generally from about 0° C. to about 45° C., with about 15-30° C. being more typical. Air streams  42 ,  44  are operative to fully quench the melt phase of film  40  such that it is in a substantially solidified state at  46 . 
     Apparatus  22  includes an exit suction orifice indicated at  48 , so that the volume of air stream  44  can exit a cavity  50  defined by cylindrical film  40  as is desired in order to control pressure. 
     There is shown in FIG. 3 a more detailed quench apparatus  60  utilized in connection with the examples which follow. Apparatus  60  includes a die exit  62  from which molten polymer exits the die to form a cylindrical parison  64 . There is also provided an outer air ring  66  and an inner air ring  68  to provide quenching air to both sides of film parison  64 . Air paths are thus defined as shown by arrows  70 ,  72 . Each air ring defines an adjustable gap  74 ,  76  which may be set as desired and is located at an adjustable height  78 ,  80  above die exit  62  as noted in the examples which follow. The gaps are adjusted along with the various pressures and valves to regulate air flow as desired. 
     A blower  82  provides air to outer air ring  66  as shown and the pressure may be measured at  85 , while another blower  86  provides air to inner ring  68  where the pressure is measured by a gauge at  87 . Air flows to the inner ring as indicated by arrows  88 ,  89 . 
     There is further provided an exhaust blower  90  which is provided with a control valve  92  to control flow out of the interior of cylindrical parison  64  as shown by arrows  94 ,  96 ,  97  and  98  which indicate the inside air ring return path. 
     In the foregoing manner, a quenching air stream is applied to both sides of a cylindrical parison as it exits a circular die to provide a double sided quench of the molten polymer as it exits the die. Blowers  82 ,  86  are provided with chillers to cool the output air, typically to a temperature of from about 15° to about 25° C. 
     The film thickness of the parison is generally from about 0.2 to about 2 thousandths of an inch (mils). The heights  78 ,  80  are typically set at from about ½ of an inch to about 6 inches, while air gaps  74 ,  76  are typically set at from about 80 to about 250 mils. The air pressure to each air ring is typically from about 0.8 inches of water to about 8 inches of water depending on the cooling desired. With an 11.87 inch diameter die, cylindrical parison  64  typically has a diameter of about 11.5 to about 12.5 inches; while the degree of expansion can be manipulated by way of the air flow and particularly by way of valve  92 . 
     Following extrusion of the film as described above it is annealed, elongated and heat set as is further described in connection with the examples which follow. A series of multi-layer microporous battery separators were prepared as described in detail hereinafter. Product characteristics are reported in accordance with the following table: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Test Methods 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Gurley 
                 ASTM-D726(B) 
               
               
                   
                   
                 Gurley is a resistance to air flow 
               
               
                   
                   
                 measured by the Gurley densometer 
               
               
                   
                   
                 (e.g. Model 4120). Gurley is the 
               
               
                   
                   
                 time in seconds required to pass 
               
               
                   
                   
                 10 cc of air through one square inch 
               
               
                   
                   
                 of product under a pressure of 12.2 
               
               
                   
                   
                 inches of water. 
               
               
                   
                 Basis Weight 
                 Basis weight is determined by 
               
               
                   
                   
                 cutting three-one square foot 
               
               
                   
                   
                 samples across the width of the 
               
               
                   
                   
                 sample and weighing them on a 
               
               
                   
                   
                 precision balance with accuracy to 
               
               
                   
                   
                 0.0001 grams. The three (3) 
               
               
                   
                   
                 samples are averaged and compared 
               
               
                   
                   
                 to tolerance limits. 
               
               
                   
                 Thickness 
                 Method: T411 om-83 developed 
               
               
                   
                   
                 under the auspices of the Technical 
               
               
                   
                   
                 Association of the Pulp and Paper 
               
               
                   
                   
                 Industry. Thickness is determined 
               
               
                   
                   
                 using a precision micrometer with a 
               
               
                   
                   
                 ½ inch diameter, circular shoe 
               
               
                   
                   
                 contacting the sample at seven (7) 
               
               
                   
                   
                 PSI. Ten (10) individual micrometer 
               
               
                   
                   
                 readings taken across the width of 
               
               
                   
                   
                 the sample are averaged. 
               
               
                   
                 Shrinkage, MD 
                 ASTM D-1204 (60 min. @ 90° C.) 
               
               
                   
                   
                 Three separate lengths of stretched 
               
               
                   
                   
                 product of approximately ten 
               
               
                   
                   
                 centimeters are measured across the 
               
               
                   
                   
                 width of the sample in the machine 
               
               
                   
                   
                 direction (MD). The sample is 
               
               
                   
                   
                 exposed to air at 90° C. for one hour, 
               
               
                   
                   
                 the lengths are re-measured, the 
               
               
                   
                   
                 percentage shrinkage of the original 
               
               
                   
                   
                 length is calculated for each sample, 
               
               
                   
                   
                 and the results averaged. 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 1 
     A 1 mil in thickness polypropylene/polyethylene/polypropylene microporous battery separator was prepared utilizing the apparatus shown in FIGS. 1 and 2. The inner and outer quench rings were supplied with air at room temperature and were positioned slightly over 3 inches in height from the die exit. Extrusion conditions are given in Table 2. Following extrusion, the sheet was annealed, stretched in a two step process and heat set under the conditions of Table 3. Product characteristics appear in Table 4. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Summary of Materials and Extrusion Conditions 
               
               
                 for Battery Separator of Example 1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment: 
                 Alpine 3 layer Co-extrusion 
               
               
                   
                 die; 210 mil Mandrel gap, 
               
               
                   
                 300 mm die diameter 
               
               
                 Resin: 
                 Fina (Dallas, TX) PP 3362 
               
               
                   
                 isotactic polypropylene, 
               
               
                   
                 Mitsu Petroleum Chemical 
               
               
                   
                 (Tokyo, JP) HIZEX ® HDPE 
               
               
                   
                 5202B polyethylene 
               
               
                 Polypropylene Extruder Barrel 
                 195° C. 
               
               
                 Temperature: 
                   
               
               
                 Polyethylene Extruder Barrel 
                 185° C. 
               
               
                 Temperature: 
               
               
                 Die Temperature: 
                 185° C. 
               
               
                 Line Speed: 
                 105 ft/sec 
               
               
                 Quench Ring Height from Die Exit; inner, 
                 3 5/16 inches, 3 5/16 inches 
               
               
                 outer: 
               
               
                 Quench Ring Air Pressure; inner, outer: 
                 7.0 inches H 2 O, 
               
               
                   
                 7.0 inches H 2 O 
               
               
                 Quench Ring Gap; inner, outer (inches) 
                 0.200 inches, 0.150 inches 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Summary of Annealing/Stretching Conditions for Battery 
               
               
                 Separator of Example 1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Annealing Temperature: 
                 120° C. 
               
               
                   
                 Annealing Time: 
                 15 minutes 
               
               
                   
                 Cold Stretch Temperature: 
                 25° C. 
               
               
                   
                 Cold Stretch Elongation: 
                 25% 
               
               
                   
                 Hot Stretch Temperature: 
                 120° C. 
               
               
                   
                 Hot Stretch Elongation: 
                 100% 
               
               
                   
                 Heat Set Temperature: 
                 120° C. 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Average Product Characteristics for Battery Separator of Example 1 
               
             
          
           
               
                 Gurley 
                 Thickness 
                 Shrinkage 
                 Base Weight 
                 Porosity 
               
               
                 Secs 
                 mils 
                 % 
                 mg/cm2 
                 % 
               
               
                   
               
               
                 26 
                 0.9 
                 3.5 
                 1.26 
                 37.2 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 2 
     Following the procedure of Example 1, a bi-layer polyethylene/polypropylene microporous battery separator was prepared. Materials, equipment and extrusion conditions appear in Table 5, while annealing and stretching conditions appear in Table 6. Average product characteristics appear in Table 7. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Summary of Materials and Extrusion Conditions 
               
               
                 for Battery Separator of Example 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 equipment: 
                 Alpine two layer co-extrusion Die, 
               
               
                   
                 210 mil mandrel gap, 300 mm die 
               
               
                   
                 diameter 
               
               
                 resin: 
                 Fina PP 3271 isotactic polypropylene, 
               
               
                   
                 HIZEX ® 5202B high density 
               
               
                   
                 polyethylene 
               
               
                 Polypropylene Extruder Barrel 
                 200° C. 
               
               
                 Temperature: 
                   
               
               
                 Polyethylene Extruder Barrel 
                 190° C. 
               
               
                 Temperature: 
                   
               
               
                 Die Temperature: 
                 200° C. 
               
               
                 Line Speed: 
                 100 ft/minute 
               
               
                 Quench Ring Height from Die 
                 3 inches, 3 5/16 inches 
               
               
                 Exit; inner, outer: 
               
               
                 Quench Ring Air Pressure: inner, 
                 4 inches H 2 O, 8 inches H 2 O 
               
               
                 outer: 
               
               
                 Quench Ring Gap; inner, outer: 
                 0.200 inches, 0.150 inches 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Summary of Annealing/Stretching Conditions 
               
               
                 for Battery Separator of Example 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Annealing Temperature: 
                 120° C. 
               
               
                   
                 Annealing Time: 
                 12 minutes 
               
               
                   
                 Cold Stretch Temperature: 
                 25° C. 
               
               
                   
                 Cold Stretch Elongation: 
                 25% 
               
               
                   
                 Hot Stretch Temperature: 
                 120° C. 
               
               
                   
                 Hot Stretch Elongation: 
                 130% 
               
               
                   
                 Heat Set Temperature: 
                 120° C. 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Average Product Characteristics for Battery Separator of Example 2 
               
             
          
           
               
                 Gurley 
                 Thickness 
                 Shrinkage 
                 Base Weight 
                 Porosity 
               
               
                 Secs 
                 mils 
                 % 
                 mg/cm2 
                 % 
               
               
                   
               
               
                 25 
                 1.0 
                 4.5 
                 1.32 
                 43.0 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 3 
     Following the procedure of Example 1, another polypropylene/ polyethylene/polypropylene tri-layer battery separator was prepared in accordance with the present invention. A summary of the materials and extrusion conditions appear in Table 8. Annealing, stretching and heat setting conditions are given in Table 9, while Table 10 sets forth product characteristics. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 Summary of Materials and Extrusion Conditions 
               
               
                 for Battery Separator of Example 3 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment: 
                 Alpine 3 layer co-extrusion die, 
               
               
                   
                 210 mil mandrel gap, 300 mm die 
               
               
                   
                 diameter 
               
               
                 Resin: 
                 Fina 3271 isotactic polypropylene, 
               
               
                   
                 HIZEX ® HDPE 5202B high 
               
               
                   
                 density polyethylene 
               
               
                 Polypropylene Extruder Barrel 
                 190° C. 
               
               
                 Temperature: 
               
               
                 Polyethylene Extruder Barrel 
                 185° C. 
               
               
                 Temperature: 
               
               
                 Die Temperature: 
                 200° C. 
               
               
                 Line Speed: 
                 85 ft/sec. 
               
               
                 Quench Ring Height from Die Exit; 
                 3 13/16 inches, 3 1/2 inches 
               
               
                 inner, outer: 
               
               
                 Quench Ring Air Pressure; inner, 
                 4 inches H 2 O; 4 inches H 2 O 
               
               
                 outer: 
               
               
                 Quench Ring Gap; inner, outer: 
                 0.250, 0.100 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 Summary of Annealing/Stretching Conditions 
               
               
                 for Battery Separator of Example 3 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Annealing Temperature: 
                 122° C. 
               
               
                   
                 Annealing Time: 
                 12 minutes 
               
               
                   
                 Cold Stretch Temperature: 
                 25° C. 
               
               
                   
                 Cold Stretch Elongation: 
                 45% 
               
               
                   
                 Hot Stretch Temperature: 
                 122° C. 
               
               
                   
                 Hot Stretch Elongation: 
                 120% 
               
               
                   
                 Heat Set Temperature: 
                 122° C. 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Average Product Characteristics for Battery Separator of Example 3 
               
             
          
           
               
                 Gurley 
                 Thickness 
                 Shrinkage 
                 Base Weight 
                 Porosity 
               
               
                 Secs 
                 mils 
                 % 
                 mg/cm2 
                 % 
               
               
                   
               
               
                 30 
                 0.86 
                 2.1 
                 1.25 
                 37 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 4 
     A polyethylene/polypropylene/polyethylene tri-layer battery separator was prepared in according to the Example 1 above. Extrusion particulars appear in Table 11, along with the equipment and materials employed. Annealing, stretching and heat setting conditions are given in Table 12, while representative product characteristics appear in Table 13. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                 Summary of Materials and Extrusion Conditions 
               
               
                 For Battery Separator of Example 4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment: 
                 Alpine Three layer co- 
               
               
                   
                 extrusion Die; 140 mil 
               
               
                   
                 gap/300 mm die diameter 
               
               
                 Resin: 
                 Fina, 3271 isotactic polypro- 
               
               
                   
                 pylene HIZEX ® 5202B high 
               
               
                   
                 density polyethylene 
               
               
                 Polypropylene Extruder Barrel 
                 195° C. 
               
               
                 Temperature: 
               
               
                 Polyethylene Extruder Barrel 
                 185° C. 
               
               
                 Temperature: 
               
               
                 Die Temperature: 
                 200° C. 
               
               
                 Line Speed: 
                 80 ft/min 
               
               
                 Quench Ring Height from Die Exit; 
                 2 3/4 inches, 3 5/16 inches 
               
               
                 inner, outer: 
               
               
                 Quench Ring Air Pressure; inner, 
                 4 inches, 7 inches H 2 O 
               
               
                 outer: 
               
               
                 Quench Ring Air Gap; inner, outer: 
                 0.150 inches, 0.200 inches 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 12 
               
               
                   
               
               
                 Summary of Annealing/Stretching Conditions 
               
               
                 for the Battery Separator of Example 4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Annealing Temperature: 
                 120° C. 
               
               
                   
                 Annealing Time: 
                 12 minutes 
               
               
                   
                 Cold Stretch Temperature: 
                 25° C. 
               
               
                   
                 Cold Stretch Elongation: 
                 25% 
               
               
                   
                 Hot Stretch Temperature: 
                 120° C. 
               
               
                   
                 Hot Stretch Elongation: 
                 130% 
               
               
                   
                 Heat Set Temperature: 
                 120° C. 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 13 
               
             
             
               
                   
               
               
                 Average Product Characteristics for Battery Separator of Example 4 
               
             
          
           
               
                 Gurley 
                 Thickness 
                 Shrinkage 
                 Base Weight 
                 Porosity 
               
               
                 Sec 
                 mils 
                 % 
                 mg/cm2 
                 % 
               
               
                   
               
               
                 33 
                 1.07 
                 3 
                 1.4 
                 41 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 5 
     Another polyethylene/polypropylene/polyethylene tri-layer battery separator was prepared in accordance with Example 4. Materials, equipment and extrusion conditions are listed in Table 14. Table 15 gives annealing, heat treating and stretching conditions, while Table 16 lists available product characteristics. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 14 
               
               
                   
               
               
                 Summary of Materials and Extrusion Conditions 
               
               
                 for Battery Separator Example 5 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment: 
                 Alpine 3 layer co-extrusion 
               
               
                   
                 die; 140 mil mandrel gap, 
               
               
                   
                 300 mm die diameter 
               
               
                 Resin: 
                 Fina 3271 isotactic polypro- 
               
               
                   
                 pylene HIZEX ® 5202B high 
               
               
                   
                 density polyethylene 
               
               
                 Polypropylene Extruder Barrel 
                 195° C. 
               
               
                 Temperature: 
               
               
                 Polyethylene Extruder Barrel 
                 190° C. 
               
               
                 Temperature: 
               
               
                 Die Temperature: 
                 193° C. 
               
               
                 Line Speed: 
                 90 ft/min. 
               
               
                 Quench Ring Height from Die Exit; 
                 3 1/2 inches, 3 5/16 inches 
               
               
                 inner, outer: 
               
               
                 Quench Ring Air Pressure; inner, 
                 4 inches H 2 O, 6 inches H 2 O 
               
               
                 outer: 
               
               
                 Quench Ring Gap; inner, outer: 
                 0.200 inches, 0.080 inches 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 15 
               
               
                   
               
               
                 Summary of Annealing/Stretching Conditions 
               
               
                 for Battery Separator of Example 5 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Annealing Temperature: 
                 120° C. 
               
               
                   
                 Annealing Time: 
                 12 minutes 
               
               
                   
                 Cold Stretch Temperature: 
                 25° C. 
               
               
                   
                 Cold Stretch Elongation: 
                 25% 
               
               
                   
                 Hot Stretch Temperature: 
                 120° C. 
               
               
                   
                 Hot Stretch Elongation: 
                 130% 
               
               
                   
                 Heat Set Temperature: 
                 120° C. 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 16 
               
             
             
               
                   
               
               
                 Average Product Characteristics for Battery Separator of Example 5 
               
             
          
           
               
                 Gurley 
                 Thickness 
                 Shrinkage 
                 Base Weight 
                 Porosity 
               
               
                 Sec 
                 mils 
                 % 
                 mg/cm2 
                 % 
               
               
                   
               
               
                 17 
                 0.50 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 6 
     Following the procedure of Example 1, another polypropylene/ polyethylene/polypropylene tri-layer battery separator was prepared in accordance with the invention. Particulars as to equipment, materials, and extrusion conditions appear in Table 17. Table 18 lists annealing, stretching and heat setting parameters. Table 19 lists product characteristics. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 17 
               
               
                   
               
               
                 Summary of Materials and Extrusion Conditions 
               
               
                 for Battery Separator of Example 6 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment: 
                 Alpine 3 layer co-extrusion 
               
               
                   
                 die, 140 mil mandrel gap, 
               
               
                   
                 300 mm die diameter 
               
               
                 Resin: 
                 Fina 3271 isotatic polypro- 
               
               
                   
                 pylene, HIZEX ® 5202B 
               
               
                   
                 high density polyethylene 
               
               
                 Polypropylene Extruder Barrel 
                 195° C. 
               
               
                 Temperature: 
               
               
                 Polyethylene Extruder Barrel 
                 185° C. 
               
               
                 Temperature: 
               
               
                 Die Temperature: 
                 198° C. 
               
               
                 Line Speed: 
                 80 ft/min 
               
               
                 Quench Ring Height from Die Exit; 
                 3 inches, 3 5/16 inches 
               
               
                 inner, outer: 
               
               
                 Quench Ring Air Pressure; inner, 
                 2 inches H 2 O, 3 inches H 2 O 
               
               
                 outer: 
               
               
                 Quench Ring Gap; inner, outer: 
                 0.080 inches, 0.200 inches 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 18 
               
               
                   
               
               
                 Summary of Annealing/Stretching Conditions 
               
               
                 for Battery Separator of Example 6 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Annealing Temperature: 
                 125° C. 
               
               
                   
                 Annealing Time: 
                 8 minutes 
               
               
                   
                 Cold Stretch Temperature: 
                 25° C. 
               
               
                   
                 Cold Stretch Elongation: 
                 45% 
               
               
                   
                 Hot Stretch Temperature: 
                 118° C. 
               
               
                   
                 Hot Stretch Elongation: 
                 200% 
               
               
                   
                 Heat Set Temperature: 
                 125° C. 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 19 
               
             
             
               
                   
               
               
                 Average Product Characteristics for Battery Separator of Example 6 
               
             
          
           
               
                 Gurley 
                 Thickness 
                 Shrinkage 
                 Base Weight 
                 Porosity 
               
               
                 Sec 
                 mils 
                 % 
                 mg/cm2 
                 % 
               
               
                   
               
               
                 24 
                 15 
                 8 
                 0.70 
                 48 
               
               
                   
               
             
          
         
       
     
     Electrical Resistance 
     Despite the industry interest in shut down or fusible battery separators, there has been little work done on characterizing the phenomena involved. It is believed that in a thermal runaway situation in a battery, the temperature may rise at 100° C. per minute or more, perhaps multiples of such a heating rate. It is believed desirable for battery separators to exhibit large increases in electrical resistance at 120-130° C. and maintain such high resistance, say 10,000 ohms square centimeter or so, as long as possible. Polypropylene/polyethylene membranes have been employed wherein polyethylene melts at a lower temperature so as to block the microporous structure, while the polypropylene maintains it structure to a higher temperature. FIGS. 4A and 4B illustrate a measurement cell useful for characterizing the electrical properties of a battery separator versus temperature. Cell  100  includes four 0.375 inch diameter nickel disks  102 ,  104 ,  106  and  108  all of which are 3 mils thick embedded in a Kapton® polyimide film,  110 ,  112  which is also 3 mils thick. FIG. 4A illustrates the top of the test cell, while FIG. 4B illustrates the bottom of the test cell. The disks are fitted with nickel tabs indicated at  109 . 
     Top assembly  114  and bottom assembly  116  are used to sandwich a separator by placing the separator there between and pressing the electrodes in a Carver press at a 125 psi with heated platens. The platens were heated at a constant rate from 60° C. to 200° C. using a Eurotherm® model 808 controller. The temperature of the electrode surface was sensed by two type E thermocouples indicated at  118 ,  120  (0.5 mil thick, not shown) which were located between a pair of electrodes positioned adjacent to the electrodes holding the separator. 
     The apparatus of FIGS. 4A and 4B is operated as described by Geiger et. al., “Advanced Separators for Lithium Batteries”, 11 th  International Seminar on Primary and Secondary Battery Technology and Application, Feb. 28-Mar. 3, 1994. An electrolyte consisting of 1M lithium trifluoromethanane- sulfonimide (HQ-115, 3M3 Co.) in a 1:1 by volume solution of propylene carbonate (Aldrich), and triethylene glycol dimethyl ether (Aldrich) was used. The conductivity of this electrolyte does not vary significantly over the temperature range employed. Electrical resistance measurements were made using an RLC bridge (GenRad Model 1659) at 1 KHZ as described by Laman et. al., “Impedance Studies for Separators in Rechargeable Lithium Batteries”, J. Electrochem. Soc., Vol. 140, No. 4, April, 1993, pp. L51-53. 
     FIG. 5 shows typical electrical resistance curves versus temperature for commercially available battery separators measured as above at a heating rate or scan rate of 60° C. per minute. Separator A is a polypropylene battery separator, Separator B is a polyethylene battery separator and Separator C is a polypropylene/polyethylene/polypropylene tri-layer separator. Separator C was also tested at a scan rate of 1° per minute and it was found that the high resistance exhibited above 130° C. or so exhibited a slightly lower upper temperature limit. 
     FIG. 6 shows electrical properties of five different separators measured as described above at scan rates of 60° C. per minute. Separators  6 ( a ),  6 ( b ), and  6 ( c ) were polyethylene/polypropylene/polyethylene membranes prepared in accordance with Example 5. Separator D was a polypropylene/polyethylene/polypropylene commercially available separator, while Separator E was a commercially available high molecular weight polyethylene separator. 
     It can be seen from FIG. 6 that only the membranes in accordance with the invention exhibit electrical resistance greater than 10,000 ohms-square centimeters between 130° C. and 185° C. This characteristic is unique to the separators of the present invention which is further defined in the appended claims.