Patent Publication Number: US-2018043598-A1

Title: Method of producing microporous plastic film

Description:
TECHNICAL FIELD 
     This disclosure relates to a method of producing a microporous plastic film. 
     BACKGROUND 
     Microporous plastic films have been broadly used as a substance separation membrane, permselective membrane, separator material of an electrochemical element such as an alkali rechargeable battery, lithium rechargeable battery, fuel cell and capacitor or the like. A separator for a lithium ion battery is a particularly suitable application. 
     JP 2009-249480 A and JP 2013-530261 A disclose a wet process for producing a microporous film made of plastic like polyolefin resin. In the wet process, a diluent such as liquid paraffin is added to a polymer, kneaded and dispersed, and then discharged through a die lip onto a cooling drum to be cooled and solidified to form a gel sheet, which is oriented uniaxially or biaxially by a roller or tenter method to improve strength to produce a film having micropores by extracting the diluent. 
     Particularly by the roller method drawing the sheet in the travelling direction by a plurality of rollers, the lengthwise draw ratio can be changed freely by only changing a roller speed. By the other tenter method, the microporous plastic film can be improved in mechanical properties because polyolefin molecules can be oriented with strong orientation in the drawing direction. As disclosed in JP &#39;480, the roller method applied to the wet process might make the diluent bleed out from a gel sheet surface under a pressure caused by heat or tension while the conveying and drawing are performed in the presence of the diluent between film and roller. To draw such a gel sheet, the sheet is sufficiently cooled below the crystallization ending temperature of the polymer and is heated again below the melting point as disclosed in JP &#39;261 where it is heated above the crystallization dispersion temperature. 
     The wet roller method disclosed in JP &#39;480 might have a problem that the diluent intervening between the roller and film lubricates to meander or fail a desirable draw ratio because of slip. 
     According to JP &#39;480, the above-described slip can be prevented when a tension over the lengthwise draw tension is given between the lengthwise drawing machine and the lateral drawing machine (tenter), wherein the tension should preferably be greater than the draw tension by 20%. However, we found that such a great tension over the draw tension drew the sheet downstream of the lateral drawing machine to cause an undesirable slip adversely and therefore the slip couldn&#39;t be prevented. 
     According to JP &#39;261, a predetermined range of contact time, contact angle and contact length between the lengthwise drawing roller and the sheet can prevent the slip and the sheet surface from being damaged. However, we found that even such measures couldn&#39;t fully prevent slip when we had an increased draw speed, a decreased draw temperature or an increased draw ratio to improve physical properties and mechanical properties of the microporous plastic film. 
     Accordingly, it could be helpful to provide a method of producing a microporous plastic film excellent in physical properties and mechanical properties by a high-speed drawing without slip under a high productivity. 
     SUMMARY 
     We thus provide a method of producing a microporous plastic film comprising kneading a diluent and a polyolefin resin with an extruder; discharging the polyolefin resin kneaded with the diluent from a die lip in a sheet shape; cooling and solidifying the sheet discharged from the die lip on one or plurality of cooling drums; reheating and drawing the solidified sheet with a plurality of rollers in a sheet conveying direction; cooling the sheet drawn in the sheet conveying direction; gripping both ends of the sheet with clips; introducing the sheet into a tenter; and washing the diluent out to prepare a uniaxially or biaxially oriented microporous plastic film, wherein the method further comprises: driving a nip roller with a motor, the nip roller pressing a sheet interposing between the nip roller and at least one of the rollers. 
     It is preferable that the method further comprises driving with a motor a nip roller pressing a sheet interposing between the nip roller and at least one of the rollers, the rollers heating the sheet in a process upstream from the drawing with the rollers in the sheet conveying direction. 
     We can produce, with high productivity, a microporous plastic film having excellent physical properties and mechanical properties by preventing slip even in wet roller drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a production process of our microporous plastic film. 
         FIG. 2  is a schematic front view of an example of nip-roller-driving method in a production process of our microporous plastic film. 
         FIG. 3  is a schematic cross-section view of an example of nip-roller-driving method shown in  FIG. 2  in a production process of our microporous plastic film. 
     
    
    
     EXPLANATION OF SYMBOLS 
     
         
           11 : microporous plastic film 
           12 : gelled sheet (film) 
           13 : uniaxially oriented sheet (film) 
           14 : biaxially oriented sheet (film) 
           15 : microporous plastic film roll 
           21 : extruder 
           22 : gear pump 
           23 : die lip 
           31 : first cooling drum 
           32 : second cooling drum 
           33 : refrigerant-flowing nip roller 
           4 : lengthwise drawing process 
           41 : heating roller group 
           42 : drawing roller group 
           421 : first drawing roller 
           422 : second drawing roller 
           43 : cooling roller group 
           44 : nip roller 
           5 : lateral drawing process 
           6 : washing/drying process 
           61 : washing solvent 
           7 : re-drawing heat-treatment process 
           8 : take-up process 
         P 1 : pulley provided by drawing roller 
         P 2 : pulley for driving nip roller 
         P 3 : auxiliary pulley 
         C: press cylinder 
         F: stand frame 
           81 : bearing for drawing roller 
           82 : bearing for nip roller double as cylinder rod connecting part 
         B: driving belt 
         M: motor 
       
    
     DETAILED DESCRIPTION 
     Hereinafter, desirable examples of our microporous plastic film will be explained with reference to the figures. 
       FIG. 1  is a schematic side view showing an example of production process of our microporous plastic film. 
     To desirably produce microporous plastic film  11 , polyolefin resin is mixed with diluent and heated to melt to prepare polyolefin resin solution. The diluent decides a structure forming micropores of the microporous plastic film and contributes to improvement of drawability (for example, improvement as reduction of unevenness at a draw ratio for exhibiting a strength) at the time of drawing film. 
     The diluent is not limited particularly, as far as it can be mixed or dissolved with polyolefin resin. The diluent may be mixed with polyolefin resin in a melt-kneading state. Alternatively, it may be mixed with solid solvent at room temperature. Such prepared solid diluent may be stearyl alcohol, ceryl alcohol, paraffin wax or the like. It is preferable that the diluent is a liquid at room temperature from the viewpoints of prevention of uneven drawing and coating convenience at a later stage. The liquid diluent may be a fatty series such as nonane, decane, decalin, paraxylene, undecane, dodecane and liquid paraffin; a cyclic fatty series or an aromatic hydrocarbon; a mineral oil fraction having a boiling point of the same range as the compounds thereof; or a phthalate ester such as dibutyl phthalate and dioctylphthalate which are liquids at room temperature. To obtain a gelled sheet stably containing liquid diluent, it is preferable to employ a nonvolatile diluent such as liquid paraffin. For example, it is preferable that the liquid diluent has a viscosity of 20 to 200 cSt at 40° C. 
     It is preferable that the polyolefin resin of 10 to 50 mass % is mixed with the diluent in total 100 mass % of polyolefin resin and diluent, from a viewpoint of good formability of extruded product. The polyolefin resin solution may be melt-kneaded uniformly by a calendar, mixer or extruder  21  having a screw as shown in  FIG. 1 . 
     It is preferable that the polyolefin resin solution in extruder  21  has a predetermined temperature of 140 to 250° C. for polyethylene composition and 190 to 270° C. for polypropylene-containing composition, depending on resin kind. A thermometer is provided inside extruder  21  or in a cylinder section to monitor a temperature indirectly and properly adjust the heater temperature, rotation speed and discharge rate of the cylinder section to control the temperature in a target range. 
     The polyolefin resin solution that has been melt-kneaded by extruder  21  is discharged through a slit section of die lip  23  into a sheet while it is measured with gear pump  22  as needed. Such discharged gelled sheet  12  is solidified as contacting cooling drum  31 . Gelled sheet  12  becomes a pillar part with crystal structure made of the polyolefin resin supporting pores of microporous plastic film  11 . Gelled sheet  12  becomes in a gel state since it includes the diluent melt-kneaded in extruder  21 . A part of the diluent bleeds out from sheet surface by cooling gelled sheet  12  so that the sheet with wet surface made by the diluent is conveyed on cooling drum  31 . The thickness of gelled sheet  12  can be adjusted by adjusting the cooling drum speed relative to the flow rate from the die lip slit section according to discharge rate. 
     It is preferable that cooling drum  31  has a temperature of 15 to 40° C., which may affect the crystal structure of gelled sheet  12 . This is because the final cooling temperature should be below the crystallization ending temperature. The molecular orientation tends to advance in a later drawing process when the higher-order structure is fine. To make up for cooling time, it is possible that the diameter of cooling drum  31  is enlarged, cooling drum  32  is added to cooling drum  31  or a plurality of cooling drums are further added. To make the crystal structure precise and uniform in gelled sheet  12 , it is preferable that a parameter such as conveyance speed, drum temperature, drum size and the number of drums is designed in view of the cooling speed. For example, even when a target sheet temperature is 30° C., cooling drum  31  may be around 20° C. in a case of a too high speed and insufficient heat conduction. It is preferable that humidity is lowered by air conditioning when the temperature is below 25° C. Cooling drum  31  may have a shape of a roller or belt. It is preferable that a surface of cooling drum  31  is made of a material which is excellent in shape stability and working accuracy to keep the roller speed constant. Such a material may be metal, ceramic or fiber composite material. It is particularly preferable that the surface is made of a metal excellent in heat conduction to a film. It is possible to perform non-adhesion coating or rubber coating on it to the extent that conduction is not obstructed. It is preferable that the surface of sheet and roller are made of metal including metal plating which is excellent in scratch resistance and heat conductivity and not swelled by the bled-out diluent making wet state on the surface. 
     It is preferable that the roller has a surface roughness of 0.2 to 40 μm at the maximum height. To make a mirror surface, the surface roughness is preferably 0.2 to 0.8 μm. To make a sufficiently rough surface, the surface roughness is preferably 20 to 40 μm. 
     Because the surface of the roller is wet with the diluent, the mirror surface has a low coefficient of friction caused by lubrication. The rough surface has an increased coefficient of friction caused by less or no lubrication because the diluent discharges from the unevenness. Although the mirror surface and the rough surface may be combined as needed, it is basically preferable that the mirror surface is provided from the viewpoints of maintenance such as cleaning and speed control precision, preferably with a certain lubricant with diluent to prevent the sheet from having uneven appearance. 
     It is preferable that cooling drums  31  and  32  are provided with a conventional heat pump or cooling device in addition to an internal flow path for refrigerant to control the surface temperature. The roller is driven to rotate by a rotation driving means such as motor at a predetermined speed while a speed-changing mechanism may be provided between rollers to apply draw tension or relaxation according to expansion and shrinkage of the sheet. Alternatively, each roller may be provided with a motor to achieve function like the speed-changing mechanism by accurately adjusting the speed by inverter or servo. 
     In  FIG. 1 , the top side of gelled sheet  12  is discharged from die lip  23  and contacts first cooling drum  31  which first contacts it, and then is rapidly cooled with refrigerant at the above-described temperature. On the other hand, the side opposite the side contacting the above-described first cooling drum  31  is slowly cooled with air as shown in  FIG. 1 . Although it is not illustrated in the figures, it is preferable that the side opposite the side contacting first cooling drum  31  is cooled by forced circulation with air nozzle or air chamber to make a rapid cooling of the opposite side. Such a configuration would be suitable to such a case that the conveying speed is high or the gelled sheet is too thick to conduct heat sufficiently from cooling drum  31 . Also, the opposite side can be improved in cooling ability by providing a refrigerant-flowing nip roller in which refrigerant flows inside opposite to cooling drum  31 . 
     To prevent wet gelled sheet  12  from meandering or degrading the cooling efficiency by lubrication, it is possible that the sheet is pushed onto cooling drum  31  with a contacting means such as nip roller, jet nozzle, suction chamber and electrostatic application. Such a contacting means can improve the travelling property and cooling efficiency of gelled sheet  12  to enable easy setting of the cooling speed and the final cooling temperature. 
     Next, gelled sheet  12  is oriented in a sheet-conveying direction with a plurality of roller groups in lengthwise drawing process  4 , and then both ends of sheet  13  are gripped by a conventional clip to be oriented in a sheet width direction (direction orthogonal to the conveying direction) while the sheet is conveyed in the travelling direction while heated and kept warm in an oven. Such a drawing process achieves a high productivity and characteristics such as strength and air permeability of microporous film. The drawing process in the sheet-conveying direction (which may be called “lengthwise drawing”) as well as driving process is performed with a roller having a metal surface and a temperature control mechanism such as conventional heater inside. To ensure the freedom of roller path, it is possible to provide an idler roller which is not driven and not shown in  FIG. 1 . Such an idler roller should have small bearing and inertia loss so that a small rotation power is sufficient because the coefficient of friction is small between wet film and roller, and should not be provided more than needed. 
     Alternatively, like cooling drum  31 , it is preferable that heating roller group  41  or drawing roller group  42  has an internal structure in which the roller is provided with a flow path for heat medium such as vapor and pressured hot water. It is possible that the roller is supported as rotatable with bearings and a shaft end connects to a heat-medium supply pipe with a rotary joint for supplying heat medium without obstructing the rotation of roller to supply heat medium to the inside. 
     It is preferable that the drawing is performed at a draw ratio of 5 to 12 in a sheet conveying direction although depending on the gelled sheet thickness. To improve strength and productivity, it is preferable that the drawing is performed at an area ratio of 30 or more in a sheet width direction together with the sheet conveying-directional drawing. It is preferable that the area ratio is 40 or more, preferably 60 or more. 
     It is preferable that a drawing temperature is a melting point of polyolefin resin or less. It is preferable that the temperature is in a range of [crystal dispersion temperature Tcd of polyolefin resin] to [melting point of polyolefin resin]. For example, it is preferable that the temperature is 80 to 130° C. for polyethylene resin, preferably 100 to 125° C. After drawing, a cooling process is performed down to these temperatures. 
     Thus performed drawing cleaves a higher order structure formed on the gelled sheet to miniaturize crystal phase and form many fibrils. The fibrils are three-dimensionally connected irregularly to form a web structure. It is suitable for a battery separator because the drawing improves mechanical strength and enlarges fine pores. 
     Thus obtained uniaxially oriented sheet  13  or biaxially oriented sheet  14  can be washed to remove diluent and dried up to make dry microporous plastic film  11  by a conventional technology as disclosed in WO2008/016174. To make microporous plastic film  11 , it is possible that dry drawing process  7  is performed to reheat and re-draw the sheet after washing process  6 . Re-drawing process  7  may be performed with a roller or a tenter. Also, heat treatment may be performed to adjust physical properties and remove residual distortion through the process. Depending on intended purposes, it is possible that the surface of microporous plastic film  11  is subject to a surface treatment such as corona discharge or functional coating with heat-resistant particles. 
     In  FIG. 1 , included diluent bleeds out from gelled sheet  12  by being cooled on cooling drums  31  and  32 . The diluent bleeds out even by stress derived from conveying tension. For the same reason, gelled sheet  12 , oriented films  13  and  14  have a surface wet with diluent after being discharged from die lip  23  before the diluent is removed in washing process  6 . In particular, gelled sheet  12  is heated to the above-described drawing temperature with heating roller group  41  or the like in lengthwise drawing process  4  so that the heating accelerates the bleeding out of the diluent. Such amount of bleeding out is great particularly at a part from first cooling drum  31  to heating roller group  41  upstream of lengthwise drawing process  4 . Because the diluent bleeding out drops along the roller surface in  FIG. 1 , it is preferable that a pan (not shown) to collect it to be discarded or reused is provided. 
     Although heating roller group  41  and drawing roller group  42  are common to each other in terms of heating and heat-keeping function of gelled sheet  12  and variable roller rotation speed, drawing roller group  42  comprises a roller to substantively draw gelled sheet  12  and generates a peripheral speed difference to permanently deform gelled sheet in the travelling direction. More particularly, drawing roller group  42  is defined as rollers which substantively draw it as generating 3% or more of peripheral speed difference. 
     As shown in  FIG. 1 , the lengthwise drawing is performed to obtain a friction force balancing to the drawing tension by pressing gelled sheet  12  interposing between at least one of drawing roller group  42  and nip roller  44 . It is preferable that nip roller  44  is provided even on drawing roller group  41  as well as cooling drums  31  and  32  shown in  FIG. 1  as needed. 
     It is preferable that the surface of nip roller is made of rubber-like elastic material that can depress gelled sheet  12  uniformly onto uneven thickness of gelled sheet  12 , deflection of roller and slight surface unevenness. The rubber-like elastic material is preferably a general vulcanized rubber such as Nitrile isoButylene-isoprene Rubber (NBR), Chloroprene Rubber (CR), Ethylene Propylene Rubber (EPDM) and Hypalon rubber (CSM). When gelled sheet  12  or conveying roller has a high temperature like 80° C. or higher, it is particularly preferable to employ the EPDM or CSM. Under a higher temperature, it is preferable to employ a silicone rubber or a fluorine rubber as well as the vulcanized rubber. It is preferable to employ a rubber which is little swelled by diluent to prevent the roller from having a distorted shape over time. 
     Gripping force (frictional force) is necessary between the roller and gelled sheet  12  to convey gelled sheet  12  without meandering in a travelling direction. Particularly in drawing roller group  42  where a high tension is generated by drawing, a high gripping force balancing the drawing tension is required to achieve a necessary draw ratio. The diluent having bled out as described above interposes between the roller and gelled sheet  12  in lubrication state to cause a deterioration of the gripping force required for conveyance and drawing. 
     The diluent intervenes between nip roller  44  and gelled sheet  12  as well as between drawing roller and sheet. The friction force required for drawing is obtained from the friction between drawing roller  42  or nip roller  44  and gelled sheet  12 . We found that the friction force required for drawing might obstruct straight travelling of gelled sheet  12  to cause meandering when the nip roller cannot rotate with lubrication of diluent between nip roller  44  and gelled sheet  12 . 
     In our method of producing a microporous plastic film, nip roller  44  which presses a sheet interposing between the nip roller and at least one of the rollers of drawing roller group  42  is driven with a motor. With such a configuration, gelled sheet  12  can be prevented from meandering without rotation of nip roller even in the presence of diluent. The driving can suppress wear debris generation that might be increased on the surface of nip roller or gelled sheet  12  when nip roller idles and slightly contacts the sheet directly even in the presence of diluent. It is preferable that a nip roller is provided on each of all rollers of drawing roller group  42  while each nip roller is driven by a motor. 
     From a viewpoint of less slip and meandering, it is preferable that each driven nip roller and the drawing roller or cooling drum provided on the nip roller rotate at the same rotation speed. In the specification, “the same rotation speed” means the difference of both rotation speed is within ±3%. 
     Further, it is preferable that nip roller  44  that presses a sheet interposing between nip roller  44  and at least one of the rollers of heating roller group  41  is driven with a motor to obtain the same effect. It is preferable that nip roller  44  is provided on each of all rollers of heating roller group  41  while each nip roller  44  is driven by a motor. 
     Nip roller  44  may be driven by a driving force transmitted through belt B and pulleys P 1 , P 2  and P 3  from drawing roller group  42  driven by a motor as shown in  FIG. 2 . In more detail, pulley P 1  is provided at a shaft end of drawing roller group  42  while pulley P 2  is provided at a shaft end of nip roller  44 . Because nip roller  44  and drawing roller group  42  should rotate in the same direction at a contact point, driving belt B should contact pulleys P 1  and P 2  as shown in  FIG. 3 . Therefore, auxiliary pulley P 3  may be provided to drive nip roller  44  and drawing roller group  42  to rotate in the same direction at a contact point. 
     Alternatively, the drawing roller or nip roller may be driven through a gear, or may be driven by magnetic force without contact for preventing the belt and gear from making dust. To prevent such dust making, it is preferable that nip roller  44  is driven directly by a motor. In any case, as shown in  FIG. 2 , belt B and the above-described driving devices should be provided not to obstruct the pressing force of a pressing cylinder by which nip roller  44  presses gelled sheet  12  interposing between the nip roller and drawing roller. For example, driving belt B is provided in a direction not to obstruct the pressing force applied from nip roller  44  onto drawing roller group  42  as shown in  FIG. 3 . 
     It is possible that a nip is performed along a tangent line substantively with a nip roller to introduce gelled sheet  12  into heating roller group  41  or drawing roller group  42  so that slip and meandering can be prevented as improving uniformity of thickness and appearance quality. That is because, when a nip roller is not provided along the tangent line, certain thickness of diluent and air is accompanied between drawing roller group  42  or heating roller group  41  and sheet  12  to make a bank by nipping with nip roller  44  afterwards. The nip roller which is provided substantively along the tangent line can suppress the thickness of diluent and air with nip roller  44  before contacting between drawing roller group  42  or heating roller group  41  and the sheet so that an oriented sheet having a higher quality can be obtained without forming a bank between gelled sheet  12  and the roller. 
     The phrase “nip along the tangent line” means a nip performed by the nip roller at a position where gelled sheet  12  starts contacting heating roller group  41  or drawing roller group  42 . The nip performed with the nip roller at this position makes gelled sheet  12  as if being a tangent line of nip roller. The phrase “a nip is performed along a tangent line substantively” means that a nip which doesn&#39;t make a bank is regarded as “a nip performed along a tangent line” even when the nipped part is slightly away from the exact position where the sheet starts contacting the roller. 
     EXAMPLES 
     Hereinafter, our methods will be explained with reference to the Examples although they are not limited to these Examples in particular. 
     Example 1 
     A mixture is prepared by dry-blending of 0.375 parts by weight of tetrakis[methylene-3-(3,5-ditertiary butyl-4-hydroxyphenyl)-propionate]methane together with 100 parts by weight of polyethylene (PE) composition which comprises 40 mass % of ultrahigh molecular weight polyethylene having mass average molecular weight (Mw) of 2.5×10 6  and 60 mass % of high-density polyethylene having Mw of 2.8×10 5 . 
     Thus obtained mixture is fed to twin-screw extruder  21  at flow rate of 97 kg/hr by a film forming method shown in  FIG. 1 . Liquid paraffin as a diluent is fed at flow rate of 388 kg/hr to be blended at 210° C. 
     Thus obtained polyethylene solution is supplied into die lip  23  as being measured by a gear pump and the polyethylene solution at 210° C. is discharged on first cooling drum  31  adjusted to 35° C. by waterflow to form gelled sheet  12 . First cooling drum  31  is driven to rotate at 10 m/min. 
     Gelled sheet  12  is subject to sampling with 10 mm square before introduction to lengthwise drawing process  4  to find that the thickness is 1.5 mm in 10 times average of contact-type thickness meter. Since bled-out diluent adheres to the surface, the above-described thickness measurement includes ±0.1 mm variation at the maximum 
     Gelled sheet  12  is heated to 110° C. on the sheet surface with heating roller group  41  and a metal waterflow roller as the first drawing roller. Between heating roller group  41  and the first roller of each drawing roller group  42 , the rotation speed of motor directly connected to the roller is controlled to make the downstream side faster by 1% of speed difference according to thermal expansion of the sheet. 
     Drawing roller group  42  consists of 2 rollers shown in  FIG. 1 . Nip roller  44  of which surface is coated with fluoro-rubber is provided onto each roller to perform lengthwise drawing by speed difference between rollers. Each nip roller is driven by a driving mechanism shown in  FIG. 2  at a rotation speed almost the same (nip roller is 2% faster) as each drawing roller contacting through a film. The speed is controlled to set to 9 the total draw ratio of uniaxially oriented sheet  13  passing lengthwise drawing process  4 , wherein the speed of first cooling drum  31  is set to 10 m/min, speed ratio from heating roller group  41  to upstream drawing roller  421  is set to 1%, the speed of upstream drawing roller  421  is set to 10.4 m/min, and the speed of drawing roller  422  is set to 10.4×8.66=90 m/min. 
     Oriented film  13  is cooled on four rollers of cooling roller group  43  including last roller  422  of drawing roller group  42  to adjust the waterflow roller temperature to make the sheet temperature 50° C. Each clip speed difference between the last drawing roller, cooling roller group  43  and lateral drawing process is 0 to make the speed same. 
     From first cooling drum  31 , surfaces of all rollers in lengthwise drawing process  4  are made of steel coated with hard chromic plating of which surface roughness is 0.4 μm (0.4S) at the maximum height. A preliminary sample has drawing tension T of 1,500N, nip roller pressure P of 2,000N and coefficient of friction μ of 0.15. 
     Both ends of oriented film  13  are gripped with a clip to perform lateral drawing at draw ratio of 6 times at 115° C. in an oven, and then biaxially oriented film  14  cooled down to 30° C. is washed in a washing bath of methylene chloride kept at 25° C. to remove liquid paraffin. The washed film is dried up in a dry kiln kept at 60° C. and then is drawn again in redrawing process  7  at areal ratio of 1.2 times in lengthwise and lateral directions. The heat treatment is performed for 20 seconds by 88.2 m/min at 125° C. to prepare microporous plastic film  11  having thickness of 16 μm and width of 2,000 mm. 
     Comparative Example 1 
     A microporous plastic film is produced by the same devices and conditions as Example 1, except that nip roller is not driven. 
     Slip on Drawing Roller 
     The speeds of sheet and roller are measured by 1% accuracy including installation accuracy with a non-contact Doppler velocimeter (made by ACT Electronics Corp., model 1522). For all Examples and Comparative Examples, slip of film  11  before drawing is evaluated according to the following standards:
         No good: The speed difference between roller and sheet is 10% or more of roller rotation speed.   Acceptable: The speed difference between roller and sheet is 5% or more and less than 10% of roller rotation speed.   Good: The speed difference between roller and sheet is less than 5% of roller rotation speed.       

     Meandering Amount in Lengthwise Drawing Process 
     The amount of meandering in lengthwise drawing process  4  is evaluated according to the following standards: 
     No good: Meandering amount is 10 mm or more. 
     Acceptable: Meandering amount is 5 mm or more and less than 10 mm. 
     Good: Meandering amount is less than 5 mm. 
     Physical Properties and Mechanical Properties of Microporous Plastic Film 
     The Gurley impermeability is determined according to JIS P8117 with Oken type Impermeability Tester (made by Asahi Seiko Co., Ltd., EGO-1T). 
     The thrust strength is determined from the maximum load applied when a needle having 1 mm diameter and spherical tip (Radius of curvature R=0.5 mm) is pricked at 2 mm/sec into a microporous membrane having membrane thickness T 1 . The measured maximum load La is converted to Lb which would be the maximum load when membrane thickness is 16 μm according to the following formula: Lb=(La×16)/T 1  to calculate the thrust strength [N/16 μm].
         Good: The Gurley impermeability is 250 sec±20 sec while the thrust strength is 6N or more.   No good: Outside the range for the above-described Good.       

     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Conveyance 
                 Physical properties, mechanical properties 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Slip on 
                   
                 Thrust 
                 Gurley 
                 Comprehensive evaluation 
               
               
                   
                 drawing roller 
                 Meandering 
                 strength [N] 
                 impermeability [sec] 
                 of physical properties 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 Good 
                 Good 
                 6 
                 150 
                 Good 
               
               
                 Comparative Example 1 
                 No good 
                 No good 
                 — 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     By comparing Example 1 and Comparative Example 1, drawing can be performed stably to suppress slip and meandering in Example 1 because the nip roller provided on each drawing roller and the drawing roller. In contrast, the slip and meandering cannot be prevented in Comparative Example 1 because the nip roller is not driven. Meandering is too great to continue the film forming and therefore a microporous plastic film cannot be sampled. 
     As described above, we make it possible to provide a microporous plastic film excellent in strength and physical properties as maintaining a travelling stability in a drawing condition where the drawing is performed to give the microporous film desirable properties. 
     INDUSTRIAL APPLICATIONS 
     Our microporous plastic film is applicable, but not limited in particular, to a separator used for electrochemical reaction device such as rechargeable battery, fuel cell and capacitor as well as a functional web such as filtration membrane, print film and clothing material.