Patent Publication Number: US-2018036933-A1

Title: Method of producing microporous plastic film

Description:
TECHNICAL FIELD 
     This disclosure relates to a method for producing a microporous plastic film. 
     BACKGROUND 
     Microporous plastic films have been broadly used as a substance separation membrane, permselective membrane, separator material of electrochemical element such as alkali rechargeable battery, lithium rechargeable battery, fuel cell and capacitor or the like. A separator for 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. In the wet process, a diluent such as liquid paraffin is added to a resin, and is 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 method 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 property because polyolefin molecules can be oriented with strong orientation in the drawing direction. As disclosed in JP 2009-249480-A, 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 resin and is heated again below the melting point as disclosed in JP 2013-530261-A where it is heated above the crystallization dispersion temperature. 
     The wet roller method disclosed in JP 2009-249480-A 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 2009-249480-A, 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 2013-530261-A, 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 even such measures couldn&#39;t fully prevent the slip when we had an increased draw speed, a decreased draw temperature or an increased draw ratio in order 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 
     Our method of producing a microporous plastic film comprises: 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 a drum; 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, wherein the method further comprises pressing the sheet interposing between a nip roller having a surface coated with a rubber and a driving roller driven by a motor upstream from a roller at the most downstream side among the rollers, wherein number N of pairs of the driving roller and the nip roller satisfies the following formula: 
         N&gt;T /(μ· P ), where
 
     T [N]: drawing tension required for drawing sheet in sheet conveying direction, 
     μ: coefficient of friction between driving roller sheet, and 
     P [N]: nip pressure between driving roller and nip roller. 
     Another of our methods of producing a microporous plastic film comprises: 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 a drum; 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, wherein the method further comprises pressing the sheet interposing between a nip roller having a surface coated with a rubber and a driving roller driven by a motor upstream from a roller at the most downstream side among the rollers, wherein 5 or more pairs of the driving roller and the nip roller are provided. 
     It is preferable that the driving roller has a surface made of a metal or a ceramic and has a surface roughness less than 1 S. 
     “Nip” means to press a sheet as being interposed with rollers. “Nip roller” means one of the two rollers used to press the sheet interposing therebetween, the one being pressed onto the other one facing to the one by moving to press the sheet interposing therebetween. 
     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 showing a production process of our microporous plastic film. 
         FIG. 2  is a schematic side view showing an example of lengthwise drawing process in producing our microporous plastic film. 
         FIG. 3  is a schematic side view showing an example of cooling process of gelled sheet in producing our microporous plastic film. 
         FIG. 4  is a schematic side view showing an example of lengthwise drawing process in producing a conventional 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 : upstream drawing roller 
           422 : downstream 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 
       
    
     DETAILED DESCRIPTION 
     Hereinafter, desirable examples of our microporous plastic film will be explained with reference to figures. 
       FIG. 1  is a schematic side view showing a production process of our microporous plastic film. 
     To desirably produce microporous plastic film  11 , polyolefin resin is mixed with diluent and heated to melt and prepare a polyolefin solution. The diluent creates 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 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 view point 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 has a predetermined temperature of 140 to 250° C. for a polyethylene composition and 190 to 270° C. for a polypropylene-containing composition, as depending on resin kind. A thermometer is provided inside the extruder or in a cylinder section to indirectly monitor temperature 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 which 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 first cooling drum  31 . Gelled sheet  12  becomes a pillar part with a crystal structure made of the polyolefin supporting pores of macroporous 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 a wet surface made by the diluent is conveyed on first 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 first cooling drum  31  has a temperature of 15 to 40° C., which may affect on the crystal structure of gelled sheet  12 . It 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 first cooling drum  31  is enlarged, second cooling drum  32  is added to first 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., first cooling drum  31  may be around 20° C. because too high a speed might cause insufficient heat conduction. It is preferable that humidity is lowered by air conditioning when the temperature is below 25° C. First cooling drum  31  may have a shape of a roller or belt. It is preferable that a surface of first cooling drum  31  is made of metal, ceramic or fiber composite material, which is excellent in shape stability and working accuracy to keep the roller speed constant. It is particularly preferable that the surface is made of a metal which is 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 is not swelled by the bled-out diluent making wet state on the surface. 
     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 a 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 a 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 to 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 such as when the conveying speed is high or the gelled sheet is too thick to conduct heat sufficiently to first cooling drum  31 . Also, the opposite side can be improved in cooling ability by providing refrigerant-flowing nip roller  33  in which refrigerant flows inside opposite to cooling drum  31  as shown in  FIG. 3 . 
     To prevent wet gelled sheet  12  from meandering or degrading cooling efficiency by lubrication, it is possible that the sheet is pushed onto first 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. 
     Other than cooling drum  31 , it is preferable that gelled sheet  12  is depressed with a nip roller onto second cooling drum  32  or other conveying roller to increase coefficient of friction which has been decreased on the mirror surface. It is preferable that the surface of nip roller is made of rubber-like elastic material which 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. 
     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 uniaxially oriented 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 being heated and kept warm in an oven. In  FIG. 1 , drawing may be performed between upstream drawing roller  421  and downstream drawing roller  422  in lengthwise drawing process  4 . Such a drawing process can achieve 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 to supply heat medium without obstructing rotation of the roller to supply heat medium to the inside. 
     It is preferable that 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 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 the melting point of polyolefin resin or less. It is preferable that the temperature is [crystal dispersion temperature Ted 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 the 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 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 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 up 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 an amount of bleeding out is great at a part from 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 relative to the upstream roller. 
     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 thawing 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. 
     When the drawing is performed between upstream drawing roller  421  and downstream drawing roller  422  in lengthwise drawing process  4 , the drawing tension at the downstream side is balanced by receiving with the clip which grips the end of uniaxially oriented sheet  13  mainly in lateral drawing process  5 . Although cooling roller group  43  may bear the drawing tension, it may not have a gripping force required to prevent the slip. Therefor; it is preferable that the roller group doesn&#39;t receive the tension to equal the speeds of lateral drawing process  5 , downstream drawing roller  422  and uniaxially oriented sheet  13 . It is preferable that the drawing roller such as drawing roller  422  in  FIG. 1  at the most downstream side is a cooling roller. Before the oriented sheet is fed to lateral drawing process  5 , it is possible that the sheet is once cooled and conveyed to a tenter oven to make the process paper feed of uniaxially oriented sheet  13  easy while the crystal structure formed by the lengthwise drawing is solidified in lateral drawing so that a highly oriented and highly strengthened microporous plastic film can be prepared. When drawing roller  422  is also a cooling roller, it can be cooled at the same time of finishing the drawing to prevent unnecessary size distortion and tension change. Cooling rollers after drawing roller  422  should have almost the same speed as tenter clip so that the tenter clip primarily receive the downstream drawing tension to prevent slip on the cooling roller. 
     On the other hand, the upstream drawing tension should be received by heating roller group  41  as well as upstream drawing roller  421  so that the total force thereof balances to the drawing tension. The upstream drawing tension should be balanced to these roller gripping forces. Otherwise, because there is no speed regulation section which is nonslip like tenter clip provided at the downstream side, the tension might be transmitted to molten gel sheet  12  between die lip  23  and first cooling drum  31  to cause problems such as varying thickness. 
     Accordingly, we obtain a gripping force balancing to the upstream drawing tension by pressing gelled sheet  12  interposing between a nip roller having a surface coated with rubber and a driving roller driven by a motor upstream from drawing roller  422  as the most downstream roller among drawing roller group  42  in  FIG. 1 . 
     According to Coulomb&#39;s law, the nip pressure of nip roller generates friction force of each driving roller calculated by product μi·Pi, where μi is a coefficient of friction between each driving roller and sheet and Pi [N] is a nip pressure of nip roller provided on each driving roller. Not to transmit the tension to gelled sheet  12  discharged from the die lip, it is possible that the total of gripping force generated by the friction force is more than drawing tension T [N]. Because the friction force is generated only as a reactive force against the tension, gelled sheet  12  cannot be drawn upstream even with a force greater than the tension. This fact makes the following formula: 
       μ1· P 1+μ2· P 2+ . . . +μ N·PN=Σμi·Pi&gt;T  
 
     wherein i indicates a numbering ordinal to identify each driving roller like i=1 to N, wherein N pieces of driving rollers are necessary for the gripping. When the above-described inequality is satisfied, reactive force against the friction can be balanced to the drawing tension at the upstream side while reactive force against the friction of tenter clip can be balanced at the downstream side so that lengthwise drawing process  4  has no slip and no trouble such as meandering and insufficient drawing. 
     It is possible that drawing tension T is calculated from a sheet cross-section area sampled from a stress-distortion diagram made by a tension tester capable of displacing and measuring load after heating gelled sheet  12  which has been formed and cooled to a supposed drawing temperature in advance. The tension meter can be provided at any part from drawing roller group  42  to lateral drawing process  5 . 
     Although nip pressure Pi can naturally be adjusted for each driving roller, it is conventionally known that the nip pressure is preferably 300 to 2,000 [N/m] per unit width to prevent meandering caused by excessively pressing with a pair of rollers. To calculate the number of nip rollers required for the gripping, the pressure can be assumed as constant at averaged value of pressure P. The above-described formula can be changed as follows: 
       μ1· P 1+μ2· P 2+ . . . +μ N·PN=Σμi·Pi=P·Σμi&gt;T.  
 
     Of course, coefficient of friction μi between each driving roller and gelled sheet  12  can have different values for each sheet and driving rollers. However, the measurement cannot be performed as conveying the sheet. So, gelled sheet  12  is sampled before drawing after forming and cooling in advance so that the measurement can be performed between gelled sheet  12  and a driving roller, plate or the like made of the same material as driving roller. The coefficient of friction may be measured by a conventional method disclosed by formula 2 and FIG. 5 of WO 2012/133097 pamphlet. The measurement of coefficient of friction is performed by hanging 2 kg weight on a sheet which has been cut into 100 mm width and wound by contact angle of 90° on the lengthwise drawing roller (of which surface is coated with hard chrome plating at 0.4 μm at the maximum height) not in operation at normal temperature of 25° C. The roller under suspension is monitored to be prevented from rotating by the suspended motor resistance in the measurement. Assuming that obtained measured value μ=constant, the formula becomes: 
     
       
      
       P·Σμi=P·μ·N&gt;T.  
      
     
     Therefore, if number N of pairs of driving roller and nip roller, or the number of nip rollers is as follows, the gripping force at the upstream side to balance to the drawing tension can be ensured while lengthwise drawing process  4  under lubrication with diluent is prevented from having slip and meandering: 
         N&gt;T /(μ· P ).
 
     For example, when the measured value range of μ is represented by the lower limit, N should be greater to increase the safety grade against slip but facility cost increases. When the range of μ is represented by the upper limit, N can be smaller to reduce facility cost but the slip might be caused by slight change of μ or increase of tension. 
     We found that it is possible that a polyethylene-based microporous plastic film is formed at a drawing tension per unit width T of 1000 to 4000 N/m and a coefficient of friction p around 0.05 to 0.4. 
     From a viewpoint of facility cost saving, when nip pressure P is set to 2,000 N/m which is the maximum in the desirable pressure range and when the coefficient of friction p is set to 0.4 in a case of relatively high friction, number N of nip roller pairs required to grip relatively high drawing tension of 4,000 N/m should be more than five pairs according to: 
         N&gt; 4,000/(0.4×2,000)=5 pairs.
 
     It is more preferable that nip roller number N is normally 10 pairs or more, by assuming drawing tension T of 2,000 N/m, nip roller pressure of 1,000 N/m and μ of around 0.2. Because too many rollers may increase facility cost, it is preferable that N is 5 to 25 pairs, preferably 10 to 20 pairs. 
     With such a configuration, it is possible that the gripping force is exhibited by friction force from the heating roller to the upstream drawing roller as shown in  FIG. 1  to partially receive gripping force of drawing tension at the upstream side so that gelled sheet  12  which is in a melting state between die lip  23  and first cooling drum  31  is prevented from having uneven thickness derived from the drawing tension. 
     It is preferable that the surface of the nip roller is made of soft 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. Because lengthwise drawing process  4  accompanies conveyance above the heat dispersion temperature in particular, the material is preferably a heat-resistant rubber such as EPDM and Hypalon rubber. It is preferably silicone rubber or fluoro-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. 
     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 or drawing roller as shown in  FIG. 2  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 as shown in  FIG. 1 , certain thickness of diluent and air is accompanied between drawing roller group  42  or heating roller group  41  and gelled sheet  12  to make a bank by nipping with nip roller  44  afterwards. 
     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 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. 
     To prevent the surface of gelled sheet from being roughed by nipping when gripping force of nip roller is ensured, it is particularly preferable that the driving roller has a mirror surface of which maximum height is 1 S, or 1 μm or less, preferably 0.2 to 0.8 μm. To make such a mirror surface, the nip roller surface may be made of metal or ceramic as described above. 
     EXAMPLES 
     Hereinafter, our methods will be explained with reference to Examples although it is 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 to twin-screw extruder  21  at flow rate of 291 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 at the first roller of drawing roller group  42 . 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 rubber is provided onto each roller to perform lengthwise drawing by speed difference between rollers. 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 10 μm (10 S) 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. Therefore, it becomes T/(μ×P)=1500/(0.15×20,000)=5. Therefore as shown in  FIG. 1 , number N of nip roller  44  is set to 6 (&gt;5) at a part upstream from the drawing tension generating part between upstream drawing roller  421  and downstream drawing roller  422 . 
     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 90 m/min at 125° C. to prepare microporous plastic film  11  having thickness of 16 μm and width of 2,000 mm. 
     Example 2 
     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 (0.8 S) at the maximum height. The other conditions are the same as Example 1. 
     Comparative Example 1 
     As shown in  FIG. 4 , a nip roller is provided only on drawing roller group  42  to perform lengthwise drawing on drawing roller group  42 . The other conditions are the same as Example 1. 
     Comparative Example 2 
     As shown in  FIG. 4 , a nip roller is provided only on drawing roller group  42  to perform lengthwise drawing on drawing roller group  42 . It was said that other conditions were the same as Example 2. 
     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 
     Appearance Quality of Biaxially Oriented Sheet  14   
     The appearance quality of biaxially oriented sheet  14  is evaluated. To inspect appearance, LED light source having 150 lumen is employed. The upper surface of travelling biaxially oriented sheet  14  is inspected to evaluate the appearance quality. The LED light source is irradiated orthogonally to the sheet travelling direction from distance of 1 m to the film to evaluate it according to the following standards. 
     No good: Appearance of biaxially oriented sheet  14  travelling at the exit of lateral drawing process  5  is visually observed to distinguish light and dark. 
     Acceptable: Appearance of biaxially oriented sheet  14  exited from lateral drawing process  5  is cut and visually observed in a resting state to distinguish light and dark although light and dark cannot be distinguished on the travelling biaxially oriented sheet  14 . 
     Good: Light and dark on biaxially oriented sheet  14  cannot be distinguished even in a resting state. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Physical properties, mechanical properties 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Conveyance 
                   
                   
                 Comprehensive 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Slip on 
                   
                   
                 Thrust 
                 Gurley 
                 evaluation  
               
               
                   
                 drawing 
                   
                   
                 strength 
                 impermeability 
                 of physical 
               
               
                   
                 roller 
                 Meandering 
                 Appearance 
                 [N] 
                 [sec] 
                 properties 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Good 
                 Good 
                 Acceptable 
                 7 
                 250 
                 Good 
               
               
                 Example 2 
                 Good 
                 Good 
                 Good 
                 7 
                 250 
                 Good 
               
               
                 Comparative 
                   
                   
                   
                   
                   
                   
               
               
                 Example 1 
                 No  
                 Acceptable 
                 Acceptable 
                 5 
                 300 
                 No good 
               
               
                 Comparative 
                 good 
                   
                   
                   
                   
                   
               
               
                 Example 2 
                 No  
                 No good 
                 Good 
                 5 
                 300 
                 No good 
               
               
                   
                 good 
               
               
                   
               
            
           
         
       
     
     By comparing the Examples and Comparative Examples, we found that all Examples set the number of nip roller pairs to obtain friction force and gripping force beyond the drawing tension continuously provide microporous plastic film  11  stably with no slip and no meandering. On the other hand, all the Comparative Examples result in unacceptable slip and meandering. Accordingly, the Comparative Examples don&#39;t achieve target physical properties and mechanical properties for microporous plastic film. 
     Compared to Example 2, appearance quality is relatively poor in Example 1 because lubrication amount of diluent is less and dirt tends to deposit on irregularity of rough surface. Nevertheless travelling ability is stable as described above. 
     Compared to Comparative Example 2, the lubrication amount of diluent is less and meandering is less in Comparative Example 1. However, the gripping force in travelling direction is not enough to prevent meandering completely. 
     As described above, our methods 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.