Patent Publication Number: US-2018036930-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. 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 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 polymer 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 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 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 drawing between a roller (B) at the most downstream side in the sheet conveying direction among the rollers and a roller (A) which is adjacently upstream to the roller (B) in the sheet conveying direction and is driven by a motor is performed by a draw ratio greatest among the drawing in the sheet conveying direction, and a ratio of travelling speed of the clip of the tenter at the most upstream side in the sheet conveying direction to a peripheral speed of the roller (B) is −3% to +3%. 
     It is preferable that the drawing is performed by a total draw ratio of 4 to 12 times in the sheet conveying direction, a peripheral speed ratio of the roller (B) to the roller (A) is 2 to 6 times, and a quotient of the peripheral speed ratio of the roller (B) to the roller (A) divided by the total draw ratio in the sheet conveying direction is 0.3 or more. 
     Our method 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 DRAWING 
         FIG. 1  is a schematic side view showing an example of 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 
               4 : lengthwise drawing process 
               41 : heating roller group 
               42 : drawing roller group 
               421 : first drawing roller 
               422 : second drawing roller (drawing roller A) 
               423 : third drawing roller (drawing roller B) 
               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  FIG. 1 .  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 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 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 between those of 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 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 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 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 . That 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, that 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. because a too high speed might cause 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 cooling drums  31  and  32  are provided with a conventional heat pump or cooling device in addition to an internal flow path for flowing 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 FIGURE, 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 to 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. 
     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 heated and kept warm in an oven. 
     For example, gelled sheet  12  may be drawn lengthwise with two drawing rollers having a speed difference therebetween. When three drawing rollers are provided in lengthwise drawing process  42  as shown in  FIG. 1 , the drawing may be performed between drawing rollers  421  and  422 , between drawing rollers  422  and  423 , or both therebetween. One step of drawing is performed by either one of the two drawings while two steps of drawings are performed by both of them. 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 that 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 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 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 that substantively draw it as generating 3% or more of peripheral speed difference 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 drawing tension is required to achieve a necessary draw ratio. The diluent having bled out as described above intervenes 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 first drawing roller  421  and second drawing roller  422 , between second drawing roller  422  and third drawing roller  423 , or both therebetween 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 . When the drawing is performed only between first drawing roller  421  and second drawing roller, second drawing roller  422  is regarded as drawing roller (B) at the most downstream side in conveying direction for the drawing while first drawing roller  421  is regarded as drawing roller (A) adjacent to drawing roller (B) at the upstream side in conveying direction. The lengthwise drawing is performed only between drawing roller (A) and drawing roller (B) and, therefore, the draw ratio is regarded as greatest in the sheet conveying direction. When a ratio ([travelling speed of clip]−[peripheral speed of drawing roller (B)]/[peripheral speed of drawing roller (B)]) of travelling speed of clip at the most upstream side in conveying direction of lateral drawing process  5  to the peripheral speed of drawing roller (B) which is second drawing roller  422  is −3% to +3%, the downstream drawing tension is balanced to the clip of lateral drawing process to prevent gelled sheet  12  from slipping in the lateral drawing process. It is preferable that a ratio of tenter clip speed at the most upstream side in conveying direction to the peripheral speed of drawing roller  423  is small. 
     It is more preferable that two steps of drawing are performed both between first drawing roller  421  and second drawing roller  422  and between second drawing roller  422  and third drawing roller  423  as shown in  FIG. 1 . With such a configuration, the drawing tension can be dispersed and the slip can be prevented. Drawing should be performed by the greatest draw ratio between drawing rollers (A) and (B) wherein third drawing roller  423  is regarded as drawing roller (B) at the most downstream side while second drawing roller  422  is regarded as drawing roller (A) adjacent to drawing roller (B) at the upstream side in conveying direction. With such a configuration, primary drawing tension in the process is generated between second drawing roller  422  and third drawing roller  423  so that the downstream drawing tension can be balanced to the tenter clip. On the other hand, when the drawing is also performed between first drawing roller  421  and second drawing roller  422 , the above-described balance of primary upstream drawing tension can increase the friction force generated by the first step drawing tension between gelled sheet  12  and first drawing roller  421  or second drawing roller  422  to prevent slipping. It is more preferable that nip roller  44  is provided on a roller upstream from second drawing roller  422  as shown in  FIG. 1  so that the upstream drawing tension can be balanced and slip can be prevented. 
     Although cooling roller group  43  may partially bear the drawing tension, it may not have a gripping force required to prevent the slip. Therefore, it is preferable that the speed is equaled to the speeds of lateral drawing process  5 , second drawing roller  422  and uniaxially oriented sheet  13 . When drawing is performed only between first drawing roller  421  and second drawing roller  422  in  FIG. 1 , it is preferable that all the speeds from third drawing roller  423  to cooling roller group  43  are the same as speed of second drawing roller  422  and the tenter clip at the most upstream side in conveying direction. 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 rollers after second drawing roller  422  are also a cooling roller group, it can be cooled at the same time of finishing the drawing to prevent unnecessary size distortion and tension change. 
     Further, because two steps of drawing are performed both between first drawing roller  421  and second drawing roller  422  and between second drawing roller  422  and third drawing roller  423  as shown in  FIG. 1 , it is preferable that the speeds of all rollers from drawing roller  423  as drawing roller (B) to cooling roller group  43  are the same as the tenter clip at the most upstream side in conveying direction. It is also preferable that they are cooled. It is further preferable that the three or four steps of drawing are performed to disperse and reduce primary drawing tension so that the upstream gripping force can be improved. However even in this case, drawing should be performed by the greatest draw ratio between drawing roller (B) at the most downstream side in conveying direction and drawing roller (A) adjacent to drawing roller (B) at the upstream side in conveying direction. 
     With such a configuration, uniaxially oriented sheet  13  travelling in lateral drawing process  5  after drawing roller (B) has almost the same tension as the drawing tension generated between drawing rollers (A) and (B). In  FIG. 1 , drawing tension generated between second drawing roller  422  and third drawing roller  423  as drawing roller (B) is maintained from third drawing roller  423  to the tenter clip at the same speed and then is received by the tenter clip without slip. 
     The ratio of travelling speed of clip at the most upstream side in conveying direction to the peripheral speed of drawing roller (B) is set to −3% to +3% because the drawing tension is not very different in such a range. Also, the speeds of sheet and roller are equaled to each other, preferably by setting the speed ratio to a minus value in 0 to −3%, while gelled sheet  12  shrinks in conveying direction when the sheet is cooled so that the drawing tension and the tension downstream the lengthwise drawing process are equaled to make it balanced easily. It is preferable that a ratio of tenter clip speed at the most upstream side in conveying direction to the peripheral speed of drawing roller  423  is small. It is preferable that a tension meter is provided at a part from drawing roller group  42  to lateral drawing process  5  to obtain measured values so that the speed ratio can be adjusted accurately to balance the tension. 
     It is preferable that the lengthwise draw ratio is 4 to 12 times in terms of total draw ratio. In performing multi steps of drawing as described above, the greatest speed ratio of drawing roller (A) and drawing roller (B) should be 2 to 6 times while quotient of the ratio of drawing roller (A) and drawing roller (B) divided by the total draw ratio should be 0.3 or more. The total draw ratio of 4 or more can disperse uneven thickness of gelled sheet  12  and sufficiently achieve mechanical properties such as strength and elastic modulus of sheet obtained by drawing. The total draw ratio of 12 or less can make it hard to generate breakage, excessive drawing tension and slip in the drawing process. 
     For example, the total draw ratio of 10 makes 3 (=10×0.3) or more of the ratio of drawing roller (A) and drawing roller (B). Because the ratio of drawing roller (A) and drawing roller (B) is the greatest in two steps of drawing, it is possible that the first step of drawing is performed by 2.85 times of the ratio while the second step of drawing (which means drawing between drawing roller (A) and drawing roller (B)) is performed by 3.5 times of the ratio. The first and second ratios may be adjusted to satisfy the above-described condition. For example, it is possible that the first step ratio of 2.5 times makes the second step ratio 4 times. Any ratio between drawing roller (A) and drawing roller (B) should be 2 to 6 times. The ratio of 6 or more might not prevent slip in one drawing section so that the clip can grip the downstream side but there might be slipping at the upstream side. In contrast, although the lower limit of total draw ratio of 4 between drawing roller (A) and drawing roller (B) multiplied by 0.3 equals 1.2, the greatest draw ratio requires 2 or more times in the same section. 
     When the lengthwise draw ratio is 10 times, it is preferable that more than two steps of drawing are performed although such a configuration is not shown in  FIG. 1 . For example, three steps of drawing in the drawing section will be considered. It is preferable that the third step of draw ratio in a section of drawing roller (A) and drawing roller (B) is 2 to 6 times, preferably 3 or more times because of 10×0.3 or more. For example, when the third step ratio is 3 times, the product of ratios of the upstream first and second steps are 3.33 (=10/3) and then each should be 1.83 (=3.33̂0.5). Therefore the ratio is 10 times (=1.83×1.83×3). Alternatively, it is possible that the ratio is 10 times (=1.67×2×3) as increasing the ratio from upstream side to downstream side. 
     To balance the upstream tension, it is preferable that nip roller  44  is provided from the heating roller group to drawing roller (A). It is preferable that the nip pressure is 300 to 2,000 [N/m] per unit width without meandering by excessively pressing with a pair of rollers conventionally, although the nip pressure may be adjusted and changed for each driving roller naturally. As shown in  FIG. 1 , cooling roller group  43  may comprise nip rollers. 
     It is preferable that the surface of 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 the lengthwise drawing process 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 prevent air bank from generating when introducing gelled sheet  12  into heating roller or drawing roller as shown in  FIG. 1  so that slip and meandering can be prevented as improving uniformity of thickness and appearance quality. 
     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 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 to twin-screw extruder  21  at flow rate of 291 kg/hr to be blended with twin-screw extruder 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 cooling drum  31  adjusted to 35° C. by waterflow to form gelled sheet  12 . 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 of first drawing roller  421 . Between heating roller group  41  and first drawing roller  421 , 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 3 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 of first cooling drum  31  is set to 10 m/min, speed ratio from heating roller group  41  to first drawing roller  421  is set to 1%, the speed of first drawing roller  421  is set to 10.4 m/min, and the speed of second drawing roller  422  is set to 10.4×1.23=12.8 m/min. The speed is controlled to set to 9 the total draw ratio of uniaxially oriented sheet  13  passing lengthwise drawing process  4 , wherein the drawing is performed by the maximum draw ratio between second drawing roller  422  as drawing roller (A) and third drawing roller  423  as drawing roller (B) while the speed of third drawing roller  423  is 12.8×7=90 m/min. 
     Oriented film  13  is cooled on four rollers of cooling roller group  43  including third drawing roller  423  as the last roller of drawing roller group  42  to adjust the waterflow roller temperature to make the sheet temperature 50° C. The most upstream clip speed in conveying direction of lateral drawing process  5  is set to −2% so that third drawing roller  423  as last drawing roller (B) and cooling roller group  43  have the same speed of 88.2 m/min as uniaxially oriented sheet  13  to shrink. 
     From first cooling drum  31 , all rollers in lengthwise drawing process  4  are made of steel coated with hard chromic plating of which surface roughness is 0.4 μm (0.4 S) at the maximum height. 
     As shown in  FIG. 1 , six upstream nip rollers  44  are facing from heating roller group  41  to second drawing roller  422  while four downstream nip rollers are facing four cooling roller group including third drawing roller  423 . 
     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. 
     Example 2 
     In a configuration shown in  FIG. 1 , the speed of first cooling drum  31  is set to 10 m/min, speed ratio from heating roller group  41  to first drawing roller  421  is set to 1%, the speed of first drawing roller  421  is set to 10.4 m/min, and the speed of second drawing roller  422  is set to 10.4×2.163=22.5 m/min. The speed is controlled to set to 9 the total draw ratio of uniaxially oriented sheet  13  passing lengthwise drawing process  4  so that the ratio of draw ratio between drawing roller (A) and drawing roller (B) to the total lengthwise draw ratio is 0.44 (&gt;0.3), wherein the drawing is performed by the maximum draw ratio between second drawing roller  422  as drawing roller (A) and third drawing roller  423  as drawing roller (B) while the speed of third drawing roller  423  is 22.5×4=90 m/min by the ratio is 2 to 6 times. The other conditions are the same as Example 1. 
     Example 3 
     In a configuration shown in  FIG. 1 , the travelling speed ratio of the most upstream clip in conveying direction of lateral drawing process relative to third drawing roller  423  is set to +3% so that the speed after lateral drawing process  5  is 92.7 m/min. The other conditions are the same as Example 2. 
     Example 4 
     In a configuration shown in  FIG. 1 , the travelling speed ratio of the most upstream clip in conveying direction of lateral drawing process relative to third drawing roller  423  is set to −3% so that the speed after lateral drawing process  5  is 87.3 m/min. The other conditions are the same as Example 2. 
     Comparative Example 1 
     In a configuration shown in  FIG. 1 , the travelling speed ratio of the most upstream clip in conveying direction of lateral drawing process relative to third drawing roller  423  is set to +5% so that the speed after lateral drawing process  5  is 94.5 m/min. The other conditions are the same as Example 1. 
     Comparative Example 2 
     In a configuration shown in  FIG. 1 , 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 first drawing roller  421  is set to 1%, the speed of first drawing roller  421  is set to 10.4 m/min, the speed of second drawing roller  422  is set to 10.4×4=41.6 m/min, and the speed of third drawing roller  423  is set to 41.6×2.163=90 m/min. The other conditions are the same as Example 1. 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: 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 
               
             
            
               
                   
               
               
                   
                   
                   
                 Quotient 
                 Ratio of 
               
               
                   
                   
                   
                 of speed 
                 tenter clip 
               
               
                   
                   
                   
                 ratio of  
                 speed most 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 drawing  
                 upstream in  
               
            
           
           
               
               
               
               
               
            
               
                   
                 Sectional draw ratio 
                   
                 roller 423  
                 conveying 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Between 
                 Between 
                   
                 to drawing  
                 direction to 
               
               
                   
                 drawing 
                 drawing 
                   
                 roller 422 
                 peripheral 
               
               
                   
                 roller 421 and 
                 roller 422 and 
                 Total 
                 divided by  
                 speed of 
               
               
                   
                 drawing roller  
                 drawing roller 
                 Lengthwise 
                 total draw 
                 drawing  
               
               
                   
                 422 
                 423 
                 draw ratio 
                 ratio 
                 roller 423 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 1.23  
                 times 
                 7  
                 times 
                 9 times 
                 0.778 
                 −2% 
               
               
                 Example 2 
                 2.163 
                 times 
                 4  
                 times 
                 9 times 
                 0.44 
                 −2% 
               
               
                 Example 3 
                 2.163  
                 times 
                 4  
                 times 
                 9 times 
                 0.44 
                 +3% 
               
               
                 Example 4 
                 2.163  
                 times 
                 4  
                 times 
                 9 times 
                 0.44 
                 −3% 
               
               
                 Comparative 
                 1.23  
                 times 
                 7  
                 times 
                 9 times 
                 0.778 
                 +5% 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 4  
                 times 
                 2.163  
                 times 
                 9 times 
                 0.24 
                 −2% 
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 2 
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                   
                   
                 Physical properties, mechanical properties 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Comprehensive 
               
               
                   
                 Conveyance 
                   
                 Gurley 
                 evaluation of 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Slip on 
                   
                 Thrust strength 
                 impermeability 
                 physical 
               
               
                   
                 drawing roller 
                 Meandering 
                 [N] 
                 [sec] 
                 properties 
               
               
                   
               
               
                 Example 1 
                 Acceptable 
                 Acceptable 
                 7 
                 250 
                 Good 
               
               
                 Example 2 
                 Good 
                 Good 
                 7 
                 250 
                 Good 
               
               
                 Example 3 
                 Good 
                 Good 
                 7 
                 250 
                 Good 
               
               
                 Example 4 
                 Good 
                 Good 
                 7 
                 250 
                 Good 
               
               
                 Comparative 
                 No good 
                 No good 
                 4 
                 350 
                 No good 
               
               
                 Example 1 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 No good 
                 No good 
                 — 
                 — 
                 — 
               
               
                 Example 2 
               
               
                   
               
            
           
         
       
     
     By comparing Examples and Comparative Examples, we found that in all Examples the drawing is performed by the greatest draw ratio between second drawing roller  422  and third drawing roller  423 . Thus, primary drawing tension is generated in the section while the speeds of second drawing roller  422  and third drawing roller  423  are controlled to substantively the same speed as the tenter clip speed. As a result, tenter clip receives downstream drawing tension to continuously provide microporous plastic film  11  stably with no slip and no meandering. Particularly in Examples 2-4, the speed ratio is adjusted in a range of 2 to 6 times in the drawing section to stably prevent slip caused by upstream drawing tension. 
     In contrast, because of the speed ratio of tenter clip to third drawing roller  423  in Comparative Example 1, a tension greater than the drawing tension is generated between lengthwise drawing process  4  and lateral drawing process  5 . As a result, microporous plastic film  11  having desirable physical properties and mechanical properties cannot be provided with no slip and no meandering. 
     Even in Comparative Example 2, drawing cannot be performed by the greatest draw ratio in the most downstream section of drawing roller. Therefore, slip is generated near second drawing roller  422  before the clip receives the drawing tension and both slip and meandering cannot be prevented. As a result, a microporous plastic film cannot be sampled because of particularly great meandering obstructing the continuation of film forming. Thus the strength and permeability cannot be measured. 
     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.