Patent Publication Number: US-2018036932-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 surface. 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. 
     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 one or plurality of cooling drums; reheating and drawing the solidified sheet with a plurality of rollers in a sheet conveying direction; cooling the sheet drawn in the sheet conveying direction; gripping both ends of the sheet with clips; introducing the sheet into a tenter; and washing the diluent out to prepare a uniaxially or biaxially oriented microporous plastic film, wherein the method further comprises: driving each of at least two of the rollers by a motor; bringing each surface of two rollers (A) and (B) among the at least two of the rollers each driven by the motor into contact with a surface of the sheet opposite to another surface contacting a first cooling drum defined as a cooling drum which first contacts the sheet discharged from the die lip among the cooling drums; and controlling each rotation speed of the rollers (A) and (B) to draw the sheet substantively between the rollers (A) and (B). 
     It is preferable that the sheet is nipped with a nip roller substantively along a tangent line to start contacting at least one of the drawing rollers (A) and (B). 
     It is also preferable that the roller (A) and the roller (B) are adjacent to each other. 
     It is also preferable that a plurality of pairs of roller (A) and roller (B) are provided in a lengthwise drawing process. 
     It is also preferable that a cooling roller is provided as a roller most downstream among one or more pairs of roller (A) and roller (B). 
     “Nip” means pressing 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 cooling process of a gelled sheet in producing our microporous plastic film. 
         FIG. 3  is a schematic side view showing an example of lengthwise drawing process in producing our microporous plastic film. 
         FIG. 4  is a schematic side view showing another example of lengthwise drawing process in producing our microporous plastic film. 
         FIG. 5  is a schematic side view showing yet another example of lengthwise drawing process in producing our microporous plastic film. 
         FIG. 6  is a schematic side view showing yet another example of lengthwise drawing process in producing our microporous plastic film. 
         FIG. 7  is a schematic side view showing an example of lengthwise drawing process in producing a conventional microporous plastic film. 
         FIG. 8  is a schematic side view showing an example of lengthwise drawing process in producing another 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 
               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 
             A: upstream drawing roller contacting surface of sheet opposite to surface contacting first cooling drum 
             B: downstream drawing roller contacting surface of sheet opposite to surface contacting second cooling drum 
             C: assistant roller of drawing section to convey sheet between rollers (A) and (B) 
           
         
       
    
     DETAILED DESCRIPTION 
     Hereinafter, desirable examples of our microporous plastic film will be explained with reference to the figures. 
       FIG. 1  is a schematic side view showing 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 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 of the same range as the compounds thereof; or a phthalate ester such as dibutyl phthalate and dioctylphthalate which are liquids at room temperature. To obtain a gelled sheet stably containing liquid diluent, it is preferable to employ a nonvolatile diluent such as liquid paraffin. For example, it is preferable that the liquid diluent has a viscosity of 20 to 200 cSt at 40° C. 
     It is preferable that the polyolefin resin of 10 to 50 mass % is mixed with the diluent in total 100 mass % of polyolefin resin and diluent, from a viewpoint of good formability of extruded product. The polyolefin resin solution may be melt-kneaded uniformly by a calendar, mixer or extruder  21  having a screw as shown in  FIG. 1 . 
     It is preferable that the polyolefin solution in extruder  21  has a predetermined temperature of 140 to 250° C. for polyethylene composition and 190 to 270° C. for polypropylene-containing composition, depending on resin kind. A thermometer is provided inside extruder  21  or in a cylinder section to monitor a temperature indirectly and properly adjust the heater temperature, rotation speed and discharge rate of the cylinder section to control the temperature in a target range. 
     The polyolefin 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 first 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 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 the crystal structure of gelled sheet  12 . This is because the final cooling temperature should be below the crystallization ending temperature. The molecular orientation tends to advance in a later drawing process when the higher-order structure is fine. To make up for cooling time, it is possible that the diameter of first cooling drum  31  is enlarged, another 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 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. 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 excellent in heat conduction to a film. It is possible to perform non-adhesion coating or rubber coating on it to the extent that conduction is not obstructed. It is preferable that the surface of sheet and roller are made of metal including metal plating which is excellent in scratch resistance and heat conductivity and not swelled by the bled-out diluent making wet state on the surface. 
     It is preferable that the roller has a surface roughness of 0.2 to 40 μm at the maximum height. To make a mirror surface, the surface roughness is preferably 0.2 to 0.8 To make a sufficiently rough surface, the surface roughness is preferably 20 to 40 Because the surface of the roller is wet with the diluent, the mirror surface has a low coefficient of friction caused by lubrication. The rough surface has an increased coefficient of friction caused by less or no lubrication because the diluent discharges from the unevenness. Although the mirror surface and the rough surface may be combined as needed, it is basically preferable that the mirror surface is provided from the viewpoints of maintenance such as cleaning and speed control precision, preferably with a certain lubricant with diluent to prevent the sheet from having uneven appearance. 
     It is preferable that first cooling drum  31  and second cooling drum  32  are provided with a conventional heat pump or cooling device in addition to an internal flow path for refrigerant to control the surface temperature. The roller is driven to rotate by a rotation driving means such as motor at a predetermined speed while a speed-changing mechanism may be provided between rollers to apply draw tension or relaxation according to expansion and shrinkage of the sheet. Alternatively, each roller may be provided with a motor to achieve function like the speed-changing mechanism by accurately adjusting the speed by inverter or servo. 
     In  FIG. 1 , the top side of gelled sheet  12  is discharged from die lip  23  and contacts first cooling drum  31  which first contacts it, and then is rapidly cooled with refrigerant at the above-described temperature. On the other hand, the side opposite the side contacting the above-described first cooling drum  31  is slowly cooled with air as shown in  FIG. 1 . Although it is not illustrated in the figures, it is preferable that the side opposite the side contacting first cooling drum  31  is cooled by forced circulation with air nozzle or air chamber to make a rapid cooling of the opposite side. Such a configuration would be suitable to such a case that the conveying speed is high or the gelled sheet is too thick to conduct heat sufficiently 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 first cooling drum  31  as shown in  FIG. 2 . 
     To prevent wet gelled sheet  12  from meandering or degrading the cooling efficiency by lubrication, it is possible that the sheet is pushed onto 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 first 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, while the 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. 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 first 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 as needed 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 first cooling drum  31  and second cooling drum  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. 
     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 the drawing part where a high tension is generated by drawing, a high gripping force balancing the drawing tension is required to achieve a necessary draw ratio. The diluent having bled out as described above interposes between the roller and gelled sheet  12  in lubrication state to cause a deterioration of the gripping force required for conveyance and drawing. 
     We found that there is a correlation between the cooling speed by first cooling drum  31  and surface state of gelled sheet  12  causing the deterioration of gripping force. As described above, the temperature of first cooling drum  31  greatly influences the crystal structure of gelled sheet  12 . When gelled sheet  12  in a fusion state is solidified by cooling on first cooling drum  31 , the faster the cooling speed is the more precise the crystal structure is, while the slower it is the more coarse the crystal structure is. We found that the lubricating diluent tends to make it slippery when the cooling speed is faster and the crystal structure is more precise. We also found that the amount of diluent bleeding out of a surface contacting first cooling drum  31  is greater at a part from cooling drum  31  to the heating section of lengthwise drawing process  4 . 
     Gelled sheet  12  is quenched on first cooling drum  31 . It is preferable that a surface of gelled sheet  12  opposite to the surface contacting first cooling drum  31  is cooled by air nozzles or refrigerant-flowing nip roller  33  as described above so that inner layer part of gelled sheet  12  can be crystalized as uniformly as possible in the thickness direction. First cooling drum  31  contacts gelled sheet  12  to directly transfer heat at a cooling efficiency higher than air nozzles and air chambers. The refrigerant-flowing nip roller as shown in  FIG. 2  has the same heat-transfer ability as first cooling drum  31 , but has a cooling efficiency less than first cooling drum  31  because of shorter contact length. It is preferable that the contact length is increased by providing a plurality of refrigerant-flowing nip rollers or a metal belt between nip rollers. The cooling speed naturally increases at the side of first cooling drum  31  where refrigerant flows densely in the circumferential direction. Thus, in a process producing a microporous plastic film, the surface contacting first cooling drum  31  has a cooling speed greater than the surface opposite to the surface contacting first cooling drum  31  and has a precise crystal structure of gelled sheet  12  while the amount of diluent bleeding out is great. Therefore, gelled sheet  12  has a slippery surface contacting first cooling drum  31  because of smaller gripping force and smaller frictional force to the roller. 
     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 relative to the upstream roller. 
     Rotation speeds of roller (A) and roller (B) are controlled to perform a drawing while surfaces of roller (A) and roller (B) contact the surface opposite to the surface contacting first cooling drum of gelled sheet  12  in lengthwise drawing process  4 , wherein roller (A)/(B) may be a roller at upstream/downstream side of drawing roller group  42  in  FIG. 1 , respectively. With such a configuration, gripping force required for conveyance and drawing can be obtained because gelled sheet  12  has a surface contacting drawing rollers (A) and (B) opposite the surface contacting first cooling drum  31 . Although upstream drawing tension is partially loaded on heating roller group  41 , the gripping force of roller (A) becomes greater to stabilize the drawing as keeping necessary draw ratio and preventing conveyance problems such as meandering. Even when the drawing tension is balanced downstream, the clip to grip the end section of uniaxially oriented sheet  13  greatly receives the load. However, the gripping force of roller (B) becomes greater to perform a nonslip drawing even downstream as preventing the problems such as meandering. 
     When roller (C) which contacts the surface contacting first cooling drum  31  is provided between roller (A) and roller (B) among rollers contacting the surface opposite to the surface contacting first cooling drum  31  of gelled sheet  122  in  FIG. 3 , drawing can be performed substantively with roller (A) and roller (B) while the rotation speed of roller (C) should be controlled so that roller (C) doesn&#39;t contribute to the drawing. Specifically, the rotation speed of roller (B) is set to 5 times of that of roller (A) when the drawing is performed at 5 times of draw ratio to roller (A). Gelled sheet  12  between roller (A) and roller (B) increases the speed as drawing in the travelling direction. Therefore, the speed of roller (C) may be set to a value in the middle of the speeds of roller (A) and roller (B) so that roller (C) doesn&#39;t contribute to the drawing. Since the drawing speed of gelled sheet  12  does not always increase linearly, it is preferable that the speed of roller (C) is adjusted not to vary from a previous torque before drawing by monitoring a torque of motor directly connected to roller (C). With such a configuration, roller (C) doesn&#39;t give any torque or drawing force to the sheet being oriented so that the drawing can be performed substantively with roller (A) and roller (B) without contribution of roller (C). The previous torque before drawing means torque when the roller is rotated with no sheet or that a sheet is conveyed without drawing. That is mainly derived from rotation resistance of a bearing of a motor or roller, or rotation resistance of a rotary joint. It is preferable that roller (C) is thus provided between roller (A) and roller (B) so that the neck-in can be reduced by frictional resistance of roller (C) in the width direction relative to the configuration shown in  FIG. 1 . Even when the speed of roller (C) is adjusted so that roller (C) doesn&#39;t contribute to the drawing, roller (C) may be forced to contribute to the drawing slightly because perfect adjustment is difficult. Even such a state can be regarded that “the drawing is performed substantively with roller (A) and roller (B)” because each roller rotates in a speed condition designated to perform the drawing substantively with roller (A) and roller (B) without contribution of roller (C). 
     It is preferable that gelled sheet  12  is pressed with a nip roller between heating roller group  41  or drawing roller group  42  as well in the cooling process so that the frictional force which has deteriorated by lubrication of diluent can greatly be increased. Slipping can be prevented even when the drawing generates tension because the surface opposite to the surface contacting first cooling drum  31  of which gripping force is higher contacts the drawing roller. Further, the gripping force can be increased by using a nip roller as shown in  FIGS. 1 and 3  to increase the frictional force to the drawing roller. Although not shown in the Figs, it is possible that a nip roller is provided appropriately at heating roller group  41  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 possible that a nip is performed along a tangent line substantively with a nip roller to introduce gelled sheet  12  into heating roller group  41  or drawing roller group  42  as shown in  FIG. 4, 5 or 6  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  FIGS. 1 and 3 , certain thickness of diluent and air is accompanied between drawing roller group  42  or heating roller group  41  and sheet  12  to make a bank by nipping with nip roller  44  afterwards. The nip roller which is provided substantively along the tangent line can suppress the thickness of diluent and air with nip roller  44  before contacting between drawing roller group  42  or heating roller group  41  and the sheet so that an oriented sheet having a higher quality can be obtained without forming a bank between gelled sheet  12  and the roller. Not to make gelled sheet  12  oriented greatly while contacting the drawing roller, it is preferable that two nip rollers are provided to drawing roller (A) to perform the nip along the tangent line even at the downstream side. 
     The phrase “nip along the tangent line” means a nip performed by the nip roller at a position where gelled sheet  12  starts contacting heating roller group  41  or drawing roller group  42 . The nip performed with the nip roller at this position makes gelled sheet  12  as if being a tangent line of nip roller. The phrase “a nip is performed along a tangent line substantively” means that a nip that doesn&#39;t make a bank is regarded as “a nip performed along a tangent line” even when the nipped part is slightly away from the exact position where the sheet starts contacting the roller. 
     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 that is little swelled by diluent to prevent the roller from having a distorted shape over time. 
     It is preferable that roller (C) which doesn&#39;t contribute to drawing is provided between roller (A) and roller (B) as shown in  FIG. 3  so that the neck-in can be reduced by a gripping force with the roller in the width direction while shortening the travelling distance of gelled sheet  12  in the air. When roller (A) is adjacent to roller (B) as shown in  FIG. 1 or 4 , the speed of roller (C) which doesn&#39;t contribute to drawing becomes not required to be controlled to simplify the device and reduce cost. 
     It is preferable that the speed is controlled by a plurality of pairs of roller (A) and roller (B) in the lengthwise drawing process. For example, drawing is performed as providing speed difference between roller (A) at the most upstream side and roller (B) at the next among drawing roller group  42  as shown in  FIG. 5 . Next, for the second stage of drawing, the speed is controlled by roller (A′) and roller (B′) while roller (A′) is the above-mentioned roller (B) and roller (B′) is a roller at the next, as shown in  FIG. 3 . For example, the draw ratio is decreased at the first stage and the draw ratio is decreased at the second stage so that physical properties required to draw films are exhibited at the ratio of the second stage to receive drawing tension by increasing the gripping force to the roller with the drawing tension at the first stage to improve the stability against slip caused by lubrication of diluent. The roller pairs may be made by combining the layouts of drawing roller group  42  shown in  FIGS. 1, 3 and 4 . It may be as shown in  FIG. 5 or 6 . 
     After the lengthwise drawing, it is possible that the sheet is once cooled with cooling roller group  43  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 a case of lateral drawing so that a highly oriented and highly strengthened microporous plastic film can be prepared. 
     It is particularly preferable that the cooling is performed right after the drawing. The highly-strengthening effect can be maximized by providing the most downstream roller as a cooling roller among pairs of roller (A) and roller (B) in the lengthwise drawing process. 
     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 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. Oriented film  13  is cooled on four rollers of cooling roller group  43  including the last roller 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  5  is 0 to make the speed same. As shown in  FIG. 1 , the nip roller provided on roller (A) is not substantively on the tangent line, but nip roller  44  is nipped on gelled sheet  12  at the position of which winding angle is 45° from starting contact between gelled sheet  12  and roller (A). Nip roller  44  provided on roller (B) is nipped on gelled sheet  12  along the tangent line. 
     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 at the maximum height. 
     Among drawing roller group  42 , drawing is performed at 8.66 of draw ratio between upstream roller (A) contacting the surface of gelled sheet  12  opposite to the surface contacting first cooling drum  31  and downstream roller (B) contacting the surface opposite to the surface contacting drum  31 . In other words, 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 cooling drum  31  is set to 10 m/min, speed ratio from heating roller group  41  to roller (A) is set to 1%, the speed of roller (A) is set to 10.4 m/min, and the speed of roller (B) is set to 10.4 x 8.66=90 m/min. 
     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 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. 
     The amount of diluent bleeding out from the top and bottom surfaces of gelled sheet  12  is measured after casting the film formed with the above-described composition and extrusion. The bleeding-out amount per unit time is measured by the following process, in which a sharp-tipped scraper is contacted to each top and bottom surfaces of gelled sheet  12  by pressing at 20 N/m at a part between second cooling drum  32  and lengthwise drawing process  4  in  FIG. 1  and the diluent on the surface is scraped off so that the weight of diluent dropped from the side surface of gelled sheet  12  is measured for 1 min. The scraper made of fluoro-resin having thickness of 5 mm is polished to sharpen the tip to contact gelled sheet  12  to achieve tan θ=5/10. Chamfering of 0.1 mm is performed at the front tip to prevent the chipping and scratch onto the sheet. The amount of bled-out diluent is measured by pressing gelled sheet  12  at the above-described pressure and 30° of angle from the parallel direction. Table 1 shows the results, in which the amount of bleeding out on the surface contacting first cooling drum  31  triples relative to the opposite surface. The diluent bleeds out of gelled sheet  12  at heating roller group  41  in lengthwise drawing process  4  even after scratch, in which the bleeding out on the surface contacting first cooling drum  31  is greater as well. 
     Table 1 also shows a measurement result of coefficient of friction between the surface of gelled sheet  12  and roller. In the process shown in  FIG. 1 , gelled sheet  12  is sampled by cutting gelled sheet  12  formed by casting at the downstream side of second cooling drum  32 . 
     The coefficient of friction is measured by a conventional method disclosed by formula 2 and FIG. 5 of WO2012/133097. The measurement 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. The coefficients of friction are compared between two samples, where the first sample is wiped with a waste cloth “KIM WIPE” made by Kimberly-Clark Corporation to make the diluent liquid invisible and the second sample is not wiped. 
     Table 1 shows the result in which the surface of gelled sheet  12  contacting the first cooling drum always has the lower coefficient of friction although there is some difference between before and after wiping with the waste cloth. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Surface opposite to contacting 
               
               
                   
                 Surface of sheet 12 
                 surface of sheet 12 with first 
               
               
                   
                 contacting first drum 31 
                 drum 31 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Bleed-out 
                 0.025 
                 0.08 
               
               
                 amount of 
               
               
                 diluent[g/m 2 ] 
               
            
           
           
               
               
               
               
               
            
               
                 Coefficient 
                 Before 
                 After 
                 Before 
                 After 
               
               
                 of friction 
                 wiping with 
                 wiping with 
                 wiping with 
                 wiping with 
               
               
                 μs 
                 KIM WIPE 
                 KIM WIPE 
                 KIM WIPE 
                 KIM WIPE 
               
               
                   
                 0.06 
                 0.17 
                 0.17 
                 0.37 
               
               
                   
               
            
           
         
       
     
     Example 2 
     A microporous plastic film is produced by the same devices and the same condition as Example 1, except that drawing roller group  42  consists of three rollers as shown in  FIG. 3  and the drawing is performed substantively with roller (A) and roller (B) as adjusting the speed to keep the motor rotation torque of drawing roller (C) within a torque range under no-load operation. 
     Example 3 
     A microporous plastic film is produced by the same devices and the same condition as Example 1, except that nip roller  44  is provided on each heating roller group  41  and drawing roller group  42  along a tangent line substantively as shown in  FIG. 4  and the drawing is performed between drawing roller (A) and roller (B). 
     Example 4 
     As shown in  FIG. 5 , drawing roller group  42  consists of three rollers, the first-stage drawing is performed between roller (A) and roller (B) and the second-stage drawing is performed with the second roller (B) as roller (A′) and the third roller as roller (B′). The first-stage draw ratio is 2 times and the second-stage draw ratio is 4.33 times so that the total draw ratio from the first to the third roller is 2×4.33=8.66 times the same as Example 1. The other conditions are the same as Example 1. 
     Comparative Example 1 
     The drawing is performed between roller (A) and roller (D) provided as drawing roller group  42  shown in  FIG. 7 . The other conditions are the same as Example 1. 
     Comparative Example 2 
     The drawing is performed between roller (A) and roller (D) among five rollers provided as drawing roller group  42  shown in  FIG. 8 . The other conditions are the same as Example 1. 
     Table 2 shows evaluation results of produced microporous plastic films according to the following standards. 
     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). 
     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. 
     Bank 
     The bank in lengthwise drawing process  4  is evaluated according to the following standards. 
     No good: Air bank is observed on a roller in lengthwise drawing process  4  while uneven thickness defined as difference between the thickest part and the thinnest part is more than 5% of average thickness. 
     Acceptable: Air bank is observed on a roller in lengthwise drawing process  4  while uneven thickness defined as difference between the thickest part and the thinnest part is 5% or less of average thickness. 
     Good: Air bank is not observed on a roller in lengthwise drawing process  4 . 
     Neck-in 
     Neck-in in lengthwise drawing process  4  is evaluated according to the following standards. 
     No good: The width difference between gelled sheet  12  entering lengthwise drawing process  4  and uniaxially oriented sheet  13  from lengthwise drawing process  4  to lateral drawing process  5  is more than 150 mm. 
     Acceptable: The width difference between gelled sheet  12  entering lengthwise drawing process  4  and uniaxially oriented sheet  13  from lengthwise drawing process  4  to lateral drawing process  5  is 100 mm or more and less than 150 mm. 
     Good: The width difference between gelled sheet  12  entering lengthwise drawing process  4  and uniaxially oriented sheet  13  from lengthwise drawing process  4  to lateral drawing process  5  is less than 100 mm. 
     Physical Properties and Mechanical Properties of Microporous Plastic Film 
     The 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 T1. The measured maximum load La is converted to Lb which would be the maximum load when membrane thickness is 16 μm according to formula: Lb=(La×16)/T1 to calculate the thrust strength [N/16 μm]. 
     Good: The 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 2 
               
             
            
               
                   
                   
               
               
                   
                 Conveyance 
                 Physical properties, mechanical properties 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Slip on 
                   
                   
                   
                 Thrust 
                   
                 Comprehensive 
               
               
                   
                 drawing 
                   
                   
                   
                 strength 
                 Impermeability 
                 evaluation of 
               
               
                   
                 roller 
                 Meandering 
                 Bank 
                 Neck-in 
                 [N] 
                 [sec] 
                 physical properties 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Acceptable 
                 Acceptable 
                 No good 
                 Acceptable 
                 7 
                 250 
                 Good 
               
               
                 Example 2 
                 Good 
                 Acceptable 
                 No good 
                 Good 
                 7 
                 250 
                 Good 
               
               
                 Example 3 
                 Acceptable 
                 Acceptable 
                 Good 
                 Acceptable 
                 7 
                 250 
                 Good 
               
               
                 Example 4 
                 Good 
                 Good 
                 Good 
                 Acceptable 
                 8 
                 230 
                 Good 
               
               
                 Comparative 
                 No good 
                 No good 
                 No good 
                 Good 
                 5 
                 300 
                 No good 
               
               
                 Example 1 
               
               
                 Comparative 
                 No good 
                 Acceptable 
                 No good 
                 Good 
                 6 
                 300 
                 No good 
               
               
                 Example 2 
               
               
                   
               
            
           
         
       
     
     By comparing Examples and Comparative Examples, we found that all Examples have good conveyance characteristics with almost no slip and no meandering since the drawing is performed substantively with the surface opposite to the surface contacting first cooling drum  31 . On the other hand, all Comparative Examples result in unacceptable slip and meandering. Accordingly, Comparative Examples don&#39;t achieve target physical properties and mechanical properties for microporous plastic film. 
     Also, compared to Examples 1 and 2, microporous films excellent in appearance and uniform thickness are produced without air bank because gelled sheet  12  is nipped substantively along a tangent line to start contacting the drawing roller in Examples 3 and 4. 
     The neck-in amount is suppressed with drawing roller (C) in Example 2 while the speed adjustment operation according to the torque of roller (C) is not necessary in Examples 1, 3 and 4 because a pair of roller (A) and roller (B) or another pair of roller (A′) and roller (B′) are adjacent. 
     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 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. These examples do not limit applications of our methods.