CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This invention claims priority to application Serial No. 61/093,903, filed September 3, 2008 and application Serial No. 61/103,462, filed October 7, 2008, which are hereby incorporated by reference. FIELD OF THE INVENTION [0002] The present invention relates to an apparatus and method for drying a thermoplastic sheet. More specifically, embodiments of the invention relate to apparatus and methods of drying a thermoplastic sheet wherein a gas is provided at an angle, β, formed by the direction of the stream and the line formed by the center of a roller and a midpoint of the region where the sheet contacts roller. BACKGROUND OF THE INVENTION [0003] General methods for producing thermoplastic sheets are well-known. In one such method, a thermoplastic composition is mixed with what is typically referred to as a process solvent or diluent. The solution or mixture of the thermoplastic composition and the diluent is then extruded through at least one die to form a gel-like sheet. [0004] Conventional tenter equipment as described, e.g., in U.S. Pat. No. 4,065,838, may then be used to stretch the film. Generally, tenter equipment includes a plurality of tenter clips mounted on two endless chains guided by a pair of diverging tracks to convey the tenter clips. The tenter clips are adapted to shift from an open position to a closed position to grip the edges of the web of material such as polymeric extrudate or film.

[0005] Following stretching, at least a portion of the diluent is removed from the thermoplastic sheet generally with a high volatility solvent, for example, by evaporation. When dry orientation is used, e.g. when the film is stretched in at least one planar direction after a part of the diluent has been removed, a portion of the washing solvent may evaporate while the film is gripped the tenter clips. The sheet can also be conveyed to a series of rollers where additional drying is accomplished. [0006] While such methods of forming thermoplastic sheet have been long known, conventional apparatus and methods can result in non-uniformity or shrinkage of the sheet during the drying process. In the production of thermoplastic microporous membranes, such shrinkage or nonuniformity can adversely affect porosity and permeability of the microporous membrane.

[0007] One source of such shrinkage is believed to be the wash solvent. A highly volatile wash solvent is generally used in the production of the thermoplastic microporous membranes. Consequently, it has been thought that the shrinkage force of the microporous membrane accompanied by volatilization of the wash solvent can be relatively large. Whatever its cause, this shrinkage force can cause the gripping action of tenter systems to damage the microporous membrane during dry orientation. Further, when the microporous membrane is dried with hot air while being carried by the multistage rollers, the microporous membrane shrinks in the transverse-direction as well as the machine direction, potentially resulting in deterioration of porosity and permeability of the microporous membrane.

[0008] To address shrinkage, Japanese Patent Application Laid-Open No. 2004- 106265, describes a method that includes casting a polymer-containing solution over a support, peeling the resulting film, and blowing air onto the film. A tenter adjusts the solvent content of the film to about 3 to 8 wt. % (based on the weight of the entire film). Drying is accomplished by blowing hot air onto the film while carrying the film with numerous multistage rolls. While drying in the multistage rolls, the surface temperature of the film ranges from a temperature of 15°C less than the glass transition point temperature (Tg) to the Tg. The elongation in the machine-direction is set within the range of -2% to 3%.

[0009] While there have been gradual improvements in the design of drying apparatus and methods, there is still a need for improved drying devices and methods, particularly ones that can provide efficient removal of the solvent yet reduce shrinkage of the sheet. Such systems would additionally benefit from environmental control of the volatile wash solvent.

SUMMARY OF THE INVENTION

[0010] In one aspect, the invention provides an apparatus for drying a microporous thermoplastic sheet. Such an apparatus includes a means for providing a porous sheet comprising at least one thermoplastic composition and having an amount of diluent therein; a roller for contacting a first surface of the sheet and conveying the sheet in a travel direction during drying; and at least one means for directing a stream of inert gas to a second surface of the sheet, said means positioned to provide the stream at an angle, β, ranging from about 30° to about 150°; wherein β is the angle formed by the direction of the stream and the line formed by the center of the roller and the median point where the film contacts roller.

[0011] In another aspect, the invention provides a method of drying a microporous thermoplastic sheet. Such a method includes continuously contacting the thermoplastic sheet having an amount of diluent in the pores thereof with a roller; passing an inert gas stream over a portion of a second surface of the sheet at an angle, β, ranging from about 30° to about 150°; where β is the angle formed by the direction of the stream and the line formed by the center of the roller and the median point where the film contacts roller. [0012] In another aspect, embodiments of the invention provide and apparatus and process designed to ensure that the washed thermoplastic sheet does not dry before reaching the drying roller. Such embodiments are achieved at least in part through the appropriate selection of the distance between the surface of the wash solvent and the drying roller. [0013] The apparatus and method may additionally include one or more of the following features.

[0014] The inert gas may travel in substantially the machine direction or in substantially opposite the machine direction in forming the angle β. In particular embodiments, the stream of inert gas travels in a direction substantially the opposite the machine direction. [0015] In some embodiments of the apparatus and method described herein the inert gas is provided by two or more, particularly four or more, independently operable means located at two or more predetermined positions spanning the width of the thermoplastic sheet. [0016] In still other particular embodiments of the apparatus and method described herein, the apparatus and method are operated in a manner whereby the roller and at least one means for directing the stream or inert gas are configured to provide for drying where a desired solvent level is attained over a relatively narrow region rl in the machine direction over the width of the sheet. In particular embodiments, rl ranges from 1 to about 300, preferably 150 mm.

[0017] In some embodiments, reduced values of rl may be provided by one or more of the following features. For example, in particular embodiments, the at least one means for directing the stream of inert gas includes a nozzle having a nozzle body, a slit, and a guide plate capable of directing the stream of inert gas flowing out of the slit. In some embodiments, the at least one means for directing the stream of inert gas includes an outlet port operably connected to the guide plate, guide plate being curved and extending substantially along the surface of the roller. In some other embodiments the inert gas is distributed in the travel direction prior to being directed in a direction substantially opposite the travel direction. The stream of inert gas may be provided at any convenient flow velocity, and in certain embodiments, the flow velocity of inert gas exiting the at least one means for directing the stream inert gas is about 200 to about 500 m/sec.

[0018] In some embodiments, the apparatus and method of the invention also include a tension adjustment roller and a drive roller for adjusting the tension of the film on the roller, and the at least one means for providing a stream of inert gas is positioned to provide the stream of inert gas to a location between the roller and the tension adjustment roller.

[0019] Particular embodiments include a nip roller for holding the thermoplastic sheet against the roller and are operated in a manner wherein the thermoplastic sheet wound on the roller has a tension of about 100 to 3000, preferably 200 to 2000, more preferably 300 to 1000 N-m. While any suitable carrying speed may be employed, typical carrying speeds of the thermoplastic sheet may range from about 5 to about 30 m/min. Typically, tension and speed are provided by one or more of the roller, the tension adjustment roller and the drive roller. To avoid tearing of the film, the roller, the tension adjustment roller and the drive roller cooperate to provide the desired film tension and speed.

[0020] Still other embodiments include an exhaust device wherein the intake portion of the exhaust device is positioned to receive the stream of inert gas after the stream contacts the thermoplastic sheet. [0021] Some embodiments of the invention include a washing tank located prior to the roller with respect to the travel direction of the sheet and contain a wash solvent. The roller is heated to a temperature ranging from 30 ⁰ C, preferably 10°C, less than the boiling point of the wash solvent to 20°C, preferably 10°C, higher than the boiling point of the wash solvent. The washing tank may include a second solvent that is substantially immiscible with the wash solvent. In particular embodiments the wash solvent is methylene chloride and the second solvent is water.

[0022] In some embodiments of the invention, the temperature of the inert gas exiting the at least one means for directing the stream of inert gas ranges from 0 ⁰ C to about 20 ⁰ C higher than the boiling point of the wash solvent, preferably from about 23 °C to about 10 ⁰ C higher than the boiling point of the wash solvent.

[0023] The apparatuses and methods described herein are particularly suitable for making thermoplastic microporous membranes such as battery separator films. BRIEF DESCRIPTION OF THE DRAWINGS [0024] Figure 1 illustrates a schematic representation of an embodiment of the apparatus for drying a thermoplastic sheet as described herein.

[0025] Figure 2 illustrates a schematic representation of the relationship of the angle, β, to the direction of gas flow in embodiments of the apparatus of drying the thermoplastic sheet described herein. [0026] Figure 3 illustrates a schematic representation of means for providing the gas and the guide plate employed in an embodiment of the invention. [0027] Figure 4 illustrates a schematic representation of an embodiment of the apparatus for drying the thermoplastic sheet that employs a solvent collection device as described herein.

[0028] Figure 5 illustrates a schematic representation depicting the region, rl, over which the desired solvent level may be obtained in the thermoplastic sheet. DETAILED DESCRIPTION OF THE INVENTION

[0029] Embodiments of the invention relate to apparatus and processes for drying a thermoplastic sheet. One such apparatus includes a means for providing a thermoplastic sheet comprising at least one thermoplastic composition and having an amount of diluent therein; a roller for contacting a first surface of the sheet; and at least one means for directing a stream of inert gas to a second surface of the sheet, said means positioned to provide the stream at an angle, β, ranging from about 30° to about 150°; wherein β is the angle formed by the direction of the stream and the line formed by the center of the roller and a midpoint of the region where the film contacts roller. [0030] For the purposes of this disclosure, the term "inert gas" is defined as any composition of matter that contacts the membrane in the gaseous state and does not substantially alter the chemical or physical structure of the membrane other than to facilitate the removal of solvent therefrom. Some particular inert gases include nitrogen, argon, and air. [0031] In the method of the present invention for producing a microporous membrane, a solution is prepared by combining a polyolefin composition and a diluent (sometimes referred to as a process solvent), generally while heating. Examples of the solvents suitable for dissolving polyolefins include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, paraffin oils, etc., and fractions of mineral oils having boiling points substantially equal to those of the above hydrocarbons. These solvents have a viscosity of 30 to 500 cSt at 25°C, preferably 50 to 200 cSt. Uneven extrusion can result when viscosity is below 30 cSt at 25 °C, making it difficult to knead the solution, whereas the solvent is difficult to remove in the post treatment when viscosity is above 500 cSt. [0032] The polyolefin or a composition thereof may be combined in any convenient manner. One such way to combine the polyolefm is to mix the polyolefm with the diluent by stirring the mixture at a temperature at which a homogeneous mixture is obtained, typically by mixing in an extruder. Combining the polyolefm and the diluent using an extruder is especially preferable where high concentrations of the polyolefin composition are employed. The polyolefin composition may also be prepared by dry blending or melt blending of the polymer components before being combined with the diluent. [0033] The mixing temperature varies depending on the types of polymers and solvents used. When mixing is to be effected in an extruder, the polyolefin composition is first charged in the extruder at a temperature preferably 30 to 100 ⁰ C above melting point of the polyolefin, although varying depending on type of the polyolefin used. For example, it is 160 to 23O ⁰ C, preferably 170 to 22O ⁰ C, in the case of polyethylene, and 190 to 270 ⁰ C ₅ preferably 190 to 250 ⁰ C, in the case of polypropylene. Then, a liquid solvent is added to the polyolefin composition downstream of the polyolefin mixing region.

[0034] The concentration of the polyolefin composition is typically 10 to 50 wt. % based on the total solution, preferably 20 to 40 wt. %, or conversely the concentration of the solvent is 90 to 50 wt. %, preferably 80 to 60 wt. %. When the polyolefm composition concentration is less than 10 wt. % (or the solvent concentration is more than 90 wt. %), swelling and neck-in are likely to take place at the exit of a die in the process of forming sheets. Accordingly, it is difficult to keep good molding processability and self-supporting characteristics of the sheet produced. On the other hand, when the polyolefin composition concentration is more than 50 wt. % (or the solvent concentration is less than 50 wt. %), the sheet produced can suffer various problems, such as excessive shrinkage in the thickness direction, lowered porosity, and deteriorated molding processability. Accordingly, it is difficult to produce the microporous membrane having large pores. It is possible to control permeability of the membrane by changing the polyolefin composition concentration in the above range. [0035] Next, the heated solution of the molten and kneaded polyolefin composition is extruded through a die or the like, in such a way to secure the final membrane thickness ranging from 3 to 250 μm, either directly or via another extruder. [0036] Usually a sheet die having a rectangular orifice is used as a die, but any suitable die such as a double-pipe hollow die or inflation die may be used. When the sheet die is used, a die gap of about 0.1 to 5 mm is used, and the die is heated to a temperature ranging from 140°C to 250°C in the extrusion process. The temperature of the die will vary according to the selection of the polyolefin composition as is known in the art. In an embodiment, a casting roll speed of 20-30 cm/minute to 15 m/minute is employed.

[0037] The mixture is cooled as it is extruded through the die and forms an extrudate having gel-like characteristics. Typically, the extrudate is formed by cooling to 90 ⁰ C or below, preferably to 80 to 30 ⁰ C ₅ at a speed of at least 50°C/minute. In one embodiment, the extrudate is cooled by direct contact with cooling air, cooling media, e.g. water or other liquid, contact with a roll operating at a reduced temperature by a coolant. In particular embodiments, contacting the extrudate with a cooling roll is preferred. But other suitable means of cooling the extrudate will be apparent to the ordinarily skilled person. [0038] Where the extrudate is cooled by contact with a cooling roll, the roll can be operated within 40 to 90°C, particularly within 30 ⁰ C, of the Tm of the polyolefin having the lowest Tm in the extrudate or within 40 to 90 ⁰ C, particularly within 30 ⁰ C, of the Tm of the polyolefin comprising a majority of the extrudate. When the cooling roll temperature is excessively high, the sheet will be cooled too slowly to sufficiently increase thickness of the walls defining the lamella structure of the polyolefin. As a result, the solvent becomes difficult to remove and membrane permeability decreases. When the cooling roll temperature is excessively low, the extrudate is quenched too rapidly, resulting in excessively densified extrudate having decreasing pore size and permeability. The extrudate is withdrawn at 1 to 20 m/minute, preferably -3 to 10 m/minute. Lower withdrawal rate within these ranges are generally preferred. [0039] The extrudate is then stretched, as desired, by an ordinary method, such as a tenter, roll, calendar method or a combination thereof at a given stretching ratio to form a sheet. The stretching may be monoaxial or biaxial, in other words in only the machine direction, the transverse direction or both. In the case of biaxial stretching, the extrudate may be stretched in the longitudinal and transverse directions simultaneously or sequentially. In particular embodiments simultaneous stretching is preferred. [0040] The stretching temperature should be in a range from the polyolefin crystal dispersion temperature to 10 ⁰ C above the crystal melting point, preferably in a range from the crystal dispersion temperature to the crystal melting point. For a polyethylene composition containing an ultra-high-molecular- weight polyethylene, for example, it is preferably in a range from 90 to 140°C, more preferably from 100 to 130 ⁰ C. In the case of polyethylene compositions, it is preferably in a range from 90 to 14O ⁰ C, more preferably from 90 to 125°C. If the stretching temperature is more than about 10°C higher than the melting point, it is more difficult to achieve the desired molecular orientation. If the stretching temperature is lower than the crystal dispersion temperature, on the other hand, it is more difficult to produce a sheet that does not tear during the stretching process.

[0041] The stretching ratio is not limited in either the machine direction (MD) or the transverse direction (TD). Preferably the total stretching ratio (i.e., ratio of the area of the stretched film compared to the area of the film prior to stretching) is about 2 to about 400, more preferably about 15 to about 400.

[0042] The stretched sheet is then washed with a solvent to remove the residual diluent. Solvents used for this solvent-removing treatment may be volatile ones, including hydrocarbons such as pentane, hexane and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; fluorinated hydrocarbons such as trifluoroethane; and ethers such as diethyl ether and dioxane. These volatile solvents may be used individually or in combination, and their selection depends on the types of the nonvolatile solvents used to dissolve the polyolefin composition. Washing methods with the solvents include an extraction method with solvent, a method of spraying solvent or a combination thereof. While any desired level may be obtained, washing generally is designed to reduce the content of the residual diluent to less than about 1 wt. %.

[0043] . Figures 1 and.2 show one specific embodiment of an apparatus for drying a thermoplastic sheet 1 containing a wash solvent 20. It should be noted, however, that the figures herein are merely schematic and are not intended to convey any information regarding the microporous structure of the thermoplastic sheet 1 or resulting membrane. The apparatus of this embodiment includes a washing bath 2 containing a wash solvent 20 and an optional second solvent 21. The second solvent 21 typically has a lower density than the wash solvent 20 and is substantially immiscible with the wash solvent 20. For the purposes of this specification, two solvents are "substantially immiscible" if at operating conditions there is a visible phase separation between the two solvents. [0044] The wash solvent 20 is used to remove at least a portion, preferably a major portion, more preferably substantially all, of the diluent from the stretched sheet 1. Removing the diluent phase from the stretched sheet 1 is performed in order to replace at least a portion, preferably a major portion, more preferably substantially all, the diluent in the pores of the sheet with the more volatile wash solvent 20. Thus, appropriate selection of the wash solvent 20 improves the porosity and permeability of the finished membrane since fewer pores remain clogged with diluent. Some suitable wash solvents 20 include volatile organic solvents, preferably halogenated organic solvents. For example, suitable wash solvents 20 include chlorinated hydrocarbons, such as methylene chloride or carbon tetrachloride; hydrofluoroethers, such as C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , and perfluoroethers, such as C ₄ F ₉ OCF ₃ or C ₄ F ₉ OC ₂ F ₅ . Methylene chloride is a suitable wash solvent 20 in particular embodiments. In some embodiments an azeotrope of methylene chloride and hydrofluoroether is very effective for removing the liquid paraffin diluent.

[0045] The amount of wash solvent 20 lost from the washing bath 2 can be reduced by providing the wash tank 2 with the substantially immiscible second solvent 21. By selecting the second solvent 21 that is immiscible with and has a density that is lower than the wash solvent, smaller amounts of the wash solvent 20 escape the wash tank 2. Any solvent which is poor in compatibility to the wash solvent 20 and has a lower density than the wash solvent 20 can be used without particular limitation. Preferably, the second solvent 21 has a higher boiling point than the wash solvent 20. One suitable, although not critical, second solvent 21 is water. [0046] The amount of wash solvent 20 in the wash tank 2 is not critical. However, in an embodiment of the invention, the wash tank contains about 300 to 30,000 parts by wt. per 100 parts by wt. of the stretched sheet. Typically, the washing temperature is about 5°C to 40 ⁰ C, preferably about 15°C to about 30 ⁰ C. The washing temperature may be increased by heating the washing bath 2 using conventional methods. Typically, however, the wash solvent 20 has a temperature that is preferably not higher than 80°C.

[0047] Although not necessary, the washing step may be performed by sequentially passing the stretched sheet 1 through a series of wash baths 2 until the residual amount of the diluent is about 1 wt. % or less of the amount of the diluent to dissolve the thermoplastic composition. In some embodiments, one or more wipers 61 are positioned between the wash tank 2 and the roller 3 in order to reduce the amount of wash solvent 20 carried to the roller 3.

[0048] The amount of the substantially immiscible second solvent 21 is also not typically critical. Where it is present, the amount of the second solvent 21, however, should be sufficient to cover the surface of the wash solvent 20 in the wash bath 2. [0049] After passing through the wash tank 2, the washed sheet 1 is conveyed to a roller 3. In one embodiment, a nip roller 30 for providing traction and holding the washed sheet 1 to the roller 3 is also included. Generally, such an arrangement is desirable in order to reduce shrinkage of the sheet during the process. The length L from the level of the substantially immiscible second solvent 21 to the roller 3 is generally in the range of from 30 to 500 mm. When the length L exceeds 500 mm, volatilization of the wash solvent 20 may cause shrinkage of the sheet 1. [0050] In particular embodiments the length L is selected to ensure that the sheet 1 does not dry before reaching the roller 3. Thus, in particular embodiments, the length L range from about 30 mm to about 250 mm, 30 mm to about 100 mm, or 30 mm to about 75 mm. The lower limit of the range of L values can be 30 mm, 35, mm, 40 mm, 45 mm, 50 mm, or 100 mm. The upper limit on the length L may be 40, 45, 50, 60, 75, 80, 90, 100, 200, or 300 in some embodiments. It is envisioned that ranges spanning these upper and lower limits are expressly disclosed. It will be appreciated that the selection of suitable length L will depend on process conditions, such as for example travel speed and the choice of wash solvent.

[0051] The roller 3 is fixed in a constant position, typically so that its rotating axis is substantially parallel to the transverse direction of the washed sheet 1. The axial length of the roller 3 should be at least as wide as the width of the washed sheet 1, so that the entire width of the sheet 1 comes in contact with the roller 3. [0052] The roller 3 typically has a diameter of 30 to 400 mm, or 50 to 300 mm, 75 to 150 mm. When the diameter is less than 30 mm, the contact area of the roller 3 with the sheet 1 is too small and it can be difficult to sufficiently transfer the heat of the roller 3 to the sheet 1 , and when the diameter exceeds 400 mm, it can be difficult to apply an appropriate tension to the sheet 1. While the carrying speed of the sheet 1 can be adjusted according to the specific requirements of individual processes, generally the carrying speed ranges from to 5 to 30 m/min. Carrying speeds of about 8 to 25 m/min are used in some specific embodiments. When the carrying speed is less than 5 m/min, production efficiency suffers. On the other hand, if the carrying speed exceeds about 30 m/min, it can be difficult to sufficiently dry the sheet. For suppressing the shrinkage of the sheet 1 , a single roller 3 is generally used, however, two or more rolls may be provided where shrinkage is not unacceptably impacted.

[0053] The roller 3 is typically, although not necessarily heated. The desired heating temperature depends on the vapor pressures of the diluent, wash solvent 20, and the second solvent 21 and to some extent on the amount of these solvents in the sheet 1. Typically, the roller 3 is operated at a temperature ranging from 30 ⁰ C less than the boiling point of the wash solvent to not more than 2O ⁰ C higher than the boiling point of the wash solvent 20. Drying roll temp is preferably higher than 0 ⁰ C to avoid freezing of water. In some embodiments the lower limit of the temperature range for the roller 3 is 5 ⁰ C less than the boiling point of the wash solvent 20. Likewise, the upper operational limit of the temperature of the roller 3 is not more than 5 ⁰ C higher than the boiling point of the wash solvent 20. In particular embodiments, the temperature of the roller 3 is maintained at a temperature in the range of 5 ⁰ C less than the temperature of the wash solvent 20 to a temperature of about 10°C greater than the temperature of the wash solvent 20. A roller temperature that is too high can cause rapid volatilization of the wash solvent 20 and may induce undesirable shrinkage. Of course, if the temperature of the roller 3 is too low, it can be difficult to dry the sheet 1 during the drying process. [0054] The invention also includes a means for providing a stream of inert gas to the surface of the sheet 1. Any conventional means can be used but should include at least a nozzle 4 that directs the inert gas stream toward the surface of the sheet 1. The nozzle 4 preferably includes curved guide plate 5 configured so that the stream flows over substantially the entire width of the sheet 1.

[0055] Figure 2 illustrates the relationship between the angle, β, in which the gas flows and roller 3. As illustrated in figure 2 the means for providing the inert gas does so at an angle, β, with respect to the imaginary line, AB , formed by the center, B, of the roller 3 and the midpoint, A, of the region where the sheet 1 is in contact with the roller 3. Typically, the means for providing the inert gas is positioned so that the gas flow at an angle, β, of about 30° to about 150°, about 45° to about 135°, about 60° to about 115°, or about 85° to about 95°. Where the curved guide plate 5 is employed, the angle, β, is measured based on the direction of the inert gas flow as it exits the nozzle 4.

[0056] In addition to flowing over substantially the entire width of the sheet 1 , the stream of inert gas flows in a direction that is substantially opposite the direction in which the sheet 1 travels (i.e., the machine direction). As used herein the term "substantially opposite the machine direction" means a direction within about 10°, within 5 or fewer degrees, or parallel to the direction opposite the machine direction of the film and at an angle within 10°, with about 5° or 0° of a tangent to the surface of the film or sheet. Typically, the determination of the direction is determined at a point where the stream of inert gas exits the at least one means for directing a stream of inert gas. [0057] Now referring to Figure 3, the particular configuration of the means for proving the stream of inert gas that is used in a particular embodiment is described. In the embodiment depicted in Figure 3, the nozzle 4 includes the nozzle body 40 having a slit 400, a guide plate 41 of L-shaped cross-section for changing the direction of the inert gas flowing out of the slit 400, and a guide plate 42 which forms the slit-like outlet port 43 with the guide plate 41. The slit 400 can be a continuous or intermittent, but a continuous slit is preferred. The shape of the nozzle 4 is generally tubular, but is not critical as long as it allows blowing of inert gas in a direction substantially opposite the machine direction at the desired flow velocity. In some embodiments, the nozzle 4 can include a number of cylindrical outlet ports instead of the slit-like outlet port 43. Cylindrical outlet port may take advantage of the Venturi effect, thereby aiding removal of the wash solvent 20 from the pores of the sheet 1.

[0058] With continuing reference to Figure 3, a particular embodiment of the invention uses the Venturi effect to help remove residual wash solvent 20 from the pores of sheet 1 by providing that the space d2 between the outlet port 43 of the nozzle 4 and the sheet 1 ranges from about 1 to about 5 mm, preferably about 1 to about 3 mm. [0059] Embodiments of the invention optionally include a curved guide plate 5 extending from the outlet port 43 of the nozzle 4 substantially along the surface of the drying roll 3. The curved guide plate 5 can be in contact with the end surface of the guide plate 42 on the upper side of the outlet port 43 of the nozzle 4. The inert gas can be uniformly distributed along the roller 3 by providing the curved guide plate 5 and the contact area of the air/inert gas flow with the microporous sheet 1 can be increased. Therefore, wash solvent 20 and second solvent 21 can be removed near terminal end part (upstream side end part) of the curved guide plate 5 by the inert gas that is flowing substantially the opposite direction with respect to the direction sheet 1 travels.

[0060] As also depicted in Figure 3, the space d3 between the downstream end part of the curved guide plate 5 and the sheet 1 is set preferably to 2 to 10 mm, more preferably to 3 to 7 mm. Likewise, the space d4 between the terminal end of the curved guide plate 5 and the sheet 1 is set preferably to 2 to 10 mm, more preferably to 3 to 7 mm. To enhance the visibility of the sheet, the curved guide plate 5 is preferably formed of a transparent material such as glass or acrylic resin. It may be beneficial in some embodiments to configure the space d4 between the terminal end of the curved guide plate 5 and the sheet 1 to be larger than the space d2 between the outlet port 43 of the nozzle 4 and the sheet 1. In particular embodiments, providing a spacing d4 that is larger than d2 may be beneficial.

[0061] In some embodiments, the flow velocity of the inert gas at the outlet port 43 of the nozzle 4 can be 200 to 500 m/sec, or 250 to 400 m/sec, although the flow velocity is not critical. When a flow velocity is less than 200 m/sec, it can be difficult to dry the sheet sufficiently. On the other hand, high flow velocity may make it difficult to prevent undesirable shrinkage of the sheet 1. The flow velocity of the inert gas can be adjusted by changing the gas pressure within a nozzle body 40 and the gap dl of the outlet port 43. In one embodiment, the gap dl of the outlet port 43 of the nozzle 4 can be 1 to 5 mm, or 1 to 3 mm. [0062] Figure 4 shows an embodiment of the apparatus that includes an optional baffle 6 advantageously positioned to remove the vapors of the solvents 20 and 21 and the stream of inert gas. One such position for the baffle is near the upper side of the nip roller 30. The construction of the baffle 6 is not critical, thus any baffle capable of collecting portions of the solvent vapors may be used. In one embodiment, the baffle 6 is positioned so that amount of the wash solvent 20 and the substantially immiscible second solvent 21 that adheres to the sheet 1 from the nip roller 30 can be reduced.

[0063] Also as depicted in Figure 4, the optional baffle 6 can be used along with a suction pump 7 and a separation tank 8 connected to the baffle 6. In another configuration, option baffle 6 can be replaced with a ventilation device, such as a hood or exhaust fan. In either case, the vapors of wash solvent 20 and second solvent 21 can be recovered by the optional suction pump 7 and passed to the optional separation tank 8 where the solvents can be recovered and reused.

[0064] Now turning to Figure 5, it has been observed that typically, the appearance of sheet 1 will change from generally transparent to opaque when the residual solvent level has been reduced to an acceptable level. Thus, while the level of solvent could be measured any number of ways, optical determination of the change from transparent to opaque provides a convenient way to determine that an acceptable level of residual solvent has been obtained. In particular embodiments, one or more of the above described features are arranged so that the desired residual solvent level in the microporous membrane is achieved over a relatively narrow band or region, rl, over the width of the film. In some embodiments, the features of the invention are arranged so that the region rl where the desired solvent level is obtained occurs on the roller 3. Because the properties of the film should not substantially deviate over the level of the film 1, smaller values of the region rl where the desired solvent level is achieved are preferred. Typically, rl ranges from 1 mm to 300 mm, preferably 3 mm to 200 mm, more preferably 5 mm to 100 mm, particularly 50 to 105 mm. In other particular arrangements, rl of 10 mm to about 80 mm, or about 20 to 30 mm. [0065] Because the difficulty of uniformly drying generally increases as the width of the film increases, it can be useful to consider the range rl relative to the width of the film. In some embodiments, the range rl over which the change from transparent to opaque is within the range :

0.001 < — < 0.1 W where W is the width of the film. In particular embodiments, the ratio rl/W has an upper limit of about 0.01, about 0.033, about 0.015, or about 0.006 and a lower limit on the range of about 0.001, 0.006, 0.01, or 0.02. A ratio rl/W ranges from 0.005 to about 0.04 can be achieved by particular arrangements of the features of the invention.

[0066] It will also be appreciated that it may be advantageous to provide two or more of any of the features to the described apparatus. Thus, embodiments of the invention may provide two or more individual means for providing a stream of inert gas. For example, one such embodiment includes two or more, preferably three, four or five independent nozzles 4, guide plates 41 or curved guide plates 5 positioned adjacently across the width of the membrane 1. In such configurations, each means for providing the stream of inert gas may be independently controlled or regulated, thereby providing greater flexibility to adjust the process parameters in order to reduce the region rl over which the microporous membrane achieves the desired level of residual solvent. [0067] After drying, the membrane 1 may be subjected to any number of optional treatments, including but not limited to, dry orientation, thermal setting heat treatment, winding, or slitting. [0068] As demonstrated above, embodiments of the invention provide new methods and apparatuses that are useful for making thermoplastic films. The methods and apparatuses have one or more of the following advantages. First, shrinkage in the transverse direction can be improved. In some instances, the microporous films have improved porosity or shutdown temperature. The microporous polyolefin membrane produced as mentioned above is a highly permeable one. Desired properties of the microporous films may be obtained by adjusting the parameters of the various components and process variable associated with the apparatus and method described herein. Some such variables include the line speed, inert gas flow rate and temperature, placement of the means for providing the inert gas and its relative angle to the path of film travel. While, these conditions have been described, adjustments to the particular values may be necessary depending implementation details. Such adjustments are within the ability of the skilled person. The skilled person may also recognize other characteristics and additional advantages of one or more embodiments of the invention. [0069] While the invention has been described with respect to a limited number of embodiments, the specific features of one embodiment should not be attributed to other embodiments of the invention. No single embodiment is representative of all aspects of the inventions. Moreover, variations and modifications therefrom exist. For example, the resulting microporous polyolefin membrane may be, if necessary, subjected to a hydrophilic treatment by plasma irradiation, impregnation with a surface active agent, surface grafting, or other process to enhance or modify its properties. In other embodiments, the composition consists essentially of or consist of the recited elements. Finally, it should be considered that embodiments lacking one or more components or steps that are not specifically enumerated herein are expressly disclosed. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention.