WO2010001722A2

WO2010001722A2 - Chill roll assembly and process for producing a microporous membrane

Info

Publication number: WO2010001722A2
Application number: PCT/JP2009/060980
Authority: WO
Inventors: Tatsuro Yagi; Hiroshige Kuzuno; Kotaro Takita; Koichi Kono
Original assignee: Tonen Chemical Corporation
Priority date: 2008-07-03
Filing date: 2009-06-10
Publication date: 2010-01-07
Also published as: WO2010001722A3; JP5451652B2; JP2011526547A

Abstract

An assembly for transferring heat from an extrtidate fornicd by extmding a polyolefin mixture through an extrusion die. The chill roll assembly includes an upstream roll positioned to contact and receive the extnidate, the. upstream roll having an external surface roughness of equal or less than 1.0 s; and at least one downstream roll positioned so as to contact and receive the extrudate from the upstream roll, the downstream roll having an. external surface roughness of equal or greater than 5.0 s. A process for producing a microporous membrane is also provided.

Description

CHILL ROLL ASSEMBLY AND PROCESS FOR PRODUCING A MICROPOROUS MEMBRANE

FIELD OF THE INVENTION

[0001] This disclosure relates generally to a system and method for producing microporous membranes, such as those useful as battery separators. BACKGROUND OF THE INVENTION [0002] Microporous membranes, particularly microporous polymeric membranes, are useful as separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver- zinc secondary batteries, etc. When the microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, the membrane's characteristics, such as . flatness and thickness uniformity, significantly affect the properties, productivity and safety of the battery. Generally, the aim is to provide a microporous membrane having well-balanced characteristics, where the term "well-balanced" means that the optimization of one membrane characteristic does not result in a significant degradation in another.

[0003] Microporous polymeric membranes can be produced according to a wet process, where at least one polymer (such as one or more polyolefin) and at least one diluent (or solvent) are combined to form a polymeric mixture and which is then extruded to form an extrudate. The extrudate is then stretched in at least one planar direction. Following stretching, at least a portion of the diluent is removed from the stretched extrudate to form the membrane. Additional steps such as membrane drying, further stretching, thermal treatments, etc. can be used downstream of the diluent removal step. Examples of references disclosing conventional wet processing include U.S. Patent No. 5,051,183, U.S. Patent No. 6,096,213, WO 2008/016174, and WO 2005/113657. [0004] U.S. Patent No. 5,830,554 discloses cooling the gel-like sheet following extrusion and before stretching. The reference discloses cooling at a rate of 5O⁰C or more; slower cooler rates are said to result in a loss of thickness uniformity, i.e., the gel- like sheet becomes rough. [0005] U.S. Patent No. 4,734,196 discloses a method for producing a relatively uniform microporous film from ultra-high-molecular-weight alpha-olefin polymer having a weight-average molecular weight greater than 5x10⁵. The microporous membrane is obtained by forming a gel-like object from a mixture of an alpha-olefin polymer having a weight-average molecular weight greater than 5x10⁵, removing at least 10 wt.% of the solvent contained in the gel-like object so that the gel-like object contains 10 to 90 wt.% of alpha-olefin polymer, orientating the gel-like object at a temperature lower than that which is 10°C above the melting point of the alpha-olefin polymer, and removing the residual solvent from the orientated product. A film is produced from the orientated product by pressing the orientated product at a temperature lower than that of the melting point of the alpha-olefin polymer, to provide a relatively uniform product.

[0006] U.S. Patent Publication No. 2007/0012617 proposes a method for producing a microporous thermoplastic resin membrane comprising the steps of extruding a mixture obtained by melt-blending a thermoplastic resin and a membrane- forming solvent through a die, cooling an extrudate to form a gel-like molding, removing the membrane-forming solvent from the gel-like molding by a washing solvent, and removing the washing solvent, the washing solvent having (a) a surface tension of 24 mN/m or less at a temperature of 25°C, (b) a boiling point of 100°C or lower at the atmospheric pressure, and (c) a solubility of 600 ppm (on a mass basis) or less in water at a temperature of 16°C; and the washing solvent remaining in the washed molding being removed by using warm water. The molten polymer is fed into a first inlet at an end of a first manifold and a second inlet at the end of a second manifold on the opposite side of the first inlet. Two slit currents flow together inside the die. It is theorized that due to the absence of flow divergence of the melt inside the manifold, it may be possible to achieve uniform flow distribution within the die. This is said to result in improved thickness uniformity in the transverse direction the film or the sheet.

[0007] While improvements have been made in producing microporous membranes having a relatively uniform thickness, further improvements are desired. SUMMARY OF THE INVENTION

[0008] In one aspect, the invention relates to an assembly for transferring heat away from an extrudate. For example, in one such an assembly for transferring heat from an extrudate, the assembly comprising: a) at least one upstream roll positioned to receive the extrudate, the upstream roll having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601(Rmax); and b) at least one downstream roll, the upstream and downstream rolls being aligned so that the downstream roll receives the extrudate from the upstream roll, the downstream roll having an external surface roughness of > 5.0 s, as determined in accordance with JIS B 0601(Rmax). [0009] Since the rolls act to transfer heat way from the extrudate, they can be referred to as "chill rolls," particularly when the rolls comprise cooling means.

[0010] In another aspect, a process for producing a microporous membrane is provided. In one form, the process includes the steps of combining one or more polymer and a diluent to form a polymeric solution. The combined polymer and diluent is extruded through an extrusion die to form an extrudate. The extrudate is cooled by transferring heat from the extrudate through a plurality of rolls to form a cooled extrudate. The plurality of rolls include an upstream roll positioned to receive the extrudate, the upstream roll having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601(Rmax), and a downstream roll positioned to receive the extrudate from the upstream roll, the downstream roll having an external surface roughness of > 5.0 s, as determined in accordance with JIS B 0601(Rmax). Following cooling, the cooled extrudate is oriented in at least one direction by about 2 to about 400 fold at a temperature of about Ted to about Tm + 1O⁰C, and at least a portion of the diluent is removed from the cooled extrudate to form a membrane. [0011] Various advantages, features and attributes of the disclosed processes and systems and their advantageous applications and/or uses will be apparent from the detailed description that follows, particularly when read in conjunction with the figures appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The disclosure is further explained in the description that follows with reference to the drawings illustrating, by way of non-limiting examples, various forms wherein:

[0013] FIG. 1 is a schematic view of one form of a system for producing an oriented film or sheet of thermoplastic material, in accordance herewith;

[0014] FIG. Ib is a schematic view of one form of an assembly for transferring heat from an extrudate formed by extruding a polyolefm mixture through an extrusion die, in accordance herewith,

[0015] FIG. 2 is a side view of an assembly for transferring heat from an extrudate in accordance herewith; and

[0016] FIG. 3 is an end view of the assembly for transferring heat from an extrudate of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

[0017] It has been observed that producing microporous polyolefin membranes by the "wet process," can lead to problems with the cooling operation, particularly relating to the design of the chill roll apparatus. Depending on the surface finish of the chill rolls, poor cooling efficiency or trapped air may occur. Additionally, a portion of the solvent may bleed out on the surface of the extrudate, causing slippage or movement of the sheet over the chill roll surface. When slip or movement occurs, sheet operability becomes unstable, cooling efficiency worsens and sheet non-uniformities such as distortion, creasing, and warping occurs, negatively impacting the properties of the microporous film. It has been discovered that these problems can be overcome by using a plurality of chill rolls, with at least one upstream roll having a smoother surface than a downstream roll.

[0018] Various aspects will now be described with reference to specific forms selected for purposes of illustration. It will be appreciated that the spirit and scope of the process and assembly disclosed herein is not limited to the selected forms. Moreover, it is to be noted that the figures provided herein are not drawn to any particular proportion or scale, and that many variations can be made to the illustrated forms.

[0019] When an amount, concentration, or other value or parameter is given as a list of upper values and lower values, this is to be understood as specifically disclosing all ranges formed from any pair of an upper value and a lower value, regardless whether ranges are separately disclosed

[0020] Reference is now made to FIGS. 1-3, wherein like numerals are used to designate like parts throughout. [0021] Referring now to FIG. 1, a system 10 for producing a microporous film or sheet of thermoplastic material is shown. System 10 includes an extruder 12, extruder 12 having a feed hopper 15 for receiving one or more polymeric materials, processing additives, or the like, fed by a line 14. Extruder 12 also receives a nonvolatile diluent (which can be a solvent), such as paraffin oil, through a solvent feedline 16. A polymeric solution is prepared within extruder 12 by combining the polymer and diluent, with mixing and heating as desired.

[0022] The heated mixture of polymer and solvent or diluent is then extruded into a sheet 18 from a die 20 of extruder 12. The extruded sheet 18 is cooled by a chill roll assembly 100, having a plurality of chill rolls, to a temperature lower than the extrudate gelling temperature, so that the extruded sheet 18 gels. The cooled extrudate 18' optionally passes to a first orientation apparatus 24, which may be a roll-type stretching machine, as shown. The cooled extrudate 18' is oriented with heating in the machine direction (MD) through the use of the roll-type stretching machine 24 or, optionally, through the use of a tenter-type stretching machine (not shown) and then the cooled extrudate 18' may optionally pass to a second orientation apparatus 26, for orientation in at least the transverse direction (TD), to produce an oriented film or sheet 18". Second orientation apparatus 26 may be a tenter-type stretching machine and may be utilized for further stretching in the MD. The first and second orientation apparatus can be used in combination, if desired, as shown in the figure. [0023] The oriented film or sheet 18" next passes to a solvent extraction device

28 where a readily volatile solvent such as methylene chloride is fed in through line 30. The volatile solvent containing extracted nonvolatile solvent is recovered from a solvent outflow line 32. The oriented film or sheet 18" next passes to a drying device 34, wherein the volatile solvent 36 is evaporated from the oriented film or sheet 18".

[0024] Optionally, the oriented film or sheet 18" next passes to dry orientation device 38 where the dried membrane is stretched to a magnification of from about 1.1 to about 2.5 fold in at least one direction to form a stretched membrane. Next, the oriented film or sheet 18" passes to the heat treatment device 44 where the oriented film or sheet 18" is annealed so as to adjust porosity and remove stress left in the film or sheet 18", after which oriented film or sheet 18" is rolled up to form product roll 48. [0025] FIGS. Ib, 2 and 3 show particular forms of chill roll assemblies 100 for transferring heat from an extradate 18 formed by extruding the combined polymer and diluent through an extrusion die 20. Chill roll assembly 100 includes at least one support frame 102 and a first (or "upstream") chill roll 104 mounted on the at least one support frame 102 and positioned to contact and receive the extrudate. The assembly also includes a second (or downstream) chill roll 112 mounted on the at least one support frame 102 and positioned so as to contact and receive the extrudate 18 from first chill roll 104. The first chill roll 104 has an external surface 106 has having an external surface roughness of = 1.0 s, as determined in accordance with JIS B 0601(Rmax). In particular embodiments, the first chill roll 104 has an external surface roughness ranging from 0.2 to 0.8 s, as determined in accordance with JIS B 0601(Rmax), more particularly ranging from 0.5 to 0.7 s, as determined in accordance with JIS B 0601 (Rmax). The downstream chill roll 112 has an external surface 114, external surface 114 having an external surface roughness of = 5.0 s, as determined in accordance with JIS B 0601 (Rmax). In particular embodiments, the chill roll 112 has an external surface roughness ranging from 7.0 to 15.0 s, more particularly, from 8.5 s to 12 s as determined in accordance with JIS B 0601 (Rmax). Other chill rolls, which are optional, can also be included as illustrated in the figure. [0026] For example, downstream of first chill roll 104, the chill roll assembly 100 can also include a third chill roll 108 mounted on the at least one support frame 102 and positioned so as to contact and receive the extrudate 18 from first chill roll 104. The chill roll 108 has an external surface 110, external surface 110 having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601(Rmax). In particular embodiments, the first chill roll 104 has an external surface roughness ranging from 0.2 to 0.8 s, as determined in accordance with JIS B 0601(Rmax), more particularly ranging from 0.5 to 0.7 s, as determined in accordance with JIS B 0601(Rjtnax). [0027] Still referring to FIGS. 2 and 3, chill roll assembly 100 further includes a drive means mounted on support frame 102 and associated with at least one of upstream chill roll 104, third chill roll 108 and downstream chill roll 112. The drive means rotates at least one of upstream chill roll 104, third chill roll 108 and downstream chill roll 112 to cause extrudate 18 to move through the chill roll assembly 100 in contact with first chill roll 104, third chill roll 108 and second chill roll 112. As shown, the drive means may include a plurality of motors 116 that drive a plurality of gears 118 through a chain and sprocket arrangement, as those skilled in the art will plainly recognize. In one form, only the downstream roll is driven in rotation. In another form, a plurality of the rolls is driven. The plurality of rolls can be driven by a single drive, e.g., using suitable linkages, or, alternatively, second, third, fourth, etc. drives can be used. When two or more rolls have independent drives, the drives are generally synchronized to reduce the risk of extrudate tearing.

[0028] To assist in regulating the temperature of extrudate 18 (e.g., by cooling the extrudate), chill roll assembly 100 can further include a cooling means associated with upstream chill roll 104, third chill roll 108 and downstream chill roll 112 for cooling extrudate 18. As shown, the cooling means may include a plurality of pumps 120 to circulate a coolant through one or more cooling circuits (not shown), the cooling circuits in fluid communication with upstream chill roll 104, third chill roll 108 and downstream chill roll 112, each of which have internal passages for circulating coolant and transfer heat from extrudate 18. In one form, the upstream and downstream rolls have associated cooling means. In another form, at least two of (i) the upstream roll, (ii) the downstream roll), or (iii) the third roll can comprise cooling means. In yet another form, the upstream roll, the downstream roll, and the third roll each comprise cooling means. [0029] As may be appreciated, in operation, extrudate 18 moves in arcuate paths around upstream chill roll 104, third chill roll 108 and downstream chill roll and 112 and in linear paths between upstream chill roll 104, third chill roll 108 and downstream chill roll 112. These paths may be seen in FIG. 2, where extrudate 18 is shown as a dotted line. As is conventional, upstream chill roll 104 and third chill roll 108 are positioned closely adjacent to each other to define a nip therebetween. In general, a nip roll can be used to increase friction to prevent slippage of the sheet over the chill roll surface. In this form, a gap is established between at least two chill rolls (e.g., between the upstream and third chill rolls), the gap being equal to or less than thickness of the sheet. In this form, the third roll can be referred to as the nip roll. Since the downstream roll has relatively rough surface, it produces a relatively large frictional force capable of conveying the sheet through the chill roll system. Consequently, the use of one or more nip rolls is optional. In one form, a gap between upstream chill roll 104 and third chill roll 108 is more than sheet thickness, e.g., 1.05 times the sheet thickness or more, or in the range of 2 to 200 times the sheet thickness, or in the range of 4 to 100 times the sheet thickness. [0030] Still referring to FIGS. 2 and 3, in one form, chill roll assembly 100 may optionally include a fourth chill roll 122 mounted on the at least one support frame 102 and positioned so as to contact and receive the extrudate 18 from the downstream chill roll 112. Fourth chill roll 122 has an external surface 124, external surface 124 having an external surface roughness of > 5.0 s, as determined in accordance with JIS B 0601(Rmax). While not required, the fourth roll can comprise cooling means and can be driven in rotation by drive means.

[0031] In another form, chill roll assembly 100 may optionally include a fifth chill roll 126 mounted on the at least one support frame 102 and positioned so as to contact and receive the extrudate 18 from fourth chill roll 122. Fifth chill roll 126 has an external surface 128, external surface 128 having an external surface roughness of > 5.0 s, as determined in accordance with JIS B 0601(Rmax). While not required, the fifth roll can comprise cooling means and can be driven in rotation by drive means. In one form, upstream chill roll 104 and third chill roll 108 each are provided with an external surface roughness of about 0.6 s, as determined in accordance with JIS B 0601(Rmax). In another form, downstream chill roll 112, fourth chill roll 122 and fifth chill roll 126 each are provided with an external surface roughness of about 10 s, as determined in accordance with JIS B 0601 (Rmax). In one form, the downstream chill roll, fourth chill roll, and fifth chill roll have progressively increasing external surface roughness. For example, the downstream chill roll 112 can have a surface roughness of > 5 s, the fourth chill roll 122 can have a surface roughness of > 6 s, and the fifth chill roll 126 can have a surface roughness of > 7 s.

[0032] Specification JIS B 0601, the contents of which are hereby incorporated in their entirety, provides a standard for the determination of surface roughness. In particular, the Maximum Height or Peak (Rmax), as used herein, is determined by taking a section of standard length, taken from the mean line on the roughness chart. The distance between the peaks and valleys of the sampled line is measured in the y direction. The value is expressed in micrometers (μm).

[0033] As indicated above and shown in FIGS. 2 and 3, a plurality of tandemly- disposed chill rolls are employed. This multi-stage operation, compared to a more conventional one-stage operation, provides the advantages of uniform cooling on both surfaces of the extrudate (as when upstream, downstream, and third rolls are used), while keeping the extrudate adhered onto the entire surface of the chill roll. This despite the fact lower tension may be employed, thus reducing distortion and warping of the extrudate, resulting in improved thickness uniformity in the extrudate and finished membrane.

[0034] Generally, different levels of chill roll surface roughness are employed depending on the films produced. For example, in a cast polypropylene system, satin- fϊnished chill rolls having a surface roughness of 2 s to 8 s are typically employed, rather than mirror-finished chill rolls. When biaxially oriented films are produced from a polymer extrudate without diluent, mirror-finished chill rolls (0.1 s to 0.5 s) are widely used.

[0035] For chill rolls used in the wet method of manufacturing microporous polymeric film, lower surface roughness is preferable for cooling, due to the adhesiveness of the sheet and the rolls. That is to say that a mirror finish is preferred over a satin-finish. In the mirror finish, the entire surface of the sheet is uniformly contacted with such rolls, achieving uniform cooling along the TD direction. For a satin-finish, although the contact area with the surface of such a roll is large, the sheet cannot be firmly attached to the roll surface, resulting in poor cooling efficiency. Another issue that arises results from the trapped air that may occur in a contact area between the molten resin and the roll surface.

[0036] Conversely, from the viewpoint of operation stability, moderately higher surface roughness is preferable. In a wet method of manufacturing microporous polyolefin films, a portion of the solvent may bleed out on the surface of the extrudate. This solvent bleed-out may accumulate between the sheet and the chill roll surface, causing slip or other undesirable movement of the sheet over the same roll surface. When the slip or movement occurs, cooling efficiency worsens and distortion and warping of the sheet occur, thereby negatively impacting the properties of the microporous film. In other words, the roll surface cannot provide enough friction to hold the sheet, thereby resulting in a meandering of the extrudate. Moreover, a satin- finish can provide appropriate asperity on the roll surface, resulting in easier removal of the excess film forming solvent interposed between such roll surface and the sheet. Additionally, the roughness of the satin finish provides an air escape path between the surface of the extrudate and the roll surface that may serve to prevent occurrence of the air trap.

[0037] However, when a mirror finish is applied to the entire surface of the sheet, a weak grip results, causing slippage and meandering of the extrudate. Although a satin-finished roll prevents the occurrence of slippage, when the satin-finish is used for the upstream roll or rolls, the patterns of the roll surface are transferred onto the sheet, resulting in the formation of uneven surface finishes of the resultant films or sheets, and a decrease in film thickness uniformity.

[0038] The chill roll assemblies disclosed herein overcome these issues by providing at least one roll having mirror-finished surface for the initial cooling stage, and at least one roll having a satin-finished surface for the latter stage. Specifically, a significant amount but less than all of the cooling solidification process is conducted using the upstream chill roll, which is substantially mirror finished, to ensure smoothness on the surface of the extrudate. It is generally desired to cool the extrudate from the temperature of the extrudate at the downstream end of the extrusion die (generally at or near the die lip) "T_d" until the extrudate reaches its gelation temperature (i.e., the temperature at which the extrudate sheet begins to gel) "T_g" or lower. In one form, the average temperature T of the extrudate following the upstream roll is T_g or lower (cooler). [0039] The chill roll assembly has at least two rolls, e.g., the upstream roll and the downstream roll. The extrudate is cooled at the temperature T following the upstream roll and the temperature T_f following the downstream roll. This temperature reduction (T_d - T_f) can be represented by the parameter ΔT_d-_f. In one form, the average temperature T of the extrudate following the upstream roll is in the range of the average temperature of the extrudate at the downstream end of the extrusion die minus KΔTd-_f, where K is a multiplicative constant in the range of 50% to 95%, or 50% to 85%, or 55% to 75% where the average temperature of the extrudate following the upstream roll is Tg or lower (colder). In one form, the downstream roll is driven in rotation by rotation means to translate the extrudate through the cooling assembly, driving the upstream roll in rotation is optional. In another form, both the upstream and downstream rolls are driven in rotation. While not wishing to be bound by any theory or model, it is believed that using the upstream roll with a relatively smooth surface to transfer a significant amount of heat from the extrudate results in suitably smooth extrudate surfaces. Moreover, using the downstream roll having a relatively rough surface to transfer a relatively smaller amount of heat away from the extrudate (generally less than 50% of the heat transferred), the amount of trapped air and bled solvent is reduced and appropriate friction is present to translate the extrudate, resulting in greater extrudate thickness uniformity which leads to greater membrane thickness uniformity. [0040] In another form, the chill roll assembly comprises the upstream roll, the downstream roll and an intermediate third roll, i.e., the third roll is located between the upstream roll and downstream roll. This embodiment differs from the previous embodiment in that extrudate cooling by the downstream (and optional fourth and fifth rolls) is optional. In other words, all of the extrudate cooling can occur by way of the upstream and third rollers. In this case, the average temperature of the extrudate translating away from the third roll is T_g or colder. This is beneficial, because, as shown in Fig. 2, both faces of the extrudate can be cooled by relatively smooth rolls, resulting in a further increase in thickness uniformity over the previous embodiment. Moreover, since solidification (extrudate gellation) is substantially complete before the extrudate contacts the downstream roll, the accumulation of air and excess solvent or diluent is reduced, which allows improved synchronization between the speed of the extrudate sheet and the roll to prevent extrudate slippage and meandering. Like the previous embodiment, though, driving the rolls (in rotation) that are upstream of the downstream roll is optional. Likewise, driving the fourth and fifth rolls (when used) in rotation is optional. [0041] Referring again to FIGS. 2 and 3, chill rolls 104 and 108 are mirror- finished to a surface roughness of approximately 0.6 s, while chill rolls 112, 122 and 126 are satin-finished to a surface roughness of approximately 8 s to 10 s. Advantageously, to achieve a surface roughness level of 0.6 s, equivalent to the specification for guide rolls, no special grinding is required. A satin-finish of 10 s provides a sufficient oil escape path, while also providing a sheet having excellent surface properties.

[0042] The films and sheets disclosed herein find particular utility in the critical field of battery separators, e.g., in lithium ion primary and secondary batteries. Such batteries are useful as power sources for, e.g., electric vehicles and hybrid electric vehicles. The films and sheets disclosed herein provide a good balance of key properties, including improved surface smoothness and thickness uniformity. [0043] While the focus of the chill roll assembly described hereinabove has been with respect to the production of monolayer films and sheets, it is within the scope of this disclosure to provide multilayer coextruded or laminated films and sheets, as those skilled in the art can plainly understand. [0044] Representative starting materials having utility in the production of the afore-mentioned films and sheets will now be described. As will be appreciated by those skilled in the art, the selection of a starting material is not critical. In one form, the starting material contains polyethylene. In another form, the starting materials contain a first polyethylene ("PE-I") having an Mw value of less than about 5 x 10⁶ or a second polyethylene ("UHMWPE-I") having an Mw value of at least about 1 x 10⁶. In one form, the starting materials can contain a first polypropylene ("PP-I"). In one form, the starting materials comprise one of (i) a polyethylene (PE), (ii) an ultra high molecular weight polyethylene (UHMWPE), (iii) PE-I and PP-I, or (iv) PE-I, UHMWPE-I, and PP-I. [0045] In one form of the above (ii) and (iv), UHMWPE-I can have an Mw in the range of from about 1 x 10⁶ to about 15 x 10⁶ or from about 1 x 10⁶ to about 5 x 10⁶ or from about 1 x 10⁶ to about 3 x 10⁶, and contain about 0 wt.% to about 40 wt.%, or about 1 wt.% to about 30 wt.%, or about 1 wt.% to 20 wt.%, on the basis of total amount of PE-I and UHMWPE-I in order to obtain a film or sheet having a hybrid structure defined in the later section, and can be at least one of homopolymer or copolymer. In one form of the above (iii) and (iv), PP-I can be at least one of a homopolymer or copolymer, or can contain no more than about 50 wt. %, on the basis of the total amount of the microporous film or sheet material. In one form, the Mw of polyolefin in the microporous film or sheet material can have about 1.5 x 10⁶ or less, or in the range of from about 1.0 x 10⁵ to about 2.0 x 10⁶ or from about 2.0 x 10⁵ to about 1.5 x 10 in order to obtain a microporous film or sheet having a hybrid structure defined in the later section. In one form, PE-I can have an Mw ranging from about 1 x 10⁴ to about 5 x 10⁵, or from about 2 x 10⁵ to about 4 x 10⁵, and can be one or more of a high-density polyethylene, a medium-density polyethylene, a branched low-density polyethylene, or a linear low-density polyethylene, and can be at least one of a homopolymer or copolymer.

[0046] In one form, the polypropylene can be, for example, one or more of (i) a propylene homopolymer or (ii) a copolymer of propylene and a fifth olefin. The copolymer can be a random or block copolymer. The fifth olefin can be, e.g., one or more of α-olefins such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, and styrene, etc.; and diolefins such as butadiene, 1, 5-hexadiene, I₅ 7-octadiene, 1, 9-decadiene, etc. The amount of the fifth olefin in the copolymer may be in a range that does not adversely affect the properties of the microporous membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc. For example, the amount of the fifth olefin can be less than 10% by mol, based on 100% by mol, of the entire copolymer. Optionally, the polypropylene has one or more of the following properties: (i) the polypropylene has an Mw ranging from about 1 x 10⁴ to about 4 x 10⁶, or about 3 x 10⁵ to about 3 x 10⁶, or about 6 x lO⁵ to about 1.5 x 10⁶, (ii) the polypropylene has an Mw/Mn ranging from about 1.01 to about 100, or about 1.1 to about 50, or about 3 to about 30; (iii) the polypropylene's tacticity may be isotactic; (iv) the polypropylene may have a heat of fusion of at least about 90 Joules/gram or about 100 J/g to 120 J/g; (v) the polypropylene may have a melting peak (second melt) of at least about 160⁰C, (vi) the polypropylene may have a Trouton's ratio of at least about 15 when measured at a temperature of about 230°C and a strain rate of 25 sec^"1; and/or (vii) the polypropylene may have an elongational viscosity of at least about 50,000 Pa sec at a temperature of 230⁰C and a strain rate of 25 sec^"1. Optionally, the polypropylene has an Mw/Mn ranging from about 1.01 to about 100 or from about 1.1 to about 50. [0047] In one form, the microporous film or sheet has a hybrid structure, which is characterized by a pore size distribution exhibiting relatively dense domains having a main peak in a range of 0.01 μm to 0.08 μm and relatively coarse domains exhibiting at least one sub-peak in a range of more than 0.08 μm to 1.5 μm or less in the pore size distribution curve. The ratio of the pore volume of the dense domains (calculated from the main peak) to the pore volume of the coarse domains (calculated from the sub-peak) is not critical, and can range, e.g., from about 0.5 to about 49.

[0048] The microporous film or sheet material can optionally contain one or more additional polyolefins, identified as the seventh polyolefin, which can be, e.g., one or more of polybutene-1, polypentene-1, poly-4-methylpentene-l, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethylene α-olefin copolymer (except for an ethylene-propylene copolymer) and can have an Mw in the range of about 1 x 10⁴ to about 4 x 10⁶. In addition to or besides the seventh polyolefin, the microporous film or sheet material can further comprise a polyethylene wax, e.g., one having an Mw in the range of about 1 x 10³ to about 1 x 10⁴.

[0049] In one form, a process for producing a monolayer microporous membrane is provided. The process includes the steps of combining a polymer (e.g., polyolefin) and a diluent to form a polymeric solution. The polymer can be a polyolefin composition comprising at least a first polyethylene having a crystal dispersion temperature (T_cd) and optionally polypropylene. The polyolefin solution can be extruded through an extrusion die to form an extrudate. These steps can be conducted under conventional conditions using conventional starting materials, as described, e.g., in U.S. Patent No. 5,051,183 and in U.S. Patent No. 6,096,213, which are incorporated by reference herein in their entirety. Following extrusion, the extrudate is cooled by, transferring heat from the extrudate through a plurality of chill rolls to form a cooled extrudate, the plurality of chill rolls comprising i) an upstream chill roll positioned to contact and receive the extrudate, the upstream chill roll having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601 (Rmax); ii) an optional third chill roll positioned so as to contact and receive the extrudate from the upstream chill roll, the third chill roll having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601 (Rmax), and iii) a downstream chill roll positioned so as to contact and receive the extrudate from the preceding chill roll, the downstream chill roll having an external surface roughness of > 5.0 s, as determined in accordance with JIS B 0601 (Rmax), orienting the cooled extrudate in at least one direction by about two to about 400 fold at a temperature of about T_Cd to T_m+10°C, and removing at least a portion of the solvent or diluent from the cooled extrudate to form a membrane.

[0050] In particular forms of the processes described herein, the upstream chill roll has an external surface roughness of ranging from 0.2 to 0.8 s, as determined in accordance with JIS B 0601(Rmax), more particularly ranging from 0.5 to 0.7 s, as determined in accordance with JIS B 0601(Rmax).

[0051] In other forms of the processes described herein, the upstream chill roll has an external surface roughness of ranging from 0.2 to 0.8 s, as determined in accordance with JIS B 0601(Rmax), more particularly ranging from 0.5 to 0.7 s; and the optional third chill roll is positioned so as to contact and receive the extrudate from the upstream chill roll. The third chill roll has an external surface roughness of 0.2 to 0.8 s, as determined in accordance with JIS B 0601(Rmax), more particularly ranging from 0.5 to 0.7 s, as determined in accordance with JIS B 0601(Rmax). [0052] In still other forms of the processes described herein, the upstream chill roll has an external surface roughness of ranging from 0.2 to 0.8 s, as determined in accordance with JIS B 0601(Rmax), more particularly ranging from 0.5 to 0.7 s, as determined in accordance with JIS B 0601(Rmax); and the downstream chill roll has a surface roughness ranging from 7 to 15 s, more particularly ranging from 8.5 to 12 s, as determined in accordance with JIS B 0601(Rmax). [0053] In one form, the microporous membrane is a two-layer membrane. In another form, the microporous membrane has at least three layers. Such membranes and production methods are described, for example, in PCT Patent Application WO 2008/016174, which is incorporated by reference in its entirety. For the sake of brevity, the production of the microporous membrane will be mainly described in terms of two- layer and three-layer membranes, although those skilled in the art will recognize that the same techniques can be applied to the production of monolayer membranes or membranes having at least four layers.

[0054] In one form, the three-layer microporous membrane comprises first and third microporous layers constituting the outer layers of the microporous polymeric membrane and a second layer situated between (and optionally in planar contact with) the first and third layers. In another form, the first and third layers are produced from the first polymeric mixture and the second (or inner) layer is produced from the second polymeric mixture. In another form, the first and third layers are produced from the second polymeric mixture and the second layer is produced from the first polymeric mixture.

[0055] A first method for producing a multi-layer membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a diluent to prepare a first polyolefin mixture, (2) combining a second polyolefin composition and a second diluent to prepare a second polyolefin mixture, (3) extruding the first and second polyolefin mixtures through at least one die to form an extrudate, (4) transferring heat from the extrudate through a plurality of chill rolls to form a cooled extrudate, e.g., a multi-layer, gel-like sheet, the plurality of chill rolls comprising i) an upstream chill roll positioned to contact and receive the extrudate, the first chill roll having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601(Rmax); ii) a third chill roll positioned so as to contact and receive the extrudate from the first chill roll, the third chill roll having an external surface roughness of < 1.0 s, as determined in accordance with JIS B 0601(Rmax), and iii) a downstream chill roll positioned so as to contact and receive the extrudate from the second chill roll, the downstream chill roll having an external surface roughness of > 5.0 s, as determined in accordance with JIS B 0601(Rmax), (5) removing at least a portion of the diluent from the multi-layer, sheet to form the microporous membrane, and optionally removing at least a portion of any volatile species from the multi-layer, microporous membrane. An optional stretching step (7), and an optional hot solvent treatment step (8), etc. can also be conducted between steps (4) and (5), if desired. After step (6), an optional step (9) of stretching a multi-layer, microporous membrane, an optional heat treatment step (10), an optional cross-linking step with ionizing radiation (11), and an optional hydrophilic treatment step (12), etc., can be conducted if desired. The order of the optional steps is not critical. [0056] In the first step of the process, polyolefin (e.g., a composition of two or more polyolefin species optionally containing other non-polyolefin or non-polymeric species) generally in the form of polyolefin resins as described above are combined, e.g., by dry mixing or melt blending with an appropriate process solvent or diluent to produce the first polyolefin mixture. Optionally, the first polyolefm mixture (which can be described as a mixture, slurry, etc.) can contain various additives such as one or more antioxidant, fine silicate powder (pore-forming material), etc., provided these are used in a concentration range that does not significantly degrade the desired properties of the multi-layer, microporous membrane.

[0057] The first diluent which can be a "membrane-forming solvent" or "process solvent" is typically liquid at room temperature, though this is not required. Conventional diluents can be used, such as those described in WO 2008/016174. In another form, the resins, etc., used to produce to the first polyolefin composition are dry mixed or melt-blended in, e.g., a double screw extruder or mixer before they are combined with the diluent. Conventional mixing, melt-blending, dry mixing, etc. conditions can be used, such as those described in WO 2008/016174. In one form, the first diluent is liquid paraffin. [0058] The amount of the first polyolefin composition in the first polyolefin mixture is not critical. In one form, the amount of first polyolefin composition in the first polyolefin mixture can range from about 1 wt.% to about 75 wt.%, based on the weight of the polyolefin mixture, for example from about 20 wt.% to about 70 wt.%. The amount of the first polyethylene resin in the first polyolefin mixture is not critical, and can be, e.g., 1-50% by mass, or 20-40% by mass, per 100% by mass of the first polyolefin mixture.

[0059] The second polyolefin mixture can be prepared by the same methods used to prepare the first polyolefin mixture. The second diluent can be selected from among the same diluents as the first diluent. And while the second diluent can be (and generally is) selected independently of the diluent, the second diluent can be the same as the first diluent, and can be used in the same relative concentration as the first diluent is used in the first polyolefin mixture.

[0060] The second polyolefin composition is generally selected independently of the first polyolefin composition. The second polyolefin composition comprises the second polyethylene resin and the second polypropylene resin. [0061] In another form, the method for preparing the second polyolefm mixture differs from the method for preparing the first polyolefin mixture, only in that the mixing temperature may be in a range from the melting point (T_m2) of the second polypropylene to T₀₁₂ + 90°C, and that the amount of polyolefin composition in the second polyolefin solution is in the range of 1-50% by mass, or 20-40% by mass, based on the mass of the second polyolefin solution.

[0062] In another form, the first polyolefin mixture is conducted from a first extruder to a first die and the second polyolefin mixture is conducted from a second extruder to a second die. A layered extrudate in sheet form (i.e., a body significantly larger in the planar directions than in the thickness direction) can be extruded from the first and second die. Optionally, the first and second polyolefin mixtures are co- extruded from the first and second die with a planar surface of a first extrudate layer formed from the first polyolefin mixture in contact with a planar surface of a second extrudate layer formed from the second polyolefin mixture. A planar surface of the extrudate can be defined by a first vector in the machine direction of the extrudate and a second vector in the transverse direction of the extrudate.

[0063] In another form, a die assembly is used where the die assembly comprises the first and second die, as for example when the first die and the second die share a common partition between a region in the die assembly containing the first polyolefin mixture and a second region in the die assembly containing the second polyolefin mixture.

[0064] In another form, a plurality of dies is used, with each die connected to an extruder for conducting either the first or second polyolefin mixture to the die. For example, in one form, the first extruder containing the first polyolefin mixture is connected to a first die and a third die and a second extruder containing the second polyolefin mixture is connected to a second die. As is the case in the preceding form, the resulting layered extrudate can be co-extruded from the first, second, and third die (e.g., simultaneously) to form a three-layer extrudate comprising a first and a third layer constituting surface layers (e.g., top and bottom layers) produced from the first polyolefin mixture; and a second layer constituting a middle or intermediate layer of the extrudate situated between and in planar contact with both surface layers, where the second layer is produced from the second polyolefm mixture.

[0065] In yet another form, the same die assembly is used but with the polyolefm mixtures reversed, i.e., the second extruder containing the second polyolefm mixture is connected to the first die and the third die, and the first extruder containing the first polyolefin mixture is connected to the second die.

[0066] In any of the preceding forms, die extrusion can be conducted using conventional die extrusion equipment, e.g., those disclosed in WO 2008/016174. [0067] While the extrusion has been described in terms of forms producing two and three-layer extrudates, the extrusion step is not limited thereto. For example, a plurality of dies and/or die assemblies can be used to produce multi-layer extrudates having four or more layers using the extrusion methods of the preceding forms. In such a layered extrudate, each surface or intermediate layer can be produced using either the first polyolefin mixture and/or the second polyolefin mixture in patterns such as A/B, A/B/A, B/ AfB, A/B/A/B, B/A/B/A/B, etc., where "A" represents layers formed from the first polyolefin solution and "B" represents layers formed from the second polyolefin solution.

[0068] The multi-layer extrudate can be formed into a multi-layer, gel-like sheet by cooling, for example, using the chill roll system of the invention. Cooling rate and cooling temperature are not particularly critical. For example, the multi-layer, gel-like sheet can be cooled at a cooling rate of at least about 50°C/minute until the temperature of the multi-layer, gel-like sheet (the cooling temperature) is approximately equal to the multi-layer, gel-like sheet's gelation temperature (or lower). In another form, the extrudate is cooled to a temperature of about 100°C or lower in order to form the multi- layer gel-like sheet. While not wishing to be bound by any theory or model, it is believed that cooling the layered extrudate sets the polyolefm micro-phases of the first and second polyolefin mixtures for separation by the membrane-forming solvent or solvents. It has been observed that in general a slower cooling rate (e.g., less than 50°C/minute) provides the multi-layer, gel-like sheet with larger pseudo-cell units, resulting in a coarser higher-order structure. On the other hand, a relatively faster cooling rate (e.g., 80°C/minute) results in denser cell units. Although it is not a critical parameter, when the cooling rate of the extrudate is less than 50°C/minute, increased polyolefm crystallinity in the layer can result, which can make it more difficult to process the multi-layer, gel-like sheet in subsequent stretching steps. The choice of cooling method is not critical. For example conventional sheet cooling methods can be used.

[0069] In another form, at least a portion of the first and second diluents are removed (or displaced) from the multi-layer gel-like sheet in order to form a solvent- removed gel-like sheet. A displacing (or "washing") solvent can be used to remove (wash away, or displace) the first and second diluents. Conventional washing solvent and washing techniques can be used, e.g., those described in WO 2008/016174. [0070] While the amount of diluent removed is not particularly critical, generally a higher quality (more porous) membrane will result when at least a major amount of first and second diluent is removed from the gel-like sheet. In another form, the diluent is removed from the gel-like sheet (e.g., by washing) until the amount of the remaining diluent in the multi-layer gel-like sheet becomes less than 1 wt.%, based on the weight of the gel-like sheet.

[0071] In another form, the solvent-removed multi-layer, gel-like sheet is dried in order to remove at least a portion of the volatile species in the sheet, e.g., remaining washing solvent. Any method capable of removing a portion of the washing solvent can be used, including conventional methods such as heat-drying, wind-drying (moving air), etc. as described in WO 2008/016174.

[0072] Although it is not critical, drying can be conducted until the amount of remaining washing solvent is about 5 wt. % or less on a dry basis, i.e., based on the weight of the dry multi-layer, microporous polyolefm membrane. In another form, drying is conducted until the amount of remaining washing solvent is about 3 wt. % or less on a dry basis. Insufficient drying can be recognized because it generally leads to an undesirable decrease in the porosity of the multi-layer, microporous membrane. If this is observed, an increased drying temperature and/or drying time should be used. Removal of the washing solvent, e.g., by drying or otherwise, results in the formation of the multi-layer, microporous polyolefin membrane.

[0073] Prior to the step for removing the first and second diluents (namely prior to step 5), the multi-layer, gel-like sheet can be stretched in order to obtain a stretched, multi-layer, gel-like sheet. It is believed that the presence of the first and second diluents in the multi-layer, gel-like sheet results in a relatively uniform stretching magnification. Heating the multi-layer, gel-like sheet, especially at the start of stretching or in a relatively early stage of stretching (e.g., before 50% of the stretching has been completed) is also believed to aid the uniformity of stretching. [0074] Neither the choice of stretching method nor the degree of stretching magnification is particularly critical, and conventional stretching methods can be used, such as those described in WO 2008/016174.

[0075] The stretching magnification is not critical. In another form where monoaxial stretching is used, the linear stretching magnification can be, e.g., about 2 fold or more, or about 3 to about 30 fold. In another form where biaxial stretching is used, the linear stretching magnification can be, e.g., about 3 fold or more in any planar direction. In another form, the area magnification resulting from stretching is at least about 9 fold, or at least about 16 fold, or at least about 25 fold. Although it is not a critical parameter, when the stretching results in an area magnification of at least about 9 fold, the multi-layer microporous polyolefin membrane has relatively higher pin puncture strength. When attempting an area magnification of more than about 400 fold, it can be more difficult to operate the stretching apparatus.

[0076] The temperature of the multi-layer, gel-like sheet during stretching

(namely the stretching temperature) is not critical. In another form, the temperature of the gel-like sheet during stretching can be about (T_m + 10⁰C) or lower, or optionally in a range that is higher than the crystal dispersion temperature T_ccj of the polyethylene but lower than Tm, wherein T_m is the lesser of the melting point T_ml of the first polyethylene and the melting point T_m2 of the second polyethylene (when used). Although this parameter is not critical, when the stretching temperature is higher than approximately the melting point T_m + 10°C, at least one of the first or second polyethylenes can be in the molten state, which can make it more difficult to orient the molecular chains of the polyolefin in the multi-layer gel-like sheet during stretching. And when the stretching temperature is lower than approximately T_Cd, at least one of the first or second polyethylenes can be so insufficiently softened that it is difficult to stretch the multi-layer, gel-like sheet without breakage or tears, which can result in a failure to achieve the desired stretching magnification. In another form, the stretching temperature ranges from about 90⁰C to about 140⁰C or from about 100⁰C to aboutl30°C. [0077] Although it is not required, the multi-layer, gel-like sheet can be treated with a hot solvent between steps (4) and (5) as described in WO 2008/016174 and in WO 2000/20493.

[0078] In another form, the dried multi-layer, microporous membrane of step (6) can be stretched, at least monoaxially. Biaxial stretching can be used, and the amount of stretching along each axis need not be the same. The stretching method selected is not critical, and conventional stretching methods can be used such as by a tenter method, etc. While it is not critical, the membrane can be heated during stretching. While the choice is not critical, the stretching can be monoaxial or biaxial. When biaxial stretching is used, the stretching can be conducted simultaneously in both axial directions, or, alternatively, the multi-layer, microporous polyolefin membrane can be stretched sequentially, e.g., first in the machine direction and then in the transverse direction. In another form, simultaneous biaxial stretching is used. When the multilayer gel-like sheet has been stretched as described in step (7) the stretching of the dry multi-layer, microporous polyolefin membrane in step (9) can be called dry-stretching, re-stretching, or dry-orientation. Conventional stretching techniques and conditions can be used, e.g., those described in WO 2008/016174.

[0079] The temperature of the dry multi-layer, microporous membrane during stretching (the "dry stretching temperature") is not critical. In another form, the dry stretching temperature is approximately equal to the melting point T_m or lower, for example in the range of from about the crystal dispersion temperature T_Cd - 30⁰C to the about the melting point T_m. When the dry stretching temperature is higher than T_m, it can be more difficult to produce a multi-layer, microporous polyolefin membrane having relatively high air permeability characteristics, particularly in the transverse direction when the dry multi-layer, microporous polyolefin membrane is stretched transversely. When the stretching temperature is lower than T_Cd - 30⁰C₅ it can be more difficult to sufficiently soften the first and second polyolefins, which can lead to tearing during stretching, and a lack of uniform stretching. In another form, the dry stretching temperature ranges from about 60⁰C to about 135°C or from about 90⁰C to aboutl30°C. [0080] In another form, the dried multi-layer, microporous membrane can be heat-treated following step (6). Conventional heat treatments such as heat set and annealing can be used, as described in WO 2008/016174.

[0081] In another form, the multi-layer, microporous membrane can be subjected to a hydrophilic treatment (i.e., a treatment which makes the multi-layer, microporous membrane more hydrophilic). The hydrophilic treatment can be, for example, a monomer-grafting treatment, a surfactant treatment, a corona-discharging treatment, etc. In another form, the monomer-grafting treatment is used after the cross-linking treatment.

[0082] When a surfactant treatment is used, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used, for example, either alone or in combination. In another form, a nonionic surfactant is used. The choice of surfactant is not critical. For example, the multi-layer, microporous membrane can be dipped in a mixture of the surfactant and water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, etc., or coated with the mixture, e.g., by a doctor blade method. [0083] A second method for producing the multi-layer, microporous membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a first diluent to prepare a first polyolefin mixture, (2) combining a second polyolefin composition and a second diluent to prepare a second polyolefin mixture, (3) extruding the first polyolefin mixture through a first die and the second mixture through a second die and then laminating the extruded first and second polyolefin mixtures to form a multi-layer extrudate, and then conducting steps 4 and 5 as previously described in connection with the first production method. Optional steps 6 through 12 can also be conducted if desired, either alone or in combination. [0084] In the second production method, laminating the first and second polyolefin mixtures is conducted before the cooling step. [0085] The process steps and conditions of the second production method are generally the same as those of the analogous steps described in connection with the first production method, except for step (3). Consequently, step (3) will be explained in more detail. [0086] The type of die used is not critical provided the die is capable of forming an extrudate that can be laminated. In one form, sheet dies (which can be adjacent or connected) are used to form the extrudates. The first and second sheet dies are connected to first and second extruders, respectively, where the first extruder contains the first polyolefin mixture and the second extruder contains the second polyolefin mixture. While not critical, lamination is generally easier to accomplish when the extruded first and second polyolefin mixture are still at approximately the extrusion temperature. The other conditions may be the same as in the first method. [0087] In another form, the first, second, and third sheet dies are connected to first, second and third extruders, where the first and third sheet dies contain the first polyolefin mixtures, and the second sheet die contains the second polyolefin mixture. In this form, a laminated extrudate is formed constituting outer layers comprising the extruded first polyolefin mixture and one intermediate comprising the extruded second polyolefin mixture.

[0088] In yet another form, the first, second, and third sheet dies are connected to first, second, and third extruders, where the second sheet die contains the first polyolefin mixture, and the first and third sheet dies contain the second polyolefin mixture. In this form, a laminated extrudate is formed constituting outer layers comprising the extruded second polyolefin mixture and one intermediate comprising extruded first polyolefin mixture. [0089] The third method for producing the multi-layer, microporous polyolefin membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a first diluent to prepare a first polyolefin mixture, (2) combining a second polyolefin composition and a second diluent to prepare a second polyolefin mixture, (3) extruding the first polyolefin mixture through at least one first die to form at least one first extrudate, (4) extruding the second polyolefin mixture through at least one second die to form at least one second extrudate, (5) transferring heat from the first and second extrudates through a plurality of chill rolls of the type described herein above to form at least one first gel-like sheet and at least one second gel-like sheet, e.g., a multi-layer, gel-like sheet, (6) removing at least a portion of the first and second diluents from the first and second gel-like sheets to form first and second membranes, (7) optionally removing at least a portion of any remaining volatile from the first and second membranes, and (8) laminating the first and second membranes in order to form the multi-layer, microporous membrane. The previously described optional steps can also be used, if desired, to further process the multi-layer microporous membrane. [0090] The steps of the third production method can generally be carried out under the relevant conditions described for the first, second, and third production methods.

[0091] In all production methods, the thickness of the layers formed from the first and second polyolefin mixture (i.e., the layers comprising the first and second microporous layer materials) can be controlled by adjusting the thickness of the first and second gel-like sheets and by the amount of stretching (stretching magnification and dry stretching magnification), when one or more stretching steps are used. Optionally, the lamination step can be combined with a stretching step by passing the gel-like sheets through multi-stages of heated rollers.

Properties of the Microporous Membrane

[0092] In one form, the microporous membrane has a thickness ranging from about 3 μm to about 200 μm, or about 5 μm to about 50 μm. In addition, the membrane generally has one or more of the following desirable properties. A. Porosity of About 25% to About 80%

B. Air Permeability of About 20 Seconds/100 cm³ to About 700 Seconds/100 cm³ as measured according to JIS P8117 (Converted to Value at 20 μm Thickness) [0093] Air permeability P₁ measured on a microporous membrane having a thickness T₁ according to JIS P8117 can be converted to air permeability P₂ at a thickness of 20 μm by the equation of P₂ = (Pj x 2OyT₁.

C. Pin Puncture Strength of About 3,000 mN/20 urn or More

[0094] The pin puncture strength (converted to the value at a 20 μm membrane thickness) is the maximum load measured when the microporous membrane is pricked with a needle 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second.

D. Tensile Strength of At Least About 60,000 kPa (measured according to according to ASTM D882) E. Tensile Elongation of At Least About 100% (ASTM D882)

F. Heat Shrinkage Ratio of 10% or Less

[0095] Heat shrinkage ratio measured after holding the microporous membrane at a temperature of about 105⁰C for 8 hours.

G. Thickness Fluctuation of 0.5 or Less [0096] The thickness of each microporous membrane can be measured by a contact thickness meter, e.g., a Litematic made by Mitsutoyo Corporation. Thickness is measured at the center across the film in TD along MD for 5 meters at 100 mm intervals and averaged. A standard deviation for the measured values equal to the square root of the variance is defined as the thickness fluctuation. When the thickness fluctuation of a battery separator exceeds 0.5, it is more difficult to produce a battery having suitable protection against internal short circuiting during manufacturing and use.

H. Melt Down Temperature of At Least About 145°C

[0097] In one form, the melt down temperature can range from about 145°C to about 19O⁰C. One way to measure melt down temperature involves determining the temperature at which a microporous membrane test piece of 3 mm in the longitudinal direction and 10 mm in the transverse direction is broken by melting, under the conditions that the test piece is heated from room temperature at a heating rate of 5°C/minute while drawing the test piece in the longitudinal direction under a load of 2 grams using a thermo-mechanical analyzer such as a TMA/SS6000 available from Seiko Instruments, Inc. Particular films may have a melt down temperature ranging from 16O⁰C to 190°C.

I. Shut Down Temperature of 140⁰C or Lower

[0098] Using a thermo-mechanical analyzer (TMA/SS6000 available from Seiko

Instruments, Inc.), a test piece of 10 mm in the transverse direction and 3 mm in the longitudinal direction is heated from room temperature at a rate of 5°C/minute while drawing the test piece in a longitudinal direction under a load of 2 g. A temperature at a point of inflection observed near the melting point is defined as the shutdown temperature. EXAMPLES Example 1

[0099] The chill roll assemblies and processes disclosed herein will be illustrated with the following example.

[00100] Dry-blended is 99.8 parts by mass of a polyolefin composition comprising 20% by mass of ultra-high-molecular-weight polyethylene (UHMWPE) having a weight-average molecular weight (Mw) of 2.0 x 10⁶, a molecular weight distribution (Mw/Mn) of 8.0, a melting point (T_m) of 135⁰C, and a crystal dispersion temperature (Ted) of 100⁰C, 80% by mass of high-density polyethylene (HDPE) having a Mw of 3.0 x 10⁵ and Mw/Mn of 8.6, T_m of 135°C, and T_cd of 100⁰C, and 0.2 parts by mass of tetrakis [methylene-3 -(3,5-ditertiary-butyl-4-hydroxyphenyI)-propionate] methane as an antioxidant. The polyolefin composition has a Mw/Mn of 8.6, a T_n, of 135°C, and

[00101] Thirty parts by mass of the resultant mixture is charged into a strong- blending double-screw extruder having an inner diameter of 58 mm and L/D of 52.5, and 70 parts by mass of liquid paraffin [50 cSt (40⁰C)] is supplied to the double-screw extruder via a side feeder. Melt-blending is conducted at 210°C and 200 rpm to prepare a first polyolefin mixture.

[00102] The Mw and Mw/Mn of each UHMWPE and HDPE are measured by a gel permeation chromatography (GPC) method under the following conditions.

Measurement apparatus: GPC-150C available from Waters Corporation,

Column: Shodex UT806M available from Showa Denko K.K.,

Column temperature: 135°C,

Solvent (mobile phase): o-dichlorobenzene,

Solvent flow rate: 1.0 ml/minute,

Sample concentration: 0.1% by weight (dissolved at 135°C for 1 hour),

Injected amount: 500

Detector: Differential Refractometer available from Waters Corp., and

Calibration curve: Produced from a calibration curve of a single- dispersion, standard polystyrene sample using a predetermined conversion constant. [00103] The polyolefin mixture is supplied from its double-screw extruder to a monolayer-sheet-forming T-die having a 250mm width at 21O⁰C, to form an extrudate. The extrudate is cooled, using a chill roll assembly having five chill rolls at a rotation speed of 1.4 m/minute, as shown in FIGS. 2 and 3 and described herein, by passing through chill rolls controlled at 15⁰C, to form a gel-like sheet. The properties of the chill rolls employed are shown in the table below.

TABLE 1

[00104] Using a tenter-stretching machine, the gel-like sheet is biaxially stretched at 115.O⁰C, to 5 fold in both machine and transverse directions.

[00105] The stretched gel-like sheet is immersed in a bath of methylene chloride controlled at a temperature of 25°C to remove the liquid paraffin. The resulting membrane is air-cooled at room temperature. The dried membrane is heat-set at 120°C for 30 seconds to produce a microporous polyolefin membrane having a 700mm width in the TD.

[00106] There is obtained a microporous membrane of polyethylene having properties as shown in Table 4.

Comparative Example 1

[00107] Example 1 is repeated except for chill rolls used. The properties of the chill rolls employed are shown in the table below.

TABLE 2

Comparative Example 2 [00108] Comparative Example 2 is repeated except for the chill rolls used. The properties of the chill rolls employed are shown in the table below.

TABLE 3

TABLE 4

* Sheet neck-in(rnm); Sheet width difference between the width on the roll 104 and 126 [00109] All patents, test procedures, and other documents cited herein, including priority documents, are fully incorporated by reference to the extent such disclosure is not inconsistent and for all jurisdictions in which such incorporation is permitted. [00110] While the illustrative forms disclosed herein have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside herein, including all features which would be treated as equivalents thereof by those skilled in the art to which this disclosure pertains.

[00111] When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

Claims

1. An assembly for transferring heat from an extrudate, said assembly comprising: a) at least one upstream roll positioned to receive the extrudate, said at least one upstream roll having an external surface roughness of < 1.0 s; and b) at least one downstream roll, said at least one downstream roll having an external surface roughness of > 5.0 s, said upstream and downstream rolls being aligned so that said downstream roll receives the extrudate from said upstream roll.

2. The assembly of claim I₅ further comprising at least one support frame wherein said upstream and downstream rolls are mounted on the at least one support frame, said support frame providing the alignment of said rolls for the extrudate to move in arcuate paths around a portion of said upstream roll and a portion of said downstream roll, and in a linear path between said upstream roll and said downstream roll.

3. The assembly of claim 2, further comprising a drive means mounted on said support frame for rotating said downstream roll to translate the extrudate through the roll assembly in arcuate contact with said upstream roll and said downstream roll.

4. The assembly of claim 3, wherein at least one of said upstream or downstream rolls, further comprise cooling means for the transferring of heat from the extrudate.

5. The assembly of claim 4, wherein said upstream roll is rotated by either said drive means or a second drive means synchronized with the first drive means.

6. The assembly of claim 2, further comprising a third roll, said third roll being downstream of the upstream roll and upstream of said downstream roll, and positioned so as receive and translate the extrudate from said upstream roll in an arcuate path around at least a portion of said third roll and then in a linear path toward the downstream roll, said third roll having an external surface roughness of < 1.0 s.

7. The assembly of claim 6, further comprising a fourth and a fifth roll, said fourth roll being positioned to receive the extrudate from said downstream roll, and said fifth roll being positioned to receive the extrudate from said fourth roll, said fourth and fifth rolls each having an external surface roughness of > 5.0 s.

8. The assembly of claim 7, wherein said upstream and third rolls each have an external surface roughness of about 0.6 s.

9. The assembly of claim 7, further comprising

(a) drive means for rotating said downstream roll and optionally said fourth and/or fifth roll to translate the extrudate through the roll assembly in arcuate contact with said upstream roll, said third roll, said downstream roll, said fourth roll, and said fifth roll, and optionally

(b) second drive means synchronized with said drive means for rotating said upstream roll and optionally said third roll.

10. The assembly of claim 9, wherein at least the upstream and third roll further comprise cooling means for the transferring of heat from the extrudate.

11. A process for producing a microporous membrane, comprising the steps of: a) combining polymer and at least one diluent; b) extruding the combined polymer and diluent through an extrusion die to form an extrudate; c) transferring heat from the extrudate through a plurality of rolls to form a cooled extrudate, the plurality of rolls comprising i) an upstream roll positioned to receive the extrudate, the upstream roll having an external surface roughness of < 1.0 s; and ii) a downstream roll positioned to receive the extrudate from the upstream roll, the downstream roll having an external surface roughness of > 5.0 s; d) orienting the cooled extrudate in at least one direction by about one to about ten fold; and e) removing at least a portion of the diluent from the cooled extrudate to form the membrane.

12. The process of claim 11 , further comprising the steps of: f) orienting the membrane to a magnification of from about 1.1 to about 2.5 fold in at least one direction; and g) heat-setting the membrane.

13. The process of claim 11 , wherein the polyolefin composition comprises a high density polyethylene and ultra high molecular weight polyethylene.

14. The process of claim 13, wherein the polyolefin further comprises polypropylene.

15. The process of claim 13, wherein the polyolefin comprises at least about 50 wt. % high density polyethylene and at least about 15 wt. % ultra high molecular weight polyethylene.

16. The process of claim 13, wherein the polyolefin comprises at least about 1 wt. % polypropylene.

17. The process of claim 11, wherein the polyolefin comprises i) from about 40% to about 100% of a first polyethylene, the first polyethylene having a weight- average molecular weight of from about 2 x 10⁵ to about 5 x 10⁵ and a molecular weight distribution (Mw/Mn) of from about 5 to about 50; ii) from about 5% to about 50% of a polypropylene having a weight-average molecular weight of from about 6 x 10⁵ to about 4 x 10⁶, a molecular weight distribution (Mw/Mn) of from about 3 to about 30 a heat of fusion of 90 J/g or more; and iii) from about 0% to about 40% of a second polyethylene having a weight-average molecular weight of from about 1 x 10⁶ to about 5 x 10⁶, a molecular weight distribution (Mw/Mn) of from about 3 to about 30 or more, percentages based on the mass of the polyolefin.

18. The process of claim 11, wherein the plurality of rolls includes one or more drive means associated with the downstream roll, or the downstream roll and the upstream roll, to advance the extrudate through the process.

19. The process of claim 18, wherein the plurality of rolls includes cooling means associated with the upstream roll, the downstream roll or both.

20. The process of claim 11 , wherein

(a) the plurality of rolls includes (i) a third roll located downstream of the upstream roll and upstream of the downstream roll, and positioned so as receive the extrudate from said upstream roll, the third roll having an external surface roughness of < 1.0 s, (ii) a fourth roll, and (iii) a fifth roll; the fourth roll being positioned to receive the extrudate from the downstream roll, the fifth roll being positioned to receive the extrudate from the fourth roll, the fourth and fifth rolls each having an external surface roughness of > 5.0 s; and

(b) the extrudate moves in arcuate paths around at least a portion of the upstream roll, the downstream roll, the third roll, the fourth roll, and the fifth roll, and in linear paths between the rolls.

21. The process of claim 20, wherein the plurality of rolls includes a first drive means for rotating at least one of the upstream or third rolls and a second drive means synchronized with the first drive means for rotating at least one of the downstream, fourth, or fifth rolls, to advance the extrudate through the process.

22. The process of claim 20, wherein the plurality of rolls includes cooling means associated with at least two of (i) the upstream roll, (ii) the third roll, or (iii) the downstream roll for transferring of heat away from the extrudate.

23. The process of claim 22, wherein the plurality of rolls includes cooling means associated with the fourth and/or fifth rolls.

24. The process of claim 23, wherein the upstream roll and the third roll each have an external surface roughness of about 0.6 s, as determined in accordance with JIS B 0601(Rmax).

25. The process of claim 24, wherein the downstream roll, the fourth roll and the fifth roll each have an external surface roughness of about 10 s, as determined in accordance with JIS B 0601(Rmax).