WO2010018749A1

WO2010018749A1 - Chill roll system and process for producing a microporous membrane

Info

Publication number: WO2010018749A1
Application number: PCT/JP2009/063455
Authority: WO
Inventors: Kotaro Takita; Koichi Kono; Hiroshige Kuzuno; Tetsuro Nogata
Original assignee: Tonen Chemical Corporation
Priority date: 2008-08-15
Filing date: 2009-07-22
Publication date: 2010-02-18
Also published as: JP5639578B2; JP2012500130A

Abstract

The invention relates to an assembly for transferring heat from an extrudate formed by extruding a polyolefin solution through an extrusion die. The assembly includes at least one pair of upstream rolls positioned to receive opposite surfaces of the extrudate, and at least one downstream roll, the pair of upstream rolls and the downstream roll being aligned so that the downstream roll receives the extrudate from the pair of upstream rolls. A process for producing a microporous membrane is also provided.

Description

CHILL ROLL SYSTEM AND PROCESS FOR PRODUCING A MICROPOROUS MEMBRANE

FIELD

[0001] This disclosure relates generally to a system and method for producing microporous membranes, such as those useful as battery separators.

BACKGROUND

[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 affects 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 polyolefϊn) and at least one diluent (or solvent) are combined to form a polymeric solution 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 extrudate before stretching. The reference discloses that while it is desirable to regulate cooling rate to control membrane pore diameters, cooling at a rate of less than 50°C per minute is said to result in a loss of thickness uniformity, i.e., the extrudate 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 solution 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 solution 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 then removing the washing solvent. 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 of the film or the sheet.

[0007] While improvements have been made in producing relatively uniform microporous membranes, further improvements are desired. SUMMARY [0008] In an embodiment, the invention relates to an assembly for transferring heat from a molten extrudate comprising polymers and diluent, said assembly comprising: a) first and second rolls positioned to receive opposite surfaces of the extrudate, said first roll contacting the extrudate along an arcuate path traversing first contact angle < 100° and said second roll contacting the extrudate along an arcuate path traversing a second contact angle < 100°; and b) a third roll downstream of the first and second rolls, said first, second, and third rolls being aligned so that said third roll receives the extrudate from at least one of the first and second rolls, said third roll contacting the extrudate along an arcuate path traversing a third contact angle to form a cooled extrudate. In particular embodiments, the third roll contacts the extrudate along an arcuate path of 180° or more. In other embodiments, the third roll contacts the extrudate along an arcuate path traversing a third contact angle of 90° to 180°, preferably 100° to 150°, more preferably 110° to 130°.

[0009] Since the rolls act to transfer heat away from the extrudate, they can be referred to as "chill rolls", particularly when the rolls comprise cooling means. [0010] In another aspect, the invention relates to a process for producing a microporous membrane comprising: a) combining polyolefin and diluent; b) extruding the combined polyolefin 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) first and second rolls positioned to receive opposite surfaces of the extrudate, the first roll contacting the extrudate along an arcuate path traversing a first contact angle < 100° and the second roll contacting the extrudate along an arcuate path traversing a second contact angle < 100°; and ii) a third roll downstream of the first and second rolls, the first, second, and third rolls aligned so that the third roll receives the extrudate from at least one of the first and second rolls, the third roll contacting the extrudate along an arcuate path traversing a third contact angle of > 180° to form a cooled extrudate; and d) removing at least a portion of the solvent or diluent from the cooled extrudate to form the membrane. [0011] These and other 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. 2 is a side view of a chill roll assembly for transferring heat from an extrudate formed by extruding a polyolefin solution through an extrusion die, in accordance herewith.

DETAILED DESCRIPTION

[0015] 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. When an excessive amount of extrudate cooling occurs on one surface of the extrudate before the cooling of the opposite face is achieved, a degradation in thickness uniformity of the extrudate may occur.

[0016] It has been discovered that this problem can be at least partially overcome by using one or more pairs of upstream chill rolls for receiving and contacting opposite surfaces of the extrudate from the extrusion die, along an arcuate path traversing a contact angle of 100° or less, e.g., in the range of 55° to 75°, and at least one downstream roll positioned to receive the extrudate from the upstream rolls, the downstream roll contacting the extrudate along an arcuate path traversing a contact angle that is greater than 100°, or greater than 180°, e.g., in the range of 190° to 220°. Increasing thickness uniformity of the extrudate results in improved processing and yield since, e.g., there are fewer comparatively thin regions of the film that would otherwise pose a risk of tearing during downstream stretching operations. Moreover, greater thickness uniformity of the extrudate results in a finished membrane having improved thickness uniformity, which decreases the need for slitting the membrane along the machine direction to remove abnormally thick or thin regions of the membrane.

[0017] 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. Reference is now made to the figures, wherein like numerals are used to designate like parts throughout. [0018] 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 of whether ranges are separately disclosed [0019] Reference is now made to FIGS. 1 and 2, wherein like numerals are used to designate like parts throughout.

[0020] FIG. 1 shows a system 10 for producing a microporous membrane of thermoplastic material. 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 diluent, such as paraffin oil, through a solvent feedline 16. A mixture of polymer and diluent is prepared within extruder 12 by combining the polymer and diluent with heating and mixing.

[0021] The heated mixture is then extruded into a sheet 18 from a die 20 of extruder 12. The extruded sheet 18 is cooled by an assembly for transferring heat from an extrudate 100 to a temperature lower than the gelling temperature, so that the extruded sheet 18 gels. [0022] In a form shown schematically in FIG.l, an assembly for transferring heat from an extrudate 100 includes at least one pair of upstream rolls 102 positioned to receive the extrudate 18. Optionally, each of the at least one pair of upstream rolls 102 may have a diameter of less than about 200 mm, e.g., in the range of 50 mm to 150 mm. As shown, at least one pair of upstream rolls 102 includes a first roll 104 and a second roll 106. Assembly 100 also includes at least one downstream roll 114. As may be appreciated, the at least one pair of upstream rolls 102 and the at least one downstream (or "third") roll 114 are aligned so that the at least one downstream roll 114 receives the extrudate 18 from the at least one pair of upstream rolls 102. In one form, the at least one downstream roll 114 has a diameter greater than or equal to about 250 mm, e.g., in the range of 275 mm to 400 mm. In another form, each of the at least one pair of upstream rolls 102 has a diameter < about 150 mm or < about 100 mm. [0023] Optionally, as shown schematically in FIG.l, assembly 100 may also include a second pair of upstream rolls 108. As shown, the second pair of upstream rolls 108 includes a first roll 110 and a second roll 112. The second pair of upstream rolls 108 may be positioned downstream of the at least one pair of upstream rolls 102 and upstream of the downstream roll 114. In one form, each of the second pair of upstream rolls 108 has a diameter of < about 200 mm, e.g., in the range of 50 mm to 150 mm. In another form each of the second pair of upstream rolls 108 has a diameter < about 100 mm. Although it is not required, the rolls of the first pair of upstream rolls can have equal diameters. Likewise, the rolls of the second pair of upstream rolls can have equal diameter, and the diameter is optimally the same as the diameter selected for the first pair of upstream rolls. In one form, the rolls constituting the first and second pair of upstream rolls are all of the same diameter and contact the extrudate along arcuate paths of equal contact angle.

[0024] If desired, the cooled extrudate 18' can be conducted 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. [0025] The cooled extrudate (or optionally 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 contacts the extrudate to remove at least a portion of the diluent from the extrudate. The volatile solvent containing extracted diluent (generally a nonvolatile solvent) is recovered from a solvent outflow line 32. The oriented film or sheet 18" next passes to a drying device 34, wherein at least a portion of any remaining volatile species (e.g., the volatile solvent 36) are evaporated from the oriented film or sheet 18".

[0026] 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. [0027] Referring now to FIG. 2, another form of an assembly 200 for transferring heat from an extrudate 18 formed by extruding a polyolefin solution through an extrusion die 20 is shown. Assembly 200 includes at least one support frame 220 and at least one pair of upstream rolls 202 positioned to receive the extrudate 18. Each of the at least one pair of upstream rolls 202 generally has a diameter of less than or equal to about 150 mm. As shown, the at least one pair of upstream rolls 202 includes a first roll 204 and a second roll 206 mounted on the at least one support frame 220 and positioned to contact and receive the extrudate 18. In another form each of the at least one pair of upstream rolls 102 has a diameter less than or equal to about 125 mm or less than or equal to about 100 mm.

[0028] Optionally, as shown schematically in FIG.2, assembly 200 may also include a second pair of upstream rolls 208. As shown, the second pair of upstream rolls 208 includes a first roll 210 and a second roll 212. The second pair of upstream rolls 208 may be positioned downstream of the at least one pair of upstream rolls 202 and upstream of the downstream roll 214. In one form, each of the second pair of upstream rolls 208 has a diameter of less than about 250 mm.

[0029] Assembly 200 also includes at least one downstream roll 214 mounted on the at least one support frame 220. The at least one pair of upstream rolls 202 and the at least one downstream roll 214 are aligned so that the at least one downstream roll 214 receives the extrudate 18 from the at least one pair of upstream rolls 202. In one form, the at least one downstream roll 214 has a diameter > about 250 mm. The assembly 200, as shown, may also include a second downstream roll 222 mounted on the at least one support frame 220 and positioned so as to contact and receive the extrudate 18 from the at least one downstream roll 214. Additional optional downstream rolls 224 and 226 may also be provided. In one form, the second downstream roll 214 has a diameter > about 300 mm. Other chill rolls, which are optional, can also be included as illustrated in the figure. [0030] Still referring to FIG. 2, chill roll assembly 200 further includes one or more drive motors 230 mounted on support frame 220 and associated with the at least one pair of upstream rolls 202 and the second pair of upstream rolls 208, if provided. One or more drive motors 232 may be provided to rotate the at least one downstream roll 214 and the second downstream roll 222, if provided, through the use, for example of gear box 234. One or more drive motors 236 may be provided to rotate the additional downstream rolls 224 and 226, if provided, through the use, for example of a chain drive mechanism 238, or a gear box (not shown). The drive mechanisms cause extrudate 18 to move through the assembly 200 in contact with the at least one pair of upstream rolls 202 and the at least one downstream roll 214. As shown, the drive means may include a plurality of motors 216 that drive a plurality of gears 218 through a chain and sprocket arrangement, as those skilled in the art will plainly recognize. In one form, only the at least one downstream roll 214 is driven in rotation. In another form, additional rolls may be driven. The 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.

[0031] To assist in regulating the temperature of extrudate 18 (e.g., by cooling the extrudate), assembly 200 can further include a cooling means associated with the at least one pair of upstream rolls 202 and the at least one downstream roll 214 for cooling extrudate 18. The cooling means may include a plurality of pumps (not shown) to circulate a coolant through one or more cooling circuits (not shown), the cooling circuits in fluid communication with the at least one pair of upstream rolls 202 and the at least one downstream roll 214, 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, the at least one pair of upstream rolls 202 or the at least one downstream roll 214 can comprise cooling means. While not required, the second downstream roll 222 can comprise cooling means and can be driven in rotation by drive means. [0032] As may be appreciated, in operation, extrudate 18 moves in arcuate paths around the at least one pair of upstream rolls 202 and the at least one downstream roll 214 and in linear paths between the at least one pair of upstream rolls 202 and the at least one downstream roll 214. In one form, the pair of upstream rolls have equal diameters in the range of 50 mm to 150 mm, or 75 mm to 125 mm, and the downstream roll has a diameter in the range of 275 mm to 400 mm. In this form, the opposite surfaces of the extrudate contact the upstream rolls along equal arcuate paths traversing a contact angle (as measured along the circumference of the roll from the point where the extrudate first contacts the roll to the point where the extrudate exits the roll) in the range of 55° to 75°, and at least one surface of the extrudate contacts the downstream roll along an arcuate path traversing a contact angle in the range of 120° to 250°, e.g. in the range of 190° to 220°. In another form the opposite surfaces of the extrudate contact the upstream rolls along equal arcuate paths traversing a contact angle in the range of 60° to 70°, and at least one surface of the extrudate contacts the downstream roll along an arcuate path traversing a contact angle in the range of 195° to 205°. [0033] These paths may be seen in FIG. 2, where extrudate 18 is shown as a solid line. As is conventional, the at least one pair of upstream rolls 202 can be 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 or movement of the sheet over the roll surface. In this form, a gap is established between the first upstream roll 204 and the second upstream roll 206, the gap being equal to or less than thickness of the sheet. The at least one downstream roll 214 may be provided with a relatively rough surface, to produce a relatively large frictional force capable of conveying the sheet through the apparatus 200. Consequently, the use of one or more nip rolls is optional. In one form, a gap between first upstream roll 204 and second upstream roll 206 is more than sheet thickness of 1.05 times or more, or 2 to 200 times, or 4 to 100 times. [0034] As indicated above and shown in FIG. 2, a plurality of tandemly-disposed 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, while keeping the extrudate adhered onto the entire surface of the roll. This despite the fact lower tension may be employed, thus minimizing distortion and warping of the extrudate, resulting in improved thickness uniformity in the extrudate and finished membrane. [0035] In operation, a significant amount but less than all of the cooling solidification process is conducted using the upstream chill roll. 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) "Tg" or lower. In one form, the average temperature T on the surface of the extrudate following the upstream roll is T_g or lower (cooler).

[0036] The roll assembly has three rolls, e.g., one pair of upstream rolls and the one downstream roll. The extrudate conducted away from the die lip has a surface temperature Ti in the range of 200°C to 235°C. The extrudate conducted away from the upstream rolls has a surface temperature T₂ that is cooler than T₁, with T₂ in the range of 25°C to 120°C, e.g., 65°C to 115°C. The extrudate conducted away from the downstream roll has a surface temperature T₃ that is less than T₂, with T₃ in the range of 20°C to 100°C. The temperature reduction (Ti - T₃) can be represented by the parameter ΔTi_-3. In one form, T₂ is equal to KATi_-3, where K is a multiplicative constant in the range of 40% to 95%, or 45% to 85%, or 50% to 75%. Generally, the average surface temperature of the extrudate conducted away from the pair of upstream rolls is T_c or lower (colder), where T_c is the polyethylene' s crystallization temperature. [0037] While not wishing to be bound by any theory or model, it is believed that using at least one pair of upstream cooling rolls to transfer a significant amount (50% or more) of heat from the extrudate results in suitably uniform surfaces. Moreover, using the downstream roll 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. [0038] 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. [0039] 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. [0040] 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 1 x 10⁶ or a second polyethylene ("PE-2") 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 or more of (i) PE-I (PE), (ii) PE-2, (iii) PE-I and PP-I, (iv) PE-I, PE-2, and PP-I, or (v) PE-I and PE-2.

[0041] In one form, the PE-2 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. When PE-2 is combined with PE-I, the amount of PE-2 can be in the range of 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 PE-2 in order to obtain a film or sheet having a hybrid structure as hereinafter defined. The PE-2 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 be 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 10⁶ 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 in the range of from about 1 x 10⁴ to about 9 x 10⁵, or from about 2 x 10⁵ to about 8 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. The PE-2 can be an ultra-high molecular weight polyethylene having an Mw in the range of 1 x 10⁶ to 2.5 x 10⁶, or 1.5 x 10⁶ to 2 x 10⁶. [0042] In one form, the PP-I 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, 1,7-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 10⁵ 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 160°C, (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 23O⁰C 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.

[0043] 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. [0044] 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. [0045] In one form, a process for producing a monolayer microporous membrane is provided. The process includes the steps of combining a polyolefin composition and a solvent or diluent to form a polyolefin solution, the polyolefin composition comprising at least a first polyethylene having a crystal dispersion temperature (T_Cd) and polypropylene, extruding the polyolefin solution 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. The membrane can be a monolayer membrane, for example. Following extrusion, the extrudate is cooled by transferring heat from the extrudate through a plurality of rolls to form a cooled extrudate, the plurality of rolls comprising i) at least one pair of upstream rolls positioned to receive the extrudate, each of the at least one pair of upstream rolls contacting the extrudate along an arcuate path traversing a contact angle of 100° or less, and ii) at least one downstream roll, the at least one pair of upstream rolls and the at least one downstream roll being aligned so that the at least one downstream roll receives the extrudate from the at least one pair of upstream rolls, the at least one downstream roll contacting the extrudate along an arcuate path traversing a contact angle of 180° or more, orienting the cooled extrudate in at least one direction by about two to about 400 fold at a temperature of about T_C(1 to T_m + 10°C, and removing at least a portion of diluent from the cooled extrudate to form a membrane.

[0046] In one form, the microporous polyolefin membrane is a two-layer membrane. In another form, the microporous polyolefin 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. The production of the microporous polyolefin 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 membranes or membranes having at least four layers.

[0047] In one form, the three-layer microporous polyolefin membrane comprises first and third microporous layers constituting the outer layers of the microporous polyolefin 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 polyolefin solution and the second (or inner) layer is produced from the second polyolefin solution. In another form, the first and third layers are produced from the second polyolefin solution and the second layer is produced from the first polyolefin solution.

[0048] The first method for producing a multi-layer membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and diluent to prepare a first mixture, (2) combining a second polyolefin composition and a second diluent to prepare a second mixture, (3) extruding the first and second 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) at least one pair of upstream rolls positioned to receive the extrudate, each of the at least one pair of upstream rolls contacting the extrudate along an arcuate path traversing a contact angle of 100° or less, and ii) at least one downstream roll, the at least one pair of upstream rolls and the at least one downstream roll being aligned so that the at least one downstream roll receives the extrudate from the at least one pair of upstream rolls, the at least one downstream roll contacting the extrudate along an arcuate path traversing a contact angle of 180° or more, (5) removing at least a portion of the diluent(s) from the cooled extrudate, and optionally removing any remaining volatile species to form the multi-layer, microporous membrane. An optional stretching step (6) and an optional hot solvent treatment step (7), etc. can be conducted between steps (4) and (5), if desired. After step (5), an optional step (8) of stretching a multi-layer, microporous membrane, an optional heat treatment step (9), an optional cross-linking step with ionizing radiation (10), and an optional hydrophilic treatment step (11), etc., can be conducted if desired. The order of the optional steps is not critical. [0049] In the first step of the process, polyolefin (e.g., a composition of at least one 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 diluent (e.g., a solvent such as liquid paraffin) to produce the first mixture. Optionally, the first mixture (which can be described as a solution, 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. In one form, the first polyolefin composition contains PE-I, and optionally PE-2 and/or PP-I. For example, the amount of PE-I in the first polyolefin composition can be in the range of from 1 wt.% to 100 wt.%, the amount of PE-2 in the first polyolefin composition can be in the range of from 0 wt.% to 99 wt.%, and the amount of PP-I in the first polyolefin composition can be in the range of from 0 wt.% to 80 wt.%, based on the weight of the first polyolefin composition. In the second polyolefin composition, for example, the amount of PE-I can be in the range of from 1 wt.% to 100 wt.%, the amount of PE-2 can be in the range of from 0 wt.% to 99 wt.%, and the amount of PP-I can be in the range of 0 wt.% to 80 wt.%, based on the weight of the second polyolefin composition.

[0050] The first and second diluent may be a solvent for polyolefin, e.g., a solvent that is liquid at room temperature. Conventional solvents can be used, such as those described in WO 2008/016174. In another form, the resins, etc., used to produce 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 solvent or diluent. Conventional mixing, melt-blending, dry mixing, etc. conditions can be used, such as those described in WO 2008/016174. [0051] The amount of the first polyolefin composition in the first polyolefin solution is not critical. In another form, the amount of first polyolefin composition in the first mixture can range from about 1 wt.% to about 75 wt.%, based on the weight of the polyolefin solution, for example from about 20 wt.% to about 70 wt.%. The amount of the first polyethylene in the first 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 mixture.

[0052] The second mixture can be prepared by the same methods used to prepare the first 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 first 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 mixture.

[0053] The second polyolefin composition is generally selected independently of the first polyolefin composition. The second polyolefin composition can comprise, e.g., the second polyethylene and/or the second polypropylene resin.

[0054] In another form, the first mixture is conducted from a first extruder to a first die and the second 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 mixtures are co-extruded from the first and second die with a planar surface of a first extrudate layer formed from the first mixture in contact with a planar surface of a second extrudate layer formed from the second 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. [0055] 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 mixture and a second region in the die assembly containing the second mixture. [0056] In another form, a plurality of dies is used, with each die connected to an extruder for conducting either the first or second mixture to the die. For example, in one form, the first extruder containing the first mixture is connected to a first die and a third die and a second extruder containing the second 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 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 mixture. [0057] In yet another form, the same die assembly is used but with the mixtures reversed, i.e., the second extruder containing the second mixture is connected to the first die and the third die, and the first extruder containing the first mixture is connected to the second die.

[0058] In any of the preceding forms, die extrusion can be conducted using conventional die extrusion equipment, e.g., those disclosed in WO 2008/016174. [0059] 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 mixture and/or the second mixture.

[0060] The multi-layer extrudate can be formed into a cooled extrudate, e.g., a multi-layer, gel-like sheet, by cooling, for example. Cooling rate and cooling temperature are not particularly critical. In one form, the multi-layer, gel-like sheet can be cooled at a cooling rate of at least about 10°C/minute until the temperature of the multi-layer, gel-like sheet (the cooling temperature) is approximately equal to the multilayer, 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. [0061] The cooling can be accomplished using the apparatus described in any of the preceding embodiments, e.g., a first pair (and optionally a second pair) of upstream chill rolls of equal diameter, with each roll in the pair contacting the extrudate along an arcuate path traversing a contact angle of 100° or less; and a downstream chill roll contacting the extrudate along an arcuate path traversing a contact angle of 180° or more. In one form, 50% or more of the cooling of the extrudate is accomplished before the extrudate contacts the downstream roll.

[0062] Following cooling, 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 microporous membrane. A displacing (or "washing") solvent can be used to remove (wash away, or displace) the first and second diluent. Conventional washing solvent and washing techniques can be used, e.g., those described in WO 2008/016174. [0063] 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 diluents are removed from the gel-like sheet. In another form, the membrane-forming solvent is removed from the gel-like sheet (e.g., by washing) until the amount of the remaining diluent in the microporous membrane sheet becomes less than 1 wt.%, based on the weight of the gel-like sheet.

[0064] In another form, the membrane obtained by removing at least a portion of the diluent is dried in order to remove the washing solvent. Any method capable of removing the washing solvent can be used, including conventional methods such as heat-drying, wind-drying (moving air), etc. as described in WO 2008/016174.

[0065] 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 microporous 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 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 microporous membrane. [0066] Prior to the step for removing the diluent, the multi-layer, gel-like sheet can be optionally 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.

[0067] The choice of stretching method, the degree of stretching magnification, and the process conditions (temperature, etc.) during stretching are not particularly critical, and conventional stretching methods can be used, such as those described in WO 2008/016174. Since stretching results in orienting the polymer in the gel-like sheet, stretching can also be referred to as "orientation". [0068] While not wishing to be bound by any theory or model, it is believed that such stretching causes cleavage between polyethylene lamellas, making the polyethylene phases finer and forming large numbers of fibrils. The fibrils form a three-dimensional network structure (three-dimensionally irregularly connected network structure). Consequently, the stretching when used generally makes it easier to produce a relatively high-mechanical strength multi-layer, microporous polyolefin membrane with a relatively large pore size. Such multi-layer, microporous membranes are believed to be particularly suitable for use as battery separators. [0069] Although it is not required, the multi-layer, gel-like sheet can be treated with a hot solvent as described in WO 2008/016174 and in WO 2000/20493.

[0070] In another form, the dried multi-layer, microporous membrane of step (6) can be optionally 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 (6) 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. [0071] In another form, the dried multi-layer, microporous membrane can be heat-treated following step (5). Conventional heat treatments such as heat set and annealing can be used, as described in WO 2008/016174.

[0072] In another form, the multi-layer, microporous polyolefin membrane can be subjected to a hydrophilic treatment (i.e., a treatment which makes the multi-layer, microporous polyolefin 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.

[0073] 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 polyolefin membrane can be dipped in a solution of the surfactant and water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, etc., or coated with the solution, e.g., by a doctor blade method.

Structure, properties, and composition of microporous membrane

Structure

[0074] The thickness of the final membrane is generally in the range of 3 μm or more. For example, the membrane can have a thickness in the range of from about 3 μm to about 300 μm, e.g., from about 5 μm to about 50 μm. The thickness of the microporous membrane can be measured, e.g., by a contact thickness meter at 1 cm longitudinal intervals over the width of 10 cm, and then averaged to yield the membrane thickness. Thickness meters such as the Litematic available from Mitsutoyo Corporation are suitable. Non-contact thickness measurement methods are also suitable, e.g. optical thickness measurement methods.

[0075] The microporous membrane exhibits a standard deviation of thickness values 1 micron or less, for example from about 0.25 microns to about 0.75 microns, making it a superior battery separator, especially for lithium ion batteries. [0076] It has been discovered that the membranes produced by prior art processes, exhibit significant thickness variation along the membrane's TD. Some of this thickness variation results from deformities along the edge of the membrane from, e.g., the tenter clips gripping the membrane during processing. The thickness variation also results from variations in the thickness of the cooled extrudate. Selvage is cut from the edges of the extrudate and/or membrane to produce a membrane having an acceptable standard deviation of thickness values, generally less than 1 micron, e.g., in the range of 0.25 to 0.75 microns.

[0077] Selvage removal can be operated continuously, e.g., by locating cutting blades oriented parallel to the membrane's MD at a desired distance inward (along TD) from the edge of the membrane. The selvage is then conducted away from the process. For conventional microporous membranes, the ratio of the weight of the membrane per unit length to the weight of the selvage per unit length is 75% or greater. [0078] The process of the invention is advantageous because the improved thickness uniformity of the cooled extrudate is carried through the process and results in a membrane having improved thickness uniformity along TD. Thus, less slitting is required to produce a membrane having the desired amount of thickness variation or less, thereby producing less selvage and improving microporous membrane yield. For the microporous membranes of the invention, the ratio of the weight of the membrane per unit length to the weight of the selvage per unit length is in the range of 75% to 99%. In an embodiment, the slitting of the membrane to remove the selvage reduces the width of the membrane by a factor of about 90% to about 99% of the width of the membrane exiting the heat setting step. This slitting step can be referred to as a "first" slitting step when additional slitting is used downstream, e.g., in order to produce microporous membrane of a desired final width for battery manufacturing. [0079] The membrane's average thickness can be measured at selected points along TD (i.e., across the membrane). While not required, a plurality of measurement points approximately equally spaced along TD and referenced to the center line of the membrane (parallel to the machine direction) are used to determine the membrane's average thickness and the standard deviation of the measured values. Since the thickness values are measured at a plurality of points referenced to the center line of the membrane, the measurements can be made before or after slitting. [0080] In an embodiment, the selected thickness measurement points are along

TD and located between initial and final points, the initial and final points being optionally equidistant along TD from the center line of the membrane. The distance between the initial point and final point can be, e.g., about 75%, or alternatively about 80%, or alternatively 90%, or alternatively about 95% of the width of the membrane after the heat setting step but before slitting, i.e., before first slitting and any slitting downstream of first slitting. While average thickness and the standard deviation of the measured thickness values can be determined with an initial point and final point only, typically those points and a plurality of points along TD are used to determine those values; e.g., at least five points, or at least ten points, or at least 20 points, or at least 40 points. For example, the number of points can be in the range of 10 to 30 points, and, optionally, the points can be equally spaced along TD at a convenient interval, e.g., the distance between adjacent measurement points can be in the range of about 25 mm to about 100 mm.

[0081] The term "μ" refers to the arithmetic mean of the measured thickness values (measured in microns) determined at the measurement points along TD. The standard deviation of the measured thickness values "σ" is defined as the square root of the variance, i.e.,

σ = (— )^l/2 , where T represents the measured thickness values at each

measurement point and N is the number of measurement points. Thickness uniformity can also be expressed as the difference (Δ) between measured and average thickness values, i.e., Δ= T - μ.

Properties [0082] In preferred embodiments, the microporous membrane of the present invention also has at least one of the following properties.

(a) A Normalized Air Permeability of 35 sec/ 100 cmVμm or less.

[0083] Air permeability is measured according to JIS P8117, and the results are normalized to a value at a thickness of 1.0 μm using the equation A = (X)/Ti_, where X is the measured air permeability of a membrane having an actual thickness Ti and A is the normalized air permeability at a thickness of 1.0 μm. In an embodiment, the normalized air permeability is 30 sec/100 cm³/μm or less, e.g., in the range of 10 sec/100 cm³/μm to

25 sec/100 cm³/μm.

Cb) Porosity of from about 20 to about 80% [0084] The membrane's porosity is measured conventionally by comparing the membrane's actual weight to the weight of an equivalent non-porous membrane of 100% polyethylene (equivalent in the sense of having the same length, width, and thickness). Porosity is then determined using the formula: Porosity % = 100 x (w2- wl)/w2, wherein "wl" is the actual weight of the microporous membrane and "w2" is the weight of an equivalent non-porous membrane of 100% polyethylene having the same size and thickness.

(c) Normalized Pin puncture strength of 15 gF/μm or greater

[0085] Pin puncture strength is defined as the maximum load measured (in grams Force or "gF") when a microporous membrane having a thickness Of T₁ is pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second. The pin puncture strength is normalized to a value at a membrane thickness of 1.0 μm using the equation

L₂ = (Li )/Ti_j where L₁ is the measured pin puncture strength, L₂ is the normalized pin puncture strength, and Ti is the average thickness of the membrane.

[0086] In an embodiment, the normalized pin puncture strength is in the range of

20 gF to 35 gF, or from 25 gF to 30 gF.

(d) MD Tensile strength of 900 Kg/cm² or more and TD Tensile strength of 800 Kg/cm² or more [0087] Tensile strength is measured in MD and TD according to ASTM D-882A.

In an embodiment, the membrane's MD tensile strength is in the range of 1000 Kg/cm² to 2,000 Kg/cm², and TD tensile strength is in the range of 900 Kg/cm² to 1300 Kg/cm².

(e) MD and TD Tensile elongation of 50% or more

[0088] Tensile elongation is measured according to ASTM D-882A. In an embodiment, the membrane's MD and TD tensile elongation are each in the range of 50% to 350%. In another embodiment, the membrane's MD tensile elongation is in the range of, e.g., 150% to 300% and TD tensile elongation is in the range of, e.g., 150% to 400%.

(f) Shutdown temperature of 140°C or less [0089] The shutdown temperature of the microporous membrane is measured by a thermomechanical analyzer (TMA/SS6000 available from Seiko Instruments, Inc.) as follows: A rectangular sample of 3 mm x 50 mm is cut out of the microporous membrane such that the long axis of the sample is aligned with the transverse direction of the microporous membrane and the short axis is aligned with the machine direction. The sample is set in the thermomechanical analyzer at a chuck distance of 10 mm, i.e., the distance from the upper chuck to the lower chuck is 10 mm. The lower chuck is fixed and a load of 19.6 mN applied to the sample at the upper chuck. The chucks and sample are enclosed in a tube which can be heated. Starting at 30°C, the temperature inside the tube is elevated at a rate of 5°C/minute, and sample length change under the 19.6 mN load is measured at intervals of 0.5 second and recorded as temperature is increased. The temperature is increased to 200⁰C. "Shutdown temperature" is defined as the temperature of the inflection point observed at approximately the melting point of the polymer having the lowest melting point among the polymers used to produce the membrane. In an embodiment, the shutdown temperature is 140°C or less, e.g., in the range of l28°C to l33°C. (g) Meltdown temperature of 142°C or higher

[0090] Meltdown temperature is measured by the following procedure: A rectangular sample of 3 mm x 50 mm is cut out of the microporous membrane such that the long axis of the sample is aligned with the transverse direction of the microporous membrane as it is produced in the process and the short axis is aligned with the machine direction. The sample is set in the thermomechanical analyzer (TMA/SS6000 available from Seiko Instruments, Inc.) at a chuck distance of 10 mm, i.e., the distance from the upper chuck to the lower chuck is 10 mm. The lower chuck is fixed and a load of 19.6 mN applied to the sample at the upper chuck. The chucks and sample are enclosed in a tube which can be heated. Starting at 30°C, the temperature inside the tube is elevated at a rate of 5°C/minute, and sample length change under the 19.6 mN load is measured at intervals of 0.5 second and recorded as temperature is increased. The temperature is increased to 200°C. The meltdown temperature of the sample is defined as the temperature at which the sample breaks, generally at a temperature in the range of about 142°C to about 200⁰C.

[0091] In an embodiment, the meltdown temperature is in the range of from

143°C to 190⁰C. Microporous membrane composition

[0092] The microporous membrane generally comprises the same polymers used to produce the polymeric composition, in generally the same relative amounts. Washing solvent and/or process solvent (diluent) can also be present, generally in amounts less than 1 wt.% based on the weight of the microporous membrane. A small amount of polymer molecular weight degradation might occur during processing, but this is acceptable. In an embodiment where the polymer is polyolefin and the membrane is produced in a wet process, molecular weight degradation during processing, if any, causes the value of Mw/Mn of the polyolefin in the membrane to differ from the Mw/Mn of the polymer used to produce the polyolefin composition by no more than about 5%, or no more than about 1 %, or no more than about 0.1 %.

[0093] In an embodiment, the microporous membrane comprises the first and second polyethylene, for example from about 25 wt.% to about 35 wt.% of the first polyethylene and from about 65 wt.% to about 75 wt.% of the second polyethylene, based on the weight of the membrane. In an embodiment the membrane contains about 30 wt.% of the first polyethylene and about 70 wt.% of the second polyethylene. Battery separator

[0094] In an embodiment, the microporous membrane of any of the preceding embodiments is useful for separating electrodes in energy storage and conversion devices such as lithium ion batteries. Battery

[0095] The microporous membranes of the invention are useful as battery separators in e.g., lithium ion primary and secondary batteries. Such batteries are described in PCT publication WO 2008/016174. EXAMPLES The Extrudate [0096] The extrudate used in Examples 1 , 2, and 3 and in Comparative Examples

1, 2, 3, and 4 is produced as follows. Extrudate thickness (measuring at the extrusion die lip) is provided in the Table 1.

[0097] 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 5.0, a melting point (T_m) of 135°C, and a crystal dispersion temperature (T_C(j) of 100°C is dry blended with 80% by mass of high-density polyethylene (HDPE) having a Mw of 5.6 x 10⁵ and Mw/Mn of 4.6, T_m of 135°C, and T_cd of 100°C, 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 an Mw/Mn of 8.6, a T_m of 135°C, and T_C(j of lOO°C.

[0098] 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 210⁰C and 200 rpm to prepare a first polyolefin solution.

[0099] The Mw and Mw/Mn of each UHMWPE and HDPE are measured by a gel permeation chromatography (GPC) method under the following conditions. [00100] Mw and Mn of the polyethylenes are determined using a High Temperature Size Exclusion Chromatograph, or "SEC", (GPC PL 220, Polymer Laboratories), equipped with a differential refractive index detector (DRJ). Three PLgel Mixed-B columns (available from Polymer Laboratories) are used. The nominal flow rate is 0.5 cm /min, and the nominal injection volume was 300 μL. Transfer lines, columns, and the DRI detector were contained in an oven maintained at 145°C. The measurement is made in accordance with the procedure disclosed in "Macromolecules, Vol. 34, No. 19, pp. 6812-6820 (2001)".

[00101] The GPC solvent used is filtered Aldrich reagent grade 1,2,4- Trichlorobenzene (TCB) containing approximately 1000 ppm of butylated hydroxy toluene (BHT). The TCB was degassed with an online degasser prior to introduction into the SEC. Polymer solutions were prepared by placing dry polymer in a glass container, adding the desired amount of above TCB solvent, then heating the mixture at 160°C with continuous agitation for about 2 hours. The concentration of UHMWPE solution was 0.25 to 0.75mg/ml. Sample solution will be filtered off-line before injecting to GPC with 2 μm filter using a model SP260 Sample Prep Station (available from Polymer Laboratories).

[00102] The separation efficiency of the column set is calibrated with a calibration curve generated using seventeen individual polystyrene standards ranging in Mp from about 580 to about 10,000,000, which is used to generate the calibration curve. The polystyrene standards are obtained from Polymer Laboratories (Amherst, MA). A calibration curve (logMp vs. retention volume) is generated by recording the retention volume at the peak in the DRI signal for each PS standard, and fitting this data set to a 2nd-order polynomial. Samples are analyzed using IGOR Pro, available from Wave Metrics, Inc. [00103] The polyolefin solution is supplied from its double-screw extruder to a monolayer-sheet-forming T-die having a 250mm width at 210°C, to form an extrudate having a first surface and a second surface. The extrudate is cooled, using chill roll assemblies as discussed in the following Examples and Comparative Examples. Example 1 [00104] The extrudate produced at a thickness of 1.2 mm is conducted to a first chill roll (CR-I) having a diameter of 100 mm, and a surface temperature of 15⁰C, as shown in Table 1. The first surface of the extrudate contacts the chill roll at a contact angle of 65° to cool the extrudate. The cooled extrudate conducted away from the first chill roll to a second chill roll (CR-2) having a diameter of 100 mm and a surface temperature of 15°C. The second chill roll contacts the second surface of the extrudate (opposite the first surface) at a contact angle of 70°. The first and second chill rolls comprise an upstream pair of chill rolls. The extrudate is conducted away from the first upstream pair of chill rolls to a second upstream pair of chill rolls, the second pair comprising a third (CR-3) and a fourth (CR-4) chill roll. The third chill roll has a diameter of 100 mm, a surface temperature of 15°C, and contacts the first surface of the extrudate at a contact angle of 70°. From there, the extrudate is conducted to the fourth chill roll having a diameter of 100 mm and a surface temperature of 15°C. The second surface of the extrudate contacts the fourth chill roll at a contact angle of 70°. [00105] The extrudate is conducted away from the second pair of chill rolls to a downstream chill roll (CR-5) having a diameter of 300 mm, a surface temperature of 15°C, and a contact angle of 200°. The extrudate conducted away from the downstream chill roll (the "cooled extrudate") has a temperature of 35°C and an average thickness of 1040 μm and a thickness deviation from the average value in the range of 5 μm to 11 μm. [00106] The extrudate is then biaxially stretched at a temperature of 115° to a magnification factor of 5 x 5 (MD x TD). The liquid paraffin is removed from the stretched extrudate to form a membrane which is then heat set at 124.7°C for 30 seconds. The properties of the final microporous membrane are shown in Table 1. Example 2 [00107] Example 2 is the same as Example 1 except that the thickness of the extrudate as produced from the die is 0.6 mm, the thickness of the cooled extrudate prior to biaxial stretching is 520 μm, the cooled extrudate has a temperature of 30°C, the thickness deviation is in the range of 5 μm to 10 μm, and the heat set temperature is 124.5°C. The properties of the final membrane are shown in Table 1. Example 3

[00108] Example 3 is the same as Example 1 except that the thickness of the extrudate as produced from the die is 2.3 mm, the cooled extrudate has a temperature of 40°C, the thickness of the cooled extrudate is 2070 μm, the thickness deviation is in the range of 5 μm to 13 μm, the biaxial stretching temperature is 116.5°C, and the heat set temperature is 123.5°C. The properties of the final membrane are shown in Table 1. Comparative Example 1

[00109] Comparative Example 1 is the same as Example 1 except that the diameter of the first chill roll is 300 mm with a contact angle of 210°, the diameter of the second and third chill rolls are each 300 mm with a contact angle of 170°, the diameter of the fourth chill roll is 300 mm with a contact angle of 150°, the diameter of the downstream chill roll is 100 mm with a contact angle at 80°, the cooled extrudate has a temperature of 35°C, the thickness of the cooled extrudate is 960 μm, and the thickness deviation is in the range of 15 μm to 35 μm. The properties of the final membrane are shown in Table 1. Comparative Example 2

[00110] Comparative Example 2 is the same as Comparative Example 1 except that the thickness of the extrudate produced from the die is 0.6 mm, the cooled extrudate has a temperature of 30°C, the thickness of the cooled extrudate is 480 μm, the thickness deviation is 15 μm to 30 μm, and the biaxial stretching temperature is 1 14.4. The properties of the final membrane are shown in Table 1. Comparative Example 3

[00111] Comparative Example 3 is the same as Comparative Example 1 except that the thickness of the extrudate produced from the die is 0.9 mm, the cooled extrudate has a temperature of 35°C, the contact angle of the first chill roll is 165°, the contact angle of the second chill roll is 150°, the diameter of the third chill roll is 450 mm with a contact angle of 225°, the diameter of the fourth chill roll is 450 mm with a contact angle of 200°, the downstream chill roll has a diameter of 200 mm with a contact angle of 170°, the thickness of the cooled extrudate is 810 μm, the thickness deviation is in the range of 15 μm to 30 μm, the biaxial stretching temperature is 1 14.8°C, and heat setting is conducted at 125.3°C for 10 seconds. Comparative Example 4

[00112] Comparative Example 4 is the same as Comparative Example 2 except that the thickness of the extrudate produced from the die is 1.4 mm, the first chill roll has a contact angle of 40°, the fourth chill roll has a contact angle of 170°, there is no downstream chill roll, the thickness of the cooled extrudate is 1220 μm, the thickness deviation is in the range of 20 μm to 35 μm, the biaxial stretching temperature is 116.4°C, and the heat setting temperature is 125.5⁰C. The properties of the finished membrane are shown in Table 1. Table 1

[00113] The results in Table 1 show that a significant improvement in the thickness uniformity of the cooled extrudate is obtained when at least one pair of upstream chill rolls contact the extrudate at contact angles of 100° or less and at least one downstream chill roll contacts the extrudate at a contact angle of 180° or more. Moreover, this improvement in thickness uniformity is surprisingly carried through subsequent film processing (biaxial orientation, heating setting). Even though these additional process steps reduce the thickness of the film by more than an order of magnitude, the improvement in thickness uniformity is preserved. [00114] 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. [00115] 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.

[00116] When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

Claims

CLAIMSWhat is claimed is:

1. A process for producing a microporous membrane, comprising: a) combining polyolefin and diluent; b) extruding the combined polyolefin 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) first and second rolls positioned to receive opposite surfaces of the extrudate, said first roll contacting the extrudate along an arcuate path traversing a first contact angle of < 100° and said second roll contacting the extrudate along an arcuate path traversing a second contact angle of < 100°; and ii) a third roll downstream of said first and second rolls, said first, second, and third rolls aligned so that said third roll receives the extrudate from at least one of said first and second rolls, said third roll contacting the extrudate along an arcuate path to form a cooled extrudate; and d) removing at least a portion of the diluent from the cooled extrudate to form the membrane.

2. The process of claim 1 , wherein the third roll contacts the extrudate along an arcuate path traversing a third contact angle of > 180°.

3. The process of claim 1, further comprising orienting the cooled extrudate in at least one direction by one to ten fold at a temperature in the range of the polyefin's T_Cd to T_m+10°C, orienting the membrane to a magnification of from 1.1 to 2.5 fold in at least one direction, and heat-setting the membrane.

4. The process of claim 3, wherein the polyolefin composition comprises a high density polyethylene and ultra high molecular weight polyethylene and the T_C(j is the polyethylene's T_Cd.

5. The process of claim 4, wherein the polyolefin further comprises a polypropylene.

6. The process of claim 5, wherein the polyolefin comprises at least 50 wt.% of a polyethylene having an Mw < 1 x 10 and at least 15 wt.% polyethylene having an Mw > 1 x 10⁶.

7. The process of claim 6, wherein the polyolefin comprises at least 1 wt.% polypropylene.

8. The process of claim 1 , wherein each of said first and second rolls has a diameter in the range of 50 mm to 150 mm, said third roll has a diameter in the range of 275 mm to 400 mm, said first and second contact angles are in the range of 55° to 75°, and said third contact angle is in the range of 190° to 220°.

9. The process of claim 1 , wherein said first and second rolls have equal diameters.

10. The process of claim 1, wherein said first and second rolls constitute a first pair of rolls with said first roll positioned to receive the extrudate from said die and said second roll positioned to receive the extrudate from said first roll, and further comprising a second pair of rolls, said second pair of rolls being positioned downstream of said first pair of rolls and upstream of said third roll, each of said first and second pair of rolls having a diameter of < 200 mm.

11. The process of claim 10, wherein said first and second pair of rolls have equal diameters that are < 100 mm and said third roll has a diameter that is > 300 mm.

12. An assembly for transferring heat from a molten extrudate comprising polymers and diluent, said assembly comprising: a) first and second rolls positioned to receive opposite surfaces of the extrudate, said first roll contacting the extrudate along an arcuate path traversing first contact angle of < 100° and said second roll contacting the extrudate along an arcuate path traversing a second contact angle of < 100°; and b) a third roll downstream of said first and second rolls, said first, second, and third rolls being aligned so that said third roll receives the extrudate from at least one of said first and second rolls, said third roll contacting the extrudate along an arcuate path traversing a third contact angle

13. The assembly of claim 12, wherein said third contact angle is > 180°.

14. The assembly of claim 12, wherein said first, second, and third rolls are mounted on at least one support frame, said support frame providing alignment of said rolls for the extrudate.to move said arcuate paths around said first, second, and third rolls, and in a linear path toward said third roll.

15. The assembly of claim 12, further comprising a drive means mounted on said support frame for rotating said third roll to translate the extrudate through said roll assembly.

16. The assembly of claim 15, wherein at least one of said first and second rolls comprise cooling means for the transferring of heat from the extrudate.

17. The assembly of claim 16, wherein at least one said first and second rolls is rotated by either said drive means or a second drive means synchronized with said first drive means.

18. The assembly of claim 12, wherein (i) each of said first and second rolls has a diameter < 200 mm, and said third roll has a diameter > 250 mm;

(ii) said first and second contact angles are equal and in the range of 55° to 75°; and

(iii) said third contact angle is in the range of 190° to 220°.

19. The assembly of claim 18, wherein said first and second rolls have equal diameters in the range of 50 mm to 150 mm, and said third roll has a diameter in the range of 275 mm to 400 mm.

20. The assembly of claim 12, wherein each of said first and second rolls has an average surface temperature of 40°C or less.

21. The assembly of claim 12, wherein said first and second rolls comprise a first pair of rolls with said first roll positioned to receive the extrudate from said die and said second roll positioned to receive the extrudate from said first roll and further comprising a second pair of rolls, said second pair of rolls being positioned downstream of said first pair of rolls and upstream of said third roll, each of said second pair of rolls contacting the extrudate along an arcuate path traversing a contact angle that is (i) 100° or less and (ii) greater than or equal to said contact angle of said first pair of rolls.

22. The assembly of claim 21, wherein each of said second pair of rolls has a diameter < 100 mm and said third roll has a diameter > 300 mm.

23. The process of claim 1 or 12, wherein the third roll contacts the extrudate along an arcuate path traversing a third contact angle of 90° to 180°, preferably 100° to 150°, more preferably 110° to 130°.