Patent Application
20180043598
This disclosure relates to a method of producing a microporous plastic film.
Microporous plastic films have been broadly used as a substance separation membrane, permselective membrane, separator material of an electrochemical element such as an alkali rechargeable battery, lithium rechargeable battery, fuel cell and capacitor or the like. A separator for a lithium ion battery is a particularly suitable application.
JP 2009-249480 A and JP 2013-530261 A disclose a wet process for producing a microporous film made of plastic like polyolefin resin. In the wet process, a diluent such as liquid paraffin is added to a polymer, kneaded and dispersed, and then discharged through a die lip onto a cooling drum to be cooled and solidified to form a gel sheet, which is oriented uniaxially or biaxially by a roller or tenter method to improve strength to produce a film having micropores by extracting the diluent.
Particularly by the roller method drawing the sheet in the travelling direction by a plurality of rollers, the lengthwise draw ratio can be changed freely by only changing a roller speed. By the other tenter method, the microporous plastic film can be improved in mechanical properties because polyolefin molecules can be oriented with strong orientation in the drawing direction. As disclosed in JP '480, the roller method applied to the wet process might make the diluent bleed out from a gel sheet surface under a pressure caused by heat or tension while the conveying and drawing are performed in the presence of the diluent between film and roller. To draw such a gel sheet, the sheet is sufficiently cooled below the crystallization ending temperature of the polymer and is heated again below the melting point as disclosed in JP '261 where it is heated above the crystallization dispersion temperature.
The wet roller method disclosed in JP '480 might have a problem that the diluent intervening between the roller and film lubricates to meander or fail a desirable draw ratio because of slip.
According to JP '480, the above-described slip can be prevented when a tension over the lengthwise draw tension is given between the lengthwise drawing machine and the lateral drawing machine (tenter), wherein the tension should preferably be greater than the draw tension by 20%. However, we found that such a great tension over the draw tension drew the sheet downstream of the lateral drawing machine to cause an undesirable slip adversely and therefore the slip couldn't be prevented.
According to JP '261, a predetermined range of contact time, contact angle and contact length between the lengthwise drawing roller and the sheet can prevent the slip and the sheet surface from being damaged. However, we found that even such measures couldn't fully prevent slip when we had an increased draw speed, a decreased draw temperature or an increased draw ratio to improve physical properties and mechanical properties of the microporous plastic film.
Accordingly, it could be helpful to provide a method of producing a microporous plastic film excellent in physical properties and mechanical properties by a high-speed drawing without slip under a high productivity.
We thus provide a method of producing a microporous plastic film comprising kneading a diluent and a polyolefin resin with an extruder; discharging the polyolefin resin kneaded with the diluent from a die lip in a sheet shape; cooling and solidifying the sheet discharged from the die lip on one or plurality of cooling drums; reheating and drawing the solidified sheet with a plurality of rollers in a sheet conveying direction; cooling the sheet drawn in the sheet conveying direction; gripping both ends of the sheet with clips; introducing the sheet into a tenter; and washing the diluent out to prepare a uniaxially or biaxially oriented microporous plastic film, wherein the method further comprises: driving a nip roller with a motor, the nip roller pressing a sheet interposing between the nip roller and at least one of the rollers.
It is preferable that the method further comprises driving with a motor a nip roller pressing a sheet interposing between the nip roller and at least one of the rollers, the rollers heating the sheet in a process upstream from the drawing with the rollers in the sheet conveying direction.
We can produce, with high productivity, a microporous plastic film having excellent physical properties and mechanical properties by preventing slip even in wet roller drawing.
Hereinafter, desirable examples of our microporous plastic film will be explained with reference to the figures.
To desirably produce microporous plastic film 11, polyolefin resin is mixed with diluent and heated to melt to prepare polyolefin resin solution. The diluent decides a structure forming micropores of the microporous plastic film and contributes to improvement of drawability (for example, improvement as reduction of unevenness at a draw ratio for exhibiting a strength) at the time of drawing film.
The diluent is not limited particularly, as far as it can be mixed or dissolved with polyolefin resin. The diluent may be mixed with polyolefin resin in a melt-kneading state. Alternatively, it may be mixed with solid solvent at room temperature. Such prepared solid diluent may be stearyl alcohol, ceryl alcohol, paraffin wax or the like. It is preferable that the diluent is a liquid at room temperature from the viewpoints of prevention of uneven drawing and coating convenience at a later stage. The liquid diluent may be a fatty series such as nonane, decane, decalin, paraxylene, undecane, dodecane and liquid paraffin; a cyclic fatty series or an aromatic hydrocarbon; a mineral oil fraction having a boiling point of the same range as the compounds thereof; or a phthalate ester such as dibutyl phthalate and dioctylphthalate which are liquids at room temperature. To obtain a gelled sheet stably containing liquid diluent, it is preferable to employ a nonvolatile diluent such as liquid paraffin. For example, it is preferable that the liquid diluent has a viscosity of 20 to 200 cSt at 40° C.
It is preferable that the polyolefin resin of 10 to 50 mass % is mixed with the diluent in total 100 mass % of polyolefin resin and diluent, from a viewpoint of good formability of extruded product. The polyolefin resin solution may be melt-kneaded uniformly by a calendar, mixer or extruder 21 having a screw as shown in
It is preferable that the polyolefin resin solution in extruder 21 has a predetermined temperature of 140 to 250° C. for polyethylene composition and 190 to 270° C. for polypropylene-containing composition, depending on resin kind. A thermometer is provided inside extruder 21 or in a cylinder section to monitor a temperature indirectly and properly adjust the heater temperature, rotation speed and discharge rate of the cylinder section to control the temperature in a target range.
The polyolefin resin solution that has been melt-kneaded by extruder 21 is discharged through a slit section of die lip 23 into a sheet while it is measured with gear pump 22 as needed. Such discharged gelled sheet 12 is solidified as contacting cooling drum 31. Gelled sheet 12 becomes a pillar part with crystal structure made of the polyolefin resin supporting pores of microporous plastic film 11. Gelled sheet 12 becomes in a gel state since it includes the diluent melt-kneaded in extruder 21. A part of the diluent bleeds out from sheet surface by cooling gelled sheet 12 so that the sheet with wet surface made by the diluent is conveyed on cooling drum 31. The thickness of gelled sheet 12 can be adjusted by adjusting the cooling drum speed relative to the flow rate from the die lip slit section according to discharge rate.
It is preferable that cooling drum 31 has a temperature of 15 to 40° C., which may affect the crystal structure of gelled sheet 12. This is because the final cooling temperature should be below the crystallization ending temperature. The molecular orientation tends to advance in a later drawing process when the higher-order structure is fine. To make up for cooling time, it is possible that the diameter of cooling drum 31 is enlarged, cooling drum 32 is added to cooling drum 31 or a plurality of cooling drums are further added. To make the crystal structure precise and uniform in gelled sheet 12, it is preferable that a parameter such as conveyance speed, drum temperature, drum size and the number of drums is designed in view of the cooling speed. For example, even when a target sheet temperature is 30° C., cooling drum 31 may be around 20° C. in a case of a too high speed and insufficient heat conduction. It is preferable that humidity is lowered by air conditioning when the temperature is below 25° C. Cooling drum 31 may have a shape of a roller or belt. It is preferable that a surface of cooling drum 31 is made of a material which is excellent in shape stability and working accuracy to keep the roller speed constant. Such a material may be metal, ceramic or fiber composite material. It is particularly preferable that the surface is made of a metal excellent in heat conduction to a film. It is possible to perform non-adhesion coating or rubber coating on it to the extent that conduction is not obstructed. It is preferable that the surface of sheet and roller are made of metal including metal plating which is excellent in scratch resistance and heat conductivity and not swelled by the bled-out diluent making wet state on the surface.
It is preferable that the roller has a surface roughness of 0.2 to 40 μm at the maximum height. To make a mirror surface, the surface roughness is preferably 0.2 to 0.8 μm. To make a sufficiently rough surface, the surface roughness is preferably 20 to 40 μm.
Because the surface of the roller is wet with the diluent, the mirror surface has a low coefficient of friction caused by lubrication. The rough surface has an increased coefficient of friction caused by less or no lubrication because the diluent discharges from the unevenness. Although the mirror surface and the rough surface may be combined as needed, it is basically preferable that the mirror surface is provided from the viewpoints of maintenance such as cleaning and speed control precision, preferably with a certain lubricant with diluent to prevent the sheet from having uneven appearance.
It is preferable that cooling drums 31 and 32 are provided with a conventional heat pump or cooling device in addition to an internal flow path for refrigerant to control the surface temperature. The roller is driven to rotate by a rotation driving means such as motor at a predetermined speed while a speed-changing mechanism may be provided between rollers to apply draw tension or relaxation according to expansion and shrinkage of the sheet. Alternatively, each roller may be provided with a motor to achieve function like the speed-changing mechanism by accurately adjusting the speed by inverter or servo.
In
To prevent wet gelled sheet 12 from meandering or degrading the cooling efficiency by lubrication, it is possible that the sheet is pushed onto cooling drum 31 with a contacting means such as nip roller, jet nozzle, suction chamber and electrostatic application. Such a contacting means can improve the travelling property and cooling efficiency of gelled sheet 12 to enable easy setting of the cooling speed and the final cooling temperature.
Next, gelled sheet 12 is oriented in a sheet-conveying direction with a plurality of roller groups in lengthwise drawing process 4, and then both ends of sheet 13 are gripped by a conventional clip to be oriented in a sheet width direction (direction orthogonal to the conveying direction) while the sheet is conveyed in the travelling direction while heated and kept warm in an oven. Such a drawing process achieves a high productivity and characteristics such as strength and air permeability of microporous film. The drawing process in the sheet-conveying direction (which may be called “lengthwise drawing”) as well as driving process is performed with a roller having a metal surface and a temperature control mechanism such as conventional heater inside. To ensure the freedom of roller path, it is possible to provide an idler roller which is not driven and not shown in
Alternatively, like cooling drum 31, it is preferable that heating roller group 41 or drawing roller group 42 has an internal structure in which the roller is provided with a flow path for heat medium such as vapor and pressured hot water. It is possible that the roller is supported as rotatable with bearings and a shaft end connects to a heat-medium supply pipe with a rotary joint for supplying heat medium without obstructing the rotation of roller to supply heat medium to the inside.
It is preferable that the drawing is performed at a draw ratio of 5 to 12 in a sheet conveying direction although depending on the gelled sheet thickness. To improve strength and productivity, it is preferable that the drawing is performed at an area ratio of 30 or more in a sheet width direction together with the sheet conveying-directional drawing. It is preferable that the area ratio is 40 or more, preferably 60 or more.
It is preferable that a drawing temperature is a melting point of polyolefin resin or less. It is preferable that the temperature is in a range of [crystal dispersion temperature Tcd of polyolefin resin] to [melting point of polyolefin resin]. For example, it is preferable that the temperature is 80 to 130° C. for polyethylene resin, preferably 100 to 125° C. After drawing, a cooling process is performed down to these temperatures.
Thus performed drawing cleaves a higher order structure formed on the gelled sheet to miniaturize crystal phase and form many fibrils. The fibrils are three-dimensionally connected irregularly to form a web structure. It is suitable for a battery separator because the drawing improves mechanical strength and enlarges fine pores.
Thus obtained uniaxially oriented sheet 13 or biaxially oriented sheet 14 can be washed to remove diluent and dried up to make dry microporous plastic film 11 by a conventional technology as disclosed in WO2008/016174. To make microporous plastic film 11, it is possible that dry drawing process 7 is performed to reheat and re-draw the sheet after washing process 6. Re-drawing process 7 may be performed with a roller or a tenter. Also, heat treatment may be performed to adjust physical properties and remove residual distortion through the process. Depending on intended purposes, it is possible that the surface of microporous plastic film 11 is subject to a surface treatment such as corona discharge or functional coating with heat-resistant particles.
In
Although heating roller group 41 and drawing roller group 42 are common to each other in terms of heating and heat-keeping function of gelled sheet 12 and variable roller rotation speed, drawing roller group 42 comprises a roller to substantively draw gelled sheet 12 and generates a peripheral speed difference to permanently deform gelled sheet in the travelling direction. More particularly, drawing roller group 42 is defined as rollers which substantively draw it as generating 3% or more of peripheral speed difference.
As shown in
It is preferable that the surface of nip roller is made of rubber-like elastic material that can depress gelled sheet 12 uniformly onto uneven thickness of gelled sheet 12, deflection of roller and slight surface unevenness. The rubber-like elastic material is preferably a general vulcanized rubber such as Nitrile isoButylene-isoprene Rubber (NBR), Chloroprene Rubber (CR), Ethylene Propylene Rubber (EPDM) and Hypalon rubber (CSM). When gelled sheet 12 or conveying roller has a high temperature like 80° C. or higher, it is particularly preferable to employ the EPDM or CSM. Under a higher temperature, it is preferable to employ a silicone rubber or a fluorine rubber as well as the vulcanized rubber. It is preferable to employ a rubber which is little swelled by diluent to prevent the roller from having a distorted shape over time.
Gripping force (frictional force) is necessary between the roller and gelled sheet 12 to convey gelled sheet 12 without meandering in a travelling direction. Particularly in drawing roller group 42 where a high tension is generated by drawing, a high gripping force balancing the drawing tension is required to achieve a necessary draw ratio. The diluent having bled out as described above interposes between the roller and gelled sheet 12 in lubrication state to cause a deterioration of the gripping force required for conveyance and drawing.
The diluent intervenes between nip roller 44 and gelled sheet 12 as well as between drawing roller and sheet. The friction force required for drawing is obtained from the friction between drawing roller 42 or nip roller 44 and gelled sheet 12. We found that the friction force required for drawing might obstruct straight travelling of gelled sheet 12 to cause meandering when the nip roller cannot rotate with lubrication of diluent between nip roller 44 and gelled sheet 12.
In our method of producing a microporous plastic film, nip roller 44 which presses a sheet interposing between the nip roller and at least one of the rollers of drawing roller group 42 is driven with a motor. With such a configuration, gelled sheet 12 can be prevented from meandering without rotation of nip roller even in the presence of diluent. The driving can suppress wear debris generation that might be increased on the surface of nip roller or gelled sheet 12 when nip roller idles and slightly contacts the sheet directly even in the presence of diluent. It is preferable that a nip roller is provided on each of all rollers of drawing roller group 42 while each nip roller is driven by a motor.
From a viewpoint of less slip and meandering, it is preferable that each driven nip roller and the drawing roller or cooling drum provided on the nip roller rotate at the same rotation speed. In the specification, “the same rotation speed” means the difference of both rotation speed is within ±3%.
Further, it is preferable that nip roller 44 that presses a sheet interposing between nip roller 44 and at least one of the rollers of heating roller group 41 is driven with a motor to obtain the same effect. It is preferable that nip roller 44 is provided on each of all rollers of heating roller group 41 while each nip roller 44 is driven by a motor.
Nip roller 44 may be driven by a driving force transmitted through belt B and pulleys P1, P2 and P3 from drawing roller group 42 driven by a motor as shown in
Alternatively, the drawing roller or nip roller may be driven through a gear, or may be driven by magnetic force without contact for preventing the belt and gear from making dust. To prevent such dust making, it is preferable that nip roller 44 is driven directly by a motor. In any case, as shown in
It is possible that a nip is performed along a tangent line substantively with a nip roller to introduce gelled sheet 12 into heating roller group 41 or drawing roller group 42 so that slip and meandering can be prevented as improving uniformity of thickness and appearance quality. That is because, when a nip roller is not provided along the tangent line, certain thickness of diluent and air is accompanied between drawing roller group 42 or heating roller group 41 and sheet 12 to make a bank by nipping with nip roller 44 afterwards. The nip roller which is provided substantively along the tangent line can suppress the thickness of diluent and air with nip roller 44 before contacting between drawing roller group 42 or heating roller group 41 and the sheet so that an oriented sheet having a higher quality can be obtained without forming a bank between gelled sheet 12 and the roller.
The phrase “nip along the tangent line” means a nip performed by the nip roller at a position where gelled sheet 12 starts contacting heating roller group 41 or drawing roller group 42. The nip performed with the nip roller at this position makes gelled sheet 12 as if being a tangent line of nip roller. The phrase “a nip is performed along a tangent line substantively” means that a nip which doesn't make a bank is regarded as “a nip performed along a tangent line” even when the nipped part is slightly away from the exact position where the sheet starts contacting the roller.
Hereinafter, our methods will be explained with reference to the Examples although they are not limited to these Examples in particular.
A mixture is prepared by dry-blending of 0.375 parts by weight of tetrakis[methylene-3-(3,5-ditertiary butyl-4-hydroxyphenyl)-propionate]methane together with 100 parts by weight of polyethylene (PE) composition which comprises 40 mass % of ultrahigh molecular weight polyethylene having mass average molecular weight (Mw) of 2.5×106 and 60 mass % of high-density polyethylene having Mw of 2.8×105.
Thus obtained mixture is fed to twin-screw extruder 21 at flow rate of 97 kg/hr by a film forming method shown in
Thus obtained polyethylene solution is supplied into die lip 23 as being measured by a gear pump and the polyethylene solution at 210° C. is discharged on first cooling drum 31 adjusted to 35° C. by waterflow to form gelled sheet 12. First cooling drum 31 is driven to rotate at 10 m/min.
Gelled sheet 12 is subject to sampling with 10 mm square before introduction to lengthwise drawing process 4 to find that the thickness is 1.5 mm in 10 times average of contact-type thickness meter. Since bled-out diluent adheres to the surface, the above-described thickness measurement includes ±0.1 mm variation at the maximum
Gelled sheet 12 is heated to 110° C. on the sheet surface with heating roller group 41 and a metal waterflow roller as the first drawing roller. Between heating roller group 41 and the first roller of each drawing roller group 42, the rotation speed of motor directly connected to the roller is controlled to make the downstream side faster by 1% of speed difference according to thermal expansion of the sheet.
Drawing roller group 42 consists of 2 rollers shown in
Oriented film 13 is cooled on four rollers of cooling roller group 43 including last roller 422 of drawing roller group 42 to adjust the waterflow roller temperature to make the sheet temperature 50° C. Each clip speed difference between the last drawing roller, cooling roller group 43 and lateral drawing process is 0 to make the speed same.
From first cooling drum 31, surfaces of all rollers in lengthwise drawing process 4 are made of steel coated with hard chromic plating of which surface roughness is 0.4 μm (0.4S) at the maximum height. A preliminary sample has drawing tension T of 1,500N, nip roller pressure P of 2,000N and coefficient of friction μ of 0.15.
Both ends of oriented film 13 are gripped with a clip to perform lateral drawing at draw ratio of 6 times at 115° C. in an oven, and then biaxially oriented film 14 cooled down to 30° C. is washed in a washing bath of methylene chloride kept at 25° C. to remove liquid paraffin. The washed film is dried up in a dry kiln kept at 60° C. and then is drawn again in redrawing process 7 at areal ratio of 1.2 times in lengthwise and lateral directions. The heat treatment is performed for 20 seconds by 88.2 m/min at 125° C. to prepare microporous plastic film 11 having thickness of 16 μm and width of 2,000 mm.
A microporous plastic film is produced by the same devices and conditions as Example 1, except that nip roller is not driven.
The speeds of sheet and roller are measured by 1% accuracy including installation accuracy with a non-contact Doppler velocimeter (made by ACT Electronics Corp., model 1522). For all Examples and Comparative Examples, slip of film 11 before drawing is evaluated according to the following standards:
The amount of meandering in lengthwise drawing process 4 is evaluated according to the following standards:
No good: Meandering amount is 10 mm or more.
Acceptable: Meandering amount is 5 mm or more and less than 10 mm.
Good: Meandering amount is less than 5 mm.
The Gurley impermeability is determined according to JIS P8117 with Oken type Impermeability Tester (made by Asahi Seiko Co., Ltd., EGO-1T).
The thrust strength is determined from the maximum load applied when a needle having 1 mm diameter and spherical tip (Radius of curvature R=0.5 mm) is pricked at 2 mm/sec into a microporous membrane having membrane thickness T1. The measured maximum load La is converted to Lb which would be the maximum load when membrane thickness is 16 μm according to the following formula: Lb=(La×16)/T1 to calculate the thrust strength [N/16 μm].
By comparing Example 1 and Comparative Example 1, drawing can be performed stably to suppress slip and meandering in Example 1 because the nip roller provided on each drawing roller and the drawing roller. In contrast, the slip and meandering cannot be prevented in Comparative Example 1 because the nip roller is not driven. Meandering is too great to continue the film forming and therefore a microporous plastic film cannot be sampled.
As described above, we make it possible to provide a microporous plastic film excellent in strength and physical properties as maintaining a travelling stability in a drawing condition where the drawing is performed to give the microporous film desirable properties.
Our microporous plastic film is applicable, but not limited in particular, to a separator used for electrochemical reaction device such as rechargeable battery, fuel cell and capacitor as well as a functional web such as filtration membrane, print film and clothing material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-031391 | Feb 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/051779 | 1/22/2016 | WO | 00 |
