Method for preparing microporous polyolefin film with improved productivity and easy control of physical properties

Abstract

Provided is a method of manufacturing a microporous polyolefin film useable as a battery separator, which is easy to control strength, permeability and shrinking properties of the microporous film and embodies excellent quality uniformity and production stability in fabricating the microporous film.

1 - 11 . (canceled) 
     
     
         12 . A microporous polyolefin film manufactured by a method by using a roll type stretching machine, the method at least including:
 (a) melting/mixing/extruding a sheet composition containing a polyolefin composition having a weight-average molecular weight of not less than 1×10 5  to less than 1×10 6  (component I) and a diluent (component II) in a ratio by weight of 15˜50:85˜50 and shaping these compositions into a sheet form through a T-die;   (b) using the stretching machine of roll type consisting of at least three rolls to stretch the sheet to the machine direction, and then, stretch the film stretched uni-axially to the machine direction into the transverse direction by a stretching machine of tenter type, in order to form a film by sequential biaxial stretching; and   (c) extracting the diluent from the film stretched to the machine and transverse direction and drying the film,   wherein the stretching machine to the machine direction includes at least one preheating roll, at least one stretching roll and at least one cooling roll or air-cooled device, a total roll contact time at the preheating rolls ranges from 1 to 500 seconds, the stretching is executed in at least one section between rolls, a stretching ratio in one section between two rolls ranges from 1.05 to 6 times, a roll contact coefficient of the sheet on the stretching roll ranges from 1.5 to 54 degree (of angle)×m, and a coefficient of non-contact section between rolls ranges from 0.1×10 −3  to 200×10 −3  min.   
     
     
         13 . A microporous polyolefin film manufactured by a method by using a roll type stretching machine, the method at least including:
 (a) melting/mixing/extruding a sheet composition containing a polyolefin composition having a weight-average molecular weight of not less than 1×10 5  to less than 1×10 6  (component I) and a diluent (component IS) in a ratio by weight of 15˜50:85˜50 and shaping these compositions into a sheet form through a T-die;   (b) using the stretching machine of roll type consisting of at least three rolls to stretch the sheet to the machine direction, and then, stretch the film stretched uni-axially to the machine direction into the transverse direction by a stretching machine of tenter type, in order to form a film by sequential biaxial stretching; and   (c) extracting the diluent from the film stretched to the machine and transverse direction and drying the film,   wherein the stretching machine to the machine direction includes at least one preheating roll, at least one stretching roll and at least one cooling roll or air-cooled device, a total roll contact time at the preheating rolls ranges from 1 to 500 seconds, the stretching is executed in at least one section between rolls, a stretching ratio in one section between two rolls ranges from 1.05 to 8 times, a roll contact coefficient of the sheet on the stretching roll ranges from 1.5 to 54 degree (of angle)×m, and a coefficient of non-contact section between rolls ranges from 0.1×10 −3  to 200×10 −3  min, and   wherein the polyolefin composition in (a) includes at least one polyolefin resin component, or 80 to 99.9 wt % of at least one polyolefin resin and 0.1 to 20 wt % of a component selected from an inorganic material or other resins except for the polyolefin resin.   
     
     
         14 . A microporous polyolefin film manufactured by a method by using a roll type stretching machine, the method at least including:
 (a) melting/mixing/extruding a sheet composition containing a polyolefin composition having a weight-average molecular weight of not less than 1×10 5  to less than 1×10 6  (component I) and a diluent (component II) in a ratio by weight of 15˜50:85˜50 and shaping these compositions into a sheet form through a T-die;   (b) using the stretching machine of roll type consisting of at least three rolls to stretch the sheet to the machine direction, and then, stretch the film stretched uni-axially to the machine direction into the transverse direction by a stretching machine of tenter type, in order to form a film by sequential biaxial stretching; and   (c) extracting the diluent from the film stretched to the machine and transverse direction and drying the film,   wherein the stretching machine to the machine direction includes at least one preheating roll, at least one stretching roll and at least one cooling roll or air-cooled device, a total roll contact time at the preheating rolls ranges from 1 to 500 seconds, the stretching is executed in at least one section between rolls, a stretching ratio in one section between two rolls ranges from 1.05 to 6 times, a roll contact coefficient of the sheet on the stretching roll ranges from 1.5 to 54 degree (of angle)×m, and a coefficient of non-contact section between rolls ranges from 0.1×10 −3  to 200×10 −3  min,   wherein the stretching to the machine direction is performed by stretching the sheet introduced info the stretching machine while applying a tensile strength of 550N/cm 2  in the machine direction for 4˜12 times, and the stretching to the transverse direction is performed by stretching the film introduced into the stretching machine while applying a tensile strength of 100˜700N/cm 2  in the machine direction for 4˜9 times,   wherein the method further includes a heat setting process that uses heat setting equipment of tenter type, after the extracting in (c), to stretch the film for 20˜100% to the transverse direction, and then, shrink the film to have a width of 5˜50%, compared to the film after stretching, with conducting thermal relaxation in a range of temperatures in which crystals of 5˜50% in the film are molten,   wherein the stretching includes: stretching to the machine direction wherein the temperature of the preheating roll ranges from a melting temperature of the resin in the sheet −50° C. to a temperature at which 70% of crystals contained in the resin of the sheet are molten, the temperature of the stretching roll ranges from the melting temperature of the resin in the sheet −50° C. to a temperature at which 90% of crystals contained in the resin of the sheet are molten, the temperature of the cooling roll is a temperature of the stretching roll −5° C., or less, or air-cooling is conducted, so that a sheet width holding ratio between rolls in the stretching roll after stretching ranges from 70 to 99% and an overall sheet width holding ratio after stretching to the machine direction ranges from 50 to 95%; and stretching to the transverse direction wherein the stretching machine to the machine direction includes a preheating part, a stretching part, and a heat setting part, and the temperature of the preheating part is a melting temperature of the resin in the sheet +20° C., or less, the temperature of the stretching part ranges from a melting temperature of the resin in the sheet −20° C. to the melting temperature of the resin in the sheet +10° C., and the temperature of the heat setting part is the melting temperature of the resin in the sheet, or less.