Microporous polyethylene film with improved strength, permeability and surface energy

Abstract

The present invention relates a microporous polyethylene film with improved mechanical strength, porosity, pore size and, particularly, improved surface energy, thereby having improved electrolyte wettability and being adequate for use as separators in high-capacity and long lifetime lithium secondary batteries. The microporous polyethylene film of the present invention is characterized by having a surface energy of at least 50 dynes / cm 2 , an air permeability (Darcy's permeability constant) of at least 2.0x10 -5 , a puncture strength of at least 0.17 N / m, a product of the air permeability and the puncture strength of at least 0.34x10 -5 Darcy N / m, a weighted average pore size of at least 30 nm, and a film shrinkage in the transverse and machine directions of not more than 5% at 105 DEG C for 10 minutes and not more than 15% at 120 DEG C for 60 minutes, respectively. The microporous polyethylene film is prepared by compounding raw materials in an extruder such that a thermodynamic single phase is formed above the temperature of liquid-liquid phase separation, inducing sufficient phase separation in a phase separation zone formed inside the extruder by controlling the temperature below the temperature of liquid-liquid phase separation, forming through a die, and carrying out plasma treatment in order to enhance surface energy.

Description

[0001] Cross References to Related Applications

[0002] This application claims priority from Korean Patent Application No. 10-2007-0117268 filed with the Korean Intellectual Property Office on November 16, 2007, the entire contents of which are hereby incorporated by reference. technical field

[0003] The invention relates to a microporous polyethylene film and a preparation method thereof. More specifically, the present invention relates to microporous polyethylene membranes having improved mechanical strength, porosity, pore size, and especially improved surface energy, and thus improved electrolyte wettability and suitable for use as high capacity , Separators in durable lithium secondary batteries. Background technique

[0004] Microporous polyethylene membranes have been widely used as battery separators, separation filters, and ultrafiltration membranes due to their chemical stability and excellent physical properties.

[0005] The production of microporous membra...

Examples

preparation example Construction

[0035] The preparation of the microporous polyethylene membrane of the present invention includes the steps of: preparing a membrane with high porosity, large pore size and excellent mechanical strength by optimizing the liquid-liquid phase separation conditions in the wet process and the molecular weight and concentration of polyethylene , and plasma-treating the membrane under atmospheric pressure to enhance the surface energy of the membrane.

[0036] First, the steps to prepare the separator are described as follows:

[0037] Low molecular weight organic substances partially compatible with polyethylene (hereinafter referred to as diluents) can form a thermodynamically single phase with polyethylene at temperatures above the melting point of polyethylene. When a thermodynamically single-phase solution of polyethylene and diluent is slowly cooled, phase separation of polyethylene and diluent occurs before the polyethylene crystallizes into a solid. Since the phase separati...

Embodiment approach

[0083] The following examples describe the present invention in more detail, but they are not intended to limit the scope of the present invention.

Embodiment 1

[0129] The weight average molecular weight is 2.1×10 5 , a high-density polyethylene having a melting temperature of 135° C. was used as component I, and dibutyl phthalate (component A in the table below) was used as component II. The contents of component I and component II were 40% by weight and 60% by weight, respectively.

[0130]Zone 15 to The temperature of the 20th zone was set at 185° C. (below the temperature of liquid-liquid phase separation) for phase separation. The stretching temperature was 127°C, and the stretching ratios in the longitudinal direction and transverse direction were each 6 times. After extraction of Component II by impregnation with dichloromethane for two minutes, heat setting was carried out at 120° C. for 15 seconds. Then, plasma was generated by discharge at atmospheric pressure (power=3.6kW, voltage=12kV) by using a dielectric barrier discharge type electrode, supplying nitrogen as a carrier gas at 300ml / min and oxygen as a reactive gas at...

[0001] Cross References to Related Applications

[0002] This application claims priority from Korean Patent Application No. 10-2007-0117268 filed with the Korean Intellectual Property Office on November 16, 2007, the entire contents of which are hereby incorporated by reference. technical field

[0003] The invention relates to a microporous polyethylene film and a preparation method thereof. More specifically, the present invention relates to microporous polyethylene membranes having improved mechanical strength, porosity, pore size, and especially improved surface energy, and thus improved electrolyte wettability and suitable for use as high capacity , Separators in durable lithium secondary batteries. Background technique

[0004] Microporous polyethylene membranes have been widely used as battery separators, separation filters, and ultrafiltration membranes due to their chemical stability and excellent physical properties.

[0005] The production of microporous membra...

Examples

preparation example Construction

[0035] The preparation of the microporous polyethylene membrane of the present invention includes the steps of: preparing a membrane with high porosity, large pore size and excellent mechanical strength by optimizing the liquid-liquid phase separation conditions in the wet process and the molecular weight and concentration of polyethylene , and plasma-treating the membrane under atmospheric pressure to enhance the surface energy of the membrane.

[0036] First, the steps to prepare the separator are described as follows:

[0037] Low molecular weight organic substances partially compatible with polyethylene (hereinafter referred to as diluents) can form a thermodynamically single phase with polyethylene at temperatures above the melting point of polyethylene. When a thermodynamically single-phase solution of polyethylene and diluent is slowly cooled, phase separation of polyethylene and diluent occurs before the polyethylene crystallizes into a solid. Since the phase separati...

Embodiment approach

[0083] The following examples describe the present invention in more detail, but they are not intended to limit the scope of the present invention.

Embodiment 1

[0129] The weight average molecular weight is 2.1×10 5 , a high-density polyethylene having a melting temperature of 135° C. was used as component I, and dibutyl phthalate (component A in the table below) was used as component II. The contents of component I and component II were 40% by weight and 60% by weight, respectively.

[0130]Zone 15 to The temperature of the 20th zone was set at 185° C. (below the temperature of liquid-liquid phase separation) for phase separation. The stretching temperature was 127°C, and the stretching ratios in the longitudinal direction and transverse direction were each 6 times. After extraction of Component II by impregnation with dichloromethane for two minutes, heat setting was carried out at 120° C. for 15 seconds. Then, plasma was generated by discharge at atmospheric pressure (power=3.6kW, voltage=12kV) by using a dielectric barrier discharge type electrode, supplying nitrogen as a carrier gas at 300ml / min and oxygen as a reactive gas at...

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