Polyolefin Microporous Membrane Suppresses Curling
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Solution Overview
Problem
Polyolefin microporous membranes used in lithium ion battery separators face challenges in achieving simultaneous high pin puncture strength and air permeability, while also experiencing curling issues that affect processability and appearance when formed into rolls, especially as widths increase.
Innovation Solution
A polyolefin microporous membrane is developed with a composition of ultra-high molecular weight polyethylene and high-density polyethylene, processed by melt-kneading, molding, stretching, and solvent extraction to achieve specific crystallization and drying conditions, resulting in a membrane with enhanced pin puncture strength, air permeability, and reduced curling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the width of polyolefin microporous membrane is increased to produce wide separators for larger batteries, then productivity and battery capacity are improved, but the membrane exhibits marked curling that impairs processability and makes it difficult to obtain a roll with good winding
Solution Approach 1:
The patent applies parameter changes by controlling the molecular weight distribution of polyethylene (weight average molecular weight of 2.0 x 10^5 to 5.0 x 10^5) and adjusting crystallization conditions (crystallization half time of 10 to 30 minutes at 117°C) to produce a membrane with balanced mechanical properties that resists curling while maintaining high strength and permeability
Solution Approach 2:
The patent uses composite materials by combining ultra-high molecular weight polyethylene (weight average molecular weight of 1.0 x 10^6 to 5.0 x 10^6) with regular polyethylene in a specific ratio (30-80 wt.% UHMWPE content) to create a microporous membrane that achieves both high pin puncture strength and suppressed curling
2Productivity
If membrane thickness is decreased to improve productivity, then production efficiency increases, but pin puncture strength and structural integrity deteriorate
Solution Approach 1:
The patent changes the parameter of molecular weight distribution by using polyethylene with weight average molecular weight of 2.0 x 10^5 to 5.0 x 10^5 and controlling crystallization kinetics (half time of 10-30 minutes) to achieve high strength in thin membranes (16 μm normalized thickness) without sacrificing pin puncture strength
Solution Approach 2:
The patent applies local quality by creating a specific microporous structure with controlled pore distribution and size through the crystallization process, where the pore structure provides both high permeability for productivity and sufficient structural support for pin puncture strength in thin membranes
3Productivity
If production speed is increased to improve productivity, then output increases, but curling and winding defects become more pronounced
Solution Approach 1:
The patent applies parameter changes by optimizing the crystallization half time to 10-30 minutes at 117°C, which controls the crystallization rate to prevent excessive curling even during high-speed production and rapid winding processes
Solution Approach 2:
The patent applies preliminary action by pre-controlling the molecular weight distribution and crystallization characteristics of the polyethylene before membrane formation, which pre-establishes the membrane's dimensional stability and resistance to curling during subsequent high-speed production and winding operations
4Strength
If ultra-high molecular weight polyethylene content is increased to improve pin puncture strength, then membrane strength increases, but air permeability and processing difficulty may worsen
Solution Approach 1:
The patent applies parameter changes by controlling the weight average molecular weight of the polyethylene matrix to 2.0 x 10^5 to 5.0 x 10^5 and adjusting the crystallization half time to 10-30 minutes, which optimizes the balance between strength and processability by preventing excessive viscosity while maintaining mechanical properties
Solution Approach 2:
The patent uses composite materials by combining ultra-high molecular weight polyethylene (providing strength) with regular polyethylene (providing processability) in a optimized ratio, where the regular polyethylene acts as a matrix that facilitates processing while the UHMWPE provides the required pin puncture strength
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The membrane exhibits excellent pin puncture strength, air permeability, and suppressed curling, ensuring a smooth roll formation with improved appearance and suitability as a separator for lithium ion secondary batteries.
Implementation Method 1
the crystallization half time t1/2 during isothermal crystallization at 117°C
Implementation Method 2
air permeability normalized to a thickness of 16 μm of from 100 to 220 sec/100 cc measured in accordance with JIS P8117
Data Source
Figure 1
AI summary
The present invention relates to a polyolefin microporous membrane having an air permeability normalized to a thickness of 16 µm of from 100 to 220 sec/100 cc, a pin puncture strength normalized to a thickness of 16 µm of not less than 550 gf, and a crystallization half time t1/2 of from 10 to 35 minutes when subjected to isothermal crystallization at 117°C; a separator for a battery using the same; and a production method thereof. The polyolefin microporous membrane of the present invention has excellent pin puncture strength and air permeability, suppresses curling, and has an excellent appearance without wrinkles or winding deviation when formed into a roll.