Microporous Polyethylene Film via Liquid-Liquid Phase Separation
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Solution Overview
Problem
Existing methods for producing microporous polyethylene films for battery separators face challenges in achieving simultaneous high permeability and mechanical strength, with limitations in controlling pore size and structure, leading to defects like needle holes and reduced productivity.
Innovation Solution
A method involving liquid-liquid phase separation of a polyethylene and diluent mixture at controlled temperatures and residence times in an extruder, followed by stretching and heat-setting, to produce films with enhanced mechanical strength and permeability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If solid-liquid phase separation method is used to manufacture microporous films, then the manufacturing process is simple, but the pore size is very small and cannot be controlled variously, limiting application to high-capacity batteries
Solution Approach 1:
The patent changes the fundamental parameter of phase separation type from solid-liquid to liquid-liquid, enabling pore size control while maintaining manufacturing feasibility. By using liquid-liquid phase separation with a diluent that remains liquid at processing temperatures, the invention achieves controllable pore sizes (1-100 μm) compared to the uncontrolled small pores of solid-liquid methods.
Solution Approach 2:
The patent utilizes liquid-liquid phase separation instead of solid-liquid phase separation. The polyolefin and diluent form a single-phase melt during extrusion, then separate into two liquid phases during cooling, creating controllable pore structures. This phase transition approach enables precise control over pore size and distribution.
2Strength
If ultrahigh-molecular-weight polyethylene is used to increase mechanical strength, then the strength increases, but the extrusion load increases greatly and compoundability decreases
Solution Approach 1:
The patent introduces a diluent as an intermediary substance that facilitates processing. The diluent acts as a processing aid during extrusion and phase separation, then is completely extracted afterward, leaving no residue. This intermediary enables the use of high-molecular-weight polyethylene with improved processability while maintaining final product strength.
Solution Approach 2:
The patent changes the molecular weight parameter of polyethylene from ultrahigh to high or medium range. This parameter change reduces extrusion load and improves compoundability with the diluent, while still achieving sufficient mechanical strength for battery separator applications through the optimized microporous structure.
3Reliability
If liquid-liquid phase separation is used to increase pore size and permeability, then permeability improves, but the mechanical strength may be compromised
Solution Approach 1:
The patent optimizes multiple parameters simultaneously: polyolefin molecular weight (20,000-1,000,000), diluent content (10-90 parts by weight per 100 parts polyolefin), and processing temperatures. These parameter changes enable achieving both high permeability (through controlled pore size 1-100 μm) and adequate mechanical strength.
Solution Approach 2:
The patent creates a composite system during processing where polyolefin and diluent form a single-phase melt that subsequently separates into two liquid phases. This temporary composite structure enables controlled phase separation and pore formation, after which the diluent is extracted, leaving a pure polyolefin microporous structure with optimized mechanical and permeability properties.
4Reliability
If high-capacity battery separator requirements are met with high permeability, then battery performance improves, but the mechanical strength and processibility are reduced
Solution Approach 1:
The diluent serves as a processing intermediary that enables high-capacity battery separator manufacturing. During extrusion, the diluent maintains a single-phase melt structure for good processibility. During cooling, it enables liquid-liquid phase separation for controlled pore formation. After extraction, no residue remains, ensuring high permeability and battery performance without compromising final mechanical 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 method results in microporous polyethylene films with superior mechanical strength and permeability, suitable for high-capacity battery separators, while also improving processibility and reducing defects.
Implementation Method 1
liquid-liquid phase separation of a polyethylene and diluent mixture at controlled temperatures and residence times in an extruder
Implementation Method 2
mixing and extrusion, and this molten resin material is cooled through a casting roll
Implementation Method 3
this molten resin material is cooled through a casting roll
Data Source
AI summary
Disclosed in the present invention are a microporous polyethylene film and a method of manufacture thereof. The polyethylene microporous film manufactured according to the present invention may contribute to an increased productivity of stable products as its extrusion and stretching may be done readily. And thus manufactured product may be used for battery separators and various filters owing to its high gas permeability, superior puncture strength, and small ratio of shrinkage.