Microporous Polyolefin Membrane Thickness Uniformity
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Solution Overview
Problem
Microporous polyolefin membranes used as battery separators face challenges in achieving optimal mechanical strength, permeability, and meltdown properties, particularly when containing ultra-high-molecular-weight polyethylene and polypropylene, which often result in poor thickness uniformity and increased risk of short-circuiting and low production yield.
Innovation Solution
A microporous polyolefin membrane comprising polyethylene and a sufficient amount of polypropylene with specific molecular weight and heat of fusion characteristics, produced through a method involving extrusion, cooling, stretching, and solvent removal, allowing for improved mechanical properties and meltdown temperatures without compromising permeability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If ultra-high-molecular-weight polyethylene and polypropylene are added to microporous polyolefin membranes to improve mechanical strength, then mechanical properties are improved, but thickness uniformity deteriorates
Solution Approach 1:
The patent controls the weight-average molecular weight of polypropylene within a specific range (5×10^5 to 5×10^6) and limits low-molecular-weight fractions to 20% or less. This parameter optimization allows the membrane to achieve both improved mechanical strength from ultra-high-molecular-weight polyethylene and adequate thickness uniformity by preventing excessive viscosity and processing difficulties that would otherwise occur with higher molecular weight polypropylene.
2Temperature
If polyethylene and polypropylene are combined in microporous membranes to improve meltdown properties, then meltdown temperature is improved, but thickness uniformity and production yield deteriorate
Solution Approach 1:
The patent specifies that polypropylene should have a weight-average molecular weight of 5×10^5 to 5×10^6 and low-molecular-weight fractions (5×10^4 or less) of 20% or less by mass. These parameter controls enable the membrane to achieve high meltdown temperature (above 160°C) while maintaining adequate thickness uniformity and production yield by optimizing the molecular weight distribution to balance thermal properties with processability.
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 solution enables the production of membranes with enhanced mechanical strength, permeability, and meltdown properties, addressing issues of thickness uniformity and production yield, thereby improving the performance and safety of battery separators.
Implementation Method 1
stretching the gel-like molding without removing the solvent
Implementation Method 2
removing the solvent
Data Source
Figure 1~2
Figure 3
AI summary
A microporous polyolefin membrane comprising a polyethylene, and polypropylene having a weight-average molecular weight of 6 x 105 or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 5 x 104 or less being 5% or less by mass of the polypropylene.