Biaxially Stretched Polypropylene Film for Uniform Surface Roughness
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Solution Overview
Problem
Polypropylene films used in capacitors for electric and hybrid vehicles require thinner films with larger electrode areas, higher voltage endurance, especially at high temperatures, and improved yield rates, while existing films suffer from air bubble entry and non-uniform stretching that leads to surface roughness differences causing film rupture and reduced voltage endurance.
Innovation Solution
A biaxially stretched polypropylene film with controlled surface features: average maximum length of elliptical spots ≤3.0 mm, reduced peak height difference ≤0.040 µm, slow axis angle variation 0.3°-2.8°, and specific molecular weight and thickness ranges, produced using a casting drum with controlled microcracks and biaxial stretching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the film is made thinner to increase electrode area, then the capacitor size and weight are reduced, but the film becomes more susceptible to air bubble entry and surface defects during manufacturing
Solution Approach 1:
The cast sheet is pre-heated to form β-crystals on the surface before stretching, creating a more stable and uniform surface structure that is less susceptible to defects during subsequent processing. This preliminary thermal treatment ensures the thin film maintains structural integrity throughout manufacturing.
Solution Approach 2:
The patent controls specific parameters including heating temperature (90-110°C for β-crystal formation), stretching ratios (MD 2.0-4.0 times, TD 3.0-6.0 times), and cooling rates to optimize the film structure. These parameter controls ensure uniform surface properties and reduce defect formation in thin films.
2Productivity
If high-speed stretching is used to increase productivity, then manufacturing efficiency is improved, but air bubbles enter between the sheet and conveying roll causing non-uniform contact and surface defects
Solution Approach 1:
The cast sheet undergoes pre-heating to form β-crystals before stretching, creating a more stable surface that maintains uniform contact with the conveying roll at high speeds. This preliminary structural preparation prevents air bubble entrapment and surface defects even during rapid processing.
Solution Approach 2:
The patent creates uniform β-crystal formation across the entire sheet surface through controlled heating, ensuring consistent local properties throughout the material. This uniform local quality prevents localized defects and maintains surface uniformity during high-speed manufacturing.
3Ease of manufacture
If the conveying roll is heated to form β-crystals and impart plasticity, then the sheet becomes more formable, but non-uniform contact due to elliptical depressions causes differences in β-crystal formation and surface roughness
Solution Approach 1:
The cast sheet is pre-heated in a controlled manner to form β-crystals uniformly across the surface before the stretching process begins. This preliminary crystallization ensures that the entire sheet, including areas that will later contact the conveying roll, has uniform structural properties and plasticity.
Solution Approach 2:
The patent precisely controls the heating temperature range (90-110°C) and duration to achieve uniform β-crystal formation. By optimizing these parameters, the sheet develops consistent surface properties that prevent localized roughness variations even when contact with the conveying roll occurs at high speeds.
4Area of stationary object
If the film is stretched to increase electrode area, then the capacitor capacity is improved, but portions with different thermal histories have non-uniform plasticity causing film rupture
Solution Approach 1:
The cast sheet is pre-heated to form β-crystals uniformly across the entire surface before stretching. This preliminary treatment ensures that all portions of the sheet, regardless of future contact variations, have uniform plasticity and structural properties, preventing film rupture during the stretching process that increases electrode area.
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
Stable film formation with higher voltage endurance and yield rates, reducing film rupture and electric field concentration, ensuring consistent performance and reliability.
Implementation Method 1
a method for producing a biaxially stretched polypropylene film, the method comprising obtaining a cast sheet and subjecting the cast sheet to biaxial stretching
Implementation Method 2
the conveying roll is heated in order to form β-crystals on the surface of the cast sheet and to impart plasticity to the sheet before stretching
Implementation Method 3
the conveying roll is heated in order to form β-crystals on the surface of the cast sheet
Implementation Method 4
subjecting the cast sheet to biaxial stretching
Data Source
Figure 1~2
Figure 3
AI summary
Provided is a polypropylene film that enables stable film formation for a longer period of time, and that has higher voltage endurance (in particular, voltage endurance at high temperatures) and a higher yield rate. The biaxially stretched polypropylene film has a first surface and a second surface, the average maximum length of substantially elliptical spots on the first surface being 3.0 mm or less, and the difference between the average reduced peak height Rpk outside the spots and the average reduced peak height Rpk within the spots being 0.040 µm or less.