Polyester Film Refractive Index Control for Foldable Displays
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Solution Overview
Problem
Conventional foldable displays suffer from image distortion and creasing issues due to film deformation when repeatedly folded, making them unsuitable for mass production and portable devices.
Innovation Solution
A polyester film with specific refractive index ranges (1.590-1.620 in the bending direction, 1.670-1.700 in the folding direction, and 1.520 or less in the thickness direction, along with a hard coating film and laminated film configurations, that provide durability and protection against deformation and cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a flexible hard coating film is used to protect the foldable display surface, then surface protection functionality is improved, but the film deforms over time causing image distortion
Solution Approach 1:
The patent applies parameter changes by precisely controlling the refractive index of the polyester film in different directions (1.590-1.620 in bending direction, 1.670-1.700 in folding direction, and 1.520 or less in thickness direction) to balance flexibility and deformation resistance, preventing image distortion while maintaining surface protection
Solution Approach 2:
The patent uses composite materials by combining polyester film with specific refractive index characteristics and hard coating layers to create a laminated structure that provides both flexibility for folding and resistance to deformation, solving the contradiction between surface protection and image distortion
2Manufacturing precision
If the refractive index in the bending direction is decreased to reduce deformation, then image distortion is reduced, but pencil hardness decreases lowering surface protection
Solution Approach 1:
The patent applies local quality by creating different refractive index characteristics in different directions of the film - lower refractive index (1.590-1.620) in the bending direction to reduce deformation, while maintaining higher refractive index (1.670-1.700) in the folding direction and controlling thickness direction refractive index (1.520 or less) to preserve surface hardness and protection functionality
3Manufacturing precision
If the refractive index in the folding direction is decreased to reduce deformation, then image distortion is reduced, but the folding direction becomes more uniaxially oriented forming cracks
Solution Approach 1:
The patent applies parameter changes by setting the refractive index in the folding direction to a specific range (1.670-1.700) that balances deformation reduction with maintaining sufficient orientation flexibility, preventing both image distortion and crack formation during repeated folding
4Strength
If a non-flexible material like glass is used to protect the display, then surface protection is improved, but the display cannot be folded
Solution Approach 1:
The patent applies flexible shells and thin films by using a polyester film with controlled refractive index characteristics that provides glass-like surface protection functionality while maintaining flexibility to enable folding, replacing rigid glass with a sophisticated flexible polymer structure
Data Source
AI summary
Provided is a polyester film for foldable displays that is unlikely to have cracks in the folding portion with the aim of providing a foldable display that is excellent in mass production applicability, and that does not have image distortion that may appear in the folding portion after the display is repeatedly folded. The polyester film has a thickness of 10 to 80 μm, a refractive index in the bending direction of 1.590 to 1.620, a refractive index in the direction of a folding portion of 1.670 to 1.700, a refractive index in the thickness direction of 1.520 or less, and a density of 1.380 g/cm3 or more, wherein the bending direction refers to a direction orthogonal to the folding portion of the polyester film to be folded.