Polyester Film for Foldable Display Substrate
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Solution Overview
Problem
Foldable displays experience image distortion and cracking at folding portions due to film deformation when repeatedly folded, posing challenges for mass production and portability in mobile 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 density of 1.380 g/cm3 or more, and an elastic modulus of 2.7 GPa or less in the bending direction and 4.5 GPa or more in the folding direction, ensuring no cracks or deformation during repeated folding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a non-flexible material such as glass is applied to protect the display surface, then surface hardness and protection are improved, but flexibility and ability to withstand repeated folding are worsened
Solution Approach 1:
The patent applies a flexible polyester film as the touchscreen substrate instead of rigid glass, enabling the display to be folded repeatedly while maintaining surface protection. The film is stretched in specific directions to achieve both flexibility and surface hardness, resolving the contradiction between rigidity and foldability.
2Adaptability or versatility
If the film is made flexible to enable folding, then adaptability and portability are improved, but image distortion and cracking at folding portions occur due to deformation
Solution Approach 1:
The patent changes the physical parameters of the polyester film by controlling its refractive index in specific directions. The film is stretched in the bending direction to achieve a refractive index of 1.590-1.620, which reduces deformation at folding portions while maintaining flexibility. This parameter control prevents image distortion and cracking during repeated folding.
Solution Approach 2:
The patent uses a composite structure with a polyester film base and an easy-to-adhere layer on the surface. This composite material combination provides both flexibility for folding and sufficient surface hardness, while the specific molecular orientation achieved through stretching enhances resistance to deformation at folding portions.
3Reliability
If the refractive index in the bending direction is decreased to reduce deformation, then image distortion is reduced, but pencil hardness and surface protection function are worsened
Solution Approach 1:
The patent applies different refractive index characteristics to different directions of the film. The refractive index in the bending direction is controlled to be 1.590-1.620 to reduce deformation, while the refractive index in the thickness direction is kept at 1.520 or less to maintain surface hardness. This directional differentiation resolves the contradiction between reducing image distortion and maintaining surface protection.
4Reliability
If the refractive index in one direction is decreased to ameliorate deformation, then image distortion is reduced, but uniaxial orientation is enhanced causing cracking or fracture at folding portions
Solution Approach 1:
The patent carefully controls multiple refractive index parameters simultaneously: the refractive index in the bending direction is set to 1.590-1.620, in the folding portion direction to 1.670-1.700, and in the thickness direction to 1.520 or less. This multi-parameter control achieves uniform molecular orientation that reduces both image distortion and the risk of cracking, preventing uniaxial orientation problems.
Data Source
AI summary
Provided is a polyester film for a touchscreen module substrate of a foldable display which is free of occurrence of a crack at a folding portion thereof, in order to provide a foldable display excellent in mass productivity, and free of a risk of causing distortion in an image displayed at a folding portion thereof after being repeatedly folded. The polyester film for a touchscreen module substrate of a foldable display satisfies the following conditions: (1) a refractive index in a bending direction is from 1.590 to 1.620; (2) a refractive index in a folding portion direction is from 1.670 to 1.700; (3) a refractive index in a thickness direction is 1.520 or less; and (4) a density is 1.380 g/cm or more, wherein the bending direction refers to a direction orthogonal to a folding portion of the polyester film at a time when the polyester film is folded.