Polyester Film Elasticity for Foldable Display Covers
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Solution Overview
Problem
Conventional materials used in flexible display devices, such as polyester resins, suffer from whitening and cracking due to compressive and tensile stresses during folding, making it difficult to maintain original characteristics after repeated deformation.
Innovation Solution
A polyester film with specific tensile stress values in the in-plane directions is developed, achieved by molding and biaxially stretching a polyester resin to satisfy certain stress relationships, allowing for increased elasticity and maintaining original characteristics during repeated folding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a polyester resin with large modulus at room temperature is used for the protective film, then the film has high strength and rigidity, but whitening and cracks are easily generated during folding
Solution Approach 1:
The patent changes the tensile stress characteristics of the polyester film by controlling the ratio of tensile stress at different strain levels. Specifically, it adjusts the ratio of tensile stress at 20% strain to tensile stress at 6% strain in the machine direction, and the ratio of tensile stress at 8% strain to tensile stress at 4% strain in the transverse direction, both to be 1.05 or more. This parameter optimization enables the film to maintain original characteristics after repeated folding while preserving adequate strength.
2Reliability
If the modulus of the protective film is low at room temperature, then whitening and cracks are reduced, but the film cannot maintain its original characteristics after repeated folding
Solution Approach 1:
The patent optimizes the tensile stress parameters of the polyester film to achieve a balance between elasticity and structural stability. By controlling the stress ratios at specific strain levels (20%/6% in MD and 8%/4% in TD) to be 1.05 or more, the film gains sufficient elasticity to resist whitening and cracking, while maintaining the structural integrity needed to preserve original characteristics after repeated folding operations.
3Ease of manufacture
If conventional polyester resin is used for flexible display protective film, then the film provides basic protection, but it cannot withstand frequent bending or folding without deterioration
Solution Approach 1:
The patent modifies the tensile stress characteristics of conventional polyester resin through controlled biaxial stretching to achieve superior folding durability. By optimizing the stress ratios at specific strain levels, the film maintains ease of manufacture using standard polyester resin and processing methods, while dramatically improving reliability under repeated folding conditions through enhanced elastic recovery properties.
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 polyester film exhibits excellent elasticity, preventing whitening and cracking, and maintaining original characteristics even after numerous folds, suitable for flexible and foldable display devices.
Implementation Method 1
the tensile stress of the film is adjusted such that the slope of the curve after the point (B) is higher than a certain level, it is possible to make the film have excellent elasticity that maintains its original characteristics even after a plurality of repeated folding
Data Source
AI summary
The polyester film according to the embodiment achieves the elasticity required for a cover of a flexible display by increasing the rate of increase in stress in an initial stage of strain to a certain level or higher. As a result, it is possible to maintain the original characteristics when it is applied to a cover of a flexible display device and subjected to a plurality of repeated folding. Accordingly, the polyester film can be applied to a cover of a flexible display device, in particular, a foldable display device to exhibit excellent characteristics.