Adhesive Sheet Viscoelastic Modulus for Foldable Display Delamination
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Solution Overview
Problem
Existing adhesive sheets for flexible image display devices, particularly those with curved or foldable surfaces, face issues such as delamination and cracking due to stress between layers during folding, especially at low temperatures, where the storage modulus of the adhesive sheet increases, leading to breakage and poor restorability.
Innovation Solution
An adhesive sheet with a storage shear modulus at 80°C between 1 kPa and 20 kPa and a loss tangent less than 0.50, and at −20°C between 1 kPa and 1000 kPa, combined with a member sheet having a tensile strength of 10 MPa to 900 MPa, to reduce stress and prevent delamination and breakage, while ensuring good restorability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the storage modulus of the adhesive sheet is increased to improve adhesion strength, then adhesion strength is improved, but stress between layers increases causing delamination and cracking during folding
Solution Approach 1:
The patent applies parameter changes by precisely controlling the storage shear modulus within a specific range (1-20 kPa at 80°C, 1-1000 kPa at -20°C) and limiting the loss tangent to less than 0.50 at 80°C. These parameter specifications optimize the adhesive sheet's viscoelastic properties to balance adhesion strength with flexibility during folding operations across different temperatures.
Solution Approach 2:
The patent employs composite materials by formulating the adhesive sheet with specific polymer compositions including polyisobutylene, polybutadiene, or acrylic polymers in controlled ratios. This composite approach creates a material that simultaneously provides sufficient adhesion and appropriate elasticity to accommodate folding stresses without delamination or cracking.
2Stability of the object's composition
If the adhesive sheet is made more rigid to maintain structural stability, then structural stability is improved, but restorability after folding deteriorates
Solution Approach 1:
The patent resolves this contradiction by changing the viscoelastic parameters of the adhesive sheet, specifically setting the storage shear modulus to 1-20 kPa at 80°C and limiting the loss tangent to less than 0.50. These parameter changes enable the adhesive to maintain structural stability while possessing sufficient elasticity to restore the laminated sheet to its original flat state after folding operations.
3Strength
If the storage modulus at low temperature is increased to improve adhesion, then adhesion is improved, but stress concentration increases causing member sheet breakage
Solution Approach 1:
The patent addresses this contradiction by specifying that the storage shear modulus at -20°C should be between 1-1000 kPa, which is significantly lower than conventional adhesives. This parameter change ensures the adhesive remains flexible at low temperatures, maintaining adhesion while preventing stress concentration that would lead to member sheet breakage during folding operations.
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 proposed adhesive sheet and laminated structure effectively reduce stress between layers, preventing delamination and breakage under various temperature conditions, and enhance restorability, making them suitable for flexible image display devices.
Implementation Method 1
the storage shear modulus at 80° C. (G′ (80° C.)) obtained by dynamic viscoelasticity measurement in shearing mode at a frequency of 1 Hz is 1 kPa or more and less than 20 kPa, and a loss tangent at 80° C. (tan δ (80° C.)) is less than 0.50
Data Source
AI summary
An adhesive sheet and a laminated sheet wherein stress applied to a member sheet is reduced in the laminated sheet formed by laminating the member sheet and the adhesive sheet, and the adhesive sheet and the laminated sheet do not break or delaminate even though a folding operation is conducted under low temperature and high temperature conditions are provided, and an adhesive sheet wherein the restorability when the folded adhesive sheet is opened is good is further provided. An adhesive sheet satisfying the following requirements (1) to (2) is proposed.(1) A storage shear modulus at 80° C. (G′ (80° C.)) obtained by dynamic viscoelasticity measurement in shearing mode at a frequency of 1 Hz is 1 kPa or more and less than 20 kPa, and a loss tangent at 80° C. (tan δ (80° C.)) is less than 0.50.(2) A storage shear modulus at −20° C. (G′ (−20° C.)) obtained by dynamic viscoelasticity measurement in shearing mode at a frequency of 1 Hz is 1 kPa or more and less than 1000 kPa.