Photocurable Adhesive Sheet for Foldable Display Step Followability
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Solution Overview
Problem
Existing adhesive sheets for image display devices lack sufficient unevenness followability, flexibility, and high refractive index, leading to issues such as bubble formation, light scattering, and reduced durability during folding, especially in flexible image display devices like OLEDs and QDs.
Innovation Solution
A photocurable adhesive sheet composed of a (meth)acrylic copolymer with specific refractive index, creep strain, and shear modulus properties, combined with a mold release film, to enhance followability and durability while reducing light scattering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the adhesive sheet is made thin to meet the demand for thinner image display devices, then the device thickness is reduced, but the adhesive sheet has low followability to steps and bubbles are generated
Solution Approach 1:
The patent changes the physical parameters of the adhesive sheet by controlling the glass transition temperature (Tg) to be -50°C or lower and the viscosity at 23°C to be within a specific range. This allows the adhesive to maintain low viscosity at room temperature for good followability, while achieving appropriate viscosity at application temperature for bubble-free bonding, thus resolving the contradiction between thinness and followability.
Solution Approach 2:
The patent employs temperature-dependent viscosity changes to dynamically adapt the adhesive's flow characteristics. At application temperature, the adhesive exhibits low viscosity for excellent followability to steps, and after bonding, the viscosity increases to provide strong adhesion. This dynamic property allows thin adhesive sheets to achieve high followability without generating bubbles.
2Reliability
If the adhesive sheet has high fluidity to follow steps and fill the entire area, then the followability is improved, but the adhesive sheet bleeds out from between mold release films during storage
Solution Approach 1:
The patent uses parameter changes by controlling the glass transition temperature to -50°C or lower, which ensures the adhesive maintains appropriate viscosity at storage temperature to prevent bleeding, while achieving low viscosity at application temperature for excellent followability. This resolves the contradiction between followability and shape retention.
Solution Approach 2:
The adhesive exhibits periodic changes in viscosity based on temperature conditions. During storage at room temperature, the adhesive maintains high viscosity for shape retention. During application, the temperature increase triggers a transition to low viscosity for excellent followability. This periodic viscosity change resolves the contradiction between preventing bleeding and achieving followability.
3Illumination intensity
If the adhesive sheet has high refractive index to reduce light scattering, then the light transmittance is improved, but the manufacturing complexity increases
Solution Approach 1:
The patent employs composite materials by incorporating specific polymer components with high refractive index characteristics into the adhesive formulation. This allows the adhesive to achieve high light transmittance and reduced light scattering at interfaces, while the composite structure enables control over other properties like viscosity and Tg, balancing optical performance with manufacturing feasibility.
Solution Approach 2:
The patent changes the chemical composition parameters of the adhesive to achieve a refractive index of 1.50 or higher. By adjusting the polymer component ratios and selecting specific high refractive index materials, the adhesive achieves improved light transmittance while maintaining manufacturability through controlled synthesis processes.
4Adaptability or versatility
If the adhesive sheet is made flexible for foldable devices, then the flexibility is improved, but the adhesive sheet may be peeled off or cause cracks in the member during repeated folding
Solution Approach 1:
The patent changes the glass transition temperature parameter to -50°C or lower, which provides the adhesive with exceptional flexibility and low-temperature performance. This ensures the adhesive can withstand repeated folding operations without peeling or causing cracks, while maintaining strong adhesion through controlled viscosity and molecular structure.
Solution Approach 2:
The patent uses composite material formulation combining specific polymers with low Tg and appropriate molecular weight. This composite structure provides both the flexibility needed for foldable devices and the cohesive strength to prevent peeling and cracking during repeated folding, resolving the contradiction between flexibility and durability.
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 adhesive sheet provides high refractive index, exceptional unevenness followability, and flexibility, ensuring stable bonding and reduced light scattering, even in flexible devices with surface irregularities, and maintains durability during repeated folding operations.
Implementation Method 1
a photocurable adhesive sheet formed from an adhesive composition... when the photocurable adhesive sheet is irradiated with an active energy ray having a wavelength of 365 nm
Data Source
AI summary
The present disclosure relates to a photocurable adhesive sheet satisfying the following requirements (1), (2), (3), and (4): Requirement (1): a refractive index of the photocurable adhesive sheet is 1.480 or more, Requirement (2): when a thickness of the photocurable adhesive sheet is set to 0.7 to 1.0 mm, a strain (creep strain) by applying a pressure of 2 kPa at a temperature of 60° C. for 600 seconds is 1,000% or more and 100,000% or less, Requirement (3): in a holding power measurement in accordance with JIS-Z-0237 (ISO29863), a peel-off time when the photocurable adhesive sheet is adhered to an SUS plate with an area of 20 mm×20 mm and a load of 500 gf is applied in an atmosphere of 40° C. is 30 seconds or more, and Requirement (4): when the photocurable adhesive sheet is irradiated with an active energy ray having a wavelength of 365 nm with an irradiation amount within an integrated light amount of 1,000 to 5,000 mJ/cm2, a storage shear modulus at −20° C. ((G′1(−20° C.)), which is obtained by a dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is 10 kPa or more and 2,000 kPa or less.