Radiation-Curable Adhesive Sheet for Storage-Stable Stress Relaxation
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Solution Overview
Problem
Radiation curable pressure-sensitive adhesive sheets deteriorate due to curing progression during storage, leading to reduced level difference absorbability and stress relaxation properties, resulting in display unevenness and bubble formation.
Innovation Solution
Incorporating a photopolymerization initiator with a specific pKa range into the adhesive layer to suppress curing during storage, maintaining high fluidity and stress relaxation properties until irradiation, followed by curing upon irradiation to enhance adhesive reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the pressure-sensitive adhesive layer is made soft to improve level difference absorbability and stress relaxation properties, then the adhesive can follow stepped spaces and reduce display unevenness, but gas generation from plastic films cannot be suppressed enough, causing the adhesive layer to float and form bubbles
Solution Approach 1:
The patent applies parameter changes by controlling the elastic modulus of the pressure-sensitive adhesive layer within a specific range (10^5 to 10^7 dyne/cm²) to balance between softness for level difference absorbability and hardness for gas suppression. This quantitative parameter control resolves the contradiction between being soft enough to absorb stress and hard enough to prevent bubble formation.
2Duration of action of stationary object
If the pressure-sensitive adhesive sheet is stored for a relatively long period of time, then it can be shipped and used later, but curing gradually proceeds during storage due to environmental factors such as heat and light, resulting in deterioration of level difference absorbability and stress relaxation properties
Solution Approach 1:
The patent applies preliminary action by adding a specific amount of photopolymerization initiator (0.01-5 wt%) and crosslinking agent to the adhesive layer before storage, but controlling their reaction conditions so that curing does not significantly proceed during storage. This allows the adhesive to maintain its required properties during storage while being prepared for subsequent curing during lamination.
Solution Approach 2:
The patent controls the storage stability by managing the concentration and activity of photopolymerization initiators and crosslinking agents, ensuring that the curing reaction rate remains low during storage (maintaining level difference absorbability) but can be activated during lamination. This parameter control enables long-term storage without significant curing progression.
3Reliability
If the pressure-sensitive adhesive layer is made hard to suppress gas generation from plastic films, then adhesive reliability is enhanced, but level difference absorbability and stress relaxation properties deteriorate
Solution Approach 1:
The patent resolves this contradiction by precisely controlling the elastic modulus parameter within the range of 10^5 to 10^7 dyne/cm², which provides optimal balance between hardness for gas suppression and softness for stress relaxation. This quantitative control allows the adhesive to be hard enough to suppress gas generation while maintaining sufficient flexibility for stress relaxation.
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 maintains excellent level difference absorbability and stress relaxation properties for a long time, suppressing curing progression during storage and improving storage stability.
Implementation Method 1
the pressure-sensitive adhesive layer is cured by irradiation with radiation, wherein the pressure-sensitive adhesive layer contains a photopolymerization initiator and a crosslinking agent, and the curing by irradiation with radiation is curing by a reaction between the photopolymerization initiator and the crosslinking agent
Data Source
AI summary
A radiation curable pressure-sensitive adhesive sheet 1 of the present invention has a pressure-sensitive adhesive layer 10 that is cured by irradiation with radiation. The pressure-sensitive adhesive layer 10 contains a photopolymerization initiator 11a and a crosslinking agent 11b, and the curing by irradiation with radiation is curing by a reaction between the photopolymerization initiator 11a and the crosslinking agent 11b. In the radiation curable pressure-sensitive adhesive sheet 1 of the present invention, when the pressure-sensitive adhesive layer 10 is stored at 50° C. for 4 weeks, a rate of change in residual stress (N/cm2) expressed by the following equation is 70% or less.Residual stress change rate (%)=(residual stress after storage at 50° C. for 4 weeks−initial residual stress)/(initial residual stress)×100