Soft acrylate monomer-based optically clear adhesive for foldable electronics: Mechanical characterization and fractography analysis under large strain

Abstract

Touchscreen panels and foldable smartphones rely heavily on optically clear adhesives (OCAs) to function effectively. Therefore, these applications have created a demand for OCAs that are flexible, bendable, or stretchable with high transparency. Developing highly viscoelastic, and optically clear adhesives is crucial for flexible electronics to become commercially viable. This study focuses on synthesizing UV-curable soft acrylate-based OCAs using 2-ethylhexyl acrylate (EHA) and 2-hydroxyethyl acrylate (HEA) as functional monomers without any solvent or crosslinker. To make the OCAs stronger under various mechanical loading types, liquid OCAs with primary and final curing under pressure were used to endure high shear stress and strain. Based on the dynamic mechanical analysis results, the shear stresses of samples at room temperature and 65 °C increased from 9.41 ± 0.6 and 2.91 ± 0.31 (90 wt% EHA, cohesive failure) to 97.5 ± 8.54, and 57.81 ± 1.52 kPa (70 wt% EHA, not failed), respectively. Besides, it should be noted that the OCA hyper-viscoelasticity properties were studied in all of the mechanical experiments, and 500 % large strain was applied to match the actual strain loading in the foldable screens. In this condition, the OCA layer's large deformation behavior and other mechanical properties are more relevant for real-world applications. The fabricated OCAs in this study exhibit competitiveness comparable to commercial OCAs, such as TMS and V0, in terms of performance and quality. Moreover, the potential utilization of 70 wt% EHA (49.7 kPa) in various applications, including stretchable displays, wearable electronic devices, and foldable smartphones, is demonstrated through 3-point tests (100 cycles), wrist bending tests (100–120 cycles), and fatigue tests for up to 10,000 cycles.