Characterization and flexibility properties of UV LED cured acrylic pressure-sensitive adhesives for flexible displays

The era of the Internet of Things (IoT) is becoming a reality, and the display equipment is at its center. The display device is the window that connects the IoT and humans, and is evolving into various form factors convenient to human use, such as foldable mobile phone, flexible wearable device, paper-like display, and rollable TV. Even though transparent flexible electrodes and substrates have been developed, their assembly remains a challenge. Therefore, the importance of optically clear acrylic pressure-sensitive adhesives (PSAs) that ensure device integrity by easily stretched and mitigating the stress during repeated stretching and bending process is being emphasized. In this study, elongation and recovery features were identifiable as key flexible properties of PSAs. These properties were estimated by measuring shear strain and strain recovery using a universal testing machine and by dynamic mechanical analytics respectively. Compared to conventional UV metal halide curing, a newly introduced ultraviolet light-emitting diode (UV LED) curing process resulted in different PSA property variables, namely UV exposure intensity and UV exposure time. Fully cured acrylic PSA realized under UV LED exposure level 30 for 8 s demonstrated the highest shear strain with excellent strain recovery of ~23,500% and 94% respectively. Under these conditions, effective material adhesion performance was achieved while retaining optical transparency and thus fulfilling the dual property requirement for non-opaque material adhesiveness necessary for use in flexible displays.