Design of high capacitance hard coating layer for improving touch sensitivity of foldable display cover window film

Abstract

Touch screen panel is a simple and convenient input device which is being applied for wide electronic devices. Especially for mobile devices, touch screen panel allows wider display by replacing physical buttons. However, for foldable display devices, display materials are being replaced from glass to colorless polyimide (CPI) or cover film which has low permittivity. Permittivity of the display surface affects sensitivity of touch. In this reason, evolution of display structure and material is required which can maintain durability and increase capacitance of foldable display. In this research, new foldable display cover film with hard coating (HC) layer was designed to achieve high capacitance and high touch sensitivity. Main strategy was to add thiol group in the hard coating layer to increase permittivity of the layer. Thiol group contains sulfur atom which has high permittivity. Based on the main strategy, two different types of hard coating layer were designed. First method was to use additive containing thiol group to hard coating layer. Increasing amount of thiol additive could increase capacitance of total hard coating layer. Second method is to fabricate additional high permittivity layer between the substrate and hard coating layer, which increase total capacitance of the total film. Oligosiloxane resin made by silane with thiol group was mixed with oligosiloxane resin made by silane with methacrylate group or crosslinker containing methacrylate groups, and this resin was cured by thiol-ene reaction to fabricate high permittivity layer. Permittivity, thermal stability, mechanical properties were investigated while controlling thiol resin ratio and thickness of the high permittivity layer.