Assessment of Fatigue Strain in Hard Coating on Colorless Polyimide Film under Folding via Digital Image Correlation Method

Abstract

Foldable display is an emerging technology as a next generation of flexible display. Though, one of the limitations of current foldable devices is its vulnerability of the outermost surface under repeated external forces such as folding stress or scratch. To protect the display from external stresses, various materials are suggested as flexible hard coating. Recently, Bae et al. reported a siloxane based flexible hard coating which has plastic-like flexibility and glass-like wear resistance.1,2 Along with the high scratch resistance and transparency, low fatigue after repeated folding makes Flex9H® hard coating one of the potential candidate hardcoating for foldable display. In this study, folding fatigue of Flex9H® hard coating on colorless polyimide (CPI) film was assessed by digital image correlation (DIC) method. DIC is an optical technique of non-destructive mechanical examination, which effectively measure deformations of materials by tracking relative displacement of standard points. With the advantage of size-independent nature of DIC analysis, microscale deformation of film cross-section was measured. Flex9H® hard coating on CPI film was assessed before and after dynamic or static folding test. The remaining strain on Flex9H® hard coating on CPI film was almost zero, even in various stress environments, such as cyclic test of 1×105 times folding and 10 days of static folding under various temperature and humidity conditions.