Dual-cured organic–inorganic hard coating with high flexibility and transparency based on polysilsesquioxane and a functional polymer

Transparent hard-coating materials with glass-like wear resistance and plastic-like flexibility have attracted attention owing to their applicability in flexible/foldable displays. Materials with wear-resistant and bendable properties are rare because inorganic materials are normally hard and brittle, whereas organic materials, although flexible, typically lack wear resistance. This study entailed the development of a flexible hard coating, prepared by crosslinking a cycloaliphatic epoxy-functionalized oligosiloxane (CEOS) and hydroxylic-functionalized acrylic polymer with phthalic anhydride (PA) to form a highly crosslinked organic–inorganic coating network. CEOS, a UV-curable high-hardness inorganic base coating material, was synthesized by a sol–gel reaction. Thermally curable acrylic polymer and PA as a crosslinker were then introduced to simultaneously improve flexibility and hardness. Two differently curable materials were blended to maximize the coating properties through the synergistic effect of the different curing characteristics. To optimize the curing system, a dual-curing process involving UV irradiation and heat treatment was introduced. In the first step, UV irradiation initiated the ring-opening polymerization of the epoxide rings of CEOS. In the second step, the epoxide ring of CEOS, the hydroxylic group of the acrylic polymer resin, and the anhydride group of PA were thermally cured, and a multi-crosslinked network coating film was fabricated. The prepared coating exhibited high hardness (nano-indentation hardness of 0.63 GPa and a pencil hardness of up to 8H) and outstanding optical transparency (up to 91 % at 500 nm). The coated layer on the PET film exhibited high flexibility and could be bent to form a U-shaped cross-section without coating damage.