Synthesis and properties of silsesquioxane-based hybrid urethane acrylate applied to UV-curable flame-retardant coatings

The silsesquioxane-based hybrid urethane acrylate (SHUA) was synthesized by modifying silsesquioxane-based hybrid polyol (SBOH) with the half adduct of isophorone diisocyanate and 2-hydroxyethyl acrylate, and characterized by Fourier transfer infrared (FTIR) and ¹H NMR spectroscopy. The SHUA was mixed with a phosphorus-containing trifunctional acrylate, TAEP, with different ratios to prepare a series of UV-curable organic–inorganic hybrid flame-retardant resins. Both the maximum photopolymerization rate and final unsaturation conversion in the UV-cured films determined by photo-DSC decreased along with SHUA content due to the decrease in functionality density and the increase in viscosity. The limiting oxygen index indicated the flame-retardant property reached to a high level of over 32 even with 40% SHUA addition, while the initial decomposition temperature and the char residue at 850 °C increased. The chemical structure change during the thermal degradation process was monitored by in situ FTIR analysis to demonstrate the condensed-phase flame-retardant mechanism. The dynamic mechanical thermal analysis results showed that the elastic storage modulus on the rubbery plateau region and glass transition temperature of the cured film increased by incorporating a small amount of SHUA, but decrease with over 10 wt.% addition of SHUA. Moreover, the addition of SHUA led to an increase in elongation at break and enhancement in abrasion resistance and hardness of film, even though no enhancement in tensile strength.