Chemical modification of cured MY720/DDS epoxy resins using fluorinated aromatic compounds to reduce moisture sensitivity

Blocking the hydroxyl, amine, and epoxide functional groups of cured MY720/DDS epoxy thin films with various fluorinated aromatic reagents reduces the equilirium moisture absorption by as much as 75%. The blocking reactions were temperature‐, time‐, and diffusion‐dependent. 2,4‐Difluorophenyl isocyanate was a more effective blocking reagent than trifluoromethylphenyl isocyanates (i.e., ortho‐, meta‐, and para‐) due to its higher reactivity. Among the trifluoromethylphenyl isocyanates, meta‐trifluoromethylphenyl isocyanate exhibited the greatest extent of reaction based on the reduction of IR peaks at 3400 and 904 cm−1, presumably due to the slightly poorer inductive effect inherent in meta‐trifluoromethylphenyl isocyanate, permitting a greater extent of diffusion into the bulk polymer. Pentafluorobenzoyl chloride was a more efficient blocking reagent compared to the fluorinated aromatic isocyanates. DSC thermogram data indicated that reaction of pentafluorobenzoyl chloride at 90°C for 2 days with the epoxy groups was essentially complete. The treated films were stable to hydrolysis even after immersion in distilled water at room temperature for 2 months as measured by IR spectroscopy.