Preparation of moisture curable polysilazane coatings Part I. Elucidation of low temperature curing kinetics by FT-IR spectroscopy

Moisture-curing of perhydropolysilazane and polyorganosilazanes to pre-ceramics has been studied at temperatures ranging from room temperature to about 150 degrees C. Results of FT-IR experiments with a diamond ATR adapter clearly showed that rapid curing required the presence of a moisture-containing atmosphere of about 80% relative humidity and the introduction of a catalytic curing agent such as ammonia or an amine. With respect to the curing rate, the effect of temperature increase can be compensated by a decrease of humidity, that is why moistening furnace are recommended for polysilazane transformation. The mechanism of moisture-cure mainly based on the hydrolysis of Si-H and Si-N bonds by water vapor to promote subsequent cross-linking of intermediately formed silanols via condensation reactions. Compared to neat polyorganosilazanes, blends with perhydropolysilazane revealed both higher curing rate and improved surface hardness. Polysilazane films of about 3 mu m thickness coated on polycarbonate plates and cured at 50 degrees C in air of about 80% relative humidity for 1 h exhibited microhardness of about 450 N mm(-2) compared to about 250N mm(-2) obtained for acrylate/silica nano-composite coatings. These findings point to dense polysilazane films what makes them very promising as coatings for technical applications, e.g., reduced gas permeability and improved corrosion resistance. (c) 2005 Elsevier B.V. All fights reserved.