Development of fast switching photochromic coatings on transparent plastics and glass

A new photochromic organic-inorganic nanocomposite (Nanomer®) coating system has been developed. It is based on an epoxysilane as network former, an organic bisepoxide as spacer, an organic amine as thermal cross-linker and surface modified SiO2 nanoparticles as fillers. This coating system is compatible with different photochromic dyes, like oxazines and pyrans.

Photochromic coatings with blue, yellow, red, green, violet and neutral tints were prepared on flat glass and PMMA substrates by dip coating and cured for 3 h at 100°C. The photochromic coatings show the following properties: the transmittance changes reversibly between 80 and 20% with half fading times of about 2–20 s (comparable to the half fading time of the appropriate dye in ethanolic solution) after UV irradiation for 15 s with 5 mW/cm2.

The coatings show a scratch hardness of about 15 g (scratch test with Vickers indenter, coating thickness 10 μm). The addition of only 3 wt.% SiO2 nanoparticles relative to epoxysilane increases the scratch resistance of the coatings to about 20 g without changing the fast kinetics of the incorporated photochromic dye(s).

The long term stability of the photochromic dye(s) in the matrix system can be considerably improved by the introduction of additives like antioxidants, hindered amine light stabilizer (HALS) and UV stabilizers. The half lifetime (decay to 50% of the initial photochromic intensity) of a blue spirooxazine dye (Blue A) measured in a dry sun-test (75 mW/cm2) could be increased from 20 h without any additive up to 200 h with an UV absorber (Tinuvin 327) as a stabilizer, which is assumed to be sufficient for ophthalmic applications.