Multilayer NIR re ̄ective coatings on transparent plastic substrates from photopolymerizable nanoparticulate sols

A new synthesis and processing route for preparing multilayer interference coatings on plastic substrates has been developed. For this purpose, alcoholic sols of surface modi®ed (3 glycidoxypropyltrimethoxysilane, GPTS) SiO2 and TiO2 particles with sizes of 10 and 4 nm, respectively, were synthesized. Layers were prepared by dip coating, subsequent UV curing (2.1 J/cm) and thermal post treatment at 808C for 15 min. Refractive indices of nD ˆ 1:47 for SiO2 layers and nD ˆ 1:93 for TiO2 were measured. As an example, plastic sheets were coated by angle dependent dip coating with withdrawal speeds from 3 up to 6 mm/s and an inclination angle of 48. By this way, one side of the substrate was coated with six quarterwave thick layers (peak wavelength at 750 nm), producing a re ̄ective interference ®lter with a re ̄ectivity of 72% between 650 and 900 nm. On the other side of the substrate, an interference ®lter with a re ̄ectivity of 66% between 800 and 1100 nm was produced simultaneously. The interference coatings do not show delamination or defects after boiling water test (H2O 1 5 wt.% NaCl, 8 h) and excellent adhesion (GT 0, TT 1) was obtained in the cross cut tape test. Yellowing did not occur (Dg , 3) and the mechanical properties of the interference ®lters were not altered after dry sun test at 760 W/m for 270 h. For mechanical protection a nanocomposite hardcoat can be applied on top of the NIR re ̄ectance ®lters without changing the optical properties remarkably.