Multilayer NIR reflective 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, alcoho lic sols of surface modified (3-glycidoxypropyltrimethoxysilane, GPTS) SiO2 and TiO2 particles with sizes of 10 and 4 nm, respectively, were synthesiz ed. Layers were prepared by dip coating, subsequent UV curing (2.1 J/cm(2)) acid thermal post treatment at 80 degrees C for 15 min. Refractive indices of n(D) = 1.47 for SiO2 layers and n(D) = 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 sin inclination angle of 4 degree s. By this way, one side of the substrate was coated with six quarterwave t hick layers (peak wavelength at 750 nm), producing a reflective interferenc e filter with a reflectivity of 72% between 650 and 900 nm. On the other si de of the substrate, an interference filter with a reflectivity of 66% betw een 800 and 1100 nm was produced simultaneously. The interference coatings do nor show delamination or defects after boiling water test (H2O + 5 wt.% NaCl, 8 h) and excellent adhesion (GT 0, TT 1) was obtained in the cross cu t tape test. Yellowing did not occur (Delta(g) < 3) and the mechanical prop erties of the interference filters were not altered after dry sun test at 7 60 W/m(2) for 270 h. For mechanical protection a nanocomposite hardcoat can be applied on top of the NIR reflectance filters without changing the opti cal properties remarkably. 0 1999 Elsevier Science S.A. All rights reserved .