FLEXIBLE MATERIALS FOR OPTICAL APPLICATIONS
A flexible material for optical applications in a wavelength range of λ 1 to λ 2 , λ 1 being smaller than λ 2 , composed of a flexible support and at least one multilayer that comprises a porous or nanoporous layer which has a low refractive index and contains inorganic nanoparticles and at least one binder, and a non-porous polymer layer which has a high refractive index and is in direct contact with the porous or nanoporous layer; said flexible material is characterized in that the maximum thicknesses of the boundary layers, in which the refractive index changes from one value to the other and which are located between the porous or nanoporous layers and the non-porous polymer layers that are in direct contact therewith, amount to 0.2 times wavelength λ 2 ; and the difference in the refractive indices of the porous or nanoporous layers and the non-porous polymer layers is at least 0.20, 200 nm and 2500 nm being typical values for λ 1 and λ 2 .
1 . Flexible material for optical applications in a wavelength range of λ 1 to λ 2 , λ 1 being smaller than λ 2 , composed of a flexible support and at least one multilayer that comprises a porous or nanoporous layer having a low refractive index and contains inorganic nanoparticles and at least one binder, and a non-porous polymer layer having a high refractive index and which is in direct contact with the porous or nanoporous layer, wherein the maximum thicknesses of the boundary layers, in which the refractive index changes from one value to the other and which are located between the porous or nanoporous layers and the non-porous polymer layers that are in direct contact, amount to maximally 0.2 times wavelength λ 2 .
2 . Material according to claim 1 , wherein λ 1 is above 200 nm and λ 2 is below 2500 nm.
3 . Material according to claim 1 , wherein the difference of the refractive indices of the porous or nanoporous layer and the non-porous polymer layer is at least 0.20 in the wavelength range λ 1 to λ 2 .
4 . Material according to claim 1 , wherein the material has one multilayer on the support.
5 . Material according to claim 4 , wherein there is a second multilayer on top of the first multilayer.
6 . Material according to claim 5 , wherein the sequence of the layers in the second multilayer is the same as in the first multilayer.
7 . Material according to claim 5 , wherein the sequence of the layers in the second multilayer is opposite to the sequence in the first multilayer.
8 . Material according to claim 1 , wherein the porous or nanoporous layer of the first multilayer is in direct contact with the support.
9 . Material according to claim 1 , wherein the non-porous polymer layer of the first multilayer is in direct contact with the support.
10 . Material according to claim 1 , wherein the dry thicknesses of the porous or nanoporous layers are from 0.2 μm to 60 μm and the dry thicknesses of the non-porous polymer layers are from 0.05 μm to 2.5 μm.
11 . Material according to claim 1 , wherein the inorganic nanoparticles are selected from the group consisting of precipitated or fumed silicium dioxide, aluminum oxide, aluminum oxide/hydroxide, zeolite beta, zeolite ZSM-5, zeolite mordenite, zeolite LTA (Linde type A), zeolite faujasite and zeolite LTL (Linde type L) or mixtures of these compounds.
12 . Material according to claim 11 , wherein the inorganic nanoparticles have a mean particle diameter between 5 nm and 200 nm.
13 . Material according to claim 1 , wherein the amount of binder in the porous or nanoporous layer containing inorganic nanoparticles is from 0.5 percent by weight to 60 percent by weight relative to the amount of nanoparticles contained in this layer.
14 . Material according to claim 1 , wherein the binder in the porous or nanoporous layer is selected from the group consisting of modified and non-modified polyvinyl alcohol, polyvinyl pyrrolidone or mixtures of these compounds.
15 . Material according to claim 1 , wherein the polymer in the non-porous polymer layer is selected from the group consisting of modified polyvinyl alcohol, polyurethane, (meth)acrylated polybutadiene, copolymers of (meth)acrylamide and polyacrylnitrile or mixtures of these compounds.
16 . Material according to claim 1 , wherein the non-porous polymer layer consists of water dispersible thermoplastic polymers having glass transition temperatures between 30° C. and 170° C., and where the non-porous polymer layer is formed by a heat treatment under pressure.
17 . Material according to claim 16 , wherein the water dispersible thermoplastic polymers are selected from the group consisting of particles, latices or waxes of polyethylene, polypropylene, polytetrafluoroethylene, polyamides, polyesters, polyurethanes, acrylnitriles, polymethacrylates, polyacrylates, polystyrene, polyvinyl chloride, polyethylene terephthalate, copolymers of ethylene and acrylic acid and paraffin waxes.
18 . Method of preparation of the materials according to claim 1 , wherein the porous or nanoporous layer containing inorganic nanoparticles and the non-porous polymer layer are applied to the flexible support in two separate coating passes.
19 . Method of preparation of the materials according to claim 18 , wherein the flexible support is coated by the cascade or curtain coating process.