Partially submerged bead monolayer
The invention provides a certain type of optical film, processes for making the film, and a backlight device containing the film. The film includes a substantially monolayer arrangement of beads partially submerged in a binder.
1 . A process for forming an optical film comprising the steps of:
(a) applying to a transparent substrate a coating comprising (1) a carrier (2) an aqueous dispersible or soluble polymeric binder and (3) a bead population of desired refractive index and low polydispersity, wherein the binder is selected to have a refractive index that substantially matches that of the beads upon removal of the carrier, and wherein the relative proportions of the coating components is sufficient to provide a substantially mono layer arrangement of the beads that is partially submerged in the binder upon removal of the carrier; and
(b) removing the carrier from the coating to provide a substrate coated with a substantially mono layer arrangement of the beads that is partially submerged in the binder.
2 . The process of claim 1 wherein the weight % binder is selected so as to obtain a desired degree of submersion whereby a desired angular light distribution can be obtained.
3 . The process of claim 1 wherein the proportions are selected to provide a substantially mono-layer arrangement of the beads that is partially submerged in the binder so that the weight % binder is from 15 to 80%.
4 . The process of claim 1 wherein the proportions are selected to provide a substantially mono-layer arrangement of the beads that is partially submerged in the binder so that the weight % binder is from 20 to 70%.
5 . The process of claim 1 wherein the proportions are selected to provide a substantially mono-layer arrangement of the beads that is partially submerged in the binder so that the weight % binder is from 25 to 60%.
6 . The process of claim 1 wherein the binder is selected to provide a sol-gel transition temperature below 30° C.
7 . The process of claim 1 wherein the binder is selected to provide a sol-gel transition temperature below 20° C.
8 . The process of claim 1 wherein the binder is selected to provide a sol/gel transition temperature below 10° C.
9 . The process of claim 1 wherein the binder comprises a gelatin.
10 . The process of claim 1 wherein the binder comprises a gelatin derived from a marine species.
11 . The process of claim 1 wherein the binder comprises fish gelatin.
12 . The process of claim 1 wherein the refractive index of the bead is within 0.2 of that of the binder after carrier removal.
13 . The process of claim 1 wherein the beads have a coefficient of variation of less than 0.2.
14 . The process of claim 1 wherein the beads have a coefficient of variation of less than 0.15.
15 . The process of claim 1 wherein the carrier is water.
16 . The process of claim 1 wherein the binder is subjected to a crosslinking step.
17 . The process of claim 1 wherein the carrier is removed by drying in air and the surface tension at the air/coating interface is at least 25 dynes/cm.
18 . The process of claim 1 wherein the refractive index of the beads and the binder is at least 1.2.
19 . A light redirecting device comprising in order:
(a) a light source and
(b) an optical film which comprises, in the direction of light travel, a transparent substrate bearing a layer containing a bead population of desired refractive index and low polydispersity disposed in an aqueous dispersible or soluble polymeric binder, wherein:
the bead refractive index is matched to the binder,
the bead population is present in a substantially monolayer arrangement of the beads with the beads partially submerged in the binder; and
(c) a transparent medium exhibiting a refractive index at least 0.2 less than that of the bead population.
20 . The device of claim 19 wherein the binder comprises gelatin.
21 . The device of claim 19 wherein the binder comprises gelatin having a sol-gel temperature less than 30° C.
22 . The device of claim 19 wherein the binder comprises gelatin having a sol-gel temperature less than 20° C.
23 . The device of claim 19 wherein the binder comprises gelatin having a sol-gel temperature less than 10° C.
24 . The device of claim 19 wherein the binder comprises fish gelatin.
25 . The device of claim 19 wherein the binder is a gelatin that comprises an imino content of less than 20% by weight.
26 . The device of claim 19 wherein the beads have an average diameter of from 1 to 20 μm.
27 . The device of claim 19 wherein the proportions of bead and binder are selected to provide a substantially mono-layer arrangement of the beads that is partially submerged in the binder so that the weight % binder is from 15 to 80%.
28 . The device of claim 19 wherein the proportions of bead and binder are selected to provide a substantially mono-layer arrangement of the beads that is partially submerged in the binder so that the weight % binder is from 20 to 70%.
29 . The device of claim 19 wherein the proportions of bead and binder are selected to provide a substantially mono-layer arrangement of the beads that is partially submerged in the binder so that the weight % binder is from 25 to 60%.
30 . The device of claim 19 wherein the beads are ellipsoidal.
31 . The device of claim 19 wherein the beads are spherical.
32 . The device of claim 19 wherein the beads contain a polymer or copolymer based on one selected from styrene, acrylate, methacrylate, and vinylidene chloride.
33 . The device of claim 19 wherein the beads are made by the process of “limited coalescence”.
34 . The device of claim 19 wherein a diffusive component providing uniform illumination and hiding the light source appearance is located between the light source (a) and the optical film (b),
35 . The device of claim 34 additionally comprising a reflective polarizer.
36 . The device of claim 19 wherein a component (d) comprises a pixelated light-gating device creating images.
37 . An optical film which comprises a transparent substrate bearing a layer containing a bead population of desired refractive index and low polydispersity disposed in an aqueous dispersible or soluble polymeric binder, wherein:
the bead refractive index is matched to the binder,
the bead population is present in a substantially monolayer arrangement of the beads with the beads partially submerged in the binder.
38 . A process for forming an optical film comprising the steps of:
(a) applying to a substrate a coating comprising a polymer in a solvent;
(b) drying the coating in a humid atmosphere to create a micro-voided polymer film;
(c) coating a curable composition over the micro-voided polymer film;
(d) curing the curable composition by heat or by radiation; and
(e) separating the cured optical film composition from the micro-voided polymer film.