Patent GB-1447585-A (GB)
Title: Retro-reflective fiom
IPC: G09F13/16
Filed: 1974-03-21
Abstract: 1447585 Retro-reflective sheets MINNESOTA MINING & MFG CO 21 March 1974 12677/74 Heading B2E [Also in Divisions B5 and B6] A retro-reflective sheet containing a monolayer of glass microspheres of refractive index at least 1.8 backed by a partially light transmissive mirror, has retro-reflective legend areas and retro-reflective background areas wherein the microspheres and associated mirror in the legend areas have different retro-reflective efficiency from those in the background areas, so that the areas are substantially transparent under diffuse light but become retro-reflective under retro-reflective viewing conditions, the legend areas thus being substantially invisible under diffuse light but being more or less brightly reflective under retro-reflective viewing conditions.  In Fig. 1, the beads 2 are hemispherically embedded in a removable carrier 10 of a paper web 12 with a polythene coating 14. The exposed surface of the beads is given a “ wavelength coating 26 of dielectric mirror material, e.g. cryalite on the legend areas. A coating of varnish 19 which may contain pearlescent pigment particles may optionally be provided on the legend areas before or after the dielectric mirror. A continuous layer 22 of dielectric mirror of a material e.g. zinc sulphide having a different refractive index to that of the mirror 26 is then provided over the whole surface. A further layer 24 of transparent thermoplastic adhesive material is then coated onto the article, and finally the polythene coated carrier is stripped off to expose the reflective surface. The exposed bead surface may be further coated with a layer or layers of transparent resin. In the Examples, (1) a bead layer on a polythene-coated paper carrier is vacuum coated with cryolite to give a “ wavelength mirror, followed by printing in a pattern with alkyd varnish, and an overall vacuum coated “ wavelength layer of zinc sulfide. A final layer of methyl methacrylate/ethyl methacrylate copolymer is provided, the sheet is heat laminated to a photographic print, and the polythene/ paper carrier layer then stripped off. (2) A similar bead layer on polythene/paper is silk screened with an alkyd varnish pattern, the varnish containing crushed glass particles. The whole surface is then vapour coated first with cryolite, then zinc sulphide and a backing sheet of ethylene vinyl alcohol co-polymer with epichlorohydrin homopolymer is heat calendered over the mirror layer. The laminate is heat fused to a paper document, and the polythene-paper layer stripped off. (3) A bead surface as before is printed with varnish in a pattern and a zinc sulphide mirror is vapour deposited thereon, followed by an adhesive coating of the acrylic polymer used in (1), and the sheet is heat calendered to a substrate followed by removal of the carrier layer. (4) A polyethylene terephthalate biaxially oriented transparent film is coated with a mixture of polyvinylidene fluoride/hexafluoropropylene copolymer with polyethylmethacrylate in solvents, and a monolayer of beads is embedded therein. The beaded sheet is vacuum coated with cryolite, a legend is screen printed thereon with alkyd varnish, and a zinc sulphide layer is vacuum coated. An acrylate pressure sensitive adhesive is applied and the sheet is pressure laminated to a document. (5) A carrier web is coated with N-butyl methacrylate, beads are applied and a further acrylate layer is applied to embed the beads. A legend is printed on, of polyvinyl butyral containing pearlescent pigment, followed by another overall layer of the same composition. An adhesive layer of acrylate copolymer is applied, the sheet is bonded to a printed page, and the carrier web stripped off. (6) As in (5), except that in place of the pearlescent legend layer, a legend of transparent methacrylate polymer followed by vacuum deposited zinc sulphide is present. (7) A polythene/paper carrier layer with partially embedded beads is coated with a mixture of vinyl chloride/vinyl acetate/maleic acid copolymer, polyurethane resin and nacreous pigment of lead carbonate in polyvinyl chloride plastisol. After drying a legend of the same mixture is stencilled on, then a final coating of acrylic adhesive is applied. The sheet is heat bonded to a chart, and the carrier is stripped off.