Patent GB-614459-A (GB)
Title: Improvements in and relating to the production of opaque optical components
IPC: G02B1/04
Filed: 1946-07-09
Abstract: A polymerizable syrup for coating a metal core to form a diffraction grating or mirror base comprises (a) one or more polymerizable organic compounds which are in liquid form and contain, per molecule, one or more CH2= C< groups and no hydroxyl, amine, carboxyl or other hydrophilic groups, and (b) 1-30 per cent by weight of the syrup of one or more unsaturated carboxylic acids and/or carboxylic acid anhydrides that are capable of copolymerizing with the polymerizable constituents of the syrup.  The syrup may contain also one or more dissolved thermoplastic polymers or copolymers containing the group CH2=C<.  The said organic compounds may be styrene, methyl methacrylate, cyclohexyl methacrylate, 2,2,2-trifluoroethyl methacrylate, esters of alpha-fluoroacrylic acid and fluorinated alcohols, allyl methacrylate, diallyl fumarate, diethylene di-(allyl carbonate) and di-(methacryloxy ethylene) phthalate. The said acids may be acrylic, chloroacrylic, methacrylic, maleic and fumaric acids. The syrup may be made by dissolving polymer in monomer, and/or by heating monomer, and/or by subjecting monomer to light.  The polymer that is dissolved in the monomer may be of low molecular weight, e.g. 5000 to 20,000.  Such low molecular weight polymers may be obtained by heating high molecular weight polymers, e.g. on hot mixing rolls, or by polymerizing monomer in the presence of either large amounts of catalyst such as benzoyl peroxide or a chain-ending compound such as turpentine.  The coating is polymerized by heat and/or light.  The presence of bubbles in the polymerized coating may be minimized by subjecting the syrup to vacuum treatment before polymerization and by applying pressure to the syrup during polymerization, e.g. in an autoclave.  As the syrup polymerizes and contracts, there is a tendency to draw air into the coating; this may be offset by providing a reservoir of the syrup around the edge of the coating, so that syrup rather than air is drawn in between the core and the mould that is used to form the coating.ALSO:A film of synthetic resin is trapped between the surface of a metal preform and a mould surface and is polymerized by heat and/or light at a temperature insufficient to cause the formation of bubbles in the film of syrup or to melt the preform. Diffraction gratings composed essentially of metal may be made thus, or the radius of curvature of a metal surface to be used as a base for a mirror may be corrected.  The coated metal preform may be made into a mirror by coating the resin film with metal, e.g. by evaporating the metal from a heated filament on to the film in a highly evacuated chamber.  The syrup is composed essentially of one or more polymerizable organic compounds which are in liquid form and contain, per molecule, one or more CH2=C< groups and no hydroxyl, amine, carboxyl or other hydrophilic groups, e.g. styrene, methyl methacrylate, cyclohexyl methacrylate, 2, 2, 2-trifluorethyl methacrylate, esters of alpha fluoroacrylic acid and fluorinated alcohols, allyl methacrylate, diallyl fumarate, diethylene di (allyl carbonate) or di (methacryloxy ethylene phthalate).  The syrup contains also 1 - 30 per cent. by weight of one or more unsaturated carboxylic acids and/or acid anhydrides that are capable of co-polymerizing with the polymerizable constituents of the film, e.g. acrylic, chloroacrylic, methacrylic, maleic or fumaric acid.  The syrup may contain also one or more dissolved thermoplastic polymers, e.g. polymethyl methacrylate.  The metal preform has an accuracy of shape and surface such that the maximum allowable variation in the preform from the ideal surface of the resin-coated preform is equal to the maximum variation allowed (in the finished optical component) from the ideal surface multiplied by dp/dp-ds, where d p is the density of the polymer produced from the syrup and d s is the density of the syrup.  Thus, if it is desired to make a plane mirror in which the maximum variation from a true plane must not be greater than 0.00004 inch, using a syrup the specific gravity of which is 1.027 and the specific gravity of the polymer produced from which is 1.189, the maximum permissible variation in the surface of the preform from the true plane is 0.0004 x 1.189/1.189-1.027=0.00029 inch.  The mould has an accuracy such that deviations of its surface from the ideal are negligible compared with deviations of the preform surface.  The presence of bubbles in the film of resin that coats the metal preform may be minimized by subjecting the syrup to vacuum treatment before polymerization and by applying pressure to the material during polymerization, e.g. in an autoclave.  The film may be applied to the preform by placing a drop of syrup on the shaped surface of the mould, placing the preform in the drop, and holding the preform in this position (spaced from the mould surface) before and during polymerization.  The syrup may be allowed to stand as a film between the mould and preform for half an hour before polymerization, to ensure uniform thickness of the film.  As the syrup polymerizes and contracts, there is a tendency to draw air into the film; this may be offset by providing a reservoir of syrup around the edge of the film, so that syrup rather than air is drawn in.  The metal of the preform may be steel, cast iron, copper, brass, lead or light alloy.  The mould may be made of glass, and may be coated with lubricant, e.g. ethyl palmitate.