Patent GB-618477-A (GB)
Title: Improvements in or relating to the production of optical elements
IPC: B29D11/00
Filed: 1946-09-02
Abstract: A rigid preform is coated with a polymerizable adhesive composition, the coating is polymerized and adheres to the preform, then a film of polymerizable syrup is trapped between the coated preform and a mould surface, and finally the film is polymerized by heat and/or light at a temperature insufficient to cause the formation of bubbles.  If the preform is transparent, lenses and prisms may thus be made; the radius of curvature of the surface of a lens, or the angle between the two surfaces of a prism may be corrected.  If the preform is opaque, front reflecting mirrors and diffraction gratings may be made.  The polymerizable adhesive composition is composed essentially of one or more compounds which are in liquid form and contain, per molecule, one or more CH2=C< groups and no hydroxyl, amino, 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 polymerizable adhesive composition comprises 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 adhesive composition, e.g. acrylic, chloroacrylic, methacrylic, maleic or fumaric acids.  The polymerizable adhesive composition may contain also one or more dissolved thermoplastic polymers having the CH2=C< group. The syrup is composed essentially of one or more liquid compounds having one or more CH2=C< groups, e.g. styrene, methyl methacrylate or the other compounds mentioned above.  The preform may be heated and then cooled, prior to coating, to remove moisture and adsorbed gases.  The adhesive composition and syrup may be subjected to vacuum treatment to remove dissolved gases and air bubbles.  The initial coating may be in the form of a dilute solution in a volatile solvent and may be applied to the preform by brushing, dipping or spraying; the solvent is allowed to evaporate whilst the preform is rotated at uniform speed to ensure the formation of a uniform coating.  The film may be applied to the coated preform by placing a drop of syrup on the shaped surface of the mould, placing the coated 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 coated preform for half-an-hour before polymerization, to ensure uniform thickness of the film.  Formation of bubbles in the film may be minimized by applying pressure to the material during polymerization, e.g. in an autoclave.  As the film 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.ALSO:A rigid preform, e.g. of glass, metal, moulded thermosetting resin, ceramic material, plaster or crystalline mineral, is coated with a polymerizable adhesive composition, the coating is polymerized and adheres to the preform, then a film of polymerizable syrup is trapped between the coated preform and a mould surface, and finally the film is polymerized by heat and/or light at a temperature insufficient to cause the formation of bubbles.  If the preform is transparent, lenses and prisms may thus be made; the radius of curvature of the surface of a lens, or the angle between the two surfaces of a prism may be corrected. If the preform is opaque, front reflecting mirrors and diffraction gratings may be made.  A mirror may be made by evaporating the metal from a heated filament on to the (doubly) coated preform in a highly-evacuated chamber.  The polymerizable adhesive composition 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 polymerizable adhesive composition comprises 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 adhesive composition, e.g. acrylic, chloroacrylic, methacrylic, maleic or fumaric acid.  The polymerizable adhesive composition may contain also one or more dissolved thermoplastic polymers having the CH2=C< group. The syrup is composed essentially of one or more liquid polymerizable organic compounds having one or more CH2=C< groups, e.g. styrene, methyl methacrylate or the other compounds mentioned above.  The accuracy of the solid coated surface of the preform (i.e. after the first coating but before the film of syrup has been moulded thereon) is such that the maximum allowable variation in the said surface from the ideal surface of the finished optical element (i.e. with the film of syrup moulded thereon) is equal to the maximum variation allowed (in the finished optical element) 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.  The accuracy of the mould must be such that its deviation from the ideal is negligible compared with the deviation of the coated preform.  In an example, the maximum deviation of the mould is 0.0001 inches, that of the coated preform is 0.00073 inches, d p is 1.189 and ds is 1.027.  The preform may be heated and then cooled, prior to coating, to remove moisture and adsorbed gases.  The adhesive composition and syrup may be subjected to vacuum treatment to remove dissolved gases and air bubbles.  The initial coating may be in the form of a dilute solution in a volatile solvent and may be applied to the preform by brushing, dipping or spraying; the solvent is allowed to evaporate whilst the preform is rotated at uniform speed to ensure the formation of a uniform coating.  The film may be applied to the coated preform by placing a drop of syrup on the shaped surface of the mould, placing the coated 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 coated preform for half an hour before polymerization, to ensure uniform thickness of the film.  Formation of bubbles in the film may be minimized by applying pressure to the material during polymerization, e.g. in an autoclave.  As the film 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.  Specifications 582,248, 585,580 and 614,459 are referred to.