Patent GB-582248-A (GB)
Title: Improvements in the moulding of optical elements from synthetic resins
IPC: G02B1/04
Filed: 1944-02-04
Abstract: A preform of thermoplastic synthetic resin is made (or selected) having a certain accuracy of shape, and between the surface of the preform and the surface of a mould also having a certain accuracy of shape is trapped a film of syrup composed essentially of one or more liquid polymerisable organic compounds having one or more CH2=C< groups per molecule, the syrup being capable of dissolving and/or swelling the surface of the preform.  The film of syrup is then polymerized by heat and/or light at a temperature insufficient to cause the formation of bubbles.  The syrup may contain dissolved organic polymerized compounds containing one or more CH2=C< groups per molecule, plasticizers, dyes, polymerization catalysts and (if transparent films are not required) fillers and inorganic pigments. The syrup liquid may be methyl methacrylate, styrene, allyl methacrylate or diallyl fumarate, the two last-named being preferably in admixture with methyl methacrylate.  The dissolved polymer may be polymethyl methacrylate.  Lenses, prisms, mirrors, flats, and reflecting gratings may be made.  In setting up the mould, a drop of the syrup is placed in the bottom part of the mould, the preform (of polymethyl methacrylate or polystyrene, cast, cut, ground or press-moulded, and preferably having been freed from internal stresses by annealing in an inert liquid as described in Specification 580,855) is placed on the drop, another drop is placed on the preform, and the top of the mould is placed in position.  The whole is then allowed to stand for 1/4 hour under non-polymerizing conditions, and then the film is polymerized by heat and/or light. The formation of bubbles in the film may be minimized by the application of pressure to the mould, e.g. in an autoclave.  As the syrup film polymerizes and p  contracts, there is a tendency for air to be drawn into the edges of the film; this can be overcome by making the mould surfaces larger than the preform surfaces, so that syrup, not air, is sucked in.  The refractive index of the film may be the same as or different from that of the preform.ALSO:A preform of thermoplastic synthetic resin is made (or selected) having a certain accuracy of shape, and between the surface of the preform and the surface of a mould also having a certain accuracy of shape is trapped a film of syrup composed essentially of one or more liquid polymerizable organic compounds having one or more CH2=C< groups per molecule, the syrup being capable of dissolving and/or swelling the surface of the preform.  The film of syrup is then polymerized by heat and/or light at a temperature insufficient to cause the formation of bubbles.  The syrup may contain dissolved organic polymerized compounds containing one or more CH2=C< groups per molecule, plasticizers, dyes, polmerization catalysts and (if transparent films are not required) fillers and inorganic pigments. The accuracy of shape of the preform must conform to the equation:- Maximum variation from the ideal surface in the preform of the dimension to be corrected (in degrees, inches &c.} = {Maximum variation from the ideal surface in the finished optical element of the same dimension (in degrees, inches &c.)} X dp/dp-ds where dp is the density of the polymer produced from the syrup and ds is the density of the syrup.  Thus, if the syrup has a density of 1.027 and the polymer produced from the syrup has a density of 1.189, and it is desired to produce a prism having the angle between a pair of faces correct to within plus or minus 40 seconds, the maximum permissible variation in the corresponding angle in the preform is 40 seconds x 1.189/1.189-1.027 i.e., 294 seconds.  The accuracy of the shape of the mould must be such that its deviation from the ideal surface is negligible compared with the deviation of the preform from the ideal surface.  The syrup liquid may be methyl methacrylate, styrene, allyl methacrylate or diallyl fumarate, the two last-named being preferably in admixture with methyl methacrylate.  The dissolved polymer may be polymethyl methacrylate.  Lenses, prisms, mirrors, flats, and reflecting gratings may be made.  In setting up the mould, a drop of the syrup is placed in the bottom part of the mould, the preform (of polymethyl methacrylate or polystyrene, cast, cut, ground or press-moulded, and preferably having been freed from internal stresses by annealing in an inert liquid as described in Specification 580,855, [Group IV], is placed on the drop, another drop is placed on the preform, and the top of the mould is placed in position.  The whole is then allowed to stand for 1/4 hour under non-polymerizing conditions, and then the film is polymerized by heat and/or light.  The mould may be of glass, steel or soft metal coated with hard metal, and may be lubricated. The formation of bubbles in the film may be minimized by the application of pressure to the mould, e.g., in an autoclave.  As the syrup film polymerizes and contracts, there is a tendency for air to be drawn into the edges of the film; this can be overcome by making the mould surfaces larger than the preform surfaces, so that syrup, not air, is sucked in.  The refractive index of the film may be the same as or different from that of the preform. Specifications 482,836; 567,777 and 567,778, [both in Group IV], also are referred to.