Patent GB-1359166-A (GB)
Title: Panels incorporating a glass or vitrocrystalline sheet and the manufacture thereof
IPC: C03C21/00
Filed: 1971-05-21
Abstract: 1359166 Glass laminates GLAVERBEL 21 May 1971 16283/71 Heading B5N [Also in Division Cl] A panel comprising a plurality of sheet components in facing relationship and including a sheet of glass or glass-ceramic in external layers of which compressive surface stresses have been produced by a chemical tempering treatment, one side of such first sheet providing an exterior face of the panel, is characterized in that in at least one region of the first sheet, the tensile strength of the sheet expressed in terms of the tensile force, imposable by flexing the sheet, which can be sustained by the compressively stressed external layers on the exterior side is less than the tensile force necessary for reducing the compressive stresses to nil. Preferably the weakening treatment required is provided by surface abrasions, such as scratches or grooves which may extend over the whole of that side of the sheet. The panels may be plane or curved in one or more directions; they may be secured together by an intervening organic sheet or may be secured in a frame. The other sheet or sheets may be glass, glass-ceramic or plastics material and such glass or glass-ceramic sheet may also have been chemically strengthened and have similar weakening treatment preferably opposite that in the first sheet. The inner face of such sheet preferably is also subjected to a treatment to reduce impairment of such face and this treatment may either be by flame polishing or by chemical dissolution of a surface portion as by contact with a medium containing fluorine ions such as ammonium fluoride or an aqueous solution of hydrofluoric acid plus, e.g. sulphuric acid which is sprayed on. The treatment may be over only a portion of the surface, the remainder being protected as by paraffin wax, a peelable varnish or a water or like fluid film flowing over the area to be protected. In place of the chemical tempering of the sheet, a thermal tempering may be effected. Either or both of the sheets of the panel may be in a state of elastic flexure i.e. initially flat sheets hold or vice versa, either in a flame or by the other sheet. As shown in Fig. 2, sheets 5 and 7 of glassceramic are secured to an intervening sheet of polyvinylbutyral 6, both sheets 5 and 7 having beeing chemically tempered, and the external faces 1 to 4 respectively abraded over areas 9, 8 and the inner face 7 of sheet may also be abraded. As shown in Figs. 9 and 10, two sheets 20, 22 of glass of different thickness were secured either side of an organic sheet 21 and the face 4 of sheet 20 weakened by a continuous groove or scratch 19, the sheets 20, 22 then being chemically tempered and prevent dust accumulating in the groove 19, the face 4 may be covered by a thin sheet 24 of polymethylmethacrylate. In the secured position, Fig. 10, the sheets are held flat by a frame (not shown) or may be left with a curvature between that of the two sheets in their initial state. In other embodiments, the two sheets are held separately in a frame; the second sheet is of an acrylic plastics sheet glued direct to a soda lime glass sheet or, as shown in Fig. 13, sheets 27, 28 of glass and glassceramic respectively are secured together by thin copper films 29, 30 on marginal zones held together by tin solder 31, the space 32 being dehydrated or filled with glass wool. Both sheets were chemically tempered and face 4 of sheet 28 weakened by a continuous scratch 33. The abrasive material used is preferably cerium oxide or alumina or a mixture thereof, but iron oxide is also referred to. The chemical tempering treatment is preferably by contact with a molten salt, either larger, e.g. K ions, or smaller, e.g. Li ions or Na ions. Baths of molten salts, used in examples, are of KNO 3  for glass-ceramic, and for soda lime glass, KNO 3  alone or with K 2 CO 3  or LiNO 3  + NaCl. Fire polishing of the sheet is effected by placing the sheet with the appropriate side, 2, face downwards over a flame obtained by passing a mixture of propane and compressed air through a porous refractory member and igniting the mixture above the member, the sheet being held in the flame for 45 seconds. In addition to polyvinyl butyral, the intervening sheet may be polyvinylchloride or a high molecular weight polycarbonate of a bisphenol and the adhesive used in a polyvinylacrylate. Specifications referred to are 1,274,731, 1,359,165, 1,359,168, 1,359,169 and 1,359,170.