Photochromic glasses

US4092174A · kind A · utility

Assignee

• Pilkington Brothers Limited · GB

Inventors

• Edric Ellis · Huntingdon, GB
• Richard Gelder
• Allan Hale

Key dates

• Filing date: Sep 21, 1977
• Grant date: May 30, 1978
• Expiry date: Sep 21, 1997

Classification

• Technology area (CPC Y): Emerging Cross-Sectional Technologies
• CPC primary: Y10S501/903
• WIPO field: Materials, metallurgy
• WIPO sector: Chemistry

Abstract

Fast-response alumino-phosphate photochromic glasses having silver halide crystals dispersed throughout the glass consisting essentially of, in weight percentages: PA0 SiO.sub.2 : 8.5 to 25%, Al.sub.2 O.sub.3 : 13 to 36.5%, P.sub.2 O.sub.5 : 7.5 to 33.5%, B.sub.2 O.sub.3 : 7 to 28%; R.sub.2 O: 7 to 20.5%, R'O:0 to 21%, TiO.sub.2 : 0 to 6%, ZrO.sub.2 : 0 to 10%, PbO:0 to 8%, Where R.sub.2 O represents at least one of Na.sub.2 O, K.sub.2 O and Li.sub.2 O, the maximum content of Li.sub.O being 5%; and R'O represents at least one of MgO,CaO,SrO and BaO, within the following individual limits: MgO: 0 to 4%, CaO: 0 to 6.5%, SrO: 0 to 10%, BaO: 0 to 21%; the amount of SiO.sub.2 is not less than 16% when the B.sub.2 O.sub.3 content is less than 8%; and the silver content of the glass, expressed as Ag.sub.2 O, is not less than 0.05% by weight. Such glasses in which Al.sub.2 O.sub.3 is the largest constituent are preferred for ophthalmic purposes, but it is also possible for either B.sub.2 O.sub.3 or P.sub.2 O.sub.5 to be the largest constituent. The refractive index can be corrected to n.sub.D = 1.523.