Patent GB-1057871-A (GB)
Title: Glass mirrors having improved resistance to tarnishing
Inventor: JOHNSON THOMAS EDWIN
IPC: C03C17/38
Filed: 1962-05-09
Abstract: The resistance to tarnishing by S and S compounds of a mirror comprising a metal layer on a glass substrate is improved by contacting a chelating agent with the external surface of the metal layer so that the chelating agent and surface react.  The mirror comprises glass coated e.g. with Ag alone or Ag and then Cu.  The metal layer may be treated with the chelating agent dissolved in an inert solvent such as butanol and subsequently be given a protective lacquer coating. Alternatively, it may be treated with chelating agent incorporated in lacquer optionally followed by further coating with lacquer.  Specified chelating agents are nitrogenous 5-membered ring compounds having in the 5-membered ring two non-adjacent N atoms, one at least of which is attached to N, and two adjacent C atoms each of which is attached to a different N atom, and having as the fifth ring-closing atom either a tetravalent C atom or a trivalent N atom.  In the case of Ag, the chelating agent may be mercaptobenzthiazole. In the case of Cu it may be benztriazole, E.D.T.A. or guanidino benzimidazole. Many lacquers are described.ALSO:The resistance to tarnishing of a mirror is improved by contacting it with a chelating agent incorporated in a lacquer. The lacquer may be a stoving lacquer produced from 85-15% alkyd resins (e.g. based on saturated or unsaturated fatty acids) and 15-85% thermosetting resins (e.g. urea formaldehyde) in admixture. In Example 1, castor oil alkyd resin containing phthalic anhydride and dissolved in an aromatic hydrocarbon solvent is mixed with a melamine formaldehyde resin dissolved in a mixture of xylene and butanol. Benzatriazole is added as chelating agent.  In other examples the chelating agent is mercaptobenzthiazole (IIa), benzotriazole (IIb), guanidino-benzimidazole (IIc) and EDTA (IId). This laquer is over-coated with another composition comprising red lead, a non-drying oil alkyd, phthalic anhydride, an aromatic hydrocarbon, a melamine/formaldehyde resin, xylene and butanol.ALSO:The resistance to tarnishing by S and S compounds of a mirror comprising a metal layer on a glass substrate is improved by contacting a chelating agent with the external surface of the metal layer so that the chelating agent and surface react.  The mirror comprises glass coated, e.g. with Ag alone or Ag and then Cu.  The metal layer may be treated with the chelating agent dissolved in an inert solvent such as butanol and subsequently be given a protective lacquer coating.  Alternatively, it may be treated with chelating agent incorporated in lacquer optionally followed by further coating with lacquer.  Specified chelating agents are nitrogenous 5-membered ring compounds having in the 5-membered ring two non-adjacent N atoms, one at least of which is attached to N, and two adjacent C atoms each of which is attached to a different N atom, and having as the fifth ring-closing atom either a tetravalent C atom or a trivalent N atom.  In the case of Ag, the chelating agent may be mercaptobenzthiazole.  In the case of Cu it may be benztriazole, E.D.T.A. or guanidino benzimidazole. The lacquers may comprises: 1) oxygen convertible coatings such as a) varnishes prepared from a drying oil and a hard resin or b) alkyd resins based on drying oils; 2) heat convertible coatings such as a) lacquers prepared from a blend of a saturated fatty acid alkyd and an amine resin or b) epoxide resins blended with amine resins; 3) catalysed convertible coatings such as a) lacquers prepared from epoxide resins and catalysed with organic amines or polyamide resins; 4) non-convertible coatings such as a) shellac in alcoholic solution, b) an acrylic acid ester polymer in xylene solution or c) a vinyl acetate polymer dispersed in water. The lacquers may be stoving lacquers produced from 85-15% alkyd resins (e.g. based on saturated or unsaturated fatty acids) and 15-85% thermosetting resins (e.g. urea formaldehyde or melamine formaldehyde) in admixture.  In Example 1, 85 parts by weight of a drying castor oil alkyd resin containing 40% of phthalic anhydride and dissolved in 85 parts by weight of an aromatic hydrocarbon solvent were mixed with 15 parts by weight of a melamine formaldehyde resin dissolved in 10 parts by weight of a mixture of xylene and butanol.  To the mixture was added 3 parts of benzotriazole dissolved in 27 parts of butanol.