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Cobalt Glass Compositions for Coatings Kobaltglaszusammensetzungen Fur Beschichtungen Compositions De Verre Au Cobalt Pour Revetements

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Cobalt Glass Compositions for Coatings Kobaltglaszusammensetzungen Fur Beschichtungen Compositions De Verre Au Cobalt Pour Revetements Europaisches Patentamt (19) European Patent Office Office europeenpeen des brevets EP 0 721 430 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) intci.6: C03C 3/093, C03C 3/064, of the grant of the patent: C03C 8/04, C03C 4/02, 04.08.1999 Bulletin 1999/31 C03C 10/04 (21) Application number: 94927732.1 (86) International application number: PCT/GB94/02105 (22) Date of filing: 28.09.1994 (87) International publication number: WO 95/09131 (06.04.1995 Gazette 1995/15) (54) COBALT GLASS COMPOSITIONS FOR COATINGS KOBALTGLASZUSAMMENSETZUNGEN FUR BESCHICHTUNGEN COMPOSITIONS DE VERRE AU COBALT POUR REVETEMENTS (84) Designated Contracting States: (56) References cited: DE ES FR GB IT NL EP-A- 0 001 755 EP-A- 0 049 041 EP-A- 0 358 933 EP-A- 0 425 927 (30) Priority: 28.09.1993 GB 9319971 GB-A- 1 187 834 (43) Date of publication of application: DATABASE WPI Section Ch, Week 8211, 17.07.1996 Bulletin 1996/29 Derwent Publications Ltd., London, GB; Class L, AN 82-21 842E & SU,A,833 617 (GLASS RES (73) Proprietor: JOHNSON MATTHEY PUBLIC INST) 30 May 1981 LIMITED COMPANY CHEMICAL ABSTRACTS, vol. 96, no. 8, 22 London SW1Y5BQ (GB) February 1982, Columbus, Ohio, US; abstract no. 5681 9y, page 302 ; & JP,A,56 109 840 (NGK (72) Inventors: INSULATORS LTD.) 31 August 1981 • SMITH, Peter, John DATABASE WPI Section Ch, Week 9022, Oxfordshire OX5 2JY (GB) Derwent Publications Ltd., London, GB; Class L, • MANGAT, Harbans, Kaur AN 90-170643 & SU,A,1 525 123 (CONG Oxfordshire OX5 1EJ (GB) CERAMICS RES) 30 November 1989 CHEMICAL ABSTRACTS, vol. 110, no. 20, 15 May (74) Representative: Wishart, Ian Carmichael et al 1989, Columbus, Ohio, US; abstract no. 1 78391 f, Patents Department page 329 ; & PL,A,141 766 (INSTITUT SZKLA I Johnson Matthey Technology Centre CERAMIKI)30 June 1988 Blount's Court Sonning Common Reading, RG4 9NH (GB) DO o CO CM Is- Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice the Patent Office of the Notice of shall be filed in o to European opposition to European patent granted. opposition a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. a. 99(1) European Patent Convention). LU Printed by Jouve, 75001 PARIS (FR) EP 0 721 430 B1 Description [0001] The present invention relates to glass compositions and, in particular to glass compositions which are of minimal toxicity and are substantially free of lead and cadmium and which can be used either as blue coloured glasses, 5 for mixing with pigments to provide other colour compositions for the decoration of ceramic materials and/or glass, or as pigments per se. [0002] Occasional episodes of lead poisoning have resulted from the use of improperly formulated and fired lead- containing glazes and/or decoration enamels on ceramic ware. Whilst lead containing glazes and decoration enamels can be prepared which are safe, and meet current requirements for permissible lead release to food with which they 10 come into contact, the problem of lead poisoning is avoided if lead itself is avoided. In addition, various pollution controls regarding the use of lead and limits on the content of lead in waste water can be avoided by the use of lead-free glazes and decoration enamels. [0003] Consequently, glaze and enamel decorating suppliers have strived to provide lead-free products which meet the necessary technical requirements, particularly the more exacting demands on durability. However, the development is of a durable and intense royal blue colour has proved to be an exception. In the past, royal blue on-glaze enamels were generally produced using high-leaded, transparent glassy fluxes and pigments based on the crystalline CoO- ZnO-Si02 system. For the new lead-free colours, transparent lead-free borosilicate fluxes have been used in place of the leaded versions. However, as the CoO-ZnO-Si02 phase is soluble in mild acid, replacement of leaded fluxes with lead-free equivalents does not fully address the durability problem. Moreover, the standard practice of increasing colour 20 intensity by increasing the pigment loading serves only to degrade the chemical resistance still further. Thus, the con- ventional pigmented flux approach is incapable of producing a durable and intense blue enamel without recourse to elaborate coating techniques for the pigment. Hence, an alternative method of producing a durable intense blue enamel for tableware decoration is required. The use of a durable lead-free intense blue glass in place of a conventional pigmented flux is herein disclosed. 25 [0004] Various attempts have been made to produce blue glasses for enamel decoration of jewellery, glassware and tableware. However, the widespread use of on-glaze blue glass enamels for tableware has been limited by shortcom- ings in certain key areas. [0005] In addition to being lead-free and cadmium-free, a coloured glass to be used for on-glaze enamel decoration must simultaneously satisfy a number of key requirements. Firstly, the softening point of the glass should be such as 30 to allow maturation within the temperature range of from 750° to 900°C, which are typical on-glaze enamelling tem- peratures for decoration on bone china, earthenware, hotelware or porcelain pieces. [0006] Secondly, the coloured glass should be compatible with a wide variety of glazes. [0007] Thirdly the coloured glass should display good resistance to pronounced opacity or mattness during a con- ventional enamel firing schedule in order to promote good colour development with a glossy finish. 35 [0008] Fourthly, the linear coefficient of thermal expansion (CTE) of the coloured glass should be compatible, or made compatible through additions of filler materials, with that of the glaze coated thereon to prevent crazing and/or spalling. [0009] Finally, the durability of the coloured glass should be such as to enable the fired colours to withstand attack by acidic foodstuffs and alkali detergents. With controls on permissable levels of metal release into acidic and alkali 40 solutions becoming more stringent, more exacting demands are being placed on the durability requirements of deco- rated pieces. [0010] In the light of the present invention, it is important to stress that it is the durability of the final product, i.e fired colour, which is of concern, not that of the glass frit which may change in use. [0011] We have now developed a glass composition which can be used as a blue coloured on-glaze enamel which 45 simultaneously meets, or at least substantially meets, the aforementioned key requirements. It relies upon the presence of Co2+ in a network-forming position in the glass to produce the blue colour, with oxygen donors such as Zn02, alkaline earth oxides and alkali oxides increasing the amount of cobalt occupying network-forming positions, and Zr02 to provide chemical durability, and or the ability to crystallize. [0012] Accordingly the present invention provides a crystallizing glass composition which is essentially free from so lead and cadmium and which comprises the following components:- Weight per cent Si02 30-70 B203 1 -25 Al203 0-1 0 Bi203 0-50 2 EP 0 721 430 B1 (continued) Weight per cent CoO 5-15 At least one of Li20 0-10 Na220 0-20 1 -30 (in total) K20 0-20 Optionally one or more of MgO, CaO, SrO or BaO 0-15 ZnO 5-15 Zr02 up to 10 TiQ2 0-1 optionally one or more of Sn02, W03, Mo03 or Nb2Os 0-5 optionally one or more of Y203, La203 or Ce02 0-5 P2O5 0-5 [0013] The glass of the present invention relies on the presence of Co2+ in the glass network to provide a blue colour. [0014] In the glasses of the present invention the intensity of the blue colour of the glass is increased by altering the glass chemistry to promote precipitation of the (Co,Zn)2Si04 (phenacite) phase either during enamel firing or by a prior thermal treatment of the glass. Optionally a nucleating agent such as Zr02, P205 or other agents known to those skilled in the art of glass ceramics may be employed to assist in the (Co,Zn)2Si04 precipitation. [0015] Preferred crystallizing glass compositions of the present invention are detailed below: Preferred Range More Preferred Range wt% wt% Si02 35-60 35-55 B203 1-20 2-10 Al203 1-10 2-6 CoO 5-15 5-12 R20 1-20 2-15 ZnO 5-15 7-12 Zr02 1 -7 2-5 R20 + ZrQ2 1 -25 2-20 [0016] For glasses with CTE's below 7.0 X 10"6/°C, the presence of Bi203 is generally required in order to provide a glossy fired appearance. The preferred range is 5-25 wt%., the more preferred range is 3-15 wt%. [0017] The composition of the present invention contains Zr02 in order to improve the resistance of the compositions to attack by alkali detergent solutions. [0018] The glasses of the invention can be prepared from natural (benificated) rocks and minerals e.g. limestone, silica sand etc. as well as "fine chemicals". [0019] The various minerals and/or oxides used in the preparation of the compositions of the present invention are usually in the form of fine powders. Precursors of these oxides can also be useful, providing that they decompose to form the required oxides at temperatures below the melting temperature of the glass. Suitable "fine chemical" precursors are the nitrites, nitrates, carbonates, metal organic salts, for example citrates, acetates, etc. EP 0 721 430 B1 [0020] The glass of the present invention may be prepared by mixing together materials, charging the mixture of materials to a glass melting furnace at a temperature sufficiently high to produce the fluxed glass and then fritting the glass by pouring it into water, or by passing it through water cooled rolls.
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