| Title of Invention | A GLASS COMPOSITION FOR COATING A ZINC OXIDE VARISTOR AND A METHOD OF MAKING THE SAME |
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| Abstract | A glass composition for coating a zinc oxide varistor. The coating composition comprises 47-50% of pbO, 29-33% of ZnO, 10-21% of B2O3, 4-6% of SiO2, 2 to 3% of AL2O3, 0.04-1% of MgO, 1 to 5% of Bi2O3 and 3 to 5% of SnO2 and 3 to 4 to 5wt% of a binder. |
| Full Text | FORM 2 THE PATENTS ACT, 1970 (39 of 1970) As amended by the Patents (Amendment) Act, 2005 & The Patents Rules, 2003 As amended by the Patents (Amendment) Rules, 2006 COMPLETE SPECIFICATION (See section 10 and rule 13) TITLE OF THE INVENTION A glass composition for coating a zinc oxide varistor and a method of making the same INVENTORS Roy Pradip Kumar, Datta Partha, Nikam Murlidhar Narayan and Singal Vivek, all of Crompton Greaves Limited, Advanced Material and Process Technology Centre, CG Global R and D Centre, Kanjur Marg (E), Mumbai -400042, Maharashtra, India, all Indian nationals APPLICANTS Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400030, Maharashtra, India, an Indian Company PREAMBLE TO THE DESCRIPTION The following specification particularly describes the nature of this invention and the manner in which it is to be performed: FIELD OF THE INVENTION This invention relates to a glass composition for coating a zinc oxide varistor and a method of making the same This invention also relates to a method of coating a zinc oxide varistor with the glass composition and to zinc oxide varistor coated with the glass composition. PRIOR ART DESCRIPTION Varistors are electrical devices which are used as surge arresters or lightening arresters to protect transmission and distribution lines and peripheral devices thereof from the destructive voltage levels induced or caused by lightening impulses or switching surges. Zinc oxide varistors have lately replaced the silicon carbide varistors because of the several advantages they posses like cost effectiveness, flexibility of being fabricated into a variety of shapes, excellent non- linearity with respect to voltage, discharge withstand current rating properties or life characteristics under voltage. A zinc oxide varistor comprises a sintered varistor body and electrodes in contact with the varistor body. The varistor body comprises zinc oxide as the many components and a high resistive coating formed or applied on the varistor body. The high resistive coating provides electric insulation to prevent flash over or Creepage when a high voltage appears on the varistor body. US5294908 describes a glass composition for coating zinc oxide varistor comprising 50.0 to 75.0 percent by weight of PbO, 10.0 to 30.0 percent by weight of ZnO, 5.0 to 10.0 percent by weight of B2O3 and 6.0 to 15.0 percent by weight of Si02. US5547907 describes a glass composition for coating zinc oxide varistor comprising 50.0 to 64 percent by weight of PbO, 25 to 30.0 percent by weight of ZnO, 5.0 to 10.0 percent by weight of B203 and 6.0 to 15.0 percent by weight of Si02. US 5594406 describes a glass composition for coating zinc oxide varistor comprising a mixture of lead borosilicate-type glass particulate material and at least one metal oxide selected from the group consisting of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. In order to reduce energy consumption and cost and to reduce cycle time of coating the varistor and to increase productivity of the varistor, it is very much desirable to have glass compositions of low melting temperatures. There is, therefore, scope for development of such glass compositions. OBJECTS OF THE INVENTION An object of the invention is to provide a glass composition for coating a zinc oxide varistor, whose composition has low melting temperature and is cost effective. Another object of the invention is to provide a glass composition for coating a zinc oxide varistor, whose coating composition reduces cycle time of coating the varistor and increases productivity of the varistor. Another object of the invention is to provide a method of making the above glass coating composition. Another object of the invention is to provide a method of coating a zinc oxide varistor with the above glass composition. Another object of the invention is to provide a zinc oxide varistor coated with the above glass composition. DETAILED DESCRIPTION OF THE INVENTION According to the invention there is provided a glass composition for coating a zinc oxide varistor, the coating composition comprising 47-50% of PbO, 29-33%) of ZnO, 10-21%) of B203,4-6% of Si02, 2 to 3% of A1203, 0.04 - 1% of MgO, 1 to 5% of Bi203 and 3 to 5% of Sn02 and 4 to 5% of a binder. According to the invention there is also provided a method of making a glass composition for coating zinc oxide varistor, the method comprising i) melting a mixture of 47-50% of PbO, 29-33% of ZnO, 10-21% of B203> 4- 6% of Si02, 2 to 3% of A1203, 0.04 - 1% of MgO, 1 to 5% of Bi203 and 3 to 5% of Sn02 at 700° to 800°C. ii) quenching the melt of step (i) in water; iii) crushing the beads formed in step (ii) and sieving the powder to obtain a powder of 2-4 microns; and iv) mixing the powder obtained in step (iii) with 4 to 5%> of a binder in water solution to make slurry. According to the invention there is also provided a method of coating a zinc oxide varistor with the above glass composition the method comprising the following steps; i) precleaned and demoisturised zinc oxide varistor at 100 - 120°C. ii) applying the glass slurry on the varistor. iii) curing the varistor at 475-525°C. Preferably the glass composition is applied on the varistor by dipping the varistor in glass slurry or by spraying the glass slurry or by applying the glass slurry with a brush. The binder is selected from polyvinyl acetate, methyl cellulose, and polyester resin. Preferably the binder is methyl cellulose. Preferably the glass composition 47-50%) of PbO, 29-33% of ZnO, 10-21% of B203, 4-6% of Si02, 2 to 3% of A1203, 0.04 - 1% of MgO, 1 to 5% of Bi203 and 3 to 5% of Sn02 and 4 to 5wt % of a binder. In one embodiment of the invention the glass composition comprises 47.81%) of PbO, 32.47% of ZnO, 20.74% of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of Bi203 and 3.18% of Sn02 and 4 wt% of methyl cellulose. In another embodiment of the invention, the glass composition comprises 47.81%) of PbO, 32.47% of ZnO, 10.74%) of B203> 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of Bi203 and 3.18% of Sn02 and 4 wt% of methyl cellulose. Prior to applying the glass composition, the varistor is surface cleaned by acetone and demoisturised by heating it at 100 tol20°C. According to the invention there is also provided a zinc oxide varistor coated with the above glass composition by the method described above. The glass composition of the invention is distinctly different and novel in terms of composition and constitution and is endowed with low melting temperature of the order of 400-450°C. Because of the low melting temperature, application temperature of the glass composition on the varistor is also reduced. Therefore, there is considerable saving in the thermal energy consumption and the glass composition of the invention is cost effective. Also the cycle time of coating of the varistor is reduced and productivity of the varistor is increased. The following experimental examples are illustrative of the invention but are not limitative of the scope of the invention: Example 1 (a) A mixture of 47.81% of PbO, 32.47% of ZnO 20.74% of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of Bi203 and 3.18% of Sn02, was melted at 700°C in a furnace and the melt was quenched in water. The beads formed were crushed in a ball mill crusher and sieved to obtain a powder of 2-4 microns. The powder was mixed with 4wt% of methyl cellulose in water solution. The melting temperature of the glass composition was 410°C. (b) A zinc oxide varistor block was surface cleaned by acetone and demoisturised by heating to 100-120°C. The above glass composition was coated on the varistor block by spraying method. The coated varistor block was cured at 500°C in a furnace. Adhesion test was performed by Cross Cut Adhesion test {ASTM- D 3359}at room temperature and at 250°C. The coating passed the above test. Durability was conformed by giving a thermal shock at 250°C to the sample. No crack was found on the surface. Example 2 (a) The procedures of example 1 were followed with a mixture of 47.81% of PbO, 32.47% of ZnO, 10.74 of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of Bi203 and 3.18%o of Sn02. The powder was mixed with 4wt% of methyl cellulose. The melting temperature of the glass composition was 450°C. Adhesion test was performed by Cross Cut Adhesion test {ASTM- D 3359}at room temperature and at 250°C. The coating passed the above test. Durability was conformed by giving a thermal shock at 250°C to the sample. No crack was found on the surface. We claim: 1. A glass composition for coating a zinc oxide varistor, the coating composition comprising 47-50% of PbO, 29-33% of ZnO, 10-21% of B203>4-6% of Si02, 2 to 3% of A1203, 0.04 - 1% of MgO, 1 to 5% of Bi203 and 3 to 5% of Sn02 and 3 to 4 to 5wt % of a binder. 2. The glass composition as claimed in claim 1 which comprises 47 to 48 % of PbO, 29 to 31 of ZnO, 10 to 15% of B203, 4 to 5 % of Si02, 1.5 to 2.5 % of A1203, 0.04 tol % of MgO, 1 to 1.5 % B12O3 and 3 to 3.5 % of Sn02 and 4 to 4.5wt % of a binder. 3. The glass composition as claimed in claim 1 or claim 2 which comprises, 47.81% of PbO, 32.47% of ZnO, 20.74% of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of B12O3 and 3.18% of Sn02 and 4wt % of methyl cellulose. 4. The glass composition as claimed in claim 1 or claim 2 which comprises, 47.81% of PbO, 32.47% of ZnO, 10.74% of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of B12O3 and 3.18% of Sn02 and 4wt % of methyl cellulose. 5. A method of making a glass composition for coating zinc oxide varistor, the method comprising i) Melting a mixture of 47-50% of PbO, 29-33% of ZnO, 10-21% of B203; 4-6% of Si02, 2 to 3% of AI2O3, 0.04 - 1% of MgO, 1 to 1.5% of Bi203 and 3 to 4% of Sn02 at 700° to 800°C. ii) Quenching the melt of step (i) in water; iii) Crushing the beads formed in step (ii) and sieving the powder to obtain a powder of 2-4 microns; and v) Mixing the powder obtained in step (iii) with 4 to 5 wt% of a binder in water solution to make slurry. 6. The method as claimed in claim 5, which comprises melting a mixture of 47 to 48 % of PbO, 29 to 31 of ZnO, 10 to 15% of B203, 4 to 5 % of Si02, 1.5 to 2.5 % of A1203, 0.04 tol % of MgO, 1 to 1.5 % B12O3 and 3 to 3.5 % of Sn02 and 4 to 4.5wt % of a binder. 7. The method as claimed in claim 5 or 6 which comprises melting a mixture of 47.81% of PbO, 32.47% of ZnO, 20.74% of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of Bi203 and 3.18% of Sn02 and mixing the powder with 4wt% of methyl cellulose. 8. The method as claimed in claim 5 or 6, which comprises melting a mixture of 47.81% of PbO, 32.47% of ZnO, 10.74% of B203, 5% of Si02, 2% of A1203, 0.75% of MgO, 1.5% of Bi203 and 3.18% of Sn02 and mixing the powder with 4wt% of methyl cellulose. 9. A method of coating a zinc oxide varistor with the glass composition as claimed in any one of claims lto 4, the method comprising the following steps; i) Precleaned and demoisturised zinc oxide varistor at 100 - 120°C. ii) Applying the glass slurry on the varistor. iii) Curing the varistor at 475-525°C. 10. The method as claimed in claim 9, wherein the glass composition is applied on the varistor by dipping the varistor in glass slurry or by spraying the glass slurry or by applying the glass slurry with a brush. 11. A zinc oxide varistor coated with the glass composition as claimed in anyone of claims 1 to 4 by the method as claimed in claim 9 or 10. Abstract A glass composition for coating a zinc oxide varistor. The coating composition comprises 47-50% of PbO, 29-33% of ZnO, 10-21% of B203,4-6% of Si02, 2 to 3% of A1203, 0.04 - 1% of MgO, 1 to 5% of Bi203 and 3 to 5% of Sn02 and 3 to 4 to 5wt % of a binder. |
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588-MUM-2008-ABSTRACT(14-6-2012).pdf
588-MUM-2008-CLAIMS(AMENDED)-(14-6-2012).pdf
588-MUM-2008-CORRESPONDENCE(23-5-2008).pdf
588-MUM-2008-CORRESPONDENCE(27-9-2011).pdf
588-MUM-2008-CORRESPONDENCE(6-11-2009).pdf
588-MUM-2008-CORRESPONDENCE(6-5-2010).pdf
588-MUM-2008-CORRESPONDENCE(7-8-2012).pdf
588-mum-2008-correspondence-received.pdf
588-mum-2008-description (complete).pdf
588-MUM-2008-FORM 1(23-5-2008).pdf
588-MUM-2008-FORM 1(24-3-2008).pdf
588-MUM-2008-FORM 1(27-9-2011).pdf
588-MUM-2008-FORM 13(27-9-2011).pdf
588-MUM-2008-FORM 18(6-5-2010).pdf
588-MUM-2008-FORM 2(TITLE PAGE)-(14-6-2012).pdf
588-MUM-2008-FORM 2(TITLE PAGE)-(24-3-2008).pdf
588-MUM-2008-FORM 26(27-9-2011).pdf
588-MUM-2008-FORM 26(6-5-2010).pdf
588-MUM-2008-MARKED COPY(14-6-2012).pdf
588-MUM-2008-REPLY TO EXAMINATION REPORT(14-6-2012).pdf
588-MUM-2008-SPECIFICATION(AMENDED)-(14-6-2012).pdf
| Patent Number | 254293 | |||||||||||||||
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| Indian Patent Application Number | 588/MUM/2008 | |||||||||||||||
| PG Journal Number | 42/2012 | |||||||||||||||
| Publication Date | 19-Oct-2012 | |||||||||||||||
| Grant Date | 17-Oct-2012 | |||||||||||||||
| Date of Filing | 24-Mar-2008 | |||||||||||||||
| Name of Patentee | CROMPTON GREAVES LTD | |||||||||||||||
| Applicant Address | CG HOUSE, DR ANNIE BESANT ROAD, WORLI, MUMBAI | |||||||||||||||
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| PCT International Classification Number | C03C3/064; C03C8/02; H01C7/10 | |||||||||||||||
| PCT International Application Number | N/A | |||||||||||||||
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