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 high resistive dip coating composition for ZnO varistor block
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Roy Pradip Kumar, Chaudhari Lokesh Kisan and Singal Vivek, all ofCrompton Greaves Limited, AMPTC, CG Global R&D Centre, Kanjurmarg (E), Mumbai 400 042, Maharashtra, India, all Indian nationals
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 high resistive dip coating composition for ZnO varistor block.
BACKGROUND OF THE INVENTION
A surge arrester or lightening arrester is a protective electrical device used to protect transmission and distribution lines and peripheral devices thereof from destructive voltage levels induced or caused by switching surges or lightening impulses. A surge arrester or lightening arrester comprises a plurality of varistors stacked together within an insulator housing, usually made of porcelain. Zinc oxide varistors have lately replaced the silicon carbide varistors because of their several advantages 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 includes a varistor body comprising zinc oxide as the main component in combination with several other metallic oxides including Bi203, Si02, C02O3, Sb203, O2O3 and Mn02- A zinc oxide varister is made by mixing zinc oxide with the other metallic oxides and binder and granulating the mixture using a spray dryer. The granules are compacted into blocks, the binder is burnt off and the blocks are calcined, sintered and cooled to ambient temperature. The sintered blocks are preheated and their periphery is coated with a high resistive coating composition. The coated blocks are dried either by leaving them in the atmosphere or by heating them upto 150°C. Following this, the coating layer is cured or baked usually at 400 to 450°C. The sides of the blocks are polished and formed with electrodes usually made of aluminium.

US 4041436 describes a zinc oxide varistor composition including a resistive borosilicate glass matrix. The glass matrix is formed during the making of the varistor itself and is, therefore, embedded or incorporated into the varistor instead of being on the surface thereof. As a result, the resistive properties of the varistor is reduced. An organic carrier like tridecanol is used as the solvent for borosilicate glass and the solvent loading is high. Use of the organic solvent in high loading is not only expensive but also creates environmental problems. US 5294908 relates to a zinc oxide varistor which employs a glass PbO composition comprising 15% by weight of a mixture of ethyl cellulose and butyl carbitol acetate as organic binder. Because of the high binder loading, cost of the varistor is increased. High binder loading also increases pollution. The baking temperatures of the coated blocks are also high. As a result, energy consumption is high, cost is increased, baking time is increased and productivity is reduced. US 6224937 relates to zinc oxide varistor which employs an iron based high resistance coating composition including a binder of polyvinyl alcohol, methyl cellulose, hydroxyl propyl cellulose and water soluble acryl from 2.5 - 15% by weight. Because of the high percentage of the binder, cost and pollution problems are increased. The baking temperatures of the coated varistor blocks are also very high thereby correspondingly increasing energy consumption and cost and reducing productivity.

OBJECTS OF THE INVENTION
An object of the invention is to provide a high resistive dip coating composition for 2n0 varistor block, which composition is economical, energy efficient and environment friendly.
Another object of the invention is to provide a high resistive dip coating composition for ZnO varistor block, which composition increases productivity.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a high resistive dip coating composition for ZnO varistor block, the composition comprising 30 to 80% of a lead based glass, 0.010 to 4% by weight of a binder selected from hydroxylethylcellulose, hydroxylmethylcellulose, carboxymethylceilulose or natural gum, 0 to 1.5% by weight of an anti forming agent and 20 to 70% by weight of water and having a viscosity of 150 to 1000 cps.
Preferably the composition comprises 35 to 70% by weight of the lead based glass, 0.08 to 2.5% by weight of the binder, 0.25-1% by weight of the antiforming agent and 29 to 65% by weight of water. Preferably the natural gum is selected from guar gum or modified guar gum. Preferably the antiforming agent is silicon based. Preferably the composition has a viscosity of 200 to 500 cps.

The varistor blocks are dip coated with the composition of the invention by immersing the blocks in the coating slurry contained in a tank for 1-5 minutes. The blocks are removed from the tank and allowed to dry by leaving them in the atmosphere or by heating them upto 150°C. The dried blocks are cured or baked by heating them at 400-450°C.
The coating composition of the invention is water based and comprises low glass content and low binder content. The curing temperature is reduced so also the curing time. As a result, the composition is economical and energy efficient. It also reduces environmental pollution and is eco friendly. As the curing time is reduced productivity is also increased.
The following experimental examples are illustrative of the invention but not limitative of the scope thereof.
Example 1
A slurry of 70% by weight of lead based glass, 0.4% by weight of hydromethylcellulose, 29.5% by weight of water and 0.1% by weight of silicon based antiforming agent was formed by thorough mixing of the components in a ball mill at 120 rpm for 5 hours until the viscosity of the slurry was 50-500cps. Sintered varistor blocks were immersed in the slurry for 1-5 minutes, removed from the slurry and allowed to drain. The coated blocks were allowed to dry at ambient temperature for 2 hr and cured at 400°C for 1.5 hours in a

furnace. The thickness of the coating was 145 microns. The coated blocks passed all the electrical characteristics required for varistor applications.
Example 2
Sintered zinc oxide varistor blocks were coated as described in Example 1 with a slurry comprising 36.8% by weight of lead based glass, 2.2% by weight of natural guar gum, 0.5% by weight of silicon based antiforming agent and 60% by weight of water prepared as described in Example 1. The coated varistor blocks were dried at 150°C in a furnace oven for 0.5 hr and cured at 450°C for 1 hour in the same furnace. The thickness of coating was 110 microns. The coated blocks passed all the electrical characteristics required for varistor applications.

We Claim:

1. A high resistive dip coating composition for ZnO varistor block, the composition comprising 30 to 80% by weight of a lead based glass, 0.010 to 4% by weight of a binder selected from hydroxylethylcellulose, hydroxylmethylcellulose, carboxymethylcellulose or natural gum, 0 to 1.5% by weight of an antiforming agent and 20% to 70% by weight water and having a viscosity of 200 to 1000 cps.
2. The composition as claimed in claim I, which comprises 35 to 70% by weight of the lead based glass, 0.08 to 2.5% by weight of the binder, 0.25-1% by weight of the antiforming agent and 29 to 65% by weight of water.
3. The composition as claimed in claim 1 or 2, wherein the natural gum comprises guar gum or modified guar gum.
4. The composition as claimed in any one of claims 1 to 3, wherein the antiforming agent is silicon based.
5. The composition as claimed in anyone of claims 1 to 4, which has a viscosity of 200 to 500 cps.