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 spray 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 of Crompton 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 spray 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 B12O3, Si02, C02O3, Sb203, Q2O3 and Mn02. A zinc oxide varistor is made by mixing zinc oxide with the other metallic oxides and binder and granulating the mixture using 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 up to 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 spray coating composition for ZnO varistor block, which composition is economical, energy efficient and environment friendly.
Another object of the invention is to provide a high resistive spray 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 spray coating composition for ZnO varistor block, the composition comprising 25 to 70% of a lead based glass, 0.010 to 4% by weight of a binder selected from hydroxylethylcellulose, hydroxylmethylcellulose or carboxymethylcellulose and 40 to 70% by weight of water and having a viscosity of 150 to 1000 cps.
Preferably the composition comprises 50 to 70% by weight of the lead based glass, 0.08 to 2.5% by weight of the binder and 40 to 65% by weight of water. Preferably the composition has a viscosity of 200 to 500 cps.
Prior to spray coating the composition of the invention on ZnO varistor blocks, the sintered blocks are preheated at 50-120°C. The coated blocks are dried 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 1% by weight of hydromethylcellulose, 59% by weight of lead based glass, and 40% by weight of water 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 150-500cps. Zinc oxide varistor blocks were preheated at 100°C for 2 hours in an oven and were spray coated with the slurry using a spray gun keeping the blocks at a distance of (30-50) cms from the nozzle of the spray gun. Spraying was done with pressurized air at 60-80psi. The coated blocks were allowed to dry at ambient temperature for 2 hr and cured at 400°C for 1 hour in a furnace. The thickness of the coating was 120 microns. The coated blocks passed all the electrical characteristics required for varistor applications.

Example 2
Zinc oxide varistor blocks were preheated and coated as described in Example I with a slurry comprising 3.8% by weight of hydroethylcellulose, 29.2% by weight of lead based glass and 67% by weight of water prepared as described in Example 1. The coated varistor blocks were dried at 150°C in a furnace for 0.5 hr and cured at 450°C for 1 hour in the same furnace. The thickness of coating wasl40 microns. The coated blocks passed all the electrical characteristics required for varistor applications.

We Claim:

1. A high resistive spray coating composition for ZnO varistor block, the composition comprising 25 to 70% of a lead based glass, 0.010 to 4% by weight of a binder selected from hydroxylethylcellulose, hydroxylmethylcellulose or carboxymethylcellulose and 40 to 70% by weight of water and having a viscosity of 150 to 1000 cps.
2. The composition as claimed in claim 1 which comprises 50 to 70% by weight of the lead based glass, 0.08 to 2.5% by weight of the binder, and 40 to 65 % by weight of water.
3. The composition as claimed in claim 1 or 2, which has a viscosity of 200 to 500 cps.