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
An electrical grade shrinkage resistant casting composition and process for preparing the same
APPLICANTS
Company Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
JAISWAL .Rajendra Gulabchand, CHAUDHAR1 Lokesh Kisan and SINGAL Vivek, all of Crompton Greaves Limited, Advanced Materia! and Process technology centre, CG Global R&D Centre, Kanjur Marg (East), Mumbai 400 042, Maharashtra, India, all indian nationals.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF INVENTION
The invention relates to an electrical grade shrinkage resistant casting composition and a process for preparing the same. The invention also relates to electrical equipment encapsulated and/or impregnated with the composition.
BACKGROUND OF THE INVENTION
Electrical equipments such as dry-type transformers have windings which are encapsulated in a casting composition, such as a polymer resin. Casting compositions based on polymer resins are widely used for encapsulating dry-type transformers. Conventionally, epoxy resin based compositions and unsaturated polyester resin based compositions are used for such encapsulation.
US 20070190332 discloses a dry-type transformer wherein the transformer coils are encapsulated with a cured mineral filler containing cyanate ester resin composition which optionally is a cured mineral filler containing epoxy modified cyanate ester resin composition. The composition is, however, prone to shrinkage at sub-ambient temperatures. Such shrinkage leaves air gaps between the equipment and the encapsulant. Encapsulants are usually electrically insulating but thermally conducting to enable dissipation of heat from the transformer windings to the surroundings. Hence, presence of air gaps between the equipment and the encapsulant results in reduced heat transfer from the equipment to the surroundings which often results in local over-heating that is detrimental to the performance of the equipment.

There is still a need for an electrical grade casting composition which can be used at low temperatures without the composition shrinking.
OBJECTS OF THE INVENTION
An object of the invention is to provide an electrical grade shrinkage resistant casting composition.
Another object of the invention is to provide a process for preparing an electrical grade shrinkage resistant casting composition.
Another object of the invention is to provide a process for preparing an electrical grade shrinkage resistant casting composition which is simple, economical and suitable to meet mass production requirements.
Another object of the invention is to provide electrical equipment encapsulated and/or impregnated with the composition.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided an electrical grade shrinkage resistant casting composition comprising 0.05 % to 1 % by weight of modified clay mixed with 15% to 55 % by weight of an unsaturated polyester resin and 40% to 80% by weight of silica powder, in combination with 0.1% to 0.4 % by weight of compatibilizing agent and

0.5% to 1.5% by weight of reactive diluent, and optionally, 8% to 18 % by weight of additives.
According to the invention there is also provided a process for preparing an electrical grade shrinkage resistant casting composition, the process comprising:
a. mixing 0.05 % to 1 % by weight of modified clay with 15% to 55 % by
weight of an unsaturated polyester resin, 0.1% to 0.4 % by weight of
compatibilizing agent and 0.5% to 1.5% by weight of reactive diluent in the
presence a catalyst to obtain a uniform mixture;
b. mixing the uniform mixture obtained in step (a) with 40% to 80% by weight
of silica powder in the presence of methyl ethyl ketone peroxide and
optionally 8% to 18 % by weight of additives to obtain a uniform mixture;
and
c. casting the uniform mixture obtained in step (b) in a mold, curing the mixture
at room temperature following by de-molding and post-curing at 80 to 90 °C.
According to the invention there is also provided electrical equipment encapsulated and/or impregnated with the composition prepared by the above process. The electrical equipments that can be encapsulated and/or impregnated with the composition include dry-type transformers, current transformers, and the like.
Preferably, the unsaturated polyester resin is selected from maleic anhydride, isophthalic acid, phthalic anhydride, o-phthalic anhydride, polyethylene terephthalate or copolyesters

thereof. Preferably, the modified clay is obtained by modifying siliceous clay or non-siliceous clay or combinations thereof by intercalation with organic modifiers, preferably the organic modifiers are quarternary amines, long chain quarternary ammonium salts, tallow amines or combinations thereof. Preferably, the modified clay is modified montmorillonite clay. Preferably, the reactive diluent is styrene. Preferably, the compatibilizing agent is silane. Preferably, the additives include flame retardants like aluminium trihydrate, coloring agents like titanium dioxide or combinations thereof. Preferably, the catalyst used in the above process is cobalt naphthanate.
The shrinkage resistance of the composition of the invention ensures that no air gap is formed on curing of the cast around the electrical equipment. Further, the composition does not shrink even on exposure to low temperatures upto -40°C. As the composition does not shrink, no cracks are formed in the cast. Therefore, the casting composition of provides excellent protection to electrical equipments from weathering, mechanical damage, current leakage and short circuits. Thus, the casting composition provides enhanced safety and stability apart from providing mechanical protection and compactness to the encapsulated and/or impregnated electrical equipments. The invention is thus advantageous in several respects.
The following examples are illustrative of the invention:
Example 1:
50 g of montmorillonite clay was dispersed in 2 liters of distilled water at 80°C and
stirred for 1 hour. 0.06 moles of intercalating agent namely stearyl amine modified

quaternary ammonium chloride was added to the dispersion and the resultant mixture was stirred for 4 hours. The solution was then filtered and the modified clay was washed with hot water 3 to 4 times to remove residual intercalating agent. The complete removal of the intercalating agent was confirmed by testing for chloride in the washings. The modified clay obtained was then dried in an air circulating oven at 90 to 110 °C for 8 to 10 hours and then ground into a fine powder which was then passed through a 200 mesh sieve.
Example 2
36 parts by weight of unsaturated polyester resin, 4 parts by weight of styrene, 1 part by weight of silane, 0.6 parts by weight of modified Montmorillonite clay as obtained in Example 1 and 0.6 parts by weight of cobalt naphthanate were sonicated for 3 hours followed by high speed mixing for half an hour to yield a uniform mixture. The uniform mixture was then blended with 0.6 parts by weight of methyl ethyl ketone peroxide (MEKP) and 119 parts by weight of silica powder, 4.4 parts by weight of titania and 27 parts by weight of aluminium trihydrate to yield a uniform mixture which was stirred for 5 to 10 minutes and then poured in a mold for casting. The mixture was kept for 3 to 4 hours for curing at room temperature. De-molding was done next followed by keeping the composition for 24 hours at room temperature. The post-curing was then done at 80 to 90°C for 3 hours.
Example 3:
Phase transition in the composition of Example 2 was studied at different temperatures and compared with the phase transition observed in a composition not comprising

modified montmorillonite clay. Volumetric shrinkage is related to molecular movements and phase transition in a material. The phase transition was mapped using a differential scanning calorimeter. The temperature was dropped at the rate of 5°C/minute starting from l0°Cto-90°C.
The phase transition in the composition prepared according to Example 2 but not comprising modified montmorillonite clay is shown in Fig. 1. The phase transition, of the composition without the modified clay was found to start at 5 °C and continued upto approximately -51 °C as is evident in Fig. 1.
On the other hand as shown in Fig. 2, no phase transition was observed upto -50 °C for the composition prepared according to Example 2 having a loading of modified clay of around 0.6 %. The phase transition of the composition without the modified clay was found to start at -50 °C and continued upto approximately -63 °C as is evident in Fig. 2. Studies were also conducted at a loading of 0.2% and also 0.5% of modified clay and it was found that the composition exhibited nophase transition up to -40 °C.
The above examples are non-limiting. The invention is defined by the claims that follow.

We Claim
1. An electrical grade shrinkage resistant casting composition comprising 0.05 % to 1 % by weight of modified clay mixed with 15% to 55 % by weight of an unsaturated polyester resin and 40% to 80% by weight of silica powder, in combination with 0.1% to 0.4 % by weight of compatibilizing agent and 0.5% to 1.5% by weight of reactive diluent, and optionally, 8% to 18 % by weight of additives.
2. The composition as claimed in claim 1, wherein the unsaturated polyester resin is selected from maleic anhydride, isophthalic acid, phthalic anhydride, o-phthalic anhydride, polyethylene terephthalate or copolyesters thereof.
3. The composition as claimed in claim 1 or 2, wherein the modified clay is obtained by modifying siliceous clay or non-siliceous clay or combinations thereof by intercalation with organic modifiers, preferably the organic modifiers are quarternary amines, long chain quarternary ammonium salts, tallow amines or combinations thereof.
4. The composition as claimed in any one of claims 1 to 3, wherein the modified clay is modified montmorillonite clay.
5. The composition as claimed in any one of claims 1 to 4, wherein the reactive diluent is styrene.

6. The composition as claimed in any one of claims 1 to 5, wherein the compatibilizing agent is silane.
7. The composition as claimed in any one of claims 1 to 6, wherein the additives include flame retardants like aluminium trihydrate, coloring agents like titanium dioxide or combinations thereof.
8. A process for preparing an electrical grade shrinkage resistant casting composition, the process comprising:
a. mixing 0.05 % to 1 % by weight of modified clay with 15% to 55 % by
weight of an unsaturated polyester resin, 0.]% to 0.55 % by weight of
compatibilizing agent and 0.5% to 2.5% by weight of reactive diluent in the
presence a catalyst to obtain a uniform mixture;
b. mixing the uniform mixture obtained in step (a) with 40% to 80% by weight
of silica powder in the presence of methyl ethyl ketone peroxide and
optionally 8% to 18 % by weight of additives to obtain a uniform mixture;
and
c. casting the uniform mixture obtained in step (b) in a mold, curing the mixture
at room temperature following by de-molding and post-curing at 80 to 90 °C.

9. The process as claimed in claim 8, wherein the unsaturated polyester resin is selected from maleic anhydride, isophthalic acid, phthalic anhydride, o-phthalic anhydride, polyethylene terephthalate or copolyesters thereof.
10. The process as claimed in claim 8 or 9, wherein the modified clay is obtained by modifying siliceous clay or non-siliceous clay or combinations thereof by intercalation with organic modifiers, preferably the organic modifiers are quarternary amines, long chain quarternary ammonium salts, tallow amines or combinations thereof.
11. The process as claimed in any one of claims 8 to 10, Wherein the modified clay is modified montmorillonite clay.
12. The process as claimed in any one of claims 8 to 11, wherein the reactive diluent is styrene.
13. The process as claimed in any one of claims 8 to 12, wherein the compatibilizing agent is silane.
14. The process as claimed in any one of claims 8 to 13, wherein the additives include flame retardants like aluminium trihydrate, coloring agents like titanium dioxide or combinations thereof.

15. The process as claimed in any one of claims 8 to 14, wherein the catalyst is cobalt naphthanate.
16. Electrical equipment encapsulated and/or impregnated with the electrical grade shrinkage resistant casting composition prepared by the process as claimed in any one of claims 8 to 15.