FORM 2 THE PATENTS ACT, 1970
(39 of 1970) As amended by the Patents (Amendment) Act, 2005
AND
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2005
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
CURABLE EPOXY COMPOSITION AND ITS USE AS CASTING RESIN
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Keshawatkar Baburao, Athikkan Venkatasami, Bhatia Anilkumar; ail of Crompton
Greaves Ltd, Conition Monitoring & Diagnostic Centre, Global R&D Centre,
Kanjur (E), Mumbai 400042, Maharashtra, India, and Thekkeveetil Sethumadhavan of Crompton Greaves Limited, Vacuum Interrupters and Instrument Transformers Division: Power Systems, Aurangabad-431136, 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:

TECHNICAL FIELD OF THE INVENTION:
The present invention relates to curable cpoxy composition and its use as casting resin in the manufacture of" transformer and other electric components and to the cross-linked products obtained by curing the compositions, which simultaneously exhibit the features of good flame retardancy, high mechanical strength and low dielectric losses at high operating temperature.
The present invention also relates to insulation system of cast resin transformer which is made from curable composition.
BACKGROUND OF THE INVENTION:
Electric coils, particularly electromagnet coils made up of multi-layer windings such as for transformers, motors, solenoids and the alike, are usually impregnated and covered with a synthetic resin to enhance insulation between the windings and to protect them against external forces and moisture. In addition to providing insulation, the synthetic resin should also contribute to the mechanical strength of the windings and also have flame retardant properties. A low-viscosity resin is commonly employed for such purpose, under vacuum in most cases, to avoid occluding air within the windings and resin, or to prevent resulting breakdown of insulation. As a result, the coils are placed and kept in a metal mould either individually or several together during soaking in the resin and subsequent heat curing in conventional methods for the impregnation so that the resin may not drain away from the coils before completion of hardening.
The critical factors for encapsulating synthetic resin for high performance transformer are thermal conductivity for heat dissipation, oxygen index for combustibility, Burning characteristics and arc resistance for flame retardant activity. Tan delta , which is measure of resistance to temperature variation. BDV for better electrical insulation, water absorption for Non porous casting and CTI for workability of the casting in highly polluted environments.
Flame- retardant casting resins are commonly used in transformer and are generally based on bisphenol A epoxy resins, reinforcing fillers and flame retardants. US 3,202,947 describes

flame retardant compositions consisting of liquid bisphenol A diglycidyl ethers, hexahydrophlhalic acid, hydrated alumina and iris(chloroalkyl) phosphates.
US 4,009.141 describes electrically insulating curable compositions consisting of selected cycloaliphatic epoxy resins and dicarboxylic anhydrides, which are reinforced with large amounts of zirconium silicate fillers and contain finely divided hydrated alumina as additional second filler. They are suitable for the encapsulating insulation of electrical components, such as, for example, of metal transformer components, or transformer bushings.
FR 2 630 578 describes curable, flame-retardant compositions for air-cooled transformers. These compositions contain at least 20% by weight pretreated aluminium hydroxide, based on the total composition consisting of resin, hardener and reinforcing additives.
Since in such systems the dielectric loss factor tan increases considerably at higher temperature, those systems are not suitable for transformers having high operating temperatures.
JP10292090 discloses an epoxy resin composition for casting which has a curing time decreased in terms of improving productivity, exhibits flame retardance and electrical properties, and is suitable for insulating a high-tension coil such as one of a flyback transformer. This composition essentially comprises an epoxy resin, bisphenol A glycidyl ether; a composite flame retardant containing aluminum hydroxide, tetrabromobisphenol, and antimony Irioxide and melbylietrahydrophthalic anhydride as the curative, and 1.8-diazabicyclo{5,4,0)undecene-7(D) as the cure accelerator.
There is therefore a need for casting resin formulations that exhibit simultaneously the features of flame retardance. low dielectric losses and good mechanical and electrical properties including thermal conductivity, especially has better cooling with non-burning
properties.

OBJECTS OF THE INVENTION:
An object of the invention is to provide a curable epoxy resin composition in which a product upon curing has high mechanical strength, good electrical properties and better thermal conductivity for excellent heat dissipation properties.
It is another object of the invention to provide the curable epoxy resin composition which is easily castable
It is another object of the invention to provide a method of making the described epoxy resin composition.
It is another object of the invention to provide a crosslinked product obtained by curing epoxy composition of the invention.
It is another object of the invention to provide insulation system to cast resin transformer.
Ft is still another object of the invention to provide insulation system to cast resin transformer having better thermal conductivity, good electrical properties and mechanical properties.
It is still another object of the invention to provide insulation system to cast resin transformer having better (lame retardance and has acceptable oxygen index.
It is still another object of the invention to provide insulation system to cast resin transformer which gives better cooling with non burning properties.
It is still another object of the invention to provide method for production of insulation system for cast resin transformer wherein the method is easy.
Other objects and advantages will become apparent from the following description and the appended claims.

DETAILED DESCRIPTION OI THE INVENTION:
According to the invention there is provided a curable epoxy composition comprising
a. 100 parts by weight of base resin, unmodified epoxy resin of medium viscosity
based on bispheno! A and cpichlororohydrin;
b. 100 parts by weight of modified carboxylic anhydride as hardener;
c. thermally pretreated filler comprising 30 to 40 parts by weight of hydrated
alumina, 35 to 50 parts by weight of alumina, 10 to 15 parts by weight of
magnesium hydroxide, and 100 to 125 parts by weight of silica along with the
additives like plasticizer 10 parts by weight and
d. accelerator 0.2 parts by weight.
The epoxy resins used as base resin in the present invention are unmodified epoxy resin of medium viscosity based on bisphenol A and cpichlororohydrin. These are short chain epoxy resin and readily available in the market under tradename of Araldite and CY 205. However, one can use any available short chain epoxy resin other than Araldite and CY 205. The epoxy resin used in the specification is intend to cover short chain liquid diglycidyl ethers of bisphenol A which has equivalent molecular weight in the range of 182 to 196 g/cquivalents and viscosity in the range of 9000 13000 cps.
The hardner used for this epoxy resin is typically an anhydride of dicarboxylic acid. Such anhydrides may be of various known types, such as cycloaliphatic (aiicyclic), aromatic, and aliphatic, and mixtures thereof, lixamplcs of such anhydrides are hexahydrophthalic anhydride, phlhalic anhydride, telrahydrophthalic anhydride, methyitetrahydrophthalic anhydride, methyitetrahydrophthalic anhydride, methyitetrahydroendomethylenephthalic anhydride, malcic anhydride, and succinic anhydride. A number of other anhydrides of this type which may be used are disclosed in the text of Lee and Neville, Handbook of Epoxy Resins, published 1967 by McGraw-Hill Book Company, New York City, Chapter 12. The present invention uses modified hardener (HY 905) based on carboxylic acid anhydrides.
An accelerator is also preferably included in the composition to accelerate the curing reaction of the epoxy resin and the anhydride curing agent. Various known accelerators for curing epoxy resins may be used for this purpose, as for example, tertiary amines. Tertiary amine

salts may also be used. Other compounds which may be found useful as curing catalysts are stannous octoale. thioglycols, and phenols, among others. Such accelerators and others which may be used are disclosed in the aforementioned Lee and Neville text. Chapter 12-3. Preferably, the accelerator used to cure the curable composition is DT 061.
The curable epoxy composition has better thermal conductivity, good electrical properties and mechanical properties, better flame retardance and has acceptable oxygen index.
According to the invention, there is also provided a method of making a curable epoxy resin composition which comprises
a) Preparing a first mixture comprising 100 parts by weight of base resin, unmodified epoxy resin of medium viscosity based on bisphenol A and epichlororohydrin, and 50% of thermally pretreated filler comprising alumina, aluminum hydroxide, magnesium hydroxide, and silica along with the additive such as plasticizer;
b) Preparing a second mixture comprising 100 parts by weight of an anhydride curing agent and remaining 50% of thermally pretreated filler material comprising alumina, aluminum hydroxide, magnesium hydroxide, and silica along with the additive such as plasticizer; and
c) Mixing together said first mixture and said second mixture at 60 to 70°C and followed by adding accelerator and mixing for at least 30 minutes to obtain curable epoxy resin composition.
The curable epoxy composition is used immediately for casting any electrical insulating products such as transformer bushings, winding, etc.
"Thermally pretreated" in this context means that, by means of heat treatment, water is removed from the fillers advantageously in an amount of from 0.001% to about 0.05 % by weight based on the original weight before removal of water.
In the present invention the filler are subjected to heating at 120 °C to 125DC for 4 hours followed by cooling down and maintaining the fillers to 60°C to obtained thermally pretreated fillers.

The filler comprising 30 to 40 parts by weight of hydrated alumina, 35 lo 50 parts by weight of alumina, 10 to 15 parts by weight of magnesium hydroxide, and 100 to 125 parts by weight of silica along with the additives like 10 parts by weight of plasticizer.
Apart from plasticizer, any other additives can be used to obtain desired characteristics.
The curable composition gets hardened / cured on adding accelerator in it. The accelerator is required in 0.2 parts by weight to cure the composition.
According to the invention, there is also provided cross linked product obtained by curing composition according to the invention.
According to the invention, there is also provided insulation system of cast resin transformer wherein electrical coils are impregnated with curable composition of the invention.
According to the invention, there is also provided method for production of insulation system for cast resin transformer, the method comprises
a. Impregnating the electrical coil with curable composition of the claim 1 under
vacuum of 10 milli bar followed by curing at 105 to 115°C for 16 hours and
b. Subject the insulation system lo post curing at 135 to I45°C for 8 hours followed by
finishing ol'lhe product.
According to the invention there is also provided an improved method for the production of an electrical components utilizing the curable casting resin, wherein the improvement comprises the use of curable composition according to claim I as the casting resin.
The electrical components are selected from the group consisting of electrical components of transformer including winding, bushings, insulators, switches, sensors, converters etc.
In making electrical insulating products such as transformer bushings, windings, etc which are formed of epoxy resin compositions, and in which metal parts are embedded (e.g., axial conductor members and hardware support members), it has been found desirable to incorporate in the cpoxy resin composition. a filler comprising 30 to 40 parts by weight of

hydrated alumina, 35 to 50 parts by weight of alumina, 10 to 15 parts by weight of magnesium hydroxide, and 100 to 125 parts by weight of silica. The curable composition of the invention not only adds high mechanical strength to the product, but also imparts a low coefficient of thermal expansion to the resin material so that it more closely matches the thermal expansion coefficient of the embedded metal parts, thereby providing a more effective seal at the joints. Such formulations at room temperature are too viscous to pour into the mould for making the desired product, and in order to reduce the viscosity it is necessary to heat the material at temperatures of about 60 to 70°C. However, at such temperatures the epoxy resin begins to react with the curing agent and the pot life or available handling time is very short, often only a few minutes. This problem may be alleviated to some extent by storing the resin and the curing agent in separate heated vessels (with one or both components premixed with the filler material) and bringing the two components together only as they pass through a mixing vessel of small volume before being poured into the moulds. In accordance with a feature of the invention, it was found that the incorporation in the resin-filler mixture of a specific amount of hydrated alumina, alumina and magnesium hydroxide in particular ration increases flame retardance of the curable composition of the invention at the same time giving better cooling to transformer. The composition also has good thermal conductivity, flame retardance, mechanical and electrical properties with acceptable oxygen index and thus insulation system also. The invention thus makes it possible to hold a relatively large amount of the epoxy resin-fillcr mixture ready for use over a substantial period of time and avoid the necessity for preparing the mixture each time it is to be poured into the mould. The method of casting any electrical component including insulation system of transformer is easy.
The following experimental examples arc illustrative of the invention but not limitative of the scope thereof:
Example 1:
Existing formulation of curable composition
I) Drying fillers:
DY 40 (plasticizer) and silica are subjected to heating 125°C for 4 hours to remove water. The mixture is cooled down to 60°C for further use.

II) First Mixture:
10D parts by weight of CY 205 base resin is mixed with 50% of filler composition comprising 5 parts by weight of DY 40 (plasticizer) and 100 parts by weight of silica to obtain first mixture.
III) Second Mixture:
100 parts by weight of HY 905 as hardener is mixed with 50% of filler composition comprising 5 parts by weight of DY 40 (plasticizer) and 100 parts by weight of silica to obtain second mixture.
IV) Curable Composition:
First mixture is mixed with the second mixture at 65°C and 0.2 parts by weight of Accelerator DT061 is added to the mixture. The mixture is used immediately for casting of insulation system.
V) Casting of insulation system
Electrical coil of the transformer is impregnated with the above-mentioned curable composition under vacuum of 10 milti bar. The casted object is cured at 110°C for 16 hours followed by the post curing at 140°C for 8 hours. The casted insulation system is subjected to finishing. The insulation system is tested for Thermal conductivity (W/m K), Burning characteristics UL 94, Tan Delta. BDV Oxygen index (%), Water absorption @100°C for 1 hour (%), Arc resistance (Seconds) and CTI and the results of the lest are given in table I. Table 1: Test results of existing insulation system

Sr. No. Test Results
1 Thermal conductivity. W/m K 0.29
2 Burning Characteristics UL94 V3
3 Tan Delta @ 0.004
4 BDV 16
5 Oxygen Index. % 39
6 Water Absorption @ 100C for 1 Hr. . % 0.35

7 Arc Resistance, Seconds 117- 140
8 CTl >600
Example 2:
Formulation of curable composition
1) Drying fillers
DY 40 (plasticizer), alumina, hydrated alumina and silica are subjected to heating 125°C for
4 hours to remove water. The mixture is cooled down to 60°C for further use.
II) First Mixture
100 parts by weight of CY 205 base resin is mixed with 50% of filler composition comprising
5 parts by weight of DY 40 (plasticizer). 87.5 parts by weight of silica. 7,5 parts of hydrated
alumina and 5 parts by weight of Alumina to obtain first mixture.
III) Second Mixture
100 parts by weight of HY 905 as hardener is mixed with 50% of filler composition comprising 5 parts by weight of DY 40 (plasticizer), 87.5 parts by weight of silica, 7.5 parts of hydrated alumina and 5 parts by weight of Alumina to obtain second mixture.
IV) Curable Composition
First mixture is mixed with the second mixture at 65°C and 0.2 parts by weight of Accelerator DT06l is added to the mixture. The mixture is used immediately for casting of insulation system.
V) Casting ol insulation system
Electrical coil of the transformer is impregnated with the above-mentioned curable composition under vacuum of 10 milli bar. The casted object is cured at 110°C for 16 hours followed by the post curing at 140° C for 8 hours. The casted insulation system is subjected to finishing. The insulation system is tested for Thermal conductivity (W/m K), Burning characteristics UL 94, Tan Delta. BDV. Oxygen index (%), Water absorption @100°C for I hour (%), Arc resistance (Seconds) and CTl and the results of the test are given in table 2.

Table 2: Test results of insulation system

Sr. Test Results
No.
1 Thermal conductivity, W/m K 0.37
2 Burning Characteristics UL 94 V3
3 Tan Delta @ 0.0042
4 BDV 17
5 Oxygen Index. % 37
6 Water Absorption @ 100 C for 1 Hr., 0.35
%
7 Arc Resistance, Seconds 123-147
8 CTI >600
Example 3:
Formulation of curable composition
I) Drying fillers
DY 40 (plasticizer), alumina, hydrated alumina, magnesium hydroxide and silica are subjected to heating 125°C for 4 hours to remove water. The mixture is cooled down to 60°C for further use.
II) Firsl Mixture
100 parts by weight of CY 205 base resin is mixed with 50% of filler composition comprising 5 parts by weight of DY 40 (plasticizer). 87.5 parts by weight of silica, 15 parts of hydrated alumina, 17.5 parts by weight of Alumina and 5 parts by weight of magnesium hydroxide to obtain first mixture.
III) Second Mixture
100 parts by weight of HY 905 as hardener is mixed with 50% of filler composition
comprising 5 parts by weight of DY 40 (plasticizer). 87.5 parts by weight of silica, 15 parts of
11

hydrated alumina, 17.5 parts by weight of Alumina and 5 parts by weight of magnesium hydroxide to obtain second mixture.
IV) Curable Composition
First mixture is mixed with the second mixture at 65°C and 0.2 parts by weight of Accelerator DT061 is added to the mixture. The mixture is used immediately for casting of insulation system.
V) Casting of insulation system
Electrical coil of the transformer is impregnated with the above-mentioned curable composition under vacuum of 10 milli bar. The casted object is cured at 110°C for 16 hours followed by the post curing at 140°C for 8 hours. The casted insulation system is subjected to finishing. The insulation system is tested for Thermal conductivity (W/m K), Burning characteristics UL 94, fan Delta, BDV, Oxygen index (%), Water absorption @ 100°C for 1 hour (%), Arc resistance (Seconds) and CT1 and the results of the test are given in table 3.
Table 3: Test results of insulation system

Sr. No. Test
Thermal conductivity. W/m K Results
1.
0.59
2 Burning Characteristics UL94 VI
3 Tan Delta @ 0.0047 i
4
5
8 BDV
Oxygen Index, % 17.9

35

Water Absorption @ 100 C for 1 Hr.,
% 0.38

Arc Resistance. Seconds CTI 135-155

>600

Sr.
No. Test Results
1 Thermal conductivity, W/m K 0.97
2 Burning Characteristics UL 94 Tan Delta @ V I
3
0.005
4 BDV 19
5 Oxygen Index, % 31
6 Water Absorption @ 100 C for ] Hr., % 0.40
7 1 Arc Resistance, Seconds 180-185

CTI >600
Example 5:
Formulation of curable composition
I) Drying fillers
DY 40 (plasticizcr). alumina, hydrated alumina, magnesium hydroxide and silica are subjected to heating 125°C for 4 hours to remave water. The mixture is cooled down to 60°C for further use.
II) First Mixture
100 parts by weight of CY 205 base resin is mixed with 50% of filler composition comprising 5 parts by weight of DY 40 (ptasticizer), 50 parts by weight of silica, 20 parts of hydrated alumina. 22.5 parts by weight of Alumina and 7.5 parts by weight of magnesium hydroxide to.obtain first mixture
III) Second Mixture
100 parts by weight of MY 905 as hardener is mixed with 50% of filler composition
comprising 5 parts by weight of DY 40 (ptasticizer), 50 parts by weight of silica. 20 parts of
hydrated alumina, 22.5 parts by weight of Alumina and 7.5 parts by weight of magnesium
hydroxide to obtain second mixture.
14

IV) Curable Composition
First mixture is mixed with the second mixture at 65°C and 0.2 parts by weight of Accelerator DT061 is added to the mixture. The mixture is used immediately for casting of insulation system.
V) Casting of insulation system
Electrical coil of the transformer is impregnated with the above-mentioned curable composition under vacuum of 10 milli bar. The casted object is cured at 110°C for 16 hours followed by the post curing at 140°C for 8 hours. The casted insulation system is subjected to finishing. The insulation system is tested for Thermal conductivity (W/m K), Burning characteristics UL 94, Tan Delta, BDV, Oxygen index (%), Water absorption @100°C for 1 hour (%), Arc resistance (Seconds) and CTI and the results of the test are given in table 5.
Tabic 5: Test results of insulation system

Sr.
No. Test
I
Thermal conductivity, W/m K Burning Characteristics U1.94 Results
1

1.11
2

V O

Tan Delta @ 0.0049
4 BDV 19.7
5 Oxygen Index, % 28
6
— Water Absorption @ 100 C for
%
Arc Resistance, Seconds
CTI 1 Hr., 0.33

190-195
8 I

>600

As will be understood, other ingredients may be incorporated in the described formulations for particular purposes as desired without departing from the scope of the invention, as for example, dyes or other fillers such as quartz powder, glass powder, glass fibers, mica, pigments, etc.
15

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

We claim:
1. A curable epoxy composition comprising
a) 100 parts by weight of base resin, unmodified epoxy resin of medium viscosity based on bisphenol A and cpichlororohydrin;
b) 100 parts by weight of modified carboxylic anhydride as hardener;
c) thermally pretreated filler comprising 30 to 40 parts by weight of hydrated alumina, 35 to 50 parts by weight of alumina, 10 to 15 parts by weight of magnesium hydroxide, and 100 to 125 parts by weight of silica along with the additives like 10 parts by weight of plasticizer and
d) 0.2 parts by weight of accelerator.

2. The curable epoxy composition as claimed in claim I, wherein the filler are subjected to heating at 120 lo 125°C for 4 hours followed by cooling down and maintaining the fillers to 60°C to obtained thermally pretreated fillers.
3. A method of making a curable epoxy resin composition which comprises

a) Preparing a first mixture comprising 100 parts by weight of base resin, unmodified epoxy resin of medium viscosity based on bisphenol A and cpichlororohydrin, and 50% of thermally pretreated filler comprising alumina, aluminum hydroxide, magnesium hydroxide, and silica along with the additive such as plasticizer:
b) Preparing a second mixture comprising 100 parts by weight of an anhydride curing agent and remaining 50% of thermally pretreated filler material comprising alumina, aluminum hydroxide, magnesium hydroxide, and silica along with the additive such as plasticizer; and
c) Mixing together said first mixture and said second mixture at 60 to 70°C and followed by adding 0.2 parts of accelerator and mixing for at least 30 minutes to obtained curable epoxy resin composition.
4. The method as claimed in claim 3. wherein the filler a.re subjected to heating at 120 to
125°C for 4 hours followed by cooling down and maintaining the fillers to 60°C to obtain thermally pretreated fillers.

5. The method as claimed in claim 3. wherein the filler comprising 30 to 40 parts by weight of hydraicd alumina. 35 to 50 parts by weight of alumina, 10 to 15 parts by weight of magnesium hydroxide, and 100 to 125 parts by weight of silica along with the additives like 10 parts by weight of plasticizer.
6. A crosslinked product obtained by curing epoxy composition according to the claim I.
7. An insulation system of cast resin transformer wherein electrical coils are
impregnated with curable composition of the claim 1.
8. A method for production of insulation system for east resin transformer, the method
comprises
a. Impregnating the electrical coil with curable composition of the claim I under
vacuum of 10 milli bar followed by curing at 105 to 115°C for 16 hours and
b. Subject the insulation system to post curing at 135 to 145°C for 8 hours followed
by finishing of the product.
9. An improved method for the production of an electrical components utilizing the
curable casting resin, wherein the improvement comprises the use of curable
composition according to claim I as the casting resin.
10. An improved method as claimed in claim 9, wherein the electrical components are
selected from the group consisting of electrical components of transformer including
winding, bushings, insulators, switches, sensors, converters etc.