Description:FIELD OF THE INVENTION
The present invention generally relates to a lubricant composition for electrical vehicle. More specifically, the present invention relates to a lubricant composition comprises of a base oil combination of suitable viscosity, and a suitable mix of an additive system, and a viscosity modifier. The ratio of sulfur to a total of nitrogen and phosphorus [S/(N+P)] in the lubricant composition is 1 to 2, makes it suitable for e-motor application.

BACKGROUND OF THE INVENTION
Electric and hybrid vehicles gained ever increasing attention over conventional vehicles. The difference between electric and conventional vehicles is that the former operates on electric motor and the later operates with internal combustion engine. Due to this difference, conventional lubricants requirements such as detergency, dispersion property, anti-wear property, rust inhibition, friction reduction, etc., may not suffice the requisites of electric vehicle applications.

Lubricant in electric vehicle applications may come in contact with electrical winding of the e-motor and gear reduction components which poses new challenges to formulators to develop lubricants with special characteristics i.e., electrical and thermal properties apart from the conventional requirements. These properties should be maintained under all the circumstances including entire operating temperature range, humidity, and oxidation conditions. Additionally, the lubricant composition must be compatible with the components of the power train and power train components are also required to be protected at high temperatures. Some of the prior arts related to subject invention are discussed below.

JP-2017020045A relates to the dispersant additive composition in combination with the metallic detergent in a lubricant composition with an electrical conductivity of less than about 300-1700 pS/m. A dispersant additive composition comprising the reaction product of a hydrocarbyl-substituted carboxylic acid or anhydride and an amine. The reaction product has a nitrogen content of about 2,400 to 10,000 ppm by weight and a boron + phosphorus: nitrogen having a weight ratio of 0: 1 to about 0.8: 1.

WO2018/067902A1 relates to the lubricating oil composition base oil as a major component, an additive package as a minor component, and an effective amount of one or more conductivity agents as a minor component. The lubricating oil has an electrical conductivity from about 10 pS/m to about 20,000 pS/m, a dielectric constant of about 1.6 to about 3.6, with a ratio of electrical conductivity-to-dielectric constant from about 5 to about 10,000.

EP2650348B1 relates to a base oil, a succinimide dispersant having a nitrogen content ranging from 2400 up to 10,000 ppm by weight, a boron plus phosphorus to nitrogen ((B+P)/N) weight ratio of from 0:1 to 0.8:1, and the dispersant that provides 300 to 900 ppm nitrogen to the lubricant composition; and an oil soluble overbased calcium sulfonate detergent having a TBN of 150 to 450, in an amount providing 60 to 600 ppm calcium to the lubricant composition, wherein the dispersant plus the overbased calcium sulfonate detergent provide the lubricant composition with an electrical conductivity of the lubricant composition, as measured at 22 °C, from 300 pS/m to 1700 pS/m.

US2021/0324293A1 discloses an electric or a hybrid-electric transmission using a lubricant, including a solvent system with a blend of one or more base oils with a branched diester and one or more poly(meth)acrylate copolymers. The lubricating compositions suitable for such applications that exhibit good lubricant properties, good electrical properties, and good cooling efficiency at the same time.

US2021/010889779B2 provides a lubricating oil composition for a transmission having excellent gear properties, clutch properties, cooling properties, and electric insulating properties. The lubricating oil composition for a transmission prepared by blending (A) a base oil, (B) a viscosity index improver having a mass average molecular weight of 10,000 to 50,000, and (C - 1) a polyamide and / or (C - 2) a polyol ester, the base oil (A) including (A - 1) a synthetic oil having a kinematic viscosity at 100 °C, of 1.0 to 10.0 mm²/s.

WO2018/067908?1 discloses a lubricating oil for electric and hybrid vehicles. The lubricating oil includes at least 80 wt % of a lubricating base oil, 0.5 to 5 wt % of one or more dispersants, 0 to 4 wt % of one or more neutral metal detergents, 0 to 1.5 wt % of zinc dialkyldithiophosphate, 0 to 0.2 wt % of molybdenum dialkyldithiocarbamate, 0 to 2 wt % of an active viscosity modifier, and 0.01 to 5 wt % of one or more other lubricating oil additives. The lubricating oil is essentially free of overbased metal detergents. The lubricating oil has an electrical conductivity of 50 to 3,000 pS/m and a kinematic viscosity of 2 to 20 cSt at 100 °C. This disclosure also relates to the method for producing the lubricating oil, methods for lubricating electric and hybrid vehicles, and methods for controlling electrical conductivity of a lubricating oil by using conductivity promoters and conductivity inhibitors.

US2018/0100118 relates to a method for minimizing the electrical drainage of charged electrical powertrain components, a method for controlling electrical conductivity over a lifetime of a lubricating oil in an electric vehicle powertrain lubricated with the lubricating oil, and a method for obtaining a desired electrical conductivity-to-dielectric constant ratio of a lubricating oil for an electric vehicle powertrain and powertrain components. The methods relate to controlling at least one of oxidation, deposit formation and corrosion over the service lifetime of the oil. The lubricating oil has a composition including a lubricating base oil as a major component, an additive package, as a minor component, and an effective amount of one or more conductivity agents, as a minor component. The lubricating oil has an electrical conductivity from 10 pS/m to 20,000 pS/m, a dielectric constant of 1.6 to 3.6, with a ratio of electrical conductivity to dielectric constant from 5 to 10,000.

It can be understood that although some of the prior art relates to lubricant composition but none of the prior art discloses about electrical and heat transfer properties of the lubricant that could be maintained in wide range of operating conditions specially in e-motor applications.

Therefore, there is a need of lubricant composition with good electrical and heat-transfer properties that could be maintained to optimum during the wide range of operating conditions in e-motor applications. Additionally, the lubricant composition must be compatible with the components of the power train which is predominantly copper.

SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended to determine the scope of the invention.

The present invention relates to a unique lubricant composition providing excellent electrical and thermo-oxidation properties which is suitable to use in the electrical vehicles wherein the lubricant is in-contact with the e-motor.

In an aspect, the present invention provides a lubricant composition for e-motors comprising of:
(i) 60 to 98 wt % of a base oil; and
(ii) 2 to 40 wt % of a combination of an additive and a viscosity modifier,
wherein a ratio of sulfur to a total of nitrogen and phosphorus [S/(N+P)] in the lubricant composition is in the range of 1 to 2, and wt % is based on the total weight of the composition.

In another aspect, the present invention provides the base oil is selected from Gp I, Gp II, Gp III, and Gp IV base oils or a combination thereof.

In another aspect, the present invention provides the base oil is a combination of Gp III and Gp IV base oil; wherein said base oil has a viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140.

In another aspect, the present invention provides the additive is selected from sulfurized isobutylenes, phosphates and monothiophosphates, dithiophosphates, ammonium phosphates, alkyl phosphates, polyisobutylene succinimide, diphenylamine, heterocyclic ethers, and acrylate ester, an antioxidant, polyisobutylene and polymethacrylate.

In another aspect, the present invention provides the additive is present in the range of 2-15 wt %, by weight of the combination of the additive and the viscosity modifier.

In another aspect, the present invention provides the viscosity modifier is selected from polyisobutylene and polymethacrylate having molecular weight in the range of 100-50000 Da.

In another aspect, the present invention provides the antioxidant is selected from a phenolic antioxidant and an aminic antioxidant; present in the range of 0.25-3.0 wt %, by weight of the combination of the additive and the viscosity modifier.

In another aspect, the present invention provides the viscosity modifier is present in the range of 0-30 wt %, by weight of the combination of the additive and the viscosity modifier.

In another aspect, the present invention provides the electrical conductivity of lubricant composition is in the range from 10 to 4000 pS/m at ambient temperature.

In another aspect, the present invention provides the dielectric constant of lubricant composition is in the range 2.0 to 7.0; volume resistivity is in the range of 0.2 x1011 to 40 x1011 ? cm; tan delta is in the range of 0.01 to 1.5; and breakdown voltage is in the range of 20 to 60 kV.

In another aspect, the present invention provides the thermal conductivity of lubricant composition is in the range of 0.1 to 0.2 W/mK; and specific heat capacity is in the range of 1.0 to 3.0 Jg-1C-1at 60 ?.

In another aspect, the present invention provides the lubricant composition maintains the electrical properties under severe thermal oxidation conditions.

In another aspect, the present invention provides the lubricant composition has excellent cooling properties and superior copper material compatibility.

In another aspect, the present invention provides a method for preparing the lubricant composition, wherein the said method comprises of:
- normalizing the temperature of the base oil blend in a container to 60-70 °C;
- adding additive components selected from the lubricant composition to obtain a reaction mixture; and
- heating the reaction mixture in a container with agitation and circulation to remove any moisture and then obtaining the lubricant composition.

OBJECTIVES OF THE PRESENT INVENTION
The main objective of the present invention is to provide a lubricant composition for e-motor.
Another objective of the present invention is to provide a lubricant composition which could maintain electrical properties under severe thermal conditions.
Another objective of the present invention is to provide a lubricant composition with excellent cooling properties and superior copper material compatibility.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1: Vapor phase copper corrosion 40 hrs @ 130 ?, Examples 3, 8, 9.
Figure 2: Vapor phase copper corrosion image 15 hrs @ 150 ?, Examples 8, 9.

DETAILED DESCRIPTION OF THE INVENTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments in the specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated process, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The composition, methods, and examples provided herein are illustrative only and not intended to be limiting.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.
Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference.
The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict, or reduce the spirit and scope of the invention.
The present invention provides, a novel lubricant composition for e-motors comprising of:
(i) 60 to 98 wt % of a base oil; and
(ii) 2 to 40 wt %. of a combination of an additive and a viscosity modifier,
wherein a ratio of sulfur to a total of nitrogen and phosphorus [S/(N+P)] in the lubricant composition is in the range of 1 to 2, and wt % is based on the total weight of the composition.

In one feature of the present invention, the base oil is selected from Gp I, Gp II, Gp III, and Gp IV base oil or a combination thereof. The physical and chemical properties of the Gp I, Gp II, Gp III, and Gp IV base oil are given in table 1.
Table 1

Group Saturate Level Sulfur Level Viscosity Index
Gp I <90% >0.03% 80-120
Gp II =90% =0.03% 80-120
Gp III =90% =0.03% =120
Gp IV 100% PAOs (Poly-Alpha-Olefins)

In another feature of the present invention, the base oil is a combination of Gp III and Gp IV base oil; wherein the said combination of base oil has a viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140.

In another feature of the present invention, the additive is selected from sulfurized isobutylenes, phosphates and monothiophosphates, dithiophosphates, ammonium phosphates, alkyl phosphates, polyisobutylene succinimide, diphenylamine, heterocyclic ethers, and acrylate ester and an antioxidant.

In another feature of the present invention, the additive is present in the range of 2-15 wt %, by weight of the combination of the additive and the viscosity modifier.

In another feature of the present invention, the viscosity modifier is selected from polyisobutylene and polymethacrylate having molecular weight in the range of 100-50000 Da.

In another feature of the present invention, the antioxidant is selected from phenolic antioxidant and aminic antioxidant. The antioxidant is present in the range of 0.25-3.0 wt %, by weight of the combination of the additive and the viscosity modifier.

In one feature of the present invention, the viscosity modifier is present in the range of 0-30 wt %, by weight of the combination of the additive and the viscosity modifier. In another feature of the present invention, the viscosity modifier is present in the range of 0.1-30 wt %, by weight of the combination of the additive and the viscosity modifier.

In another feature of the present invention, the electrical conductivity of lubricant composition is in the range from 10 to 4000 pS/m at 25 ?.

In another feature of the present invention, the dielectric constant of lubricant composition is in the range 2.0 to 7.0; volume resistivity is in the range of 0.2 x1011 to 40 x1011 ? cm; tan delta is in the range of 0.01 to 1.5; and breakdown voltage is in the range of 20 to 60 kV.

In another feature of the present invention, the thermal conductivity of lubricant composition is in the range of 0.1 to 0.2 W/mK; and specific heat capacity is in the range of 1.0 to 3.0 Jg-1C-1at 60 ?.

In one feature of the present invention, the lubricant composition maintains the electrical properties under severe thermal oxidation conditions.

In another feature of the present invention, the lubricant composition provides excellent cooling properties and superior copper material compatibility.

The present invention also provides, a method for preparing the lubricant composition, wherein the said method comprises of:
- normalizing the temperature of the base oil blend in a container to 60-70 °C;
- adding additive components selected from the lubricant composition to obtain a reaction mixture; and
- heating the reaction mixture in a container with agitation and circulation to remove any moisture and then obtaining the lubricant composition,
wherein, the lubricant composition comprises of 60 to 98 wt % of a base oil, and 2 to 40 wt % of a combination of an additive and a viscosity modifier; the base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120 -140;
the additive is selected from sulfurized isobutylenes, phosphates and monothiophosphates, dithiophosphates, ammonium phosphates, alkyl phosphates, polyisobutylene succinimide, diphenylamine, heterocyclic ethers, acrylate ester and an antioxidant; and
the ratio of sulfur to a total of nitrogen and phosphorus [S/(N+P)] in the lubricant composition is in the range of 1 to 2.

METHODOLOGY

Breakdown voltage: Breakdown voltage is measured by ASTM D 1816.
Electrical conductivity: Electrical conductivity is measured on Emcee model 1154 as per ATM D 4308.
Thermal conductivity: Thermal conductivity was measured using ASTM D 7984.
Specific heat capacity: Specific heat capacity is measured by TGA-DSC.
Dielectric constant/Tandelta/resistivity: Tandelta, volume resistivity, and dielectric constant are measured on instrument MOTOR as per the IEC 60247. Frequency range 45-65 Hz and capacitance 55.5 pF, 500 voltage, temperature 26±1 ?. Tan delta and dielectric constant were measured in AC Mode and Volume resistivity was measured in DC mode.
Vapor phase copper corrosion This test was done on modified ASTM D 130 test method.

EXAMPLES
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

Example 1: The composition of lubricant comprises of base oil (Gp I/II/III) 60-98 wt %, VM2 in the range of 10-30 wt %, additive components selected from sulfurized isobutylenes, phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates in the range 2-15 wt %. The composition is mentioned in Table 2.
Base oil (Gp I/II/III) in example 1 comprises of a combination of a primary and a secondary base oil, wherein the primary base oil has viscosity ranging from 3-11 cSt at 100 °C and viscosity index value in the range of 100-130; and wherein the secondary base oil has viscosity ranging from 20-40 cSt at 100°C and viscosity index value in the range of 80-120.

Example 2: The composition of lubricant comprises of base oil (Gp III and or IV) 85-98 wt %, VM=0 wt %, additive components selected from sulfurized isobutylenes, phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates in the range 2-15 wt %. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 3: The composition of lubricant comprises of base oil (Gp III and or IV) 60-98 wt %, VM1 = 10-30 wt %, additive components selected from sulfurized isobutylenes, phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates in the range 2-15 wt %. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 4: The composition of lubricant comprises of base oil (Gp III and or IV) 60-98 wt %, VM2 = 10-30 wt %, additive components selected from sulfurized isobutylenes, phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates in the range 2-15 wt %. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 5: The composition of lubricant comprises of base oil (Gp III and or IV) 60-98 wt %, VM1 = 10-30 wt %, additive components selected from sulfurized isobutylenes, phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates in the range 5-12 wt %, and polyisobutylene succinimide in the range of 0.25-3.0 wt %. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 6: The composition of lubricant comprises of base oil (Gp III and or IV) 60-98 wt %, VM1 = 10-30 wt %, additive components selected from diphenylamine, heterocyclic ethers, acrylate ester and polyisobutylene succinimide in the range 2-15 wt %. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 7: The composition of lubricant comprises of base oil (Gp III and or IV) 60-98 wt %, VM1 = 10-30 wt %, additive components selected from diphenylamine, heterocyclic ethers, acrylate ester and polyisobutylene succinimide in the range 2-12 wt %, and an antioxidant 1 in the range 0.25-3.0 wt %; wherein antioxidant 1 is a phenolic antioxidant. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 8: The composition of lubricant comprises of base oil (Gp III and or IV) in the range 60-98 wt %, VM1 = 10-30 wt %, additive components are selected from diphenylamine, heterocyclic ethers, acrylate ester and polyisobutylene succinimide in the range 2-12 wt %, and an antioxidant 2 in the range 0.25-3.0 wt %; wherein the antioxidant 2 is aminic antioxidant. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Example 9: The composition of lubricant comprises of base oil (Gp III and or IV) in the range 60-98 wt %, VM1 = 10-30 wt %, additive components are selected from phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates, and polyisobutylene succinimide in the range 2-15 wt %. The base oil has viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140. The composition is mentioned in Table 2.
Table 2: Composition of all the examples is given in the below table

Components Example 1 Example 2 Example 3 Example 4 Example 5
Base Oil (Gp I/II/III), wt% 60-98
Base Oil (Gp III and /or IV), wt% 85-98 60-98 60-98 60-98
VM Polymethacrylate (VM1)/ polyisobutylene (VM2), wt% 10-30
(VM2) 10-30
(VM1) 10-30
(VM2) 10-30
(VM1)
Additive Sulfurized isobutylenes, wt% 2-15 2-15 2-15 2-15 5-12
Phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates, wt%
Polyisobutylene succinimide, 0.25-3.0
Diphenylamine/ Heterocyclic ethers/ Acrylate Ester, wt%
Antioxidant 1 or 2, wt%
Ratio of S/(N+P) after test 14 20.2 24 11.39 8.14

Components Example 6 Example 7 Example 8 Example 9
Base Oil (Gp I/II/III)
Base Oil (Gp III and or IV) 60-98 60-98 60-98 60-98
VM (polymethacrylate/ polyisobutylene), wt% 10-30
(VM1) 10-30
(VM1) 10-30
(VM1) 10-30
(VM1)
Additive Sulfurized isobutylenes, wt%
Phosphates, ammonium phosphates, alkyl phosphates and mono/di/tri thiophosphates, wt% 2-15
Polyisobutylene succinimide, 2-15 2-12
2-12

Diphenylamine/ Heterocyclic ethers/ Acrylate Ester, wt%
Antioxidants 1 wt% 0.25-3.0
Antioxidants 2, wt% 0.25-3.0
Ratio of S/(N+P) after test 1.54 1.65 1.75 1.43

Physico-chemical and electrical properties of the compositions for example 1-9 are mentioned in Table 3.
Table 3: Physico-chemical and electrical properties of the examples 1 to 9 and Industry reference oil
Example Kinematic viscosity at @ 40 ?, cSt Kinematic viscosity @ 100 ?, cSt Viscosity Index Density@15 ?, gm/cm3 Conductivity, pS/m Breakdown Voltage, kV
1 117.1 15.64 142 0.8676 421 29.05
2 30.62 5.878 139 0.8483 1192 29.10
3 91.99 15.37 177 0.8722 521 51.32
4 98.75 14.12 146 0.8604 604 43.32
5 91.24 15.34 178 0.8734 3770 51.70
6 89.92 14.13 162 0.8538 2580 52.30
7 89.21 14.01 162 0.8533 2660 52.77
8 79.39 14.12 185 0.8588 1790 54.61
9 96.15 16.18 181 0.8688 868 51.45
Industry reference oil 149.3 14.31 95 0.9011 1109 NA

The physico-chemical and electrical properties for examples 1-9, after accelerated oxidation test (ISOT@150 ?, 96 hrs) are mentioned in Table 4-7.
Table 4: Physico-chemical and electrical properties after accelerated oxidation test (ISOT@ 150?, 96 hrs) Examples 1- 9.
Parameter Example 1 Example 2 Example 3 Example 4 Example 5
Fresh After test Fresh After test Fresh After test Fresh After test Fresh After test
KV @ 40 ?, cSt 117.1 135.1 30.62 31.03 91.99 93.06 98.75 106.8 91.24 100.8
KV @ 100 ?, cSt 15.64 16.95 5.878 5.924 15.37 15.60 14.12 14.99 15.34 16.50
VI 142 136 139 139 177 178 146 146 178 177
Conductivity, pS/m 421 334 1192 35.5 521 15 604 260 3770 990
Cu, ppm 915 374 436 310 323
% loss of additives (P, Ca, B, N) 4 50 56 21 14.7

Parameter Example 6 Example 7 Example 8 Example 9
Fresh After test Fresh After test Fresh After test Fresh After test
KV @ 40 ?, cSt 89.92 93.08 89.21 92.7 79.39 79.49 96.15 97.34
KV @ 100 ?, cSt 14.13 14.31 14.01 14.21 14.12 14.14 16.18 16.12
VI 162 159 162 160 185 185 166 178
Conductivity, pS/m 2580 1650 2660 2140 1790 1820 868 821
Cu, ppm 29 13 7 8
% loss of additives (P, Ca, B, N) 2.7 3.4 0.17 11

The electrical properties for example 1-9 after accelerated oxidation test (ISOT@150 ?, 96 hrs) are mentioned in Table 5.

Table 5: Electrical properties after accelerated oxidation test (ISOT@150 ?, 96 hrs) Examples 1-9.

Parameter Example 1 Example 2 Example 3 Example 4 Example 5
Fresh After test Fresh After test Fresh After test Fresh After test Fresh After test
Volume Resistivity, ohm cm 2.49 x1011 3.81 x1011 0.88 x1011 21.18x1011 1.59x1011 37.5x1011 0.76x1011 4.79x1011 0.29 x1011 1.10 x1011
Dielectric constant 2.467 2.546 2.488 2.457 2.626 2.918 2.512 2.446 3.014 2.809
Tan delta 0.03 0.033 0.0717 0.0094 0.0685 0.0509 0.1062 0.0181 0.2124 0.0863

Parameter Example 6 Example 7 Example 8 Example 9
Fresh After test Fresh After test Fresh After test Fresh After test
Volume Resistivity, ohm cm 0.2552 x1011 0.3647 x1011 0.2582 x1011 0.4073 x1011 0.4512 x1011 0.4620 x1011 0.9961x1011 1.113x1011
Dielectric constant 3.082 2.7 2.9 2.7 2.723 2.723 2.606 2.591
Tan delta 0.6645 0.2817 0.3429 0.2268 0.2042 0.2009 0.1089 0.0266

The heat transfer properties for example 1, 3, 4, 6, 7, and 9 after accelerated oxidation test (ISOT@150 ?, 96 hrs) are mentioned in Table 6.

Table 6: Heat-transfer properties after accelerated oxidation test (ISOT @ 150 ?, 96 hrs) Examples 1, 3, 4, 6, 7, 8, and 9.

Parameter Example 1 Example 3 Example 4 Example 6
Fresh After test Fresh After test Fresh After test Fresh After test
Thermal Conductivity, (W/mK) @ 60 ? 0.2439 0.1474 0.1526 0.1544 0.1478 0.1482 0.1485 0.1486
Specific heat capacity, (Jg-1C-1) @ 60 ? 2.51 1.82 2.78 2.03 2.25 2.22 1.80 1.77

Parameter Example 7 Example 8 Example 9
Fresh After test Fresh After test Fresh After test
Thermal Conductivity, (W/mK) @ 60 ? 0.1484 0.1485 0.1523 0.1526 0.1528 0.1541
Specific heat capacity, (Jg-1C-1) @ 60 ? 2.04 1.79 2.14 2.18 2.65 2.15

The liquid phase copper strip % wt change for example 3, 8, and 9 after 144 hrs@130 ? are mentioned in Table 7.
Table 7: % wt change of copper strip in liquid phase, after 144 hrs@130 ?, Examples 3, 8 & 9.
Example 3 Example 8 Example 9
1.9 3X10-5 1.3X10-4

From table 3, it is clear that examples 3 and 5-9 have shown high breakdown voltage which signifies appropriate combination of base oils and viscosity modifiers. Table 4, 5 and 6 indicate that the conductivities of example 1 to 9 fall in the range of 10 to 4000 pS/m, dielectric constant in the range of 2.0 to 7.0; volume resistivity in the range of 0.2 x1011 to 40 x1011 ? cm; tan delta in the range of 0.01 to 1.5 at 25 ?; thermal conductivity in the range of 0.1 to 0.2 W/mK; and specific heat capacity in the range of 1.0 to 3.0 Jg-1C-1 at 60 ?. Table 4 showed that examples 6-9 have resulted minimal electrical properties changes, negligible wear, and significantly less additive depletion under severe thermal oxidation conditions. Retention in electrical properties and, additive elements and less wear in the examples 6-9, preferably in example 8 is due to presence of right combination of base oils and additive chemistries which ultimately led to suitable ratio of S/(N+P).
Table 6 disclosed that heat transfer properties as demonstrated by the thermal conductivity and specific heat capacity are maintained among examples 2-9 due to high quality base oils. Table 7 & figures 1 and 2 confirmed that minimal copper wt% loss and better copper rating is observed in example 8 and 9 preferably in example 8 due to right combination of additive chemistry.
The ratio of [S/(N+P)] influences the electrical properties (i.e., electrical conductivity, volume resistivity, tan delta and dielectric constant) and material compatibility of lubricants during service period. It is observed that low [S/(N+P)] ratio maintains the electrical properties and material compatibility which further leads to longer drain of oil.

ADVANTAGES OF THE PRESENT INVENTION
1. Lubricant composition is suitable for the electric vehicle transmission, in which e-motor is in contact with fluid.
2. Proposed unique combination of base oils and additives can maintain electrical properties even after accelerated thermal ageing which indicates excellent electrical properties retention.
3. The unique lubricant composition provides superior cooling properties as demonstrated by the thermal conductivity and specific heat capacity.
4. Carefully selected composition provides superior copper material compatibility.
5. Low wear (Cu) and additive retention post ageing analysis due to suitable combination of additives and base oils.
, Claims:WE CLAIM

1. A lubricant composition for e-motors comprising of:
(i) 60 to 98 wt % of a base oil; and
(ii) 2 to 40 wt % of a combination of an additive and a viscosity modifier,
wherein a ratio of sulfur to a total of nitrogen and phosphorus [S/(N+P)] in the lubricant composition is in the range of 1 to 2, and wt % is based on the total weight of the composition.

2. The lubricant composition as claimed in claim 1, wherein the base oil is selected from Gp I, Gp II, Gp III, and Gp IV base oils or a combination thereof, preferably the base oil is selected from Gp III, and Gp IV base oils or a combination thereof.

3. The lubricant composition as claimed in claim 1, wherein the base oil is a combination of Gp III and Gp IV base oil; wherein the said combination of base oil has a viscosity range from 2-10 cSt at 100 °C and viscosity index value in the range of 120-140.

4. The lubricant composition as claimed in claim 1, wherein the additive is selected from sulfurized isobutylenes, phosphates and monothiophosphates, dithiophosphates, ammonium phosphates, alkyl phosphates, polyisobutylene succinimide, diphenylamine, heterocyclic ethers, acrylate ester, an antioxidant, polyisobutylene and polymethacrylate.

5. The lubricant composition as claimed in claim 1, wherein the additive is present in the range of 2-15 % by weight of the combination of the additive and the viscosity modifier.

6. The lubricant composition as claimed in claim 1 or claim 4, wherein the antioxidant is selected from a phenolic or an aminic antioxidant in the range of 0.25-3.0 % by weight of the combination of the additive and the viscosity modifier.

7. The lubricant composition as claimed in claim 1, wherein the viscosity modifier is present in the range of 0-30 % by weight of the combination of the additive and the viscosity modifier.

8. The lubricant composition as claimed in claim 1, wherein the viscosity modifier is having a molecular weight in the range of 100-50000 Da.

9. A method for preparing the lubricant composition as claimed in claim 1, wherein the said method comprises of:
- normalizing the temperature of the base oil blend in a container to 60-70 °C;
- adding additive components selected from the lubricant composition as defined in claims 1-8 to obtain a reaction mixture; and
- heating the reaction mixture in a container with agitation and circulation to remove any moisture and then obtaining the lubricant composition.