DESC:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents [Amendment] Rules, 2006

PROVISIONAL SPECIFICATION
(See Section 10 and rule 13)

TITLE OF THE INVENTION : “Process for preparing Multi Layered Graphene and lubricant composition thereof.”

APPLICANT :

NAME: Indian Oil Corporation Ltd.
NATIONALITY: IN
ADDRESS: G-9, Ali Yavar Jung Road, Bandra (East), Mumbai-400 051

PREAMBLE TO THE DESCRIRTION

Complete
The following specification describes the invention:

FIELD OF INVENTION
The present invention relates to the field of lubricant or grease that involves hydrocarbon oil based systems. In particular, the invention provides a process of preparing in situ dispersion of multi layered graphene in different hydrocarbon oils and their inclusion in grease, gear oil, differential oil, forging oil system for an improved thermal conductivity. The graphene system consists of a mixture of MLG and SLG which is a result of delamination/exfoliation of natural graphite.

BACKGROUND OF INVENTION
Graphite is a layered material with single layers (graphene) stacked along c-axis by van der Waal’s force. For decades it has been the main constituent of lubricants and grease systems since it aids to the tribological as well as the thermal diffusivity of the system. However, stability of the same in nonpolar hydrocarbon oils has always been a challenge in lubricant and grease applications. Recently discovered and widely studied material graphene is very unique with respect to its properties and applications. This material has the highest thermal conductivity at room temperature, even more than the diamond. Hence, inclusion of this material to the lubricant or grease system would provide unique thermal conductivity property since heat removal from the system has been a biggest challenge in industries and also perceived as primary function of a lubricant. It is to be noted that graphene dispersion in polar medium has been reported widely, but the same in hydrocarbon oil is still a taboo. Few reports are there where shear ball milling has been used to prepare either graphite nano-sheets or exfoliation of graphite to prepare graphene. However, all of these reports use a wet ball milling method and the medium has always been water. The planetary ball mills favor the exfoliation of the graphite sheets due to the shear component of the applied stress. Moreover, a number of in-plane defects are created due to bombardment with the balls that accelerates the cleavage as well as the exfoliation. This process results in nanographite with reduced stacking, which in turn forms MLG and SLG sheets. The stirred media bead mill system with its unique build provides more shear than the impact, which would be more effective for exfoliation. Peukert et. al in US Publication 2012/00220198 A1 has reported the use of stirred media mill for producing platelets comprising a layered material. Graphite has been used in water in presence of an ionic surfactant as a model and demonstrated that the shear component is essential for better exfoliation of layered materials in polar mediums.
SUMMARY OF THE INVENTION
The present invention provides a lubricant or grease that involves hydrocarbon oil based systems. In particular, the invention provides a process of preparing in situ dispersion of multi layered graphene in different hydrocarbon oils and their inclusion in grease, gear oil, differential oil, forging oil system for an improved thermal diffusivity. The graphene system consists of a mixture of MLG and SLG which is a result of delamination/exfoliation of natural graphite.

Accordingly, the present invention provides a lubricant composition comprising:
a hydrocarbon base oil;
exfoliated graphite; and
a dispersant;
characterized in that the exfoliated graphite has an extent of exfoliation of a minimum of 5 in the merit scale of 10; and the exfoliated graphite exhibits significant peaks at 1351, 1578, 2700, as observed by Raman Spectroscope.

In an embodiment, the present invention provides a process for preparing a dispersion comprising multi-layered graphene sheets and/or single-layered graphene sheets, said process comprising:
a) adding a dispersant to a hydrocarbon base oil and mixing the same in a batch or inline mixer to obtain a first mixture;
b) adding graphite to the first mixture under constant stirring and continuing the stirring for a predetermined period of time to obtain a second mixture;
c) subjecting the second mixture to grinding in a bead mill system to obtain a third mixture, wherein the grinding comprises subjecting the second mixture to an initial grinding by beads having average size of 1000µ and to a progressive grinding by beads having a size in the range of 200 to 900µ;
d) subjecting the third mixture to processing in a vertical mill comprising beads having a size in the range of 20 to 100 µ to obtain a forth mixture; and
e) subjecting the forth mixture to processing in a homogenizer under a pressure in the range of 500 – 1800 bar to obtain a dispersion comprising MLG and / or SLG sheets having a size in the range of 297 to 671; with extent of exfoliation in the range of 4 to 9 to obtain the said dispersion.

In another embodiment, the amount of dispersant added to the hydrocarbon base oil is in the range of 2 to 5 weight percent, the percentage being expressed in terms of the total quantity of the dispersion.

In still another embodiment, the amount of dispersant added to the hydrocarbon base oil is 3-4 weight percent, the percentage being expressed in terms of the total quantity of the dispersion.

In yet another embodiment, the dispersant is soluble in hydrocarbon medium.

In further another embodiment, the dispersant is selected from the group comprising of amine, amide, imide, succinimide, hydroxyl succinimide.

In even another embodiment, the dispersion is poly isobutylene succinimide.

In yet another embodiment, the hydrocarbon base oil is selected from the group of lubricating oils, and solvent-treated or acid-treated mineral lubricating oils of the paraffinic, naphthenic or mixed paraffinic-naphthenic types and polyalphaolefins (PAOs) derived from monomers having from about 4 to about 30 carbon atoms having a viscosity in the range from about 1.5 to about 150 mm2/s (cSt) at 100ºC and esters of dicarboxylic acids (e.g., etc.) a mixture thereof.
In another embodiment, the ester is selected from adipic acid, phthalic acid and succinic acid.

In still another embodiment, the amount of graphite added to the first is in the range of 0 .1 to 20 weight percent, the percentage being expressed in terms of the total quantity of the dispersion.

BRIEF DESCRIPTION OF DRAWINGS:
Fig 1: Particle size analysis of samples collected after different intervals of energy consumed and bead size;
Fig 2: Dispersive Raman spectra of dispersion after grinding. High resolution peak for 2D (Inset);
Fig 3: TEM images of the sample (a) sheet on lacy carbon coated grid, (b) few sheets, (c) HRTEM of graphene, (d) SAED pattern of the sample;
Fig 4: Thermal conductivity of base oil as well as the graphene dispersion;
Fig 5: (a) Thermal conductivity of grease containing graphite and different conc. of MLG in 150 neutral oil, (b) Thermal conductivity of grease containing graphite and different conc. of MLG in bright stock;
Fig 6 (a) LFW 1 test for bright stock formulation top treated with 0.15 and 0.5 % MLG (b) and 6(c) LFW 1 test for gear oil as well as formulations top treated with 0.15 and 0.5 % MLG;
Fig 7: Viscosity at 40°C of the base oils and the 16 % MLG dispersions;

DESCRIPTION OF THE INVENTION
The present invention discloses the composition and the process of preparation of a dispersion of multilayered graphene sheets in low viscous hydrocarbon oils i.e LAB (linear alkyl benzene) using poly isobutylene succinimide (PIBSI) as a dispersant. The invention also discloses the preparation of a dispersion of multilayered graphene sheets in high viscous oil i.e, bright stock, and the dispersant used remaining the same.

The invention also provides a grease composition where graphite has been replaced by the prepared dispersions to improve the thermal conductivity and tribological aspects of the final compositions.

One aspect of the invention discloses the preparation of the dispersion of graphene in oil. The dispersion is prepared by adding the graphite slowly to a mixture of dispersant and oil with constant stirring.

The resulting mixture is further processed with agitated bead mill system. The vertical stirred media mill generally can be filled with 70-90% of the chamber volume with the stirring media which is generally beads. These beads in combination with inbuilt fins, enhances the interaction with material and provide shear. More is the shear component applied to the material, better is exfoliation of layered materials.

The bead size in vertical stirred media mill can vary from 1mm to 0.05 mm. Smaller the size of the beads, more close packed is the beads which help in exfoliation. The size of the beads can also be adjusted depending upon the required extent of particle size reduction as well as the exfoliation for layered materials.

Vertical stirred media mill induces the defects in graphite which includes breaking of its particles and its special internal design. This results in exfoliation of graphite and generation of multi layered or single layered Graphene. The shear force is promoted more through vertical stirred media mill as compared to planetary ball mill or horizontal media mill. Suitable dispersant used wraps the sheets in stirred media bead mill and prevent them from re-agglomeration.

Operating RPM of the mill also plays a vital role. A high RPM can lead to more breaking of particles instead of exfoliation. In a nutshell, maximum exfoliation for a layered material can be obtained by optimizing the size of beads, rotational speed and concentration of material to be ground.

Another aspect of the invention discloses the preparation of improved grease or lubricant composition from hydrocarbon oil based systems using graphene system which further comprises of a mixture of MLG and SLG. Apart from greases and lubricants, the graphene dispersion can also be used in the formulation of gear oil, differential oil, forging oil system. The resulting compositions demonstrate improved thermal diffusivity.

Another aspect of the invention studies the thermal conductivity of MLG dispersion w.r.t the base oil using laser flash method. The process requires substantial temperature drop across the plane. The thermal conductivity of base oil LAB, 8.5 % dispersion as such and the 0.025 % graphene dispersion was measured in the temperature range 30-100 °C. It can be observed from the obtained graphs as shown in Fig 4 that the thermal conductivity of base oil LAB and 0.025 % graphene dispersion is more or less same. However, with 8.5 % graphene load, the thermal conductivity increased nearly three times which further supports the use of graphene dispersion for thermal management in industries.

The hydrocarbon base oil is selected from the group comprising base oil which may include oils derived from animal sources and vegetable sources (e.g., rapeseed oil, fish oil, coconut oil, castor oil, lard oil), as well as mineral lubricating oils (Group I, Group II, Group III), and solvent-treated or acid-treated mineral lubricating oils of the paraffinic, naphthenic or mixed paraffinic-naphthenic types. The oils may be biodegradable in nature. The polyalphaolefins (PAOs) derived from monomers having from about 4 to about 30 carbon atoms having a viscosity in the range from about 1.5 to about 150 mm2/s (cSt) at 100ºC can also be used in this invention. The esters of dicarboxylic acids (e.g., adipic acid, phthalic acid, succinic acid etc.) with a variety of alcohols are also useful. The base oil can be an individual oil or may be a mixture of different proportion of these mentioned oils. In specific embodiment, the organic media is low viscosity mineral oil, or low viscosity di-iso decyl di-adipate (DIDA), or a mixture thereof.

In another embodiment of the invention thermal conductivity of grease containing various concentrations of MLG were compared with that of graphite. Two systems have been selected, one grease composition in low viscous oil (150 neutral oil) and another in high viscous oil i.e bright stock. Grease composition containing 7 % graphite content was taken as a base value since this is the composite on being generally used.

It can be noted that in case of 150 neutral oil, the thermal conductivity of the grease for 2 % of the MLG composition is same or little on lower side as shown in Fig 5a. However for the 5% MLG composition, the value obtained is 15-20 % higher than the composition 7% graphite.

The grease composition containing MLG in high viscous oil behaved little differently. Both 2 and 5 % MLG dispersion containing greases showed good improvement over the grease containing 7 % graphite as depicted in Fig 5 b. The improvement for 5 % MLG grease was varied in the range 12-25 % for different temperature.

The tribological properties of grease composition containing graphite were also compared with that containing MLG and excellent tribological quality was found in case of MLG, where the weld-load maintained though the MLG quantity was significantly lowered. In addition, the anti-wear properties of the compositions were also found to be excellent and wear scar dia found lowered for a lower dose of MLG.

Additionally, LFW 1 test of the bright stock oil and formulated gear oil top treated (doped) with different percentage of MLG were done on a block on ring system. In both the cases maximum temperature reached during the process was significantly lowered compared to the neat oils as shown in Fig 6a and 6b. This could be due to efficient heat dissipation of the MLG containing formulations. As it is shown in Fig 6c, further the temperature for the top treated samples of gear oil lowered and saturated at much lower values than the neat oil. As also can be seen in Fig 6c, there is a dramatic drop in coefficient of friction for the formulations containing MLG.

In another embodiment of the invention the variation of viscosity with shear rate was also studied. Bead milling changes the shape and size of the graphitic particles and has found to significantly control the viscosity of the final dispersion. The viscosity of the oils and respective dispersions were measured. The two mineral oils chosen are, 150 neutral oil with lower viscosity and bright stock with higher viscosity. Both the oils showed the Newtonian behavior, which changed to non-Newtonian for the MLG dispersions. However, it can be noted that viscosity of 16 % dispersion in both increased significantly, though more significant for the oil of lower viscosity (150 neutral oil) as shown in Fig 7.

On evaluation of the properties like tribological properties, heat dissipation properties and reduction of coefficient of friction etc. in oil composition formulated with MLG have provided promising results and encourages its application in gear oils, automotive oils, metal working oils, and other industrial lubricants.

Hence, on a positive note it is good to simply top treating MLG into existing product line and giving a value addition. A method for producing MLG or SLG dispersion at industrial scale in hydrocarbon oil medium would be a breakthrough for use in grease, lubricant and related applications.

Following examples further illustrate the present invention without limiting the scope of the invention:

Example 1
In an example of the present invention, the dispersant PIBSI (3% w/w) was mixed well with the oil (LAB/150 neutral oil/bright stock) with an over head stirrer mixture. After that, graphite (16% w/w) was added to the system slowly with constant stirring and the stirring was continued for 2 hours. The resulting dispersion was further processed with bead mill system having beads of different sizes i.e. 1000 µ, 300 µ and 100 µ gradually starting with the larger one. The first step starting with the addition of biggest size of the beads (1000 µ) was primarily to de-agglomerate the graphite and to reduce the size due to impact force with bombardment of beads.

Further the products have been processed in a vertical mill of special design, where the applied shear force is dominant over the impact force. This step helped in both size reduction as well as de-stacking of the graphite layers. The final step with smaller beads (100 µ) applies maximum shear force that helps major destacking of the graphene layers. The dispersant present in the oil helps keeping the delaminated layers apart and making the dispersion stable. Size analysis of the dispersion taken at different consumed energy levels and bead sizes has been provided in Fig 1. The consistent peak at around 4.5 µ in all the graphs is due to the presence of the dispersant that doesn’t undergo any change during the grinding process. However, it can be seen that at end of processing with beads of smaller sizes the peak gradually shifts to left confirming the effective grinding of the material. The particle sizes for different energy levels during a step of grinding were measured, though only the final size at end of each step has been documented here. Initial size of the agglomerated graphite taken was around 4 µ, and with the initial processing with beads of 1 mm size it get de- agglomerated and further reduced in size to give a single peak at 840 nm and average particle size of 560 nm. The peak is quite broad and the tail of peak stretched beyond one micron size.

On subsequent processing with bead size of 300 µ the peak shifts towards left and two peaks becomes evident as seen in Fig 1, the major being around 480 nm and minor centered at 120 nm. However, the tail region is quite at higher side that accounts for higher average size. It can be noticed that at the end of processing with 100 µ beads, a single peak centered on 310 nm exists. The peak is quite narrow and the tail shifted remarkably to left the average size being around 610 nm. It is worth mentioning that due to presence of polymeric dispersant during each measurement, the average size calculated by the software is at higher end and the true particle size can be considered around the major peak for the material. Further, on subjecting the dispersion to high pressure homogenizer, working at 1500 bar the particle size reduced to 297 nm with the dispersion being thoroughly delaminated and homogenized.

Raman spectroscopy can been used to determine the exact number of graphene layers. It has also been evident from studies that the dispersion contains mixture of multilayer (graphite) layers as well as single layered graphene sheets. Graphitic materials can be characterized by D (defect)-band (~1350 cm-1), G (Graphitic)-band (~1580 cm-1) and a 2D-band (~2700 cm-1). Raman spectra of the mechanically exfoliated graphite were taken from dried film of diluted sample on a glass slide using a laser line of 514.5 nm on SEKI 750 Raman analyzer. The spectrum shows the G band of at 1582 and 1574 cm-1, and D band at 1357 and 1351 cm-1 for graphite and exfoliated graphite (MLG) respectively suggesting the nearly graphitic nature of the samples. However, for the exfoliated sample the peaks are shifted to left indicating more ordering and increase in sp2 carbon density. As documented widely, the symmetry as well as the full width at half maximum (FWHM) of the 2D peak can be used to distinguish between graphene (monolayer, bilayer or multilayer) and bulk graphite. The 2D peak for graphite appears at 2720 cm-1 and same for exfoliated graphite is at 2700 cm-1 indicating delamination and formation of few layers or monolayered graphene. Moreover, as it can be seen in the high resolution peak for 2D (inset), the peak appears to be single in nature confirming the sample to be monolayered graphene. Moreover, presence of D band and D* indicates increase in defect in the sample. Various part of the diluted film deposited has been examined, where the single 2D peak has been found in many places. In some places the graphitic nature of the peak has also been found.

The shape and size of particle as well as the number of layers can be studied by TEM. The TEM studies were carried out on the sample of a lacy as well as normal carbon coated copper grid. In most of places thin flat structures of lateral dimension few to several hundred nanometers size could be found with different thickness (based on transparency) as shown in Fig 3a. In Fig 3b a few sheets of nearly 100 nm width and few hundred nanometer length with a well demarcated edge can be seen. However, at some points the sheets are curled owing to high surface energy at the edges. They also tend to stack on each other due to high surface energy after being exfoliated. The HRTEM gives the lattice fringes having planar distance 0.35 nm suggesting graphitic nature of the sample.

The number of sheets present in graphene can be estimated through SAED pattern, through analyzing the intensity ratio of brag reflection. Studies have shown that for a monolayer graphene, I[1100]/ I[2100] > 1. In the present SAED pattern collected from the sample, the planes marked in the image have intensity ratio ~1.3, which is unique feature for monolayer graphene, confirming presence of monolayer graphene. In addition, the well demarcated hexagonal pattern maintaining the six fold symmetry indicates good crystallinity of the graphene sheets. In a nut shell the TEM study suggests a mixture of well crystalline few layered as well as single layered graphene sheets of few hundred nanometers cross sectional dimension.

Example 2:
The test was done on a Falex instrument with 0.15 %, 0.30% and 0.5 % MLG dispersion top treated to the existing brightstock oil containing 0.6% antioxidant. The test design was a block-on-ring system done for one hour duration at 200 lb load with a RPM of 1125. Initially the top-treatment was done by adding MLG in different concentration (w/w) followed by sonication with a sonic horn. The test results are shown in FigFig 6a and 6b. It can be noted that temperature rise for the top treated formulations are dramatically reduced. The maximum temperature reached for 0.15% MLG treated oil was 118 °C and it was reduced to 94 °C at the end of test. Similarly for 0.3 & 0.5% MLG top treated oil, temperature changes from maximum 113 to 73 °C & 111 to 70°C respectively. This is also well supported by the friction of coefficient measured where the same has been reduced to 0.01 µ from 0.08 µ. The results are encouraging for application in gear oils and other industrial lubricants.

Example 3:
The same test was done with 0.15 % and 0.5 % MLG dispersion top treated to the existing gear oil formulation with a change in duration to 3 hrs. The results are shown in FigFig 6c. It can be noted that temperature rise for the top treated formulations are dramatically reduced. The maximum temperature reached for neat gear oil was 114 °C and it was reduced to 102 and 92 respectively for 0.15 and 0.5 % top treated formulations. This also well supported by the friction of coefficient measured where the same has been reduced to 0.02 µ from 0.07 µ. The test was done on a formulated oil composition with MLG top treated to it and the results are promising in terms of heat dissipation and reduction of coefficient of friction. Hence, on a positive note it is good to simply top treating MLG into existing product line and giving a value addition.

Example 4:
The prepared graphene dispersions are then evaluated for stability using centrifuge at 600C for 1 hour at a RPM of 4000. The vials are removed and visually evaluated for sedimentation and graded on a merit scale of 1 to 10 for the extent of exfoliation. The average merit Rating is based on average rating to one significant number based on Examples tested. Typically, better results (merit rating) are obtained for samples that have a higher degree of exfoliation having better stability & lower merit rating are for samples with less exfoliation and more prone to unstability. The results obtained are:

Type of Processing Time of Processing Particle Size (nm) Predominant Peaks Extent of Exfoliation on merit of 10
Ball Mill 2 hour 671 766 4
Horizontal Media Mill 2 hour 474 570 4
Homogenizer 2 hour Not stable for PS determination
1
Ultra Sonication 2 hour Not stable for PS determination
3
High Pressure Homogenizer 2 hour Not stable for PS determination
2
Horizontal Media Mill + Vertical Media Mill 1 Hour + 1 Hour (Not exceeding 2 hours) 310 422 7
Horizontal Media Mill + Homogenizer 1 Hour + 1 Hour (Not exceeding 2 hours) 470 562 4
Horizontal Media Mill + Ultra Sonication 1 Hour + 1 Hour (Not exceeding 2 hours) 452 538 5
Horizontal Media Mill + High Pressure Homogenizer 1 Hour + 1 Hour (Not exceeding 2 hours) 447 531 5
Horizontal Media Mill + Vertical Media Mill + Homogenizer (Not exceeding 2 hours) 328 408 7
Horizontal Media Mill + Vertical Media Mill + High Pressure Homogenizer (Not exceeding 2 hours) 297 346 9

Advantages:
• The present invention provides a stable graphene dispersion.
• The present invention provides a graphene dispersion with improved thermal conductivity.
,CLAIMS:WE CLAIM:

1. A lubricant composition comprising:
a hydrocarbon base oil;
exfoliated graphite; and
a dispersant;
characterized in that:
the exfoliated graphite has an extent of exfoliation of a minimum of 5 in the merit scale; and the exfoliated graphite exhibits significant peaks at 1351, 1578, 2700, as observed by Raman Spectroscope.

2. A process for preparing a dispersion comprising multi-layered graphene sheets and/or single-layered graphene sheets, said process comprising:
a. adding a dispersant to a hydrocarbon base oil and mixing the same in a batch or inline mixer to obtain a first mixture;
b. adding graphite to the first mixture under constant stirring and continuing the stirring for a predetermined period of time to obtain a second mixture;
c. subjecting the second mixture to grinding in a bead mill system to obtain a third mixture, wherein the grinding comprises subjecting the second mixture to an initial grinding by beads having average size of 1000µ and to a progressive grinding by beads having a size in the range of 200 to 900µ;
d. subjecting the third mixture to processing in a vertical mill comprising beads having a size in the range of 20 to 100 µ to obtain a forth mixture; and
e. subjecting the forth mixture to processing in a homogenizer under a pressure in the range of 500 – 1800 bar to obtain a dispersion comprising MLG and / or SLG sheets having a size in the range of 297 to 671; with extent of exfoliation in the range of 4 to 9 to obtain the said dispersion.
3. The process as claimed in claim 1, wherein an amount of dispersant added to the hydrocarbon base oil is in the range of 2 to 5 weight percent, the percentage being expressed in terms of the total quantity of the dispersion.
a. The process as claimed in claim 3, wherein the amount of dispersant added to the hydrocarbon base oil is 3-4 weight percent, the percentage being expressed in terms of the total quantity of the dispersion.
4. The process as claimed in claim 1, wherein the dispersant is soluble in hydrocarbon medium.
5. The process as claimed in claim 5, wherein the dispersant is selected from the group comprising of amine, amide, imide, succinimide and hydroxyl succinimide.
6. The process as claimed in claim 6, wherein the dispersion is poly isobutylene succinimide.
7. The process as claimed in claim 1, wherein the hydrocarbon base oil is selected from the group of lubricating oils, and solvent-treated or acid-treated mineral lubricating oils of the paraffinic, naphthenic or mixed paraffinic-naphthenic types and polyalphaolefins (PAOs) derived from monomers having from about 4 to about 30 carbon atoms having a viscosity in the range from about 1.5 to about 150 mm2/s (cSt) at 100ºC and esters of dicarboxylic acids (e.g., etc.) a mixture thereof.

8. The process as claimed in claim 7, wherein the ester is selected from adipic acid, phthalic acid and succinic acid.

9. The process as claimed in claim 1, wherein an amount of graphite added to the first is in the range of 0 .1 to 20 weight percent, the percentage being expressed in terms of the total quantity of the dispersion.