Field of the Invention

The present invention relates to a grease

composition containing a specified clay mineral, and it concerns a grease composition which has high

environmental compatibility and is provided with the main properties of a grease. The present invention also relates to a method for the production of the grease composition, together with an agent added to enhance the properties of said grease composition.

Background of the Invention

Along with the advances in machine technology, the environments under which greases are used are changing dramatically year by year. For example, in the case of motor vehicles and electrical machinery, etc., as the various components have become more compact and

lightweight, and their output raised, their operating conditions have shifted to higher temperatures and the lubricating conditions employed have become more severe. In the case of the rolling mills, etc., for iron/steel continuous casting and hot rolling facilities, use thereof in environments of over 150°C are not rare, and the grease used therein needs to be a grease which is outstanding in its heat resistance and oxidation

stability. Furthermore, there have been increasing demands recently for materials which are not only satisfactory in terms of enhanced grease performance at high temperature but which are also environmentally compatible (i.e. highly safe during use, and of little environmental impact in production, etc.) . Currently, over 50% of this grease market is occupied by lithium grease. Lithium grease can be used at high temperatures of up to a maximum of around 120°C, and it is comparatively good in terms of shear stability and water resistance, in addition to which its oil/fat or fatty acid starting materials are readily available and easy to handle and production costs are comparatively low, so it is regarded as a grease of general

applicability. Furthermore, lithium complex greases, etc., have also been proposed as greases which can be employed over a temperature range still broader than that of ordinary lithium grease (as described in JP-A-1- 170691) . However, with regard to the lithium hydroxide used for the purposes of bringing about the

saponification reaction with the oil/fat or fatty acid starting material for the lithium grease, the demand for lithium is steadily rising as it becomes more widespread, and there are concerns that in the future changes will arise in the position of lithium grease as a general- purpose product.

As examples of greases other than lithium grease, there are sodium and aluminium greases, etc., but with sodium grease there is the problem that when mixed with water the grease changes to a fluid form and flows out from, for example, the bearings, and so it has gradually been pushed out of the grease market. On the other hand, the temperature range of use of aluminium grease is the same or less than that of calcium grease, so it is restricted to special applications.

Urea grease may be given as an example of a grease which can be employed as a heat-resistant grease. Urea grease can be used at temperatures even higher than a lithium complex grease, so it is a high-performance grease composition frequently employed in heat-resistant applications. For example, in JP-A-2008-94991there is disclosed a grease composition where calcium carbonate, which is an inorganic compound, and a solid lubricant like polytetrafluoroethylene or graphite, are mixed along with the polyurea compound. This grease composition is outstanding in its heat resistance and extreme-pressure properties when used under high temperature and high load environments, and hardening can be suppressed even where the grease is exposed to high local temperatures.

However, with this kind of urea grease there are problems in terms of the safety and handling of the isocyanate and amines which constitute the starting materials, so low environmental compatibility has been an issue. Furthermore, since sophisticated production techniques and equipment are required, costs are raised and the applications thereof are limited.

As a means for giving a grease high environmental compatibility, consideration can be given to employing a naturally derived mineral, or a synthetic inorganic analogue thereof, as a grease starting material. Thus, in JP-A-2012-224834 , there is disclosed a grease composition which for example employs, as a grease thickener, calcium carbonate, which is a material used as a feed for domestic animals or as a cosmetic material, etc., and so said grease can meet the demand for a lowering of environmental impact .

However, with the naturally derived minerals which have been used in greases hitherto, and the synthetic inorganic analogues thereof, even where they show a thickening effect (a viscosity raising effect) as in the case of the inorganic compounds described in JP-A-2012- 224834, etc., when a high viscosity silicone oil or a synthetic oil is used as the base oil grease thixotropy is only secured with the inclusion of a wax or other such thickener, and it has been difficult to achieve a high thickening effect substantially with the inorganic compound on its own. Even in the case where a certain degree of thickening effect is shown, the properties demanded of a grease (such as heat resistance and oxidation stability) have not necessarily been excellent.

The present invention has been made against this background, and its objective lies in providing a grease composition which is outstanding both in terms of its main properties as a grease (worked penetration, heat resistance, oxidation stability, etc.) and also in its environmental compatibility.

As a result of painstaking research to attain the aforesaid objective, it has been discovered that

specified naturally produced inorganic compounds both possess environmental compatibility and also enhance the main properties of a grease composition such as the worked penetration, and the present invention has been perfected based on this discovery.

More specifically, the present invention provides [1] to [12] below.

[1] A grease composition, characterized in that it contains a particulate hydrotalcite type compound represented by formula (I): [M ² V _X M ³⁺ _X (OH) ₂ ] [ (A ^n- ) _{x n} -mH ₂ 0] (where M ²⁺ is a divalent metal ion, M ³⁺ is a trivalent metal ion, A ^n- is an anion of valency n, x is a number which satisfies 0 < x < 1, and m is a number equal to 0 or above) and/or a derivative thereof.

[2] A grease composition as described in [1] where the mixed amount of the aforesaid hydrotalcite type compound and/or derivative thereof is between 0.5 and 50 parts by mass, taking the grease composition as a whole as 100 parts by mass.

[3] A grease composition as described in [1] or [2] where the aforesaid hydrotalcite type compound and/or derivative thereof is included as a thickener.

[4] A grease composition as described in any of [1] to [3] where the aforesaid hydrotalcite type compound is hydrotalcite, which is represented by formula (II) :

[Mg ₆ Al ₂ (OH) ₁₆ ] [ (C0 ₃ )■ 4 (H ₂ 0) ] .

[5] An agent for adding to a grease composition, characterized in that

it includes a particulate hydrotalcite type compound represented by formula (I): [M ² V _X M ³⁺ _X (OH) ₂ ] [ (A ^n- ) _{x n} ■mH ₂ 0] (where M ²⁺ is a divalent metal ion, M ³⁺ is a trivalent metal ion, A ^n- is an anion of valency n, x is a number which satisfies 0 < x < 1, and m is a number equal to 0 or above) and/or a derivative thereof, and

when said agent is added to a base oil belonging to Groups 1 to 5 of the base oil categories stipulated by the American Petroleum Institute (API), or to a mixture thereof, the grease composition satisfies at least one condition selected from the following group (A) to (C) .

(A) The worked penetration in the consistency test specified in JIS K 2220 7 is between 175 and 430.

(B) The evaporation loss following the heating for

24 hours at 150°C of an SPCC steel plate, as specified in the wet test method of JIS K 2246, is less than 10%.

(C) In the oxidation stability test (99°C, 100 hours) specified in JIS K 2220 12, the reduction in pressure of oxygen consumed by the oxidation reaction is no more than 35 kPa.

[6] An agent as described in [5] which, in terms of a base oil belonging to Groups 1 to 5 of the base oil categories stipulated by the American Petroleum Institute (API), or a mixture thereof, satisfies condition (A) above .

[7] An agent as described in [6] which, in terms of a base oil belonging to Groups 1 to 5 of the base oil categories stipulated by the American Petroleum Institute (API), or a mixture thereof, also satisfies conditions (B) and/or (C) above.

[8] An agent as described in any of [5] to [7] above where the aforesaid hydrotalcite type compound is hydrotalcite, which is represented by formula (II) :

[Mg ₆ Al ₂ (OH) ₁₆ ] [ (C0 ₃ )■ 4 (H ₂ 0) ] .

[9] A method for the production of a grease

composition which is characterized in that it includes a process wherein a grease composition which satisfies at least one of the conditions selected from the following group (A) to (C) is obtained by adding a particulate hydrotalcite represented by formula (I) : [M ²⁺ i_

xM ³⁺ _x (OH) ₂ ] [ (A ^n- ) _x/n -mH ₂ 0] (where M ²⁺ is a divalent metal ion, M ³⁺ is a trivalent metal ion, A ^n- is an anion of valency n, x is a number which satisfies 0 < x < 1, and m is a number equal to 0 or above), and/or a derivative thereof, to a base oil belonging to Groups 1 to 5 of the base oil categories stipulated by the American Petroleum Institute (API), or to a mixture of these.

(A) The worked penetration in the consistency test specified in JIS K 2220 7 lies between 175 and 430.

(B) The evaporation loss following the heating for 24 hours at 150°C of an SPCC steel plate, as specified in the wet test method of JIS K 2246, is less than 10%.

(C) In the oxidation stability test (99°C, 100 hours) specified in JIS K 2220 12, the reduction in pressure of oxygen consumed by the oxidation reaction is no more than 35 kPa.

[10] A method as described in [9] where the grease composition obtained in the aforesaid stage satisfies condition (A) above.

[11] A method as described in [10] where the grease composition obtained in the aforesaid stage also

satisfies conditions (B) and/or (C) above.

[12] A method as described in any of [9] to [11] above where the aforesaid hydrotalcite type compound is hydrotalcite, which is represented by formula (II) :

[Mg ₆ Al ₂ (OH) ₁₆ ] [ (C0 ₃ ) ■ 4 (H ₂ 0) ] .

In accordance with the present invention it is possible to provide a grease composition which is outstanding in its main properties as a grease (worked penetration, heat resistance, oxidation stability, etc.) and also in its environmental compatibility.

The grease composition of the invention is formed by the addition of a hydrotalcite type compound and/or a derivative thereof. Below, a detailed explanation is provided in relation to the specific components, amounts of said components, production method, properties and applications of the grease composition, but the invention is not to be restricted in any way thereto. For example, in the grease composition, the hydrotalcite type compound and/or a derivative thereof may be especially

incorporated as a thickener but, as long as the effects of the invention are manifested, even where the

hydrotalcite type compound and/or a derivative thereof is incorporated into the grease composition in applications other than as a thickener, it should be understood as belonging within the scope of the invention. There are no particular restrictions on the base oil employed in the grease composition of the invention. For example, it is possible to employ, as appropriate, the mineral oils, synthetic oils, animal/vegetable oils and mixtures thereof used in ordinary grease compositions .

Specific examples thereof are those belonging to Groups 1 to 5 of the base oil categories of the API (American Petroleum Institute) . The API base oil categories constitute a broad classification of base oil materials defined by the American Petroleum Institute to create a guide for lubricating oil grade base oils.

In the present invention, the types of synthetic oil are not particularly restricted but, as preferred examples, there are poly-α-olefins (PAOs) and

hydrocarbon-based synthetic oils (oligomers) . PAOs are the homopolymers or copolymers of α-olefins. For example, the OC-olefins comprise compounds where the C-C double bond is at the chain terminal, and butene, butadiene, hexene, cyclohexene, methylcyclohexene, octene, nonene, decene, dodecene, tetradecene, hexadecane, octadecene, eicosene and the like may be given as examples thereof. As examples of the hydrocarbon-based synthetic oils (oligomers) there are the homopolymers or copolymers of ethylene, propylene or isobutene. These various compounds can be used on their own, or they can be employed in the form of mixtures of two or more types thereof. Moreover, providing the C-C double bond lies at the terminal, they may have any of the structures that isomeric structures can adopt, and they may be of branched or linear

structure. It is also possible to jointly employ two or more such structural isomers or double bond positional isomers. Amongst these olefins, those with 5 or fewer carbons have a low ignition point, whereas those with 31 or more carbons have high viscosity and low practicality, so the use of linear olefins with from 6 to 30 carbons is more preferred.

In the present invention, there are no particular restrictions on the type of mineral oil, but preferred examples are the paraffinic or naphthenic mineral oils obtained by subjecting the lubricating oil fraction obtained by the atmospheric distillation or reduced pressure distillation of crude oil to a refining means, or suitable combination of two or more types of refining means, from amongst solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulphuric acid treatment, clay treatment and the like.

Moreover, in the present invention there may also be used, as the base oil, GTL (gas-to-liquid) which is synthesized from natural gas by liquid fuel conversion technology utilizing the Fischer-Tropsch method. When compared to a mineral oil base oil obtained by refining crude oil, GTL has extremely low sulphur and aromatic contents, and its paraffin structural content is

extremely high, so it has outstanding oxidation stability and very low evaporation loss, and therefore can be favourably employed as the base oil in the present invention.

The thickener employed in the invention is a hydrotalcite type compound (also known as a layered double hydroxide) . Hydrotalcite type compounds constitute a type of clay mineral having a structure represented by [M ²⁺ _1-x M ³⁺ _x (OH) ₂ ] [ (A ^n- ) _x/n ■mH ₂ 0] , and they are layered inorganic clay compounds comprising positively charged main layers [M ²⁺ ₁ _ _x M ³⁺ _x (OH) ₂ ] and negatively charged intermediate layers [ (A ^n- ) _x/n -mH ₂ 0] . Here, M ²⁺ is a divalent metal ion, M ³⁺ is a trivalent metal ion, A ^n- is an interlayer anion of valency n, x is a number which satisfies 0 < x < 1, and m is a number equal to 0 or above. Examples of M ²⁺ include Mg ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ , Ca ²⁺ and the like. Examples of M ³⁺ include Al ³⁺ ,

Cr ³⁺ , Mn ³⁺ , Fe ³⁺ , Co ³⁺ , Ni ³⁺ , La ³⁺ , V ³⁺ , Ga ³⁺ and the like. Examples of A ^n- include inorganic anions (ΟΕΓ, N0 ₃ -, C0 ₃ ^2- , SO ₄ ^2- , PO ₄ ^3- , F-, Cl-, Br-, I-, At-) , and organic anions {carboxylate anions (such as formate, salicylate, oxalate or citrate ions)}, and the like.

The greases most widely used in general terms are soap-based greases. Of these, lithium soap grease is outstanding in its consistency yield (the extent to which the grease is hardened) and in its shear stability, and it is the grease most frequently employed as a general- purpose grease. The main reason for this lies in the features mentioned. With regard to the structure of this lithium soap grease, the lithium stearate which functions as the thickener is dispersed in the base oil in a string-shaped fashion, and adopts an intertwined three- dimensional fibre structure. In terms of the basic form of the grease, the semi-solid grease properties are secured by the holding of the base oil within this fibre structure. Thus, the composition of such a soap-based grease is largely made up of comparatively long chain- length stearate. This derives from the fact that the balance between the base oil holding power possessed by the hydrocarbon and the intermolecular forces of the micelles which constitute the fibres lies in an optimum relation. For this reason, the consistency yield is good, and an effective action operates too in terms of shear stability. On the other hand, an inorganic material does not adopt a three-dimensional fibre structure, and usually the grease structure is secured by the gelling brought about by intermolecular forces and other

interactions between the particles dispersed in the base oil. For example, in the case of bentonite, in a solvent (of an aqueous kind) electrostatic coupling occurs and swelling of the crystals takes place forming a "card- house" structure, as a result of which gelling takes place and there is a change to a semi-solid state.

However, such electrostatic coupling does not readily come about in the base oil from which the grease is composed and so swelling/gelling and the formation of a firm grease structure is not possible. Consequently, often a water-soluble polar solvent is added as a binder in order to promote the swelling. However, most inorganic materials show little action of value in terms of swelling in a base oil and thereby manifesting the main properties of a grease (forming a semi-solid of

measurable consistency) so that, except for the bentonite case above, the only grease currently commercially available which employs an inorganic thickener is the silica grease in circulation. Against this background, the hydrotalcite type compound used as the thickener of the present invention has a considerable effect as a grease thickener. Taking this effect into consideration, it is thought that since this compound is a layered compound with a surface charge it forms a card house structure similar to that of bentonite, and the primary particles form firmly aggregated secondary particles and adopt a structure whereby the gaps produced between these secondary particles readily draw-in the lubricating oil.

Additionally, since the bulk specific gravity is high (and surface area large), the dispersibility in the lubricating oil is high. Consequently, it is thought that the outstanding thickening action is manifested without the need for the addition of a polar solvent, or the like, due to these combined effects.

Furthermore, hydrotalcite type compounds possess the property of enhancing the heat resistance and the oxidation stability of the grease composition. A possible reason for the enhancement in the oxidation stability of the grease composition is thought to be because of the high inherent heat stability possessed by the

hydrotalcite type compound employed as the thickener (as a result of this high heat stability, a stable thickening effect is manifested without structural breakdown even in a hot environment) . Again, another possible reason for the enhancement in the oxidation stability of the grease composition is thought to be the acid-receptive action of the hydrotalcite type compound (that is to say, the action of neutralizing acidic materials in contact with the grease composition by means of an ion-exchange reaction with the hydrotalcite) .

The hydrotalcite type compound is not restricted in any way providing it is represented by aforesaid formula (I), and suitable selection can be made from known materials. Examples thereof include hydrotalcite which is represented by Mg ₆ Al ₂ (OH) ₁₆ (CO ₃ )■ 4 (H ₂ O) , stichtite

{Mg ₆ Cr ₂ (OH) is (CO ₃ ) -4(H ₂ O)}, pyroaurite

{Mg ₆ Fe ₂ (OH) ₁₆ (C0 ₃ ) ^■ 4 (H ₂ O) } , iowaite

{Mg ₄ Fe (OH) a (OC1) · 4 (H ₂ O) } , desautelsite {Mg ₆ Mn ₂ (OH) ₁₆ (CO ₃ ) •4(H ₂ O)}, manasseite {Mg ₆ Al ₂ (OH) ₁₆ (CO ₃ ) · 4 (H ₂ O) } ,

barbertonite {Mg ₆ Cr ₂ (OH) _i6 (C0 ₃ ) · 4 (H ₂ 0) } , reevesite

{Ni ₆ Fe ₂ (OH) is (C0 ₃ ) · 4 (H ₂ O) } , honessite {Ni ₆ Fe ₂ (OH) ₁₆ (SO ₄ )

•4(H ₂ 0)}, takovite {Ni ₆ Al ₂ (OH) ₁₆ (CO ₃ ) · 4 (H ₂ O) } , comblainite

{Ni ₆ Co ₂ (OH) is (CO ₃ ) · 4 (H ₂ 0) } , green rust

{Fe ^II ₄ Fe ^III ₂ (OH) 12 (CO ₃ ) · 3 (H ₂ O) } and the like. Of these hydrotalcite type compounds, those of formula [Mg ²⁺ ₁ _ _X A1 ³⁺ _X (OH) ₂ ] [ (A ^n- ) _{x n} -mH ₂ 0] are preferred, with

Mg ₆ Al ₂ (OH) 16 (C0 ₃ ) · 4 (H ₂ 0) being particularly preferred.

The hydrotalcite type compounds relating to the present invention may also be derivatives thereof.

Examples of these derivatives in the present invention include the oxides (e.g. fired products) or modified compounds (organo-modified or alkali metal-modified materials), etc. These derivatives are also layered compounds, with a surface charge and possessing ion- exchange properties, so the mechanism of their effect is identical to that for the case of the hydrotalcite type compounds above. Below, any reference just to

hydrotalcite type compounds is to be taken to include derivatives of these hydrotalcite type compounds.

The particulate hydrotalcite type compound may be material synthesized in the form of particles, material naturally occurring as particles, or material obtained by the pulverizing of bulk material (such as material in mineral form) or by some other usual method. The average particle diameter of this particulate hydrotalcite type compound is preferably from 0.1 to 50 um, with 0.1 to 30 um more preferred, and 0.1 to 5 um particularly preferred. If it exceeds 50 um, the surface area is lowered, so the amount of oil drawn into the gaps between the secondary particles is reduced and, furthermore, dispersibility in the lubricating oil is impaired. The average particle diameter here is that measured by means of a laser diffraction/scattering type particle size distribution measurement device (volume standard particle size distribution; measurement range 0.02 to 2000 μηι) .

In the grease composition there may also be used, along with the aforesaid thickener (i.e. the hydrotalcite type compound) , a thickener other than a hydrotalcite type compound (referred to as the other thickener) .

Examples of this other thickener include tribasic calcium phosphate, alkali metal soaps, alkali metal complex soaps, alkaline earth metal soaps, alkaline earth metal complex soaps, alkali metal sulphonates, alkaline earth metal sulphonates, and other metal soaps, terephthalamate metal salts, triureamonourethane, diurea, tetraurea, and polyureas other than these, or clay, silica aerogel and other types of silica (silicon oxide), polytetra- fluoroethylene and other types of fluoropolymer, etc., and one of these, or combinations of two or more may be jointly employed. Moreover, as well as these, it is also possible to use any other material having a thickening effect on liquid materials.

It is also possible to include optional additives in the grease composition, such as antioxidants, corrosion inhibitors, oiliness conferrers, extreme pressure agents, anti-wear agents, solid lubricants, metal deactivators, polymers, metal-based cleaning agents, metal-free cleaning agents, colouring agents, water-repellents and the like, where, taking the grease composition as a whole as 100 parts by mass, the total amount of said optional components lies in the range from about 0.1 to 20 parts by mass. As examples of the antioxidants there are 2,6- di-tert-butyl-4-methylphenol, 2, 6-di-tert-butyl-p-cresol, ρ,ρ' -dioctyldiphenylamine, N-phenyl- α-naphthylamine , phenothiazine and the like. Examples of the corrosion inhibitors include oxidized paraffin, metal carboxylates , metal sulphonates, carboxylate esters, sulphonate esters, salicylate esters, succinate esters, sorbitan esters, and various types of amine salts, etc. Examples of the oiliness conferrers, extreme pressure agents and anti- wear agents include sulphurized zinc dialkyl

dithiophosphates , sulphurized zinc diallyl dithio- phosphates, sulphurized zinc dialkyl dithiocarbamates, sulphurized zinc diallyl dithiocarbamates, sulphurized molybdenum dialkyl dithiophosphates, sulphurized

molybdenum diallyl dithiophosphates, sulphurized

molybdenum dialkyl dithiocarbamates, sulphurized

molybdenum diallyl dithiocarbamates, organo-molybdenum complexes, sulphurized olefin, triphenyl phosphate, triphenyl phosphorothionate, tricresyl phosphate, other phosphate esters, sulphurized oils and fats, and the like. Examples of solid lubricants include molybdenum disulphide, graphite, boron nitride, melamine-cyanurate, PTFE (polytetrafluoroethylene) , tungsten disulphide, graphite fluoride and the like. Examples of the metal deactivators include N, N' -disalicylidene-1, 2- diaminopropane, benzotriazole, benzimidazole,

benzothiazole , thiadiazole, and the like. Examples of the polymers include polybutene, polyisobutene,

polyisobutylene, polyisoprene, polymethacrylate and the like. Examples of the metal-based cleaning agents include metal sulphonates, metal salicylates, metal phenates and the like. Examples of the metal-free cleaning agents are succinimide and the like.

Next, explanation is provided of the mixed amounts of the base oil and thickener in the grease composition of the invention. With regard to the amounts of the optional components included, these may be suitably incorporated as required in the amounts discussed above.

Taking the total grease composition as 100 parts by mass, the mixed amount of the base oil is preferably from 50 to 95 parts by mass, more preferably from 60 to 90 parts by mass, and still more preferably from 70 to 85 parts by mass .

Taking the total grease composition as 100 parts by mass, the mixed amount of total thickener is preferably from 1 to 50 parts by mass, more preferably from 3 to 30 parts by mass, and still more preferably from 5 to 20 parts by mass .

As described above, the grease composition is formed by including at least a hydrotalcite type compound as a thickener, with other thickener being suitably used in combination therewith, but even in the case where the hydrotalcite type compound is the only thickener, that is to say even in the case where essentially no thickener other than a hydrotalcite type compound is included, it is possible to achieve a high thickening effect (that is to say, it is possible to produce a grease composition having a high thickening effect) . Furthermore, since the hydrotalcite type compound has the property of enhancing the heat resistance and the oxidation stability of the grease composition as discussed above, it is possible to achieve greater heat resistance and oxidation stability of the grease composition the greater the proportion of the hydrotalcite type compound and the smaller the proportion of the other thickener within the amount of thickener as a whole. Moreover, since the hydrotalcite type compound possesses high environmental compatibility, reducing the proportion of other thickener within the thickener as a whole is advantageous too in terms of environmental compatibility. Consequently, within the aforementioned amount of thickener as a whole

incorporated into the grease composition, and taking the total grease composition as 100 parts by mass, it is preferred that the hydrotalcite type compound be from 0.5 to 30 parts by mass, and more preferably from 1 to 20 parts by mass. In the same way, the thickener other than hydrotalcite type compound (i.e. the other thickener) is preferably no more than 20 parts by mass, and more preferably no more than 10 parts by mass.

Next is discussed a method of producing the grease composition where substantially no thickener other than the hydrotalcite type compound is included.

An existing means can be suitably employed as the method for producing the grease composition, so, for example, production may be conducted by the following process. The base oil and the thickener are blended together, and introduced into a dedicated grease- production device (a programmable grease-production prototype) . Next, stirring is carried out at room temperature (e.g. about 25°C) (examples of the stirring conditions being a stirring rotation rate of 20-100 rpm, for a mixing time of 10-15 minutes), after which further treatment is performed by means of homogenizing equipment (such as a triple roll mill, or the like), and then vacuum degassing conducted and a homogeneous grease composition obtained. In the case where other optional components (additives, etc.) are employed, the base oil and the additives may be mixed together beforehand at a suitable temperature (for example, a temperature of between 80 and 100°C), after which the temperature is returned to room temperature, and then the hydrotalcite type compound added (or the hydrotalcite type compound may be mixed with the base oil at room temperature, and thereafter the temperature raised and the mixing of the additives performed) .

In this case where the hydrotalcite type compound essentially does not include thickener other than the hydrotalcite type compound, there is no particular accompanying chemical reaction at the time of the thickener addition and mixing, so it is also possible to produce the grease without including a stage in which the temperature is raised (or, in the case where high temperature is required for the mixing of the additives, it is possible to shorten the process in which a high temperature is maintained) , so energy savings and a lowering of costs become possible. Treatment in the stage in which the hydrotalcite type compound and the base oil are mixed and stirred need not necessarily be conducted at room temperature, and a heat treatment may also be performed (for example, at less than about 140°C) .

Next is discussed a method of producing the grease composition where the hydrotalcite type compound is used in combination with other thickener.

The method for producing a grease in the case where the hydrotalcite and a thickener other than a

hydrotalcite (referred to as the other thickener) are jointly used as the grease composition thickener is explained, taking the example where a urea thickener is used as said other thickener. First of all, the starting materials for the urea thickener (the diisocyanate, primary monoamine, monoalcohol, etc.) are suitably blended and a synthesis reaction performed within the base oil, following which the temperature is raised to about 180°C and thereafter lowered, and the mixing of the additives, etc., performed at a temperature between 80 and 100°C, and, having carried out thorough mixing and stirring, the temperature is then reduced to room temperature. Subsequently, the hydrotalcite type compound is mixed therewith and stirring performed to obtain a dispersion, and by subjecting this to homogenization using a kneading machine (for example, a triple roll mill) the grease composition can be obtained.

When a conventional thickener and the hydrotalcite type compound thickener are jointly employed in this way, it is also possible, depending on the conventional thickener, to first form a grease composition by an ordinary grease production method, after which the consistency is then raised by the introduction of the hydrotalcite type compound, to complete the production of the grease composition. Moreover, the grease composition may also be produced in accordance with an ordinary grease production method with the hydrotalcite type compound and the other thickener incorporated in the same process (timing) . Furthermore, it is also possible to separately prepare a grease composition using the aforesaid hydrotalcite type compound as the thickener, and a conventional grease composition where a thickener other than a hydrotalcite type compound is used as the thickener, and then to mix these together.

The grease composition preferably has a dropping point of at least 180°C or beyond, with at least 200°C or beyond being further preferred, and at least 220°C or beyond being particularly preferred. When the dropping point of the grease composition is at least 180°C, it is thought that there is less possibility of lubrication problems arising, for example loss of viscosity at high temperature and accompanying leakage, or seizure, etc. The dropping point refers to the temperature at which the thickening agent structure is lost as the temperature of a viscous grease is raised. Measurement of the dropping point here can be carried out in accordance with JIS K In the test of consistency, the grease has a consistency of preferably between No 00 and No 4 (175- 430), and more preferably a consistency of No 2 or No 3 (220-295) . The consistency denotes the apparent firmness of the grease. As the consistency here, there is used the worked penetration value measured in accordance with JIS K 2220 7.

The grease composition preferably has an evaporation loss of less than 10% in a thin film heating test (150°C, 24 hours) . The thin film heating test method is as follows. 3.0 g ± 0.1 g of the sample is applied to the central area region (50 mm X 70 mm) of one face of an SPCC steel plate test piece of dimensions 1.0 mm

thickness X 60 mm length X 80 mm width, as specified in the wet test method of JIS K 2246, then heating is carried out at 150°C X 24 hours, with the weight of the SPCC steel plate being measured before and after said heating, and the amount of evaporated material,

determined by means of the following formula, taken as the evaporation loss.

Amount evaporated (%) = { (weight in g before heating - weight in g after heating) /weight in g before heating} X 100

In the case of the grease composition, in the oxidation stability test (at 99°C for 100 hours) the reduction in the pressure of oxygen by consumption in the oxidation reaction is preferably no more than 35 kPa, more preferably no more than 30 kPa, and still more preferably no more than 25 kPa. The oxidation stability of a grease refers to the resistance of the grease to oxidation by reaction with the oxygen in the air. While the degradation of a grease composition due to oxidation is also influenced by the base oil, oxidative decomposition of the thickener has a particularly great effect. The basic function of the thickener is to hold the base oil and maintain physical hardness as a grease, so that the grease is retained at the lubricating sites of the machine and performs the role of suitably

supplying the base oil component held by the thickener to the sliding surfaces. If the thickener is broken down by oxidation, the inherent hardness of the grease can no longer be maintained and the base oil holding function is lost, so it flows out from the lubricating sites and a suitable lubricating state can no longer be sustained. This behaviour is greatly influenced by the environment in which the grease is used and, in particular, as the temperature increases oxidative degradation occurs at an accelerating rate. When oxidation of the grease

progresses due to the heating, oxidation products are generated, and as a result of an increase in the

viscosity of the base oil component, the generation of a sludge, and/or the breakdown of the network structure, etc., hardening or softening of the grease takes place so that the lubricating life is reached. Ultimately, the service life or operational reliability of the machine employing the grease may be impaired. Thus, high

oxidation stability of the grease composition is

extremely important in terms of maintaining an

appropriate lubrication state at the lubricating sites and in enhancing the lubrication life. The measurement of the oxidation stability here can be carried out in accordance with JIS K 2220 12.

The grease composition can of course be employed for generally used machines, bearings, gears, and the like, but it can also exhibit outstanding properties under still more severe conditions, such as under high- temperature conditions. For example, in the case of motor vehicles, it can be employed for the lubrication of the starter, alternator and various actuator components of the engine periphery, the propeller shaft, constant velocity joints (CVJ) , wheel bearings, the clutch and other powertrain components, the electric power steering (EPS) , braking equipment, ball joints, door hinges, handle parts, cooling fan motors, brake expanders and various other parts/components. Furthermore, it can also be used at the various high temperature/high load sites of construction equipment such as excavators, bulldozers and cranes, etc., and in the iron/steel industry and papermaking industry, as well as for forestry machinery, agricultural machinery, chemical plants, power generation facilities, drying ovens, copiers, railway carriages, seamless pipe screw joints, and the like. Other

applications where it is suitably employed include its use for hard disk bearings, for plastic lubrication, and as a cartridge grease, etc.

Examples

Next, the present invention is explained in further detail by means of working examples and comparative examples, but the invention is not to be restricted in any way by these examples.

The starting materials employed in Working Examples

1 to 10, and in Comparative Examples 1 to 4, were as follows .

Thickeners

• Thickener A: Mg ₆ Al ₂ (OH) ₁₆ (CO ₃ ) -4 (H ₂ O) of average particle diameter 0.5 μm

• Thickener B: Mg ₆ Al ₂ (OH) ₁₆ (CO ₃ ) -4 (H ₂ O) of average particle diameter 30 μm • Magnesium silicate: first-class reagent grade, of average particle diameter 10 um

• Calcium carbonate: first-class reagent grade, of average particle diameter 10 um

• Aluminium oxide: special reagent grade, of average particle diameter 75 um

• Synthetic mica: commercial product for industrial use, of average particle diameter 10 um

Base Oil

· Base oil A: a paraffinic mineral oil obtained by dewaxing solvent refining, belonging to Group 1; of 100°C kinematic viscosity 11.25 mm ² /s, and viscosity index 97

• Base oil B: a poly- α-olefin belonging to Group 4; of 100°C kinematic viscosity 6.34 mm ² /s, and viscosity index 136

• Base oil C: GTL (gas to liquid) synthesized by the Fischer-Tropsch method, belonging to Group 3; of 100°C kinematic viscosity 7.77 mm ² /s, 40°C kinematic viscosity 43.88 mm ² /s, and viscosity index 148

· Base oil D: naphthenic mineral oil obtained by dewaxing solvent refining, belonging to Group 1; of 100°C kinematic viscosity 10.71 mm ² /s, and viscosity index 30.34

Working Example 1

Base oil A and thickener A were weighed out in the mixing proportions shown in the table such that their combined total was 500 g, and then these were introduced into a dedicated grease-production device of internal capacity 1.0 kg. The dispersion formed by mixing for 15 minutes at a mixing rotation rate of 200 rpm at room temperature was then treated using a triple roll mill, after which vacuum degassing was carried out and a homogeneous grease of No 2 consistency obtained.

Working Example 2

As starting materials, base oil B and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 2 consistency.

Working Example 3

As starting materials, base oil C and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 2 consistency.

Working Example 4

As starting materials, a base oil formed by mixing together base oil A : base oil C = 1 : 1, and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 2 consistency.

Working Example 5

As starting materials, a base oil formed by mixing together base oil A : base oil D = 11 : 5, and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 2 consistency. Working Example 6

As starting materials, a base oil formed by mixing together base oil A : base oil B : base oil C : base oil D = 1 : 1 : 1 : 1, and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 2 consistency.

Working Example 7

As starting materials, base oil A and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 4 consistency.

Working Example 8

As starting materials, base oil A and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 3.5 consistency.

Working Example 9

As starting materials, base oil A and thickener A were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 00 consistency.

Working Example 10

As starting materials, base oil A and thickener B were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1 to obtain a homogeneous grease of No 1.5 consistency.

Comparative Example 1

As starting materials, base oil A and magnesium silicate were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1, but a fluid state (non-grease state) material was formed.

Comparative Example 2

As starting materials, base oil A and calcium carbonate were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1, but a fluid state material was formed.

Comparative Example 3

As starting materials, base oil A and aluminium oxide were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1, but a fluid state material was formed.

Comparative Example 4

As starting materials, base oil A and synthetic mica were introduced into a grease-production kettle in the mixing proportions shown in the table, and production was carried out in the same way based on the production method in Working Example 1, but a fluid state material was formed. Comparative Example 5

This was a commercial general-purpose lithium grease (produced by Showa Shell Sekiyu K.K.) where the thickener was lithium 12-hydroxystearate soap and a mineral oil type lubricating oil was used as the base oil, with the base oil viscosity being 12.2 mm ² /s at 100°C.

Comparative Example 6

This was a commercial general-purpose urea type grease (produced by Showa Shell Sekiyu K.K.) where a mineral oil type lubricating oil was used as the base oil, and where the base oil viscosity was 11.3 mm ² /s at 100°C .

The consistency, dropping point, oxidation stability (oxidation stability test), and thermal stability (thin- film heating test) of the grease compositions prepared from the starting material constituents and by the production method described above were respectively measured by the methods described earlier, and the results are shown in Table 1 and Table 2. Reference to "not measurable" in the case of Comparative Examples 1 to 4 means that measurement of the grease consistency and of the dropping point was not possible since the grease had a fluid state .

Table 2

It has been confirmed by means of Working Examples 1 to 10 that hydrotalcite has properties as a thickener. Furthermore, it is clear that the grease compositions relating to the working examples have a high dropping point, and high heat resistance and oxidation stability, when compared to Comparative Examples 5 and 6 (the commercial grease compositions), and they have the performance demanded of a grease. Moreover, no inherent thickening effect was noted in the comparative examples employing common inorganic materials or mica, which is an ordinary clay mineral.