JP7522030B2 - Lubricating oil additive composition and lubricating oil composition - Google Patents

Description

The present invention relates to a lubricating oil additive composition and a lubricating oil composition containing the lubricating oil additive composition.

Lubricating oils are blended with various lubricating oil additives for the purpose of imparting or supplementing the properties and performance required for the lubricating oil.
One representative example of a lubricating oil additive is a load-bearing additive that imparts wear resistance and extreme pressure properties to the lubricating oil. Examples of the load-bearing additive that are commonly used include phosphorus-based compounds such as phosphates, phosphites, and phosphonates, and sulfur-phosphorus-based compounds such as thiophosphates.

However, phosphorus and sulfur are highly corrosive elements. Therefore, in recent years, various studies have been conducted on polymer-based compounds as load-bearing additives to replace phosphorus-based compounds and sulfur-phosphorus-based compounds (see Patent Documents 1 and 2).

JP 2013-124266 A Japanese Unexamined Patent Publication No. 6-65588

However, the polymer compounds currently being investigated as load-bearing additives do not yet provide sufficient balance between wear resistance and extreme pressure properties.

The present invention aims to provide a lubricating oil additive composition that is suitable as a load-bearing additive and has excellent wear resistance and extreme pressure properties, as well as a lubricating oil composition that contains the lubricating oil additive composition.

The present inventors conducted extensive research to solve the above problems. As a result, they discovered that a boronized poly(meth)acrylate copolymer having specific structural units and specific side chains can solve the above problems, leading to the completion of the present invention.

That is, the present invention relates to the following [1] to [4].
[1] A lubricating oil additive composition comprising a boronated poly(meth)acrylate copolymer (X) including a structural unit (a) derived from an alkyl (meth)acrylate (A) represented by the following general formula (a-1), a structural unit (b) derived from a hydroxyl-containing (meth)acrylate (B) represented by the following general formula (b-1), and a structural unit (c) derived from a borate ester (C) of the hydroxyl-containing (meth)acrylate (B), wherein the structural unit (c) has a side chain in which one or more hydroxyl groups are bonded to a boron atom:

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.]
[2] A method of using the lubricating oil additive composition according to the above [1] as a load-bearing additive.
[3] A lubricating oil composition comprising the lubricating oil additive composition according to [1] above and a lubricating oil base oil.
[4] A method for producing a lubricating oil additive composition, comprising: a step (S) of polymerizing an alkyl (meth)acrylate (A) represented by the following general formula (a-1) and a hydroxyl-containing (meth)acrylate (B) represented by the following general formula (b-1) in the presence of an inorganic boric acid compound (y) to produce a boronated poly(meth)acrylate copolymer (X).

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.]

The present invention makes it possible to provide a lubricating oil additive composition that is suitable as a load-bearing additive and has excellent wear resistance and extreme pressure properties, as well as a lubricating oil composition that contains the lubricating oil additive composition.

The upper and lower limit values of the numerical ranges described in this specification can be combined in any manner. For example, when numerical ranges "A to B" and "C to D" are described, the numerical ranges "A to D" and "C to B" are also included in the scope of the present invention.
In addition, unless otherwise specified, a numerical range of "lower limit value to upper limit value" described in this specification means not less than the lower limit value and not more than the upper limit value.
In this specification, the numerical values in the examples are numerical values that can be used as upper or lower limits.
In this specification, "(meth)acrylate" means acrylate or methacrylate, and other similar terms have the same meaning.

[Embodiments of lubricating oil additive composition]
The lubricating oil additive composition of the present embodiment comprises a constituent unit (a) derived from an alkyl (meth)acrylate (A) represented by the following general formula (a-1), a constituent unit (b) derived from a hydroxyl-containing (meth)acrylate (B) represented by the following general formula (b-1), and a constituent unit (c) derived from a borate ester (C) of the hydroxyl-containing (meth)acrylate (B), wherein the constituent unit (c) contains a boronated poly(meth)acrylate copolymer (X) having a side chain in which one or more hydroxyl groups are bonded to a boron atom.

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.]

The present inventors have conducted intensive research to solve the above problems. As a result, it has been found that a boronized poly(meth)acrylate copolymer (X) containing a structural unit (a) derived from an alkyl (meth)acrylate (A) represented by the above general formula (a-1), a structural unit (b) derived from a hydroxyl-containing (meth)acrylate (B) represented by the above general formula (b-1), and a structural unit (c) derived from a borate ester (C) of the hydroxyl-containing (meth)acrylate (B) is excellent in both wear resistance and extreme pressure properties, and that the boronized poly(meth)acrylate copolymer (X) can be suitably used as an additive for lubricating oils (particularly as a load-bearing additive).
The reason why the boronated poly(meth)acrylate copolymer (X) has excellent abrasion resistance and extreme pressure properties is presumably because the inclusion of the structural unit (a) ensures oil solubility, the inclusion of the structural unit (b) makes it a multi-point adsorption type polymer, and the inclusion of the structural unit (c) provides a side chain in which boron is contained and one or more hydroxyl groups are bonded to the boron atom.
According to the study by the present inventors, it has been confirmed that it is difficult for the non-boronized poly(meth)acrylate copolymer consisting of the above-mentioned structural units (a) and (b) to achieve both wear resistance and extreme pressure properties.In addition, it has been confirmed that even for boronized poly(meth)acrylate copolymers, if they do not have a side chain in which one or more hydroxyl groups are bonded to a boron atom, it is difficult to achieve both wear resistance and extreme pressure properties.

In the following description, the alkyl (meth)acrylate (A), the hydroxyl group-containing (meth)acrylate (B), and the borate ester (C) of the hydroxyl group-containing (meth)acrylate (B) are also referred to as "monomer (A)," "monomer (B)," and "monomer (C)," respectively.

In the present embodiment, the boronated poly(meth)acrylate copolymer (X) may be composed only of the constituent unit (a) derived from the monomer (A), the constituent unit (b) derived from the monomer (B), and the constituent unit (c) derived from the monomer (C). However, it may also contain other constituent units than the constituent units (a), (b), and (c) as long as the effects of the present invention are not impaired.
In this embodiment, the total content of the structural units (a), (b), and (c) in the boronated poly(meth)acrylate copolymer (X) is preferably 70 mol % to 100 mol %, more preferably 80 mol % to 100 mol %, and even more preferably 90 mol % to 100 mol %, based on all the structural units of the boronated poly(meth)acrylate copolymer (X).

The following describes the alkyl (meth)acrylate (A), the hydroxyl group-containing (meth)acrylate (B), and the borate ester (C) of the hydroxyl group-containing (meth)acrylate (B).

<Alkyl (meth)acrylate (A)>
The alkyl (meth)acrylate (A) used in this embodiment is represented by the following general formula (a-1).

The structural unit (a) derived from the alkyl (meth)acrylate (A) mainly functions to impart oil solubility to the boronated poly(meth)acrylate copolymer (X).
The alkyl (meth)acrylate (A) may be used alone or in combination of two or more kinds. Therefore, the boronized poly(meth)acrylate copolymer (X) may contain one kind of the structural unit (a) derived from the alkyl (meth)acrylate (A) alone or two or more kinds.

In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group, that is, the alkyl (meth)acrylate (A) has an acryloyl group or a methacryloyl group as a polymerizable functional group.
Monomers in which R ^a1 is a substituent other than a hydrogen atom or a methyl group are difficult to obtain, and since such monomers have low reactivity, it is also difficult to polymerize them.
In this embodiment, from the viewpoint of easily adjusting the molecular weight of the boronized poly(meth)acrylate copolymer (X), R ^a1 is preferably a hydrogen atom. That is, the alkyl(meth)acrylate (A) preferably has an acryloyl group as a polymerizable functional group.

In the above general formula (a-1), R ^a2 represents an alkyl group having 8 to 20 carbon atoms.
When the alkyl group has less than 8 carbon atoms, or when the alkyl group has more than 20 carbon atoms, it becomes difficult to ensure the oil-solubility of the boronated poly(meth)acrylate copolymer (X).

Examples of the alkyl group having 8 to 20 carbon atoms that can be selected as R ^a2 include an octyl group, a nonyl group, a decyl group, a undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, and an icosyl group. These may be linear or branched.

Here, from the viewpoint of making it easier to ensure the oil solubility of the boronized poly(meth)acrylate copolymer (X), the number of carbon atoms in the alkyl group is preferably 10 to 18, more preferably 10 to 16, and even more preferably 10 to 14.

<Hydroxyl Group-Containing (Meth)Acrylate (B)>
The hydroxyl group-containing (meth)acrylate (B) used in this embodiment is represented by the following general formula (b-1).

The structural unit (b) derived from the hydroxyl group-containing (meth)acrylate (B) has the function of making the boronized poly(meth)acrylate copolymer (X) a multi-point adsorption type polymer, and is presumed to contribute to improving the wear resistance and extreme pressure properties.
The hydroxyl group-containing (meth)acrylate (B) may be used alone or in combination of two or more kinds. Therefore, the boronized poly(meth)acrylate copolymer (X) may contain one kind of the structural unit (b) derived from the hydroxyl group-containing (meth)acrylate (B) alone or two or more kinds.

In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. That is, the hydroxyl group-containing (meth)acrylate (B) has an acryloyl group or a methacryloyl group as a polymerizable functional group.
Monomers in which R ^b1 is a substituent other than a hydrogen atom or a methyl group are difficult to obtain, and since such monomers have low reactivity, it is also difficult to polymerize them.
In this embodiment, from the viewpoint of facilitating adjustment of the molecular weight of the boronated poly(meth)acrylate copolymer (X), R ^b1 is preferably a hydrogen atom. That is, the hydroxyl group-containing (meth)acrylate (B) preferably has an acryloyl group as a polymerizable functional group.

In the above general formula (b-1), R ^b2 represents an alkylene group having 2 to 4 carbon atoms.
When the alkylene group has one carbon atom, the polarity becomes high and the oil solubility decreases.
Furthermore, if the alkylene group has 5 or more carbon atoms, the oil solubility increases too much, resulting in a decrease in adsorbability to metals.
Here, from the viewpoint of easily ensuring appropriate oil solubility and appropriate adsorbability to metals, the alkylene group preferably has 2 to 3 carbon atoms, and more preferably 2 carbon atoms.

m1 represents an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different. In addition, the bonding mode between the moieties represented by -(OR ^b2 ) _m1 may be a random bond or a block bond, but from the viewpoint of ease of polymerization, a random bond is preferable.
When m1 is 0, the hydroxyl group-containing (meth)acrylate (B) becomes a carboxylic acid, and therefore the oil solubility decreases.
Furthermore, when m1 is an integer of 11 or more, the polarity increases due to the influence of the -(OR ^b2 )- moiety, and the oil solubility decreases.
Here, from the viewpoint of easily ensuring appropriate oil solubility, m1 is preferably 1 to 6, more preferably 1 to 4, even more preferably 1 to 2, and still more preferably 1.

<Borate Ester (C) of Hydroxyl-Containing (Meth)Acrylate (B)>
The borate ester (C) of the hydroxyl-containing (meth)acrylate (B) used in this embodiment can be synthesized, for example, by reacting the hydroxyl-containing (meth)acrylate (B) with an inorganic boric acid compound (y). The structural unit (c) derived from the borate ester (C) of the hydroxyl-containing (meth)acrylate (B) can also be produced by reacting the poly(meth)acrylate copolymer (x) containing the structural units (a) and (b) with an inorganic boric acid compound (y) to boronize a part of the structural unit (b).

Examples of the inorganic boric acid compound (y) include compounds represented by the following general formula (y-1).
(R ^y1 O) _n -B-(OH) _3-n (y-1)
[In the above general formula (y-1), R ^y1 is an alkyl group having 1 or 2 carbon atoms. n is 0 or 1.]
Specific examples of the inorganic boric acid compound (y) include orthoboric acid, monomethyl borate, monoethyl borate, etc. Among these, orthoboric acid is preferred from the viewpoint of making it easier to exhibit the effects of the present invention.
In addition, when n=2, the boron atom is not bonded to one or more hydroxyl groups in the boric acid ester (C) of the hydroxyl group-containing (meth)acrylate (B). In other words, the boron atom is not bonded to one or more hydroxyl groups in the structural unit (c). Therefore, it is not possible to obtain a lubricating oil additive composition having excellent wear resistance and extreme pressure properties.

<Requirement (α)>
From the viewpoint of making it easier to exert the effects of the present invention and from the viewpoint of solubility in base oil, the boronated poly(meth)acrylate copolymer (X) of this embodiment preferably satisfies the requirement (α), i.e., the content ratio of the structural unit (a) to the structural unit (b) [(a)/(b)] is preferably 20/80 to 80/20 in molar ratio.
In order to improve the solubility of the boronated poly(meth)acrylate copolymer (X) in the base oil, the ratio (a)/(b) is more preferably 40/60 or more, and even more preferably 50/50 or more.
Furthermore, from the viewpoint of making it easier to exert the effects of the present invention, [(a)/(b)] is more preferably 75/25 or less, and even more preferably 70/30 or less.
The upper and lower limits of these numerical ranges can be combined in any combination. Specifically, the ratio is more preferably 40/60 to 75/25, and even more preferably 50/50 to 70/30.

<Requirements (β)>
From the viewpoint of making it easier to exert the effects of the present invention and from the viewpoint of solubility in base oil, the boronized poly(meth)acrylate copolymer (X) of this embodiment preferably satisfies the requirement (β). That is, the boron content of the boronized poly(meth)acrylate copolymer (X) is preferably 0.10% by mass to 1.70% by mass based on the total amount of the boronized poly(meth)acrylate copolymer (X). In addition, from the viewpoint of further improving the solubility of the boronized poly(meth)acrylate copolymer (X) in base oil, the boron content in the boronized poly(meth)acrylate copolymer (X) is more preferably 1.60% by mass or less, even more preferably 1.40% by mass or less, and even more preferably 1.20% by mass or less based on the total amount of the boronized poly(meth)acrylate copolymer (X).
Furthermore, from the viewpoint of making it easier to exert the effects of the present invention, the boron content of the boronated poly(meth)acrylate copolymer (X) is more preferably 0.20 mass % or more, further preferably 0.60 mass % or more, and even more preferably 0.80 mass % or more, based on the total amount of the boronated poly(meth)acrylate copolymer (X).
The upper and lower limits of these numerical ranges can be arbitrarily combined. Specifically, the range is more preferably 0.20% by mass to 1.60% by mass, even more preferably 0.60% by mass to 1.40% by mass, and even more preferably 0.80% by mass to 1.20% by mass.
The boron content of the boronated poly(meth)acrylate copolymer (X) can be calculated based on the result of dissolving a predetermined amount of the boronated poly(meth)acrylate copolymer (X) in an organic solvent (e.g., a lubricating oil base oil), measuring the amount of boron in the organic solvent in accordance with JIS-5S-38-03, and the amount of the boronated poly(meth)acrylate copolymer (X) dissolved in the organic solvent.

<Other Monomers>
The boronized poly(meth)acrylate copolymer (X) may contain, in addition to the above-mentioned structural units (a), (b), and (c), structural units derived from other monomers, as long as the effects of the present invention are not impaired. Examples of the other monomers include functional group-containing monomers other than the monomers (A), (B), and (C). Examples of the other functional group-containing monomers include functional group-containing (meth)acrylates other than the monomers (A), (B), and (C).
However, from the viewpoint of making it easier to exert the effects of the present invention, the poly(meth)acrylate copolymer (X) preferably has a total content of the structural units (a), (b), and (c) of more than 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, even more preferably 70% by mass to 100% by mass, still more preferably 80% by mass to 100% by mass, and even more preferably 90% by mass to 100% by mass, based on all structural units of the boronated poly(meth)acrylate copolymer (X).
[0043] In order to more easily exert the effects of the present invention, the content of constituent units derived from functional group-containing monomers other than the monomers (A), (B) and (C) in the boronated poly(meth)acrylate copolymer (X) is preferably 50% by mass or less, more preferably less than 40% by mass, even more preferably less than 30% by mass, still more preferably less than 20% by mass, and even more preferably less than 10% by mass, based on the total amount of constituent units.

<Physical Properties of Boronated Poly(meth)acrylate Copolymer (X)>
(Mass average molecular weight (Mw), molecular weight distribution (Mw/Mn))
The mass average molecular weight (Mw) of the boronated poly(meth)acrylate copolymer (X) of the present embodiment is preferably 5,000 to 100,000, more preferably 5,000 to 50,000, and even more preferably 5,000 to 30,000, from the viewpoint of making it easier to exhibit the effects of the present invention and from the viewpoint of solubility in the base oil.
From the viewpoint of making it easier to exert the effects of the present invention, the molecular weight distribution (Mw/Mn) of the boronated poly(meth)acrylate copolymer (X) of this embodiment is preferably 3.0 or less, more preferably 2.7 or less, and even more preferably 2.4 or less. The molecular weight distribution (Mw/Mn) of the poly(meth)acrylate copolymer (X) of this embodiment may be 1.01 or more, 1.3 or more, or 1.4 or more.
The mass average molecular weight (Mw) and the molecular weight distribution (Mw/Mn) are values measured or calculated by the method described in the examples described below.

(Polymerization mode)
The polymerization mode of the boronized poly(meth)acrylate copolymer (X) of the present embodiment is not particularly limited, and may be any of block copolymerization, random copolymerization, and block/random copolymerization. Among these, random copolymerization is preferred from the viewpoint of ease of polymerization reaction.

[Method of manufacturing lubricating oil additive composition]
The method for producing the lubricating oil additive composition of the present embodiment is not particularly limited, but for example, it is preferable to use a production method including a step (S) of polymerizing an alkyl (meth)acrylate (A) represented by the following general formula (a-1) and a hydroxyl group-containing (meth)acrylate (B) represented by the following general formula (b-1) in the presence of an inorganic boric acid compound (y) to produce a boronated poly(meth)acrylate copolymer (X).

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.]

Steps (S1) and (S2) are described in detail below.

<Step (S1)>
In the step (S1), a poly(meth)acrylate copolymer (x) is produced as a raw material for producing a boronized poly(meth)acrylate copolymer (X).
The method for producing the poly(meth)acrylate copolymer (x) (polymerization method) is not particularly limited, and any of the known methods can be used for production. Examples of such methods include emulsion polymerization, suspension polymerization, and solution polymerization, and from the viewpoint of simplicity, it is preferable to use the solution polymerization method.

(Solution Polymerization Method)
The solution polymerization method is carried out, for example, by charging the monomers (A) and (B), a solvent and an initiator into a reactor, replacing the atmosphere in the reactor with nitrogen, and then reacting with stirring for 2 to 10 hours at 60° C. to 100° C. Monomers other than the monomers (A) and (B) may also be charged into the reactor as desired.

Examples of the solvent used in the solution polymerization method include alcohols such as methanol, ethanol, propanol, 2-propanol, and butanol; hydrocarbons such as benzene, toluene, xylene, and hexane; esters such as ethyl acetate, butyl acetate, and isobutyl acetate; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ethers such as methoxybutanol, ethoxybutanol, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, propylene glycol monobutyl ether, and dioxane; mineral oil; and synthetic oils such as poly-α-olefins, ethylene-α-olefin copolymers, alkylbenzenes, alkylnaphthalenes, polyphenyl ethers, alkyl-substituted diphenyl ethers, polyol esters, dibasic acid esters, hindered esters, monoesters, and GTL base oils.
These may be used alone or in combination of two or more.

Examples of initiators used in the solution polymerization method include azo initiators such as 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2-methylpropionitrile), 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis-(N,N-dimethyleneisobutylamidine) dihydrochloride, and 1,1'-azobis(cyclohexyl-1-carbonitrile); hydrogen peroxide; organic peroxides such as benzoyl peroxide, t-butyl hydroperoxide, cumene hydroperoxide, methyl ethyl ketone peroxide, and perbenzoic acid; persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; redox initiators of hydrogen peroxide- ^Fe2+ ; and other existing radical initiators.

The molecular weight of the poly(meth)acrylate copolymer (x) (the molecular weight of the boronized poly(meth)acrylate copolymer (X)) is controlled by a known method. For example, the molecular weight of the poly(meth)acrylate copolymer (x) (the molecular weight of the boronized poly(meth)acrylate copolymer (X)) can be controlled by the reaction temperature, reaction time, amount of initiator, amount of each monomer, type of solvent, use of a chain transfer agent, etc.

(Preferred embodiment in step (S1))
In the step (S1), from the viewpoint of producing a poly(meth)acrylate-based copolymer (x) (boronated poly(meth)acrylate-based copolymer (X)) satisfying the above-mentioned requirement (α), it is preferable to adjust the blending ratio [(A)/(B)] of the alkyl (meth)acrylate (A) to the hydroxyl group-containing (meth)acrylate (B) to a molar ratio of 20/80 to 80/20.
In addition, from the viewpoint of improving the solubility of the poly(meth)acrylate copolymer (x) (boronated poly(meth)acrylate copolymer (X)) in the base oil, [(A)/(B)] is more preferably 40/60 or more, and even more preferably 50/50 or more.
Furthermore, from the viewpoint of making it easier to exert the effects of the present invention, [(A)/(B)] is more preferably 75/25 or less, and even more preferably 70/30 or less.
The upper and lower limits of these numerical ranges can be combined in any combination. Specifically, the ratio is more preferably 40/60 to 75/25, and even more preferably 50/50 to 70/30.

<Step (S2)>
In the step (S2), the poly(meth)acrylate copolymer (x) is reacted with an inorganic boric acid compound (y) to produce a boronated poly(meth)acrylate copolymer (X).
The inorganic boric acid compound (y) may be the same as those described above, and is preferably orthoboric acid.
The reaction between the poly(meth)acrylate copolymer (x) and the inorganic boric acid compound (y) can be carried out, for example, in a solvent at a reaction temperature of 100° C. to 150° C. The reaction time is, for example, 3 hours to 10 hours. Examples of the solvent include those similar to those listed in the above step (S1).

(Preferred embodiment in step (S2))
In the step (S2), from the viewpoint of producing a boronated poly(meth)acrylate copolymer (X) satisfying the above requirement (β), it is preferable to adjust the ratio [(y)/(x)] of the number of moles of hydroxyl groups in the poly(meth)acrylate copolymer (x) to the number of moles of the inorganic boric acid compound (y) to be equal to or greater than 1/3 and less than 3/3.
From the viewpoint of the solubility of the boronated poly(meth)acrylate copolymer (X) in the base oil, [(y)/(x)] is preferably 2.9/3 or less, more preferably 2.7/3 or less, and even more preferably 2.5/3 or less.
Furthermore, from the viewpoint of making it easier to exert the effects of the present invention, [(y)/(x)] is preferably 1.5/3 or more, more preferably 1.7/3 or more, and even more preferably 2.0/3 or more.
The upper and lower limits of these numerical ranges can be combined in any combination. Specifically, the ratio is more preferably 1.5/3 to 2.9/3, even more preferably 1.7/3 to 2.7/3, and even more preferably 2.0/3 to 2.5/3.

<Content of boronated poly(meth)acrylate copolymer (X) in lubricating oil additive composition>
In the lubricating oil additive composition of this embodiment, from the viewpoint of making it easier to exert the effects of the present invention when added to a lubricating oil base oil, the content of the boronized poly(meth)acrylate copolymer (X) is preferably 50 mass% or more, more preferably 60 mass% or more, even more preferably 70 mass% or more, even more preferably 80 mass% or more, even more preferably 90 mass% or more, and even more preferably 95 mass% or more based on the total amount of the lubricating oil additive composition. Considering the purity of the boronized poly(meth)acrylate copolymer (X), the content of the boronized poly(meth)acrylate copolymer (X) is usually less than 99 mass% based on the total amount of the lubricating oil additive composition.
The lubricating oil additive composition of the present embodiment may be diluted with a diluting solvent from the viewpoint of solubility in the lubricating oil base oil and ease of handling. The content of the boronized poly(meth)acrylate copolymer (X) in the lubricating oil additive composition means the content based on the total amount of the active ingredient in the lubricating oil additive composition excluding the diluting solvent.

<Uses of the lubricating oil additive composition>
The lubricating oil additive composition of the present embodiment is excellent in both anti-wear properties and extreme pressure properties, and is therefore useful as a load-bearing additive.
Thus, in this embodiment, there is provided a method of using the lubricating oil additive composition as a load-bearing additive.

[Lubricating Oil Composition]
The lubricating oil composition of the present embodiment contains a lubricating oil additive composition containing the boronated poly(meth)acrylate copolymer (X) and a lubricating oil base oil.
In order to obtain a good effect of the lubricating oil additive composition, the content of the boronized poly(meth)acrylate copolymer (X) is adjusted to be preferably 0.30% by mass or more, more preferably 0.40% by mass or more, even more preferably 0.50% by mass or more, and even more preferably 0.60% by mass or more based on the total amount of the lubricating oil composition. Also, it is preferably adjusted to be 3.0% by mass or less, more preferably 2.5% by mass or less, even more preferably 2.0% by mass or less, and even more preferably 1.5% by mass or less.
The upper and lower limits of these numerical ranges can be arbitrarily combined. Specifically, the range is preferably 0.30% by mass to 3.0% by mass, more preferably 0.40% by mass to 2.5% by mass, even more preferably 0.50% by mass to 2.0% by mass, and still more preferably 0.60% by mass to 1.5% by mass.
In addition, with respect to the content of the lubricating oil additive composition, from the viewpoint of favorably exerting the additive effect of the lubricating oil additive composition, the amount of boron derived from the boronated poly(meth)acrylate copolymer (X) is preferably 20 ppm by mass to 400 ppm by mass, more preferably 30 ppm by mass to 380 ppm by mass, and even more preferably 40 ppm by mass to 350 ppm by mass, based on the total amount of the lubricating oil composition.

<Lubricant base oil>
The lubricating base oil may be any base oil commonly used in lubricating oil compositions without any particular limitations. Specific examples of the lubricating base oil include one or more types selected from the group consisting of mineral oils and synthetic oils.
The kinematic viscosity of the lubricating base oil at 100° C. is preferably in the range of 1 mm ² /s to 50 mm ² /s, more preferably in the range of 2 mm ² /s to 30 mm ² /s, and even more preferably in the range of 3 mm ² /s to 20 mm ² /s. The viscosity index of the lubricating base oil is preferably 80 or more, more preferably 90 or more, and even more preferably 100 or more.
The kinematic viscosity and viscosity index of the lubricating base oil are values measured or calculated in accordance with JIS K2283:2000.

Specific examples of lubricating base oils are listed below.
Examples of mineral oils include distillate oils obtained by distilling paraffin-based crude oil, intermediate-based crude oil, or naphthene-based crude oil under atmospheric pressure and/or reduced pressure, refined oils obtained by refining the distillate oils according to conventional methods, etc. Refining methods for obtaining refined oils include, for example, solvent dewaxing, hydroisomerization, hydrofinishing, clay treatment, etc.
Examples of synthetic oils include hydrocarbon oils, aromatic oils, ester oils, ether oils, etc. In addition, as the synthetic oil, GTL (Gas To Liquids) obtained by isomerizing wax (GTL wax, Gas To Liquids WAX) produced from natural gas by the Fischer-Tropsch process or the like may be used.

<Other additives>
The lubricating oil composition of this embodiment may contain other additives such as antioxidants, oiliness agents, detergent dispersants, viscosity index improvers, rust inhibitors, metal deactivators, and antifoaming agents, within the range that does not impair the effects of the lubricating oil additive composition. These may be used alone or in combination of two or more.
In addition, in this embodiment, there is provided an additive package for a lubricating oil composition containing the boronated poly(meth)acrylate-based copolymer (X) and, as an additive other than the lubricating oil additive composition containing the boronated poly(meth)acrylate-based copolymer (X), one or more additives selected from antioxidants, oiliness agents, detergents and dispersants, viscosity index improvers, rust inhibitors, metal deactivators, and antifoaming agents.

(Antioxidant)
As the antioxidant, amine-based antioxidants, phenol-based antioxidants, etc., which are used in conventional lubricating oil compositions, can be used. These antioxidants may be used alone or in combination of two or more.
Examples of the amine-based antioxidant include monoalkyldiphenylamine-based compounds such as monooctyldiphenylamine and monononyldiphenylamine; dialkyldiphenylamine-based compounds such as 4,4'-dibutyldiphenylamine, 4,4'-dipentyldiphenylamine, 4,4'-dihexyldiphenylamine, 4,4'-diheptyldiphenylamine, 4,4'-dioctyldiphenylamine, and 4,4'-dinonyldiphenylamine; polyalkyldiphenylamine-based compounds such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, and tetranonyldiphenylamine; and naphthylamine-based compounds such as α-naphthylamine, phenyl-α-naphthylamine, butylphenyl-α-naphthylamine, pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine, and nonylphenyl-α-naphthylamine.
Examples of the phenol-based antioxidant include monophenol-based compounds such as 2,6-di-tert-butyl-4-methylphenol and 2,6-di-tert-butyl-4-ethylphenol; and bisphenol-based compounds such as 4,4'-methylenebis(2,6-di-tert-butylphenol) and 2,2'-methylenebis(4-ethyl-6-tert-butylphenol). The content of the antioxidant may be the minimum amount required to maintain the oxidation stability of the lubricating oil composition. Specifically, for example, the content is preferably 0.01 to 1 mass% based on the total amount of the lubricating oil composition.

(Oil-based agent)
Examples of oily agents include fatty alcohols; fatty acid compounds such as fatty acids and fatty acid metal salts; ester compounds such as polyol esters, sorbitan esters, and glycerides; and amine compounds such as fatty amines.
From the viewpoint of the effect of adding the oiliness agent, the content thereof is usually from 0.1 to 20 mass %, preferably from 0.5 to 10 mass %, based on the total amount of the lubricating oil composition.

(Detergent/Dispersant)
Detergent-dispersants include metal sulfonates, metal salicylates, metal phenates, and succinimides.
From the viewpoint of the effect of adding the detergent-dispersant, the content is usually from 0.01 to 10 mass %, preferably from 0.1 to 5 mass %, based on the total amount of the lubricating oil composition.

(Viscosity index improver)
Examples of viscosity index improvers include polymethacrylates, dispersion-type polymethacrylates, olefin copolymers (e.g., ethylene-propylene copolymers, etc.), dispersion-type olefin copolymers, and styrene copolymers (e.g., styrene-diene hydrogenated copolymers, etc.).
The content of the viscosity index improver is preferably 0.3 to 5 mass % based on the total amount of the lubricating oil composition.

(anti-rust)
Examples of the rust inhibitor include metal sulfonates, succinic acid esters, and alkanolamines such as alkylamines and monoisopropanolamine.
From the viewpoint of the effect of adding the rust inhibitor, the content of the rust inhibitor is usually 0.01 to 5 mass %, preferably 0.03 to 3 mass %, based on the total amount of the lubricating oil composition.

(Metal deactivator)
Examples of metal deactivators include benzotriazole and thiadiazole.
The preferred content of the metal deactivator is usually 0.01 to 5 mass %, and preferably 0.01 to 1 mass %, based on the total amount of the lubricating oil composition, from the viewpoint of the effect of adding the metal deactivator.

(Antifoaming agent)
Examples of the antifoaming agent include methylsilicone oil, fluorosilicone oil, and polyacrylate.
From the viewpoint of the effect of adding the antifoaming agent, the content is usually 0.0005 to 0.01 mass % based on the total amount of the lubricating oil composition.

<Grease Composition>
The lubricating oil additive composition of the present embodiment can also be used by blending it into a grease composition.
That is, in this embodiment, a grease composition containing the lubricating oil additive composition, a thickener, and a lubricating oil base oil can be provided.

<Physical properties of lubricating oil composition>
(Kinematic viscosity, viscosity index)
The lubricating oil composition of this embodiment preferably has a 100° C. kinematic viscosity of 1.0 mm ² /s to 50 mm ² /s, more preferably 2.0 mm ² /s to 30 mm ² /s, and even more preferably 3.0 mm ² /s to 20 mm ² /s.
The lubricating oil composition of this embodiment preferably has a viscosity index of 90 or greater, more preferably 100 or greater, and even more preferably 110 or greater.
The kinematic viscosity and viscosity index of the lubricating oil composition are values measured or calculated in accordance with JIS K2283:2000.

(Wear resistance)
The lubricating oil composition of this embodiment preferably has a wear scar diameter of 0.57 mm or less, more preferably 0.53 mm or less, and even more preferably 0.50 mm or less, as determined by the Shell abrasion test described in the Examples section below.

(Extreme pressure)
The lubricating oil composition of this embodiment preferably has a maximum non-seizure load (LNL) of 490 N or more, more preferably 618 N or more, and even more preferably 785 N or more, as determined by the Shell four-ball load carrying capacity (EP) test described in the Examples below.

[Uses of the lubricating oil composition]
The lubricating oil composition of the present embodiment contains the boronated poly(meth)acrylate copolymer (X) and therefore has excellent wear resistance and extreme pressure properties.
For this reason, the lubricating oil composition of the present embodiment can be suitably used for a variety of applications, including, for example, drive system oils such as gear oils (manual transmission oil, differential oil, etc.), automatic transmission oils (automatic transmission oil, etc.), continuously variable transmission oils (belt CVT oil, toroidal CVT oil, etc.), power steering oil, shock absorber oil, and electric motor oil; internal combustion engine (engine) oils such as for gasoline engines, diesel engines, and gas engines; hydraulic oils; turbine oils; compressor oils; fluid bearing oils; and rolling bearing oils; and can be suitably used as a lubricating oil composition that is filled into equipment used in each of these applications and lubricates the various parts of the equipment.

[Lubrication method using lubricating oil composition]
A preferred example of a lubrication method using the lubricating oil composition of the present embodiment is a method in which the lubricating oil composition is filled into equipment used for each of the above-mentioned applications and lubricates the components associated with each of the equipment.

[One aspect of the present invention provided]
According to one aspect of the present invention, the following [1] to [10] are provided.
[1] A lubricating oil additive composition comprising a boronated poly(meth)acrylate copolymer (X) including a structural unit (a) derived from an alkyl (meth)acrylate (A) represented by the following general formula (a-1), a structural unit (b) derived from a hydroxyl-containing (meth)acrylate (B) represented by the following general formula (b-1), and a structural unit (c) derived from a borate ester (C) of the hydroxyl-containing (meth)acrylate (B), wherein the structural unit (c) has a side chain in which one or more hydroxyl groups are bonded to a boron atom:

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.]
[2] The lubricating oil additive composition according to the above [1], wherein the boronized poly(meth)acrylate copolymer (X) further satisfies the following requirement (α):
<Requirement (α)>
The content ratio of the structural unit (a) to the structural unit (b) [(a)/(b)] is, in molar ratio, 20/80 to 80/20.
[3] The lubricating oil additive composition according to the above [1] or [2], wherein the boronized poly(meth)acrylate copolymer (X) further satisfies the following requirement (β):
<Requirements (β)>
The boron content of the boronated poly(meth)acrylate copolymer (X) is 0.10% by mass to 1.70% by mass based on the total amount of the boronated poly(meth)acrylate copolymer (X).
[4] The lubricating oil additive composition according to any one of the above [1] to [3], wherein the boronized poly(meth)acrylate copolymer (X) has a mass average molecular weight (Mw) of 5,000 to 100,000.
[5] The lubricating oil additive composition according to any one of the above [1] to [4], which is used as a load-bearing additive.
[6] A method for using the lubricating oil additive composition according to any one of the above [1] to [4] as a load-bearing additive.
[7] A lubricating oil composition comprising the lubricating oil additive composition according to any one of [1] to [5] above and a lubricating oil base oil.
[8] A step (S1) of producing a poly(meth)acrylate copolymer (x) by polymerizing an alkyl (meth)acrylate (A) represented by the following general formula (a-1) and a hydroxyl group-containing (meth)acrylate (B) represented by the following general formula (b-1):
and (S2) reacting the poly(meth)acrylate copolymer (x) with an inorganic boric acid compound (y) to produce a boronated poly(meth)acrylate copolymer (X).

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.]
[9] In the step (S1), a blending ratio [(A)/(B)] of the alkyl (meth)acrylate (A) and the hydroxyl group-containing (meth)acrylate (B) is adjusted to a molar ratio of 20/80 to 80/20. The method for producing a lubricating oil additive composition according to the above item [8].
[10] The method for producing a lubricating oil additive composition according to the above [8] or [9], wherein in the step (S2), a ratio [(y)/(x)] of the number of moles of the inorganic boric acid compound (y) to the number of moles of the hydroxyl groups in the poly(meth)acrylate copolymer (x) is adjusted to 1/3 or more and less than 3/3.

The present invention will be described in detail with reference to the following examples, but the present invention is not limited to these examples.

[Methods for measuring various physical properties]
The properties of the raw materials used in each Example and Comparative Example and the lubricating oil compositions in each Example and Comparative Example were measured according to the procedures described below.

(1) Kinematic Viscosity and Viscosity Index The 40° C. kinematic viscosity, 100° C. kinematic viscosity, and viscosity index of the lubricating oil compositions were measured or calculated in accordance with JIS K2283:2000.

(2) Boron Content The boron content of the lubricating oil composition was measured in accordance with JIS-5S-38-03.
The boron content of the lubricating oil additive composition was calculated based on the result of measuring the boron amount in the lubricating oil composition in accordance with JIS-5S-38-03 and the amount of the lubricating oil additive composition added (dissolved amount) to the lubricating oil composition.

(3) Mass average molecular weight (Mw), molecular weight distribution (Mw/Mn)
A Waters 1515 isocratic HPLC pump and a Waters 2414 refractive index (RI) detector were equipped with one Tosoh TSKguard column Super HZ-L and two Tosoh TSKSuper Multipore HZ-M columns. The tubes were attached in this order from the upstream side, and measurements were performed under the conditions of a measurement temperature of 40° C., a mobile phase of tetrahydrofuran, a flow rate of 0.35 ml/min, and a sample concentration of 1.0 mg/ml, and the results were calculated in terms of standard polystyrene.

[Examples 1 to 4, Comparative Examples 1 to 3]
The lubricating oil base oil and lubricating oil additive composition shown below were thoroughly mixed in the blending amounts (mass %) shown in Table 1 to prepare the lubricating oil compositions of Examples 1 to 4 and Comparative Examples 1 to 3, respectively.
Details of the lubricating oil base oils and lubricating oil additive compositions used in Examples 1 to 4 and Comparative Examples 1 to 3 are as follows.

<Lubricant base oil>
Mineral oil (150N) classified as Group II by API classification

<Lubricating Oil Additive Composition>
Poly(meth)acrylate copolymer (x): Produced by the method explained in Comparative Production Example 1.
Boronated poly(meth)acrylate copolymer (X)-1: Produced by the method explained in Production Example 1.
Boronated poly(meth)acrylate copolymer (X)-2: Produced by the method explained in Production Example 2.
Boronated poly(meth)acrylate copolymer (X′): Produced by the method explained in Comparative Production Example 2.

<Production Examples 1 and 2, Comparative Production Examples 1 and 2>
(Monomers, etc. used in Production Examples 1 and 2)
"Dodecyl acrylate": a compound in which R ^a1 is a hydrogen atom and R ^a2 is a dodecyl group (an alkyl group having 12 carbon atoms) in the above general formula (a-1). Used as alkyl (meth)acrylate (A).
"2-hydroxyethyl acrylate": a compound in which, in the above general formula (b-1), R ^b1 is a hydrogen atom, R ^b2 is an ethylene group (an alkylene group having 2 carbon atoms), and m1 = 1. Used as the hydroxyl group-containing (meth)acrylate (B).
Inorganic boric acid compound (C): orthoboric acid (monomer used in Comparative Production Examples 1 and 2, etc.)
"1-(methacryloylethoxy)-4,4,6-trimethyl-dioxaborinane": a monomer corresponding to the compound of Reference Example 1 in JP-A-6-65588.
Dodecyl methacrylate: a compound in which R ^a1 is a methyl group and R ^a2 is a dodecyl group in the above general formula (a-1).
"Stearyl methacrylate": a compound in which R ^a1 is a methyl group and R ^a2 is a stearyl group in the above general formula (a-1).

(Comparative Production Example 1: Production of poly(meth)acrylate copolymer (x))
Into a 200 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, and a stirrer, 30.3 g (126 mmol) of dodecyl acrylate, 8.6 g (74 mmol) of 2-hydroxyethyl acrylate, and 39.9 g of 2-propanol as a solvent were placed.
Next, the atmosphere in the flask was replaced with nitrogen, 0.17 g of 2,2'-azobis(isobutyronitrile) was added as an initiator, and the temperature was slowly raised with stirring, and the mixture was reacted for 4 hours under reflux at a temperature of 75 to 85°C. After completion of the reaction, the solvent was distilled off under reduced pressure, thereby obtaining a poly(meth)acrylate copolymer (x).
The poly(meth)acrylate copolymer (x) had a mass average molecular weight (Mw) of 12,800 and a molecular weight distribution (Mw/Mn) of 1.5.
The poly(meth)acrylate copolymer (x) was diluted with mineral oil so that the content of the poly(meth)acrylate copolymer (x) was 50 mass %, and then mixed with the lubricating base oil.

(Production Example 1: Production of boronated poly(meth)acrylate copolymer (X)-1)
Into a 500 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, a Dean-Stark tube, and a stirrer, 25 g of the poly(meth)acrylate copolymer (x) obtained in Comparative Production Example 1 diluted with mineral oil, 0.97 g (15.7 mmol) of orthoboric acid, and 260.0 g of toluene as a solvent were placed.
The temperature was slowly raised with stirring, and the mixture was allowed to react for 4 hours while refluxing at a temperature of 110 to 120°C. After completion of the reaction, the solvent was distilled off under reduced pressure to obtain a mineral oil-diluted product containing 50% by mass of boronated poly(meth)acrylate copolymer (X)-1 (50% by mass diluted with mineral oil).
The boronated poly(meth)acrylate copolymer (X)-1 had a mass average molecular weight (Mw) of 12,300 and a molecular weight distribution (Mw/Mn) of 1.5. The boron content of the boronated poly(meth)acrylate copolymer (X)-1 was 0.29% by mass.
In Production Example 1, the ratio [(y)/(x)] of the number of moles of hydroxyl groups in the poly(meth)acrylate copolymer (x) to the number of moles of the inorganic boric acid compound (y) was 1/3.
Since there is almost no difference between the mass average molecular weight (Mw) of the boronated poly(meth)acrylate copolymer (X)-1 and the mass average molecular weight of the poly(meth)acrylate copolymer (x), no crosslinked structure is formed by the reaction of the poly(meth)acrylate copolymer (x) with orthoboric acid, and one or more hydroxyl groups are bonded to the boron atoms of the boronated poly(meth)acrylate copolymer (X)-1.
The boronized poly(meth)acrylate copolymer (X)-1 was mixed with the lubricating base oil in the state of a 50% by mass diluted product with mineral oil.

(Production Example 2: Production of boronated poly(meth)acrylate copolymer (X)-2)
A boronated poly(meth)acrylate copolymer (X)-2 was obtained in the same manner as in Production Example 1, except that 1.93 g (31.2 mmol) of orthoboric acid was added.
The boronated poly(meth)acrylate copolymer (X)-2 had a mass average molecular weight (Mw) of 12,300 and a molecular weight distribution (Mw/Mn) of 1.5. The boron content of the boronated poly(meth)acrylate copolymer (X)-2 was 0.57% by mass.
In Production Example 2, the ratio [(y)/(x)] of the number of moles of hydroxyl groups in the poly(meth)acrylate copolymer (x) to the number of moles of the inorganic boric acid compound (y) was 2/3.
Since there is almost no difference between the mass average molecular weight (Mw) of the boronated poly(meth)acrylate copolymer (X)-2 and the mass average molecular weight of the poly(meth)acrylate copolymer (x), no crosslinked structure is formed by the reaction of the poly(meth)acrylate copolymer (x) with orthoboric acid, and one or more hydroxyl groups are bonded to the boron atoms of the boronated poly(meth)acrylate copolymer (X)-2.
The boronized poly(meth)acrylate copolymer (X)-2 was mixed with the lubricating base oil in the state of a 50% by mass mineral oil dilution.

(Comparative Production Example 2: Production of boronated poly(meth)acrylate copolymer (X'))
A 200 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, and a stirrer was charged with 5.3 g (20.7 mmol) of 1-(methacryloylethoxy)-4,4,6-trimethyl-dioxaborinane, 39.2 g (154.3 mmol) of dodecyl methacrylate, 4.4 g (13 mmol) of stearyl methacrylate, and 48.9 g of mineral oil as a solvent.
Next, the atmosphere in the flask was replaced with nitrogen, 0.5 g of Perbutyl O was added as an initiator, and the temperature was slowly raised with stirring, and the mixture was allowed to react for 4 hours while refluxing at a temperature of 95 to 100°C to obtain a mineral oil dilution product (50 mass% mineral oil dilution product) containing 50 mass% of boronated poly(meth)acrylate copolymer (X'). The mass average molecular weight (Mw) of the boronated poly(meth)acrylate copolymer (X') was 155,000, and the molecular weight distribution (Mw/Mn) was 3.3. The boron content of the boronated poly(meth)acrylate copolymer (X') was 0.25 mass%.
The boronized poly(meth)acrylate copolymer (X') was mixed with the lubricating base oil while being diluted with the dilution solvent used in the polymerization (i.e., in the form of a 50% by mass mineral oil dilution).

[Evaluation method]
The tests described below were carried out to evaluate the wear resistance and extreme pressure properties.

<Shell abrasion test>
The wear resistance of the lubricating oil compositions was evaluated using a Shell abrasion tester in accordance with ASTM D 4172 under test conditions of a load of 30 kg, a rotation speed of 1,200 rpm, a temperature of 80° C., and a test time of 30 minutes. The results were expressed as the wear scar diameter (mm) of the test ball.

<Shell four-ball load-bearing (EP) test>
In accordance with ASTM D2783-03 (2014), the maximum non-seizure load (LNL) was measured using a four-ball tester at a rotation speed of 1,800 rpm and an oil temperature of 25±5°C. The higher this value, the better the extreme pressure properties.

<Evaluation criteria>
Those that satisfied the following criteria (I) or (II) were deemed to be acceptable.
Criterion (I): The wear scar diameter in the shell wear test is less than 0.50 mm, and the maximum non-seizure load (LNL) is 490 N or more.
Criterion (II): The wear scar diameter in the shell wear test is 0.50 mm or more and 0.60 mm or less, and the maximum non-seizure load (LNL) is 618 N or more.

The results are shown in Table 1. In Table 1, the values in parentheses for the content of the lubricating oil additive composition refer to the content converted into resin content.

The following can be seen from Table 1.
From the results shown in Examples 1 to 4, it is seen that the lubricating oil composition containing the boronated poly(meth)acrylate copolymer (X)-1 or the boronated poly(meth)acrylate copolymer (X)-2 satisfies either of the above criteria (I) or (II) and is excellent in wear resistance and extreme pressure properties.
From the results shown in Comparative Example 1, it can be seen that the lubricating oil composition containing poly(meth)acrylate copolymer (x) does not meet either of the above criteria (I) and (II), and does not achieve both wear resistance and extreme pressure properties. Also, from the results shown in Comparative Example 2, it can be seen that by increasing the amount of poly(meth)acrylate copolymer (x), wear resistance is improved, but extreme pressure properties are significantly reduced. From the above, it can be seen that it is difficult to achieve both wear resistance and extreme pressure properties in the lubricating oil composition containing poly(meth)acrylate copolymer (x).
Moreover, from the results shown in Comparative Example 3, it can be seen that the lubricating oil composition containing the boronized poly(meth)acrylate copolymer (X') does not satisfy either of the above criteria (I) or (II), and does not achieve both wear resistance and extreme pressure properties. Since the boronized poly(meth)acrylate copolymer (X') does not have a side chain with at least one hydroxyl group bonded to a boron atom, it is believed that the boronized poly(meth)acrylate copolymer (X)-1 and (X)-2, which have a side chain with at least one hydroxyl group bonded to a boron atom, are unable to achieve both wear resistance and extreme pressure properties.

Claims (10)

A lubricating oil additive composition comprising a boronated poly(meth)acrylate copolymer (X) including a structural unit (a) derived from an alkyl (meth)acrylate (A) represented by the following general formula (a-1), a structural unit (b) derived from a hydroxyl-containing (meth)acrylate (B) represented by the following general formula (b-1), and a structural unit (c) derived from a borate ester (C) of the hydroxyl-containing (meth)acrylate (B), wherein the structural unit (c) has a side chain in which one or more hydroxyl groups are bonded to a boron atom:

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.] The lubricating oil additive composition according to claim 1, wherein the boronized poly(meth)acrylate copolymer (X) further satisfies the following requirement (α):
<Requirement (α)>
The content ratio of the structural unit (a) to the structural unit (b) [(a)/(b)] is, in molar ratio, 20/80 to 80/20. The lubricating oil additive composition according to claim 1 or 2, wherein the boronized poly(meth)acrylate copolymer (X) further satisfies the following requirement (β):
<Requirements (β)>
The boron content of the boronated poly(meth)acrylate copolymer (X) is 0.10% by mass to 1.70% by mass based on the total amount of the boronated poly(meth)acrylate copolymer (X). The lubricating oil additive composition according to any one of claims 1 to 3, wherein the boronized poly(meth)acrylate copolymer (X) has a mass average molecular weight (Mw) of 5,000 to 100,000. The lubricating oil additive composition according to any one of claims 1 to 4, which is used as a load-bearing additive. A method of using the lubricating oil additive composition according to any one of claims 1 to 4 as a load-bearing additive. A lubricating oil composition comprising the lubricating oil additive composition according to any one of claims 1 to 5 and a lubricating oil base oil. A step (S1) of producing a poly(meth)acrylate copolymer (x) by polymerizing an alkyl (meth)acrylate (A) represented by the following general formula (a-1) and a hydroxyl group-containing (meth)acrylate (B) represented by the following general formula (b-1);
and (S2) reacting the poly(meth)acrylate copolymer (x) with an inorganic boric acid compound (y) to produce a boronated poly(meth)acrylate copolymer (X).

[In the above general formula (a-1), R ^a1 is a hydrogen atom or a methyl group. R ^a2 is an alkyl group having 8 to 20 carbon atoms.]

[In the above general formula (b-1), R ^b1 is a hydrogen atom or a methyl group. R ^b2 is an alkylene group having 2 to 4 carbon atoms. m1 is an integer of 1 to 10. When m1 is an integer of 2 or more, multiple R ^b2 may be the same or different.] The method for producing a lubricating oil additive composition according to claim 8, wherein in the step (S1), the blending ratio [(A)/(B)] of the alkyl (meth)acrylate (A) and the hydroxyl group-containing (meth)acrylate (B) is adjusted to a molar ratio of 20/80 to 80/20. The method for producing a lubricating oil additive composition according to claim 8 or 9, wherein in step (S2), the ratio [(y)/(x)] of the number of moles of the inorganic boric acid compound (y) to the number of moles of hydroxyl groups in the poly(meth)acrylate copolymer (x) is adjusted to 1/3 or more and less than 3/3.