JP7583355B2 - Ester base oil for lubricating oil and lubricating oil composition - Google Patents

Description

The present invention relates to an ester base oil used in lubricants, and more specifically to an ester base oil that is excellent in anti-emulsibility, hydrolytic stability, and lubricity, as well as excellent durability even in the presence of water.

Lubricants used in metal processing oils, hydraulic oils, etc. consist of base oils and additives with various functions. Esters are widely used as base oils for lubricants because they have properties such as high lubricity, high heat resistance, and high volatility resistance due to the ester bond, which is a polar group, in their structure.

In the field of metalworking, the inclusion of water is a major problem, causing a decrease in tool life and a decrease in machining accuracy. Ester-based oils are more polar than mineral oils, so they are more likely to emulsify with water. When emulsification occurs, the oil film performance on the metal sliding surface decreases, and machining accuracy may decrease. In addition, when water is mixed in, hydrolysis occurs, breaking the ester bond, and organic acids are generated. The organic acids generated can cause metal corrosion, leading to a decrease in tool life. For this reason, when ester-based oils are used in the field of metalworking, various studies are being conducted to improve the anti-emulsification properties (water separation properties) and hydrolysis stability.

For example, Patent Document 1 discloses a lubricating oil composition in which two types of phosphorus-based compounds are added to a specific ester base oil to address the issue of demulsification. In addition, Patent Document 2 discloses a lubricating oil composition in which a fatty acid amide compound and a benzotriazole derivative are added to a specific ester base oil to address the issue of hydrolysis, and Patent Document 3 discloses a lubricating oil composition in which a specific compound is added to a fatty acid ester base oil made of glycerol and oleic acid.

JP 2017-008229 A JP 2008-115301 A JP 2017-165912 A

However, since metalworking oils may be used repeatedly, even if the performance is good at the beginning of use, the lubricant may deteriorate when exposed to high temperature conditions during processing or when water is mixed in, and performance may decrease over time. There is a demand for lubricants with a longer life, and there is a demand for a higher performance ester base oil that is excellent in anti-emulsification properties, hydrolysis stability, and lubricity, as well as excellent durability even in the presence of water.

The objective of the present invention is to provide an ester base oil for lubricating oils that is excellent in anti-emulsification properties, hydrolytic stability, and lubricity, and that also has excellent durability even in the presence of water.

As a result of intensive research into solving the above problems, the inventors discovered that an ester base oil in which a specific ester (A) and a specific ester (B) are mixed in a specific ratio can achieve the above objective, and thus completed the present invention.

That is, the present invention relates to the following [1] to [ 2 ].
[1] An ester base oil for lubricating oils, characterized in that the mass ratio ((A):(B)) of the following ester (A) and the following ester (B) is 80:20 to 30:70, the total amount of the following ester (A), the following ester (B) and the following ester (C) is 95 to 100 mass% based on the total mass of the ester base oil for lubricating oils, and when the following ester (C) is contained, the content of the following ester (C) based on the total mass of the ester base oil for lubricating oils is 10 mass% or less .

Ester (A): A monoester formed by the ester reaction of a branched monoalcohol having 8 to 14 carbon atoms with linoleic acid.

Ester (B): A monoester formed by the ester reaction of a branched monoalcohol having 8 to 14 carbon atoms with oleic acid.

Ester (C): A monoester formed by the ester reaction of a monoalcohol having 4 to 22 carbon atoms with linoleic acid.

(2) A lubricating oil composition comprising 80 mass % or more of the ester base oil for lubricating oils according to (1).

The ester base oil for lubricating oils of the present invention has excellent anti-emulsification properties, hydrolytic stability, and lubricity, and also has excellent durability even in the presence of water.

The ester base oil for lubricating oil of the present invention contains ester (A) and ester (B) in a specific ratio. It may contain any component other than ester (A) and ester (B) as long as it does not impair the effects of the present invention. Hereinafter, a preferred embodiment of the present invention will be described in detail.
In this specification, a numerical range defined using the symbol "to" is intended to include both the upper and lower limits of the range. For example, "2 to 5" means "2 or more and 5 or less."

[Ester (A)]
The ester (A) is a monoester formed from an ester reaction product of a branched monoalcohol having 8 to 14 carbon atoms with linoleic acid.

When the branched monoalcohol, which is the raw material of the ester (A), has 15 or more carbon atoms, sufficient abrasion resistance may not be obtained, and when the carbon number is less than 8 , sufficient demulsification properties may not be obtained. Furthermore, linear monoalcohols may not provide sufficient hydrolysis stability. Examples of branched monoalcohols having 8 to 14 carbon atoms include isooctanol, 2-ethylhexanol, isononanol, 3,5,5-trimethylhexanol, isodecanol, isododecanol, and isotridecanol. One of these alcohols may be used alone, or two or more may be used in combination.

The branched monoalcohol which is the raw material for the ester (A) preferably has 8 to 14 carbon atoms, and particularly preferably has 12 to 14 carbon atoms, from the viewpoints of anti-emulsification properties and hydrolysis stability.
For example, 2-ethylhexanol, 3,5,5-trimethylhexanol, isododecanol, and isotridecanol are preferred, and isododecanol and isotridecanol are particularly preferred.

The linoleic acid used in the ester (A) may be any industrially available product. From the viewpoint of the duration of the effect, however, it is preferable that the linoleic acid in the product has a linoleic acid purity of 95% or more.
The mass of the ester (A) is calculated based on the mass of linoleic acid calculated from the purity of linoleic acid.

[Ester (B)]
The ester (B) is a monoester formed from an ester reaction product of a branched monoalcohol having 8 to 14 carbon atoms with oleic acid.

If the branched monoalcohol, which is the raw material of the ester (B), has 15 or more carbon atoms, sufficient abrasion resistance may not be obtained, and if the branched monoalcohol has less than 8 carbon atoms, sufficient demulsibility may not be obtained. Furthermore, if a linear monoalcohol is used, sufficient hydrolysis stability may not be obtained.
Examples of branched monoalcohols having 8 to 14 carbon atoms, which are raw materials for the ester (B), include isooctanol, 2-ethylhexanol, isononanol, 3,5,5-trimethylhexanol, isodecanol, isododecanol, isotridecanol, etc. Among these alcohols, one type may be used alone, or two or more types may be used in combination.

From the viewpoints of anti-emulsification properties and hydrolytic stability, the branched monoalcohol used as the raw material for the ester (B) preferably has a carbon number of 8 to 14, and particularly preferably has a carbon number of 10 to 14. For example, 2-ethylhexanol, 3,5,5-trimethylhexanol, isododecanol, and isotridecanol are preferred, and isododecanol and isotridecanol are particularly preferred.

As the oleic acid which is the raw material of the ester (B), any commercially available oleic acid can be used. From the viewpoint of the durability of the effect, however, oleic acid having a main component purity of 95% or more is preferable.
The mass of the ester (B) is calculated based on the mass of oleic acid calculated from the purity of oleic acid.

From the viewpoint of anti-emulsification properties, it is preferable that the composition further contains an ester other than ester (A) and ester (B) (hereinafter also referred to as ester (C)).

[Ester (C)]
It is a monoester formed from the ester reaction product of a monoalcohol having 4 to 22 carbon atoms and linolenic acid.
When the monoalcohol used as the raw material for the ester (C) has 23 or more carbon atoms, sufficient abrasion resistance may not be obtained, whereas when the monoalcohol has less than 4 carbon atoms, sufficient demulsibility may not be obtained.

The monoalcohol having 4 to 22 carbon atoms, which is the raw material for ester (C), may be saturated or unsaturated, and linear or branched. Examples of monoalcohols include linear saturated alcohols such as 1-butanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, 1-undecanol, 1-dodecanol, 1-tridecanol, 1-tetradecanol, 1-pentadecanol, 1-hexadecanol, 1-heptadecanol, 1-octadecanol, 1-nonadecanol, 1-eicosanol, and 1-docosanol; branched saturated alcohols such as 2-methylpropanol, isobutanol, isooctanol, 2-ethylhexanol, isononanol, 3,5,5-trimethylhexanol, isodecanol, isododecanol, isotridecanol, isostearyl alcohol, 2-octyldodecanol, and 2-hexyldecanol; linear unsaturated alcohols such as palmitoleyl alcohol, oleyl alcohol, elaidyl alcohol, linoleyl alcohol, linolenyl alcohol, and erucyl alcohol. These alcohols may be used alone or in combination of two or more.

From the viewpoint of anti-emulsification properties, the monoalcohol that is the raw material of the ester (C) preferably has a carbon number of 8 to 18, more preferably 10 to 14. For example, 1-octanol, 2-ethylhexanol, 1-nonanol, 3,5,5-trimethylhexanol, 1-dodecanol, isododecanol, 1-tridecanol, and isotridecanol are preferred, and 1-dodecanol, isododecanol, 1-tridecanol, and isotridecanol are particularly preferred.
As the linolenic acid which is the raw material of the ester (C), any commercially available linolenic acid can be used, but from the viewpoint of the durability of the effect, linolenic acid having a main component purity of 95% or more is preferable.

The esters (A), (B) and (C) constituting the ester base oil for lubricating oils of the present invention can be produced by a conventional esterification reaction and transesterification reaction.
That is, the equivalent ratio of the monoalcohol having 4 to 22 carbon atoms to the specific carboxylic acid is preferably 0.8 to 1.5 equivalents of the specific carboxylic acid relative to the alcohol, and more preferably 0.9 to 1.2 equivalents from the viewpoints of production efficiency and economic efficiency. The reaction is adjusted to such an equivalent ratio, and an esterification catalyst is added as necessary to carry out the reaction.

As esterification catalysts, Bronsted acids such as sulfuric acid, methanesulfonic acid, and p-toluenesulfonic acid, and Lewis acid catalysts such as titanium, zirconium, hafnium, tin, and zinc can be used.

The esterification reaction is carried out at 160°C or higher under a nitrogen stream until the rate of decrease in the acid value or hydroxyl value of the reaction solution per hour is 2.0 mgKOH/g or less. In order to efficiently remove excess fatty acids and alcohols, it is preferable to carry out the reaction until the rate of decrease per hour is 1.0 mgKOH/g or less.

In order to remove excess alcohol present in the ester crude product after the reaction is completed and by-products generated during the reaction, it is preferable to distill them off under reduced pressure conditions under a nitrogen stream. The alcohol is preferably removed at a liquid temperature of 160°C or higher and a vacuum of 100 Torr or less.

In addition, in order to remove excess carboxylic acid in the ester crude product, it is preferable to carry out neutralization purification of the carboxylic acid with an alkali. Examples of the alkali to be used are sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate, and it is preferable to use an aqueous solution of 5 to 15% by mass. In neutralization purification, the above-mentioned aqueous alkali solution is added to the ester crude product, stirred and allowed to stand, and the separated lower layer of carboxylic acid soap aqueous solution is removed. Thereafter, it is preferable to carry out water washing (hot water washing) in order to further remove the carboxylic acid soap in the ester crude product. Water washing is carried out by adding hot water of 60 to 90°C to the ester crude product, stirring and allowing to stand, and removing the lower water layer.

After neutralizing the carboxylic acid with an alkali and washing with water, the esters (A), (B) and (C) can be obtained by performing adsorption treatment using activated clay, acid clay and synthetic adsorbents, steaming or other operations, either alone or in combination.

The ester base oil for lubricating oils of the present invention is obtained by mixing the ester (A) and the ester (B). From the viewpoints of demulsification property and hydrolysis resistance, the mass ratio of the ester (A) to the ester (B) ((A):(B)) is preferably 80:20 to 30:70, more preferably 70:30 to 35:65, and particularly preferably 60:40 to 45:55.
In the case where the ester (C) is contained, the content of the ester (C) is, from the viewpoint of anti-emulsification property and hydrolysis resistance, 10 mass % or less , more preferably 8 mass % or less, and particularly preferably 5 mass % or less, based on the total mass of the ester base oil.

The lower limit of the total content of the esters (A), (B) and (C) is 95% by mass.

As long as the effect of the present invention is not impaired, an ester other than ester (A), ester (B), and ester (C) (hereinafter also referred to as ester (D)) may be contained. Examples of ester (D) include esters of monoalcohols having 4 to 22 carbon atoms and saturated fatty acids having 12 to 22 carbon atoms.

The ester base oil for lubricating oils of the present invention can be appropriately blended with known additives, such as antioxidants such as phenols, amines, and quinolines, metal deactivators such as benzotriazoles, thiadiazoles, and dithiocarbamates, acid scavengers such as epoxy compounds or carbodiimides, and phosphorus-based extreme pressure agents, depending on the purpose.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.
(Synthesis Example 1: Synthesis of isotridecyl linoleate)
A 100 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, a stirrer, a Dimroth condenser, and a 10 mL oil-water separation tube was charged with 41.4 g of linoleic acid (NOF Corp., EXTRA LINOLEIC 99) and 28.6 g of isotridecanol (KH Neochem Corp., tridecanol) (equivalent ratio of carboxylic acid/alcohol = 1.03). Then, 0.1 g of p-toluenesulfonic acid was charged as a catalyst. While removing the reaction water accumulated in the oil-water separator, the reaction solution was heated to 220°C, and the acid value of the reaction solution was measured every hour. The reaction was continued until the decrease in acid value per hour was 0.5 mgKOH/g or less.
The reaction solution was then decompressed to 30 Torr at 220° C., and the alcohol and volatile reaction by-products were removed. After cooling the reactor to 85° C., 1.5 equivalents of the amount of sodium hydroxide calculated from the acid value was diluted with ion-exchanged water to prepare a 10% by mass aqueous solution, which was added to the reaction solution and stirred for 1 hour. After stopping the stirring, the solution was left to stand for 30 minutes, and the aqueous layer separated into the lower layer was removed.

Next, ion-exchanged water was added in an amount equivalent to 20% by mass to the reaction solution, which was then stirred at 85°C for 10 minutes, allowed to stand for 15 minutes, and the separated aqueous layer was removed. This procedure was repeated five times. The solution was then dehydrated by stirring at 100°C and 30 Torr for one hour. Finally, activated clay was added in an amount equivalent to 2% by mass to the reaction solution, which was then stirred at 80°C and 30 Torr for one hour, and the adsorbent was removed by filtration to obtain tridecyl linoleate.

(Synthesis Example 2: Synthesis of isotridecyl oleate)
A 100 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, a stirrer, a Dimroth condenser, and a 10 mL oil-water separation tube was charged with 41.4 g of oleic acid (EXTRA OLEIN 99, NOF Corp.) and 28.6 g of isotridecyl alcohol (Tridecanol, KH Neochem Corp.) (carboxylic acid/alcohol equivalent ratio = 1.03). Then, 0.1 g of p-toluenesulfonic acid was charged as a catalyst. The subsequent steps were carried out in the same manner as in Synthesis Example 1, and tridecyl oleate was obtained.

(Synthesis Example 3: Synthesis of 2-ethylhexyl linoleate)
In a 100 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, a stirrer, a Dimroth condenser, and a 10 mL oil-water separator, 47.1 g of linoleic acid (NOF Corp., EXTRA LINOLEIC 99) and 22.9 g of 2-ethylhexanol (Mitsubishi Chemical Corp., 2-ethylhexanol) were charged (carboxylic acid/alcohol equivalent ratio = 0.95). Then, 0.1 g of p-toluenesulfonic acid was charged as a catalyst. The subsequent steps were carried out in the same manner as in Synthesis Example 1 to obtain 2-ethylhexyl linoleate.

(Synthesis Example 4: Synthesis of 2-ethylhexyl oleate)
In a 100 mL four-neck flask equipped with a thermometer, a nitrogen inlet tube, a stirrer, a Dimroth condenser, and a 10 mL oil-water separation tube, 47.1 g of oleic acid (EXTRA OLEIN 99, manufactured by NOF Corp.) and 22.9 g of 2-ethylhexanol (2-ethylhexanol, manufactured by Mitsubishi Chemical Corp.) were charged (carboxylic acid/alcohol equivalent ratio=0.95). Then, 0.1 g of p-toluenesulfonic acid was charged as a catalyst. The subsequent steps were carried out in the same manner as in Synthesis Example 1, and 2-ethylhexyl oleate was obtained.

(Synthesis Example 5: Synthesis of tridecyl linoleate)
A 100 mL four-neck flask equipped with a temperature system, a nitrogen inlet tube, a stirrer, a Dimroth condenser, and a 10 mL oil-water separation tube was charged with 41.4 g of linolenic acid (EXTRA γ-LINOLEIC 99, manufactured by NOF Corp.) and 28.6 g of tridecyl alcohol (1-tridecanol, manufactured by TCI) (equivalent ratio of carboxylic acid/alcohol=1.03). Then, 0.1 g of p-toluenesulfonic acid was charged as a catalyst. The subsequent steps were carried out in the same manner as in Synthesis Example 1, and tridecyl linolenate was obtained.

[Preparation of ester base oil for lubricating oil]
Each of the esters in Synthesis Examples 1 to 5 was weighed into a flask so as to have the mass ratio shown in Table 1, and the mixture was heated to 80° C. and stirred for 30 minutes to prepare each of the ester base oils for lubricating oils in Examples 1 to 4 and Comparative Examples 1 and 2.

[Acid value]
The acid value of each base oil was measured in accordance with JIS C2101.
[Anti-emulsification test]
The demulsification property was evaluated based on JIS K 2520 and was evaluated based on the time it took for oil and water to separate. The evaluation was made as follows.
"◎": Less than 1 hour, "◯": 1 hour or more to less than 5 hours, "X": 5 hours or more. In addition, 50 ml of the lubricating ester base oil and 0.1 ml of water were placed in a 100-mL glass bottle, sealed in an air atmosphere, and left to stand in a constant temperature bath at 80° C. for 4 days, after which the demulsibility of the lubricating ester base oil was measured under the same conditions.

[Hydrolytic stability]
The hydrolytic stability of each base oil was evaluated. 50 ml of ester base oil for lubricating oil and 0.1 ml of water were placed in a 100 mL glass bottle, sealed in an air atmosphere, and left in a thermostatic bath at 80° C. for 4 days, after which the acid value was measured. Evaluation was performed by measuring the increase in acid value due to hydrolysis. The evaluation was as follows.
"◎": Less than 0.05 mgKOH/g "○": 0.05 mgKOH/g or more to less than 0.15 mgKOH/g "×": 0.15 mgKOH/g or more

[Wear resistance test]
Wear resistance was evaluated using an SRV tester (Optimol Schwingungs Reihungundund Verschleiss Tester Type 4). The SRV test was performed using a ball/disk, and the test pieces were made of SUJ-2. The test conditions were a test temperature of 120°C, a load of 100N, an amplitude of 1mm, and a vibration frequency of 50Hz, and the wear scar diameter was measured after a test time of 25 minutes. The evaluation was as follows.
"◎": Less than 500 μm "◯": 500 μm or more to less than 550 μm "X": 550 μm or more Furthermore, 50 ml of the lubricating ester base oil and 0.1 ml of water were placed in a 100-mL glass bottle, sealed in an air atmosphere, and allowed to stand in a thermostatic bath at 80° C. for 4 days. The wear resistance of the lubricating oil composition was then measured under the same conditions.

As is clear from the results shown in Table 2, the lubricating oil ester base oils 1 to 4 according to the present invention have excellent anti-emulsification properties, hydrolytic stability, and lubricity, and also have excellent durability even in the presence of water.

On the other hand, the ester base oils for lubricating oils having a mass ratio of ester (A) to ester (B) outside the range had good hydrolytic stability and wear resistance, but were poor in demulsibility and durability of hydrolytic stability.

Claims (2)

An ester base oil for lubricating oils, characterized in that the mass ratio ((A):(B)) of the following ester (A) and the following ester (B) is 80:20 to 30:70, the total amount of the following ester (A), the following ester (B) and the following ester (C) is 95 to 100 mass% based on the total mass of the ester base oil for lubricating oils, and when the following ester (C) is contained, the content of the following ester (C) based on the total mass of the ester base oil for lubricating oils is 10 mass% or less .

Ester (A): A monoester formed by the ester reaction of a branched monoalcohol having 8 to 14 carbon atoms with linoleic acid.

Ester (B): A monoester formed by the ester reaction of a branched monoalcohol having 8 to 14 carbon atoms with oleic acid.

Ester (C): A monoester formed by the ester reaction of a monoalcohol having 4 to 22 carbon atoms with linoleic acid.
A lubricating oil composition comprising 80 mass % or more of the ester base oil for lubricating oils according to claim 1.