Oil composition for damper

Abstract

A working oil composition for shock absorbers, which contains the following components (A) and (B): (A) an ester base oil having a pour point of-45 DEG C or less and a 10% distillation temperature of 300 DEG C or more as determined by the gas chromatography distillation test method specified in JIS K 2254: 1998; and (B) zinc dialkyl dithiophosphate.

Description

TECHNICAL FIELD

[0001] The present application relates to a working oil composition for a damper. BACKGROUND

[0002] Generally, a damper (shock absorber) used for a vehicle such as a four-wheeled vehicle, a two-wheeled vehicle, and the like absorbs an impact applied in association with a body motion such as rotation, acceleration / deceleration, and unevenness of a road surface by a damping force generated when working oil filled in the damper passes through a piston valve and an orifice by reciprocation of a piston rod. Further, friction with an oil seal is generated at the time of such reciprocation of the piston rod, but if the friction is too large, the oil seal is easily worn, or steering stability, ride comfort, and the like of the vehicle are affected. On the other hand, even in the case where the friction is too small, the steering stability of the vehicle is reduced, and ride comfort and the like are affected. Therefore, in order to impart the damper with friction within an appropriate range, various working oil compositions for a damper have been proposed in the past.

[0003] For example, in Japanese Patent Application Publication No. 2022-182560 (Patent Literature 1), a working oil composition for a damper containing a base oil and a friction modifier is proposed, and as the base oil, it is disclosed that mineral oil and / or synthetic oil can be used.

[0004] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Publication No. 2022-182560 SUMMARY

[0005] PROBLEMS TO BE SOLVED BY THE INVENTION However, the conventional working oil composition for a damper described in the above-described Patent Literature 1 has room for improvement in terms of simultaneously achieving flowability at low temperatures and reduction in the evaporation amount of the composition at the time of work at a high level.

[0006] Further, in recent years, the requirement for such low-temperature properties has become more stringent. From such a background, in recent years, a shock absorber oil composition having excellent low-temperature properties even at an outside air temperature of -40°C using a shock absorber has been required. Moreover, from the viewpoint of making the shock absorber exhibit excellent cushioning action or damping force even at low temperatures, for example, it is preferable that the pour point of the composition be -45°C or lower. With such a requirement, the conventional shock absorber oil composition has reduced the viscosity of the base oil by, for example, incorporating a solvent component having low viscosity, and has incorporated a viscosity index improver to some extent, thereby making the pour point of the composition -45°C or lower. However, if the solvent component is incorporated in a large amount according to the conventional formulation described above, the evaporation amount of the composition at the time of use increases and low evaporation cannot be achieved. In the case of using such a shock absorber oil composition having a large evaporation amount, there is a problem that the prescribed damping force cannot be maintained for a long period of time, and sometimes the situation that the shock absorber has to be replaced arises. From such a viewpoint, a new shock absorber oil composition having excellent low evaporation and low-temperature fluidity and excellent friction properties (mainly, properties capable of improving the followability of the load applied to the oil seal to the movement (displacement) of the piston rod, and capable of adjusting the friction to an appropriate size) has been required.

[0007] The present application was made in view of the problems of the prior art described above, and aims to provide a shock absorber oil composition capable of making all of the properties of low-temperature fluidity, low evaporation, and friction properties excellent.

[0008] Means for solving the problems The present inventors have conducted intensive studies in order to achieve the above-mentioned object, and as a result, have found that by making a shock absorber oil composition contain the following components (A) and (B), it is possible to make the low-temperature fluidity of the composition excellent and achieve a reduction in the evaporation amount, and that excellent friction properties (mainly, properties capable of improving the followability of the load applied to the oil seal to the movement (displacement) of the piston rod, and capable of adjusting (increasing) the friction to an appropriate size) can be obtained using the composition, thereby completing the present application.

[0009] That is, the present application provides the following solutions.

[0010] [1] A shock absorber oil composition containing the following components (A) and (B): (A) an ester base oil having a pour point of -45°C or lower and a 10% distillation temperature of 300°C or higher, which is determined according to the gas chromatography distillation test method prescribed in JIS K 2254:1998; and (B) zinc dialkyldithiophosphate.

[0011] [2] The working oil composition for a shock absorber according to [1], further comprising the following component (C): (C) at least one metal-based detergent selected from the group consisting of alkali metal sulfonates and alkaline earth metal sulfonates.

[0012] [3] The working oil composition for a shock absorber according to [1] or [2], wherein the ester base oil has a kinematic viscosity at 40°C of 5.0 mm 2 / s or more and 30.0 mm 2 / s or less.

[0013] [4] The working oil composition for a shock absorber according to any one of [1] to [3], wherein the ester base oil is composed of a carboxylic acid ester which is a reaction product of a carboxylic acid having 1 to 24 carbon atoms and an alcohol having 1 to 20 carbon atoms.

[0014] [5] The working oil composition for a shock absorber according to any one of [1] to [4], wherein the ester base oil is composed of a carboxylic acid ester which is a reaction product of a carboxylic acid having 1 to 18 carbon atoms and an alcohol having 1 to 6 carbon atoms.

[0015] [6] The working oil composition for a shock absorber according to any one of [1] to [5], wherein the content of the above-mentioned component (B) is 100 mass ppm or more and 1500 mass ppm or less in terms of zinc element based on the total amount of the composition.

[0016] Effects of Invention According to the present application, it is possible to provide a working oil composition for a shock absorber which has excellent all properties of low-temperature fluidity, low evaporation, and friction properties. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a schematic diagram showing an outline of a test machine used in a test for measuring friction properties.

[0018] Figure 2 is a schematic sectional view showing a relationship between an oil seal in a region R in Figure 1 and a piston rod when the test machine shown in Figure 1 is used.

[0019] Figure 3 is a graph showing a relationship between displacement and time and a relationship between load and time for approximately 1 cycle at a time of 30000 cycles obtained when the working oil composition for a shock absorber of Example 1 is used.

[0020] Figure 4are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 2 was used.

[0021] Figure 5 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 3 was used.

[0022] Figure 6 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 4 was used.

[0023] Figure 7 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 5 was used.

[0024] Figure 8 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 6 was used.

[0025] Figure 9 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 7 was used.

[0026] Figure 10 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 8 was used.

[0027] Figure 11 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 9 was used.

[0028] Figure 12 are graphs each showing a displacement-time relationship and a load-time relationship of approximately 1 cycle at 30000 cycle times obtained in the case where the working oil composition for a damper of Example 10 was used.

[0029] Figure 13are graphs each showing a displacement vs. time relationship and a load vs. time relationship for approximately 1 cycle at a time of 30000 cycles obtained in the case where the buffer operating oil composition of Example 11 was used.

[0030] Figure 14 are graphs each showing a displacement vs. time relationship and a load vs. time relationship for approximately 1 cycle at a time of 30000 cycles obtained in the case where the buffer operating oil composition of Comparative Example 1 was used.

[0031] Figure 15 are graphs each showing a displacement vs. time relationship and a load vs. time relationship for approximately 1 cycle at a time of 30000 cycles obtained in the case where the buffer operating oil composition of Comparative Example 4 was used. DETAILED DESCRIPTION

[0032] Hereinafter, the present application will be described in detail according to a preferred embodiment thereof. Note that in the present specification, as long as not particularly stated, the expression "X to Y" with respect to numerical values X and Y means "X or more and Y or less". In the case where only the unit of numerical value Y is added in the expression, the unit is also applied to the numerical value X.

[0033] The buffer operating oil composition of the present application contains the following components (A) and (B): (A) an ester base oil having a pour point of -45°C or lower and a 10% distillation temperature of 300°C or higher, which is obtained according to the gas chromatography distillation test method prescribed in JIS K 2254:1998; and (B) zinc dialkyldithiophosphate.

[0034] The buffer operating oil composition of the present application contains as an essential component component (A): an ester base oil having a pour point of -45°C or lower and a 10% distillation temperature (temperature at which the distillation amount reaches 10 area %) of 300°C or higher, which is obtained according to the gas chromatography distillation test method prescribed in JIS K 2254:1998.

[0035] In this way, the ester base oil (component (A)) contained as an essential component in the present application needs to have a pour point of -45°C or lower. By making the pour point of such a base oil be the above upper limit value or lower, even without using a solvent or the like, the low-temperature fluidity of the composition using the base oil can be made excellent. In particular, in the case where an ester base oil having a pour point of -45°C or lower is used, even without using a pour point depressant, the pour point of the finally obtained composition can be easily adjusted to be -45°C or lower, and a composition having a high level of low-temperature fluidity as required in the field of buffer operating oil compositions can be easily obtained. Note that in the present specification, as the "pour point" of a base oil and a composition, the pour point determined according to JIS K 2269:1987 is adopted.

[0036] The above ester base oil (component (A)) contained as an essential component in the present application needs to satisfy the condition that the 10% distillation temperature is 300°C or higher as determined by the gas chromatography distillation test method prescribed in JIS K2254:1998. When the 10% distillation temperature of such an ester base oil is 300°C or higher, the evaporation loss of the base oil at the time of use can be highly suppressed compared to the case where it is lower than 300°C, and the evaporation amount of the resulting composition can be reduced. From the same viewpoint, a higher effect can be obtained when the 10% distillation temperature of such an ester base oil is preferably 350°C or higher. Note that, as the "10% distillation temperature" referred to here, the temperature at which the distillation amount of the base oil reaches 10 area% relative to the entire chromatogram (total area) based on the percentage of the integrated area of the chromatogram is adopted, which is determined by the gas chromatography distillation test method prescribed in JIS K2254:1998.

[0037] In addition, the kinematic viscosity at 100°C of the above ester base oil (component (A)) is preferably 1.5 to 20 mm 2 / s, more preferably 2.0 to 11 mm 2 / s. By setting the kinematic viscosity at 100°C of the above ester base oil to 1.5 mm 2 / s or higher, a higher effect can be obtained in terms of suppression of evaporation loss compared to the case where it is less than 1.5 mm 2 / s. In addition, if the kinematic viscosity at 100°C is set to 20 mm 2 / s or lower, a higher effect can be obtained in terms of viscosity-temperature characteristics and friction characteristics at low oil temperature compared to the case where it exceeds 20 mm 2 / s.

[0038] In addition, the kinematic viscosity at 40°C of the above ester base oil is preferably 5.0 to 50 mm 2 / s, more preferably 5.0 to 30.0 mm 2 / s, further preferably 7.0 to 15.0 mm 2 / s. By setting the kinematic viscosity at 40°C of the above ester base oil to 5.0 mm 2 / s or higher, a higher effect can be obtained in terms of damping force compared to the case where it is less than 5.0 mm 2 / s. In addition, if the kinematic viscosity at 40°C is set to 50 mm 2 / s or lower, a higher effect can be obtained in terms of damping force compared to the case where it exceeds 50 mm 2 / s.

[0039] Further, the viscosity index of the above-mentioned ester base oil is preferably 100 or more, more preferably 110 or more, further preferably 120 or more, and particularly preferably 125 or more. By setting the viscosity index to the above lower limit or more, the change in viscosity of the working oil with respect to temperature change can be reduced, and a higher effect in maintaining stable damping force can be obtained.

[0040] Note that, in the present specification, as the "kinematic viscosity at 40°C", "kinematic viscosity at 100°C", and "viscosity index", the values measured in accordance with JIS K 2283: 1993 are used.

[0041] Further, the hydroxyl value of the ester base oil of the present application is preferably 20 or less, more preferably 15 or less, and further preferably 10 or less. By setting the hydroxyl value of the ester base oil to the above upper limit or less, the thermal / oxidative stability of the base oil can be improved. Note that, such a hydroxyl value can be set to a value less than 5, but in view of the relationship between the effect and economy, the value is preferably set to 0.01 or more (more preferably 0.50 or more). Note that, in the present specification, as the "hydroxyl value", the hydroxyl value measured by the indicator titration method of JIS K 0070 "Acid value, saponification value, iodine value, hydroxyl value, and unsaponifiable value of chemicals" is used.

[0042] Further, the above-mentioned ester base oil (component (A)) is preferably composed of a carboxylic acid ester which is a reaction product of at least one alcohol (may be a mixture of aliphatic alcohol and aromatic alcohol) selected from the group consisting of aliphatic alcohol and aromatic alcohol, and at least one carboxylic acid (may be a mixture of aliphatic carboxylic acid and aromatic carboxylic acid) selected from the group consisting of aliphatic carboxylic acid and aromatic carboxylic acid. As a preferable one of such carboxylic acid esters, for example, a reaction product of aliphatic alcohol and aliphatic acid can be cited. Further, as another one of such carboxylic acid esters, for example, a reaction product of aliphatic alcohol and aliphatic carboxylic acid and / or aromatic carboxylic acid can be cited. Note that, the hydrocarbon group in such aliphatic alcohol and aliphatic carboxylic acid can be a saturated hydrocarbon group or an unsaturated hydrocarbon group. Further, in the case where the above-mentioned alcohol is a mixture of two or more kinds of alcohol, it can be a mixture of saturated aliphatic alcohol and unsaturated aliphatic alcohol. Similarly, in the case where the above-mentioned carboxylic acid is a mixture of two or more kinds of carboxylic acid, it can be a mixture of saturated aliphatic carboxylic acid and unsaturated aliphatic carboxylic acid. Further, the hydrocarbon group in such aliphatic alcohol can be linear or branched.

[0043] As the above carboxylic acid, the number of carbon atoms is preferably 1 to 24 (more preferably 1 to 18, further preferably 5 to 16). Also, as the above alcohol, the number of carbon atoms is preferably 1 to 20 (more preferably 1 to 10, further preferably 2 to 6). By making the number of carbon atoms (C) of such a carboxylic acid be below the above upper limit, a higher effect can be obtained in terms of flowability at low temperatures, as compared with the case where the number of carbon atoms exceeds the above upper limit. Note that in the case where the number of carbon atoms (C) of the carboxylic acid is set to 5 or more, a higher effect can be obtained in terms of evaporation. Also, by making the number of carbon atoms (C) of the above alcohol be below the above upper limit, a higher effect can be obtained in terms of flowability at low temperatures, as compared with the case where the number of carbon atoms exceeds the above upper limit. Note that in the case where the number of carbon atoms (C) of the alcohol is set to 2 or more, a higher effect can be obtained in terms of evaporation. Note that such an alcohol and carboxylic acid are each preferably appropriately selected within the above preferred range of the number of carbon atoms, taking into account the valence number and the like, so that the carboxylic acid ester obtained by combining them satisfies the above conditions for the ester base oil (A).

[0044] Also, as one preferred mode of the above ester base oil, from the viewpoint of further improving flowability and evaporation at low temperatures, an ester base oil composed of a carboxylic acid ester that is a reaction product of a carboxylic acid having a number of carbon atoms of 1 to 24 (more preferably 1 to 18, further preferably 5 to 16) and an alcohol having a number of carbon atoms of 1 to 20 (more preferably 1 to 10, further preferably 2 to 6) can be cited.

[0045] Also, as the carboxylic acid ester that constitutes such an ester base oil (component (A)), the carboxylic acid esters of the modes described in (i) to (vii) below (in addition, they can be used alone or in combination with two or more) can be cited as preferred modes: (i) a carboxylic acid ester (so-called "monoester") that is a reaction product of at least one of linear or branched C1 to C24 aliphatic monocarboxylic acids and at least one of linear or branched C1 to C20 aliphatic monohydric alcohols; (ii) a carboxylic acid ester (so-called "diester") that is a reaction product of at least one of linear or branched C2 to C20 aliphatic dicarboxylic acids and at least one of linear or branched C1 to C20 aliphatic monohydric alcohols; (iii) a carboxylic acid ester (so-called "polyester") that is a reaction product of at least one of linear or branched C1 to C24 aliphatic monocarboxylic acids and at least one of C2 to C20 aliphatic polyhydric alcohols having 2 to 10 hydroxyl groups; (iv) a carboxylic acid ester (so-called "complex ester") which is a reaction product of a mixture of carboxylic acids composed of at least one of linear or branched C1-C24 aliphatic monocarboxylic acids and at least one of linear or branched C2-C20 aliphatic dicarboxylic acids, and at least one of C2-C20 aliphatic polyols having 2 to 10 hydroxyl groups; (v) a carboxylic acid ester which is a reaction product of at least one of linear or branched C2-C20 aliphatic dicarboxylic acids and at least one of C2-C20 aliphatic polyols having 2 to 10 hydroxyl groups; (vi) a carboxylic acid ester which is a reaction product of a mixture of carboxylic acids composed of at least one of linear or branched C2-C24 aliphatic monocarboxylic acids and at least one of linear or branched C2-C20 aliphatic dicarboxylic acids, and a linear or branched C1-C20 aliphatic monohydric alcohol; (vii) a carboxylic acid ester which is a reaction product of a mixture of carboxylic acids composed of linear or branched C2-C24 aliphatic monocarboxylic acids and linear or branched C2-C20 aliphatic dicarboxylic acids, and a mixture of alcohols composed of at least one of C2-C20 aliphatic polyols having 2 to 10 hydroxyl groups and at least one of C1-C20 aliphatic monohydric alcohols.

[0046] In addition, as one mode of the monoester which can be used for such an ester base oil, for example, the following monoester, which is a reaction product of at least one carboxylic acid (linear or branched C5 to C20 aliphatic monocarboxylic acid) selected from the group consisting of pentanoic acid, isopentanoic acid, hexanoic acid, isohexanoic acid, heptanoic acid, isoheptanoic acid, octanoic acid, isooctanoic acid, nonanoic acid, isononanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, palmitic acid, heptadecanoic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, nonadecanoic acid, and eicosanoic acid and at least one alcohol (linear or branched C1 to C18 aliphatic monoalcohol) selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, 1-octanol, 3-methyl-1-butanol, nonanol, decanol, undecanol, dodecanol, tridecanol, tetradecanol, pentadecanol, hexadecanol, heptadecanol, octadecanol, isopropyl alcohol, isobutyl alcohol, 2-octanol, 3-octanol, isononyl alcohol, isodecyl alcohol, isoundecyl alcohol, isododecyl alcohol, isotridecyl alcohol, isotetradecyl alcohol, isopentadecyl alcohol, isohexadecyl alcohol, isopentadecyl alcohol, isooctadecyl alcohol, neo-pentyl alcohol, tert-butyl alcohol, 2-methyl-2-butanol, 2,3-dimethyl-2-butanol, 2-methyl-2-pentanol, 3-methyl-3-pentanol, 3-ethyl-3-pentanol, 2,3-dimethyl-2-pentanol, 2,3-dimethyl-2-pentanol, 2,3-dimethyl-2-pentanol, 2,3-dimethyl-3-pentanol, 2,3,4-trimethyl-3-pentanol, 2-methyl-2-hexanol, and 3-methyl-3-hexanol can be exemplified. Note that the monoester which can be used for the ester base oil is not limited to the above mode, and as other modes, for example, the following can be exemplified: a reaction product of at least one of linear or branched C12 to C20 monocarboxylic acid and at least one of linear or branched C6 to C16 monoalcohol; a reaction product of at least one of linear or branched C14 to C18 monocarboxylic acid and at least one of linear or branched C6 to C12 monoalcohol; and the like. In addition, as the monoester which can be preferably used for the ester base oil, for example, 2-ethylhexyl oleate, 2-ethylhexyl coco fatty acid ester, 2-ethylhexyl palmitate, 2-ethylhexyl stearate, 2-ethylhexyl tallow fatty acid ester, and the like can be exemplified.

[0047] Further, as one mode of the diester usable for such an ester base oil, for example, the following diester, which is a reaction product of at least one kind of carboxylic acid (linear or branched C3 to C12 aliphatic dicarboxylic acid) selected from the group consisting of malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, azelaic acid, sebacic acid, brassic acid, docdecanedioic acid, diglycolic acid, 1,4-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, and 2,6-decahydro-naphthalene dicarboxylic acid, and at least one kind of alcohol (linear or branched C1 to C14 aliphatic monohydric alcohol) selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, isoamyl alcohol, isohexyl alcohol, isoheptyl alcohol, 2-ethylhexyl alcohol, 2-propylheptyl alcohol, 2-propyl-4-methyl-hexyl alcohol, 2-propyl-5-methyl-hexyl alcohol, 2-isopropyl-4-methyl-hexyl alcohol, 2-isopropyl-5-methyl-hexyl alcohol, 2-propyl-4,4-dimethylpentyl alcohol, 2-ethyl-2,4-dimethylhexyl alcohol, 2-ethyl-2-methyl-heptyl alcohol, 2-ethyl-2,5-dimethylhexyl alcohol, 2-isopropyl-heptyl alcohol, 2-butyl-1-octanol, and 2-pentyl-1-nonanol, can be exemplified. Note that the diester usable for the above ester base oil is not limited to the above mode, and as other modes, for example, the following diester, which is a reaction product of at least one kind of linear or branched C3 to C12 aliphatic dicarboxylic acid and at least one kind of branched C5 to C16 aliphatic monohydric alcohol; the following diester, which is a reaction product of linear or branched C6 to C8 aliphatic dicarboxylic acid and branched C6 to C14 aliphatic monohydric alcohol; and the like, can be exemplified. Further, as the diester that can be preferably used for the ester base oil, for example, diisodecyl adipate, diisotridecyl adipate, di(isopropylheptyl)adipate (DPHA), and diisononyl adipate (DNA) can be exemplified.

[0048] Further, as other modes of the diester usable for the ester base oil, for example, the following diester (dicarboxylic acid ester), which is a reaction product of at least one kind of dicarboxylic acid selected from the group consisting of adipic acid, phthalic acid, pimilic acid, suberic acid, azelaic acid, and sebacic acid and a branched aliphatic alcohol represented by the following general formula (1) (hereinafter, sometimes referred to as "branched aliphatic alcohol R-OH" for convenience) can be exemplified. Note that as such a branched aliphatic alcohol R-OH represented by the general formula (1), a C7 to C12 primary alcohol can be preferably used.

[0049] (1) [a, b and c in formula (1) are values selected in such a manner that the sum (a+b) of a and b is any integer from 4 to 9, a is any integer from 1 to 8, b is any integer from 1 to 6, and c is any integer from 0 to 5, satisfying all of the above conditions.] Further, as the residue R (branched alkyl group) after removal of the hydroxyl group (-OH) in the above general formula (1), depending on the values of a, b and c in the formula, one of the branches becomes the main chain and the other forms a side chain, and the alkyl group (alkyl side chain) on the side chain side is preferably a C1 to C6 alkyl group. Further, the part of the main chain of the above residue R is a C6 to C11 straight chain alkyl group in formula (1) where a+b is 4 to 9 and c is 0 to 5.

[0050] Further, as the residue R (branched alkyl group) after removal of the hydroxyl group (-OH) in the above general formula (1), depending on the values of a, b and c in the formula, one of the branches becomes the main chain and the other forms a side chain, and the alkyl group (alkyl side chain) on the side chain side is preferably a C1 to C6 alkyl group. Further, the part of the main chain of the above residue R is a C6 to C11 straight chain alkyl group in formula (1) where a+b is 4 to 9 and c is 0 to 5.

[0051] Further, the diester (dicarboxylic acid ester) of the above mode (ii) which is one of the preferred modes of the above ester base oil is a reaction product of at least one of straight chain or branched C2 to C20 aliphatic dicarboxylic acids and at least one of straight chain or branched C1 to C20 aliphatic monohydric alcohols. Here, as a preferred one of such diesters, a diester obtained using any one of adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid and a mixture thereof as the above "straight chain or branched C2 to C20 aliphatic dicarboxylic acid" can be cited. Further, as a preferred one of such diesters, a diester obtained using a straight chain or branched C5 to C20 fatty alcohol (more preferably a straight chain or branched C9 to C15 fatty alcohol) as the above "straight chain or branched C1 to C20 aliphatic monohydric alcohol" can be cited. Furthermore, as a particularly preferred one of such diesters, a diester obtained using any one of nonanol, isodecyl alcohol, isotridecyl alcohol and 2-propylheptanol as the above "straight chain or branched C1 to C20 aliphatic monohydric alcohol" can be cited.

[0052] Further, as one example of the polyester which can be used for the above-mentioned ester base oil, for example, a reaction product of at least one carboxylic acid (linear or branched C1-C24 aliphatic monocarboxylic acid) selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, oleic acid, palmitoleic acid, erucic acid, ricinoleic acid, linoleic acid, and linolenic acid, and at least one alcohol (C2-C20 aliphatic polyhydric alcohol having 2 to 10 hydroxyl groups) selected from the group consisting of ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, neopentyl glycol, trimethylolpropane, trimethylolethane, pentaerythritol, sorbitol, and dipentaerythritol can be exemplified.

[0053] Further, as another example of the polyester which can be used for the above-mentioned ester base oil, a reaction product of at least one linear or branched C8-C18 aliphatic monocarboxylic acid and at least one C3-C16 polyhydric alcohol having 2 to 6 hydroxyl groups can be exemplified. Further, as another example of the above-mentioned polyester, a reaction product of at least one linear or branched C14-C18 monocarboxylic acid and at least one C4-C12 polyhydric alcohol having 3 to 5 hydroxyl groups can be exemplified.

[0054] As a preferable example of the polyester which can be used for the ester base oil, a neopentyl glycol type ester can be exemplified. In such a neopentyl glycol ester, as a preferable example, a reaction product of neopentyl glycol and a C5-18 (more preferably C5-16) aliphatic monocarboxylic acid (for example, neopentyl glycol 2-ethylhexanoate) can be exemplified. Further, as another preferable example of the polyester, a pentaerythritol type ester can be exemplified. As such a pentaerythritol type ester, as a preferable example, a reaction product of pentaerythritol and a C5-18 aliphatic monocarboxylic acid (for example, pentaerythritol 2-ethylhexanoate) can be exemplified.

[0055] Further, as one of the complex esters usable in the above-mentioned ester base oil, for example, a reaction product of at least one carboxylic acid selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, oleic acid, palmitoleic acid, erucic acid, ricinoleic acid, linoleic acid and linolenic acid (linear or branched C1 to C24 aliphatic monocarboxylic acid) and at least one carboxylic acid selected from the group consisting of malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid (octanedioic acid), azelaic acid and sebacic acid (linear or branched C3 to C10 aliphatic dicarboxylic acid) and at least one alcohol selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, neopentyl glycol, trimethylolpropane, trimethylolethane, pentaerythritol, sorbitol and dipentaerythritol (C2 to C20 polyhydric alcohol having 1 to 10 hydroxyl groups) and the like.

[0056] Thus, the ester base oil of the above-mentioned component (A) can be appropriately used from the ester base oils composed of the carboxylic acid esters of various modes as described above, and is not particularly limited, and the ester base oil composed of the above-mentioned polyesters is preferable, and the ester base oil composed of the reaction product (polyester) of at least one of C1 to C18 aliphatic monocarboxylic acid and at least one of C1 to C10 (further preferably C2 to C6) aliphatic polyhydric alcohol having 2 to 6 (more preferably 2 to 4) hydroxyl groups is more preferable. Note that, among the raw materials of the above-mentioned reaction product (polyester) preferably used for the ester base oil, i.e., C1 to C18 aliphatic monocarboxylic acid, from the viewpoint of fluidity at low temperature, formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, 2-ethylhexanoic acid are preferable, and more preferably formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, 2-ethylhexanoic acid. Further, as the above-mentioned C2 to C6 aliphatic polyhydric alcohol used as the raw material of the above-mentioned reaction product (polyester) preferably used for the ester base oil, ethylene glycol, propylene glycol, glycerol, butanediol, neopentyl glycol, trimethylolethane, pentaerythritol, trimethylolpropane, sorbitol are more preferable. Note that, the ester base oil of component (A) can be composed of only one of the above-mentioned carboxylic acid esters, or two or more of the above-mentioned carboxylic acid esters can be combined.

[0057] In addition, in the working oil composition for a shock absorber of the present application, the content of the above-mentioned component (A) is preferably 70 to 99 mass%, more preferably 80 to 99 mass%, further preferably 85 to 99 mass%, particularly preferably 90 to 99 mass%, based on the total amount of the working oil composition for a shock absorber (based on the total amount of the composition). If the content of such component (A) is set to be the above-mentioned lower limit or more, a higher effect in terms of being able to impart stable friction characteristics can be obtained compared to the case where it is less than the above-mentioned lower limit. On the other hand, if the content of component (A) is set to be the above-mentioned upper limit or less, a higher effect in terms of performance impartment by the additive can be obtained compared to the case where it exceeds the above-mentioned upper limit.

[0058] In the working oil composition for a shock absorber of the present application, the above-mentioned component (A) is a necessary component, and therefore the component of the base oil constituting the working oil for a shock absorber contains at least the above-mentioned component (A). Here, the base oil (total base oil) of the working oil for a shock absorber can be composed only of component (A), or can also be a mixed base oil containing component (A) and other base oil components within a range that does not impair the effects obtained by using component (A). As such other base oil components, there is no particular limitation, and mineral oil-based base oils, synthetic base oils, and the like can be cited.

[0059] As the mineral oil-based base oil that can be used as such other base oil components, for example, solvent refined mineral oils, hydrocracked mineral oils, hydrorefined mineral oils, solvent dewaxed base oils, and the like having a kinematic viscosity at 100°C of 1 to 100 mm 2 / s can be cited.

[0060] In the case where the base oil (total base oil) of the working oil composition for a shock absorber contains component (A) and other base oil components, the other base oil components can be used alone by one kind, or can also be used in combination with two or more kinds. In addition, in the case where the base oil (total base oil) of the working oil composition for a shock absorber is composed of a mixed base oil of component (A) and other base oil components, the content ratio of component (A) in the mixed base oil is preferably 75 mass% or more, more preferably 81 mass% or more, further preferably 85 mass% or more, particularly preferably 90 mass% or more, based on the total amount of the mixed base oil.

[0061] In addition, from the viewpoint of improving the friction characteristics, the base oil (total base oil) of the above-mentioned working oil composition for a shock absorber is more preferably composed only of component (A).

[0062] Further, in the case where the base oil (total base oil) of the above-mentioned shock absorber operating oil composition is constituted by a mixed base oil of component (A) and another base oil component, the pour point thereof is preferably -45°C or lower, in order not to impair the effects obtained by using component (A). By making the pour point of such a base oil be the above-mentioned upper limit or lower, the low-temperature fluidity of the composition using this base oil can be efficiently improved, as compared with the case where it exceeds the above-mentioned upper limit.

[0063] Further, in the case where the base oil (total base oil) of the above-mentioned shock absorber operating oil composition is constituted by a mixed base oil of component (A) and another base oil component, the evaporation property thereof is preferably such that the 10% distillation temperature of the mixed base oil is 300°C or higher, from the viewpoint of not impairing the effects (particularly, the low evaporation property) of the present application by using the mixed base oil thereof. When the 10% distillation temperature in gas chromatography distillation is lower than the above-mentioned lower limit value, there is a tendency that the evaporation property of the composition as a whole using this mixed base oil decreases.

[0064] In the case where the base oil (total base oil) of the above-mentioned shock absorber operating oil composition is constituted by a mixed base oil of component (A) and another base oil component, it is preferable that the kinematic viscosity at 100°C, the kinematic viscosity at 40°C and the viscosity index of the mixed base oil be substantially the same ranges as the preferable ranges of the kinematic viscosity at 100°C, the kinematic viscosity at 40°C and the viscosity index of the ester base oil of component (A), respectively. Note that, in the case where the base oil (total base oil) of the above-mentioned shock absorber operating oil is a mixed base oil of component (A) and another base oil component, the content of the mixed base oil is preferably 81 to 99% by mass, more preferably 90 to 99% by mass, based on the total amount of the composition.

[0065] The shock absorber operating oil composition of the present application contains the above-mentioned component (A) and, as component (B), a zinc dialkyldithiophosphate (ZnDTP) as essential components. As such a zinc dialkyldithiophosphate (component (B)), in the field of lubricating oils and operating oils, a zinc dialkyldithiophosphate known as so-called anti-wear agent can be appropriately used, and there is no particular limitation, and a zinc dialkyldithiophosphate (ZnDTP) represented by the following general formula (2) is particularly preferable.

[0066] (2) [In formula (2), R 1 ~R 4 each independently represents a linear or branched alkyl group having 1 to 24 carbon atoms (more preferably 3 to 18, further preferably 3 to 8). R 1 ~R 4each independently represents a linear or branched alkyl group having 1 to 24 carbon atoms, and can be a combination of different groups. In addition, R 1 ~R 4 The number of carbon atoms of R 1 ~R 4 may be any one of a primary alkyl group, a secondary alkyl group, and a tertiary alkyl group, is preferably a primary alkyl group or a secondary alkyl group or a combination thereof, and is further preferably a primary alkyl group and a secondary alkyl group in a molar ratio (primary alkyl group: secondary alkyl group) of 100:0 to 50:50. The ratio can be a combination ratio of alkyl chains within a molecule, or can be a mixing ratio of ZnDTP having a primary alkyl group and ZnDTP having a secondary alkyl group. In this way, by making the primary alkyl group predominant, fuel consumption performance can be further improved. In addition, the method of producing ZnDTP is not particularly limited, and a known method can be appropriately used. For example, the following method can be employed: an alcohol having an alkyl group corresponding to R 1 ~R 4 is reacted with phosphorus pentasulfide to synthesize dithiophosphoric acid, which is neutralized with zinc oxide to thereby synthesize.

[0067] As such a zinc dialkyldithiophosphate, a zinc dithiophosphate having a primary alkyl group (primary type ZnDTP) or a zinc dithiophosphate containing a secondary alkyl group (secondary type ZnDTP) is preferred, in which a higher effect is obtained in terms of friction characteristics (mainly in terms of increasing the friction in a moderate range at the contact portion of the rubber oil seal of the damper and the piston rod) and improvement in oxidation stability (ISOT), and thus the primary type ZnDTP is more preferred. Note that in the case where the zinc dialkyldithiophosphate is used as a mixture of the primary type ZnDTP and the secondary type ZnDTP, the content of the primary type ZnDTP in the above mixture is preferably 50% by mass or more (more preferably 70% by mass or more), particularly from the viewpoint of improving the oxidation stability (ISOT).

[0068] In addition, in the working oil composition for a shock absorber of the present application, the content of the above-mentioned (B) component is preferably 100 mass ppm or more and 1500 mass ppm or less (more preferably 150 mass ppm or more and 1500 mass ppm or less, further preferably 200 mass ppm or more and 1500 mass ppm or less) in terms of zinc element based on the total amount of the composition. In addition, from the viewpoint of friction characteristics, the lower limit of the content of the above-mentioned (B) component is more preferably 250 mass ppm, and the upper limit of the content of the above-mentioned (B) component is more preferably 1400 mass ppm, further preferably 1200 mass ppm, and particularly preferably 1000 mass ppm or less. Thus, as the content of the above-mentioned (B) component, as a preferable example, 250 mass ppm or more and 1500 mass ppm or less (more preferably 250 mass ppm or more and 1200 mass ppm or less, further preferably 250 mass ppm or more and 1000 mass ppm or less) in terms of zinc element based on the total amount of the composition can be cited. By setting the content of the component (B) to the above-mentioned lower limit or more, a higher effect of increasing the friction in a moderate range can be obtained compared to the case of being lower than the above-mentioned lower limit. On the other hand, by setting the content of the component (B) to the above-mentioned upper limit or less, a higher effect of heat / oxidation stability can be obtained compared to the case of exceeding the above-mentioned upper limit.

[0069] In addition, in the working oil composition for a shock absorber of the present application, from the viewpoint of further improving the effect of increasing the friction to a moderate range, the content of the above-mentioned (B) component is preferably 0.1 to 1.0 mass % (more preferably 0.5 to 1.0 mass %) based on the total amount of the composition.

[0070] In addition, the working oil composition for a shock absorber of the present application preferably further contains at least one metal-based detergent (metal sulfonate-based detergent) selected from the group consisting of alkali metal sulfonates and alkaline earth metal sulfonates as a component (C). By using such a component (C) in combination with the component (B), the friction between the piston rod and the oil seal in the shock absorber can be adjusted to a moderate range, and the friction can be made more stable.

[0071] As the above-mentioned alkali metal sulfonate, as a preferable example, an alkali metal salt of an alkyl aromatic sulfonic acid obtained by sulfonating an alkyl aromatic compound, an alkali salt thereof, and a high alkali salt thereof can be cited. In addition, as the above-mentioned alkaline earth metal sulfonate, as a preferable example, an alkaline earth metal salt of the above-mentioned alkyl aromatic sulfonic acid, an alkali salt thereof, and a high alkali salt thereof can be cited. In addition, as the alkali metal, sodium or potassium is preferable, and as the alkaline earth metal, calcium or magnesium is preferable.

[0072] Further, the weight average molecular weight of the above-mentioned alkyl aromatic compound is preferably 400 to 1500, more preferably 700 to 1300. As examples of the above-mentioned alkyl aromatic sulfonic acid, for example, so-called petroleum sulfonic acid, synthetic sulfonic acid can be listed. As the petroleum sulfonic acid mentioned here, for example, those obtained by sulfonating alkyl aromatic compounds of lubricating oil fractions of mineral oil, so-called petroleum acid produced as a by-product when white oil is manufactured, and the like can be listed. Further, as one example of the synthetic sulfonic acid, those obtained by recovering a by-product in an alkylbenzene manufacturing facility of a raw material for a detergent, or by sulfonating alkylbenzene having a linear or branched alkyl group obtained by alkylating benzene with a polyolefin can be listed. As another example of the synthetic sulfonic acid, those obtained by sulfonating dialkylnaphthalene or the like can be listed. Further, as a sulfonating agent at the time of sulfonating these alkyl aromatic compounds, there is no particular limitation, and for example, fuming sulfuric acid, sulfuric anhydride can be used.

[0073] Further, among such alkali metal sulfonates and alkaline earth metal sulfonates, from the viewpoint of the friction characteristics, an alkaline earth metal sulfonate is more preferable, and a calcium sulfonate is particularly preferable.

[0074] Further, the base number of such a metal-based purifier (component (C)) is not particularly limited, and from the viewpoint of being able to obtain a higher effect in terms of the friction characteristics, it is preferably 200 to 500 mg·KOH / g (more preferably 250 to 400 mg·KOH / g). Note that, as the "base number" mentioned here, the base number determined according to JIS K2501: Base Number by Perchloric Acid Method is adopted.

[0075] Further, in the shock absorber operating oil composition of the present application, the content of the above-mentioned metal-based purifier (component (C)) is not particularly limited, and it is preferably 0.001 to 0.10% by mass, more preferably 0.001 to 0.07% by mass, further preferably 0.001 to 0.06% by mass, particularly preferably 0.001 to 0.05% by mass, and most preferably 0.001 to 0.03% by mass, based on the total amount of the composition. Further, the lower limit value of the content of component (C) is more preferably 0.003% by mass (further preferably 0.005% by mass, particularly preferably 0.007% by mass, and most preferably 0.009% by mass). By setting the content of such a metal-based purifier (component (C)) to the above-mentioned range, a higher effect in terms of the friction characteristics can be obtained.

[0076] In addition, the content of the above-mentioned metal-based purifier (component (C)) is preferably 1 mass ppm or more and 121 mass ppm or less, calculated as a metal element (for example, in the case where the metal-based purifier is calcium sulfonate, calculated as calcium element). The lower limit of the range of the content of such component (C) is more preferably 4 mass ppm, more preferably 5 mass ppm, more preferably 6 mass ppm, more preferably 8 mass ppm, more preferably 10 mass ppm, and particularly preferably 11 mass ppm. In addition, the upper limit of the range of the content of the above-mentioned component (C) is more preferably 85 mass ppm, more preferably 73 mass ppm, more preferably 61 mass ppm, more preferably 50 mass ppm, and particularly preferably 36 mass ppm. In addition, as a preferable example, the content of the above-mentioned metal-based purifier (component (C)) calculated as a metal element is 5 mass ppm or more and 50 mass ppm or less (more preferably 10 mass ppm or more and 50 mass ppm or less, and further preferably 20 mass ppm or more and 50 mass ppm or less) can be cited. By setting the content of such metal-based purifier (component (C)) to the above-mentioned lower limit or more, a higher effect on the friction characteristics can be obtained compared to the case where it is lower than the above-mentioned lower limit. On the other hand, by setting the content of the metal-based purifier (component (C)) to the above-mentioned upper limit or less, a higher effect on the friction characteristics can be obtained compared to the case where it exceeds the above-mentioned upper limit.

[0077] In addition, the working oil for a shock absorber of the present application preferably further contains a viscosity index improver (hereinafter, sometimes referred to as "component (D)"). As such viscosity index improver (component (D)), there is no particular limitation, and a publicly known compound generally used as a viscosity index improver for lubricating oil (any compound can be appropriately selected from publicly known compounds as a viscosity index improver and used) can be appropriately used.

[0078] In addition, the content of the above-mentioned viscosity index improver is not particularly limited, and for example, can be 0.01 mass% or more and 20 mass% or less, based on the composition. The content of such viscosity index improver (the content of the polymer used as a viscosity index improver) is preferably 20 mass% or less (more preferably 15 mass% or less, and further preferably 10 mass% or less), based on the total amount of the composition. In addition, the content of the viscosity index improver (the content of the polymer used as a viscosity index improver) is preferably 0.01 mass% or more (more preferably 0.1 mass% or more), based on the total amount of the composition. By setting the content of such viscosity index improver to 20 mass% or less, a composition having more excellent shear stability can be obtained, and on the other hand, by setting it to 0.01 mass% or more, the effect of improving the low-temperature viscosity characteristics can be further improved.

[0079] In addition, the shock absorber operating oil of the present application can appropriately incorporate, in addition to the above-mentioned component (C) and the above-mentioned component (D), any additive used in a shock absorber operating oil. As the additive, specifically, there can be mentioned an antioxidant, an ashless dispersant, an extreme pressure agent, an anti-wear agent other than the above-mentioned component (B), a friction modifier, a pour point depressant, a metal deactivator, an antifoaming agent, a metal-based detergent other than the above-mentioned component (C), an anticorrosive agent, an antirust agent, an antifoaming agent, a seal swell agent, a coloring agent, and the like. These additives can be used singly at 1 kind, or two or more kinds can be used in combination. Note that, from the viewpoint of improving the friction characteristics, the shock absorber operating oil of the present application is more preferably one that contains only the above-mentioned component (B) as the anti-wear agent (does not contain an anti-wear agent other than the above-mentioned component (B)). In addition, in the case where the shock absorber operating oil of the present application contains a metal-based detergent in its composition, from the viewpoint of improving the friction characteristics, as the above-mentioned metal-based detergent, it is more preferable to contain only the above-mentioned component (C) (does not contain a metal-based detergent other than the above-mentioned component (C)).

[0080] As the above-mentioned antioxidant, any compound generally used as an antioxidant in the field of lubricating oils such as shock absorber operating oils can be used, and, for example, there can be mentioned 2,6-di-tert-butyl-p-cresol, 4,4'-methylenebis(2,6-di-tert-butylphenol, octyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 3-methyl-5-tert-butyl-4-hydroxyphenyl-substituted fatty acid esters, and the like phenol-based antioxidants, phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, dialkyldiphenylamine, and the like amine-based antioxidants, and the like. Such an antioxidant can generally be contained in the shock absorber operating oil of the present application in a range of 0.01 to 5% by mass, based on the total amount of the composition.

[0081] As the above-mentioned pour point depressant, any compound generally used as a flowability improver in the field of lubricating oils such as shock absorber operating oils can be used, and, for example, there can be mentioned polymethacrylate-based flowability improvers, and the like.

[0082] Note that, in the case where the above-mentioned additive is contained in the shock absorber operating oil composition of the present application, the content (content based on the total amount of the composition) of these additives is not particularly limited, and, in the case where the above-mentioned additive is a pour point depressant, an anticorrosive agent, an antirust agent, and an antifoaming agent, it is preferable to set the range of 0.005 to 5% by mass for each additive, and, in the case where the above-mentioned additive is a metal deactivator, it is preferable to set the range of 0.005 to 1% by mass, and, further, in the case where the above-mentioned additive is an antifoaming agent, it is preferable to set the range of 0.0005 to 1% by mass.

[0083] Further, the shock absorber operating oil composition of the present application is preferably a composition having a pour point (determined according to JIS K 2269: 1987) of -45°C or lower. By making such a pour point be the above upper limit or lower, in recent years, a composition satisfying a high level of low-temperature fluidity required in the field of shock absorber operating oils can be produced.

[0084] Further, the shock absorber operating oil composition of the present application is preferably a composition having a 10% distillation temperature of 300°C or higher (more preferably 350°C or higher). By setting the 10% distillation temperature to the above lower limit or higher, generation of evaporation loss can be highly suppressed.

[0085] Further, the shock absorber operating oil composition of the present application preferably has a kinematic viscosity at 100°C of 2.0 to 15.0 mm 2 / s, more preferably 3.0 to 10.0 mm 2 / s. By setting the kinematic viscosity at 100°C of the composition to the above lower limit or higher, a higher effect in terms of damping force can be obtained compared to the case of being lower than the above lower limit. Further, by setting the kinematic viscosity at 100°C to the above upper limit or lower, a higher effect in terms of damping force can be obtained.

[0086] Further, the shock absorber operating oil composition of the present application preferably has a kinematic viscosity at 40°C of 8.0 to 50.0 mm 2 / s, more preferably 10.0 to 40.0 mm 2 / s. By setting the kinematic viscosity at 40°C of the composition to the above lower limit or higher, a higher effect in terms of damping force can be obtained compared to the case of being lower than the above lower limit. Further, by setting the kinematic viscosity at 40°C to the above upper limit or lower, a higher effect in terms of damping force can be obtained.

[0087] Further, the shock absorber operating oil composition of the present application preferably has a viscosity index of 100 or higher, more preferably 120 or higher, further preferably 140 or higher. By setting such a viscosity index to the above lower limit or higher, a higher effect in terms of suppression of viscosity change occurring with respect to temperature change can be obtained.

[0088] Examples Hereinafter, the present application will be described more specifically based on examples and comparative examples, but the present application is not limited to the following examples.

[0089] 〔About the components used in the examples and comparative examples〕 <Base oil> 〈(A) Component: Ester base oil: ester base oil satisfying both the condition of a pour point of -45°C or lower and the condition of a 10% distillation temperature of 300°C or higher〉 • Ester base oil (A-1): Ester base oil obtained by reacting neopentyl glycol with 2-ethylhexanoic acid (10% distillation temperature: 351°C, pour point: temperature of less than -45°C (< -45°C), kinematic viscosity at 100°C: 2.1 mm 2 / s, kinematic viscosity at 40°C: 7.4 mm 2 / s, viscosity index: 53) • Ester base oil (A-2): Ester base oil obtained by reacting pentaerythritol with valeric acid (10% distillation temperature: 425°C, pour point: < -45°C, kinematic viscosity at 100°C: 3.6 mm 2 / s, kinematic viscosity at 40°C: 15.7 mm 2 / s, viscosity index: 114) • Ester base oil (A-3): Ester base oil obtained by reacting trimethylolpropane with octanoic acid (55 mole%), decanoic acid (45 mole%) (10% distillation temperature: 465°C, pour point: < -45°C, kinematic viscosity at 100°C: 4.5 mm 2 / s, kinematic viscosity at 40°C: 20.4 mm 2 / s, viscosity index: 137).

[0090] (A') Component: Mineral oil base oil (hereinafter, sometimes abbreviated as "mineral oil"): Other base oil component than the (A) component • Mineral oil (A'-1): Paraffin-based mineral oil obtained by hydrogenation treatment (API-based base oil group: Group II, 10% distillation temperature: 298°C, pour point: -30°C, kinematic viscosity at 100°C: 2.5 mm 2 / s, kinematic viscosity at 40°C: 8.9 mm 2 / s, viscosity index: 94) • Mineral oil (A'-2): Paraffin-based mineral oil obtained by hydrogenation treatment (API-based base oil group: Group I, 10% distillation temperature: 295°C, pour point: -25°C, kinematic viscosity at 100°C: 2.0 mm 2 / s, kinematic viscosity at 40°C: 6.5 mm 2 / s, viscosity index: 98) • Mineral oil (A'-3): Paraffin-based mineral oil obtained by hydrogenation treatment (API-based base oil group: Group I, 10% distillation temperature: 378°C, pour point: -15°C, kinematic viscosity at 40°C: 20 mm 2 / s, kinematic viscosity at 100°C: 4.1 mm 2 / s, viscosity index: 100).

[0091] < (A") Component: Ester base oil (hereinafter, sometimes abbreviated as "other ester base oil") that does not satisfy the condition that the 10% distillation temperature is 300°C or higher > Other base oil components other than the (A) component • Other ester base oil (A"-1): Ester base oil obtained by reacting 2-ethylhexanol with butyric acid (10% distillation temperature: 224°C, pour point: <-45°C, kinematic viscosity at 100°C: 0.7 mm 2 / s, kinematic viscosity at 40°C: 1.5 mm 2 / s, viscosity index: not applicable) • Other ester base oil (A"-2): Ester base oil obtained by reacting 2-ethylhexanol with 2-ethylhexanoic acid (10% distillation temperature: 298°C, pour point: <-45°C, kinematic viscosity at 100°C: 1.1 mm 2 / s, kinematic viscosity at 40°C: 2.7 mm 2 / s, viscosity index: not applicable).

[0092] Note that, regarding the properties of the base oil and the aforementioned base oils ((A) to (A") components) that are constituent components thereof, the 10% distillation temperature is a measured value obtained by determining the relationship between the amount of distillation and the temperature (chromatogram) according to the gas chromatography distillation test method prescribed in JIS K 2254:1998, and taking the temperature at which 10% of the total area (total area) of the chromatogram is reached as the 10% distillation temperature, with the percentage of the integral area of the chromatogram as the basis. In addition, the pour point of each of the base oils and the aforementioned base oils ((A) to (A") components) that are constituent components thereof is a value measured according to JIS K 2269:1987, and the kinematic viscosity (at 40°C, at 100°C) and the viscosity index are values measured according to JIS K 2283:1993, respectively. Among these, the ester base oils (A"-1) and (A"-2) have a viscosity for which the test method for the viscosity index is not applicable, and therefore "not applicable" is noted for the viscosity index.

[0093] < (B) Component: Anti-wear agent> • Anti-wear agent (B-1): Zinc dialkyldithiophosphate (ZnDTP (I): manufactured by Chevron Oronite Co., trade name: OLOA 269R, primary ZnDTP, zinc content: 8.0 mass%, phosphorus content: 7.0 mass%, sulfur content: 14 mass%) • Anti-wear agent (B-2): Zinc dialkyldithiophosphate (ZnDTP (II): manufactured by Chevron Oronite Co., trade name: OLOA 262, secondary ZnDTP, zinc content: 7.9 mass%, phosphorus content: 7.2 mass%, sulfur content: 14 mass%) • Anti-wear agent (B-3): Dialkyl hydrogen phosphite (manufactured by SC Organic Chemicals Co., Ltd., trade name: Chelex H-18D) wherein the anti-wear agent (B-3) is used as a comparative component of zinc dialkyldithiophosphate. In addition, the content of Zn (zinc) with respect to the total amount of the composition when the above-described ZnDTP (I) is used at a proportion of 0.5 mass% is 450 mass ppm. In addition, with respect to Examples and the like in which the above-described ZnDTP (I) and (II) are used as the (B) component, the content of the (B) component in terms of zinc with respect to the total amount of the composition is shown in Tables 1 and 2.

[0094] < (C) Component: Metal deactivator > Metal deactivator (C-1): Calcium sulfonate (manufactured by LANXESS Co., Ltd., trade name: Hybase C-311, base number (TBN): 300 mgKOH / g, content of calcium atom: 28 mass%).

[0095] < (D) Component: Viscosity index improver > • Viscosity index improver (D-1): Dispersed polymethacrylate (manufactured by Sanyo Chemical Industries, Co., Ltd., trade name: ACLUBE 845, Mw: 50,000).

[0096] < (E) Component: Antioxidant > • Antioxidant (E-1): 2,6-di-tert-butyl-p-cresol (DBPC: manufactured by LANXESS Co., Ltd., trade name: Vulkanox BHT).

[0097] < (F) Component: Pour point depressant > Pour point depressant (F-1): Polymethacrylate-based pour point depressant (manufactured by DKS Co., Ltd., trade name: Lubran 141, Mw: 183,000).

[0098] (Examples 1 to 11 and Comparative Examples 1 to 11) A working oil composition for a shock absorber was prepared using each of the above-described components in such a manner as to become the composition shown in Tables 1 and 2. With respect to the obtained composition, the kinematic viscosity (40°C, 100°C) and the viscosity index of the composition measured in accordance with JIS K 2283:1993 are shown in Table 1. Note that, with respect to the item of "Composition" in Table 1, "-" indicates that the component is not used. In addition, in the item of "Composition" in Table 1, "in mass%" indicates the content (mass%) of the base oil component in terms of mass with respect to the total amount of the base oil (all base oils), and "mass%" and "mass ppm" indicate the content (mass%) in terms of mass with respect to the total amount of the working oil composition for a shock absorber (total amount of the composition).

[0099] [Evaluation of the properties of the working oil compositions for shock absorbers obtained in the various embodiments, etc.] <Determination of 10% distillation temperature: Confirmation test for evaporability> Using the buffer working oil compositions obtained in each example, the relationship between distillation yield and temperature was determined by gas chromatography distillation test method as specified in JIS K 2254:1998 (chromatogram). The temperature at which 10% of the total area of ​​the chromatogram is reached was determined as the 10% distillation temperature. The results are shown in Table 1.

[0100] <Determination of Pour Point> The pour points of the buffer working oil compositions obtained in each example were determined using the pour point test method specified in JIS K 2269:1987. The results are shown in Table 1.

[0101] <Determination of Friction Properties> The frictional properties of the buffer oil compositions obtained in the various examples were evaluated using the test pieces and testing machine described below, and by the test methods described below.

[0102] (Experimental film) The oil seal (opening diameter: 12mm) and piston rod (diameter: 12mm) of the front wheel shock absorber manufactured by Hitachi Astemo Corporation were recycled as test pieces from a car (manufactured by Matsuda Corporation, model name: MAZDA3) manufactured in 2020.

[0103] (Testing machine) Used in the determination of frictional properties Figure 1 and Figure 2 The testing machine shown. Additionally... Figure 1 This is a schematic diagram showing the outline of a testing machine with the test piece 10 (a test piece consisting of an oil seal 11 and a piston rod 12) installed. Figure 2 It is a schematic illustration of the experiment. Figure 1 A schematic cross-sectional view showing the relationship between the oil seal 11 and the piston rod 12 in region R. Additionally, in... Figure 1 For convenience, the test machine is depicted in its pre-test state, i.e., before the oil seal 11 is in contact with the piston rod 12.

[0104] Thus, as a testing machine, the following testing machine was used, which is as follows: Figure 1 The diagram shows a piston rod 12 that is positioned to move back and forth ( Figure 2linear motor 13 and a load sensor 14 for measuring the load applied to the oil seal 11. In addition, the linear motor 13 is fixed to the workbench 15 as shown, and the portion where the oil seal 11 is fixed is designed to be movable to a prescribed position (a position where the oil seal 11 and the piston rod 12 become in contact with each other) by a guide rail provided on the workbench 15. Moreover, the oil seal 11 is fixed to the piston rod 12 as shown. Figure 1 In addition, the portion where the oil seal 11 is fixed is designed to be movable to a prescribed position (a position where the oil seal 11 and the piston rod 12 become in contact with each other) by a guide rail provided on the workbench 15. Moreover, the oil seal 11 is fixed to the piston rod 12 as shown. Figure 2 In addition, the portion where the oil seal 11 is fixed is designed to be movable to a prescribed position (a position where the oil seal 11 and the piston rod 12 become in contact with each other) by a guide rail provided on the workbench 15. Moreover, the oil seal 11 is fixed to the piston rod 12 as shown. Figure 2 In addition, the portion where the oil seal 11 is fixed is designed to be movable to a prescribed position (a position where the oil seal 11 and the piston rod 12 become in contact with each other) by a guide rail provided on the workbench 15. Moreover, the oil seal 11 is fixed to the piston rod 12 as shown.

[0105] (Test method) Each of the working oil compositions for shock absorbers obtained in each of the examples was used, and a test piece 10 was mounted on the test machine as shown in FIG. 1. In addition, 0.1 ml of the working oil composition for shock absorbers was applied to the piston rod 12. Moreover, after the oil seal 11 and the piston rod 12 were arranged in a relationship as shown in FIG. 1, the position of the contact portion of the oil seal 11 and the piston rod 12 before the piston rod 12 was moved was made 0 mm, and the linear motor 13 was operated so that the displacement of the piston rod 12 reached ±0.16 mm from the position of 0 mm in the direction of the arrow shown in FIG. 1. In addition, one reciprocation of the piston rod 12 in the range of ±0.16 mm was taken as one cycle, and the operation state was controlled so that the piston rod was moved at a frequency of 10 Hz. Then, from the graph showing the relationship between the displacement (moving distance) of the piston rod and time for one cycle at the time of 30000 cycles and the graph showing the relationship between the load applied to the oil seal and time for one cycle at the time of 30000 cycles, the time difference between the peak value of the displacement (moving distance) of the piston rod and the peak value of the load applied to the oil seal (time difference between the peak values) was confirmed. Figure 1 Figure 2 Figure 2

[0106] In addition, in order to explain the time difference between the peak values, the graphs showing the relationship between the displacement and time and the relationship between the load and time for approximately one cycle at the time of 30000 cycles obtained when each of the working oil compositions for shock absorbers of Examples 1 to 11 was used were shown in FIGS. 2 to 12, respectively. Figures 3-13 Figure 3 Example 1,​​​​Figure 4 Example 2 Figure 5 Example 3 Figure 6 Example 4 Figure 7 Example 5 Figure 8 Example 6 Figure 9 Example 7 Figure 10 Example 8 Figure 11 Example 9 Figure 12 Example 10 Figure 13 Example 11 shows the displacement versus time and load versus time curves for approximately one cycle at 30,000 cycle times obtained using the buffer working oil compositions of Comparative Example 1 and Comparative Example 4, respectively. Figures 14-15 ( Figure 14 Comparative Example 1 Figure 15 (Compare Example 4). Figures 3-15 As shown, through the aforementioned periodic motion, a time difference (a delay in the peak value of displacement relative to the peak value of the load) is generated at the position of each peak value in both the positive and negative regions of the load and displacement magnitude. In this experiment, the time difference (peak value delay) between the peak values ​​in the positive and negative regions was confirmed using the working oil composition for each buffer. (It should be noted that the peak value of displacement in the positive region represents the piston rod at the front end of 30,000 cycles (refer to...) Figure 2 The position with the most movement, the peak displacement in the negative side region represents the piston rod at the rear side after 30,000 cycles (refer to...). Figure 2 (The location with the most movement). The results (time difference between peaks in the positive and negative regions) are shown in Table 1.

[0107] Furthermore, it is known that the smaller the time difference between the measured displacement and the peak value of the load, the higher the follow-through of the load applied to the oil seal to the movement (displacement) of the piston rod, and the better the ride comfort of the vehicle. Therefore, the smaller the time difference between the measured displacement and the peak value of the load, the more likely the composition is to impart the desired frictional characteristics to shock absorbers and other dampers. From this perspective, when the time difference between the aforementioned peak values ​​(the delay between the peak value of the displacement and the peak value of the load) is 0.0104 seconds or less (a further preferred level is 0.0102 seconds or less), it can be evaluated as being able to impart the desired frictional characteristics to shock absorbers and other dampers.

[0108] In addition, during the test, the waveform of the curve was also confirmed based on the relationship between displacement and time and the relationship between load and time for one cycle out of 30,000 cycles calculated for each buffer working oil composition.

[0109] Table 1 Table 2 From the results shown in Tables 1 to 2 and Figures 3-15 it was found that in the case of using the shock absorber operating oil compositions obtained in Examples 1 to 11 (corresponding to the shock absorber operating oil compositions of the present application), the time difference between the peaks was 0.0104 seconds or less (the time difference between the peaks of displacement and load was very small), and the followability of the load applied to the oil seal to the displacement of the piston rod was excellent. In contrast, it was found that in the case of the shock absorber operating oil compositions obtained in Comparative Examples 1 to 9, the time difference between the peaks in the measurement test of the friction characteristics exceeded 0.0104 seconds, and the followability of the load applied to the oil seal to the displacement of the piston rod was low.

[0110] In addition, in the case of the shock absorber operating oil compositions obtained in Examples 1 to 11 (corresponding to the shock absorber operating oil compositions of the present application), from the description of Figures 3-13 , when the graph of the relationship between the displacement and time and the relationship between the load and time at the time of 30000 cycles was confirmed for each of the shock absorber operating oil compositions, the waveforms of the graphs during reciprocating motion were not significantly disturbed regardless of which composition was used, and it was confirmed that the waveforms became neat to the same extent, and it was found that the friction stability during operation was also excellent. In contrast, in the case of the graph using the composition of Comparative Example 4 shown in Figure 15 , compared to the graph shown in Figures 3-13 , it was confirmed that the waveform of the load toward the negative side was disturbed. In addition, Comparative Examples 3 to 4 and Comparative Examples 6 to 9 all had the same degree of disturbance in the waveforms of the graphs obtained as Figure 15 . From such results shown in Figures 3-15 , in the case of using the shock absorber operating oil compositions obtained in Examples 1 to 11 (corresponding to the shock absorber operating oil compositions of the present application), the waveforms showed high followability (the difference between the load and the displacement was small) without being disturbed, and it was understood that excellent friction characteristics as required for the operating oil of a shock absorber were obtained.

[0111] From such a result, it is known that, in a case where the shock absorber working oil composition of the present application is used for a shock absorber, excellent friction properties such as excellent ride comfort can be imparted. That is, from the above-mentioned result, it is known that, according to the shock absorber working oil composition of the present application, not only the followability of the load applied to the oil seal to the operation of the piston rod can be improved, but also the friction can be adjusted to be a moderate size in which the stability of the friction at the time of operation is also excellent (the friction can be adjusted to a moderate size required for a shock absorber to avoid excessive lubrication by the effect of lubrication), and it is known that excellent friction properties can be obtained as a shock absorber working oil.

[0112] In addition, it is also confirmed from the results shown in Tables 1 to 2 that the pour point of the composition of the shock absorber working oil composition (corresponding to the shock absorber working oil composition of the present application) obtained in Examples 1 to 11 in which the ester base oil ((A) component) having a pour point of -45°C or lower and a 10% distillation temperature of 300°C or higher and the zinc dialkyldithiophosphate ((B) component: ZnDTP (I), or a mixture of ZnDTP (I) and (II)) are combined is -45°C or lower, and the 10% distillation temperature of the composition is also 300°C or higher, and the low-temperature fluidity and the low-evaporation property can be taken into consideration (note that the 10% distillation temperature of the composition is derived from the 10% distillation temperature of the ester base oil ((A) component), and becomes a value that is approximately the same degree as the 10% distillation temperature of the base oil). On the contrary, the shock absorber working oil compositions obtained in Comparative Examples 1 to 2 and Comparative Example 5 can be excellent in the low-temperature fluidity and the low-evaporation property by the ester base oil ((A) component), but cannot have high-level friction properties. In addition, the shock absorber working oil compositions obtained in Comparative Examples 3 to 4 and Comparative Examples 6 to 9 in which the mineral oil ((A' component) is used as the base oil cannot take into consideration the low-temperature fluidity and the low-evaporation property. Furthermore, the shock absorber working oil compositions obtained in Comparative Examples 10 to 11 in which only the other ester base oil ((A" component) that does not satisfy the condition of the 10% distillation temperature of 300°C or higher is used as the base oil are insufficient in the low-evaporation property, and cannot take into consideration the low-temperature fluidity and the low-evaporation property.

[0113] From such a result, it is confirmed that, by the shock absorber working oil composition of the present application, all of the low-temperature fluidity, the low-evaporation property, and the friction properties can be excellent.

[0114] Industrial applicability As explained above, according to the present application, a shock absorber working oil composition in which all of the low-temperature fluidity, the low-evaporation property, and the friction properties can be excellent can be provided. Therefore, the shock absorber working oil composition of the present application is useful as a shock absorber (for example, a shock absorber) working oil or the like for a vehicle.

[0115] Explanation of reference signs 10 test piece; 11 oil seal; 12 piston rod; 13 linear motor; 14 load cell; 15 work table; 16 guide rail.

Claims

1. A working oil composition for a shock absorber, comprising the following components (A) and (B): (A) an ester base oil having a pour point of -45°C or lower and a 10% distillation temperature of 300°C or higher according to the gas chromatography distillation test method specified in JIS K 2254: 1998; and (B) zinc dialkyldithiophosphate.

2. The working oil composition for a shock absorber according to claim 1, further comprising the following component (C): (C) at least one metal-based detergent selected from the group consisting of alkali metal sulfonates and alkaline earth metal sulfonates.

3. The working oil composition for a shock absorber according to claim 1, wherein The kinematic viscosity at 40°C of the ester base oil is 5.0 mm 2 / s or more and 30.0 mm 2 / s or less.

4. The working oil composition for a shock absorber according to claim 1, wherein The ester base oil consists of a carboxylic acid ester that is a reaction product of a carboxylic acid having 1 to 24 carbon atoms and an alcohol having 1 to 20 carbon atoms.

5. The working oil composition for a shock absorber according to claim 1, wherein The ester base oil consists of a carboxylic acid ester that is a reaction product of a carboxylic acid having 1 to 18 carbon atoms and an alcohol having 1 to 6 carbon atoms.

6. The working oil composition for a shock absorber according to claim 1, wherein The content of the component (B) is 100 mass ppm or more and 1500 mass ppm or less in terms of zinc element, based on the total amount of the composition.

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