Lubricating oil composition for shock absorber, shock absorber, and method for adjusting friction characteristics of lubricating oil for shock absorber

Abstract

To provide a lubricant composition for a shock absorber, a shock absorber, and a method for adjusting frictional property of a lubricant for the shock absorber, that can achieve both operational stability and ride comfort.SOLUTION: A lubricant composition for a shock absorber comprises a base oil and a friction modifier, wherein the friction modifier comprises a pentaerythritol ester, and the base oil is mainly composed of an ester oil.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a lubricating oil composition for a shock absorber, a shock absorber, and a method for adjusting the friction characteristics of a lubricating oil for a shock absorber. [Background technology]

[0002] It has been known that the vibration damping force of a shock absorber is the sum of the hydraulic damping force generated in the valve and the friction force generated at the sliding parts between the piston rod and oil seal or between the piston and cylinder. It is also known that if the vibration damping force of a shock absorber is large, operational stability increases but ride comfort deteriorates, while conversely, if the vibration damping force of a shock absorber is small, operational stability deteriorates but ride comfort improves. Therefore, in recent years, with a focus on ride comfort, research has been conducted to reduce the friction force of shock absorber lubricating oils and thus the vibration damping force of shock absorbers by adjusting friction modifiers added to shock absorber lubricating oils (for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Shock Absorber Technology Trends and Tribology (Hiroshi Nakanishi, Tribologist 2009 (Vol. 54) No. 9, p. 598) Summary of the Invention [Problem to be solved by the invention]

[0004] Shock absorbers exert vibration damping force through reciprocating motion. While it takes a certain amount of time for the hydraulic damping force to build up, frictional force has high responsiveness. Therefore, frictional force is an important factor in the damping force of shock absorbers when transitioning from a stationary state to a sliding state or when the vibration amplitude is small. However, as in the past, if the frictional force of shock absorber lubricating oils was reduced with a focus on ride comfort, the damping force also decreased, resulting in a problem of deterioration in operational stability. In particular, in recent years, there have been many well-maintained roads, and vibrations with smaller amplitudes than normal amplitudes often occur. Therefore, there has been a demand for a shock absorber lubricating oil composition that can achieve both operational stability and ride comfort when transitioning from a stationary state to a sliding state or when the vibration amplitude is small.

[0005] The present invention provides a lubricating oil composition for a shock absorber, a shock absorber, and a method for adjusting the friction characteristics of a lubricating oil for a shock absorber, which can achieve both operational stability and ride comfort. [Means for solving the problem]

[0006] The present invention relates to the following lubricating oil compositions for shock absorbers (1) to (8). (1) A lubricating oil composition comprising a base oil and a friction modifier, wherein the friction modifier is one or more selected from pentaerythritol diesters, pentaerythritol triesters, and pentaerythritol tetraesters; The base oil contains pentaerythritol ester, and the base oil is mainly composed of ester oil, which is an ester oil obtained by combining a fatty acid having less than 18 carbon atoms with an alcohol. and the pentaerythritol ester is contained in an amount of 2% by weight or more based on the total amount. A lubricating oil composition for a shock absorber, comprising: (2) The lubricating oil composition for a shock absorber according to (1) above, wherein the ester oil is a monoester oil. (3) The lubricating oil composition for a shock absorber according to (1) or (2) above, wherein the base oil contains the ester oil in an amount of 50% by weight or more of the total weight of the base oil, or the ester oil accounts for the largest proportion of the total weight of the base oil. (4) A lubricating oil composition for a shock absorber according to any one of (1) to (3) above, characterized in that the base oil contains 90% by weight or more of the ester oil. (5) The lubricating oil composition for a shock absorber according to any one of (1) to (4) above, wherein the pentaerythritol ester contains a pentaerythritol tetraester as a main component. (6) The pentaerythritol ester is Fatty acids with 6-12 carbon atoms The lubricating oil composition for a shock absorber according to any one of (1) to (5) above, which is a pentaerythritol ester having a medium-chain fatty acid. (7) A lubricating oil composition for a shock absorber according to any one of (1) to (6) above, wherein the base oil has a viscosity index of 60 or less. (8) The lubricating oil composition for a shock absorber according to any one of (1) to (7) above, wherein the friction modifier further contains zinc dithiophosphate.

[0007] The present invention also provides a shock absorber as described below in (9). (9) A shock absorber containing the lubricating oil composition for a shock absorber according to any one of (1) to (8) above. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a lubricating oil composition for a shock absorber, a shock absorber, and a method for adjusting the friction characteristics of a lubricating oil for a shock absorber, which can achieve both operational stability and ride comfort. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram illustrating a friction test device used in a friction test of the lubricating oil for a shock absorber according to the present invention. [Figure 2] FIG. 2 is a diagram for explaining a method for measuring the friction characteristics of a lubricating oil for a shock absorber according to the present invention. [Figure 3] 1A and 1B are diagrams showing the friction characteristics of the lubricating oil for a shock absorber according to a comparative example in the form of a circulation diagram, and 1C is a diagram showing the friction characteristics of the lubricating oil for a shock absorber according to this embodiment in the form of a circulation diagram. [Figure 4] FIG. 10 is a diagram for explaining a circulating graphic; [Figure 5] FIG. 1 is a diagram showing measurement results of friction characteristics of various lubricating oils for shock absorbers. DETAILED DESCRIPTION OF THE INVENTION

[0010] The lubricating oil composition for a shock absorber, the shock absorber, and the method for adjusting the friction characteristics of the lubricating oil for a shock absorber according to the present invention will be described below with reference to the drawings. Note that the present invention will be described below using the lubricating oil for a shock absorber as an example.

[0011] The lubricating oil according to this embodiment comprises (A) a base oil and (B) a friction modifier, and the friction modifier (B) contains (B1) zinc dithiophosphate (hereinafter also referred to as ZnDTP) and (B2) pentaerythritol.

[0012] (A) Base oil The lubricating oil for shock absorbers according to the present invention is characterized in that the base oil contains ester oil as a main component. Specifically, the base oil contains ester oil in an amount of 50% by weight or more of the total base oil, or in an amount of ester oil that accounts for the largest proportion. Note that the lubricating oil for shock absorbers according to the present invention may contain ester oil as a main component of the base oil, or may be configured to contain a base oil that is a mixture of ester oil and mineral oil. Furthermore, the base oil preferably contains ester oil in an amount of 90% by weight or more of the total base oil, and more preferably contains only ester oil. In this embodiment, the base oil is composed solely of commercially available ester oil.

[0013] The ester oil may be a monoester, diester, polyol ester, phosphate ester, or the like, with monoesters being particularly preferred. Examples of monoesters include fatty acid monoesters obtained by combining fatty acids having 6 to 22 carbon atoms, such as oleic acid, adipic acid, pelargonic acid, coconut oil fatty acid, palmitic acid, beef tallow fatty acid, and pelargonic acid, with alcohols, such as 2-ethylhexanol, isooctyl alcohol, isodecyl alcohol, and neopentyl glycol. The lubricating oil for shock absorbers according to the present invention may be a mixture of monoesters and other ester oils, but preferably contains monoesters in an amount of 50% by weight or more of the total ester oil, or in the highest proportion.

[0014] The base oil has a kinematic viscosity of 10 mm at 40°C. 2 / s or less, or a kinematic viscosity of 2mm at 100°C 2 / s or less is more preferable. By lowering the viscosity of the base oil, it is possible to add a thickener, which makes it possible to further adjust the friction characteristics of the shock absorber lubricating oil. For example, the monoester of 2-ethylhexanol and coconut oil fatty acid has a kinematic viscosity of 6 mm at 40°C. 2 / s, and the kinematic viscosity at 100°C is 1.8mm 2 / s, making it possible to adjust the friction characteristics of the shock absorber lubricant by adding a thickener.

[0015] (B) Friction modifier The lubricating oil for shock absorbers according to this embodiment contains a friction modifier. The friction modifier is not particularly limited, but various types of friction modifiers, such as phosphorus-based, amine-based, or ester-based friction modifiers, can be contained. By adjusting the amount of friction modifier added, the coefficient of friction of the lubricating oil for shock absorbers can be adjusted. In addition, the lubricating oil for shock absorbers according to this embodiment contains (B1) zinc dithiophosphate and (B2) pentaerythritol ester as friction modifiers.

[0016] (B1) Zinc dithiophosphate (ZnDTP) ZnDTP is generally a compound represented by the following formula 1, and has the function of assisting the adjustment of the friction coefficient by a friction modifier. [ka] [In the above formula 1, each R represents an individual hydrocarbon group, such as a linear primary alkyl group, a branched secondary alkyl group, or an aryl group.]

[0017] As described above, several types (structures) of ZnDTP are known, including those having a primary alkyl group, a secondary alkyl group, or an aryl group, but the lubricating oil for shock absorbers according to this embodiment contains two types of ZnDTP as described below.

[0018] That is, the lubricating oil for a shock absorber according to this embodiment contains ZnDTP represented by Chemical Formula 2 below as the first ZnDTP. [ka] [In formula 1, R 11 ~R 14 is an alkyl group, and the alkyl group has a primary alkyl group and a secondary alkyl group. That is, R 11 ~R 14 one to three of the R 11 ~R 14 The remainder are secondary alkyl groups.

[0019] In the first ZnDTP, the primary alkyl group is not particularly limited and examples thereof include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, an n-pentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group, an isoamyl group, an isobutyl group, a 2-methylbutyl group, a 2-ethylhexyl group, a 2,3-dimethylbutyl group, and a 2-methylpentyl group. However, an alkyl group having 4 to 12 carbon atoms (for example, an isobutyl group (having 4 carbon atoms) or a 2-ethylhexyl group (having 8 carbon atoms)) is preferred.

[0020] In the first ZnDTP, the secondary alkyl group is not particularly limited and examples thereof include an isopropyl group, a sec-butyl group, a 1-ethylpropyl group, a 2-ethylhexyl group, and a 4-methyl-2-pentyl group, but is preferably an alkyl group having 3 to 6 carbon atoms (for example, an isopropyl group (3 carbon atoms)).

[0021] In the first ZnDTP, the ratio of primary alkyl groups to secondary alkyl groups is not particularly limited, but it is preferable that the ratio of primary alkyl groups to secondary alkyl groups is higher.

[0022] The content of the first ZnDTP in the lubricating oil for shock absorbers is not particularly limited, but is preferably 0.1 mass% or more, more preferably 0.4 mass% or more, and is preferably 4.0 mass% or less, more preferably 2.0 mass% or less.

[0023] As described above, the lubricating oil for shock absorbers according to the present invention contains a first ZnDTP having both a primary alkyl group and a secondary alkyl group, and therefore when a friction modifier is added, the friction coefficient can be easily adjusted to a value suitable for ride comfort and handling stability. In addition, compared to lubricating oils for shock absorbers containing a ZnDTP having only a primary alkyl group and / or a ZnDTP having only a secondary alkyl group, the variation in the friction coefficient can be reduced, thereby further improving ride comfort.

[0024] Furthermore, the lubricating oil for a shock absorber according to this embodiment contains, as a friction modifier, a second ZnDTP having a structure different from that of the first ZnDTP. The second ZnDTP is represented by the following formula 3. [ka] [In formula 2, R 21 ~R 24 is a secondary alkyl group. That is, the second ZnDTP does not have a primary alkyl group, but only a secondary alkyl group.]

[0025] The number of carbon atoms in the secondary alkyl group of the second ZnDTP is not particularly limited, and examples thereof include an isopropyl group, a sec-butyl group, a 1-ethylpropyl group, a 2-ethylhexyl group, and a 4-methyl-2-pentyl group. However, the secondary alkyl group is preferably an alkyl group having 3 to 8 carbon atoms (for example, an isopropyl group (3 carbon atoms), a 2-ethylhexyl group (8 carbon atoms), or an isobutyl group (4 carbon atoms)).

[0026] Furthermore, the content of the second ZnDTP is not particularly limited, but it is preferable that it is less than that of the first ZnDTP, and it is preferable that it is 20% by weight or less of the amount of ZnDTP added (the total amount of the first ZnDTP and the second ZnDTP).

[0027] The type of alkyl group contained in ZnDTP can be determined by a known method. 13 The structure of ZnDTP can be determined using -NMR, or by analyzing the characteristics of the POC absorption band and the P=S PS absorption band in the fingerprint region of FT-IR to determine whether the alkyl group is a primary alkyl group or a secondary alkyl group.

[0028] By including a second ZnDTP having only a secondary alkyl group as the dithiophosphoric acid (B1), ride comfort can be further improved compared to when only the first ZnDTP is included. Specifically, micro-vibrations during driving can be further reduced compared to when only the first ZnDTP is included. Furthermore, by using a ZnDTP having a secondary alkyl group having 3 to 8 carbon atoms as the second ZnDTP, the difference in friction coefficient between small amplitude (low speed) and normal amplitude (high speed) can be reduced, thereby improving ride comfort.

[0029] (B2) Pentaerythritol ester Pentaerythritol esters are tetrahydric sugar alcohols, and are compounds in which the hydroxyl groups, which are terminal substituents of pentaerythritol, are ester-bonded to fatty acid residues. Pentaerythritol esters include pentaerythritol tetraesters, in which all four terminal substituents are ester-bonded to fatty acid residues, and pentaerythritol monoesters, pentaerythritol diesters, and pentaerythritol triesters, which are partial esters in which any terminal substituent is ester-bonded to a fatty acid residue. In the following explanation, pentaerythritol tetraesters will be abbreviated as PE4E, pentaerythritol triesters as PE3E, pentaerythritol diesters as PE2E, and pentaerythritol monoesters as PE1E.

[0030] In the pentaerythritol ester according to the present invention, the fatty acid residue is not particularly limited and may be, for example, a C6 to C22 fatty acid residue such as a stearic acid residue or an oleic acid residue. Further, examples of the fatty acid residue include caprylic acid, capric acid, oleic acid, stearic acid, myristic acid, palmitic acid, linoleic acid, adipic acid, pelargonic acid, tall fatty acid, palm fatty acid, coconut fatty acid, and beef tallow fatty acid.

[0031] It should be noted that, when producing PE4E, it is technically difficult to produce only PE4E, and PE4E may be mixed with PEIE, PE2E, and PE3E. Therefore, even commercially available "pentaerythritol tetraesters" do not consist solely of PE4E. They primarily contain PE4E, but also PE3E, PE2E, or PE1E. Therefore, the "pentaerythritol tetraester" of the present invention may be a mixture of pentaerythritol esters commercially available as "pentaerythritol tetraesters," or a mixture of pentaerythritol esters containing 80% or more of "pentaerythritol tetraesters." For the same reason, the "pentaerythritol diester" of the present invention may be a mixture of pentaerythritol esters commercially available as "pentaerythritol diesters," or a mixture of pentaerythritol esters containing 80% or more of "pentaerythritol diesters." The same applies to PEIE and PE3E.

[0032] (Friction characteristics of the lubricating oil for shock absorbers according to the present invention) Next, the friction characteristics of the shock absorber lubricating oil according to the present invention will be described. In this invention, the friction characteristics of the shock absorber lubricating oil were analyzed using a friction test apparatus 10 configured as shown in FIG. 1. As shown in FIG. 1, the friction test apparatus 10 is a pin-on-disk type friction test apparatus. A disk test piece 2 fixed on a slide bearing 1 is reciprocated by an electromagnetic vibrator 3, and a pin test piece 4 is pressed against the disk and slid against it. The friction force generated by this sliding motion is measured using a strain gauge 6 attached to the fixed shaft 5 of the pin test piece 4. Furthermore, since the combination of the shock absorber lubricating oil and the oil seal is a factor that affects the friction characteristics of a shock absorber, the friction test apparatus 10 shown in FIG. 1 uses acrylonitrile butadiene rubber (NBR), which is used as an oil seal in shock absorbers, for the pin test piece 4. The tip of the pin test piece 4 was cut at a 140° angle to mimic the oil lip shape. Furthermore, the disk test piece 2 was made of a hard chrome-plated film, which is used on piston rod surfaces. In the example shown in FIG. 1, the friction force (friction coefficient) between the NBR pin test piece 4 and the chrome-plated disk test piece 2 is measured, but the friction force (friction coefficient) between a copper ball and the chrome-plated disk test piece 2 may also be measured.

[0033] FIG. 2 shows an example of the results of measuring the friction force of a shock absorber lubricant by reciprocating the pin test piece 4 and the disk test piece 2 using the friction test apparatus 10 at an amplitude of ±2.0 mm, a frequency of 1.5 Hz, a load of 20 N, and a temperature of 30°C. FIG. 2 shows that the motion directions of the pin test piece 4 and the disk test piece 2 are reversed at the phases of π / 2 and 3π / 2. In the example shown in FIG. 2, the motion directions of the pin test piece 4 and the disk test piece 2 are reversed, and the shock absorber lubricant has a temporary stationary state at the timings of π / 2 and 3π / 2, and immediately thereafter transitions from the stationary state to a sliding state. As shown in FIG. 2, the shock absorber lubricant according to this embodiment has a friction characteristic in which the friction force peaks when transitioning from a sliding state to a stationary state or when transitioning from a stationary state to a sliding state. In this way, by using the friction test apparatus 10, the peak value F of the friction force when transitioning from a sliding state to a stationary state or when transitioning from a stationary state to a sliding state can be measured. saand the average friction force F in the sliding state during micro-vibration ave It is possible to measure the following.

[0034] Figure 3 shows, in cycle diagrams, the frictional properties of the shock absorber lubricating oils measured as shown in Figure 2. Specifically, (A) shows the frictional properties of a shock absorber lubricating oil (Comparative Example 1) that uses a GIII mineral oil as the base oil and does not contain pentaerythritol ester or ZnDTP. (B) shows the frictional properties of a shock absorber lubricating oil (Comparative Example 2) that uses a GIII mineral oil as the base oil and contains a pentaerythritol tetraester having an enanthic acid residue and ZnDTP as friction modifiers. (C) shows the frictional properties of a shock absorber lubricating oil (Example) that uses an ester oil of isodecyl alcohol and pelargonic acid as the base oil and contains a pentaerythritol tetraester having an enanthic acid residue and ZnDTP as friction modifiers.

[0035] Here, first, the circulation diagram shown in FIG. 3 will be described with reference to FIG. 4. FIG. 4 is a diagram for explaining the circulation diagram of the friction characteristics of the shock absorber lubricating oil. In FIG. 4, P1 represents the friction force of the shock absorber lubricating oil in a sliding state where the pin test piece 4 is accelerating and sliding in the forward direction (for example, the friction force in the phase from 3π / 4 to π in FIG. 2), and P2 represents the friction force of the shock absorber lubricating oil in a sliding state where the pin test piece 4 is decelerating and sliding in the forward direction (for example, the friction force in the phase from π to 5π / 4 in FIG. 2). Similarly, P3 represents the friction force of the shock absorber lubricating oil in a sliding state where the pin test piece 4 is accelerating and sliding in the backward direction (for example, the friction force in the phase from 7π / 4 to 2π in FIG. 2), and P4 represents the friction force of the shock absorber lubricating oil in a sliding state where the pin test piece 4 is accelerating and sliding in the backward direction (for example, the friction force in the phase from 2π to π / 4 in FIG. 2). Furthermore, P5 represents the frictional force of the lubricating oil for the shock absorber just before the shock absorber is slid in the forward direction and then brought to a standstill, P6 represents the frictional force of the lubricating oil for the shock absorber just after the shock absorber is slid in the backward direction, P7 represents the frictional force of the lubricating oil for the shock absorber just before the shock absorber is slid in the backward direction and then brought to a standstill, and P8 represents the frictional force of the lubricating oil for the shock absorber just after the shock absorber is slid in the forward direction.

[0036] In Figure 3, the shock absorber amplitude is a fine amplitude of ±2.0 mm, and the friction characteristics against vibrations occurring on roads in relatively good condition are shown. It can be seen that the shock absorber lubricating oil of Comparative Example 1 (A), which does not contain pentaerythritol ester, maintains a low friction force just before the shock absorber comes to a standstill or immediately after it starts sliding, compared to the shock absorber lubricating oil of Comparative Example 2 (B) and the shock absorber lubricating oil of the Example (C), which contain pentaerythritol ester. This shows that the shock absorber lubricating oil of Comparative Example 1 (A), which does not contain pentaerythritol ester, does not exert a strong friction force, particularly against shock absorber vibrations with a fine amplitude, compared to the shock absorber lubricating oil of Comparative Example 2 (B) and the shock absorber lubricating oil of the Example (C), and therefore tends to result in poorer operability (traction, tire contact with the ground, acceleration performance, braking performance, vehicle behavior such as roll and pitching) on ​​roads in relatively good condition.

[0037] On the other hand, the lubricating oil for shock absorbers of Comparative Example 2, which uses a GIII mineral oil (B) containing pentaerythritol ester as the base oil, exhibits high frictional force just before the shock absorber comes to a standstill or immediately after it starts sliding, and compared with the lubricating oil for shock absorbers of Comparative Example 1 (A) which does not contain pentaerythritol ester, the frictional force acts strongly against vibrations of the shock absorber during small amplitudes, resulting in good operability during small amplitudes of the shock absorber. However, the lubricating oil for shock absorbers using a GIII mineral oil (B) containing pentaerythritol ester as the base oil exhibits high frictional force and a high coefficient of friction during sliding, resulting in poor ride comfort during small amplitudes of vibration, compared with the lubricating oil for shock absorbers of the Example (C) which uses an ester oil (C) containing pentaerythritol ester as the base oil.

[0038] In contrast, the shock absorber lubricating oil of the example (C), which uses an ester oil as a base oil, exhibits high friction just before the shock absorber comes to a standstill or immediately after it starts sliding. Therefore, similar to the shock absorber lubricating oil of Comparative Example 2 (B), which contains a pentaerythritol ester, the frictional force is strong against vibrations of the shock absorber during small amplitudes, resulting in good operability during small amplitudes. Furthermore, the shock absorber lubricating oil of the example (C), which uses an ester oil as a base oil containing a pentaerythritol ester, can reduce frictional force during sliding compared to the shock absorber lubricating oil of Comparative Example 1 (B), which uses a GIII mineral oil as a base oil, also containing a pentaerythritol ester, and provides good ride comfort during small amplitudes. Thus, the shock absorber lubricating oil of the example (C), which uses an ester oil as a base oil, exhibits both good operability and good ride comfort during small amplitudes, compared to the shock absorber lubricating oil of Comparative Example 1 (B), which uses a GIII mineral oil as a base oil.

[0039] As described above, the lubricating oil for shock absorbers according to the present invention contains a pentaerythritol ester as a friction modifier and has an ester oil as the main component of the base oil, thereby improving ride comfort during small vibration amplitudes while ensuring operability during small vibration amplitudes. In the present invention, the friction characteristics based on the frictional force of the lubricating oil for shock absorbers just before the shock absorber comes to a standstill or just after the shock absorber starts to slide, which are indices of the operability of the shock absorber during small vibration amplitudes (traction, tire contact with the ground, acceleration performance, braking performance, behavioral performance such as vehicle roll and pitching, etc.), are defined as responsiveness RI. Specifically, as shown in the following formula (1), the frictional force of the lubricating oil for shock absorbers just before the shock absorber comes to a standstill or just after the shock absorber starts to slide, i.e., the peak value F of the frictional force of the lubricating oil for shock absorbers when transitioning from a stationary state to a sliding state or from a sliding state to a stationary state, is sa and the average value of the friction force of the lubricating oil for the shock absorber in a sliding state F ave and the average friction force F ave The ratio of the RI to the RI is defined as the response. Responsiveness RI = (F sa -F ave) / F ave ···(1)

[0040] In this example, the average coefficient of friction and responsiveness RI of various lubricating oils for shock absorbers were measured using friction testing apparatus 10 shown in Figure 1, and the results are plotted in Figure 5. In the example shown in Figure 5, the average coefficient of friction and responsiveness RI of various lubricating oils for shock absorbers were measured by reciprocating a pin test piece 4 and a disk test piece 2 at an amplitude of ±2.5 mm, a speed of 4.0 mm / sec, a load of 20 N, and a temperature of 30°C. In addition, the lubricating oils for shock absorbers shown below contain ZnDTP and an FM agent as friction modifiers, unless otherwise specified.

[0041] The squares in Figure 5 plot the average friction coefficient μ and responsiveness RI of shock absorber lubricants whose base oil is a mineral oil (other than GIII mineral oil) and into which common friction modifiers (but excluding ZnDTP, FM agents, and pentaerythritol esters) are added. The diamonds in Figure 5 plot the average friction coefficient μ and responsiveness RI of shock absorber lubricants whose base oil is a GIII mineral oil and into which common friction modifiers (but excluding ZnDTP, FM agents, and pentaerythritol esters) are added. In Figure 5, the area with many shock absorber lubricants, indicated by squares and diamonds, is indicated by circle C. Shock absorber lubricants that only contain common friction modifiers added to mineral oil base oil are scattered within the area indicated by circle C in Figure 5, and their responsiveness RI did not exceed 0.4, failing to achieve a high responsiveness RI.

[0042] The ● in Figure 5 plots the measurement results of the average friction coefficient μ and responsiveness RI of shock absorber lubricants in which different types of pentaerythritol esters were added to naphthenic base oil. Specifically, N1 is a shock absorber lubricant in which PE2E, which has oleic acid residues, was added to naphthenic base oil at 2 wt%, N2 is a shock absorber lubricant in which PE3E, which has oleic acid residues, was added to naphthenic base oil at 30 wt%, and N3 is a shock absorber lubricant in which PE4E, which has oleic acid residues, and PE3E, which has enanthic acid residues, were added to naphthenic base oil at 25 wt% and 5 wt%, respectively. N4 is a lubricating oil for shock absorbers in which PE4E having an oleic acid residue is added to a naphthenic base oil at a total weight % of 30%, and N5 is a lubricating oil for shock absorbers in which PE4E having an oleic acid residue is added to a naphthenic base oil at a total weight % of 29% and PE3E having an enanthic acid residue is added to a naphthenic base oil at a total weight % of 1. In addition, in Figure 5, the approximation curves of the naphthenic base oil lubricating oils for shock absorbers N1 to N5 are displayed as LN.

[0043] As shown in Figure 5, for shock absorber lubricants N1-N5, which are based on naphthenic base oils, the average friction coefficient μ and responsiveness RI tend to increase as the number of ester groups in the pentaerythritol ester increases. Furthermore, the slope of the approximate curve LN for shock absorber lubricants N1-N5, which are based on naphthenic base oils, is smaller than the approximate curve LP for shock absorber lubricants P1 and P2, which are based on PAO (described below), the approximate curve LG for shock absorber lubricants G1-G4, which are based on GIII mineral oil and contain pentaerythritol ester, and the approximate curve LE for shock absorber lubricants EA1-EA2, EB1-EB9, and EC1-EC6, which are based on ester base oils. This indicates that the degree of increase in the average friction coefficient μ increases as the responsiveness RI increases when pentaerythritol ester is used as the base oil. This indicates that when naphthenic base oils are used as base oils, improving operability at small amplitudes by using pentaerythritol esters may result in a decrease in ride comfort.

[0044] The circles in Figure 5 plot the average friction coefficient μ and responsiveness RI of shock absorber lubricants whose base oil is poly-α-olefin (PAO2C) and whose friction modifier contains pentaerythritol ester. Specifically, P1 is a shock absorber lubricant in which PE3E, which has oleic acid residues, has been added to the PAO2C base oil at a total weight percentage, and P2 is a shock absorber lubricant in which PE4E, which has oleic acid residues, has been added to the PAO2C base oil at a total weight percentage. In Figure 5, the approximate curves for shock absorber lubricants P1 and P2, which use PAO2C as the base oil, are displayed as LP.

[0045] We also found that the average friction coefficient μ and responsiveness RI tend to increase with the number of ester groups in the pentaerythritol esters in shock absorber lubricants based on PAO2C. Furthermore, the approximation curve LP for shock absorber lubricants P1 and P2 based on PAO2C has a steeper slope than the approximation curve LP for shock absorber lubricants P1 and P2 based on naphthenic base oils. However, the approximation curve slope is shallower than the approximation curve LG for shock absorber lubricants G1-G4 based on GIII mineral oil and containing pentaerythritol esters, and the approximation curve LE for shock absorber lubricants EA1-EA2, EB1-EB9, and EC1-EC6 based on ester base oils. This indicates that the greater the responsiveness RI, the greater the increase in the average friction coefficient μ. This suggests that when PAO2C is used as a base oil, improving operability at small amplitudes with pentaerythritol esters may result in a decrease in ride comfort compared to when using GIII mineral oil or ester oil as the base oil.

[0046] The triangles in Figure 5 plot the average friction coefficient μ and responsiveness RI of shock absorber lubricants containing a GIII mineral oil base oil and a pentaerythritol ester-containing friction modifier. Specifically, G1 is a shock absorber lubricant containing 2 wt% PE2E with oleic acid residues added to a GIII mineral oil base oil. G2 is a shock absorber lubricant containing 30 wt% PE4E with oleic acid residues added to a GIII mineral oil base oil. G3 is a shock absorber lubricant containing 26 wt% PE4E with oleic acid residues added to a GIII mineral oil base oil. G4 is a shock absorber lubricant containing 20 wt% PE4E with enanthic acid residues added to a GIII mineral oil base oil. In Figure 5, the approximate curves for shock absorber lubricants G1 to G4, which contain a GIII mineral oil base oil and pentaerythritol ester, are indicated as LG.

[0047] It was also found that shock absorber lubricants using GIII mineral oil as a base oil and containing pentaerythritol ester tended to have higher average friction coefficients μ and responsiveness RIs as the number of ester groups in the pentaerythritol ester increased. Furthermore, the approximation curve LG for shock absorber lubricants G1-G4 using GIII mineral oil as a base oil and containing pentaerythritol esters had a steeper slope than the approximation curves LP for shock absorber lubricants P1 and P2 using naphthenic base oils and PAO2C base oils, and the approximation curves LP for shock absorber lubricants P1 and P2 using PAO2C base oils. However, the approximation curves had a steeper slope than the approximation curves LE for shock absorber lubricants EA1-EA2, EB1-EB9, and EC1-EC6 using ester base oils. This indicates that the greater the responsiveness RI, the greater the increase in average friction coefficient μ. This suggests that when using GIII mineral oil as a base oil, improving operability at small amplitudes by using pentaerythritol esters may result in a decrease in ride comfort compared to when using ester oil as a base oil.

[0048] 5, EA1-EA2, indicated by hatched circles, EB1-EB9, indicated by solid diamonds, and EC1-EC6, indicated by solid gray squares, are plots of the measurement results of the average friction coefficient μ and responsiveness RI for shock absorber lubricating oils each having an ester oil base oil. Specifically, EA1-EA2, EB1-EB9, and EC1-EC6 are shock absorber lubricating oils having the compositions shown in Table 1 below. [Table 1]

[0049] That is, EA1 is a lubricating oil for shock absorbers that uses a monoester of 2-ethylhexanol and oleic acid as a base oil and has 30 wt% of PE4E, which has an enanthic acid residue, added to it, and EA2 is a lubricating oil for shock absorbers that uses a monoester of 2-ethylhexanol and oleic acid as a base oil and has 30 wt% of PE4E, which has an oleic acid residue, added to it. EB1 to EB9 are all lubricating oils for shock absorbers that use a monoester of isooctyl alcohol and adipic acid as a base oil. EB1 contains 2% by weight of PE2E having oleic acid residues as a total component, EB2 contains 30% by weight of PE3E having oleic acid residues as a total component, EB3 contains 17.5% by weight of PE3E having oleic acid residues as a total component, EB4 contains 30% by weight of PE4E having oleic acid residues as a total component, EB5 contains 20% by weight of PE4E having oleic acid residues as a total component, EB6 contains 5% by weight of PE4E having oleic acid residues as a total component and 25% by weight of PE4E having enanthic acid residues as a total component, EB7 contains 30% by weight of PE4E having oleic acid residues as a total component, EB8 contains 17.5% by weight of PE4E having oleic acid residues as a total component, and EB9 contains 10% by weight of PE4E having oleic acid residues as a total component. Although EB4 and EB7 have the same composition, the measurement results differ due to the different number of samples and measurement dates. Furthermore, EC1 to EC6 are all shock absorber lubricating oils with a base oil of a monoester of isodecyl alcohol and pelargonic acid. EC1 contains 30% by weight of PE3E having oleic acid residues overall, EC2 contains 2% by weight of PE2E having oleic acid residues overall, EC3 contains 30% by weight of PE4E having oleic acid residues overall, EC4 contains 15% by weight of PE4E having oleic acid residues overall and 15% by weight of PE3E having enanthic acid residues overall, EC5 contains 10% by weight of PE4E having oleic acid residues overall and 20% by weight of PE3E having enanthic acid residues overall, and EC6 contains 20% by weight of PE4E having enanthic acid residues overall. In FIG. 5, the approximate curves of the lubricating oils for shock absorbers EA1 to EA2, EB1 to EB9, and EC1 to EC6, each of which uses an ester base oil as the base oil, are indicated by LE.

[0050] Even for shock absorber lubricants based on ester oil, the average friction coefficient μ and responsiveness RI tend to increase with the number of ester groups in the pentaerythritol ester. Furthermore, the approximation curves LE for shock absorber lubricants EA1-EA2, EB1-EB9, and EC1-EC6 based on ester oil have a steeper slope (more vertical) than the approximation curves LP for shock absorber lubricants P1 and P2 based on naphthenic base oils, PAO2C base oils, and LG for shock absorber lubricants P1 and P2 based on GIII mineral oil and containing pentaerythritol ester. This indicates that the average friction coefficient μ increases less even as responsiveness RI increases when pentaerythritol ester is used as the base oil for ester oils, even when operability at small amplitudes is improved, resulting in improved ride comfort compared to other base oils.

[0051] Furthermore, compared with EA1-EA2, which use a monoester of 2-ethylhexanol and oleic acid as a base oil, EB1-EB9, which use a monoester of sodecyl alcohol and pelargonic acid as a base oil, and EC1-EC6, which use a monoester of isodecyl alcohol and pelargonic acid as a base oil, tend to show a smaller increase in the average friction coefficient μ even when the responsiveness RI increases. In particular, shock absorber lubricant EC6, which uses a monoester of isodecyl alcohol and pelargonic acid as a base oil and contains 30% by weight of PE4E, which has oleic acid residues and enanthic acid residues, had a responsiveness RI of over 0.8 while maintaining an average friction coefficient of less than 0.06, significantly improving operability and ride comfort compared to conventional models.

[0052] Furthermore, when PE4E is used as a pentaerythritol ester in shock absorber lubricating oils based on ester oil, it was found that the responsiveness RI tends to be higher when PE4E contains medium-chain fatty acid residues such as enanthic acid residues than when PE4E contains only long-chain fatty acid residues such as oleic acid residues.

[0053] As described above, the lubricating oil composition for shock absorbers according to the present invention is characterized by containing a pentaerythritol ester as a friction modifier and using an ester oil as a base oil as a main component. By using an ester oil as the base oil, it is possible to provide a lubricating oil composition for shock absorbers that can increase the responsiveness RI while keeping the average coefficient of friction μ small, as shown in Figure 5, thereby achieving both improved operability and ride comfort. In particular, when an ester oil is used as the base oil, the friction coefficient can be kept low even if the responsiveness RI is increased by changing the type of pentaerythritol ester added. Therefore, the responsiveness RI of the lubricating oil for shock absorbers can be adjusted to a desired value by changing the type of pentaerythritol ester added.

[0054] Furthermore, in the lubricating oil for shock absorbers according to the present invention, the addition of PE4E as the pentaerythritol ester can further increase the responsiveness RI of the lubricating oil for shock absorbers, and the use of a pentaerythritol ester having a medium-chain fatty acid residue rather than a long-chain fatty acid residue as the fatty acid residue of the pentaerythritol ester can further increase the responsiveness RI of the lubricating oil for shock absorbers. By adjusting the responsiveness RI of the lubricating oil for shock absorbers to a high level in this way, behavioral performance such as tire grip (road performance), acceleration performance, braking performance, and vehicle roll and pitching can be further improved, making it possible to improve maneuverability and stability.

[0055] Although the preferred embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the above-described embodiments. Various modifications and improvements can be made to the above-described embodiments, and such modifications and improvements are also included in the technical scope of the present invention.

Claims

1. Contains a base oil and a friction modifier, the friction modifier contains one or more pentaerythritol esters selected from pentaerythritol diesters, pentaerythritol triesters, and pentaerythritol tetraesters; The base oil contains ester oil as a main component, The ester oil is an ester oil obtained by combining a fatty acid having less than 18 carbon atoms with an alcohol, A lubricating oil composition for shock absorbers, characterized in that it contains 2% by weight or more of the pentaerythritol ester based on the total weight.

2. 2. The lubricating oil composition for a shock absorber according to claim 1, wherein the ester oil is a monoester oil.

3. 3. The lubricating oil composition for a shock absorber according to claim 1, wherein the base oil contains the ester oil in an amount of 50% by weight or more of the total weight of the base oil, or the ester oil accounts for the largest proportion of the total weight of the base oil.

4. 3. The lubricating oil composition for a shock absorber according to claim 1, wherein the base oil contains the ester oil in an amount of 90% by weight or more.

5. 5. The lubricating oil composition for a shock absorber according to claim 1, wherein the pentaerythritol ester contains a pentaerythritol tetraester as a main component.

6. 6. The lubricating oil composition for a shock absorber according to claim 1, wherein the pentaerythritol ester is a pentaerythritol ester having a medium-chain fatty acid that is a fatty acid having 6 to 12 carbon atoms.

7. 7. The lubricating oil composition for a shock absorber according to claim 1, wherein the base oil has a viscosity index of 60 or less.

8. 8. The lubricating oil composition for a shock absorber according to claim 1, wherein the friction modifier further contains zinc dithiophosphate.

9. A shock absorber containing the lubricating oil composition for shock absorbers according to any one of claims 1 to 8.

Images