Hydraulic device and working fluid for hydraulic device

Abstract

A hydraulic device according to one aspect of the present invention comprises: a cylinder; an oil seal member that contains an acrylonitrile butadiene rubber and that is provided to the cylinder; and a working fluid that is sealed inside the cylinder. The working fluid contains a base oil and an additive that has been added to the base oil. The oil seal member has an acrylonitrile content of 17-32 mass%. The base oil contains a fatty acid monoester which has an aniline point that is not lower than -102°C but lower than 51°C. The volume change rate of the oil seal member as measured at 100°C over 72 hours in accordance with JIS K 6258-4:2016 or ISO 1817:2015 is 0-25%.

Description

Hydraulic equipment and hydraulic fluids for hydraulic equipment

[0001] The present invention relates to a hydraulic device and a working fluid for the hydraulic device. This application claims priority to Japanese Patent Application No. 2024-116083, filed on July 19, 2024, the contents of which are incorporated herein by reference.

[0002] Patent Document 1 below discloses a lubricating base oil containing a monoester compound having a natural origin index of 100% and having one or more branched structures and having 18 or more carbon atoms. Patent Document 1 below describes the kinematic viscosity of this lubricating base oil, and also describes that the volume change rate of a test acrylic rubber when immersed in the lubricating base oil and measured under certain conditions is less than 20%.

[0003] International Publication No. 2023 / 026739

[0004] There has long been a demand for hydraulic oils for hydraulic equipment that have a low environmental impact, excellent low-temperature fluidity and abrasion resistance, and are free from problems such as oil leakage. For example, hydraulic shock absorbers have traditionally used hydraulic oils based on mineral oil, but considering the environmental impact, vegetable-based hydraulic oils could be considered as an alternative. However, when acrylonitrile-butadiene rubber is used for the sealing members of hydraulic shock absorbers, there is a risk of the sealing members swelling because the polarity of vegetable oil is closer to that of acrylonitrile-butadiene rubber than that of mineral oil. To prevent swelling of the sealing members, there is a problem in that it becomes necessary to use medium- and high-temperature nitrile rubber, which has issues with cold resistance, or expensive cold-resistant fluorine-based rubber.

[0005] The present invention aims to solve the above problems and to provide a hydraulic device equipped with a working fluid that has a low environmental impact, can suppress swelling of oil seal members, and has low viscosity and good low-temperature fluidity, and a hydraulic device working fluid for use in this hydraulic device.

[0006] One aspect of the present invention is a hydraulic device comprising: a cylinder; an oil seal member including acrylonitrile butadiene rubber and provided on the cylinder; and a working fluid sealed within the cylinder, wherein the working fluid includes a base oil and an additive added to the base oil, the oil seal member has an acrylonitrile content of 17% by mass or more and 32% by mass or less, the base oil includes a fatty acid monoester having an aniline point of -102°C or more and less than 51°C, and the volume change of the oil seal member measured under conditions of 100°C for 72 hours in accordance with JIS K 6258-4:2016 or ISO 1817:2015 is 0% or more and 25% or less. Another aspect of the present invention is a working fluid for hydraulic equipment, which includes a base oil and an additive added to the base oil, and is used in an environment where it is exposed to an oil seal member containing acrylonitrile-butadiene rubber, wherein the oil seal member has a TR10 value of -55°C or higher and -30°C or lower as measured in accordance with JIS K 6261-4:2017 or ISO 2921:2011, the base oil contains a fatty acid monoester having an aniline point of -102°C or higher and lower than 51°C, and the oil seal member has a volume change of 0% or higher and 25% or lower as measured under conditions of 100°C for 72 hours in accordance with JIS K 6258-4:2016 or ISO 1817:2015.

[0007] According to the above aspects of the present invention, it is possible to provide a hydraulic device equipped with a working fluid that has a small environmental impact, can suppress swelling of oil seal members, and has good low-temperature fluidity, and a hydraulic device working fluid for use in this hydraulic device.

[0008] 1 is a diagram showing the overall configuration of a hydraulic device according to a first embodiment of the present invention, and is a longitudinal cross-sectional view taken along a cross section including a central axis CL.

[0023] FIG. 1 is a graph showing the relationship between the aniline point of a fatty acid monoester and the volume change rate of an oil seal member. Specifically, the graph shows the tendency for each TR10 value, with the aniline point (°C) of the fatty acid monoester on the horizontal axis and the volume change rate of acrylonitrile butadiene rubber (NBR) on the vertical axis.

[0024] FIG. 2 is a graph showing the correlation between the TR10 value (°C) of acrylonitrile butadiene rubber (NBR) estimated from FIG. 2 and the aniline point (°C) of the fatty acid monoester for each volume change rate.

[0025] FIG. 2 is a table showing the substance names or general names of each fatty acid monoester shown in FIG. 2.

[0026] FIG. 3 is a graph showing the correlation between the nitrile content (mass%) of acrylonitrile butadiene rubber (NBR) used as an oil seal member and the TR10 value (°C).

[0027] FIG. 4 is a graph showing the relationship between the aniline point (°C) of a conventional petroleum-based working fluid and the volume change rate (%) of a low-nitrile rubber used as an oil seal member. The total carbon number of fatty acid monoester and 40°C kinematic viscosity (mm ２ 8 is a graph showing the correlation between the fatty acid monoesters (amount of fatty acid monoesters present in the fatty acid monoesters) and the amount of fatty acid monoesters present in the fatty acid monoesters.

[0009] A hydraulic device (shock absorber) according to one embodiment of the present invention will be described below. Note that the embodiment described below is specifically described to provide a better understanding of the gist of the present invention, and does not limit the present invention unless otherwise specified. Also, the scale of Figure 1 used in the following description may be changed as appropriate to make each part easier to see.

[0010] FIG. 1 is a longitudinal cross-sectional view of a twin-tube hydraulic device (hydraulic shock absorber) according to this embodiment, taken along a cross section including a central axis CL. This hydraulic shock absorber 1 includes a twin-tube cylinder 4, which includes a bottomed outer tube 2 and a bottomed inner tube 3 coaxially disposed within the outer tube 2. A piston rod 5 is inserted into the cylinder 4, with one end 5a inserted into the inner tube 3 and the other end protruding a predetermined distance outside the cylinder 4, and is reciprocally movable along the central axis CL, which is the axial direction of the cylinder 4. A piston 6 is connected to one end 5a of the piston rod 5, which slides along the inner surface of the inner tube 3 and moves within the inner tube 3 while remaining in axial contact with the inner tube 3. FIG. 1 illustrates the hydraulic shock absorber 1, with the central axis CL of the outer tube 2, inner tube 3, and piston rod 5 oriented vertically, with the bottoms of the outer tube 2 and inner tube 3 drawn downward and the other end of the piston rod 5 protruding from the inner tube 3 drawn upward.

[0011] The piston 6 divides the interior of the inner cylinder 3 into oil chambers A and B, and the volumes of oil chambers A and B change in response to movement of the piston 6 within the inner cylinder 3. A reservoir chamber C is provided between the outer cylinder 2 and the inner cylinder 3 on the inner circumferential side of the outer cylinder 2 and the outer circumferential side of the inner cylinder 3. A hydraulic fluid 7 for hydraulic equipment, which will be described in detail later, is filled in the oil chambers A, B, and C. The piston 6 has multiple flow paths (not shown) communicating with the oil chambers A and B, and these flow paths are provided with damping force generating mechanisms 8 and 9 incorporating valve mechanisms (not shown). When the volumes of the oil chambers A and B change as the piston 6 moves within the inner cylinder 3, the hydraulic fluid 7 for hydraulic equipment moves between the oil chambers A and B via the flow paths within the piston 6, causing the piston 6 to generate a damping force against the back-and-forth movement of the piston rod 5.

[0012] The bottom side of the inner cylinder 3 shown in Figure 1 is closed by a bottom wall member 12 incorporating a bottom valve 11. The bottom side of the outer cylinder 2 is closed by a bottom wall 13. The bottoms of the inner cylinder 3 and the outer cylinder 2 are arranged close to each other, with the bottom wall member 12 overlapping the bottom wall 13, and a communication chamber 14 is formed between the bottom wall member 12 and the bottom wall 13. This communication chamber 14 is in communication with the reservoir chamber C via a flow path member 15. The communication chamber 14 is also in communication with the oil chamber A via the bottom valve 11. When the piston 6 moves closer to the bottom wall member 12, the hydraulic fluid 7 for hydraulic equipment moves from the oil chamber A to the reservoir chamber C via the bottom valve 11, the communication chamber 14, and the flow path member 15. Furthermore, when the piston 6 moves in a direction away from the bottom wall member 12 , the hydraulic fluid 7 for hydraulic equipment moves from the reservoir chamber C to the oil chamber A via the flow path member 15 , the communication chamber 14 and the bottom valve 11 .

[0013] The upper end (opening: part) of the outer cylinder 2 is liquid-tightly closed by an oil seal member 17 made of acrylonitrile butadiene rubber (NBR) and integrated with the lid 16. As a result, a hydraulic fluid 7 for hydraulic equipment is sealed inside the cylinder 4. A through hole is formed in the center of the lid 16, and the piston rod 5 is provided to pass through this through hole. The annular oil seal member 17 is provided on the inner periphery of the through hole and in a position in contact with the outer periphery of the piston rod 5. In this specification, acrylonitrile butadiene rubber may be referred to as nitrile rubber, and the term nitrile rubber in this specification refers to acrylonitrile butadiene rubber unless otherwise specified.

[0014] As shown in Figure 1, a rod guide 10 is provided below the oil seal member 17 and within the upper end of the inner cylinder 3. This rod guide 10 has a cylindrical inner peripheral portion 10a and a cylindrical outer peripheral portion 10b. The rod guide 10 is arranged so that the inner peripheral portion 10a contacts the outer peripheral surface of the piston rod 5 and the outer peripheral portion 10b contacts the inner peripheral surface of the upper end of the outer cylinder 2. The rod guide 10 is in liquid-tight contact with the piston rod 5 at the inner peripheral surface of the inner peripheral portion 10a but allows the piston rod 5 to reciprocate.

[0015] 1, a ring-shaped plate-shaped cover 16 and an oil seal member 17 are integrally attached to the upper end of the outer cylinder 2, and the rod guide 10 is held down by the cover 16. The piston rod 5 passes through the cover 16 and protrudes to the outside of the inner cylinder 3, and an oil seal member 17 is provided integrally with the cover 16 between the inner periphery of the cover 16 and the outer periphery of the piston rod 5 to close the gap between them.

[0016] The hydraulic fluid 7 used in the hydraulic shock absorber 1 of this embodiment is primarily composed of a base oil containing a fatty acid monoester. For example, the hydraulic fluid 7 may contain approximately 90% by mass or more of base oil and 10% by mass or less of other additives. The additives may be those contained in general shock absorber working fluids of this type. Examples of additives include viscosity index improvers and pour point depressants. The base oil preferably contains a fatty acid monoester having an aniline point of -102°C or higher and lower than 51°C. When the base oil is used, the volume change of the oil seal member 17 measured at 100°C for 72 hours in accordance with JIS K 6258-4:2016 or ISO 1817:2015 is preferably 0% or higher and 25% or lower.

[0017] The oil seal member 17 is preferably made of acrylonitrile-butadiene rubber having an acrylonitrile content of 17% by mass or more and 32% by mass or less. Generally, rubber having an acrylonitrile content of less than 25% by mass is referred to as low-nitrile rubber, and rubber having an acrylonitrile content of 25% by mass or more and less than 31% by mass is referred to as medium-nitrile rubber. Therefore, the acrylonitrile-butadiene rubber having an acrylonitrile content of 17% by mass or more and 32% by mass or less can be referred to as medium-low-nitrile rubber. Generally, acrylonitrile-butadiene rubber having an acrylonitrile content of 31% by mass or more and less than 36% by mass is referred to as medium-high nitrile rubber, and acrylonitrile-butadiene rubber having an acrylonitrile content of 36% by mass or more and less than 43% by mass is referred to as high-nitrile rubber. Acrylonitrile-butadiene rubber having an acrylonitrile content of 43% by mass or more is referred to as very-high nitrile rubber.

[0018] The fatty acid monoesters having an aniline point of -102°C or more and less than 51°C were selected based on the correlation between the aniline point of the fatty acid monoester and the volume change rate of the oil seal member, as shown in the graph of Figure 2, which will be explained later. The correlation shown in Figure 2 is a relationship discovered by the present inventors. The base oil currently used in general hydraulic oils for shock absorbers is mineral oil, and mineral oils emit CO2 when disposed of. ２ In contrast, if a naturally-derived base oil is used as the base oil for the hydraulic fluid 7, the amount of CO emitted at the time of disposal is small. ２ This can reduce carbon dioxide emissions and contribute to decarbonization. Fatty acid esters can be derived from natural sources such as plants, but because fatty acid esters swell rubber more easily than mineral oils, they cannot easily be used in oil seal components made of low-nitrile rubber, which is currently in use. For this reason, if natural fatty acid esters are used as base oils, it will be necessary to use medium- to high-nitrile rubber, which has poor cold resistance, or expensive cold-resistant fluororubber for oil seal components.

[0019] Therefore, the present inventors have conducted extensive research into whether there is a means for applying a fatty acid ester to the base oil while still using oil seal members made of low- and medium-temperature nitrile rubber that are currently in use. As a result, it has been found that by using the above-mentioned aniline point fatty acid monoester base oil and employing low- and medium-temperature nitrile rubber, it is possible to ensure sealing performance at low temperatures of -30°C or below while suppressing swelling, without employing expensive oil seal members such as cold-resistant fluororubber. Furthermore, if a naturally derived fatty acid monoester base oil is used, CO2 emissions at the time of disposal can be reduced. ２ Emissions can also be reduced.

[0020] The aniline point is one of the property values ​​of oil, and is known to be related to the volume change of rubber. Measurement of the aniline point is specified in JIS K2256:2013 or ISO 2977:1997. It is also known that when the aniline point is low, it can be reliably measured by measuring the mixed aniline point using the following formula (a): Aniline point = (mixed aniline point + 69.3) / 2 ... formula (a) Here, the aniline point and mixed aniline point are both in Celsius temperature (°C).

[0021] It is known that when various types of rubber are immersed in (exposed to) various types of oil, there is a certain correlation between the aniline point of the oil and the volume change rate (%), as shown in Figure 6. In the graph shown in Figure 6, the horizontal axis shows the aniline points of petroleum-based working fluids that have been commonly used in the past, and the vertical axis shows the volume change rate of low-nitrile rubber.

[0022] As shown in Figure 6, in order to keep the volume change rate within the range of 0 to 15% for low nitrile rubber, it is necessary to use oil with an aniline point in the range of 80°C to 105°C. Also, from the results shown in Figure 6, it can be seen that for low nitrile rubber, the lower the aniline point of the oil, the greater the volume change rate. For this reason, it is generally believed that oils with low aniline points are unsuitable for oil seal members made of acrylonitrile butadiene rubber.

[0023] While understanding the above-mentioned generally known relationship, the present inventor has studied various base oils and found that there is a base oil which can suppress the volume change rate of acrylonitrile-butadiene rubber low even if it has a low aniline point, and that base oil is a base oil mainly composed of fatty acid monoester.

[0024] The fatty acid monoester of this embodiment has a structure in which a fatty acid and an alcohol are bonded by a dehydration condensation reaction. It is preferable to use a plant-derived fatty acid. The alcohol can be selected from plant-derived alcohols, but alcohols other than plant-derived alcohols may also be used. An example of the structure of the fatty acid monoester has a structure shown in the following formula (1). In formula (1), R represents an alkyl group. In formula (1), the portion enclosed by a chain line on the left side is an alcohol-derived structure, and the portion enclosed by a chain line on the right side is a fatty acid-derived structure. The alcohol-derived structure may be a plant-derived structure, or a non-plant-derived alcohol-derived structure. Considering the environmental impact, the fatty acid-derived structure is preferably a plant-derived structure.

[0025]

[0026] The fatty acid used as a raw material for the fatty acid monoester of this embodiment may be any one of lauric acid represented by the following formula (2), stearic acid represented by the following formula (3), oleic acid represented by the following formula (4), linoleic acid represented by the following formula (5), and linolenic acid represented by the following formula (6). Furthermore, the raw material alcohol may be any one of isobutanol (2-methylpropanol) represented by the following formula (7), 2-ethylhexanol represented by the following formula (8), or isotridecanol represented by the following formula (9). More specifically, the fatty acid monoester may be, for example, 2-ethylhexyl laurate represented by the following formula (10), 2-ethylhexyl oleate represented by the following formula (11), or isotridecyl stearate represented by the following formula (12). Fatty acid esters made from raw materials represented by any of these chemical formulas have an aniline point of −102° C. or higher and lower than 51° C.

[0027]

[0028]

[0029]

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037]

[0038] In addition to the fatty acids described above, examples of fatty acids used as raw materials for the fatty acid monoester used in this embodiment include myristic acid, palmitic acid, palmitoleic acid, and vegetable oil fatty acids containing these acids in combination, such as palm oil, soybean oil, and rice bran oil.

[0039] In addition to the alcohols described above, alcohols used as raw materials for the fatty acid monoesters used in this embodiment can also be used, such as linear alcohols such as n-butanol and branched alcohols derived from plants such as 2-octanol. Other examples of fatty acid monoesters that can be used in this embodiment include rice bran oil fatty acid n-butyl and methyl hepsyl myristate.

[0040] In this embodiment, a base oil primarily composed of a fatty acid monoester can be used for the working fluid 7 in the hydraulic shock absorber 1 having the above-described configuration. However, there is a demand for a low-temperature nitrile rubber with cold resistance at temperatures below -30°C to be used for the oil seal member 17. Therefore, research into the combination of a base oil primarily composed of a fatty acid monoester and a low-temperature nitrile rubber has revealed that a fatty acid monoester with an aniline point between -102°C and 51°C, such as the fatty acid monoester described above, can ensure sealing performance at low temperatures. Furthermore, the low-temperature nitrile rubber that constitutes the oil seal member 17 has a TR10 value of -55°C or higher and -30°C or lower, measured in accordance with JIS K 6261-4:2017 or ISO 2921:2011. The TR10 value refers to the Celsius temperature (°C) at which the shrinkage rate reaches 10% based on the temperature-shrinkage rate curve obtained by a TR test (Low-temperature retraction (TR test)) as specified in JIS K 6261-4:2017 or ISO 2921:2011.

[0041] Furthermore, the hydraulic equipment working fluid 7 of this embodiment may contain one or more additives such as a viscosity index improver and a pour point depressant in addition to the base oil. For example, of 100 mass% of the hydraulic equipment working fluid 7, the additives may be about 10 mass% or less, with the remainder being the base oil. As an example, the total content of additives may be 10 mass% for 90 mass% base oil. The properties of the hydraulic equipment working fluid 7 can be adjusted to some extent by mixing with a different type of base oil or by adding additives. For example, if the kinematic viscosity at 40°C of the base oil is 8.5 mm of JASO C602:2001, ２Even if the pour point of the base oil is above -30°C as specified in JASO C602:2001, a pour point of -30°C can be achieved by adding a certain amount of pour point depressant.

[0042] The base oil may include a mineral oil classified as Group I, II, or III in API 1509:2021, or a synthetic oil classified as Group IV or V. That is, in addition to containing 50 mass% or more of the base oil made of the fatty acid monoester described above, the base oil may also include the synthetic oil described above as a base oil other than the above base oil.

[0043] The fatty acid monoester preferably has a carbon number of 20 or more and 34 or less. There is a correlation between the molecular weight of the polymer material, i.e., the total carbon number, and the viscosity. ２ According to the test described below, it has been found that the total carbon number of a fatty acid monoester having a kinematic viscosity of 1 / s or less is 34 or less. Among the fatty acid monoesters described above that exhibit excellent properties (viscosity, pour point, rubber swelling), the one with the smallest total carbon number is 2-ethylhexyl laurate shown above in formula (10), which has 20 carbon atoms. Furthermore, among alcohols, the one with the smallest number of carbon atoms in the alkyl group is isobutanol shown in formula (7), which has 4 carbon atoms. Furthermore, based on the test results described below and taking into account the relationship with the 40°C kinematic viscosity, it is preferable that the number of carbon atoms in the fatty acid monoester is 20 or more and 34 or less.

[0044] The fatty acid monoester preferably contains 40% by mass or more of polyunsaturated fatty acids. The main fatty acid composition of vegetable oils is unsaturated fatty acids such as oleic acid and linoleic acid. The higher the unsaturated fatty acid content, the better the fluidity and the lower the viscosity and melting point (or freezing point). Palm oil and soybean oil account for the majority of vegetable oils currently on the market, with palm oil containing 40% by mass or more of polyunsaturated fatty acids and soybean oil containing 70% by mass or more of polysaturated fatty acids. Many other natural oils and fats, such as palm olein, olive oil, rapeseed oil, sesame oil, cottonseed oil, sunflower oil, safflower oil, rice bran oil, corn oil, linseed oil, beef tallow, lard, and fish oil, contain 40% by mass or more of polyunsaturated fatty acids. The polyunsaturated fatty acids referred to here include oleic acid, linoleic acid, and linolenic acid.

[0045] The fatty acid monoesters described above preferably have a linear structure. The main fatty acid composition of vegetable oils is the fatty acids shown in the formulas (3) to (6) above, and it is clear that they do not have a branched structure. Therefore, when using naturally occurring fatty acids and alcohols such as vegetable oils as raw materials, those with a linear structure are more readily available.

[0046] If the hydraulic shock absorber 1 uses the hydraulic fluid 7 for hydraulic equipment using the base oil and the oil seal member 17 using the medium-low nitrile rubber, it is not necessary to use the medium-high nitrile rubber, which has problems with cold resistance, and it is not necessary to use expensive cold-resistant fluororubber. When selecting fatty acids and alcohols, plant-derived ones can be selected. Therefore, according to this embodiment, it is possible to achieve excellent oil sealing properties at low temperatures while reducing CO2 emissions when disposing of the working fluid. ２ This contributes to reducing CO₂ emissions, making it possible to provide an environmentally friendly hydraulic shock absorber (hydraulic equipment) 1 at low cost. Furthermore, by adjusting the carbon number of the fatty acid monoester as described above, the viscosity and pour point can be set within ranges suitable for oil seals, thereby achieving good low-temperature fluidity. The viscosity and pour point of the hydraulic equipment working fluid 7 can also be adjusted to some extent by adjusting additives such as viscosity index improvers and pour point depressants.

[0047] In this embodiment, the hydraulic shock absorber 1 having the configuration shown in Fig. 1 is used as an example of application of the hydraulic fluid 7 for hydraulic equipment, but the application is not limited to this example. The hydraulic fluid 7 for hydraulic equipment can be applied to a wide range of hydraulic equipment as long as the hydraulic equipment has a configuration in which a hydraulic fluid is sealed in a cylinder and an oil seal member provided in the cylinder is in contact with the hydraulic fluid.

[0048] Figure 2 is a graph examining the correlation between the aniline point of fatty acid monoester and the volume change rate of acrylonitrile-butadiene rubber. Figure 3 shows the results of trial calculation of the correlation between the TR10 value of acrylonitrile-butadiene rubber (NBR) and the aniline point of fatty acid monoester for each volume change rate from the regression line drawn in Figure 2. The names of substances showing the aniline point of each plot shown in Figure 2 are as shown in Figure 4. TOSOLV-ME, TOSOLV-RB, TOSOLV-IB, and TOSOLV-89 shown in Figure 4 are all trade names of Toshin Yushi Co., Ltd., EXCEPEARL EH-L and EXCEPEARL TD-S are all trade names of Kao Corporation, and UNISTAR MB876 and UNISTAR MB881 are all trade names of NOF Corporation.

[0049] Since JASO C 602:2001 and JASO C 611:1993 stipulate that the low-temperature characteristics of automotive hydraulic shock absorbers are limited to -30°C, it was determined that the cold resistance of oil seal members must also be -30°C or less. To achieve this, a medium nitrile rubber with a TR10 value of -30°C and a volume change rate of 0% to 25% is estimated to have an aniline point of -102°C to 17°C, as shown in Figure 3. For a TR10 value of -55°C, the aniline point required to satisfy a volume change rate of 0% to 25% is estimated to be 44°C to 51°C, as shown in Figure 2. Therefore, assuming an oil seal member made of medium-low nitrile rubber, a fatty acid monoester with an aniline point of -102°C or higher and lower than 51°C can be determined to suppress the volume change rate of the oil seal member to a range of 0% to 25%.

[0050] Figure 5 is a graph showing the results of an investigation into the correlation between nitrile content and TR10 value using commercially available packings and oil seals with hardnesses of 80 to 85 as specified in JIS K 6253-3:2012 or ISO 7619-1:2010. NOK Corporation's commercially available product, product name A567 (NBR), guarantees a cold resistance of -55°C in terms of TR10 value. This cold resistance is considered to be the highest level of cold resistance among commercially available sealing materials for hydraulic equipment. The nitrile content of A567 is estimated to be approximately 17% based on the regression line in Figure 5. The nitrile content required to achieve a TR10 value of -30°C is estimated to be approximately 32% by mass based on the regression line in Figure 5.

[0051] Taking these into consideration, it is considered necessary for the oil seal member to have a TR10 value of −55° C. or higher and −30° C. or lower as measured in accordance with JIS K 6261-4:2017 or ISO 2921:2011. It is also considered preferable for the oil seal member to have an acrylonitrile content of 17% by mass or higher and 32% by mass or lower.

[0052] It is considered preferable that the minimum volume change rate (expansion rate) of the oil seal member be 0% and the maximum be 25%. Based on the lower limit of the allowable swelling rate when using high-swelling automotive test lubricant No. 3 (IRM903, hereafter referred to as No. 3 oil), it is preferable that the minimum allowable swelling rate be 0%. It was determined that the maximum allowable swelling rate should be 25% based on the upper limit of the allowable swelling rate when using No. 3 oil as specified in JIS B 2402-4:2013 or ISO 6194-4:2009. Regarding the upper limit of the allowable swelling rate when using No. 3 oil as specified in JIS B 2403:2009, the oil seal member of the present invention is 25%, including rotational movement, while V-packings are used exclusively for reciprocating movement. Therefore, for a structure intended for reciprocating movement sealing, such as the present invention, it is considered more preferable to set the upper limit of the allowable swelling rate to 20%.

[0053] The present invention is also applicable to hydraulic equipment such as automotive dampers that reciprocate at high frequencies, where internal pressure increases with temperature, where the leakage direction coincides with the direction of gravity, and where maintenance is expected to be infrequent or difficult. Furthermore, the present invention is applicable to hydraulic equipment that is mass-produced and susceptible to tolerances in oil seals. Considering such demanding applications, it is most preferable to set the upper limit of the allowable expansion coefficient at 15%.

[0054] FIG. 7 shows the relationship between the total carbon number of fatty acid monoesters and the 40°C kinematic viscosity (mm ２ 7 is a graph showing the correlation between the solubility of cellulose in water and the solubility of cellulose in water (particle size / s). The names of the substances plotted in FIG. 7 are as shown in FIG. 8. EXCEPEARL EH-L and EXCEPEARL TD-S are both trade names of Kao Corporation. UNISTAR MB876, UNISTAR MB881, UNISTAR MB816, and UNISTAR M182A are all trade names of NOF Corporation. NIKKOL GS-MHIS, NIKKOL GS-MHL, NIKKOL GS-MHM, and NIKKOL GS-MHP are all trade names of Nikko Chemicals Co., Ltd.

[0055] From the results shown in FIG. 7, it is possible to determine the kinematic viscosity (4 to 19 mm) that is desirable for the constituent material of the oil seal member. ２ It can be seen that fatty acid monoesters exhibiting the above viscosity (s) have a carbon number of 20 or more and 34 or less. Furthermore, since fatty acid monoesters obtained from alcohols with a small carbon number, such as n-butanol or isobutanol, also have a favorable viscosity range, it is preferable that the number of carbon atoms in the alkyl group of the raw material alcohol is 4 or more.

[0056] From the relationships shown in Fig. 2 and Fig. 3, it is considered that the higher the aniline point of the fatty acid monoester base oil, the more difficult it is to swell acrylonitrile-butadiene rubber. It is considered that the aniline point that does not swell the acrylonitrile-butadiene rubber with a nitrile content of 17% which is the most easily swellable (swelling ratio 0%) is the highest (51°C). It is considered that an aniline point (-102°C) that stops the swelling of the acrylonitrile-butadiene rubber with a nitrile content of 32% by mass which is the most difficult to swell at 25% is necessary, the minimum aniline point (-78°C) that stops the swelling at 20% is more preferable, and the minimum aniline point (-54°C) that stops the swelling at 15% is most preferable.

[0057] According to the above aspects of the present invention, it is possible to provide a hydraulic device and a working fluid for a hydraulic device that have a low environmental impact, can suppress swelling of oil seal members, and has a low viscosity and good low-temperature fluidity, and therefore has great industrial applicability.

[0058] REFERENCE SIGNS LIST 1... Hydraulic shock absorber (hydraulic equipment: shock absorber) 2... Outer cylinder 3... Inner cylinder 4... Cylinder 5... Piston rod 6... Piston 7... Hydraulic equipment working fluid 8, 9... Damping force generating mechanism 10... Rod guide 11... Bottom valve 17... Oil seal member A, B... Oil chamber C... Reservoir chamber

Claims

1. A hydraulic device comprising: a cylinder; an oil seal member comprising acrylonitrile butadiene rubber and provided on the cylinder; and a working fluid sealed within the cylinder, wherein the working fluid comprises a base oil and an additive added to the base oil; the oil seal member has an acrylonitrile content of 7% by mass or more and 32% by mass or less; the base oil comprises a fatty acid monoester having an aniline point of -102°C or more and less than 51°C; and the volume change of the oil seal member measured at 100°C for 72 hours in accordance with JIS K 6258-4:2016 or ISO 1817:2015 is 0% or more and 25% or less.

2. The hydraulic device according to claim 1, wherein the fatty acid monoester has a carbon number of 20 or more and 34 or less, and the alkyl group of the alcohol in the fatty acid monoester has a carbon number of 4 or more.

3. The hydraulic equipment according to claim 1, wherein the fatty acid monoester contains 40% or more of polyunsaturated fatty acids.

4. The hydraulic device according to claim 1, wherein the fatty acid monoester has a linear structure and does not have a branched structure.

5. The hydraulic device according to any one of claims 1 to 4, wherein the working fluid contains at least one of a base oil other than the base oil, a viscosity index improver, and a pour point depressant.

6. The hydraulic device according to any one of claims 1 to 4, further comprising: a piston that slides inside the cylinder; and a piston rod that is connected to the piston and moves back and forth along the axial direction of the cylinder while contacting the oil seal member.

7. A working fluid for hydraulic equipment, comprising a base oil and an additive added to the base oil, and used in an environment in which it comes into contact with an oil seal member containing acrylonitrile butadiene rubber, wherein the oil seal member has a TR10 value of -55°C or higher and -30°C or lower when measured in accordance with JIS K 6261-4:2017 or ISO 2921:2011, the base oil contains a fatty acid monoester having an aniline point of -102°C or higher and lower than 51°C, and the volume change of the oil seal member is 0% or higher and 25% or lower when measured in accordance with JIS K 6258-4:2016 or ISO 1817:2015 under conditions of 100°C for 72 hours.

8. The working fluid for hydraulic equipment according to claim 7, wherein the fatty acid monoester has 20 to 34 carbon atoms, and the alkyl group of the alcohol of the fatty acid monoester has 4 or more carbon atoms.

9. The working fluid for hydraulic equipment according to claim 7, wherein the fatty acid monoester contains 40 mass % or more of polyunsaturated fatty acids.

10. The working fluid for hydraulic equipment according to claim 7, wherein the fatty acid monoester has a linear structure and does not have a branched structure.

11. The working fluid for hydraulic equipment according to any one of claims 7 to 10, comprising at least one of a base oil other than the base oil, a viscosity index improver, and a pour point depressant.

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