JP2008255239A - Gear oil composition - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gear oil composition which simultaneously satisfies low-temperature fluidity and shear stability at a high level and is excellent in corrosion-preventing properties and oxidation stability as well as moderate extreme pressure properties. <P>SOLUTION: The gear oil composition is prepared by adding to a lubricating base oil (A) of a viscosity index of 110 or more, 0.05-0.5 mass% (in terms of the mass of phosphorus element) at least one phosphorus-containing extreme pressure additive (B) selected from the group consisting of phosphite esters and phosphate esters, 0.005-0.2 mass% nitrogen-containing corrosion inhibitor (C-1), 0.005-0.2 mass% nitrogen- and sulfur-containing corrosion inhibitor (C-2), 1-10 mass% ethylene-α-olefin copolymer (D) as a viscosity index improver, and 0.02-0.2 mass% (in terms of the mass of zinc element) zinc dialkyldithiophosphate (E) and has a sulfur content of 1.4 mass% or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gear oil composition, and more particularly to a gear oil composition used in a high speed and high load environment.

  Currently, various types of lubricating oils are used in various industrial fields in order to reduce friction and wear on the contact surface during contact sliding between metals. Of these lubricating oils, gear oils are required to be excellent in extreme pressure properties, oxidation stability, wear prevention properties, shear stability, etc. in addition to lubricity under use conditions. In order to satisfy these performances, conventionally, gear oils are blended with various additives such as extreme pressure agents, corrosion inhibitors, antioxidants, viscosity index improvers, etc., in base oils composed of mineral oils and / or synthetic oils. Manufactured.

  For example, JP-A-2002-371291 (Patent Document 1) discloses a lubricating oil composition in which a base oil composed of mineral oil and / or synthetic oil is blended with a thiadiazole derivative having a specific structure and a phosphorus-containing compound having a specific structure. ing. The lubricating oil composition is excellent in oxidation stability at high oil temperature and wear resistance, and has improved fatigue life due to prevention of pitching, and is suitable as gear oil and transmission oil for automobiles.

JP 2002-371291 A

  By the way, when the apparatus / device is started, the entire apparatus is basically at substantially the same temperature as the outside air temperature. For this reason, when using devices and equipment in cold regions, the lubricating oil used for this is not only performance under normal conditions but also fluidity at low temperatures so that gear seizure and malfunction do not occur at startup. Is also required. However, there are various problems to improve the fluidity at low temperatures while maintaining the performance under normal conditions.

  For example, when a synthetic oil typified by poly-α-olefin (PAO) is used as the base oil used in the lubricating oil composition, although the performance part of the above-mentioned problem is easy to solve, the cost increases significantly. There is a problem. On the other hand, when the mineral oil is used as the base oil to lower the viscosity and the viscosity index is improved by using a viscosity index improver represented by polymethacrylate (PMA), the fluidity at low temperature can be secured, but the gear There is a problem that the shear stability important as oil cannot be sufficiently secured.

  Furthermore, some of the extreme pressure agents necessary for securing extreme pressure properties also deteriorate the low temperature fluidity. In addition, as an extreme pressure agent, it is necessary to select a compound having reactivity suitable for the environment in which the gear oil is used. If too much importance is attached to extreme pressure properties, corrosion of the metal or oxidation stability of the gear oil will occur. May be reduced. Therefore, when the gear oil is used in a low temperature environment, the extreme pressure agent added to the gear oil composition does not deteriorate the low temperature fluidity while having the extreme pressure necessary for the use environment, and the gear oil. It is necessary to select a compound that does not significantly reduce the corrosion prevention property and oxidation stability of the material and add it in an appropriate amount.

  The object of the present invention is to solve the above-mentioned problems, and at a high level, low-temperature fluidity and shear stability are compatible, and a gear oil that has excellent anti-corrosion and oxidation stability while having moderate extreme pressure. It is to provide a composition.

  As a result of diligent investigations to achieve the above object, the present inventors have achieved a high level by adding an ethylene-α-olefin copolymer as a viscosity index improver to a lubricating base oil having a viscosity index of 110 or more. In addition to phosphorus-based extreme pressure agent, nitrogen-based corrosion inhibitor, nitrogen / sulfur-based corrosion inhibitor, zinc dialkyldithiophosphate (ZnDTP) is added as an antioxidant. As a result, it was found that a gear oil composition having an appropriate extreme pressure and excellent corrosion resistance and oxidation stability was obtained, and the present invention was completed.

That is, the gear oil composition of the present invention is
To the lubricating base oil (A) with a viscosity index of 110 or more,
0.05-0.5% by mass with at least one phosphorus-based extreme pressure agent (B) selected from the group consisting of phosphite and phosphate as phosphorus element mass,
0.005 to 0.2% by mass of nitrogen-based corrosion inhibitor (C-1),
0.005-0.2% by mass of nitrogen / sulfur corrosion inhibitor (C-2),
1 to 10% by mass of ethylene-α-olefin copolymer (D) as a viscosity index improver and 0.02 to 0.2% by mass of zinc dialkyldithiophosphate (E) as zinc element mass,
The sulfur content is 1.4% by mass or less. The gear oil composition of the present invention preferably has a sulfur content / phosphorus content of 6.5 or less, and preferably has a Brookfield (BF) viscosity at −40 ° C. of 100,000 mPa · s or less.

  In the gear oil composition of the present invention, by adding a specific amount of the ethylene-α-olefin copolymer (D) as a viscosity index improver to the lubricating base oil (A) having a viscosity index of 110 or more, Fluidity and shear stability can be achieved at a high level. Furthermore, a specific amount of a phosphorus extreme pressure agent (B), a nitrogen corrosion inhibitor (C-1), and a nitrogen / sulfur corrosion inhibitor (C-2) is added to the lubricating base oil (A). By adding a specific amount of zinc dialkyldithiophosphate (E) as an antioxidant, corrosion resistance and oxidation stability can be greatly improved while imparting moderate extreme pressure. Therefore, according to the present invention, there is provided a gear oil composition that has both low temperature fluidity and shear stability at a high level, and has excellent anti-corrosion and oxidation stability while having an appropriate extreme pressure. It becomes possible to do.

  The present invention is described in detail below. The base oil used in the gear oil composition of the present invention is a lubricating base oil (A) having a viscosity index of 110 or more. The lubricating base oil (A) may be a mineral oil or a synthetic oil of a lubricating oil fraction, and uses those conventionally used as base oils for various lubricating oils including gear oils. it can. The lubricating base oil (A) needs to have a viscosity index of 110 or more, preferably 120 or more. When a lubricating base oil having a viscosity index of less than 110 is used, the Brookfield (BF) viscosity at −40 ° C. increases remarkably, and sufficient low temperature fluidity cannot be secured. In addition, when a lubricating base oil having a viscosity index of 120 or more is used, the low temperature fluidity is particularly good. In general, the viscosity index of the lubricating base oil (A) is 180 or less.

  Specifically, as mineral oil, a fraction obtained by subjecting atmospheric distillation residue such as paraffinic crude oil to vacuum distillation is treated with a solvent extraction method such as furfural, hydrorefining, and a solvent dewaxing method such as MEK / toluene. Lubricating base oil, slack wax and FT synthesis obtained by treating the base oil obtained by treating with the above-mentioned appropriate treatment method. Highly refined base oil obtained by hydroisomerizing wax and the like, Highly refined base oil obtained by MEW / toluene solvent dewaxing or hydrodewaxing a suitable fraction obtained by hydrocracking heavy oil , And mixtures thereof can be used.

  Synthetic oils include α-olefin oligomers, diesters synthesized from dibasic acids such as adipic acid and primary alcohols, polyhydric alcohols such as neopentyl glycol, trimethylolpropane, pentaerythritol, and monovalents. Polyol esters synthesized from basic acids, alkylbenzenes, polyoxyalkylene glycols, and mixtures thereof can be used.

  Among these base oils, mineral oil is preferable in terms of cost, and mineral oil obtained by hydrocracking is more preferable.

The lubricating base oil (A) preferably has a kinematic viscosity (100 ° C.) of 4 to ²⁰ mm ² / s. If the kinematic viscosity (100 ° C.) of the lubricating base oil (A) is too low, it is necessary to increase the viscosity index improver to be added in order to ensure an appropriate product viscosity. Moreover, there is a possibility of causing an increase in wear due to a decrease in the oil film thickness at the sliding portion. On the other hand, if the kinematic viscosity (100 ° C.) of the lubricating base oil (A) is too high, the fluidity at low temperatures may not be sufficiently secured.

  The lubricating base oil (A) preferably has an aromatic content of 5% by mass or less. When the aromatic content in the lubricating base oil (A) is 5% by mass or less, the stability to heat and oxidation can be further increased. The aromatic content in the lubricating base oil (A) can be determined by ndM ring analysis as defined in ASTM D3238.

  The lubricating base oil (A) preferably has a sulfur content of 0.01% by mass or less. If the sulfur content in the lubricating base oil (A) is 0.01% by mass or less, corrosion due to the sulfur content can be sufficiently suppressed.

  The gear oil composition of the present invention contains 0.05 to 0.5% by mass of at least one phosphorus-based extreme pressure agent (B) selected from the group consisting of a phosphite and a phosphate as a phosphorus element mass, It is preferable to contain 0.1 mass%. The gear oil composition of the present invention preferably contains both a phosphite ester and a phosphate ester. If the content of the phosphorus extreme pressure agent (B) is less than 0.05% by mass as the elemental phosphorus, the extreme pressure and oxidation stability will be insufficient. On the other hand, if the content of the elemental phosphorus exceeds 0.5% by mass, corrosion or chemical There is a possibility of causing wear and / or deterioration of oxidation stability. In addition, it becomes possible to keep corrosion prevention property and oxidation stability favorable by content of phosphorus type extreme pressure agent (B) being 0.1 mass% or less as phosphorus element mass.

  The phosphorous acid ester (B-1) is a compound in which one or more hydroxyl groups of phosphorous acid are esterified, and may be a triester, a diester, or a monoester. However, diesters are preferred. Further, as the phosphite (B-1), specifically, dialkyl phosphites such as di-2-ethylhexyl phosphite and dilauryl phosphite are preferable, and among these, carbon number such as dilauryl phosphite is preferable. Particularly preferred are dialkyl phosphites having 8 to 18 alkyl groups.

  The phosphoric ester (B-2) is a compound in which one or more hydroxyl groups of phosphoric acid are esterified, and may be a triester, a diester, or a monoester. Triesters are preferred. Specific examples of the phosphate ester (B-2) include triphenyl phosphate, tricresyl phosphate, triethyl phenyl phosphate, tripropyl phenyl phosphate, tributyl phenyl phosphate, cresyl diphenyl phosphate. Triaryl phosphates such as ethyl phenyl diphenyl phosphate, propyl phenyl diphenyl phosphate, butyl phenyl diphenyl phosphate, dicresyl phenyl phosphate, diethyl phenyl phenyl phosphate, dipropyl phenyl phenyl phosphate, dibutyl phenyl phenyl phosphate Among these, a triaryl phosphate having an aryl group having 6 to 12 carbon atoms such as tricresyl phosphate is preferable. Especially preferred is a salt.

  The gear oil composition of the present invention contains 0.005 to 0.2 mass%, preferably 0.01 to 0.1 mass% of a nitrogen-based corrosion inhibitor (C-1) as the corrosion inhibitor (C). If the content of the nitrogen-based corrosion inhibitor (C-1) is less than 0.005% by mass, the corrosion prevention and / or oxidation stability will be insufficient. On the other hand, even if it exceeds 0.2% by mass, the oxidation stability And the effect of corrosion stability does not increase remarkably, and if it is added excessively, it leads to thickening of the oil after oxidative degradation, so addition within the above range is preferable. In addition, by setting the content of the nitrogen-based corrosion inhibitor (C-1) to 0.01% by mass or more, the corrosion resistance and oxidation stability of the gear oil composition are greatly improved, while on the other hand, 0.1% by mass or less. By doing so, moderate oxidation stability and corrosion stability can be imparted without thickening.

  In the present invention, the nitrogen-based corrosion inhibitor (C-1) is a nitrogen-containing corrosion inhibitor and does not contain sulfur in the molecule. 2). As the nitrogen-based corrosion inhibitor (C-1), a heterocyclic compound such as a triazole derivative having 3 or more N in the ring and containing no sulfur in the molecule, or an N—C—N bond. A heterocyclic compound such as an imidazoline derivative or pyrimidine derivative having no sulfur in the molecule, and a heterocyclic compound having a> NH group and further having one or more nitrogen atoms, Compounds that do not contain sulfur are particularly preferred. Specifically, as the nitrogen-based corrosion inhibitor (C-1), benzoic compounds such as N, N-bis (2-ethylhexyl)-(4 or 5) -methyl-1H-benzotriazole-1-methylamine are used. And triazole derivatives.

  The gear oil composition of the present invention contains 0.005 to 0.2 mass%, preferably 0.01 to 0.1 mass% of a nitrogen / sulfur corrosion inhibitor (C-2) as the corrosion inhibitor (C). If the content of the nitrogen / sulfur corrosion inhibitor (C-2) is less than 0.005% by mass, the corrosion prevention and / or oxidation stability will be insufficient. The effect of stability and / or corrosion stability does not increase remarkably, and excessive addition leads to thickening of the oil after oxidative degradation, so addition within the above range is preferable. In addition, by making the content of nitrogen / sulfur corrosion inhibitor (C-2) 0.01% by mass or more, the corrosion resistance and oxidation stability of the gear oil composition are greatly improved, while 0.1% by mass By making it below, moderate oxidation stability and corrosion stability can be imparted without increasing the viscosity even after oxidative degradation.

  In the present invention, the nitrogen / sulfur corrosion inhibitor (C-2) is a corrosion inhibitor containing nitrogen and sulfur, and the above-mentioned nitrogen corrosion inhibitor (C-2) contains sulfur in the molecule. C-1). As the nitrogen / sulfur corrosion inhibitor (C-2), a compound having an N—C—S bond is preferable, and among them, a —N═C (—X) —S—S—R structure (wherein X Is particularly preferably a compound having NH, O or S, and R is a hydrocarbon group. Specific examples of the nitrogen / sulfur corrosion inhibitor (C-2) include thiadiazole derivatives, and in particular, polysulfide structures such as alkylated 2,5-dimercapto-1,3,4-thiadiazole. Preferred are thiadiazole derivatives.

  The gear oil composition of the present invention contains 1 to 10% by mass of an ethylene-α-olefin copolymer (D) as a viscosity index improver. The ethylene-α-olefin copolymer is obtained by copolymerizing ethylene and α-olefin with a coordination anion polymerization catalyst or the like, and the viscosity can be adjusted depending on the degree of polymerization. When the content of the ethylene-α-olefin copolymer (D) is less than 1% by mass, the viscosity index cannot be sufficiently improved. On the other hand, when the content exceeds 10% by mass, the effect of improving the viscosity index is saturated. In addition, when additives other than ethylene-alpha-olefin copolymer (D) are used as a viscosity index improver, the shear stability of a gear oil composition will deteriorate.

  The ethylene-α-olefin copolymer (D) preferably has a number average molecular weight of 1,000 to 30,000, and more preferably 2,000 to 10,000. If the number average molecular weight of the ethylene-α-olefin copolymer (D) is 1,000 or more, a sufficient viscosity index improving effect can be obtained, while if it is 30,000 or less, shear stability required for gear oil is obtained. Is secured. By using an ethylene-α-olefin copolymer (D) having a number average molecular weight of 2,000 to 10,000, the balance between the effect of improving the viscosity index and the shear stability becomes the best.

  The gear oil composition of the present invention contains 0.02 to 0.2% by mass, and preferably 0.05 to 0.1% by mass, of zinc dialkyldithiophosphate (E) as an elemental zinc as an antioxidant. When the content of zinc dialkyldithiophosphate (E) is less than 0.02% by mass as the elemental zinc, the oxidation stability is insufficient. On the other hand, when the elemental zinc content exceeds 0.2% by mass, the zinc compound is also deteriorated. It increases, and there is a possibility that it becomes sludge without being dispersed in the oil and causes a malfunction of the apparatus. In addition, if the content of zinc dialkyldithiophosphate (E) is 0.02% by mass or more as the elemental zinc mass, the oxidation stability is particularly good. It will not be as much sludge as it comes.

［式中、Ｒ¹は、アルキル基であり、同一であっても異なってもよい］で表わされる。ここで、Ｒ¹におけるアルキル基としては、二級アルキル基（sec-）が好ましく、sec-ヘキシル基が特に好ましい。一般に、二級アルキル基（sec-）を有するジチオリン酸亜鉛は、一級アルキル基（ｎ-）を有するジチオリン酸亜鉛に比べて、耐摩耗性・極圧性に非常に優れる一方、酸化安定性に劣り、アリール基を有するジチオリン酸亜鉛は、一級アルキル基（ｎ-）を有するジチオリン酸亜鉛に比べて、酸化安定性に非常に優れる一方、耐摩耗性・極圧性に劣る特徴がある。そして、ギヤ油は、耐摩耗性・極圧性を重視するため、二級アルキル基（sec-）を有するジチオリン酸亜鉛が好ましいが、他のタイプのジチオリン酸亜鉛を併用してもよい。なお、アルキル基Ｒ¹の炭素数は４〜１０が好ましい。 The zinc dialkyldithiophosphate (E) is represented by the following general formula (I):

[Wherein R ¹ is an alkyl group which may be the same or different]. Here, the alkyl group in R ¹ is preferably a secondary alkyl group (sec-), and particularly preferably a sec-hexyl group. In general, zinc dithiophosphate having a secondary alkyl group (sec-) is very excellent in wear resistance and extreme pressure, but inferior in oxidation stability, compared to zinc dithiophosphate having a primary alkyl group (n-). In addition, zinc dithiophosphate having an aryl group is superior in oxidation stability to zinc dithiophosphate having a primary alkyl group (n-), but is inferior in wear resistance and extreme pressure. The gear oil is preferably zinc dithiophosphate having a secondary alkyl group (sec-) in order to place importance on wear resistance and extreme pressure, but other types of zinc dithiophosphate may be used in combination. The number of carbon atoms in the alkyl group R ¹ is preferably 4 to 10.

  The gear oil composition of the present invention further includes additives such as ashless dispersants, pour point depressants, antifoaming agents, and more specifically, boron-containing succinimides, sulfur-based oil agents, dithiophosphoric acids. Amines, organic amides, nonionic activators and the like can be added as appropriate. However, since the metal-based detergent is a basic component, it reacts with acidic components such as dilauryl phosphite belonging to the phosphorus-based extreme pressure agent (B) and causes sludge generation. It is preferable that the oil composition does not contain a metallic detergent.

  The gear oil composition of the present invention has a sulfur content of 1.4% by mass or less, and preferably 1.0 to 1.4% by mass. If the sulfur content of the gear oil composition exceeds 1.4% by mass, the oxidation stability and low-temperature fluidity deteriorate. On the other hand, when a general-purpose gear oil package is used, the sulfur content of the gear oil composition is less than 1.0% by mass. It is difficult to make. In the gear oil composition of the present invention, the mass ratio of sulfur / phosphorus is preferably 6.5 or less, more preferably 4.5 to 6.5. In order to improve the oxidation stability, corrosion resistance, and extreme pressure with a good balance, it is preferable to adjust the mass ratio of sulfur / phosphorus to the above range.

  The gear oil composition of the present invention preferably has a Brookfield (BF) viscosity at −40 ° C. of 100,000 mPa · s or less. If the BF viscosity of the gear oil composition at -40 ° C is 100,000 mPa · s or less, the fluidity at low temperatures is sufficiently high, and even when used in a device used in a cold region, It is possible to prevent malfunctions from occurring.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

[Preparation of gear oil composition]
The gear ratios of Example 1 and Comparative Examples 1 to 13 using the following lubricant base oils and additives at the blending ratio shown in the upper part of Tables 1 and 2 (addition amount is mass% based on the total amount of the composition). An oil composition was prepared.

<Lubricant base oil>
[Component A: Lubricating base oil]
A-1; mineral oil (kinematic viscosity at 100 ° C .: 7.6 mm ² / s, viscosity index: 129, aromatic content: 0.0 mass%, sulfur content: 0.0008 mass%)
a-2; mineral oil (kinematic viscosity at 100 ° C .: 7.6 mm ² / s, viscosity index: 100, aromatic content: 5.9 mass%, sulfur content: 0.16 mass%)

<Additives>
[B component: extreme pressure agent]
B-1; dilauryl phosphite B-2; tricresyl phosphate b-3; mixture of sulfurized olefin and sulfurized ester b-4; dithiophosphate b-5;

[C component: corrosion inhibitor]
C-1, N, N-bis (2-ethylhexyl)-(4 or 5) -methyl-1H-benzotriazole-1-methylamine C-2; alkylated 2,5-dimercapto-1,3,4 Thiadiazole (The alkyl group is a branched structure having 9 carbon atoms.)

[D component: viscosity index improver]
D-1; ethylene-α-olefin copolymer (number average molecular weight: 5,200)
d-2: polymethacrylate (non-dispersed, number average molecular weight: 20,000)
The amount added was adjusted so that the viscosity of the composition was 9 to 11 mm ² / s in the examples and all comparative examples.

[E component: antioxidant]
e-1; diphenylamine e-2; hindered phenol (octyl 3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate)
E-3: Zinc dialkyldithiophosphate (alkyl group: sec-C ₆ )

[Other additives]
A combination of boron-containing succinimide, sulfur-based extreme pressure agent, dithiophosphate amine, organic amide, gear oil package containing nonionic activator, ashless dispersant, pour point depressant, defoamer, It is the same composition and compounding quantity in an Example and all the comparative examples.

<Evaluation method>
(Low temperature fluidity)
The Brookfield (BF) viscosity at -40 ° C. was measured according to ASTM D2983.

[Shear stability]
According to CEC L-45-T-93, a KRL shear test was performed for 60 hours, and the rate of viscosity reduction at 100 ° C. was measured.

[Extreme pressure]
According to JIS K 2519, the Iwata-type four ball friction test was performed at 750 rpm, and the load bearing capacity was measured.

[Corrosion prevention]
According to JIS K 2513, a copper plate corrosion test was performed under the conditions of 121 ° C. and 3 hours.

[Oxidation stability]
The viscosity increase rate at 100 ° C. was measured under conditions of 150 ° C. and 48 hours in accordance with the ISOT method which is an oxidation stability test specified in JIS K 2514.

  As is apparent from the results of the examples, the gear oil composition of the present invention is compatible with low temperature fluidity and shear stability at a high level, and has an appropriate extreme pressure, but also has corrosion resistance and oxidation. Excellent performance in stability.

  On the other hand, when only one of the corrosion inhibitors is used (Comparative Examples 1 to 5), it is inferior to one or both of corrosion prevention and antioxidant properties. Moreover, when the total amount of phosphorus derived from the phosphorus extreme pressure agent is less than 0.05% by mass (Comparative Example 6), the extreme pressure property and the oxidation stability are inferior. Moreover, when the sulfur content in the whole gear oil composition exceeds 1.4 mass% (Comparative Examples 7 to 11), the oxidation stability or the low-temperature fluidity is inferior even though the extreme pressure property is excellent.

  Further, when a base oil having a low viscosity index is used (Comparative Example 12), the low temperature fluidity is poor, and when a commonly used polymethacrylate is used as a viscosity index improver (Comparative Example 13), shear stability is achieved. Inferior to sex.

  As described above, in the base oil / additive, the gear oil composition in which at least one of the components of the present invention is replaced with a different component or the addition amount is out of the specified range is the product of the present invention in one performance. Although there is a composition that is superior to the other, it is inferior in other performances, and it can be seen that the composition of the present invention is the only composition in which the performance required as a gear oil is excellent in a well-balanced manner.

Claims (1)

  0.05 to 0.5 mass by mass of at least one phosphorus-based extreme pressure agent (B) selected from the group consisting of phosphite ester and phosphate ester in lubricating base oil (A) having a viscosity index of 110 or more %, Nitrogen-based corrosion inhibitor (C-1) 0.005-0.2% by mass, nitrogen / sulfur-based corrosion inhibitor (C-2) 0.005-0.2% by mass, ethylene-α-olefin co-polymer as viscosity index improver A gear oil comprising 1 to 10% by mass of polymer (D) and 0.02 to 0.2% by mass of zinc dialkyldithiophosphate (E) as elemental zinc, and having a sulfur content of 1.4% by mass or less. Composition.