Lubricating oil composition

Abstract

This invention provides a lubricating oil composition comprising at least 80wt%, based on the overall weight of the lubricating oil composition, of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch derived base oils and mixtures thereof, one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants, and one or more of citric acid and a derivative thereof. This invention also provides use of an additive selected from one or more of citric acid and its derivatives in a lubricating oil composition comprising at least 80wt% based on the overall weight of the lubricating oil composition of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch derived base oils and mixtures thereof and one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants, in order to improve the air release properties of said lubricating oil composition as measured by ASTM D3427.

Description

[0001] SP2943 - 1 - LUBRICATING OIL COMPOSITION Field of the Invention This invention relates to an industrial lubricating oil composition and the improvement of air release in industrial lubricating oil compositions. 5 Background of the invention Lubricating oil compositions are used to protect and lubricate surfaces in contact and may also be used to transmit power. Lubricating oil compositions are required in all industrial equipment with moving parts. Industrial 10 lubricating oil compositions, therefore, include hydraulic oils, gear oils, compressor oils, circulating oils and turbine oils, etc. Such industrial lubricating oil compositions generally comprise one or more base oils and one or more additives. Typically, the additive treat rate 15 for industrial lubricating oil compositions is considerably lower than for engine oil compositions, with additive treat rates of less than 10wt% being routine and additive treat rates of less than 5wt% usually being preferred. 20 It is known that air entrainment can reduce the efficacy of lubricating oil compositions and particularly those used as industrial lubricating oil compositions. Air entrainment is a phenomenon wherein air bubbles (typically having a diameter of less than 1mm) are dispersed 25 throughout the lubricating oil. Entrained air can be distinguished from free air (a pocket of air trapped in part of the system), from dissolved air (lubricating oils may contain between 6 and 12 percent by volume of dissolved air) and from foam (air bubbles typically 30 greater than 1mm in diameter that congregate on the surface of the oil). Air entrainment can have a number of negative consequences including loss of lubricity, possible oxidation of the lubricating oil composition, noisy operation, lower efficiency, and higher oil temperatures. 5 Of particular relevance are industrial lubricating oil compositions used in hydraulic systems. Current trends in the design of hydraulic systems are towards smaller reservoirs, low oil residence times and high power output. These changes are leading to increased problems of air 10 entrainment. The air entrainment properties of a lubricating oil are typically measured using the ASTM D3427 air release test. This test measures the time needed for air entrained in the oil to reduce in volume to 0.2% under the test 15 conditions and at the specified temperature. WO2017089354 describes a method comprising mixing an alkyl acrylate additive into a lubricating oil using a high shear mixer to improve air release. WO2017089357 teaches an improvement in air release properties in a 20 lubricating oil suitable for use as a hydraulic fluid, wherein at least 70% by mass of the base oil is GTL. Citric acid is known to be a demulsifier / demetalizer for crude oil, see for example US4439345 and EP0612548. The present inventors have sought to improve the air 25 entrainment properties of lubricating oil compositions, particularly those used in hydraulic systems. Summary of the Invention The present invention provides a lubricating oil composition comprising at least 80wt%, based on the 30 overall weight of the lubricating oil composition, of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch derived base oils and mixtures thereof, one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants, and one or more of citric acid and a derivative thereof. 5 The present invention also provides the use of an additive selected from one or more of citric acid and its derivatives in a lubricating oil composition comprising at least 80wt% based on the overall weight of the lubricating oil composition of base oil, wherein the base oil is 10 selected from Group II, Group III and Fischer-Tropsch derived base oils and mixtures thereof and one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, 15 friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants, in order to improve the air release properties of said lubricating oil composition as measured by ASTM D3427. Detailed Description of the Invention 20 One or more specific embodiments of the present disclosure will be described below. These described embodiments are examples of the presently disclosed composition and methods. Additionally, in an effort to provide a concise description of these embodiments, not 25 all features of an actual implementation may be described in the specification. When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the 30 elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. The present inventors have found the addition of one 5 or more of citric acid and a derivative thereof into a lubricating oil composition comprising at least 80wt% based on the overall weight of the lubricating oil composition of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch derived base 10 oils and mixtures thereof and one or more additives selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants provides 15 an unexpected improvement with respect to the air release properties of said lubricating oil composition. The lubricating oil composition is preferably an industrial lubricating oil composition. Industrial lubricating oil compositions include, but are not limited 20 to, those used as hydraulic oils, gear oils, compressor oils, circulating oils and turbine oils. In a preferred embodiment of the invention, the lubricating oil is formulated for use in a hydraulic fluid. Hydraulic fluids are used in many different types of hydraulic machinery 25 and are used not only to lubricate the machinery but also to transmit pressure. Air entrainment can be a significant problem in hydraulic systems, causing spongy or erratic operation of the hydraulics. A lubricating oil composition according to the 30 present invention comprises at least one base oil and at least one additive as well as citric acid or a derivative thereof. Industrial lubricating oil compositions contain lower amounts of additives compared to, for example, engine oils. Therefore, the lubricating oil composition comprises at least 80wt% of base oil. Preferably, the lubricating oil composition comprises at least 85wt%, more preferably at least 90wt% of base oil based on the overall 5 weight of the lubricating oil composition. Suitably the lubricating oil composition comprises no more than 99.5wt% of base oil on the basis of the overall weight of the lubricating oil composition. The base oil used in the present invention is 10 preferably Group II or Group III, including Fischer- Tropsch derived base oils, according to the API base oil categories. Preferably, the base oil comprises at least one Fischer-Tropsch derived base oil. Fischer-Tropsch derived base oils are synthesised by the conversion of, 15 for example, natural gas or biomass to liquid fuel. They have very low sulphur content and aromatic content compared with mineral oil base oils refined from crude oil and have a very high paraffin constituent ratio. The kinematic viscosity of the Fischer-Tropsch 20 derived base oil at 100oC is preferably from 2 to 20mm2 / s, more preferably from 3 to 15mm2 / s and most preferably from 3 to 10mm2 / s. The viscosity is suitably measured using ASTM D445. Preferably, the overall amount of additives in the 25 lubricating oil composition is less than 20wt%, more preferably less than 10wt%, even more preferably less than 5wt% based on the overall weight of the lubricating oil composition. Suitably, the overall amount of additives in the lubricating oil composition is at least 0.5wt% based 30 on the overall weight of the lubricating oil composition. The additives may be incorporated into the lubricating oil composition singly or as part of one or more additive packages. Whether added singly or in additive packages, a diluent oil may be included with the additive and form part of the finished lubricating oil composition. The lubricating oil composition comprises one or more additives, other than citric acid, selected from the 5 group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, antioxidants. Such additives are known in many lubricant composition types. Typical overall 10 amounts of these one or more additives are in the range of from 0.001wt% to 10.0wt% based on the overall weight of the lubricant oil composition. Suitable anti-wear additives may be selected from zinc salts such as zinc dialkyl or diaryl 15 dithiophosphates, borates, substituted thiadiazoles, polymeric nitrogen / phosphorus compounds made, for example, by reacting a dialkoxy amine with a substituted organic phosphate, amine phosphates, sulphurised oils of natural or synthetic origin, sulphurised lard, sulphurised esters, 20 sulphurised fatty acid esters, and similar sulphurised materials, organo-phosphates and triphenyl phosphorothionate. Rust and corrosion inhibitors include zinc dithiophosphate, metal phenolates, calcium or magnesium 25 sulfonates and amines. Dispersant additives may be reaction products of polyisobutenyl succinic anhydride and an amine or ester. Extreme pressure additives are typically selected from sulfur based, for example DMTD derivatives, 30 sulfurized olefins, sulfurized fats and oils or dithiocarbamates, and molybdenum-based additives, for example dithiocarbamates or dithiophosphates. Friction modifiers, are typically selected from organomolybdenum compounds, for example molybdenum diothiocarbamates and molybdenum dithiophosphates, or are selected from ashless esters or amides such as oleyl amide or glycerol mono oleate. Viscosity index improvers include, but are not 5 limited to, olefin polymers, such a polyisobutylenes, methacrylates, dienes and alkylated styrenes. Pour point depressants include alkylated napthenic products, polymethacrylates and phenolic polymers. The antioxidant may be a so-called hindered phenolic 10 or amine antioxidant, for example naphthols, sterically hindered monohydric, dihydric and trihydric phenols, sterically hindered dinuclear, trinuclear and polynuclear phenols, alkylated or styrenated diphenylamines or ionol derived hindered phenols. 15 Preferably, the lubricant oil composition does not contain any additives classified as demulsifiers or foam inhibitors, other than citric acid. More preferably, the lubricant oil composition contains no demulsifiers selected from the list consisting of polyalkylene 20 derivatives of alkyl phenols, fatty alcohols, fatty acids, fatty amines, fatty amides, and / or polyols (including cross-linked polyols). Also more preferably, the lubricant oil composition contains no foam inhibitors selected from the list consisting of polydimethylsiloxane,25 organo-modified siloxanes, polyacrylates, and / or non- ionic / anionic surfactants. The lubricating oil composition comprises one or more of citric acid and derivatives thereof. Citric acid has the chemical formula HOC(CO2H)(CH2CO2H)2 and may also be30 known by the IUPAC name of 2-hydroxypropane-1,2,3- tricarboxylic acid. Derivatives of citric acid are preferably citrates, including salts and esters of citric acid. Such salts and esters may include mono-, di- and tri-esters and / or salts of the tricarboxyclic acid and mixtures thereof. A mixture of citric acid and its derivatives may be present in the finished lubricating oil composition. However, a preferred additive is citric acid. This is typically available in solid form but may be more 5 suitably used in a diluted form in a non-aqueous carrier fluid. Typically, such diluted form contains in the range of from 40 to 60wt% of citric acid. Preferably the citric acid and / or its derivatives is present in or added to the lubricating oil composition in 10 an amount in the range of from 0.005wt% to 0.1wt%. preferably in the range of from 0.02wt% to 0.08wt% relative to the overall weight of the lubricating oil composition. The kinematic viscosity at 40˚C (KV40) of the 15 lubricating oil composition is preferably in the range of from 20 to 75 mm2 / s, more preferably in the range of from 30 to 75 mm2 / s, most preferably in the range of from 40 to 75 mm2 / s. The present invention also provides the use of an 20 additive selected from one or more of citric acid and its derivatives in a lubricating oil composition comprising at least 80wt% based on the overall weight of the lubricating oil composition of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch 25 derived base oils and mixtures thereof and one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point 30 depressants, anti-oxidants in order to improve the air release properties of said lubricating oil composition as measured by ASTM D3427. ASTM D3247 is a standard test method for air release properties of hydrocarbon based oils. It measures the time taken for air entrained in the oil to be released and the density of the oil to return to 99.8% of the initial density. The air release properties are improved as compared 5 to air release properties achieved using the lubricating oil composition not containing one or more of citric acid and its derivatives. Preferably, the air release properties are improved with no worsening of the foaming performance as measured by ASTM D892. Also preferably, the 10 air release properties are improved and there is no worsening of the demulsification performance as measured by ASTM D1401. Most preferably, the air release properties are improved with no worsening of the foaming performance as measured by ASTM 892 and no worsening of the 15 demulsification performance as measured by ASTM D1401. The invention will now be further illustrated by reference to the following non-limiting examples. Examples A number of hydraulic oil compositions as shown in 20 Table 1 were blended according to standard procedures. The base oil used was a Fischer-Tropsch derived Group III base oil. The additive package was a standard hydraulic oil additive package comprising detergent, anti- wear, antioxidant, corrosion inhibitors and no demulsifier 25 or citric acid. The acrylic copolymer additive was supplied at a concentration of 1-2% active content. Citric acid additive was supplied at a concentration of 40-60% active content in liquid form.

[0002] Table 1 Component (wt%) Ex.1* Ex.2* Ex.3* Ex.4 Ex.5 Ex.6 Additive package 1.00 1.00 1.00 1.00 1.00 1.00 Methacrylate VM 8.50 8.50 8.50 8.50 8.50 8.50 Base oil 90.50 90.47 87.50 90.50 90.49 90.40 Acrylic copolymer additive 0 0.03 3.00 0 0 0 Citric acid additive 0 0 0 0.003 0.010 0.100 *comparative examples. The examples were all tested for demulsification and air release properties as set out in Table 2. Table 2 test Ex.1* Ex.2* Ex.3* Ex.4 Ex.5 Ex.6 ASTM D892 Seq II, Tendency 0 mins, mL 40 40 50 30 50 40 Seq II, Stability 10 mins, mL 0 0 0 0 0 0 Seq III, Tendency 0 mins, mL 310 300 330 280 280 210 Seq III, Stability 10 mins, mL 0 0 0 0 0 0 ASTM D3427 Air release @ 50˚C, min 2.1 1.9 10.1 1.7 1.4 0.7 ASTM D1401 @ 54˚C Time to ≤3mL emulsion, min 25 20 10 25 25 20 Time to ≥ 37mL water, min 25 20 10 25 25 20 Complete separation, min 25 >30 >30 >30 >30 >30 Appearance after blending Bright & Bright & Hazy / Bright & Bright & Slightly clear, clear cloudy clear, clear, hazy, very light very light yellow very light very light very light yellow yellow yellow yellow yellow As can be seen from the data reported in table 2, the addition of a citric acid containing additive increases the rate of air release from the tested hydraulic oil formulation and does not significantly affect the foaming or demulsification performance of the hydraulic oil. In some examples the foaming performance is even slightly improved. As measured by ASTM D3427 at 50°C, the time needed to release air from the tested hydraulic oils decreases with addition of the citric acid additive. Hydraulic oil with 25 ppm and 100 ppm of the citric acid-containing additive is able to release air in 1.7 min and 1.4 min, which is faster than comparative example 1 (a hydraulic oil with no demulsifier) and comparative example 2 (hydraulic oil with acrylic copolymer).

Claims

SP2943 - 12 - C L A I M S 1. A lubricating oil composition comprising at least 80wt%, based on the overall weight of the lubricating oil composition, of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch derived base 5 oils and mixtures thereof, one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point 10 depressants, anti-oxidants, and one or more of citric acid and a derivative thereof.

2. A lubricating oil composition as claimed in Claim 1, wherein the lubricating oil composition is formulated for use in a hydraulic fluid. 15 3. A lubricating oil composition as claimed in Claim 1 or Claim 2, wherein the lubricating oil composition comprises at least 85wt%, more preferably at least 90wt%, and no more than 99.5wt% of base oil based on the overall weight of the lubricating oil composition. 20 4. A lubricating oil composition as claimed in any one of Claims 1 to 3, wherein the base oil comprises at least one Fischer-Tropsch derived base oil.

5. A lubricating oil composition as claimed in any one of claims 1 to 4, wherein the one or more additives other 25 than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants are present in the lubricating 30 oil composition in an amount in the range of from 0.001wt%to 10.0wt% based on the overall weight of the lubricant oil composition.

6. A lubricating oil composition as claimed in any one of claims 1 to 5, wherein one or more of citric acid and its 5 derivatives is present in or added to the lubricating oil composition in an amount in the range of from 0.005wt% to 0.1wt% relative to the overall weight of the lubricating oil.

7. A lubricating oil composition as claimed in any one of 10 claims 1 to 6, wherein said lubricating oil composition has a kinematic viscosity at 40˚C in the range of from 20 to 75 mm2 / s, more preferably in the range of from 30 to 75 mm2 / s, most preferably in the range of from 40 to 75 mm2 / s.

8. The use of an additive selected from one or more of 15 citric acid and its derivatives in a lubricating oil composition comprising at least 80wt% based on the overall weight of the lubricating oil composition of base oil, wherein the base oil is selected from Group II, Group III and Fischer-Tropsch derived base oils and mixtures thereof 20 and one or more additives, other than citric acid, selected from the group consisting of anti-wear additives, rust and corrosion inhibitors, dispersants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, anti-oxidants, in order 25 to improve the air release properties of said lubricating oil composition as measured by ASTM D3427.

9. The use according to claim 8, wherein the air release properties are improved with no worsening of the foaming performance as measured by ASTM D892. 30 10. The use according to claim 8 or claim 9, wherein the air release properties are improved with no worsening of the demulsification performance as measured by ASTM D1401.

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