Field of the Invention

The invention relates to a method for improving the air release of a lubricating oil.

Background of the Invention

Lubricating oils are used to protect and lubricate surfaces in contact, and may also be used to transmit power. It is known that air entrainment can reduce the efficacy of lubricating oils. Air entrainment is a phenomenon wherein air bubbles (typically having a diameter of less than 1mm) are dispersed 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 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, noisy operation, lower efficiency and higher oil temperatures .

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 conditions and at the specified temperature.

The present inventors have sought to improve the air entrainment properties of lubricating oils .

Summary of the Invention

Accordingly, the invention provides a method for improving the air release of a lubricating oil as measured by ASTM D3427, said method comprising mixing an additive into the lubricating oil using a high shear mixer; wherein the additive is an alkyl acrylate polymer.

Such alkyl acrylate polymers have been previously used as anti-foam additives. Air release times are typically considered to get worse with the addition of additives, especially anti-foams such as silicone oils. This is explained by Duncanson in "Properties on Foam in Lubricating Oils", pages 9-13, May 2003 Lubrication Engineering, Journal of the Society of Tribologists and Lubrication Engineers.

It is surprising that incorporating the alkyl acrylate polymer into the lubricating oil with a high shear mixer affects the air release properties of the lubricating oil. With the present invention the skilled person can use the alkyl acrylate polymer to improve the air release of a lubricating oil.

Detailed Description of the Invention

The present invention provides a method for improving the air release of a lubricating oil as measured by ASTM D3427. In the test, compressed air is blown through the lubricating oil, which has been heated to a temperature of 50°C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is recorded as the air release time. A desirable air release value is typically less than 3 minutes, preferably less than 60 seconds and most preferably less than 20 seconds.

The air release is improved as compared to air release achieved using the lubricating oil wherein the additive has not been mixed with the lubricating oil using a high shear mixer. The inventors have found useful reductions in air release times when carrying out the method of the invention. The lubricating oil comprises a base oil component, which may be a blend of several base oils. The base oils in the base oil component may be any conventional base oils chosen from Groups I, II, III, IV or V of the API (American Petroleum Institute) base oil categories.

Preferably the base oil component comprises a GTL base oil, and more preferably the base oil component comprises at least 75wt% of a GTL base oil based upon the weight of the base oil component. GTL base oils are synthesised by the Fischer-Tropsch method of converting natural gas to liquid fuel. They have a 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 lubricating oil suitably comprises additives.

Preferably the amount of additives is less than 10wt% based upon the weight of the lubricating oil, more preferably less than 5wt%, most preferably less than 2.5wt%. The additives may include antioxidants, antiwear additives, demulsifiers , emulsifiers, rust and corrosion inhibitors, VI improvers and/or friction modifiers.

Preferably, the lubricating oil has a kinematic viscosity in the range of from 5 to 220cSt at 40°C, more preferably of from 10 to 200cSt, most preferably of from 20 to lOOcSt. The kinematic viscosity is suitably measured using ASTM D445 (ASTM D7042) .

In a preferred embodiment of the invention, the lubricating oil is formulated for use a hydraulic fluid. Hydraulic fluids are used in many different types of hydraulic machinery and are used not only to lubricate the machinery but also to transmit pressure. Air

entrainment can be a particular issue in hydraulic systems, causing spongy or erratic operation of the hydraulics .

In the method of the invention an alkyl acrylate polymer additive is mixed into the lubricating oil.

Suitable alkyl acrylate polymers are disclosed in, for example, US3166508. The molecular weight of the polymer is suitably less than 10,000, preferably less than 7,500 and more preferably less than 5,000. In one embodiment the alkyl acrylate polymer may be a homopolymer of an alkyl acrylate having at least 3 but less than 7 carbon atoms in the alkyl radical. In another embodiment the alkyl acrylate polymer may be a copolymer of at least two different alkyl acrylates in which the alkyl radical has from 1 to 18 carbon atoms and in which the average number of carbon atoms in the alkyl radicals of the copolymer molecular is at least 3 but less than 7 on a molar basis.

Suitably at least 0.0005wt% of alkyl acrylate polymer, based upon the weight of the lubricating oil, is mixed into the lubricating oil, preferably at least 0.001wt%. Suitably less than 0.1wt% of alkyl acrylate polymer, based upon the weight of the lubricating oil, is mixed into the lubricating oil, preferably less than 0.01wt%. The preferred amount of alkyl acrylate polymer will be a balance between using enough to achieve the desired improvement in air release properties and avoiding unnecessary expense by using too much alkyl acrylate polymer.

The alkyl acrylate polymer additive is mixed into the lubricating oil using a high shear mixer. The term "high shear mixer" is well-known to persons skilled in the art. A rotating impeller or high-speed rotor creates flow and shear within the liquid that is mixed. In some embodiments the rotor will turn within a stationary component known as a stator. Preferably the additive is mixed into the lubricating oil at a rate of at least 400rpm. The rate is preferably at least 600rpm and more preferably at least 800rpm. Preferably the mixing is for at least 10 minutes, more preferably at least 30 minutes, and most preferably at least 60 minutes.

The invention is further explained in detail below by means of examples, but the invention is in no way limited by these examples.

Examples

Seven different base oil blends were prepared using combinations of three base oils:

1) Chevron 220 R (a Group II mineral oil available from Chevron)

2) XHVI 8 (a Fischer-Tropsch derived oil available from Shell)

3) Spectrasyn 8 (a poly-alpha olefin fluid available from ExxonMobil)

Table 1 gives the amounts of each base oil (in weight % based upon the total weight of the blend) present in each blend. The blends in Table 1 are all ISO 46.

Table 1

An additive package containing extreme

pressure/antiwear additive, friction modifier, ashless rust inhibitor, emulsifier and antioxidant was added to the base oil blends to provide fully formulated

lubricating oils. The additive package was the same in each instance. Then 125 ppm of PC 3144 additive

(available from Allnex) , which is a 40% solution of a modified acrylic polymer in a hydrocarbon solvent, was added to the formulations at 40°C either at a stirring rate of 1200rpm using an overhead Caframo high shear mixer for an hour or at a stirring rate of 300rpm using a magnetic stirrer for an hour.

The air release time of the base oil blends, of the fully formulated oils and of the fully formulated oils plus PC 3144 was tested using the ASTM D3427 method. The results are given in Table 2:

Table 2

The results show that air release of the base oil blends varies. Adding the additive package can further increase the air release time. Adding the PC 3144 additive using a high shear mixer can substantially reduce the air release time (unless the air release time is already very low as was the case for Blend 3 and Blend 7) . Adding the PC 3144 additive with a magnetic stirrer does not seem to have the same effect.