This invention relates to an oil composition containing a particular type of antioxidant additive, and especially to electrical insulating oils contain¬ ing such an antioxidant additive.

The use of antioxidant additives in oils, such as electrical insulating oils, is well known. These additives may be mineral oil based, for example an aromatic petroleum stream, or they may be synthetic. Examples of known synthetic antioxidants include hindered phenols such as 2,6-ditertiarybutyl-4-methyl phenol and amines such as N-phenyl alpha napththalene.

In addition, a variety of nitrogen-containing compounds have been disclosed as antioxidants. US Patent 1 768 910 discloses compounds such as pyridine, quinoline and piperidene; US Patent 2 647 824 discloses the combination of a hydrogenated quinoline and an amino phenol; US Patents 3 197 475, 3 597 353, 3 720 616, 3 969 237 and 4 162 225 disclose a variety of benzotria- zole compounds, and US Patent 3 785 977 discloses a variety of amine and triazine compounds. European Patent 69507 describes oil compositions, such as elec¬ trical insulating oils, containing a specified heter- aromatic nitrogen compound, for example purine or a purine derivative, as an antioxidant. The antioxidant may be combined with other known antioxidants such as 2 ,6-ditertiarybutyl paracresol.

There is nevertheless a need for oils with improved antioxidant performance, especially in the field of electrical insulating oils.

The present invention provides an oil composition comprising :

(i) a base oil which does not contain any additional aromatic petroleum stream;

(ii) an antioxidant additive comprising a mixture of (a) phenol or a derivative thereof; (b) an amine or a derivative thereof, and (c) a triazole or a derivative thereof.

The synthetic additive according to the present invention has the advantage that it can provide the oil composition with sufficient antioxidant properties so that there is no requirement to include additional anti¬ oxidants, such as aromatic petroleum streams.

The phenol component of the antioxidant additive is preferably a hindered phenol such as a polyphenol or phenol derivative such as an alkyl phenol. Examples of suitable compounds include trimethyl phenol, 2,6-deter- tiarybutyl paracresol and 2,6-ditertiarybutyl phenol.

The amine component of the antioxidant additive is preferably a hindered amine. Examples of suitable amines include phenylnaphthylamine and aminophenolics such as 2,6-ditertiarybutyl-alpha-dimethylamino paracresol.

The triazole component of the antioxidant additive is preferably a lipophilic triazole derivative, for example benzotriazole.

The amounts of the components comprising the antioxidant additive based on the weight of the base oil are preferably as follows:

from 500 to 10,000 ppm, more preferably 1,000 to 6,000 ppm, most preferably 2,000 to 4,500 ppm phenol or phenol derivative; from 100 to 8,000 ppm, more preferably 200 to 5,000 ppm, most preferably 300 to 1,500 ppm amine or amine derivative; and from 10 to 4,000 ppm, more preferably 100 to 1,000 ppm, most preferably 150 to 500 ppm triazole or triazole derivative.

The base oil employed in the oil composition of the present invention is typically a conventional hydro¬ carbon oil of lubricating viscosity and may be mineral oil, synthetic oil or a mixture thereof. The lubricat¬ ing oils employed may be of any suitable lubricating viscosity and may range from 30 to 7,500 SUS at 37.8"C. Particularly useful are lubricating and speciality oils, preferably electrical insulating oils, such as transformer oils or circuit-breaker oils, which have relatively low viscosity values, for example a viscosity Of 40 to 100 SUS at 37.8°C.

Where the base oil is a mineral oil this is preferably a suitable petroleum fraction that has undergone conventional petroleum processing as applied to lubricating oil products, including aromatics extraction, dewaxing and hydrotreating if necessary.

Other additives conventionally added to lubricating oils, and especially to electrical insulating oils, may also be included in the oil composition, for example dispersants, pour point depressants, extreme pressure additives and gas absorbers.

The invention shall now be illustrated by the following Examples.

Examples 1 to 10

A refined electrical insulating oil, based on mineral oil, was formulated using a vacuum distillate which had undergone conventional aromatics extraction, dewaxing and hydrotreatment. The distillate had a boiling range of 280 to 400 ^β C and a viscosity of 15.0 to 15.5 cSt at 20°C.

Samples of this base oil were then formulated with additives A, B and C, as specified below, in various combinations and concentrations. The antioxidising properties of each formulation were evaluated according to standard test IEC 474. The test exposes the formula¬ tion to an oxidising environment by heating it to a temperature of 120 ^β C in the presence of oxygen gas and a copper catalyst. The induction time, which is the time taken for the formulation to create a volatile acidity equivalent to a neutralisation number of 0.28 mg KOH/g, is measured in hours. The longer the induction time, the better the antioxidant properties.

The formulations and results of test IEC 474 are given in Table 1 below in which the following abbreviations are used for the antioxidant additives :

A = 2,6-di-t-butyl phenol

B -=- 2 ,6-di-t-butyl-alpha-dimethylamino paracresol

C = benzotriazole

TABLE 1

The results show that a significant improvement in antioxidant properties of the electrical insulating oil is obtained when the three additives A, B and C are combined together.