EP0349534A1

EP0349534A1 - Hydraulic fluids.

Info

Publication number: EP0349534A1
Application number: EP88901051A
Authority: EP
Inventors: Gert Stenmark; Kari Jokinen; Heikki Kerkkonen; Eero Leppamaki; Eino Piirila
Original assignee: Raision Tehtaat Oy AB
Current assignee: Raisio Oyj
Priority date: 1987-01-28
Filing date: 1988-01-27
Publication date: 1990-01-10
Anticipated expiration: 2008-01-27
Also published as: EP0349534B1; DE3876432D1; NO884237D0; NO884237L; NO174060B; DK536188A; DK536188D0; DE3876432T2; AU1227388A; WO1988005808A1; ATE83003T1

Abstract

Hydraulic fluids based on natural triglycerides. The base composition for these hydraulic fluids consists of at least one natural triglyceride and one anti-oxidant additive. The triglyceride suitable for the composition can be defined as an ester of a straight-chain C10 to C22 fatty acid and glycerol which has an iodine number of between 50 and 128. The fraction of the anti-oxidant additives is selected preferably among hindered phenolics and aromatic amines, but can be completed by compounds selected from metal salts of dithioacids, phosphites, sulfides, amides, non-aromatic amines, hydrazides and triazols. Preferred amount of the anti-oxidant(s) in the fluid is 1.5 to 4.5 percent by weight.

Description

Hydraulic fluids

The present invention is concerned with hydraulic fluids based on oily triglycerides of fatty acids.

The hydraulic fluids commonly used are petroleum-based, chemically saturated or unsaturated, straight-chained, branched or ring-type hydrocarbons.

The petroleum-based hydraulic fluids involve, however, a number of environmental and health risks. Hydrocarbons may constitute a cancer risk when in prolonged contact with the skin, as well as a risk of damage to the lungs when inhaled with the air. Moreover, oil allowed to escape into the environment causes spoiling of the soil and the ground water, even in small quantities. They are also toxic to the aquatic life in rivers, lakes, etc.

In addition to the above, hydrocarbon oils as such have in fact a rather limited applicability for hydraulic purposes, wherefor the hydraulic fluids based on such oils contain a variety of additives in considerable amounts. Petroleum is also a non-renewable, and consequently limited, natural resource.

Thus there is an obvious need for fluids for hydraulic purposes which are based on renewable natural resources, and which are, at the same time, environmentally acceptable. One such a natural base component for hydraulic fluids is the oily triglycerides, as suggested in the patent specification GB 2 134 923.

The triglycerides described in the said specification GB 2 134 923 are glycerol esters of fatty acids, and the chemical structure of the said esters can be defined by means of the following formula:

wherein R₁, R₂ and R₃ can be the same or different and are selected from the group consisting of saturated and unsaturated straight-chained alkylr alkenyl, and alkadienyl chains of ordinarily 9 to 22 carbon atoms.

The triglyceride may also, according to the teaching of the specification GB 2 134 923, contain a small quantity of an alkatrienylic acid residue, but a larger quantity is detrimental, because it promotes oxidation of the triglyceride oil. Certain triglyceride oils, so-called drying oils, contain considerable quantities of alkatrienyl and alkadienyl groups, and they form solid films, under the effect of the oxygen in the air. Such oils, the iodine number of which is usually higher than 130 and which are used i.a. as components of special coatings, cannot be considered for use in the hydraulic fluids.

As is stated in the specification GB 2 134 923, any other oily triglyceride with an iodine number of at least 50 and no more than 128 is suitable for the purpose. Particularly suitable are the triglycerides of the oleic acid-linoleic acid type which contain no more than 20 per cent by weight of esterified saturated fatty acids calculated on the quantity of esterified fatty acids. These oils are liquids at 15 to 20°C, and their most important fatty acid residues are derived from the following unsaturated acids: oleic acid, 9-octadecenoic acid, linoleic acid, 9,12-octa-decadienoic acid. The most preferred among these triglycerides of vegetable origin, under normal temperatures of use, are described to be those that contain esterified oleic acid in a quantity in excess of 50 per cent by weight of the total quantity of fatty acids (Table 1).

Table 1

Usable triglyceride oils

Olive Peanut Maize Rape oil oil oil oil ------------------------------------------------------------------------------------------------------

Iodine number 1) 77-94 84-100 103-128 94-126 Cloud point °C 2) -5 - -6 4 - 5 4 - 6 2 - 4 -----------------------------------------------------------------------------------------------------

Fatty acids %

Saturated

Palmitic acid C 16 7-16 6-9 8-12 4-6

Stearic acid C 18 1-3 3-6 2-5 1-3

Unsaturated

Oleic acid C 18:1 65-85 53-71 19-50 51-62

Linoleic acid C 18:2 4-15 13-27 34-62 16-24 -----------------------------------------------------------------------------------------------------

1) Methods AOCS Cd 1-25, ASTM D 1959 or AOAC 28.020

2) Method AOCS Co 6-25

-----------------------------------------------------------------------------------------------------

It is characteristic of all of these oily triglycerides that their viscosities change on change in temperature to a lesser extent than the viscosities of hydrocarbon basic oils. The viscosity-to-temperature ratio characteristic of each oil can be characterized by means of the empiric viscosity index (VI), the numerical value of which is the higher the less the viscosity of the oil concerned changes with a change in temperature. The viscosity indexes of triglycerides are clearly higher than those of hydrocarbon oils with no additives, so that triglycerides are to their nature so-called multigrade oils. This is of considerable importance under conditions in which the operating temperature may vary within rather wide limits. The viscosities and viscosity indexes of certain triglycerides are given in Table 2

Table 2

Viscosity properties of oils

Viscosity mm²/s Viscosity 38°C 99°C index 1) 2) -----------------------------------------------------------------------------------------------------

Olive oil 46.68 9.09 194

Rape seed oil 50.64 10.32 210

(eruca)

Rape seed oil 36.04 8.03 217

Mustard oil 45.13 9.46 215

Cottonseed oil 35.88 8.39 214

Soybean oil 28.49 7.60 271

Linseed oil 29.60 7.33 242

Sunflower oil 33.31 7.68 227 -----------------------------------------------------------------------------------------------------

Hydrocarbon-based basic oils 0 - 120 -----------------------------------------------------------------------------------------------------

1) Method ASTM D 445 2) Method ASTM D 2270 The fume point of triglycerides is above 200°C and the flash point above 300°C (both determinations as per AOCS Ce 9a-48 or ASTM D 1310). The flash points of hydrocarbon basic oils are, as a rule, clearly lower.

The triglyceride oils differ from the non-polar hydrocarbons completely in the respect that they are of a polar nature. This accounts for the superb ability of triglycerides to be adsorbed on metal faces as very thin adhering films. A study of the operation of glide faces placed in close relationship to each other, and considering pressure and temperature to be the fundamental factors affecting lubrication, shows that the film-formation properties of triglycerides are particularly advantageous in hydraulic systems.

In addition, water cannot force a triglyceride oil film off a metal face as easily as a hydrocarbon film.

Rape seed oil has been considered as an example of the monomeric triglyceride oils used in the hydraulic fluids in accordance with the specification GB 2 134 923, which rape seed oil is also obtained from the subspecies Brassica campestris and which oil, in its present-day commercial form, contains little or no erucic acid, 13-docosenoic acid. However, it is to be kept in mind that applicable triglyceride oils differ from rape seed oil only in respect of the composition of the fatty acids esterified with glycerol, which difference comes out as different pour points and viscosities of the oils. Even oils obtained from different sub-species of rape and from their related sub-species display differences in pour points and viscosities, owing to differences in the compositions of fatty acids, as appears from Table 3. Of the rape seed oils mentioned in the table, the first one (eruca) has been obtained from a sub-species that has a high content of erucic acid (C 22:1). Table 3

Properties of certain Brassica oils

Rape Rape False White seed seed flax mustard oil oil

(eruca)

Fatty acids %

Saturated

C 16 2.2 3.5 5.4 2.5

C 18 1.1 1.0 2.2 0.8

C 20 0.8 0.5 1.1 0.6

Unsaturated

C 18: 1 11.6 59.0 13.4 22.3

C 18: 2 14.0 21.3 17.5 8.0

C 18: 3 10.0 11.9 36.5 10.6

C 20: 1 8.5 1.3 14.7 8.0

C 22: :1 48.0 0.5 3.6 43.5 -----------------------------------------------------------------------------------------------------

Pour point °C 1) - 17 - 26 - 26 - 17

Viscosity mm²/s 100ºC 10.3 8.0 9.0 9.5 -----------------------------------------------------------------------------------------------------

1) Method ASTM D 97

The characterizing data of rape seed oil are compared in Table 4 with certain commercial basic mineral oils. Table 4

Characteristic data of rape seed oil and certain basic mineral oils

Rape Gulf Gulf Nynäs Nynäs seed 300 300 S 100 H 22 oil paraTexas mid oil

Density g/cm³1) 15°C 0.9205 0.878 0.914 0.910 0.926 Viscosity mm²/s -20°C 660

40ºC 34.2 60.7 57.9 99 26

100°C 8 8.1 6.6 8.6 3.9

Viscosity index 217 101 26 31

Pour point °C -27 -12 -34 -18 -33

Flash point °C 2) > 300 238 188 215 180

Acid value mg KOH/g 3) 0.06 0.04 0.09 0.01 0.01 -------------------------------------------------------------------------------------------

1) Method ASTM D 1298 2) Method ASTM D 93

3) Method ASTM D 974

-------------------------------------------------------------------------------------------

The above data indicate that the said triglycerides have many properties which are of advantage especially in hydraulic fluids. As mentioned already before, the viscosity index (VI) of triglycerides, as compared with mineral oil products, is superior. The viscosity index of the triclyceride oils is apparently also more stable against mechanical and heat stresses existing in the hydraulic systems than the viscosity index of the hydraulic fluids based on formulated mineral oils and containing polymeric viscosity index improves. In addition it can be expected that the ability of the polar triglyceride molecule to adhere onto metallic surfaces improves the lubricating properties of these triglycerides.

The only property of the natural triglycerides which has shown to impede their intended use for hydraulic purposes is their tendency to be easily oxidized.

The oxidation has many negative effects to the properties of a natural triglyceride based hydraulic fluid, wherefore the fluid has to be replaced by fresh fluid more frequently than fluids based on hydrocarbon oils.

For instance the viscosity of the natural triglyceride hydraulic fluid is increased due to the oxidation. The oxidation causes also foaming of the fluid, the filtration properties of the fluid are decreased, and the higher water solubility causes problems in the hydraulic system. The oxidation products are also corrosive. In order to avoid these problems caused by oxidation the working temperature of the hydraulic system is to be kept lower than when hydrocarbon based oils are used.

It has, however, been noted that the tendency of the said natural triglycerides to be oxidized can be decreased essentially to the same level as that of the common hydrocarbon based hydraulic oils, by using additives in very moderate amounts, which additives have been selected according to the invention. This fact is evident from the results of the following example 1. Example 1

In this example the stability of the hydraulic fluids against oxidative degradation was tested. The fluids were tested with an apparatus according to the test method ASTM D 525 by introducing into a pressure vessel 100 ml of the fluid to be tested. The vessel was closed and placed into boiling water. During the test the oxygen pressure in the vessel was determined.

The oils tested were:

Oil number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Refined rape seed oil: 100 98.97 97.95 96.85 96.5 97.0 98.5 98.0 99.0 95.0 97.0 96.0 95.5 95.8 0 0

- additive, percent byweight

Irgalube349 0.5 1.0 1.0 0.5 0.5 0.5

IrganoxLl30 0.5 1.0 2.0 Reomet 39 0.03 0.05 0.05

Anglamol 75 1.5 0.5 1.5 1.5 1.5 1.5

EN 1235 0.1

Hitec4735 2.0 2.0 2.0 1.5 2.0 1.5 2.0 Additin10 1.5

IrganoxL57 1.0 0.5

IrganoxL180 1.0 1.0

Vanlube AZ 0.5 0.5

Irgalube TPPT 0.5

Tinuvin 770 0.2

IrganoxPS 800 2.0

Additives, tot. 1.03 2.05 3.15 3.5 3.0 1.5 2.0 1.0 4.5 3.0 4.0 4.5 4.2

Shell Tellus T 32 100%

Esso Univis HP-32 100%

The additives used were: Irgalube 349, amino phosphate derivative, Ciba-Geigy; Irganox L 130, mixture of tertiary-butyl phenol derivatives, Ciba-Geigy; Reomet 39, triazole derivative, Ciba-Geigy; Anqlamol 75, zinc dialkyldithio-phosphate, Lubrizol; EN 1235, kortacid T derivative, Akzo Chemie; Hitec 4735, mixture of tertiary-butyl phenol derivatives. Ethyl Pertoleum Additives Ltd; Irganox PS 800, dilauryl thio di propiona- te, Ciba-Geigy; Irganox L 180, triaryl phosphite, Ciba-Geigy; Irganox L 57, mixed alkyl diphenyl amine, Ciba-Geigy; Irganlube TPPT, triphenylphosphorothionate, Ciba-Geigy; Tinuvin 770, bis (2,2,6,6-tetrametyl-4-piperidyl) sebacate, Ciba-Geigy; Vanlube AZ, zinc diamyldithiocarbamate, R.T. Vanderbilt; Additin 10, 2,6-di-tertiary-butyl-4-mathylphenol, Rhein-Chemie.

The results of this test are given in Table 5.

Table 5

Oil

Time, hours

Pressure, psi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0 120 121 127 124 126 125 125 128 124 124 129 124 121 125 125 121 12 109 113 124 121 121 123 117 120 115 122 126 120 118 123 119 118 24 76 103 121 119 116 120 110 113 107 119 123 118 n6 120 118 117 36 33 97 117 116 110 118 104 107 100 117 120 115 113 117 116 116 48 16 88 114 114 106 116 92 102 92 115 118 112 114 115 114 116 60 - 80 110 112 101 114 77 96 83 113 116 no 108 113 112 114 72 - 71 107 110 97 112 64 91 76 110 114 107 106 111 111 113

As can bee seen from the results of Table 5, the compositions 3, 4, 5, 6, 8, 10, 11, 12, 13 and 14 are clearly comparable with the common mineral-oil based hydraulic oils 15 and 16 used for comparison in this example. The compositions 2 and 9 contain the anti-oxidant additives selected according to the invention, but the amounts used have not been sufficient. From the data in Table 5 it can be derived that a triglyceride complying with the definitions presented at the beginning of this description and containing a certain amount of carefully selected anti-oxidant additives can form a base for a fluid composition usable for hydraulic purposes.

According to the invention the anti-oxidant fraction in the composition forms 2.0 to 4.5 percent by weight of the composition, and the anti-oxidants are selected so that at least one compound is from the group (1) of hindered phenolics and aromatic amines, and the remaining compound (s) forming the balance in the composition, is from the group (2) of metal salts of dithioacids, phosphites and sulphides, or from the group (3) of amides, non-aromatic amines, hydrazides and triazols.

Examples of compounds which belong to the abovemention- ed groups can be named as follows:

1) 2,6-di-tert-butyl-4-methyl phenol; 2'2-methylenebis-(4-methyl-6-tert-butylphenol); N,N'-disecbutyl-p-phenylene-diamine; alkylated diphenyl amine; alkylated phenyl-alfa-naphtyl amine

2) zinc dialkyldithiophosphates; tris (nonylphenyl) phosphite; dilauryl thiodipropionate 3 ) N,N^,-diethyl-N,N'-diphenyloxamide;

N,N'-disalicylidene-1,2-propenylenediamine; N,N'-bis (beta-3,5-ditertbutyl-4-hydroxyphenylpropiono) hydrazide

One indication of the resistance towards oxidation of oils is also their ability to keep the lubrication properties in higher temperatures. This ability was tested in the following example 2.

Example 2

In the tests a Cameron Plint tester (High Frequency Friction Machine TE. 77) was used. In this tester the friction between a moving and a stationary element is determined at increasing temperatures. As the moving element is used a steel ball having a diameter of 6 mm, whereas the stationary element consists of a steel plate. The lubricant to be tested is spread on the plate, and it is exposed to the ambient air oxygen during the tests. In the tests conducted the ball was pressed towards the plate by a force of 40 N during its reciprocating movement having an amplitude of 5 mm and a frequency of 20 Hz. The temperature at the beginning of each test was adjusted to 40 °C, whereafter it was increased by 2 °C per minute. The temperature in which the friction began to increase sharply was registered, and it was used as an indication of the failure of the lubricative film between the ball and the plate. The film failure temperature is a measure of the oxidation resistance of the oil.

The results of this test are given in the following table 6. Table 6

Test No

1 2 3 4 5 6 7 8 9

Refined rape seed oil, % (W), 100 98 99 96.5 98.5 98 97.8

Additive, % (W),

Hitec 4735 2 2 1.5 2

Irganox L 57 1 1

Anglamol 75 1.5

Irgalube 349 0.5 0.5

Irgalube TPPT 0.2

Mobil DTE Heavy Medium 100

Esso Univis HP-32 100

Film failure

Temperature, °C 125 171 148 232 205 226 225 192 164 From the results of table 6 (tests 2 and 3) it can be seen, that if the oil contains an anti-oxidant which can be classified to hindered phenolics (Hitec 4735) or to aromatic amines (Irganox L 57) the film failure temperature is higher than that of pure rape seed oil. The test 3, however, shows that the percentage of the antioxidant has not been high enough. The result is clearly better if the oil contains also a small amount of other anti-oxidant (Irgalube 349, amino phosphate derivative, test 5). Higher percentages of the anti-oxidants give results which are superior to the results of commercial hydrocarbon based hydraulic oils.

The ability of the hydraulic fluids according to the invention was also tested in a full scale test, which is described in the following example 3.

Example 3

A vegetable oil based hydraulic fluid was tested using as a reference a commercial mineral oil based hydraulic fluid. In the test two new identical hydraulic driven mining loaders were used. During the test the pressures in the hydraulic circuits varied from 0 to 165 bar and the hydraulic fluid temperature from 60 to 80°C. Hydraulic pressure was generated by gear pumps and the power was taken out by means of cylinder-piston devices.

The hydraulic fluids tested were:

1. Vegetable oil

- refined rape seed oil, 98.5 % by weight

- additive, Anglamol 75, 1.5 % " 2. Vegetable oil

- refined rape seed oil 96.5 % by weight

- additive 1, Anglamol 75, 1.5 % "

- additive 2, Hitec 4735, 2.0 % "

3. Mineral oil based hydraulic fluid, Teboil OK 14 - 46

The following Table 7 gives the viscosity of the oils after a prolonged time in operation.

Table 7

Time, hours Viscosity, mm²/s / 40 °C

Fluid

1 2 3

0 34 33.2 44.6

300 36.8 33.2 38.1

600 39.5 33.5 35.2

900 44.3 33.9 34.3

1200 51.8 34.1 34.2

1500 55.6 34.3 34.2

In the same test also the volumetric efficiency of the hydraulic systems 2 and 3 was recorded during the test period and the results are given in the following table 8. Table 8

Time, hours M v / M ref

Fluid

0 1 1

300 0.960 0.94

600 0.945 0.88

900 0.940 0.84

1200 0.935 0.79

1500 0.93 0.76

Mv means efficiency recorded M ref means efficiency at the beginning of the test

The efficiency tests were conducted using a fluid pressure of 165 bar, and a temperature of 65°C.

The test results of Table 7 indicate that the durability against shear stress of the vegetable oil based fluid was better than that of the mineral oil based fluid.

The test results of Table 8 indicate that the efficiency of the system containing the vegetable oil based fluid decreased slower than that of the mineral oil based fluid.

The lubricative properties of a hydraulic fluid based on the triglyceride composition of the invention were tested by using the testing method described in the following example 4. Example 4

The suitability of rape seed oil as a hydraulic fluid was tested in a four ball tester according to the test method IP 239, in which the test period is one hour and the load 1 kg, as well as according to the standard Test Method STD No 791/6503,1, in which the load is increased stepwise during the test period of 10 seconds. The oils tested are given in the Table 9.

Table 9

No Oil

1. Refined rape seed oil, 96.5 % by weight Additives,

- Anlamol 75, 1.5 % "

- Hitec 4735, 2.0 % "

2. Shell Tellus T 32

3. Esso Univis HP-32

4. Neste Hydraulic 32 Super, manufacturer Neste, Finland

5. Teboil Hydraulic Oil 32 S

6. Mobil Flowrex Special

All the oils tested belong to the viscosity cathegory ISO VG 32 according to the test method ASTM D 2422.

The results of the said tests are given in the Table 10 Table 10

IP 239, 1 h / 50 kg STD No 791/6503,1 wear, mm load to welding of the balls

1. 0 .46 over 300 2. 0.71 200 3. 1,52 140 4. 1.49 200 5. 0.81 260 6. 0.57 200

The lubricating properties were compared also by using a gear system, which test is described in the following Example 5.

Example 5

The protective action of three hydraulic fluids on gear systems against wear was tested by using the FZG-method according to the standard DIN 51354 E (FZG gear rig test machine).

The oils used were:

Oil No

Refined rape seed oil 96.5 % by weight Anglamol 75 1.5 " Hitec 4735 2.0 " Refined rape seed oil 98.9 % by weight Irgalube 349 0.5 Additin 10 0.5 .Reomet 39 0.05 Sarkosyl 0 0.05

Mobil DTE 25

- Sarkosyl 0 is N-acyl-sarcosine, manufacturer Ciba- Geigy

The results of this test are given in the following table 11.

Table 11

Oil Load degree Specific wear, to damage mg/horsepower/hour

1 above 12 0.05

2 above 12 0.033

3 11 0.10

In addition to the basic composition the hydraulic fluid according to the invention may also comprise other constituents such as:

- Boundary lubrication additives, such as metal dialkyl dithiophosphates; metal diaryl dithiophosphates; metal dialkyl dithiocarbamates; alkyl phosphates; phosphorized fats and olefins; sulphurized fats and fat derivatives; chlorinated fats and fat derivatives

- Corrosion inhibitors, such as metal sulfonates; acid phosphate esters; amines; alkyl succinic acids

- VI (Viscosity Index) improvers, such as polymethacrylates; styrene butadiene copolymers; polyisobutylenes

- Pour point depressants, such as chlorinated polymers; alkylated phenol polymers; polymethacrylates

- Foam decomposers, such as polysiloxanes; polyacrylates

- Demulsifiers, such as heavy metal soaps; Ca and Mg sulphonates

From the base composition according to the invention can be made hydraulic fluids for different purposes by adjusting its viscosity. The following table 12 gives one example of adjusting possibilities.

Table 12

From a base composition according to the invention was made hydraulic fluids for different viscosity classes (ASTM D 2422)

Oil comp. vise. class

% by mm²/s weight

1. Refined rape seed oil 62.5 20.6 ISO VG 22

Rilanit EHO 35

Hitec 4735 2.0

Anglamol 75 1.5

2. Refined rape seed oil 96.5 34.3 ISO VG 32

Hitec 4735 2.0

Anglamol 75 1.5

3. Refined rape seed oil 73.5 45.2 ISO VG 46

Priolube 3987 23

Hitec 4735 2.0

Anglamol 75 1.5

4. Refined rape seed oil 76.5 73.0 ISO VG 68

Priolube 3987 14

Priolube 3986 6

Hitec 4735 2.0

Anglamol 75 1.5

Rilanit EHO, 2-ethylhexyl oleate, Henkel Priolube 3987, pentaerythritol ester, Unichema Priolube 3986, complex ester, Unichema

Claims

What is claimed is:

1. A base composition for hydraulic fluids consisting of:

- at least one natural triglyceride which is an ester of a straight-chain C₁₀ to C₂₂ fatty acid and glycerol, which triglyceride has an iodine number of at least 50 and not more than 128, and

- at least one anti-oxidant additive,

- characterized in

- that the anti-oxidant additive fraction forms at least 1.5 percent by weight of the composition,

- that the anti-oxidant additive fraction contains at least one compound selected from the group:

- (I) hindered phenolics and aromatic amines,

- and in that at least in anti-oxidant percentages of 1.5 to 2.0 in the composition the balance is selected from at least one of the following anti-oxidant groups:

- (II) metal salts of dithioacids, phosphites and sulfides.

- (Ill) amides, non aromatic amines, hydrazides and triazols.

2. A base composition for hydraulic fluids according to the claim 1, characterized in that the anti-oxidant fraction forms 1.5 to 4.5 percentage by weight of the composition.

3. A base composition for hydraulic fluids according to the claims 1 or 2, characterized in that the anti- oxidants selected from the group I form 1.0 to 3.0 percentage by weight of the composition.

4. A base composition for hydraulic fluids according to the claims 1 to 3, characterized in that the triglyceride is of oleic-acid-linoleic-acid type and contains saturated fatty acids of not more than 20 percent by weight calculated on the quantity of fatty acids esterified with glycerol.

5. A base composition for hydraulic fluids according to the claim 4, characterized in that the triglyceride consists of rape seed oil.

6. A hydraulic fluid made of a base composition according to any of the preceeding claims 1 to 5, characterized in that in order to comply with the ISO VG 22 viscosity class requirement it contains about 65 percent by weight of the base composition and about 35 percent by weight of 2-ethylhexyl oleate.

7. A hydraulic fluid made of a base composition according to any of the preceeding claims 1 to 5, characterized in that in order to comply with the ISO VG 32 viscosity class requirement it contains 100 percent by weight of the base composition.

8. A hydraulic fluid made of a base composition according to any of the preceeding claims 1 to 5, characterized in that in order to comply with the ISO VG 46 viscosity class requirement it contains about 77 percent by weight of the base composition and about 23 percent by weight of pentaerythritol ester.

9. A hydraulic fluid made of a base composition according to any of the preceeding claims 1 to 5, characterized in that in order to comply with the ISO VG 68 viscosity class requirement it contains about 80 percent by weight of the base composition, about 14 percent by weight of pentaerythritol ester and about 6 percent by weight of a complex ester.

10. A hydraulic fluid made of a base composition according to any of the preceeding claims 1 to 5, characterized in that the fluid in addition contains at least one component selected from: boundary lubrication additives, corrosion inhibitors, Vl-improvers, pour point depressants, foam decomposers, demulsifiers.

11. A hydraulic fluid made of a base composition according to claim 5, characterized in that it has the following composition:

- Refined rape seed oil, 97.0 percent by weight 2,6-di-tertiary-butyl- 4-methylphenol

(Additin^® 10), 1.5

Triaryl phosphite

(Irganox^® L 180), 1.0

- Amino phosphate derivative (Irgalube^® 349), 0.5

12. A hydraulic fluid made of a base composition according to claim 5, characterized in that it has the following composition: Refined rape seed oil. 96.5 percent by weight Tertiary-butyl phenol derivative (Hitec^® 4735), 2.0 Zn-dialkyldithiophosphate (Anglamol^® 75), 1.5