EP0006000B1

EP0006000B1 - Method of making silicone-containing lubricating oils

Info

Publication number: EP0006000B1
Application number: EP79300996A
Authority: EP
Inventors: Michael Stanley Bingley
Original assignee: Gully Michael Guy John Viscount Selby
Current assignee: Gully Michael Guy John Viscount Selby
Priority date: 1978-05-31
Filing date: 1979-05-30
Publication date: 1981-09-09
Also published as: EP0006000A1; DE2960776D1

Description

This invention relates to the incorporation of silicone into lubricating oils to make what are referred to as silicone oils.
Mineral oils containing silicone are known, and have very much better lubricating properties than oils without silicone. Such silicone oils may be used with great effect as ordinary engine sump oils or as additives for sump oils, but are particularly suitable for use as upper cylinder lubricants and two stroke oils, especially in modern high power to weight ratio two stroke engines which can operate at speeds up to 20,000 rpm. So far, however, there have been problems with the stability of such oils, the silicone tending to separate too readily from the oil and, in some cases, the mixture breaking down under the extreme conditions encountered in the cylinders with the production of substances which are deleterious to the operation of the enqine.
We have found that if during the manufacture of a silicone oil a particular heating cycle is followed or the mixture is subjected to a particular magnetic field, the stability of the silicone oil produced is very much improved. Although at present we have no evidence to support the theory, we suspect that perhaps the reason for the improvement is that the silicone molecules, which normally have a tightly wound spiral structure, partially unwind under the action of the thermal or magnetic energy to which they are subjected so that oil molecules can attach themselves at points along the length of each silicone molecule to prevent it from returning to a tight coil when the energy field is removed. The silicon molecules are then bound to the oil molecules and are prevented thereby from settling out of suspension.
According to the invention therefore, in a method of making a silicone oil, a suspension of silicone in a lubricating oil is formed, and the stability of the suspension is improved either by subjecting the mixture to a magnetic field or by thermal treatment as defined later. At normal temperatures however, silicones and oil do not mix at all well and, although the silicone will dissolve in the oil if the temperature is raised high enough it will be usual for the suspension to be formed by mixing together the silicone, the oil, and a solvent in which the silicone is at least partly soluble.
The oil which is used in forming the silicone oil may be any suitable mineral, vegetable or synthetic oil, such as for example, any paraffinic or naphthenic mineral lubricating oil, castor oil, polyisopropylene, or polyisobutylene. Preferably however, the oil will be a paraffinic mineral oil, and may comprise a mixture of different paraffinic mineral oils. For best results, the oil or oils used in the method in accordance with the invention should be substantially free of water (less than 30 parts per million by weight), and should preferably have a sulphur content which is less than 1%, preferably less than 0.3%, by weight. Most standard SAE 30 paraffinic oils which are on sale should be satisfactory from this point of view, particularly in respect of the low water contents.
The silicone which is used in the method in accordance with the invention preferably has a molecular chain length which substantially matches that of the oil, or in other words the viscosities of the silicone and the oil are preferably of the same order as each other, at least over the normal ambient and operating temperature range. Provided this requirement can be met, phenyl silicones, homologeous series silicones or halogenated silicones may be used, but preferably the silicone used in the invention is a dimethyl silicone. Particularly good results have been achieved with a half and half mixture of dimethyl silicone fluid having a nominal viscosity of 300 centi-stokes at 25°C and a dimethyl silicone fluid having a nominal viscosity of 50 centistokes at 25°C.
The solvent, when used, is preferably perchloroethylene, which may be of an industrial or anaiytical grade, but other common aliphatic solvents or aromatic solvents may be used, such as carbon tetrachloride, chloroform, ethylene dichloride, trichlorethylene, benzene, toluene, xylene, diethyl ether, di-isopropyl ether, or white spirit. To a lesser extent, cyclohexane, petroleum ether, petrol, amyl acetate, petroleum spirit, 2-ethylhexanol, dioxane or diethyl cellosolve may be used, and possibly some of the alcohol series may be used provided the silicone can be made to go into at least partial solution.
When it is intended that a silicone oil made in accordance with the invention is to be used as a two-stroke oil, or as an upper cylinder lubricant for four-stroke petrol engines and diesel engines, or indeed as an engine sump oil, the silicone oil produced preferably comprises, by volume, not more than 1% silicone, not more than 4% solvent, and oil (including any other additives which may be considered desirable) as the remainder. It is thought that adding silicone in an amount as little as 0.001 % to an oil by the method in accordance with the invention will have some effect in improving the lubricating properties of the oil, but generally the silicone oil should contain not less than 0.02% silicone. Preferably the silicone oil will contain about 0.2% silicone and at least 0.4% solvent.
When the method in accordance with the invention is carried out using thermal energy to improve the stability of the silicone oil, the silicone, a solvent in which the silicone is at least partly soluble, and a carrier oil are mixed and are heated at between 100°C and 160°C in the absence of oxygen to form a silicone-rich carrier comprising, by volume, not more than 20% silicone, not more than 40% carrier oil, and solvent as the remainder, and the silicone-rich carrier is subsequently added to and mixed with a base oil at a temperature between 25°C and 125°C to form a mixture comprising, by volume, not more than 1% silicone, not more than 4% solvent, and oil as the remainder, the mixture, which is the silicone oil, being allowed to cool in the absence of oxygen.
Preferably the carrier oil comprises at least a portion which is the same as the base oil, and in the preferred case where the base oil is a standard SAE 30 paraffinic mineral oil, the carrier oil is preferably a mixture of paraffinic technical white oil and a paraffinic mineral oil which is identical to the base oil, preferably in the ratio of 4:1.
In preparing the silicone-rich carrier the heating of the components in the absence of oxygen is important to prevent oxidation of the silicone. There are a number of possible ways of doing this, but the preferred method, at least when the boiling point of the solvent is below 160°C (as is the case with the preferred solvent perchloroethylene) is not to add the silicone until the carrier oil and the solvent have been mixed and raised to the boiling point of the solvent in a suitable vessel. After allowing the oxygen to be driven from the vessel by the boiling solvent, the silicone is then injected directly into the mixture in the vessel, and boiling of the mixture is maintained for a short while before sealing the vessel and allowing the contents to cool slowly. The silicone-rich carrier is preferably added to and mixed with the base oil at a temperature of 60°C, this step and the subsequent cooling of the final mixture taking place in the absence of oxygen for the same reason as mentioned above.
Preferably however, the stabilisation of the silicone suspension in the method in accordance with the invention is carried out using a magnetic energy field rather than the relatively complicated thermal treatment described above. Although it has been found that a relatively weak field of a few gauss will work, it is considered that the stronger the field the better will be the results. The mixture of the silicone, the solvent (when used), and the oil need only be placed in or passed through the magnetic field to achieve the required effect, but preferably the silicone is added to and mixed with the other components in the presence of the magnetic field.
Using a magnetic field to stabilise the silicone suspension is much easier and simpler than using heat since there is no need to take precautions to exclude oxygen from the process, and a straight mixture of the components may be treated in a single stage. If preferred however, a silicone-rich carrier of similar composition to that formed in the heat treatment process described above may first be formed, and the silicone-rich carrier subsequently added to a base oil to form the silicone oil mixture. In this case the stabilising magnetic field is applied to the silicone-rich carrier, the silicone preferably being added to and mixed with the solvent and the carrier oil in the presence of the magnetic field. If desired, the final silicone oil mixture may also be subjected to a magnetic field.
As mentioned earlier, a silicone oil manufactured in accordance with the invention is suitable for use, amongst other things, as an engine sump oil. If desired however, the silicone-rich carriers hereinbefore described may be used on their own as an engine oil additive which is rich in silicone and which would be added in small amounts to ordinary sump oils as required. In this case the additive would comprise, by volume, not more than 20% silicone, not more than 40% oil, and solvent as the remainder, it being the intention for such an additive to be added to sump oil in an amount of from 1 to 5% by volume of the sump oil. The manufacture of the additive would be exactly the same as for the silicone-rich carriers described earlier. In this case the oil will preferably be a mixture of paraffinic technical white oil and an SAE 30 paraffinic mineral oil, preferably in the ratio of 4:1. Generally it is thought that the silicone and solvent contents will be less for a diesel engine sump oil additive than for a petrol engine sump oil additive.
Three examples in which the method in accordance with the invention was carried out to form a silicone oil suitable for use as a two stroke oil, an upper cylinder lubricant, or an engine oil will now be described.

Example I.

A dimethyl silicone mixture was made consisting of 50% dimethyl silicone fluid obtainable from ICI as F111/50 (having a nominal viscosity of 50 centistokes at 25°C), and 50% dimethyl silicone fluid obtainable from ICI as F111/300 (having a nominal viscosity of 300 centistokes at 25°C). In addition, a carrier oil mixture was made consisting of, by volume, 80% paraffinic technical white oil, and 20% of a standard SAE 30 paraffinic oil. This oil was obtained as SAE 30 Castrol 1 10A, and had a water content less than 30 parts per million by weight, and a sulphur content less than 0.3% by weight.
The dimethyl silicone mixture and the carrier oil mixture were then used together with some perchloroethylene to make a silicone-rich carrier consisting of, by volume, 4% dimethyl silicone, 26% carrier oil, and 70% perchloroethylene. In making the silicone-rich carrier, the carrier oil and the perchloroethylene were mixed together, without the silicone mixture, and heated in a suitable vessel to 127°C, which is the boiling point of perchloroethylene. After allowing time for the oxygen in the vessel to be driven off, the dimethyl silicone mixture was added directly to the contents of a vessel by syringe. Boiling was maintained for a short while to ensure that the silicone mixed completely with the oil and solvent, and to prevent air from re-entering the vessel. The vessel was then sealed and the mixture allowed to cool slowly in the absence of oxygen. The resulting mixture was the silicone-rich carrier.
A standard SAE 30 paraffinic mineral oil (obtained as SAE 30 Castrol 110A) was then used as a base lubricating oil to which the silicone-rich carrer was added to form a silicone oil as follows. The base oil was placed in a closed vessel and heated to a temperature of 60°C while continuously agitating the oil, air being allowed to escape from the vessel but not to enter. When 60°C was reached, a small quantity of the silicone-rich carrier was added to the base oil in the vessel, the quantity being 5% by volume of the total mixture, and the vessel was sealed after the remaining air had been expelled. The mixture of the base oil and the silicone-rich carrier in the vessel was agitated continuously for about 15 minutes at 60°C before being allowed to cool slowly in the absence of oxygen. The resulting mixture was a high quality silicone oil containing, by volume, 0.2% silicone, 3.5% perchloroethylene, 1.04% technical white oil, and the SAE 30 paraffinic oil as the remainder.

Example II.

As in Example I, the first stage was to make a silicone-rich carrier. This carrier had exactly the same composition as that in Example I, and again the carrier oil mixture and the perchloroethylene were mixed together before the dimethyl silicone mixture was added. In this case however, the mixture of the carrier oil and the perchloroethylene was placed, at room temperature, in a test tube within a coil comprising between 5,000 and 6,000 turns of SWG 34 copper wire, and the coil energised by a 28 volt d/c supply to generate an electro-magnetic field within the coil. The dimethyl silicone mixture was then added to and mixed with the contents of the test tube in the presence of this relatively weak magnetic field and the mixture subjected to the field for about 5 minutes.
After that, some of the silicone-rich carrier so formed was added to and vigorously mixed with a quantity of a base lubricating oil in the form of a standard SAE 30 paraffinic oil (obtained as SAE 30 Castrol 11 OA), the silicone-rich carrier being added in an amount which was 5% by volume of the total mixture. In contrast to Example I however, the silicone-rich carrier was added to the base oil at room temperature and the mixture was subjected to the same magnetic field which was used in the formation of the silicone-rich carrier. The mixture was subjected to the magnetic field for about 5 minutes, and the result was a high quality silicone oil having the same composition as that produced in Example I.

Example III.

As in the previous examples the first stage was to make a silicone-rich carrier using perchloroethylene and the same dimethyl silicone mixture and carrier oil mixture as described in example I. In this case however, 10ccs of the perchloroethylene were mixed with 6ccs of the carrier oil and placed in a tube within a coil generating a much stronger magnetic field than that used in Example II, 4ccs of the dimethyl silicone mixture then being added to and mixed with the perchloroethylene and carrier oil in the tube, i.e. in the presence of the magnetic field. The resulting mixture constituted the silicone-rich carrier, and comprised, by volume 20% dimethyl silicone, 50% perchloroethylene, and paraffinic mineral oil (4 parts technical white oil and 1 part SAE 30 base oil) as the remainder.
As in the previous examples, the silicone-rich carrier so obtained was then added to and thoroughly mixed with a base oil in the form of a standard SAE 30 paraffinic mineral oil (obtained as SAE 30 Castrol 110A). In this case however, the silicone-rich carrier was added in an amount which was 1 % by volume of the total mixture, and the mixture was subjected briefly to a further magnetic field by being poured through a tube surrounded by an energised electromagnetic coil. The resulting mixture was an excellent silicone oil containing, by volume, 0.2% silicon, 0.5% perchloroethylene, 0.24% technical white oil, and the SAE 30 paraffinic base oil as the remainder.
In order to illustrate the improved nature and performance of silicone oils manufactured in accordance with the invention, a number of tests have been carried out using the oils made in Examples I to III and these are described and reported as follows.

Test 1.

Two clear glass jars were set up, one jar containing 500ccs of the silicone oil produced in Example II, and the other jar containing 500ccs of a silicone oil of identical composition and manufactured in exactly the same manner except that the silicone-rich carrier and the final mixture were not subjected to a magnetic field or any other treatment. Then, without disturbing the jars or their contents in any way, the nature and appearance of the two oils were observed over a number of days. After the first day the control oil (i.e. the oil with no magnetic treatment) appeared homogeneous except for thin horizontal clear regions, or striae, near the upper surface. This condition remained substantially the same until the fifth day when large particles (silicones) began depositing on the bottom of the jar. By the sixth day there was a fairly heavy deposit of silicones at the bottom of the jar and the striae were more pronounced, this trend continuing through to the ninth day. In contrast, throughout the whole nine day period the silicone oil produced in accordance with Example 11 remained a completely homogeneous suspension, exhibiting no striae or deposition. During this period the ambient temperature around the jars ranged between 10 and 12°C, but on the tenth day the temperature suddenly increased to 16°C, following which some of the heavy silicone particles began to deposit out from the oil in Example II. At this point the test was discontinued but it did serve to show that the magnetic field treatment during the manufacture of the silicone oil markedly improved the ability of the oil to hold the silicones in suspension.
It is to be noted that the magnetic field used in Example 11 was relatively weak, and although tests have not yet been carried out to prove the fact, it is considered that by making the magnetic field strength very much stronger the stability of the silicone oil suspension will be improved very much more, perhaps to a point where some of the silicone molecules will remain in suspension (sufficient to cause a beam of light projected through the oil to be visibly diffracted) almost indefinitely, even at fairly high ambient temperatures and the correspondingly low viscosity values. Indeed, provided that the oil is not overloaded with silicone and there is sufficient solvent present, it is considered that a major proportion of the silicone molecules could be stabilised in suspension.

Test 2.

Samples of three different oils, one being the magnetically treated silicone oil produced in Example II, another being the control oil used in Test 1, (that is the untreated silicone oil mixture), and the third being just the base oil (i.e. the SAE 30 Castrol 110A) used in making the silicone oils, were subjected to standard engine rig tests. These comprise the usual four ball tests to measure weld point, wear, and seizure under pressure, and are well known in the oil industry for determining the performance of an oil. The results of the tests are given in the following table:-
These results show very clearly that simply mixing silicones with a base lubricating oil very much improves the properties of the oil, and more importantly that by making the silicone oil in accordance with the invention the properties of the oil are improved still further quite appreciably.

Test 3.

Two motor cars whose petrol consumption had been measured over a long period of time using the recommended standard petrol and oil were then run using the silicone oil produced in Example I as an upper cylinder lubricant (3.3 to 6.6ccs per litre of petrol) and using the silicone-rich carrier of Example I as a sump and gear box oil additive (5ccs per 560 ccs), and the average petrol consumption measured over a period of about seven months. In the first motor car, a VW Golf, the average petrol consumption before using the silicone oil was 10.06 kms. per litre. In the seven month period using silicone oil as described, the petrol consumption decreased to 11.3 kms. per litre, an improvement of approximately 13%.
In the second vehicle, a Triumph TR6 the average petrol consumption without using the silicone oil was 8.3 kms. per litre, and during the period of use with silicone oil this decreased to 10.25 kms. per litre, an improvement of about 19%. In addition it was noticed that a previous tendency for the TR6 engine to pink when using 4 star petrol disappeared when using the silicone oil as an upper cylinder lubricant, indicating that a silicone oil manufactured in accordance with the invention may have the effect of increasing the octane rating of a petrol when mixed with the petrol as an upper cylinder lubricant.
It has also been found that when using the silicone oil in this way there is very much less tendency for carbon deposits to form on the valves and piston surfaces, and indeed that existing carbon deposits will be reduced or even removed altogether. Furthermore, it has been found that when using the silicone oil as an engine sump oil, the engine can be run until seizure, following loss of the oil or oil pressure, without causing any damage to the engine.

Test 4.

A series of test were carried out in which a 21 cc Ohlsson and Rice two stroke engine was run continuously at 6,000 to 7,000 rpm to drive a Champ generator at a power rating of 100 watts, the engine running until 500 ccs of fuel had been used up. In each case a different two stroke oil was used with the petrol, mixed in the ratio of 1:25, the engine being stripped before and after the test to measure the wear, if any, on the piston and to ascertain the condition of the engine surfaces.
In the case of one commercially available synthetic two stroke oil the reduction in the average diameter of the piston thrust face was 60 microns, and in the case of another commercially available synthetic two stroke oil the reduction was 25 microns, both very large amounts of wear in the relatively short duration of the tests. In addition, the piston thrust face was scratched in each case, although in general the engine was fairly clean apart from light carbon streaks on the engine bore and spark plug with the first oil.
In contrast, when using the silicone oil produced in Example I as the two stroke oil, no reduction in the average diameter of the piston thrust face was measured, indicating zero wear over the duration of the test. Furthermore there appeared a bright mirror finish on the piston surfaces, although the rest of the engine was perhaps not quite as clean as after using the synthetic oils. The spark plug however was a uniform grey colour, which is good.

Test 5.

The 21cc Ohlsson and Rice two stroke engine was run powering the Champ generator, and the engine surface temperature was measured at different power output levels using a calibrated thermistor bolted to the engine cylinder head. The test was carried out a number of times using different two stroke oils in the petrol used to fuel the engine (2% oil in the mixture), and the results are shown in the accompanying graphs which plot engine surface temperature (degrees centigrade) against generator power output (current in amps). It was found that the engine would often run at different temperatures on different days, despite using the same fuel mixture and the ambient temperature being the same. Consequently, the tests which are recorded on each graph were carried out on the same day as each other and as close as possible to each other in time in order to avoid as much as possible temperature differences arising for reasons other than the change in fuel mixture.
In the graph of Figure 1, the curve () - () represents a fuel mixture containing 2% of a commercially available synthetic two stroke oil;

the curve■―■ represents a fuel mixture containing 2% of a different commercially available synthetic two stroke oil; and,
the curve ⊙―⊙ represents a fuel mixture containing 2% of the silicone oil produced in Example III.

During these tests the ambient temperature remained 13°C. As can be seen, when using the silicone oil of Example III in the fuel, the engine ran approximately 2°C cooler than when using the commercially available synthetic oils.
In the graph of Figure 2, the curve ⊙―⊙ represents a fuel mixture containing 2% of the silicone oil made in Example III; and,

the curve■―■ represents a fuel containing 2% of a silicone oil having the same composition as that of Example III but which was made instead using the weaker magnetic field treatment of Example II.

The ambient temperature throughout these tests was 13°C and the graph indicates that the engine ran approximately 2°C cooler when using the silicone oil of Example III, that is the oil subjected to the stronger magnetic field during its manufacture.
In the graph of Figure 3, the curve x - x represents a fuel containing 2% of a commercially available synthetic two stroke oil different from those used in the tests of Figure 1;

the curve■―■ represents a fuel containing 2% of a two stroke silicone oil in accordance with the invention and consisting of a half and half mixture of the two oils used in the tests of Figure 2; and,
the curve ⊙―⊙ represents a fuel containing 2% of a two stroke silicone oil which is the same as that represented by · · in Figure 2 except that the base oil used in forming the silicone oil contained 0.2% of an aliphatic chlorinated wax (obtained from ICI as Cereclor 42).

The ambient temperature throughout the tests was 1 1 °C. As can be seen from the graph the engine temperatures at relatively low loads were much the same for the synthetic oil and the silicon oil mixture, but at higher loads were much higher for the synthetic oil and the engine would not actually produce maximum power. When run using the silicone oil containing Cereclor 42 the engine ran very cool at low loads, perhaps too cool since a fluffy deposit was found on the sparking plug. In all other cases using a silicone oil in accordance with the invention the sparking plug ended in very good condition, being a matt grey colour with no deposits. In general however, the reduction of the operating temperature of an engine, which the use of silicone two stroke oils produced in accordance with the invention appears to do, is beneficial in that there tends to be less wear in the engine and there is less carbon break down in the combustion of the fuel, resulting in less pollution.
In summary, while we accept that these tests are by no means conclusive, we think that it is true to say that not only are silicone oils manufactured in accordance with the invention out- standinglv better in performance (providing much greater engine protection and life and less fuel consumption) than oils at present commercially available, but that they are also appreciably better, both in stability and performance than simple mixtures of oil and silicones.

Claims

1. A method of making a silicone oil, in which a suspension of silicone in a lubricating oil is formed and the stability of the suspension is improved by subjecting the mixture to a magnetic field.

2. A method according to claim 1, in which the silicone is added to and mixed with the oil in the presence of the magnetic field.

3. A method according to claim 1 or claim 2, in which the oil is a paraffinic mineral oil having a water content of less than 30 parts per million by weight and a sulphur content of less than 1% by weight.

4. A method according to any one of claims 1 to 3, in which the silicone is a dimethyl silicone having a molecular chain length which substantially matches that of the oil.

5. A method according to any one of claims 1 to 4, in which the suspension includes a solvent in which the silicone is at least partly soluble.

6. A method according to claim 5, in which the silicone oil produced is intended for use as an engine sump oil, a two stroke oil, or an upper cylinder lubricant, and comprises, by volume, not more than 1% silicone, not more than 4% solvent, and oil as the remainder.

7. A method according to claim 6, in which the silicone oil contains about 0.2% silicone and at least 0.4% solvent.

8. A method according to any one of claims 5 to 7, in which the solvent is perchloroethylene.

9. A method according to any one of claims 5 to 8, in which the silicone and the solvent are first mixed with a carrier oil to form a silicone-rich carrier comprising, by volume, not more than 20% silicone, not more than 40% carrier oil, and solvent as the remainder, the silicone being added to and mixed with the solvent and the carrier oil in the presence of a magnetic field to improve the stability of the silicone suspension, and the silicone-rich carrier is subsequently mixed with a base oil to provide the silicone oil.

10. A method according to claim 9, in which the silicone-rich carrier comprises 20% silicone, 30% carrier oil, and 50% solvent, and is added to the base oil in an amount which is 1% by volume of the mixture thereof.

11. A method according to claim 9 or claim 10, in which the mixture of the silicone-rich carrier and the base oil is also subjected to a magnetic field.

12. A method of making a silicone oil, in which a lubricating oil, a silicone and a solvent in which the silicone is at least partly soluble, are mixed together to form a suspension of silicone in the oil, and the stability of the suspension is improved by thermal treatment, the silicone and the solvent being mixed with a carrier oil and heated at between 100°C and 160°C in the absence of oxygen to form a silicone-rich carrier comprising, by volume, not more than 20% silicone, not more than 40% carrier oil, and solvent as the remainder, and the silicone-rich carrier subsequently being added to and mixed with a base oil at a temperature between 25°C and 125°C to form a mixture comprising, by volume, not more than 1% silicone, not more than 4% solvent, and oil as the remainder, the mixture being allowed to cool in the absence of oxygen to form a silicone oil suitable for use as an engine sump oil, a two-stroke oil, or an upper cylinder lubricant.

13. A method according to claim 12, in which the solvent is perchloroethylene and is heated with the carrier oil to at least 127°C before the silicone is added in the absence of oxygen to form the silicone-rich carrier.

14. A method according to claim 12 or claim 13, in which the silicone-rich carrier is added to the base oil at a temperature of 60°C.

15. A method according to claim 9 or any one of claims 12 to 14, in which the silicone-rich carrier comprises 4% silicone, 26% carrier oil, and 70% solvent, and is added to the base oil in an amount which is 5% by volume of the mixture thereof.

16. A method according to any one of claims 9 to 15, in which the base oil is an SAE 30 paraffinic mineral oil having a water content of less than 30 parts per million by weight and a sulphur content of less than 1% by weight, and the carrier oil is a mixture of paraffinic technical white oil and a paraffinic mineral oil identical to the base oil.

17. A method according to claim 16, in which the paraffinic technical white oil constitutes 80% by volume, of the carrier oil.