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Title:
LUBRICATING OIL COMPOSITION AND METHOD FOR PRODUCTION THEREOF
Document Type and Number:
WIPO Patent Application WO/2008/046862
Kind Code:
A1
Abstract:
In a lubricating oil composition, an efficient defoaming action must be exerted both against bubbles on the oil surface and against bubbles within the oil. To this end, the present invention provides a lubricating oil composition, comprising polydimethylsiloxane having a kinematic viscosity of from 10 000 to 60 000 mm2/sec at 25°C in an amount of from 0.1 to 30 ppm, preferably from 0.1 to 10 ppm, calculated as content of Si, and a polyacrylate defoamant having a weight average molecular weight of from 4 000 to 150 000 in an amount of at least 60 times the aforesaid polydimethylsiloxane, calculated as content of Si (ppm).

Inventors:
WAKIZONO, Tetsuo (4052-2 Nakatsu, Aikou-GunAikou, Kanagawa, 243-0303, JP)
TAZAKI, Hiroyuki (4052-2 Nakatsu, Aikou-GunAikou, Kanagawa, 243-0303, JP)
Application Number:
EP2007/061094
Publication Date:
April 24, 2008
Filing Date:
October 17, 2007
Export Citation:
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Assignee:
SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V. (Carel van Bylandtlaan 30, HR The Hague, NL-2596, NL)
WAKIZONO, Tetsuo (4052-2 Nakatsu, Aikou-GunAikou, Kanagawa, 243-0303, JP)
TAZAKI, Hiroyuki (4052-2 Nakatsu, Aikou-GunAikou, Kanagawa, 243-0303, JP)
International Classes:
C10M145/14; C10M155/02; C10M157/10; C10M169/04
Attorney, Agent or Firm:
SHELL INTERNATIONAL B.V. (Intellectual Property Services, PO Box 384, CJ The Hague, NL-2501, NL)
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Claims:
C L A I M S

1. Lubricating oil composition, comprising polydimethylsiloxane having a kinematic viscosity of from

10 000 to 60 000 m 2 /sec at 25 0 C in an amount of from 0.1 to 30 ppm, preferably from 0.1 to 10 ppm, calculated as content of Si, and a polyacrylate defoamant having a weight average molecular weight of from 4 000 to 150 000 in an amount of at least 60 times the aforesaid polydimethylsiloxane, calculated as content of Si (ppm) .

2. Lubricating oil composition according to Claim 1, wherein the average particle size of the polydimethylsiloxane is 0.1 μm or less.

3. Method for production of a lubricating oil composition, comprising at least the steps of:

(a) diluting a polydimethyl-siloxane having a kinematic viscosity of from 10 000 to 60 000 mm 2 /sec at 25 0 C with an organic solvent;

(b) stirring the diluted liquid for at least one minute at 8 000 rpm or more so that the average particle size of the polydimethylsiloxane is 0.1 μm or less; (c) adding the stirred diluted liquid containing polydimethylsiloxane to a base oil so as to constitute 0.1 to 30 ppm, preferably from 0.1 to 10 ppm, calculated as content of Si; and (d) adding at least 60 times the amount of aforesaid polydimethylsiloxane, calculated as content of Si (ppm) , of a polyacrylate defoamant having a weight average molecular weight of from 4 000 to 150 000 to the aforesaid base oil.

4. Method according to Claim 3, wherein in step (b) the diluted liquid is stirred for at least five minutes at

20 000 rpm or more.

5. Method according to Claim 3 or 4 , wherein the aforesaid base oil is one classified by API (American Petroleum Institute) base oil category as Group 2 or Group 3.

6. Lubricating oil composition obtainable by the method of anyone of Claims 3 to 5. 7. Lubricating oil composition according to any of claims 1, 2 or 6 for improving defoaming properties. 8. Method of improving defoaming properties during lubrication using the lubricating oil composition according to any of claims 1, 2 or 6.

Description:

LUBRICATING OIL COMPOSITION AND METHOD FOR PRODUCTION

THEREOF

The present invention provides a lubricating oil composition and a method for the production thereof, and provides effective defoaming properties.

Recently, various types of industrial machinery, in particular automobiles, have been made smaller and of higher efficiency, but in connection with this the environment in which the lubricating oils are used in the machinery has become more severe .

For example, because of the size reduction of the machinery, the lubricating oil tanks have also become smaller, thus the oil volume decreased, and as a result the stress to which the oil is subjected has increased, the rate of deterioration has accelerated, and not only the lubricating performance but also the defoaming performance are lost at an earlier stage. Specifically, unpleasant abnormal noise is produced owing to blowout of bubbles due to the oil surface rising from the oil tank, or owing to cavitation due to air dragged in by the high pressure pump. In particular, in the case of automobiles, bubbles formed from the gearbox are a causative factor, and lubricating oil sometimes sprays out into the engine space, causing a risk of outbreak of fire.

Further, with the quality improvements in automobiles, noise reduction has acquired an important role, and if unpleasant abnormal noise from the gearbox is produced as aforesaid, not only does the market value of the car itself decrease, but also bubbles are dragged into the lubricated areas and may cause seizing and

abrasion, and for this reason also it is desirable that the lubricating oil maintains its defoaming properties over a long period.

Concerning the defoaming properties in this lubricating oil, countermeasures both as regards defoaming of bubbles on the oil surface and defoaming of bubbles within the oil are considered necessary, however, difficulties are encountered in simultaneously effecting defoaming of bubbles on the oil surface and defoaming of bubbles within the oil, and no clear countermeasures have been discovered previously. See 'Defoaming of Lubricating Oils', by Mitsuo Okada, Oil Chemistry [Yukagaku] , Vol.42, No.10 (1993) , 807-810.

That is to say, in contrast to the fact that dimethylpolysiloxane defoamants are effective in eliminating bubbles on the oil surface, if the quantity added becomes large, bubbles within the oil become smaller and are stabilised, as a result of which there is a tendency for the bubbles within the oil to persist instead. On the other hand, polyacrylate polymer defoamants have been used as degassing defoamants since they destabilise bubbles within the oil, but conversely these defoamants tended to stabilise bubbles on the oil surface . The present invention was made in the light of the aforesaid, and its purpose is to provide a lubricating oil composition which has efficient defoaming action both against bubbles on the oil surface and against bubbles within the oil, and is capable of maintaining excellent defoaming properties on the oil surface and within the oil for long periods, and to provide an efficient method for the production thereof.

The present invention provides lubricating oil composition, comprising polydimethylsiloxane having a kinematic viscosity of from 10 000 to 60 000 mm 2 /sec at 25 0 C (according to ASTM D445, Appendix C) in an amount of from 0.1 to 30 ppm, preferably from 0.1 to 10 ppm, calculated as content of Si, and a polyacrylate defoamant having a weight average molecular weight of from 4 000 to 150 000 in an amount of at least 60 times the aforesaid polydimethylsiloxane, calculated as content of Si (ppm) . Further, the present invention provides a method for easily producing the aforesaid lubricating oil composition combining both defoaming properties against bubbles on the oil surface and defoaming properties against bubbles within the oil, by: (a) diluting a polydimethyl-siloxane having a viscosity of from 10 000 to 60 000 mm 2 /sec at 25°C with an organic solvent such as kerosene, light oil or the like; (b) stirring the diluted liquid for at least one minute at 8 000 rpm or more so that the average particle size of the polydimethylsiloxane is 0.1 μm or less (as measured in e.g. a measuring instrument using PIDS (Polarisation Intensity Differential Scattering Technology) ) ; (c) adding the stirred diluted liquid containing polydimethylsiloxane to a base oil so as to constitute 0.1 to 30 ppm, preferably from 0.1 to 10 ppm, calculated as content of Si; and (d) adding at least 60 times the amount of aforesaid polydimethylsiloxane, calculated as content of Si (ppm) , of a polyacrylate defoamant having a weight average molecular weight of from 4 000 to 150 000 to the aforesaid base oil.

By means of the present invention, through the addition of a polydimethylsiloxane of defined viscosity and a polyacrylate defoamant of defined molecular weight,

- A - in combination with a defined ratio, to a lubricating oil, it is effectively possible to perform both defoaming against bubbles on the oil surface and defoaming against bubbles within the oil . Further, by micronising the polydimethylsiloxane by high speed stirring with a diluent, and mixing with a polyacrylate defoamant, it is possible to maintain the defoaming efficacy over a long period, and efficiently, to obtain a lubricating oil composition having long- lasting defoaming properties.

As the base oil in the lubricating oil composition of the present invention, mineral oils, synthetic oils and mixtures thereof normally used in lubricating oils can be used, and in particular base oils belonging to API (American Petroleum Institute) base oil categories Group 1, Group 2, Group 3, Group 4 and the like can be used alone or as mixtures .

Group 1 base oils for example include paraffinic mineral oils obtained by applying a suitable combination of refining techniques such as solvent refining, hydrorefining and dewaxing to a lubricating oil cut obtained by normal pressure distillation of crude oil.

The viscosity index may be 80-120, preferably 95-110 (according to JIS K2283) . The kinematic viscosity at 40°C is preferably 2-680 mm 2 /sec, more preferably 8-220 mm 2 /sec (according to JIS K2283) . Further, the total sulphur content may be less than 700 ppm, preferably less than 500 ppm. The total nitrogen content may also be less than 50 ppm, preferably less than 25 ppm. Further, those with an aniline point of 80-150 0 C, preferably 90-120 0 C, are preferably used.

Group 2 base oils for example include paraffinic mineral oils obtained by applying a suitable combination

of refining techniques such as hydrocracking and dewaxing to a lubricating oil cut obtained by normal pressure distillation of crude oil. In Group 2 base oils refined by a hydrorefining method such as the Gulf Corp. method, the total sulphur content is less than 10 ppm, the aromatics content is 5% or less, and they are suitable for use in the present invention.

There is no particular restriction as to the viscosity of these base oils, but the viscosity index may be 90-125, preferably 100-120. The kinematic viscosity at

4O 0 C is preferably 2-680 mm 2 /sec, more preferably 8-220 mm 2 /sec. Further, the total sulphur content may be less than 700 ppm, preferably less than 500 ppm, and more preferably less than 10 ppm. The total nitrogen content may also be less than 10 ppm, preferably less than 1 ppm. Further, those with an aniline point of 80-150 0 C, preferably 100-135 0 C are preferably used.

Group 3 base oils and Grade 2 plus base oils for example include paraffinic mineral oils produced by intensive hydrorefining of a lubricating oil cut obtained by normal pressure distillation of crude oil, or base oils made by subjecting waxes produced in dewaxing processes to refining by conversion to isoparaffins and dewaxing by the Isodewax process, or base oils refined by the Mobil wax isomerisation process, and these are also suitable for use in the present invention.

There is no particular restriction as to the viscosity of these base oils, but the viscosity index may be 95-145, preferably 100-140. The kinematic viscosity at 40 0 C is preferably 2-680 mm 2 /sec, more preferably 8-220 mm 2 /sec. Further, the total sulphur content may be 0-100 ppm, preferably less than 10 ppm. The total nitrogen content may also be less than 10 ppm, preferably less

than 1 ppm. Further, those with an aniline point of 80- 150°C, preferably 110-135 0 C are preferably used. As synthetic oils, for example polyolefins, alkylbenzenes, alkylnaphthalenes, esters, polyoxyalkylene glycols, polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers, polyphenyl ethers, dialkyl diphenyl ethers, fluorine-containing compounds (perfluoro polyethers, fluorinated polyolefins, etc.), silicone oils and the like are mentioned. The aforesaid polyolefins include polymers of various olefins, or hydrogenation products thereof. Any olefin can be used, and for example ethylene, propylene, butene, α-olefins with five or more carbons and the like are mentioned. In the production of the polyolefins, one of the aforesaid olefins can be used singly, or a combination of two or more can be used. Especially suitable are the polyolefins referred to as poly-α- olefins (PAO), and these are Group 4 base oils.

There is no particular restriction as to the viscosity of these synthetic base oils, but the kinematic viscosity at 40°C is preferably 2-680 mm 2 /sec, more preferably 8-220 mm 2 /sec.

GTL (gas-to-liquid) derived base oils synthesised by natural gas to liquid fuel conversion technology, the Fischer-Tropsch method, is suitable for use as a base oil in the present invention, since its oxidation stability is excellent, and evaporative losses are extremely low, as the sulphur content and aromatics content are very low, and the percentage paraffin structure very high, compared to mineral oil base oils refined from crude oil.

There is no particular restriction as to the viscosity characteristics of the GTL derived base oils, but normally the viscosity index is 130-180, more

preferably 140-175. Further, the kinematic viscosity at 40 0 C is preferably 2-680 mm 2 /sec, more preferably 5-120 mm 2 /sec. Also, normally, the total sulphur content is less than 10 ppm, and the total nitrogen content less than 1 ppm. One example of such a GTL derived base oil product is Shell XHVI (registered trademark) .

It is can be stated that aforesaid base oils of lower viscosity are generally preferable since the rate of elimination of bubbles from the liquid becomes greater, however, since defoamants are incorporated as aforesaid in the present invention, the effects of the base oil in this respect can essentially be regarded as small. Among them, those with a kinematic viscosity at 100 0 C in the range 1-50 mm 2 /sec, in particular 2-15 mm 2 /sec are normally suitable.

With regard to oxidation stability, the % CA of this base oil is preferably 20 or less, in particular 10 or less (according to ASTM D 3238-80) . Further, although there is no particular restriction as to the pour point, which is an indicator of the low temperature fluidity, this is preferably -10 0 C or below, in particular -15°C or below. Also, the viscosity index is preferably 100 or more, since the viscosity remains high at high temperature . The polydimethylsiloxanes in the defoamants forming part of the lubricant compositions of the present invention are represented by the following formula (I) . [Chem.l]

C H a C H 3 C H ,

I I I

C H ., - S i - O -(~ S i - 0 -) - S i - C H 8 • • • ( I )

I I . 1

C H J C H a C H n

(in the aforesaid formula I, n is a positive integer and is a value corresponding to the viscosity) .

The aforesaid polydimethylsiloxanes have a kinematic viscosity at 25°C of about 10 000 to 60 000 mm 2 /sec. Of course, the inclusion of polydimethylsiloxanes of viscosity lower than the aforesaid is not excluded, but if the content becomes high, the lubricating oil may become turbid, or it may be unable to disperse in the lubricating oil, and settle out, hence the content should limited appropriately so that these do not occur.

These polydimethylsiloxanes can be used singly, or as combinations of two or more with different viscosities .

Concerning the quantity thereof used, based on the total weight of the composition, they are incorporated in a proportion of about 0.1 to 30 ppm, preferably from 0.1 to 10 ppm (wt. ppm, hereinafter likewise), calculated as content of Si. With less than 0.1 ppm, there may be no defoaming effect, and if it exceeds 30 ppm, the lubricating oil composition may become turbid, and conversely there may be no defoaming effect. More preferably, it is in the range of about 3-10 ppm.

Together with the aforesaid polydimethylsiloxanes, a polyacrylate defoamant is used. As such polyacrylates, those of molecular weight of from about 4 000 to 150 000 may be used. The quantity thereof used is such that the quantity added (ppm) is at least 60 times the aforesaid polydimethylsiloxane calculated Si weight (ppm) . Further, although there is no particular upper limit provided that it is within the normal added weight range, it is normally preferable to use 5 wt. % or less.

The lubricating oil compositions of the present invention are obtained by incorporating a

polydimethylsiloxane component and a polyacrylate component into a base oil as aforesaid, however, depending on the lubricating oil use, known additives to improve its characteristics, such as metallic detergents, ash-free dispersants, oxidation inhibitors, friction modifiers, metal inactivators, viscosity index improvers and pour point depressants can also be incorporated as appropriate within a range such that the purpose of the present invention is not prejudiced. Normally, the total quantity of these supplementary additives incorporated is preferably within the range of about 0.05 to 25 wt.%, relative to the total weight of the composition.

When the aforesaid polydimethylsiloxane is incorporated into the base oil, it is preferably incorporated after first being dispersed in a solvent.

The necessary quantity of this polydimethylsiloxane is added to the aforesaid solvent, and stirred for at least one minute at a high revolution rate of 8 000 rpm or more using a stirring device such as a homogeniser. Preferably, it is stirred for at least five minutes at 20 000 rpm or more.

By means of this stirring, the polydimethylsiloxane is micronised in the solvent, the particle size can be made smaller, and a polydimethylsiloxane average particle size of 0.1 μm or less can be obtained. This average particle size may be measured with a measuring instrument which uses PIDS (Polarisation Intensity Differential Scattering Technology) . It goes without saying that other micronising methods may be used.

The solvent for the aforesaid dispersion should be of low viscosity, dissolve in the base oil, and readily

evaporate after heating, so that there is no residue in the base oil.

As such solvents, for example those of flash point 40°C or higher, which are safer to handle than gasoline, and have low viscosity, and can readily be stirred with a stirring device such as the aforesaid homogeniser, for example the kerosene defined in JIS K2203, or the light oil defined in JIS K2204, are suitable.

Also, it is desirable that this solvent can readily dissolve in a base oil such as a mineral oil or synthetic oil, and also that, as stated below, when the solvent in which the polydimethyl-siloxane has been dispersed and the aforesaid polyacrylate are added to the base oil and stirred, and heated to about 6O 0 C while the defoamants are being dispersed in the base oil, the aforesaid solvent readily evaporates, and the major part is dispersed outside the system and does not remain in the lubricating oil composition. If this solvent remains, there is a risk that the flash point of the lubricating oil composition will decrease, the kinematic viscosity will decrease, and the lubricating performance will decline.

The solvent used here is not restricted to the aforesaid kerosene and light oil, and any can be used as appropriate provided that it is an organic solvent such as another petroleum type solvent or an ether or the like, and satisfies the aforesaid requirements. Further, as kinematic viscosity lowering components, synthetic oils such as PAO (poly-alpha-olefins) and the like can also be used, provided that it is possible to incorporate them.

The polydimethylsiloxane, finely ground and dispersed in the aforesaid solvent, and the aforesaid

polyacrylate are added to the aforesaid base oil, and mixed and stirred so that the defoamants are dispersed in a homogeneous state in the whole of the base oil. The quantity of the aforesaid polydimethylsiloxane added to the base oil is such as to constitute 0.1 to 30 ppm, preferably from 0.1 to 10 ppm, calculated as Si. Further, The quantity of the aforesaid polyacrylate added is such as to constitute at least 60 times the aforesaid polydimethylsiloxane calculated Si weight (ppm) . The mixing and stirring of the aforesaid defoamants into the base oil can be effected by blending using a jet stirrer or other suitable method. As aforesaid, while the defoamants are being dispersed in the base oil by stirring, this is heated to about 6O 0 C, and as a result of this the aforesaid solvent in which the polydimethylsiloxane has been dispersed is evaporated and dispersed outside the system, such that none remains in the lubricating oil composition, and a lubricating oil composition wherein the polydimethylsiloxane and polyacrylate are dispersed and mixed in a homogeneous state can be obtained.

As aforesaid, by dispersion of the polydimethylsiloxane in the solvent, the average particle size of the polydimethylsiloxane can be made 0.1 μm or less, and the average particle size can also be made 0.1 μm or less after mixing and dispersion in the base oil.

In such polydimethylsiloxane and polyacrylate- containing lubricating oil compositions, since the polydimethylsiloxane is micronised and dispersed in the lubricating oil composition, the number of particles increases with the same added weight, as a result the probability of contact with bubbles formed from the

lubricating oil composition increases, and because of this an effective defoaraant action can be achieved.

Since the aforesaid polydimethylsiloxane is adsorbed on the surface of bubbles on the oil surface and has the function of bursting the bubbles by penetrating between the bubbles, it defoams bubbles on the oil surface particularly effectively. However, a disadvantage is that, in the case of bubbles mixed within the lubricating oil composition, when the polydimethylsiloxane breaks bubbles within the oil, the large bubbles are broken up and become smaller, the buoyancy of the bubbles themselves becomes even smaller, and they remain within the oil for long periods, thus it stabilises the bubbles within the oil. On the other hand, the polyacrylate which is simultaneously present at a defined concentration relative to the aforesaid polydimethylsiloxane is able to increase the size of the bubbles by causing any bubbles within the lubricating oil composition to combine with others, as a result of this the buoyancy of the bubbles increases, and thus they soon float up to the oil surface. Then, since they can be burst by the aforesaid polydimethylsiloxane when they reach the oil surface, bubbles both on the surface of the lubrication oil composition and within the oil can be efficiently eliminated. Examples

The present invention is explained in more detail by means of practical examples, however the present invention is no way limited by these examples.

Preparation of Polydimethylsiloxane Dispersions

The polydimethylsiloxane dispersions shown in Table 1 were prepared using the materials stated below.

Polydimethylsiloxane: Toray Silicone (Co.) SH200

Solvent 1 : kerosene

Solvent 2: light oil

Solvent 3: PAO2 (kinematic viscosity at 100°C: 2.0 mm 2 /sec) .

Table 1

Preparation of Lubricating Oil Compositions

Practical Examples 1-3 and Comparative Examples 1-3 were prepared using the compositions and preparation methods shown in Table 2.

The polydimethylsiloxane average particle size (μm) was measured with a Beckman Colour KS13320 measuring instrument using PIDS (Polarisation Intensity

Differential Scattering Technology) .

Table 2

polyacrylate defoamant; DF defoamant

Test 1: Foam Test - JIS Method

The degree of foaming of the lubricating oil compositions of the aforesaid Practical Examples 1-3 and Comparative Examples 1-3 were measured by the foam test specified in JIS K2518, and the defoaming efficacy was assessed.

(1) The degree of foaming in sequence I, sequence II and sequence III was measured for the aforesaid lubricating oil compositions directly after preparation by heating to 6O 0 C and stirring for 15 mins at 200 rpm. Sequence I : degree of foaming at 24 ± 0.5°C Sequence II: degree of foaming at 93 ± 1°C Sequence III: degree of foaming on reducing temperature to 24 ± 0.5 0 C after sequence II measurement of the degree of foaming at 93 ± I 0 C.

Method of stating degree of foaming: stated in 10 ml units, stated as trace (Tr) if 10 ml or less and stated as zero (0) if liquid surface can be seen.

(2) Degree of foaming in Sequence I, Sequence II and Sequence III measured as above on lubricating oil compositions after one day had elapsed.

(3) Degree of foaming in Sequence I, Sequence II and Sequence III measured as above on lubricating oil compositions after seven days had elapsed. (4) Degree of foaming in Sequence I, Sequence II and

Sequence III measured as above on lubricating oil compositions after one month had elapsed. Test 2 : Foam Test - Mixer Method

The within-oil defoaming properties of the aforesaid lubricating oil compositions were tested using a commercial mixer one month after their preparation by heating to 6O 0 C and stirring at 200 rpm for 15 minutes.

Firstly, a scale graduated in mm units was attached to the glass mixer vessel, 200 ml of sample were introduced into the mixer and stirred for 2 minutes, then the mixer was stopped. (1) The foam layer thickness (mm) 10 seconds from directly after stoppage of mixer was measured.

(2) The foam layer thickness (mm) 1 minute from directly after stoppage of mixer was measured.

(3) The foam layer thickness (mm) 2 minutes from directly after stoppage of mixer was measured.

Test Results

The results of test 1 and test 2 are shown in Table 3.

Table 3

P. Ex. 1 P . Ex . 2 P. Ex. 3 C. Ex. 1 C. Ex. 2 C . Ex . 3

Added PDMS Si calc . wt . , ppm 3 3 3 0 3 3

PADF wt. % 0.02 0.02 0.02 0 0 0.005

PADF/PDMS multiple (x) 66.7 66.7 66.7 0 0 16.7

PDMS av. particle size (μm) 0.07 0.07 0.07 0.07 0.07

JIS K2518 measurement of degree of foaming directly- sequence I Tr 10 10 160 Tr Tr after prep. sequence II Tr Tr 10 20 Tr 10 sequence III Tr Tr Tr 60 Tr 10 after 1 day sequence I Tr 10 10 10 10 sequence II 10 Tr 10 10 Tr sequence III Tr Tr 0 10 Tr after 7 days sequence I 10 Tr 10 10 Tr sequence II 10 Tr 10 Tr Tr sequence III Tr Tr 0 10 10 after 1 sequence I 10 10 10 10 Tr month sequence II 10 Tr Tr 10 Tr sequence III Tr 10 Tr Tr 10

Mixer method after 1 10 sees after 2 2 3 3 3 3 month stopping

1 min after 2 3 3 4 6 6 stopping

2 mins after 2 3 4 5 7 6 stopping

Key: P. Ex. practical example, C. Ex. comparative example, PDMS polydimethylsiloxane, PADF polyacrylate defoamant.

Discussion

With the combination of 3 ppm polydiraethylsiloxane and 0.02 wt . % polyacrylate defoamant of Practical Example 1, both the JIS method foam test results after one month and the within-oil defoaming efficacy by the mixer method were excellent.

As is shown by the results of Practical Example 2 and Practical Example 3, it was found that even when PAO (low viscosity poly-α-olefin) or light oil were used as the base oil instead of the kerosene of Practical Example 1, almost the same defoaming properties as in Practical Example 1 could be maintained for a long period. Since the gear oil of Comparative Example 1 contained no defoamant component whatever, it was found that the schedule I and schedule II degree of foaming in the JIS method foaming test directly after preparation was particularly great and it was of low commercial value. Also, since the results directly after preparation were poor, tests after one day, seven days and one month were not performed.

In Comparative Example 2, polydimethylsiloxane diluted solution was added, and, compared to Comparative Example 1, a rather considerable defoaming effect was obtained, however an adequate result was still not obtained, particularly in the mixer method foam test.

Further, in Comparative Example 3 also, an adequate result was still not obtained, particularly in the mixer method foam test .

From the aforesaid results, it was concluded that there is an interdependence between the quantities of polydimethylsiloxane and polyacrylate defoamant added, and that if both are used together in appropriate quantities, excellent defoaming properties are obtained.