Patent Application
20050075250
The present invention relates to lubricating grease compositions based on (per)fluoropolyethers, having low friction coefficients and reduced wears, particularly suitable to be used for example in bearings working at high speed in a wide range of temperatures.
(Per)fluoropolyethers having kinematic viscosities at 20° C. between 20 and 2,000 cSt used as lubrificating oils, are known.
Greases obtained by said (per)fluoropolyethers by the addition of thickeners are also known, among which the most commonly used is polytetrafluoroethylene (PTFE) in powder.
In U.S. Pat. No. 4,472,290 it is described a process for preparing a grease based on a (per)fluoropolyether oil and PTFE in powder, in the form of aggregates having sizes from 1 to 200 micron, consisting in dispersing said PTFE in the (per)fluoropolyether oil by using a fluorinated surfactant. Said PTFE is obtained by tetrafluoroethylene (TFE) polymerization in aqueous dispersion and subsequent polymer separation from the aqueous medium. The greases obtained with this process show the following drawback, in particular at high temperatures: the surfactant contained therein decomposes thus modifying the good grease characteristics. In practice it is observed a corrosion of the bearing mechanical parts in contact with said lubricants.
In U.S. Pat. No. 6,025,307 it is described a fluorinated grease formed of: 15-50% of PTFE particles having sizes lower than 1 micron, 30-84% by weight of a perfluoropolyether oil and 0.5-10% by weight of a surfactant or of a dispersing agent having a perfluoroalkyl or perfluoropolyether chain. The preparation process of this grease is very complicate since it involves various steps:
Said process, even though preparing greases having good lubricating properties, is disadvantageous, since it requires several steps and significant amounts of acid and bases and light solvents to be stripped. Further it is extremely difficult to obtain on an industrial scale a grease having a constant quality.
Besides in the grease obtained with this process there are high amounts of surfactant and water. The drawback of these greases is that said components, in contact with metal surfaces and/or at high temperatures, give corrosion phenomena. In practice the use of said greases is limited to applications wherein oxidizable metal parts are absent.
In EP 856,570 antiseizing pastes are described, prepared by addition to a perfluoropolyether oil having a viscosity between 20 and 2,000 cSt at 20° C. of a PTFE in powder having molecular weight between 100,000 and 700,000 and particle size from 2 to 7 micron, the PTFE being obtained by irradiation with gamma rays or with electron beam of PTFE powder and subsequent milling.
These pastes preferably containing 50-60% by weight of PTFE are not suitable to be used as lubricating greases in bearings working at high speed due to the remarkable friction coefficient owing to the high solid content (PTFE).
It was therefore desired to have available (per)fluoropolyether lubricating greases containing low amounts of the thickening agent PTFE, having a combination of low wear and friction values, suitable to be used in bearings working at high speed in a wide range of temperatures, preparable with a simple, inexpensive and insutrially reproducible process.
(Per)fluoropolyether-based greases have been surprisingly and unexpectedly found, satisfying the above requirements.
An object of the present invention are (per)fluoropolyether lubricating greases comoprising:
The surfactant content of 20 ppm represents the sensitivity limit of the analytical determination as described in the characterization of the Examples.
The drying to obtain component B) is carried out at a temperture in the range 105° C.-190° C., preferably 110-140° C. Generally powder particle aggregates are obtained having an average size between 10 and 30 micron.
The perfluoropolyether oils of component A) are selected from the following classes:
E—O—(CF(CF3)CF2O)m′(CFXO)n′—E′ (1)
The preferred perfluoropolyether oils are those of the classes (1), (4), (5) or their mixtures and are available on the market with the trademark FOMBLIN® marketed by Solvay Solexis.
Component B) is selected between the TFE homopolymer (PTFE) and the TFE copolymers with 0.01-1% by weight of a comonomer having an ethylene type unsaturation.
As comonomers having an ethylene type unsaturation, those olefinic, acrylic and styrene can be mentioned as, for example, ethylene, propylene, methylmethacrilate, (metha)acrylic acid, butylacrylate, hydroxyethylhexylacrylate, styrene, C3-C8 pefluoroolefins, as hexafluoropropene (H—FP); C2-C8 fluoroolefins containin hydrogen, as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, CH2═CH—Rf perfluoroalkylethylene, wherein Rf is a C1-C6 perfluoroalkyl; C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, as chlorotrifluoroethylene (CTFE); CF2═CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7, preferably perfluoropropylvinylether (PVE); CF2═CFOX (per)fluorooxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl; fluorodioxoles, preferably perfluorodioxoles; fluorovinylethers CFX═CXOCF2OR (MOVE) wherein R is a C2-C6 (per)fluoroalkyl or a C5-C6 cyclic group, or a C2-C6 (per)fluorooxyalkyl group containing from one to three oxygen atoms and X═F, H, preferably perfluoro-3,5-dioxa-1-heptene CF2═CF—O—CF2—O—CF2CF3 (MOVE 1) and perfluoro-3,5,8-trioxa-1-nonene CF3OCF2CF2OCF2OCF═CF2 (MOVE 2).
The preferred comonomers are perfluoropropyl-vinylether (PVE) and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD).
The aqueous latex of polytetrafluoroethylene or of the tetrafluoroethylene copolymer with another monomer having an ethylene type unsaturation, used to prepare component B), is known and is prepared according to what described in U.S. Pat. No. 6,297,334 herein incorporated by reference.
The preparation of the TFE polymer latex comprises the following steps:
The microemulsion used in the latex preparation is described in U.S. Pat. No. 4,864,006 and U.S. Pat. No. 4,990,283.
As above said, the latex is coagulated and the obtained polymer particles are dried. The coagulation is carried out by addition of an electrolyte.
The electrolyte can be an inorganic acid or an inorganic salt. Among the acids, it can be mentioned nitric acid, hydrochloric acid, sulphuric acid; among the salts, it can be mentioned potassium nitrate, ammonium carbonate, magnesium sulphate, aluminum sulphate, potassium carbonate, calcium nitrate, sodium chloride.
Preferably nitric acid or ammonium carbonate are used.
As said the drying of the coagulated latex is carried out at a temperature in the range 105° C.-190° C., preferably 110°-140° C., for a time between 20 and 60 hours.
A further object of the present invention is a process for the preparation of the invention (per)fluoropolyether greases comprising the following steps:
In step a), after the (per)fluoropolyether oil A) introduction in the mixer, generally a degassing under vacuum at 60° C. for 2 hours at 0.1 mbar is carried out.
In step b), the addition of the powder of component B) to the oil is generally carried out in at least 3 hours until reaching the desired composition.
In step c) the stirring is carried out in a constant way, for about 8 hours, under vacuum.
The refining of step d) is generally carried out in a three-cylindrical refiner.
The grease obtainable with this process has, as said, an amount of water lower than 100 ppm, preferably lower than 60 ppm and a surfactant amount lower than 20 ppm.
The greases of the present invention can be used as such or can be additioned with the known additives of the prior art, as, for example, antirust, antiwear, antioxidants, stabilizers.
The invention greases can be formulated depending on the amounts of A) and B) so to have different penetration degrees.
The penetration is determined according to the ASTM D-217 method and the greases are classified according to the NLGI scale from 000 degree, corresponding to a penetration value of 475 mm/10′, to 6 degree, corresponding to 85 mm/10′.
The greases of the present invention result more stable, in applications requiring high temperatures, since the thickener is directly formulated with (per)fluoropolyether oil without using surfactants.
The greases of the present invention contain, the penetration being equal, compared with the known (per)fluorinated greases, a much lower amount of thickening agent (PTFE).
Unexpectedly and surprisingly the invention greases show very low friction coefficient values and a very reduced wear with respect to the known (per)fluorinated greases.
The invention greases are used in the lubrication of mechanical parts and their combination of very good properties makes them particularly suitable in the lubrication of bearings working at high speed and in a wide range of temperatures.
Even operating under these conditions, the bearings have improved peformances since the stress values are reduced and a lower heat generation is obtained (see the Examples, stress and maximum temperature reached).
Furthermore it has been found that the greases of the present invention, considering their low solid content, are applicable with significant advantages in the lubrication of microbearings, microgears, preferably in plastic, for example mechanical actuators, extending their life and significantly reducing the running noise.
The invention greases show furthermore a very low toxicity since they are formed of substantially stable and inert (per)fluoropolyether oils and PTFE.
Some Examples follow for illustrative and not limitative purposes of the invention.
The following characterizations have been carried out on the components and on the greases of the invention.
Determination of the Polymer Molecular Weight
The number average molecular weight of PTFE is calculated from the first melting temperature, if lower than 327° C., as described in EP 481,509, or by the absolute specific weight datum according to what described by Doban R. C., Knight A. C., Peterson J. H, Sperati C. A. in “Meeting of the American Chemical Society” Atlantic City, September 1956.
Determination of the Average Particle Diameter in the Latex
The average particle diameter is measured by Laser Light Scattering based on the laser light diffusion (Photon Correlation Spectroscopy). The used instrument consists of an Argon laser light source having a wave length of 514.5 nm by Spectra-Physics and in a photocorrelator by Brookhaven 2030 AT model. The scattering measurement is carried out at 25° C. on latex samples diluted with filtered water and at an angle of 90°. The particle diameter in the latex is calculated by applying the cumulant method.
Determination of the Polymer Content in the Latex (Expressed in g/l H2O)
About 20 grams of latex are weighed in a glass beaker and put in a stove to dry for 1 hour at 150° C. The latex dry content, expressed as g PTFE/l water, is obtained by applying the formula:
g PTFE/l water=(Wess−Wtara)/(Winiz−Wess)*1000
wherein
0.5 g of powder are wet with ethanol and brought to basic pH with a NH4OH solution. The powder is dried under nitrogen flow, to the dried powder 2 ml of acidified methanol with some drops of concentrated H2SO4 are added, the whole is brought into a hermetically sealed test tube heated at 70° C. for 70 hours to carry out the surfactant esterification. To the obtained mixture 5 ml of Delifrene A 113 and 4 ml of water are then added. It is let stand. Two phases are separated, a μlitre of the lower fluorinated phase containing the surfactant ester is taken.
The fluorinated phase is gaschromatographically analyzed by injecting it in a 3 m long glass column, with a diameter of 3 mm, filled with a stationary phase constituted by Carbovax 20M absorbed at 25% by weight on Chromosorb W 80-100 mesh.
The method sensitivity limit is 20 ppm.
Quantitative Determination of the Water in the Grease
The ASTM E 1064-85 method has been used.
Penetration
The test is carried out according to the ASTM D 217 method.
Four-ball Wear Test
For the wear evaluation the ASTM D 2266 method has been followed using a 40±0.2 kgf (392N) load at a test temperature of 75°±2° C.
Evaluation Friction Test for Ball Bearings
A ball bearing (SKF 6303 model), cleaned with n-hexane and dried, is filled at 30% by weight, with respect to the grease weight required for the total filling, with the grease to be tested and the suitable screens are inserted to avoid the material coming out. The so assembled bearing is mounted on a shaft, connected to an engine, equipped with suitable housing couple and tightened. The cup wherein the bearing is placed is equipped with a loading cell which allows to measure the bearing stress during the running. On the outer race of the bearing a thermocouple is positioned detecting the temperature evolution during the test. The test is carried out by making the bearing to start at 5,000 rpm and maintaining such speed for 2 hours. The test result is given by the initial take-off value in mN•m, stress after steady conditions of 2 hours in mN•m, oscillation of the steady condition value and maximum temperature reached at the end of the test.
Test for Friction (Stress) Evaluation at High Speed
The same equipment and filling modalities of the bearing described in the previous test are used.
The test conditions are the following: a speed gradient is set from 0 to 16,000 rpm with step of 2000 rpm, each lasting 1 hour. The test duration is of total 8 hours and the stress value after 1 hour at 16,000 rpm and the maximum temperature reached are evaluated.
In a glass vessel there are added in sequence:
A perfectly clear microemulsion is obtained in the temperature range from 2° C. to 46° C.
In a 400 l reactor equipped with stirring system wherein vacuum has been made (−660 mmHg), 9,500 g of the microemulsion prepared according to the described process, 2 kg of a paraffin having a softening point 52°-54° C. and 270 l of demineralized water are introduced.
The reactor is maintained under mechanical stirring. 1,200 g of 2,2,4-trifluoro-5-trifluoro-methoxy-1,3-dioxole (TTD) and 100 mmHg of ethane are fed into the reactor. The reactor is pressurized at 20 bar with C2F4 and heated to 90° C. At this point 700 cc of a solution of ammonium persulphate (initiator) equal to 14 g are fed into the reactor.
After a first pressure decrease of 0.5 bar, the pressure is maintained at 20 bar by feeding C2F4.
After 30 minutes the tetrafluoroethylene (TFE) feeding is stopped, it is cooled and the reactor is discharged.
The so obtained latex has a concentration of 440 g/l and an average particle diameter of 50 nm.
The latex is diluted until bringing the polymer concentration to 7% by weight and 20 litres of said latex are introduced at a temperature of 24° C. in a 50 litre reactor. The latex is additioned under stirring with an aqueous solution of (NH4)2CO3 at 15% so to bring the pH to 8.
Stirring is continued by applying specific power of 3 KW/m3 until powder flotation. Stirring is then stopped and the water is discharged by separating it from the wet powder.
The wet powder is then subjected to two washings with 20 litres of water at the temperature of 24° C., under stirring for 5 minutes. The wet powder is then discharged and dried in a static oven at 120° C. for 48 hours.
The dry powder analyzed with the above described method resulted free from surfactant.
In a grease mixer (2, 7 litre Apinox model) 1,200 g of oil (Fomblin® Y04) are introduced, having the following structure:
CF3—[(O—CF(CF3)—CF2)n—(O—CF2)m]—O—CF3
with n/m=40 and kinematic viscosity at 40° C. of 15 cSt, and it is degassed for two hours at 0.1 mbar. 300 g of modified PTFE prepared in the Example 1A are then added, by successive steps of about 100 g at a time, starting the mixer at least 30′ after every addition. When the additions are over, the slurry is left under vacuum, mixing for 8 hours.
Said time elapsed, 1,500 g of grease are discharged, and is refined by one passage through the three cylinder refiner.
The analyzed grease shows a water content of about 50 ppm.
On the refined grease the following characterizations are carried out: penetration, wear test, steady condition test and high speed test the results of which are reported in Table 1.
The Example 1B is repeated but by introducing 900 g of the oil described in the Example 1B and 600 g of a commercial PTFE Algoflon® L203 having a number average molecular weight of 500,000, obtained from a latex having a particle diameter of 200-300 nm, by successive steps of about 100 g at a time.
On the refined grease the same characterizations of the Example 1B are carried out and the results are reported in Table 1.
From the comparison of the Example 1B with the Example 2 it results that with the invention grease remarkably improved performances are obtained (see the Table) the consistency (penetration degree) being equal.
The Example 1B is repeated but by using 1,200 g of oil (Fomblin® Z15), having the following structure:
CF3—[(O—CF2CF2)n—(O—CF2)m]—O—CF3
with n/m≈1 and kinematic viscosity at 40° C. of 92 cSt, and 300 g of PTFE of the Example 1A.
The analyzed grease shows a water content of about 50 ppm.
On the refined grease the following characterizations are carried out: penetration, wear test, steady condition test and high speed test, the results of which are reported in Table 2.
The Example 3 is repeated by introducing 900 g of the oil described in the Example 3 and 600 g of a commercial PTFE Algoflon® L203 described in the Example 2.
On the refined grease the same characterizations of the Example 3 are carried out and the results are reported in Table 2.
From the comparison of the Example 3 with the Example 4 it results that with the invention grease remarkably improved performances are obtained (see the Table) the consistency (penetration degree) being equal.
The Example 1B has been repeated but by using as PTFE the commercial PTFE of the Example 2 (comparative). The obtained grease has shown a penetration degree higher than 400.
The performances and the uses of such very fluid grease are not absolutely comparable with those of the grease according to the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2003 A 001913 | Oct 2003 | IT | national |
