EP1918944A2

EP1918944A2 - Magnetorheological Fluid (MRF)

Info

Publication number: EP1918944A2
Application number: EP07119395A
Authority: EP
Inventors: Juan de Dios García Lopéz Durán; Fernando González Caballero; Angel Vicente Delgado Mora; Guillermo Ramón Iglesias; Modesto Torcuato Lopez Lopez; María Luisa Jiménez Olivares; Luis Fernández Ruiz-Moron; Jorge Insa Monesma; Eduardo Romero Palazón
Original assignee: Repsol YPF SA
Current assignee: Repsol SA
Priority date: 2006-10-26
Filing date: 2007-10-26
Publication date: 2008-05-07
Also published as: EP1918944A3; ES2301390B1; ES2301390A1

Abstract

Magnetorheological fluid composed of magnetic particles dispersed in a liquid phase which comprises a base oil, a viscosity modifier, and a stabilizing agent.

Particularly the viscosity modifier is a styrene-fumarate copolymer and the stabilizing agent is aluminium stearate.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a magnetorheological fluid which comprises magnetic micrometric particles dispersed in a liquid phase composed of a base oil; a viscosity modifier, more particularly composed of two polymers, the first a polyester type polymer which also acts as a pour point depressor and the second a styrene-fumarate copolymer: and a stabilizing agent, more particularly aluminium stearate. In addition to the preparation method and its use as lubricant for the manufacturing of hydraulic dampers for automotion and friction shock absorbers.

PRIOR ART

Magnetic fluids are colloidal suspensions formed by ferro- or ferromagnetic particles dispersed in a carrier liquid. When the diameter of the magnetic-responsive particles is in the order of one micron, they are called magnetorheological fluids (MRF). They have the important property of varying their flow (rheological) properties quickly and reversibly on being subjected to an external magnetic field. It can be achieved that a MRF changes from the typical behaviour of a Newtonian viscous liquid to that of the plastic fluid with high threshold stress and very high viscosity due to action of the magnetic field, a phenomenon which is known as magnetorheological (MR) effect.
This property makes MRF possess a wide range of technological applications. Examples of such in engineering are: damping of periodic and non-periodic vibrations, braking, clutches, anti-seismic protection of structures, anti-vibratory settling, etc. They are also being applied in medicine to the design of intelligent prosthesis for bone joints.
The design of a magnetorheological fluid requires the prior selection of: (i) base oil, (ii) magnetic colloidal particles, and (iii) additives. To achieve that the fluid has sufficiently high MR response it is very important that it is stable to particle aggregation and segmentation processes. It must be taken into account that to achieve an intense MR effect, high concentrations (up to 20% or 30 % in fraction of volume) of magnetic-responsive particles are required which have a very high density and a low remnant magnetization. Hence the aggregation occurs between particles due to magnetic interaction and their rapid sedimentation. When the separation of phases occurs, and if afterwards it is not easy to redisperse the suspension, the MRF stops being useful for most of its potential applications. Therefore, surfactants, dispersants and other additives are usually incorporated in the formation of the MRFs to improve their stability.
There currently exist numerous patents on magnetic fluids and more specifically on magnetorheological fluids. However, very few have come to be marketed for their use in dampers as magnetorheological lubricants. Among these, we can highlight the following patents: US No. 5,683,615 (November 1997) and US No. 6,547,986 B1 (April 2003). The MRFs marketed for dampers, disclosed in said patents, have the following as most outstanding characteristics: (i) they are multi-component suspensions both in the solid and liquid phases; (ii) the solid phase usually contains: micrometric iron, with a wide range of particle size and thickening thixotropic agents (normally nanometric silica and/or organic clays; (iii) the liquid phase contains antioxidant components and other to improve their tribological properties.
These thickening additives which are described in said patents, of the silica or organic clay type, may worsen the tribological behaviour of the product when incorporated in hydraulic dampers for automotion. The silica or silicate particles are very abrasive and, due to this, are contraindicated for this type of dampers, although not so for friction dampers where the lubricant is embedded in a porous tissue or sponge.

EXPLANATION OF THE INVENTION

The authors of the present invention provide a magnetorheological fluid, very stable over time and which does not lose its rheological properties, wherein it is possible to regulate its viscosity and its viscoelastic characteristics by action of an external magnetic field. In this regard, they can be called active lubricants since their function consists of facilitating that a damper responds to external mechanical impulses in proportionate form to the intensity thereof.
The formulation which the authors of the present invention propose for the new MRF is of simple embodiment and does not have a threshold stress at zero magnetic field for which reason it functions as a typical lubricant, or inactive, in the absence of a field. When a magnetic field is applied a threshold stress appears and its viscosity increases in a very wide range depending on the content in magnetic-responsive particles and of the intensity of the magnetic field applied.
The composition of the lubricant comprises (in addition to the magnetic-responsive particles and the base oil) two additives dissolved in liquid phase to avoid the separation of phases, i.e. as dispersant and stabilizing additives. Other additives may be added, such as antioxidants, anticorrosives and friction improvers and which provide specific features to the product for the application for which it is designed.
The magnetorheological fluid of the present invention does not contain solid additives different from the magnetic-responsive particles and further avoid the processes of aggregation and sedimentation. In consequence, it can be used as lubricant both for the manufacturing of hydraulic dampers and friction dampers; in this last case it would be sufficient to increase the concentration of viscosity modifier additive to raise the product viscosity and achieve that the lubricant remains absorbed in the sponge.
In particular, aluminium stearate is an additive which is absorbed in the iron/oil interface and avoids the aggregation between iron particles induced by the remnant magnetization of said particles. The concentration of this compound is very low with respect to other MRFs existing on the market. The viscosity modifier is a polymer such as a styrene-fumarate copolymer, or mixture of polymers such as a styrene-fumarate copolymer of polyester type, soluble in the base oil which regulates the viscosity of the resulting product. Its main function consists of avoiding the sedimentation of the iron particles, due to its thickening and dispersant effect. Although aluminium stearate is an additive which is present in other formulations, the addition of a soluble compound such as the viscosity modifier of the present invention is absolutely original and has demonstrated great efficacy in improving the stability of a magnetorheological fluid. The two additives have a low cost.
In accordance with another aspect of the present invention, a magnetorheological fluid (MRF) is provided which comprises magnetic particles dispersed in a liquid phase composed of base oil, a viscosity modifier and a stabilizing agent.
"Stabilizing agent" in the present invention is understood as an additive which prevents the aggregation between magnetic particles due to absorption thereon. And "viscosity modifier" is understood in the present invention to mean a polymer soluble in base oil which decreases the sedimentation rate of the solid particles.
In a preferred embodiment, the stabilizing agent is aluminium stearate and the viscosity modifier is a styrene and fumarate copolymer, and more preferably a styrene and dialkyl-fumarate copolymer.
In an even more preferred embodiment, the viscosity modifier is a mixture of two polymers, polymer of polyester type, such as 2-alcoxy-dialkyl-succinate and a styrene and dialkyl-fumarate copolymer (viscosity modifier which hereinafter we will call "PMV"), both additives are absorbed in the magnetic particle/oil interface avoiding the aggregation between magnetic particles, and act improving the wetting of the metallic particles by the base oil.
The viscosity modifier "PMV" is characterized in that it is a mixture of two polymers, the first a 2-alcoxy-dialkyl-succinate polymer with an average molecular weight between 1000 and 5000 g/mol and the second a styrene and dialkyl-fumarate copolymer with an average molecular weight between 80,000 and 130,000 g/mol. In the present invention, 2-alcoxy-dialkyl-succinate polymer is understood to mean a polymer formed by a single monomer wherein both the alcoxy radical and the two alkylic radicals may contain between eight and twelve carbon atoms. Styrene and dialkyl-fumarate copolymer is understood to mean a block copolymer which contains two monomers which are styrene and dialkyl-fumarate, in the latter, the alkyl radical contain between eight and twelve carbon atoms. In the present invention, "magnetic particles" are understood to mean any ferromagnetic or ferrimagnetic particle for example, but not limited to iron, cobalt, nickel or alloys of the previous metals or magnetic ferrites such as, for example, magnetite, cobalt ferrite, nickel ferrite, barium ferrite. In a preferred embodiment of the present invention, the magnetic particles are iron.
The lubricants proposed are stable magnetorheological suspensions with magnetic particle contents between 10 % and 50 % of the total volume of the MRF. It has been verified that there does not exist significant sedimentation in periods of up to 20 days, see examples.
The magnetorheological lubricant proposed -as occurs with other commercial products- tends to form a sediment, composed of the most voluminous particles present in the suspension, after long periods of storage. However, said sediment is very incompact, which facilitates the redispersion and homogenization of the product quickly and without opposing high mechanical resistance. Without a doubt, the absence of additives constituted by solid colloidal particles and the efficacy as dispersants/stabilizing agents of the soluble compounds added considerably contribute to the easy redispersion of the product:
The magnetorheological behaviour of the product is characterized in that: (i) in the absence of a magnetic field it behaves as a practically Newtonian liquid with viscosity and threshold stress much less than those of other magnetorheological lubricants existing on the market; (ii) in the presence of a weak magnetic field (magnetic induction of the outer field less than 21.5 mT) threshold stresses (plastic behaviour) and viscosities are obtained lower than others on the market; (iii) low raised magnetic fields reach threshold stresses and viscosities greater than other MRFs on the market.
Therefore, the MRF of the present invention is a product which permits, by action of a magnetic field, a variation in its rheological properties which is wider than in other magnetorheological lubricants. This permits modulating a more versatile and proportional response to the action of external mechanical stresses than those that other MRFs marketed have (see examples).

In a preferred embodiment of the present invention the MRF fluid comprises a liquid phase as described in Table 1.

TABLE 1. Composition of the liquid phase of the magnetorheological fluid.

Compound	Quantity
Base oil
	50%-90% total volume MRF
Stabilizing agent: Aluminium stearate	0.1 -1 g/100mL of base oil
Viscosity modifier ("PMV"): 2-alcoxy-dialkyl-succinate polymer and styrene and dialkyl-fumarate copolymer	1 -10 g/100 mL of base oil

The more preferred concentrations of the aluminium stearate are found between 0.15 and 0.8 g/100 mL of base oil and between 1.5 and 8 g/100 mL of base oil for the PMV viscosity modifier.
The even more preferred concentrations of aluminium stearate are between 0.2 and 0.5 g/100 mL of base oil and between 2 and 5 g/100 mL of base oil for the PMV.
In the present invention, base oil is understood to mean a mineral oil or a silicon oil between 10 and 1000 mPa·s. In a preferred embodiment anticorrosive and antioxidant additives are added to the base oil such as, but not limited to, additives of aminic or phenolic nature, or their mixtures in a concentration range between 0.25 and 5 g/100 mL of base oil, but preferably between 0.35 and 3 g/100 mL of base oil, and more preferably between 0.5 and 1 g/100 mL of base oil. The viscosity of the base oil is preferably between 20 and 100 mPa·s for its subsequent use in hydraulic dampers and between 100 and 1000 mPa·s for its subsequent use in friction dampers.
The magnetic particles have a diameter which vary between 0.1 µm and 3 µm, and are in a proportion of between 10 % and 50 % of the total volume of the MRF, and more preferably between 10 % and 30 % of the total volume.
As described above, in addition to the stabilizing agent and the viscosity modifier, other additives may also be added to the magnetorheological fluid, such as antioxidants, anticorrosives and friction improvers and which provide the product with specific features for the application it is designed for.
Another aspect of the present invention provides a method of production of the magnetorheological fluid already described in the present invention which comprises the following steps:

a. Mixing base oil with the stabilizing agent by stirring;
b. Adding the viscosity modifier by stirring; and
c. Adding the magnetic particles gradually whilst keeping stirring.

A third aspect of the present invention provides the use of the magnetorheological fluid as lubricant for the manufacturing of dampers, preferably in the manufacturing of hydraulic dampers or friction dampers.
Throughout the claims and the description of the present invention, the word "comprises" and the variations thereof, do not aim to exclude other components or steps. The examples and the figures are provided by way of illustration and do not have the purpose of limiting the present invention.

DESCRIPTION OF THE FIGURES

Fig. 1.- Represents the relative frequency of the sensor oscillating circuit in accordance with time ("d" are days and "h" hours) of different samples of MRF. Where (a) 0.2 % w/v of AISt (aluminium stearate) + 2 % w/v of PMV + 32 % v/v of Fe; b) 0.2 % w/v AISt + 2 % w/v of PMV + 25 % v/v of Fe; (c) 0.4 % w/v AISt + 1 % w/v of polyalkenyl succinamide polyol polymer + 2 % w/v of PMV + 25 % v/v of Fe.
Fig. 2.- Represents the relative frequency of the sensor oscillating circuit in accordance with time of different samples of MRF. Where (1) REPSOLYPF-UGR32%, coil up; (2) REPSOLYPF-UGR32%, coil down; (3) FMRCom32%, coil up; (4) FMRCom32%, coil down.
Fig. 3.- Represents the force of penetration in sediment in accordance with time after 30 days at rest of the REPSOLYPF-UGR32% (continuous trace) and FMRCom32% (discontinuous trace) fluids. Fig. 3A.-Shows three cycles of penetration-extraction; Fig. 3B.- detail of the first cycle.
Fig. 4 Represents the force of penetration in sediment in accordance with time after 30 days at rest of the REPSOLYPF-UGR32% (continuous trace) and FMRCom32% (discontinuous trace) fluids. It includes the first cycle of penetration in samples: Fig. 4A.- recently prepared; Fig. 4B.- after 15 days at rest; Fig. 4C.- after 30 days at rest
Fig. 5.- Represents the shear stress compared with deformation velocity for the rising values ("c") of magnetic induction (B) of the applied external field indicated Fig. 5A.- For the REPSOLYPF-UGR32 % fluid; Fig. 5B.- For the FMRCom32% fluid.
Fig. 6.- Represents the shear stress (at a deformation speed of 50 s^-1) in accordance with the magnetic induction (B) for the samples Fig. 6A.-REPSOLYPF-UGR32% and Fig. 6B.- FMRCom32%, for an applied field first rising (c) and then decreasing (d).
Fig. 7.- Represents the shear stress in accordance with deformation speed for external fields of low magnetic induction (B between 0 and 21.5 mT). Fig. 7A. For the REPSOLYPF-UGR32% fluid; Fig. 7B.- For the FMRCom32% fluid.
Fig. 8.- Represents the shear stress (at a deformation speed of 50 s^-1) in accordance with the magnetic induction (B) for the samples Fig. 8A.-REPSOLYPF-UGR32% and Fig. 8B.- FMRCom32%; for low intensity magnetic fields.
Fig. 9.- Represents the force in accordance with displacement (D) in compression (Comp.) and in traction (Trac.) in a damper friction test for increasing values ("c") of current intensity (I). Fig. 9A.- For the fluid REPSOLYPF-UGR32 %; Fig. 9B.- For the FMRCom32% fluid.
Fig. 10.- Represents the force (in position x = 0) in compression and traction in accordance with current intensity in a friction damper test for: (a) Force in compression-REPSOLYPF-UGR 25%; (b) Force in traction-REPSOLYPF-UGR 25%; (c) Force in compression-REPSOLYPF-UGR 32%; (d) Force in traction-REPSOLYPF-UGR 32%; (e) Force in compression-FMRCom32%; (f) Force in traction- FMRCom32%.
Fig. 11.- Represents the force in accordance with displacement in absence of magnetic field applied in a characterization test in damper in stages (et.) 2 to 6 for the samples: Fig. 11A.- REPSOLYPF-UGR32%, Fig. 11 B.- FMRCom32%.
Fig. 12.- Represents the force in accordance with displacement in a characterization test for rising current intensities in stage 2 for samples: Fig 12A.- REPSOLYPF-UGR32%, Fig 12B.- FMRCom32%.
Fig. 13.- Represents the force in accordance with displacement for rising current intensities in stage 3 of a characterization test for samples: Fig 12A.- REPSOLYPF-UGR32%, Fig 12B.- FMRCom32%.
Fig. 14.- Represents the force in accordance with the speed for rising current intensities in stage 2 of a characterization test for samples of REPSOLYPF-UGR32 %.
Fig. 15.- Represents the maximum force in accordance with the maximum speed obtained for oscillations with different frequency, in stages 2 to 7 of a characterization test, for the current intensities indicated in a REPSOLYPF-UGR32 % fluid.
Fig. 16.- Represents the maximum force in accordance with the current intensity in stage 7 of a characterization test for the MRF: (a) Force in compression-REPSOLYPF-UGR 25%; (b) Force in traction-REPSOLYPF-UGR 25%; (c) Force in compression-REPSOLYPF-UGR 32%; (d) Force in traction-REPSOLYPF-UGR 32%; (e) Force in compression-FMRCom32%; (f) Force in traction- FMRCom32%.
Fig. 17.- Represents a hysteresis damper test with the REPSOLYPF-UGR32% fluid without applying magnetic field. Fig. 17A.- in stages 2 to 5, with a frequency of 1.5 Hz; Fig. 17B.- stages 6 to 10, with a frequency of 12 Hz.
Fig. 18.- Represents a hysteresis test with the FMRCom32% fluid without applying magnetic field. Fig. 18A.- in stages 2 to 5, with a frequency of 1.5 Hz; Fig. 18B.- stages 6 a 10, with a frequency of 12 Hz.
Fig. 19.- Represents the force in accordance with the speed for current intensities: 0, 0.5, 1, 2, 3, 4, 5 and 6 A, respectively, in stage 2 of a hysteresis test and for: Fig. 19A.- the REPSOLYPF-UGR32 % fluid; Fig 19B.- the FMRCom32% fluid.
Fig. 20.- Represents the force in accordance with the speed for current intensities: 0, 0.5, 1, 2, 3, 4, 5 and 6 A, respectively, in stage 6 of a hysteresis test and for: Fig. 20A.- the REPSOLYPF-UGR32 % fluid; Fig 20B.- the FMRCom32% fluid.
Fig. 21.- Represents the maximum force in accordance with the maximum speed in each stage of a hysteresis test, stages 2 to 5, for the current intensities indicated, for the REPSOLYPF-UGR32 % fluid.
Fig. 22.- Represents the maximum force in accordance with the maximum speed in each stage of a hysteresis test for the current intensities indicated in stages 6 to 10, respectively, for the REPSOLYPF-UGR32% fluid.
Fig. 23.- Represents the maximum force in accordance with the current intensity in stages 2 and 10 of a hysteresis test, for the REPSOLYPF-UGR and FMRCom32% fluids: (a) Force in compression-REPSOLYPF-UGR 32%, 1.5 Hz; (b) Force in traction-REPSOLYPF-UGR 32%, 1.5 Hz; (c) Force in compression-FMRCom32%, 1.5 Hz; (d) Force in traction- FMRCom32%, 1.5 Hz; (e) Force in compression-REPSOLYPF-UGR 32%, 12 Hz; (f) Force in traction- REPSOLYPF-UGR 32%, 12 Hz; (g) Force in compression-FMRCom32%, 12 Hz; (h) Force in traction-FMRCom32%, 12 Hz.

EXAMPLES

Below, the invention will be illustrated with examples of tests performed by the inventors which reveals the effectiveness of the new magnetorheological fluid (MRF) of the present invention.

EXAMPLE 1.-

Two FMR fluids were formulated with the following compositions:

REPSOLYPF-UGR32% Formulation. Iron concentration: 32% in fraction of total volume of the MRF; aluminium stearate concentration (AISt): 0.2 g in 100 mL of base oil; concentration of PMV viscosity modifier: 2 g in 100 mL of base oil. The base oil is a mineral oil from REPSOLYPF-YPF with viscosity 23.5 mPa·s.
The REPSOLYPF-UGR25% formulation. It contains 25% iron in fraction of the total volume of the MRF; the same concentration of AlSt as the previous and a PMV viscosity modifier concentration of 4 g in 100 mL of base oil. The base oil is a mineral oil from REPSOLYPF with viscosity 23.5 mPa·s.

The solid phase contains micrometric iron particles with a very wide distribution of sizes, using colloidal iron particles (produced from carbonyl iron precursors) in a concentration which varies between 10 % and 50 % in fraction of iron volume ϕ (the concentration in grams of iron for each 100 mL of suspension is obtained by multiplying ϕ by the iron density: 7.5 g/cm³). These iron particles have the following properties, described in Table 2. TABLE 2. Chemical composition, distribution of size and density of iron particles.

Element Concentration (% weight)

Iron Min. 97.5

Carbon 0.7 - 1.0

Nitrogen 0.7 - 1.0

Oxygen 0.3 - 0.5

Distribution of particle size

Percentile 10 (10% less than) <1.0 µm

Percentile 50 (50% less than) <2.0 µm

Percentile 90 (90% less than) <3.0 µm

Density

"Tap Density" 3 3.0 - 3.5 g/cm

"True Density" 3 min. 7.5 g/cm
A simple and systematic protocol was defined for preparation of the suspensions which optimize the wetting of the iron particles, which contributes to the homogeneity of the resulting MRF (REPSOLYPF-UGR32% and REPSOLYPF-UGR25%) and, in consequence, to their stability and better MR response.
We started with a base oil with a viscosity of 23 mPa·s to which the anticorrosive and antioxidant additives were added, which improved the thermal and oxidative stability thereof, these additives were a mixture of aminic and phenolic nature.
Then the base oil and aluminium stearate were mixed during a minimum time of 10 minutes by a mechanical stirrer at 200 rpm.
Then the PMV viscosity modifier was added whilst stirring for a minimum time of 10 minutes in the same conditions.
And, finally, the iron particles were added gradually whilst stirring for a minimum period of time of 10 minutes in the aforementioned conditions.
The resulting composition of this procedure was a fluid with a liquid phase, as described in Table 1, contains two specific additives for this application dissolved in the base oil: aluminium stearate and a viscosity modifier (PMV).
To compare the properties of the two formulations, one of the products marketed from the aforementioned patents ( USA No. 6,547,986 B1, April 2003) was used; this commercial product contains a concentration of 32 % iron in fraction of the total volume of the fluid and, hereinafter, we will refer to it as FMRCom32%.

EXAMPLE 2.- SEDIMENTATION.

A specific apparatus was designed to measure sedimentation in MRF concentrates (opaque) based on electromagnetic induction phenomenon (equipment claims in patent application P2006601189). The method essentially consists of measuring the resonance frequency in accordance with the sedimentation time in a parallel circuit formed by a coil and a condenser. The coil surrounds the test tube in which the following samples of the MRF are found (Figure 1):

(a) 0.2 % w/v of AlSt (aluminium stearate) + 2 % w/v of PMV + 32 % v/v of Fe ("REPSOLYPF-UGR32%");
(b) 0.2 % w/v AlSt + 2 % w/v of PMV + 25 % v/v of Fe;
(c) 0.4 % w/v AlSt + 1 % w/v of polyalkenyl succinamide polyol polymer (dispersant different from the PMV polymer which we will call "PD") + 2 % w/v of PMV + 25 % v/v of Fe.

A relative frequency f_r was calculated as the initial frequency (homogenous suspension) and the frequency at any subsequent time. The relative frequency exclusively depends on the fraction of volume of magnetic particles (ϕ) in the area of the suspension surrounded by the sensor coil, so that f_r decreases when ϕ decreases and vice-versa.
A stable MRF was achieved during a time no less than 20 days with iron contents in fraction of volume of 25 % and 32 % -see Figures 1 and 2. Figure 1 illustrates an example of this behaviour. Figure 2 compares the sedimentation tests of the REPSOLYPF-UGR32% and FMRCom32% samples. In this case, the results are shown of the measurements taken simultaneously with two sensor coils positioned in the upper and lower part of the test tube which contains the suspensions. As could be observed, for the sample of REPSOLYPF-UGR32% no significant differences were observed between the measurements taken with both coils. In contrast, in the case of the FMRCom32% sample, a significant increase was observed in the values of f_r taken in the lower coil, which indicated that the iron concentration at the bottom of the suspension grows quicker in this sample than in our product. Therefore, we can conclude that in the REPSOLYPF-UGR32% fluid, homogeneity was maintained (negligible sedimentation) during at least 20 days, whilst in FMRCom32% not such an optimum result was obtained.
The effect of the addition of other dispersants of polymeric type was tested, as seen in Figure 1, as is the case of the polyalkenyl succinamide polyol polymer ("PD"), in order to further improve the stability of the suspensions. It was verified that in a suspension that contains a fraction of iron volume of more than 20 % the addition of those dispersants caused a quicker sedimentation in the suspensions, i.e. a negative synergic effect on dissolving them in the base oil together with the other compounds mentioned in Table 1.

EXAMPLE 3.- REDISPERSION

In the bibliography on the technological applications of the MRFs, it is frequently mentioned that one of the main limiting factors of these fluids for their use as lubricants lies, even more than in avoiding their sedimentation, in the difficulty of redispersion thereof after a long time at rest. This basically comes from two phenomena: (i) the existence of aggregates between magnetic particles which coagulate due to action of the remnant magnetization thereof; and/or (ii) after a long time of use, in those MRFs which contain colloidal particles which form thickening gels (silica, organic clays) to prevent sedimentation, increasingly compact sediments usually appear due to wear of the non-magnetic solid material.
Nevertheless, as far as we know, there do not exist quantitative studies on the rigidity of the sediments formed in MRFs. Thus, we have studied the rigidity of the sediment after different times at rest of the lubricants.
This example consisted of performing penetration tests. For this, the force necessary to penetrate the sediment with a standard needle in accordance with time was measured. The needle used was composed of a cylindrical rod to which a conical point was coupled. The rod had a length of 58 mm and diameter of 3.0 mm; the conical point was 25.4 mm long and 4.00 mm in diameter. The weight of the rod-point assembly was 2.50 g. The rod was combined vertically and underneath to a Mettler AE163 analytical balance (sensitivity ± 0.1 mg). The suspensions to test were introduced in test tubes (height 40 mm; diameter 9 mm) coupled to an electric motor which permitted raising them vertically at a speed of 10 mm/s. Before starting each test, the balance was set to zero and the test tube was then raised so that the needle penetrated the suspension. The force necessary to penetrate the suspension in accordance with time was recorded. In each test three consecutive cycles of penetration/extraction of the needle were performed.
Figure 3(a) represents the force of penetration in accordance with the time in three consecutive cycles of penetration/extraction of the needle for the REPSOLYPF-UGR32% and FMRCom32% samples which were 30 days at rest. As was observed, the force of penetration was always greater in the FMRCom32% than in REPSOLYPF-UGR32% with equal iron concentration.
Nevertheless, it is convenient to analyse in detail the graphic for a more exact analysis. For this, Figure 3(b) shows a detail of the first cycle of penetration which appears in Figure 3(a). In the boxed area to the left of the graphic we can observe that, whilst in the FMRCom32% fluid a sharp and monotonous increase is observed in the force-time ratio, in the REPSOLYPF-UGR32% fluid there is first a slow increase (less force-time gradient) followed by another stretch in which the gradient is greater. This change of gradient was due to the penetration of the supernatant liquid followed by the progressive penetration in the sediment. In the square of the right of Fig. 3(b) (negative force values) another relevant difference is shown between the two fluids. The peak with negative forces in the needle extraction phase indicates that this drags fluid adhered to its walls. If we take into account that the sediment in the commercial fluid is more compact (see Fig. 3(a) than in penetration much greater force peaks are reached than for the REPSOLYPF-UGR32% fluid), it is logical that on removing the needle it drags a greater mass of sediment.
In short, Figure 3 shows that, although in the REPSOLYPF-UGR32% fluid there is an upper area of phase separation, the sediment is less compact than in the FMRCom32% commercial fluid. In consequence, it will be easier to redisperse obtaining a more homogenous fluid in the first cycle of operation in the damper, for which a more regular response is to be expected in the oscillating operation of the damper.
Figure 4 compares the results obtained in the first cycle of penetration in the same homogenized samples (Fig. 4a), after 15 days (Fig. 4b) and 30 days (Fig.4c) at rest in the test tube. The details of this figure clearly show that the sediment of the REPSOLYPF-UGR32% fluid was significantly less compact (see that the peak in the force-time graphic is produced at much lower force values) than those formed in the commercial fluid.

EXAMPLE 4.- MAGNETORHEOLOGICAL PROPERTIES.

Example 4a.- Intense magnetic fields.

In Figure 5 we show the rheograms (shear stress in accordance with deformation speed) of the two fluids which contain 32 % iron (REPSOLYPF-UGR32% and FMRCom32%) under the action of rising intensity magnetic fields. These rheograms were obtained in a magnetorreometer (model MCR300 from Physica-Anton Paar, Germany) using parallel plate geometry.
It was observed how the necessary stress to make the sample flow (threshold stress) increased by several orders of magnitude on applying external magnetic fields with magnetic induction of up to B = 431 mT. We understand threshold stress to be the shear stress necessary to produce a deformation speed, dy/dt, observable in the time scale of the industrial application of the fluid.
Taking as reference the value of the shear stress necessary for each a deformation speed of 50 s^-1, Figure 6 represents the value of said stress obtained on running a complete cycle of first rising and then decreasing magnetic field. The cycles were obtained which are shown in said Figure 6 wherein the magnetorheological results obtained for the REPSOLYPF-UGR32% fluid were compared with those of the FMRCom32% fluid. As can be observed, higher shear stress values are achieved in the REPSOLYPF-UGR32% lubricant (σ_maximum = 46300 Pa, for B = 432 mT), than in the commercial (σ_maximum = 35900 Pa). The width in the hysteresis cycle was similar in REPSOLYPF-UGR32% [Δσ (for B = 215 mT) = 3300 Pa] and in FMRCom32% [Δσ (for B = 215 mT) = 3500 Pa]. The hysteresis cycle is relatively narrow and similar in both products.
The important increase in thresholds stress was observed which was achieved on increasing the magnetic field. The fluid of the present invention behaved as a practically ideal MRF: in the absence of a field is a Newtonian fluid (negligible threshold stress) and as the magnetic field increases it becomes a plastic fluid with threshold stress that can reach very high values.

Example 4b.- Low intensity magnetic fields.

Figures 7 and 8 represent the results obtained for low intensity magnetic fields. The MRF REPSOLYPF-UGR32% developed, for magnetic fields with B < 21.5 mT, lower threshold stresses (Figures 7a and 8a) than of the commercial fluid (Figures 7b and 8b). In contrast, for high field values (Figure 6) higher threshold stress values were obtained. Therefore, a REPSOLYPF-UGR32% lubricant with a wider magnetorheological response range, from practically Newtonian lubricant, for a zero field, until non-Newtonian lubricant of plastic type with high threshold stress and viscosity, low intense magnetic field, than the commercial product we have used for comparison has.

EXAMPLE 5.- TRIBOLOGICAL PROPERTIES: DAMPER TESTS.

Tests were performed on MRFs formulated by REPSOLYPF-UGR, and in some cases they were compared with the FMRCom32% fluid, in a magnetorheological damper for automobiles of pressurized monotube, model "MagneRide" manufactured by Delphi (USA). The measurement temperature was in all cases 40 °C. For said purpose, all damper tests include a prior stage of homogenization and heating. The measurements were carried out in a MTS 835 device with MTS 505.11 pressure unit (MTS Systems Corp., USA). The tests performed are described below.

Example 5a.- Friction test.

This test was carried out at very low speed, with the object of measuring internal stresses other than those purely hydraulic ones. This would be the case of a friction test in a conventional damper. The damper was secured with double ball joints to avoid stresses due to misalignments. In this case no type of lateral load was applied. A constant speed of 0.4 mm/s and amplitude of 20 mm was maintained, so that the oscillation period is 100 s. Figure 9 shows the force results in accordance with displacement in compression and traction, in the absence of a magnetic field (current intensity in coil of the damper I = 0) and for rising magnetic fields (current intensity in the coil of the damper between 0.5 and 6 Amps: 0.5, 1, 2, 3, 4, 5 and 6A). The most outstanding is that, even for high current intensities, the damper with the REPSOLYPF-UGR32% fluid showed a regular behaviour without notable oscillations in the force-displacement curves (no peaks were observed in the curves) than with the commercial lubricant FMRCom32%. From the details of graphics such as those of Figure 9, force values were extracted on passing through the zero displacement position for the different current intensities which are shown in Figure 10, both for the FMRCom32% and for REPSOLYPF-UGR25% and REPSOLYPF-UGR32%. It could be observed that the response of the damper with the two lubricants REPSOLYPF-UGR was more intense for current intensities less than 2 A, whilst the opposite occurred for intensities greater than 3 A.
These tests showed the tendency for particle aggregation in the commercial fluid FMRCom32%. The dynamic tests showed how these structures break on applying a certain shear, giving a response in erratic force. The formation of structures which are aligned in the direction of the magnetic field and were quickly destroyed on applying small deformations, in they case of the commercial fluid, gave as result the irregular curves of graphics 9b. In the case of the fluid of the present patent REPSOLYPF-UGR, both 25 % and 32% of iron, provided a monotonously rising response and without irregularities in the entire operational range (of 0 a 6A of current intensity and 12V voltage).

Example 5b: Characterization test

In this test, after the heating stage (stage 1), sinusoidal excitation was applied with rising frequencies: 0.3678 Hz (stage 2), 0.9266Hz (stage 3), 1.853Hz (stage 4), 2.779Hz (stage 5), 3.7 Hz (stage 6) and 7.413 Hz (stage 7), with a constant amplitude of 45 mm, and speeds of 52 mm/s (stage 2), 131 mm/s (stage 3), 262 mm/s (stage 4), 393 mm/s (stage 5), 524 mm/s (stage 6) and 1048 mm/s (stage 7), respectively.
Figures 11 represent the dependency between force and displacement in stages 2 to 6 for the REPSOLYPF-UGR32% (Figure 11 a) and FMRCom32% fluids (Figure 11 b) in the absence of magnetic field. The existence of irregularities were observed in the commercial fluid FMRCom32% for the stages performed at greater speed. On the other hand, the force values were higher than for the REPSOLYPF-UGR32% fluid.
Figures 12 represents the curves in accordance with the displacement obtained in stage 2 for rising current intensities (of 0, 0.5, 1, 2, 3, 4, 5 and 6A) for the same two fluids, REPSOLYPF-UGR32% (Figure 12a) and FMRCom32% (Figure 12b). Two facts are notable: (i) higher force values were reached at a same given current intensity in the REPSOLYPF-UGR32R% fluid than in the commercial FMRCom32%; and (ii) no irregular responses were appreciated not even for high current intensities in the REPSOLYPF-UGR32% lubricant, whilst the response was irregular in the commercial fluid FMRCom32% (see the high current intensity curves in Figure 12b).
Figures 13 show the results obtained in stage 3 of this test, at a greater frequency, 0.9266Hz and speed 131 mm/s) from which similar conclusions could be drawn to those of Figure 12: (i) higher force values were reached at a same given current intensity in the REPSOLYPF-UGR32R% fluid than in the commercial FMRCom32%; and (ii) no irregular responses were appreciated not even for high current intensities in thREPSOLYPF-UGR32% lubricant, whilst the response was irregular in the commercial fluid FMRCom32%.
Figure 14 shows the results obtained in stage 2 of this force characterization test of the force in accordance with the speed for rising current intensities, of 0, 0.5, 1, 2, 3, 4, 5 and 6A, in a sample of REPSOLYPF-UGR32% lubricant, wherein the absence of irregularities in the damper response were again not observed. From graphics such as this, the data represented in Figure 15 could be obtained, wherein the ratio between maximum force and maximum speed for stages 2 to 7 for the different current intensities of 0, 0.5, 1, 2, 3, 4, 5 and 6A, for the lubricant REPSOLYPF-UGR32% were observed. From graphics such as Figure 15 the graphic of Figure 16 could be obtained, which represents the maximum force in accordance with the current intensity for oscillations of 7.413 Hz (stage 7). In this Figure 16 it is evident that the response of the lubricant is more intense (higher force values) in the REPSOLYPF-UGR lubricants (both with 25% and 32% of iron) than the commercial lubricant FMRCom32% for sinusoidal oscillations of up to 7.4 Hz.

Example 5c.- Hysteresis test.

In this test, after the heating stage 1, excitation was applied in displacement with constant frequency (1.5 Hz in stages 2 to 5 and 12 Hz in stages 6-10). For both stages, the amplitude of the stroke was increased in each one of the stages, for the frequency of 1.5 Hz being: 10.61 mm (stage 2), 21.22 mm (stage 3), 42.22 mm (stage 4), 63.66 mm (stage 5), until obtaining the following maximum speeds: 50 mm/s (stage 2), 100 mm/s (stage 3), 200 mm/s (stages 4), 300 mm/s (stage 5). For the frequency of 12 Hz, the maximum speeds and the amplitudes were respectively: 100 mm/s and 2,653 mm (stage 6), 200 mm/s and 5,305 mm (stage 7), 350 mm/s and 9,284 mm (stage 8), 550 mm/s and 14,589 mm (stage 9) and 750 mm/s and 19,89 mm(stage 10).
Figures 17 and 18 represent, by way of example, the dependency between force and speed in stages 2 to 5 (1.5 Hz) and 6 to 10 (12 Hz) for the REPSOLYPF-UGR32% and for the FMRCom32% lubricant in the absence of the applied magnetic field. In a similar way, the force-speed curves could be obtained when current was passed through (between 0 and 6A) by the damper coil.
Figures 19 show the results obtained in stage 3 for both fluids, REPSOLYPF-UGR32% and FMRCom32%, and Figures 20 for stage 6. In these figures the difference could be observed between both fluids: absence of peaks in the force-speed curves and greater force range reached in the REPSOLYPF-UGR32% fluids, which permits a more versatile response and provided to external mechanical impulses.
From graphics such as those shown in the previous Figures, Figure 19 and Figure 20, the data obtained in Figures 21 and 22 were obtained for the ratio between maximum force and maximum speed in the tests at 1.5 Hz, in Figure 21, and 12 Hz, in Figure 22, for current intensity in the range of 0 A to 6 A for the REPSOLYPF-UGR32% fluid.
Finally, Figure 23 compared the ratio between force and current intensity for the lubricants which contain 32 % iron (REPSOLYPF-UGR32% and FMRCom32%) in two of the stages carried out: stage 2 (1.5 Hz; v_max = 50 mm/s) and stage 10 (12 Hz; v_max = 1048 mm/s). It was observed that, both at high and low frequencies, the response of the REPSOLYPF-UGR32% lubricant was more intense throughout the wide range of current intensity tested than those of the commercial fluid FMRCom32%.
The wide range of tribological tests carried out in a commercial magnetorheological damper for automobiles allows concluding that the MRF described in the present invention shows a regular response, without peaks in force-displacement or force-speed cycles, under the action of external forces in a wide range of excitation amplitudes and frequencies. Subjected to sinusoidal oscillations it has a more intense response, both at low frequencies (approx. 1 Hz) and high frequencies (approx. 12 Hz), than others on the market. This entails that an intensity response can be achieved comparable to others on the market with significantly lower concentrations of iron, which reduces problems of oxidation and degradation of the lubricant.

Claims

Magnetorheological fluid (MRF) where the magnetic particles are dispersed in a liquid phase which comprises: a base oil, a viscosity modifier and a stabilizing agent.
Fluid according to claim 1, where the viscosity modifier is a styrene and fumarate copolymer .
Fluid according to claim 2, where the fumarate is dialkyl-fumarate.
Fluid according to either of claims 2 or 3, where the viscosity modifier further comprises a polyester polymer.
Fluid according to claim 4, where the polyester polymer is 2-alcoxy-dialkyl-succinate.
Fluid according to either of claim 1 to 5, where the viscosity modifier is in a proportion of between 1 % and 10 % (w/v) of base oil.
Fluid according to claim 6, where the viscosity modifier is in a proportion of between 2% and 5% (w/v) of base oil.
Fluid according to claim 1, where the stabilizing agent is aluminium stearate.
Fluid according to either of claims 1 or 8, where the stabilizing agent is in a proportion of between 0.1 % and 1 % (w/v) of base oil.
Fluid according to claim 9, where the stabilizing agent is in a proportion of between 0.2% and 0.5% (w/v) of base oil.
Fluid according to any of claims 1 to 10, where the base oil is a mineral oil or a silicon oil.
Fluid according to claim 11, where the base oil is in a proportion of between 50 % and 90 % of the total volume of the MRF.
Fluid according to any of the preceding claims, where anticorrosive and antioxidant agents are added to the base oil.
Fluid according to claim 13, where the additives are of aminic or phenolic nature, or their mixtures.
Fluid according to any of claims 13 or 14, where the additives are in a proportion less than 5 g/100 mL of base oil.
Fluid according to claim 1, where the magnetic particles are iron.
Fluid according to either of claims 1 or 16, where the diameter of the magnetic particles vary between 0.1 µm and 3 µm.
Fluid according to any of claims 1, 16-17, where the magnetic particles are in a proportion of between 10 and 50 % of the total volume of the MRF.
Fluid according to claim 18, where the magnetic particles are in a proportion of between 10 and 30% of total volume of the MRF.
Method of producing the magnetorheological fluid according to any of claims 1 to 19, which comprises the following steps:
a. Mixing oil with the stabilizing agent by stirring;

b. Adding the viscosity modifier by stirring; and

c. Adding the magnetic particles gradually whilst the stirring continues.
Use of the magnetorheological fluid according to any of claims 1 to 19 as lubricant in the manufacturing of dampers.
Use of the magnetorheological fluid according to claim 21, where the dampers are hydraulic or friction.