Abstract:
A magnetorheological (MR) fluid comprising a glycol-based carrier fluid, a magnetizeable particle, and an inorganic clay. The MR fluid is particularly suitable for use in devices that have materials incompatible with typical hydrocarbon-based MR fluids, for example in devices where natural rubber is in contact with the fluids as in automotive engine mounts. The fluid is substantially non-foaming, and an improvement over glycol fluids thickened with organoclays.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to magnetorheological (MR) fluids, and more particularly non-aqueous MR fluids containing an inorganic clay additive. 
       BACKGROUND OF THE INVENTION 
       [0002]    MR fluids based on hydrocarbon or silicone oils are well-known in the literature and numerous patents, and many device applications based on these fluids are also known. Aqueous MR fluids are also known, but there are fewer device applications for this fluid because of its limited temperature stability and its lack of lubricity. Hydrocarbon-based MR fluids have been found to be unsatisfactory in devices that contain natural rubber (e.g., automotive engine mounts) due to an incompatibility between the rubber and the hydrocarbon carrier fluid. Silicone-based fluids are more compatible with the rubber material, but they are generally more expensive and are not as desirable from a user&#39;s viewpoint because of the potential for silicone cross-contamination. 
         [0003]    Glycol-based fluids are compatible with natural rubber and have acceptable temperature stability without the drawbacks associated with silicone fluids. A previous patent for glycol-based MR fluid assigned to Delphi Corporation (U.S. Pat. No. 6,824,700 B2, Glycol-Based MR Fluids with Thickening Agent) uses organoclay as a thickening agent. Such fluids suffer from the drawback that they form a persistent foam when exposed to vacuum, which is a significant problem for vacuum-filling operations typically used by engine mount manufacturers. 
         [0004]    It would therefore be desirable to provide a glycol-based fluid with minimal settling that is non-foaming and satisfactory for use in engine mounts or similar devices. However, dispersion of inorganic clays in glycol fluids cannot be achieved with the standard process used for MR fluid preparation. Typically, a standard process, such as using high speed disperser blade, results in a poorly dispersed system, as evidenced by poor settling performance. It is therefore, also desirable to provide an improved process for incorporating inorganic clays into MR fluids such that the settling behavior is improved. 
         [0005]    It is to these perceived needs that the present invention is directed. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides an MR fluid comprising a magnetically responsive particle, a glycol carrier fluid, and an inorganic clay. 
         [0007]    In a first aspect of the present invention, an MR fluid is provided which is suitable for use in devices that have materials incompatible with typical hydrocarbon-based MR fluids, for example in devices where natural rubber is in contact with the fluids as in automotive engine mounts. The fluid is substantially non-foaming, and an improvement over glycol fluids thickened with organoclays. 
         [0008]    In a further aspect of the present invention, a process of preparing the MR fluid is provided wherein the inorganic clay is pre-dispersed in a water-glycol mixture, prior to the addition of the magnetically responsive particles and bulk of the carrier fluid. 
         [0009]    One feature and advantage of the present invention is a formulation and method that ensure nearly complete dispersion and delamination of the inorganic clay particles. 
         [0010]    As will be realized by those of skill in the art, many different embodiments of the present invention are possible. Additional uses, objects, advantages, and novel features of the invention are set forth in the detailed description that follows and will become more apparent to those skilled in the art upon examination of the following or by practice of the invention. 
         [0011]    Thus, there has been outlined, rather broadly, the more important features of the invention in order that the detailed description that follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, obviously, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. In this respect, before explaining several embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details and construction and to the arrangement of the components set forth in the following description. The invention is capable of other embodiments and of being practiced and carried out in various ways. 
         [0012]    It is also to be understood that the phraseology and terminology herein are for the purposes of description and should not be regarded as limiting in any respect. Those skilled in the art will appreciate the concepts upon which this disclosure is based and that it may readily be utilized as the basis for designating other structures, methods and systems for carrying out the several purposes of this development. It is important that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     DETAILED DESCRIPTION 
       [0013]    The present invention provides a non-aqueous MR fluid comprising a carrier fluid, a magnetically responsive particle, and an inorganic clay. 
         [0014]    In a first embodiment of the present invention, the MR fluid comprises a magnetically responsive particle. Any solid which is known to exhibit magnetorheological activity can be used, specifically including paramagnetic, superparamagnetic and ferromagnetic elements and compounds. Examples of suitable magnetic-responsive particles include iron, iron alloys (such as those including aluminum, silicon, cobalt, nickel, vanadium, molybdenum, chromium, tungsten, manganese and/or copper), iron oxides (including Fe 2  O 3  and Fe 3  O 4 ), iron nitride, iron carbide, carbonyl iron, nickel, cobalt, chromium dioxide, stainless steel and silicon steel. Examples of suitable particles include straight iron powders, reduced iron powders, iron oxide powder/straight iron powder mixtures and iron oxide powder/reduced iron powder mixtures. A preferred magnetic-responsive particulate is carbonyl iron, preferably reduced iron carbonyl. 
         [0015]    The magnetically-responsive particle size should be selected so that it exhibits multi-domain characteristics when subjected to a magnetic field. In one embodiment of the present invention, the average particle diameter sizes for the magnetic-responsive particles generally comprise between 0.1 and 1000 μm, preferably between about 0.1 and 500 μm, and more preferably between about 1.0 and 10 μm, and are preferably present in an amount between about 50 and 90 percent by weight of the total composition. 
         [0016]    In a further embodiment of the present invention, the carrier fluid comprises a glycol-based fluid. In one embodiment of the present invention, the glycol fluid comprises triols and other polyols, for example alkane, alkene, or alkyne diols and polyols. The glycol fluids suitable for use in the present invention are sufficiently hydrophilic so they can form a mixture with the aqueous component to produce a sufficiently liquid medium to act as a good carrier fluid. In an exemplary embodiment of the present invention, the glycol-based fluid comprises a mixture of propylene glycol and ethylene glycol. Due to the greater thickening effect observed for propylene glycol, the glycol-based fluid advantageously includes an ethylene glycol to propylene glycol ratio of about 0:100 to about 100:0. In another example of the present invention, the glycol-based fluid comprises at least about 50 weight percent ethylene glycol, and most preferably 70 weight percent ethylene glycol, based on the total weight of the fluid. 
         [0017]    In another embodiment of the present invention, the carrier fluid additionally comprises a small amount of water. In a first embodiment of the present invention, the water comprises less than 15 percent by volume of the MR fluid. In a preferred embodiment of the present invention, the carrier fluid comprises at least 3 volume percent water based on the total volume of the MR fluid. In a most preferred embodiment of the present invention, the carrier fluid comprises about 6 to about 10 volume percent water based on the total volume of the MR fluid. 
         [0018]    The inorganic clay component of the present invention comprises synthetic or natural clays and includes but is not necessarily limited to bentonite, hectorite, montmorillonite, or similar inorganic clays with a laminar structure and polar surface. 
         [0019]    In one embodiment of the present invention, the clay component comprises an inorganic clay. While many types of inorganic clays are suitable for use in the present invention, bentonite and hectorite clays are preferred. The bentonite or hectorite used in the composition of the invention are hydrophilic mineral clays that are anti-settling agents, thickening agents and rheology modifiers. Naturally occurring bentonites and hectorites include various metal cations which provide the clay with hydrophilic properties. They increase the viscosity and yield stress of the magnetorheological fluid compositions described herein. 
         [0020]    The bentonite or hectorite thickens the fluid composition to slow down particle settling, and provides for a soft sediment once the magnetic particles settle out. The soft sediment provides for ease of re-dispersion. Preferred bentonites or hectorites are thermally, mechanically and chemically stable and have a hardness less than that of conventionally used anti-settling agents such as silica or silicon dioxide. Compositions of the invention described herein preferably shear thin at shear rates less than 100/sec, and recover their structure after shear thinning in less than five minutes. 
         [0021]    Bentonite or hectorite clays are typically found in the form of agglomerated platelet stacks. When sufficient mechanical and/or chemical energy is applied to the stacks, the stacks can be delaminated. The delamination occurs more rapidly as the temperature of the fluid containing the clay is increased. The clays tend to be thixotropic and shear thinning, i.e., they form networks which are easily destroyed by the application of shear, and which reform when the shear is removed. The individual clay platelets have physical and mechanical properties that make them ideally suited for use in the magnetorheological fluid compositions described herein. For example, they are extremely flexible and at the same time are extremely strong. 
         [0022]    In a preferred embodiment of the present invention, the inorganic clay comprises a member of the Laponite group of synthetic hectorites produced by Southern Clay Products, Gonzales, Tex. Laponites are layered hydrous magnesium silicates that are free from natural clay impurities and can be synthesized under controlled conditions. When added to water with moderate agitation, an optimum dispersion is generally obtained in about 30 minutes. The viscosity of the Laponite suspensions will increase upon addition of the metal particulates. 
         [0023]    Preferably, the inorganic clay is present in a range of between 0.1 and 10 percent by weight of the formulation, more preferably, between 0.1 and 6 percent by weight, and most preferably, between about 0.2 and 2.0 percent by weight. In another embodiment of the present invention, the inorganic clay is used to the exclusion of [i.e. substantially no amount of] organic thickeners such as xanthan gum, carboxymethyl cellulose or other polymeric additives. 
         [0024]    Optional components include carboxylate soaps, dispersants, surfactants, corrosion inhibitors, lubricants, extreme pressure anti-wear additives, antioxidants, thixotropic agents and conventional suspension agents. Carboxylate soaps include ferrous oleate, ferrous naphthenate, ferrous stearate, aluminum di- and tri-stearate, lithium stearate, calcium stearate, zinc stearate and sodium stearate, and surfactants include sulfonates, phosphate esters, stearic acid, glycerol monooleate, sorbitan sesquioleate, laurates, fatty acids, fatty alcohols, fluoroaliphatic polymeric esters, and titanate, aluminate and zirconate coupling agents and other surface active agents. Polyalkylene diols (i.e., polyethylene glycol) and partially esterified polyols can also be included. 
         [0025]    Suitable corrosion inhibitors are described in U.S. Pat. No. 5,670,077 and include sodium nitrite, sodium nitrate, sodium benzoate, borax, ethanolamine phosphate and mixtures thereof. The corrosion inhibitor can be present in an amount between 0.1 to 10 percent by weight of the composition. 
       Method of Manufacture 
       [0026]    In a further aspect of the present invention, a method of manufacturing an MR fluid is provided. The MR fluid is made by first preparing a grease comprising an inorganic clay and water, then incorporating the grease into a carrier fluid. The grease is made by mixing the inorganic clay into glycol and water in a roughly 1:3:3 weight ratio. The incorporation is preferably performed in a mixer rotating at approximately 2500 RPM, then allowing the mixture to stand for several hours during which it thickened to form a stiff grease. The grease is then further processed using a 3-roll mill to achieve as close to full dispersion (delamination) of the inorganic clay as possible. The final MR fluid is then prepared by dispersing about 1 part by weight grease to three parts by weight glycol fluid, then dispersing an appropriate amount of a magnetically responsive particle into the mixture. Any additives, such as surfactants may be added during this process. 
         [0027]    When the composition is prepared, it may be necessary to subject the clays to high shear stress to delaminate the clay platelets. There are several means for providing the high shear stress. Examples include colloid mills and homogenizers. 
         [0028]    In addition to the process described above, for preparing pre-dispersed clay grease can include roller milling, sand milling, ball milling, high-shear rotor-stator milling, colloid milling, homogenizers, or other high-shear processes that result in delamination of the clay particles but do not fragment the delaminated particles. Alternatively, the full fluid can be milled by any similar high-shear process such that the required level of delamination of the clay particles is achieved, as measured by settling performance. 
       EXAMPLE 1 
       [0029]    A grease was prepared by mixing 21 g Laponite® RDS with a mixture containing 45 g deionized water and 45 g Glycol 7030 (Glycol 7030 is a mixture of ethylene glycol and propylene glycol (70:30 weight ratio) with various nonessential additives). This mixture was hand-mixed to give a thick stiff grease. An MR fluid was prepared by mixing 100.18 g of this grease into a dispersion containing 2478 g water-atomized iron, 511.4 g Glycol 7030, and 31 g of a dispersant/surfactant. Settling was assessed with the standard 24-hour and two-week pint can settling tests. 
         [0030]    A currently preferred embodiment of the present invention is exemplified by the formulation given in Table 1. The fluid was prepared by first making a grease consisting of 13 weight percent Laponite RDS clay, 43.5 weight percent deionized water, and 43.5 weight percent Glycol 7030. The grease was made by mixing powdered Laponite RDS into the water and glycol fluid mixture using a rotor-stator at approximately 2500 RPM, then allowing the mixture to stand for several hours during which it thickened to form a stiff grease. This grease was further processed using a 3-roll mill to achieve full dispersion of the Laponite RDS powder. The MR fluid was made by dispersing 981.4 g of this grease into an additional 2834.1 g glycol fluid and 20.0 g dispersant/surfactant, then dispersing 15,563 g of water-atomized iron into the mixture. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Formulation of Example 1 
               
             
          
           
               
                   
                   
                 Weight 
               
               
                   
                 Weight (g) 
                 Percent 
               
               
                   
                   
               
             
          
           
               
                   
                 Water Atomized Iron 
                 15562.8 
                 80.24% 
               
               
                   
                 Laponite RDS 
                 127.6 
                  0.66% 
               
               
                   
                 Dispersant/surfactant 
                 20.0 
                  0.10% 
               
               
                   
                 Deionized Water 
                 425.8 
                  2.20% 
               
               
                   
                 Glycol 7030 
                 3260.1 
                 16.80% 
               
               
                   
                 TOTAL 
                 19396.3 
                   100% 
               
               
                   
                   
               
             
          
         
       
     
       EXAMPLE 2 
       [0031]    Table 2 summarizes formulations made using 36 volume percent water-atomized iron with Glycol 7030 as the carrier fluid, with the suspension aids indicated in the “Description” column. The first five entries are organoclays, and all formed persistent foams during the vacuum degassing operation. Additionally, all the organoclays except Garamite 1958 had poor settling properties. Entry 6 involves the one organoclay with favorable settling after addition of an anti-foaming additive (BYK-A555) and shows that addition of antifoamer caused loss of the favorable settling properties. The three inorganic clays had no persistent foam formation. Additionally, the last entry shows that substantial amounts of water enhance the settling properties of the inorganic clay system. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Settling and Foaminess of MR Fluids with Different Clay Types 
               
             
          
           
               
                   
                   
                   
                   
                 2-week 
                   
               
               
                   
                   
                   
                 24-hr CL* 
                 hardness 
                 2-week 
               
               
                 Description 
                 Clay type 
                 Foam during degas 
                 (%) 
                 (N) 
                 CL (%) 
               
               
                   
               
             
          
           
               
                 2 vol % Bentone SD-2 
                 Organoclay 
                 Persisted &gt; 15 minutes 
                 12 
                 35 
                 23 
               
               
                 2 vol % Bentone 27 
                 Organoclay 
                 Persisted &gt; 15 minutes 
                 11 
                 35 
                 23 
               
               
                 2 vol % Baragel 24 
                 Organoclay 
                 Persisted &gt; 15 minutes 
                 9 
                 35 
                 21 
               
               
                 2 vol % Garamite 1958 
                 Organoclay 
                 Persisted &gt; 15 minutes 
                 1 
                 10 
                 25 
               
               
                 2 vol % Garamite 1958 + 1% A555 
                 Organoclay 
                 Did not persist 
                 10 
                 35 
                 23 
               
               
                 2 vol % Laponite RDS 
                 Inorganic clay 
                 Did not persist 
                 6 
                 35 
                 23 
               
               
                 3 vol % Laponite RDS 
                 Inorganic clay 
                 Did not persist 
                 3 
                 22 
                 16 
               
               
                 3 vol % Lap RDS + 5 vol % water 
                 Inorganic clay 
                 Did not persist 
                 4 
                 7 
                 12 
               
               
                   
               
               
                 *24-hr CL is the percentage clear layer that forms after settling. 
               
             
          
         
       
     
       EXAMPLE 3 
       [0032]    Table 3 is supplementary to Table 2 and shows the importance of clay pre-dispersion. All fluids were made with water-atomized iron in Glycol 7030. The first fluid was made by the standard process with no attempt to pre-disperse the clay. The second and third fluids were made by cycling all components except iron through the high-shear mill, then adding iron and using the standard process to finish the fluid. The fourth fluid was made using a grease formed by activation of Laponite RDS in a carrier fluid comprising 50 weight percent water and 50 weight percent Glycol 7030. The final fluid was made using a grease formed by first activating Laponite RDS in a carrier fluid comprising 50 weight percent water and 50 weight percent Glycol 7030, then further dispersing the clay by milling the grease with a three-roll-mill. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 MR Fluids (36 volume percent iron) with Laponite RDS Clay 
               
             
          
           
               
                   
                   
                 2-week 
                   
               
               
                   
                   
                 sediment 
               
               
                   
                 24-hr 
                 hardness 
                 2- week 
               
               
                 Description 
                 CL (%) 
                 (N) 
                 CL (%) 
               
               
                   
               
             
          
           
               
                 2 vol % Laponite RDS, 5 vol % water, 
                 17 
                 &gt;30 
                 26 
               
               
                 Rotor-stator 
               
               
                 2 vol % Laponite RDS, 2.0 vol % water, 
                 5 
                 22 
                 18 
               
               
                 IKA pre-mill 4 passes 
               
               
                 2 vol % Laponite RDS, 7.8 vol % water, 
                 4 
                 9 
                 14 
               
               
                 IKA pre-mill 4 passes 
               
               
                 2 vol % Laponite RDS, 7.8 vol % water, 
                 3 
                 6 
                 19 
               
               
                 Predispersed 
               
               
                 2 vol % Laponite RDS, 7.8 vol % water, 
                 1 
                 5 
                 11 
               
               
                 3-roll milled grease 
               
               
                   
               
             
          
         
       
     
         [0033]    Although the present invention has been described with reference to particular embodiments, it should be recognized that these embodiments are merely illustrative of the principles of the present invention. Those of ordinary skill in the art will appreciate that the compositions, apparatus and methods of the present invention may be constructed and implemented in other ways and embodiments. Accordingly, the description herein should not be read as limiting the present invention, as other embodiments also fall within the scope of the present invention as defined by the appended claims.