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Publication numberUS3047507 A
Publication typeGrant
Publication dateJul 31, 1962
Filing dateApr 4, 1960
Priority dateApr 4, 1960
Publication numberUS 3047507 A, US 3047507A, US-A-3047507, US3047507 A, US3047507A
InventorsWillis M Winslow
Original AssigneeWefco Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Field responsive force transmitting compositions
US 3047507 A
Abstract  available in
Previous page
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Claims  available in
Description  (OCR text may contain errors)

member when separated by narrow fluid-filled space.

the rigid or semi-rigid structure which endures.

United States Patent:

3,047,507 FIELD RESPONSIVE FORCE TRANSMITTING COMPOSITIONS Willis M. Winslow, Wheat Ridge, Colo., asslgnor to Wefco, Inc Denver, (3010., a corporation of Colorado No Drawing. Filed Apr. 4, 1960, Ser. No. 19,465 23 Claims. (Cl. 252-75) This invention relates to compositions which exhibit a pronounced change in viscosity and in resistance to shear in response to an electrical field. In particular, the invention relates to improved compositions of the type disclosed in my Patent No. 2,417,850 granted March 25, 1947.

It was disclosed in the above patent that useful fluid compositions could be made which were responsive to electric fields to such a pronounced degree as to actually tie together, as a unit, a driven member and a drivinIg 11 thus became possible by the use of such compositions not only to provide a useful power drive with various degrees of slip, but, indeed, to reduce the slip to zero, all under control of an applied field and without contact of the solid driving and driven members.

As set forth in said patent, various fluids consisting of certain finely divided non-magnetic solids, such as cornstarch, suitably dispersed as particles in a dielectric mobile vehicle, for example a light weight transformer insulating oil, exhibit an electrically variable and instantly reversible viscosity change when placed between two conductors and subjected to an electric field. Fluids which show this capability are referred to as electro-viscous, and the induced viscosity increase above the normal viscosity of the fluid is called the electro-viscosity. The compositions which demonstrate such viscosity change, for purposes of this application, will sometimes be referred to as electrofluids. It appears that the electro-viscosity varies approximately as the square of the intensity of the electric field. All degrees of viscosity change have been obtained, depending on the field strength, .and with intense fields it has been found possible to obtain an infinite viscosity at which the compositions may be characterized as a solid.

It appears that the electro-viscosity of these suspensions results, at least in part, from the electrical nature of the particles, and that best results are obtained with those particles having a high surface electrical conductivity, relative to the conductivity or dielectric constant of the oil.

In my early studies of this effect it was determined that an electric field polarizes the fluid in such manner that the normally free particles are caused to form webs or chains mechanically linking the electrodes. I later discovered that fluids could be made which fall into two classes, depending upon Whether the stiffened structure remains unbroken upon removal of the electric field. In a selfbreak-ing fluid the forces of kinetic agitation and inherent electrical forces of repulsion at the particle surfaces seem to be sufiicient to disrupt the chains. In the case of a composition which remains substantially unchanged after removing the field the normally present electrical surace charges of the particles may become interlocked to form This latter type, will, of course, break under dynamic shear due to relative movement of the electrodes. With such com-positions the corresponding electro-viscosity is, in general, higher than for a self-breaking mix. It has been found, however, that either type can be prepared from particles of the same base material, and that certain surface active materials which become adsorbed on the particle surfaces influence this character of the mix. In

3,047,507 Patented July 31, 1962 general, the greater the percentage of surface active agent, or the greater the dispersive quality of the agent, the less will be the lock-in effect.

Following the work described in my earlier Patent No. 2,417,850, it has been found that compositions, such as those disclosed in the aforesaid patent, show higher electro-viscosity when the same are partially hydrated, and there is included a suitable surfactant.

The present applicant has found it possible to prepare electro-viscous compositions ranging in viscosity from medium weight lubricating oils to light greases, which contain from about 30 to about 70 percent by weight or more of finely divided, non-magnetic, solid particles. With higher concentration of solids more chains are formed and although it might be expected that the electroviscosity of such compositions would only be proportionately increased as the proportion of solids is raised, that effect has been found to be much greater than proportional when the particle volume exceeds about 38 percent.

Fluids of the foregoing character can be used where it is desired to control the transmission of mechanical force in response to an electrical potential or current. The electroviscous fluids of the present invention have been successfully used as the variable frictional linkage between a driven electrode and a driving electrode to form a voltageresponsive clutch; between a stationary electrode and a driven electrode to form a voltage-responsive valve of a hydraulic system; in an electro-viscosity pump; and in various analogous regulating and control devices. For examples of other applications of the compositions of the present invention reference is made to my aforesaid Patent No. 2,417,850.

By the term fluid as used in the foregoing discussion and in the following specification and claims, is meant a substance which normally exhibits the flow characteristics of a viscous liquid. Such compositions differ basically from heretofore known hydraulic fluids in that their resistance to shear can be increased many fold by application thereto of an intense flux field. While in the preferred embodiment of the invention the compositions are relatively fluid in their normal state, it should be understood that some are thixotropic, i.e., set up in a gel-like structure when at rest but upon application of shearing stress are converted to a more fluid composition.

Objects of the present invention include the provision of compositions based on the foregoing discoveries; the provision of'electro-viscous fluids of high electro-viscosity as well as high ratio of electro-viscosity to normal viscosity; the provision of compositions having stable properties under high electrical stress and over long periods of use; and the provision of fluids having high resistivity and low power factor.

Other objects will become apparent from the following description of the invention.

The novel compositions of the present invention comprise as essential components at least one of each of the following four classes of ingredients: (1) An electrically stable, low dielectric, oleaginous vehicle having a viscosity suited to the intended use of the final product; (2) finely divided, non-conductive solids having an average diameter of from about 0.1 to about 5 microns and possessing the ability to adsorb a significant quantity of a substance such as water or alcohol; (3) an organic surface active dispersing agent; and (4) water or a mixture of water and a water miscible alcohol or other hydroxy compound.

In addition to the foregoing four classes of components, there may be included in such compositions a component capable of imparting viscosity stabilization to these high solids-content fluids. For this purpose certain materials heretofore found useful in solids-free lubricating oils as viscosity index improvers have found utility. Thus, such additives as acrylic and methacrylic derivatives, polyisobutylene, the reaction product of .paraflin wax, and naphthalene have been found especially desirable. These viscosity stabilizing compounds often have the property of increasing viscosity as well, while this may often be acceptable it is sometimes desired to reduce the normal viscosity to a minimum. For this purpose, either with or without added V.I. improvers, it has been found useful to add certain special surface active amines such as l-hydroxyethyl, 2-heptadecenyl imidazoline. Methyl oleate and sodium bis (tridecyl) sulfosuccinates have also shown effectiveness for this purpose. Surface active materials generally contribute to this reduction.

In addition, mention will be made later of certain other types of ingredients which although not essential to the manufacture of improved electro-fluids have been found to be useful additives for certain purposes.

As the vehicle in the instant composition may be employed, any relatively non-viscous oleaginous fluid having substantial stability to an electrical field and a dielectric constant from about 2 to about 5.5. In short, the vehicle may be selected from a very wide variety of classes of liquids and the same will be generally suitable if they are stable, of suitable viscosity and essentially non-conductive, i.e., insulating in their electrical properties. It is preferred to employ a vehicle having a dielectric constant in the range of from about 2 to 3. Indeed, the closer the dielectric constant of the vehicle is to about 2 the better the results.

Naturally, the particular application to which the ultimate composition is to be directed will have a bearing on the liquid vehicle selected as well as on the percentage of solids and the other variables. If, for example, substantial fluidity of the final composition is unnecessary when the same is not under the influence of the electrical field, the same degree of care in selecting a low viscosity vehicle is not required. Indeed, for certain applications, e.g., in chucking a metal workpiece to a bench for grinding, a relatively paste-like composition may be desired.

It is believed that the instant compositions function to transmit force under the influence of an electrical field because each of the individual particles comprise a solid center of non-conductive substance onto which water and other conductive substance has been adsorbed. In the electro-fluid it is believed that there exists around each solid an insulating sheath provided by the low dielectric vehicle and surface active agent. The theory postulated is that charges are free to move about in the surface of each particle as a result of the conductive substance, each particle thereby developing thereon a separation of charge. Conductivity between the particles or between particle and plates is prevented by the insulative effect of the surfactant and vehicle.

As would be expected, the normal viscosity of the mix, i.e., the viscosity in the absence of an applied voltage, is in general greater, the greater is the viscosity of the vehicle. It has been found that the electro-viscosity, i.e., the increment in viscosity when a voltage is applied, is generally not greatly influenced by theviscosity of the vehicle. Because it is desirable to obtain a high ratio of electro-viscosity to normal viscosity to meet the requirements of most practical applications of these compositions a vehicle will preferably be selected having a viscosity not greater than about centipoise, or 100 Saybolt seconds, at C., but it has been found that vehicles exceeding this can be employed and, indeed, may be preferred for certain applications. Also it has been found that vehicles of viscosity less than about 2 centipoise or Saybolt seconds, at 25 C., are in general too volatile for practical uses. Accordingly, the preferred viscosity range is between 2 and 20 centipoise at 25 C.

In addition to the viscosity requirement, the vehicle must be one which is sufficiently stable as to be suitable for electrical insulative uses over long periods of time.

Oleaginous vehicles which may be employed herewith are, for example, petroleum derived hydrocarbon fractions. These may be mineral oils of the lubricating viscosity range or lower, e.g., in the kerosene boiling range. These have normally been refined, for example by extraction, to substantially remove the constituents as sulfur, aromatics, etc., with phenol, furfural, B,B-dichlorodiethylether (Chlorex), liquid S0 nitrobenzene, etc. Syntheic oils resulting from polymerization of unsaturated hydrocarbons or other Oleaginous materials within the lubricating oil viscosity range or somewhat lower, e.g., high molecular weight polyoxyalkylene compounds such as polyalkylene glycols and esters thereof, aliphatic d esters of dicarboxylic acids such as the butyl, hexyl, 2- ethyl decyl, lauryl, etc., esters of sebacic acid, adipic acid, azeleic acid, etc., may be employed. Polyfluoro derivatives of organic compounds, particularly fluorinated hydrocarbons, in the lubricating oil viscosity range have shown excellent promise. Mixtures of these various vehicles may be employed if desired. Thus, for example, a mixture of diester and fluorocarbon; of various silicone fluids; a diester and a hydrocarbon oil; etc., may be employed.

The following electrically stable vehicles meet the foregoing requirements and have been found especially suitable for compounding an electro-viscous fluid: (A) A light weight hydrocarbon petroleum distillate free of alkali, acid, sulfur, unsaturated hydrocarbons, having no kerosene odor, and containing roughly 13 percent napthenes and 87 percent paraflins; this product is sold under the name Deo-Base; (B) as a medium hydrocarbon petroleum distillate prepared for use as a transformer oil and sold under the name WemcoC; (C) Type 500 silicone fluids of which the 5 centistoke grade is representative.

Properties of these vehicles which are pertinent to the present invention are listed in the following table:

density, grams! em.

To obtain intermediate properties, vehicles A and B may be mixed in any desired proportions. Alternatively, the viscosity of vehicle A can be increased by addition of dibutyl sebacate of pure grade. Vehicle B may also be modified by addition of a pure grade of 1,3 di-isopropyl benzene. The silicone fluids may be used in a wide variety of viscosity grades from 2 to 20 centipoise, depending on the requirements.

The following vehicle have been found useful in the preparation of compositions of the type herein described and claimed: alpha methyl benzyl ether; benzene, biphenyl-terphenyl; 1, bis(x-chlorophenyl) ethane; bromocyclohexane; 4, bromodiphenyl; chlorinated paraffin; di- (2 ethylhexyl) adipate; di(2 ethylhexyl)rnaleate; dibenzyl ether; dibutyl carbitol eidthylene glycol dibutyl ether; dichloroethyl ether; di-Z-ethylhexyl phthalate; 1,1-diphenylethane; tripropylene glycol methyl ether; butyl cyclohexyl phthalate; di-Z-ethylhexyl azelate; dipropylene glycol diperlargonate; di-iso-octyl azelate; tetraethylene glycol, dimethyl ether; di(2-ethylhexyl) hexahydrophthalate; tri- (Z-ethylhexyl) phosphate; triethylene glycol di(2-ethylbutyrate); trifluorovinyl polymer; heptadecanol; monomeric pentaerythritol ester; hexa(2-ethylbutoxy) disiloxane; thiodiethylene glycol; alpha methyl naphthalene; monoisopropyl biphenyl; N-butyl ether; diethylene glycol monohexyl ether; silicate ester; perchlorethylene; 2 phenylcyclohexanol; diethylene glycol monobutyl ether; toluene; tributyl phosphate; tricresyl phosphate; xylol and a variety of fractions and grades of refined petroleum fractions.

It will be understood, of course, that the electro fluids prepared fromea'ch of the foregoing vehicles, and, indeed, others which are not listed, but which may be used in accordance herewith, are not identical in their properties. Some are superior to others for certain applications. These examples are given for the purpose of bringing out the wide variations in vehicle selection which are avail able. Certain examples of electro-fluids which have been found especially satisfactory are disclosed herein. As one skilled in the art is aware, substitutions of ingredients in a composition containing four or more components requires a certain amount of adjusting and, indeed, experimentation in order to obtain the optimum results. With the informatoin given herein such a person can readily select those ingredients best suited for his desired use.

The quantity of vehicle present in the electro-fiuids of the present invention may vary rather widely with the range of from about to about 65% and preferably from about 25% to about 50% by weight of the final product.

As in the case of the vehicles, the variety of solids which may be employed in the force transmitting fluids of the present invention is great. vAs noted above, my Patent No. 2,417,850 suggests the use of starch, limestone, gypsum, flour, gelatine and carbon. As varied as these materials are it has now been found that any essentially non-conductive solid having significant ability to adsorb water or alcohol on its surface, and preferably those with substantial surface area per gram may be employed in accordance herewith.

Finely divided silica gel, of the type described hereinafter in detail, has been found especially suitable for use herein. But solid particles of the following types having an average diameter of from about 0.1 to about 5 microns have been found useful in the manufacture of electrofluids of the present invention: aluminum octoate; aluminum oleate; aluminum stearate; polystyrene carboxylic acid polymer; amorphous silica; barium titanate; colloidal silica; calcium stearate; activated charcoal; colloidal kaolin clay; crystalline D-sorbitol; dimethyl hydontoin resin; flint quartz; lauryl pyridinium chloride; lead oxide; lithium stearate; magnesium silicate; mannitol; microcel-C; micronized mica; molecular sieves (Union Carbide) nylon powder; onyx quartz; rottenstone; white bentonite; zinc stearate.

Especially effective materials for the suspended or dispersed phase of the mix are obtained by milling various commercial grades of activated silica gel which have been prepared by the Patrick process and referred to in recent years as xerogels.

As is well known, activated gels of this character contain surface areas ranging upward from 100 square meters per gram. Hence, even when milled to impalpable powders, each particle is itself highly porous and contains a pore area many times its exterior. The pore surfaces are concave and readily adsorb large amounts of liquids or vapors. It is for this reason that such materials have found use as desiccants, catalysts, etc. Advantage is taken of this porosity in the present invention to load the particles with substances which modify the electrical characteristics of the particle, such as its conductivity, dielectric constant, and surface charge.

Although substantial electro-viscosity is exhibited by mixes in which the particles have a size as large as 5 microns, best results have been found when the particle size lies within the range of 0.1 to 1.0 micron in diameter. Present commercial impalpable silica gels contain particles larger than this, as well as particles of sharp-edged form, and it is found desirable to tumble the material in a mill containing flint pebbles or steel balls for periods of from 5 to hours. This may be done with or without a vehicle. The eifect of this preliminary treatment shows up in a final mix having higher breakdown dielectric strength and higher electro-viscosity.

In one method granular activated silica gel, 8 to 16 mesh,'is milled in a water vehicle to 0.1 to 1.0 micron particle size. The resulting slurry is poured into fiat trays and allowed to settle for two days. The supernatant water is then drained oif and the settlings dried to a chalky cake which, with the aid of heat, is reduced to a water content of from 5 percent to 20 percent by weight of the anhydrous material. The chalk is readily powdered, stored in sealed containers, and is in condition for incorporation in the dielectric vehicle of the final mix. During the milling various agents may be added which are taken up by the particles and have an effect on the ultimate behaviour of the fluid. Thus, the particle pH, as indicated by the color of adsorbed dyes, may be fixed at this stage by addition of a small amount of acid, base, or buffer solution. Also, the aforementioned metallic oxides may be incorporated in the form of freshly made gelatinous hydroxide precipitates. When it is desired that the particles be loaded with a mixture of alcohol and water, the former may be added during this water grinding operation. Some improvement in the condition of the particles occurs if the aforesaid powder is allowed to age in an open tray or is subjected, four or five times, to alternate dehydration at 250 degrees C. and hydration at room temperature. Apparently the individual particles become compressed and rounded as the result.

In another method the particles are properly conditioned by milling the silica gel in the dielectric vehicle of the final mix and in the presence of the surface active agent. An open pebble or ball mill may be used for mixes containing up to about 20 percent particle volume. It is found advantageous to add small amounts of water during the grinding and finally to subject the mix to heat or low humidity air in order to reduce the water content of the particles to from 5 percent to 20 percent by weight of the anhydrous solids during the latter stages of the process. The mix is usually finished after about twenty four hours of milling. It may then be concentrated by heating off part of the vehicle, by allowing the suspension to shrink to an equilibrium volume after which the excess vehicle is poured off, or by collecting the suspension in concentrated form by means of a centrifuge. This treatment when complete brings the electrical resistivity of the mix within the range of 10 to 10 ohm-cm, it being in this range that the silica gel type of mix is most efiicient.

In still another method the materials for the final mix are ground together in their final proportions. In this case, and particularly when the solids content is very high the mix is initially very thick and paste-like and the milling is done by positive drive of rollers or balls which bear against the walls of the container. A volatile vehicle such as benzene, xylol or mineral spirits may be used for the milling operation; following this the grinding vehicle is distilled off and replaced by one of the aforementioned vehicles.

Whichever method is employed, the milling is continued until particle size is in the range of 0.1 to 1.0 micron.

While the foregoing treatment has been found desirable for conditioning particles from commercial materials which are dominantly activated silica gel, it has been found that substantially spherical active particles of silica gel can be made without prolonged milling when the precipitation of silica is carried out by the quick method described in US. Patent No. 2,114,123. It is found that the quick precipitate after washing in hot water, partially drying, and dispersing in the dielectric vehicle can be milled to a 0.1 to 1.0 micron particle size in about two hours.

The presence of a minor amount of water is essential to the effectiveness of the present compositions. The water may be added at any one of several stages in the preparation of the composition. Thus, it may be combined with the silica gel or other solids when the same are first added to the vehicle or it may be introduced as such at some stage in the preparation. It is preferred to introduce the water as adsorbed water on the surface of the solids.

With certain solids found ureful in accordance herewith, particularly silica gel, the dessicant properties of the same are such that at least a part of the water may be introduced by subjecting the solids, per se, or the mix during milling to an atmosphere of relatively high relative humidity, e.g., above about 60%. The water may also be added as water of hydration of one of the components of the composition or formed in situ, e.g., in a condensation or esterification reaction.

It has been found to be particularly advantageous to include along with the minor amount of water a relatively small quantity of a mono or polyhydroxy compound, e.g., methanol, ethanol, propanol, isopropanol, the butanols, pentanols, hexanols, the ethylene glycol, diethylene glycol, propylene glycol, glycerine, etc. Amines such as the primary and secondary aliphatic amines, triethylene tetraamine, triethanolamine, l-hydroxy-ethyl 2-heptadeceny1- imidazaline have also been found useful. The hydroxyor amino-compound may be present in an amount in excess of the water, but the total of water plus hydroxy or amino compound should not exceed about 30% by weight of the total composition and preferably should be in the range of from about 10% to about 20% by weight of the total composition. The water should be present in an amount of from about 0.5% to about by weight of the total composition and preferably from about 1.5% to about 10%.

It appears that the characteristic of the water, wateralcohol, water-glycol, etc., which have been found useful in accordance herewith as the material adsorbed on the surface of the solids, may be the hydroxyl group present therein. While this has not been established with certainty, certain additional facts seem to bear out the importance of having such a hydrophillic group present in materials added for the purpose of providing or enhancing the flow of electrons on the surface of each solid particle. Thus, for example, it has been found especially desirable to have present in the composition a small amount of an alkali hydroxide, e.g., lithium, potassium or sodium hydroxide. Such alkali hydroxide may be used with advantage in an amount up to about 1 percent of the total composition. It is, of course, preferably introduced as an aqueous solution, the water of which is included as part of the total water content of the mix in, for example, a 1 Normal solution. Calcium hydroxide has also been found useful in some compositions.

A primary function of a surface agent in this invention is to effect the dispersion of solids throughout the vehicle and in so doing to serve as a substantially anhydrous boundry lubricant to permit the free end and close admixture of high percentage of solids. No less important, however, is the function of these agents as electrical insulative skins which are attached by adsorption or chemical bond to the particle surfaces.

The hydrophyllic and also silicophyllic polar group of the surface active agent largely determines the degree of attachment to the particle. Since silica gel is amphoteric in chemical nature, as is also water or alcohol adsorbed in the micro-pores, the polar group may be acidic, neutral, or basic. For example, this group may be the carboxyl acid group COOH, a more or 'less neutral salt, or ester group COOM or COOR', where M is a metal and R is a polyhydric alcohol, or a basic amine group such as NH OH. If the silica gel matrix contains a metallic hydroxide, a true chemical bond may occur by reaction with a COOH group of the surface agent. Similarly, silica gel which has been pre-treated to show a marked acid reaction may react chemically with a primary amine or an hydroxy compound to form a chemically held film or skin.

The non-polar group of the surface active agents used in this invention is a hydrocarbon. It is this portion of the molecule, together with potential barriers at the surface, which provides the electrical insulative property of the skin. Being a hydrocarbon and hence organophyllic in nature, there is some degree of attraction between the hydrocarbon group and the molecules of the vehicle, as also between the skins of contacting particles. Hence, there is some residual tendency for the mix to become structuralized. The result is that the non-potentialized mix frequently shows the properties of a soft jelly. However, the normal gelling forces are so weak that the mix is fluid under slight shear and requires an interval of time when quiescent to re-establish the gelled condition. Such a fluid is said to be thixotropic. It has an advantage over a strictly liquid fluid in that the particles have less tendency to settle to a cake. It is found that the mixes which are nearest to true liquids in nature are those which have been prepared using a mixture of two or more surface agents which are themselves either liquids or of low melting point, such for example as various oleates. Conversely, surface agents of high melting point, such as various stearates, produce mixes of greater body or grease-like character.

So-called anionic types of surface active agents which have been found suitable in preparing electro-viscous fluids include the fatty acids of 8 to 26 carbon atoms, the naphthenic acids averaging about 18 carbon atoms, and the resinic acids (abietic, primaric, etc.) of 20 carbon atoms. Also effective are the metallic or hydroxymetal salts of these acids of the general formula (R.COO) .M where M is one of the following: Li, Na, K, NH Fe(OH) Al(OH) when x=1; Mg, Ca, Sr, Ba, Zn, Cd, Sn, Pb, Mn, Fe, Co, Ni, Fe.OH, ALOH, when x=2; and Fe or Al when x=3.

The foregoing salts may be made in known manner, as by double decomposition of the acetate and the carboxylic acid when NaOH solution is added, or by heating a mixture of the carbonate or the freshly prepared hydroxide of the metal with the carboxyllic acid. For the fatty acids, those sold under the name Neo-Fats have been found suitable; for the naphthenic acids a grade known as Oronite N is satisfactory. Pine rosin is used for preparing the resinates. A number of the salts mentioned are sold as paint and varnish driers and can be obtained on the market.

Of the surface active agents of cationic type may be mentioned the primary amines of general formula R.NH which form salts with acids adsorbed by the silica gel particles. A mixture of primary amines containing 14, 16 and 18 carbon chains, and sold under the designation AM118.5C, will react with an acid to form a specific example of this class. Another example is lauryl pyridinium chloride.

Of the non-ionic type of surface active agents may be mentioned the cresols, amino silane sold under the designation of Sylon RD-602, and the partial or complete esters of polyhydric alcohols. The latter include fatty acid esters of ethylene glycol, glycerine, mannitol and sorbitol. Specific examples are sorbitan sesquioleate and mono-oleate, sold under the names Arlacel C and Span a sorbitan monolaurate (Span 20); and a polyoxyalkalene derivative of sorbitan trioleate (Tween The strongest and least conductive electro-viscous fluids have been made by using a mixture of two or more of the foregoing agents. This is probably in part due to the composite nature of the particle surface, i.e., silica and water for reasons explained hereinafter. Another rule for choosing the most suitable agent is that strongly acidic or basic particles, as tested by indicator dyes, call for strongly basic or acidic polar groups, respectively.

The required amount of surface agent increases with the porosity of the particle. For round non-porous particles of 0.5 micron mean diameter, the computed volume of an 18 carbon chain monolayer, such as an oleate for example, is 3 percent of the particle volume; seldom more than twice thisamount is required for non-porous particle materials. However, for porous silica gel powders,

ranging from 0.1 to 1.0 micron means particle size and percent to 20 percent water by weight of silica, the necessary amounts of surface agent range from about 25 percent to 50 percent by volume of the particles, or from 15 percent to 30 percent by weight of silica. It is evident that 'the surface active agents penetrate the particle. The added weight of both loading and surface active agents to silica gel particles ranges from 20 percent for a low porosity grade to 40 percent for a high porosity grade. In general, it appears that a surfactant should be present in an amount of from about 5 to 15 percent of the total electro-fluid composition.

An appreciable excess of the surface agent beyond these limits will result in a mix of low electro-viscosity. On the other hand, a deficiency will result in a mix in which the particles lack complete boundary lubrication and tend to form agglomerates or have the lock-in habit when potentialized.

The soaps of divalent Pb, Mn, and Fe are outstanding in their ability to reduce the leakage conductivity of a mix. This may be due to the residual valence of these metals which provides a firm anchorage of the soap molecule to the particle surface.

Examples of surface active agents which have been found useful in accordance herewith are: tetrasodium N- (1,Z-dicar-boxylic)-N-octadecyl-sulfosuocinate; sodium dioctyl sulfosuccinate; sodium bis(tn'decyl)sulfosuccinate; 4,4 dimethyl 2 heptadecenyl 2 oxaline; 4 ethyl, 4 hytdroxymethyl decenyl-Z-oxazoline; 4 methyl, 4 hydroxymethyl decenyl-Z-oxacoline; 4,4-bishdroxymethyl 2 heptadecenyl 2 oxazoline; N hydroxyethyl (Z-heptadecenyl glyoxalidine); sorbitan sesquioleate; sorhitan partial fatty esters; di (2 ethylhexyl amine); polyoxyethylene ether alcohol; polyoxyethylene alkyl aryl ether; polyoxyethylene esters of mixed fatty and resin acids; polyoxyethylene ether alcohol; sorbitan monolaurate; sorbitan monopalmitate; sorbitan monostearate; sorbitan tristerate; sorbitan monooleate; sorbitan trioleate; nonyl phenol reacted with 4, 9 or 12 moles ethylene oxide; sodium tetradecyl sulfate; sodium heptadecyl sulfate; sodium 2 ethyl hexyl sulfate; sodium di-(Z-ethyl hexyl) phosphate; polyalkylene glycol ether; alkylphenylether of polyethylene glycol; trimethylnonylether of polyethylene glycol; alkyl phenyl ethers of polyethylene glycol; polyoxyethylene sorbitan monolaurate palmitate; polyoxyethylene sorbitan monolaurate stearate; polyoxyethylene sorbitan monolaurate tristearate; polyoxyethylene sorbitan monolaurate trioleate. 1

While various types of surface agents have been described in some detail, these should not be considered as exhaustive of those which can be used to achieve the dispersive and insulative effect. 'It will be understood that the surfactants contribute to uniformity of the composition and, indeed, to viscosity reduction of the overall mix.

It may sometimes be found desirable to add a material capable of increasing and stabilizing the viscosity of a fluid when the same is not under the influence of the electrical field. In general, materials which have found utility as viscosity index improving agents in lubricating oils have also been useful for this purpose in compositions of the present type. Thus, certain of the methacrylic polymers sold by Rohm & Haas under the trade names Acryloid, e.g., HE-820, HF-825 and -HF-829, have been found useful. Also polyisobutylenes such as Esso Corporations paratone have also demonstrated this property. The diesters of dicarboxylic acids containing from 6 to about 9 carbon atoms, e.g., di-isooctyl adipate, di-sec. amyl sebacate, etc., have been found to render the fluids less viscous.

The added viscosity stabilizing materials are preferably added in amounts of from about 0.5 percent to about 2 percent of the total composition.

It is important that the components of the electrofluids prepared in accordance herewith be thoroughly admixed, so as to afford a homogeneous, substantially stable fluid. This has been found at times to be a difficult task, particularly with the high levels of solids content. Ball mills, roll mills, colloid mills, as well as various types of dispersion mixers have, however, all been used with success. It has been most satisfactory to employ a dispersion mixer. With such mixers liquefication and homogeneity appear to be most readily attained in the shortest period of time. Combinations of mixing equipment have also been found satisfactory. Thus, Hobart planetary mixers or ball mills have been used while the mix is in its most viscous stage with the mix being later transferred to a colloid mill for work in the more fluid stage.

There follows, with its method of preparation, a com position which has been found especially useful. It should be understood, of course, that this example is exemplary only and not intended to limit the invention in any way.

EXAMPLE A Percent by wt. Alkaterge T 1 7.80 Tech. white mineral oil 26.30

1 A mixture in proportions of about 2: 1 of 4,4-bis hydroxymethyl-Z-heptadecenyl-2oxazoline and oleylamido triethanolmethane -product of Commercial Solvents C0.

3 Viscosity of 40-50 Saybolt sec. at 100 F.

3 Specially ground dehydration grade-about 1 micron aver age diameter. Davidson Chemical Co. SMR-55-6826.

*Primarily (about 1 hydroxyethyl-2heptadecenyl imidazoline. The remaining 10% comprises oleic acid amide gndblgaminoethyl ethanol-amine. Product sold by Union ar 1 e.

5A silicate ester base coolant-dielectric fluid sold by Monsanto Chemical Co The composition set forth in Example A was compounded as follows: The mineral oil was heated to about 180 C., the sodium stearate added, and then the two were mixed until uniform at which time the sorbitan sesquioleate was stirred in and the mixture was rapidly cooled, to room temperature at which melted Alkaterge T was mixed in. This mixture was then placed in a dispersion mixer, the mixer was started and the silica gel was added as rapidly as possible over a period of about twenty minutes. Mixing was continued for a total of one hour, beginning with the first addition of silica gel. The sodium hydroxide was then added and the mix was agitated for another hour. Then Amine 220 was added, followed by an hour of mixing and the Coolan ol 45 was added with still another hour of mixing. At the end of this period the product was ready for use. It was found to be a stable material demonstrating high electro viscosity.

There are set forth below a number of additional examples of electro-fluids of the present invention. These are exemplary only and are offered to give those skilled in the art a further idea of the scope of the invention.

The powders in the following examples were made from a commercial granular silica gel of desiccant quality and sold under the name Protek-Sorb. The particles were conditioned by methods already described. Each of these mixes normally showed fluid properties and the nonpotentialized particles moved freely and individually when the mix was subjected to slight shearing stresses. Each mix showed a many fold increase in viscosity when subjected to a strong electric field short of the breakdown strength of the mix, and each was found to have a resistivity lying within the range 10 to 10 ohm-cm. The particle volume was within the range 38 percent to 52 percent in most of the cases cited. In each case the normal viscosity of the finished mix was of the order of 10 to times that of the vehicle used.

1 1 The word powder in the following examples means .1 to 1 micron activated silica gel, dry basis, prepared by any of the methods described.

All ingredients are in parts by weight.

Example N0. 1

Powder 100 Oleic acid 10 Ferrous oleate 10 o-Cresol Water 9 Sodium oleate 1 Dec-Base 62 Example N0. 2

Powder 100 Ferrous oleate 5 Oleic acid Lead naphthenate 5 Water 5 Dec-Base 47 Example N0. 3

Powder 100 Linseed oil 10 Ferrous oleate 2 Lead naphthenate 3.5 Manganese naphthenate 2 Water =6 Deo-Base 43 Example N0. 4

Powder Ammonium oleate 3 Lauryl pyridinium chloride 3 Water 2 Deo-Base 14 Example N0 5 Powder 27 Span 80 7 5 Water 1 Ethylene glycol 2.5 Deo-Base 20 Example N0. 6

Powder 31.2 Span 80 4.2 Tween 85 4.4 Ethylene glycol 6.0 Water 1.8

Dee-Base 20.2

Example N0. 7

Powder 32 Oleic acid 8 Lead naphthenate 2 Water 2 Dec-Base 24 Example N0. 8

Powder 21 Span 20 -l 2 Tween 20 1.7 Ferrous oleate 2.5

Water 1 Isopropyl alcohol 1 Dec-Base 15 Example N0. 9

Powder Ferrous oleate 2 Manganese naphthenate 5 Water 3 Dec-Base 21 12 Example N0. 10

Powder 30 Manganese naphthenate 8 Water 3 Deo-Base 21 Example No. 1]

Powder 30 Arlacel C 4 Manganese naphthenate 4 Ethylene glycol 2 Water 1 Deo-Base 22 Example N0. 12

Powder 60 Spun 2O 10 Ferrous oleate 5 Water 5 Dec-Base 32 Dibutyl sebacate 8 Example N0. 13

Powder 3O Oleic acid 1.5 Ferrous oleate 1.5 Ferrous naphthenate 3 Water 4 X tri-isopropyl benzene 22 Example N0. 14

Powder 30 Linseed oil 1.5 Ferrous oleate 3 Sodium oleate 1 Cobalt naphthenate .2 Water 4 Deo-Base 21 Example N0. 15

Powder 23.4 Oleic acid 2 Naphthenic acid (acid N0. 200) 2 Water 1.2

- Triethanolamine 2 Deo-Base 15 Example N0. 16

Powder 30.2 Ferrous oleate 4.8 ArlacelC 4.8 Water 4 Wemco C 24 Example N0. 17

Powder 50 Magnesium di-oleate 4 Ferrous oleate 4 Oleic acid 4 Water 7 Dec-Base 29 Example N0. 18

Powder 24 Zinc resinate 2.5 Calcium oleate 2.5 Water 1.8 Type 500 silicone, 20 cp. 24

Example N0. 19

Silica gel powder 30 Sylon RD-602 7 Water 1.5 Deo-Base 22 13 Example N0. 20

Silica gel powder 36 Lead linoleate 3.6 Iron naphthenate 3.6 Sylon RD-602 4.5 Sodium oleate .7 Oleic. acid 1.7 Water 2.5 Wernco C. transformer oil 22 Example No. 2]

Powder 100 Lithium oleate 10 Ferrous oleate 10 Oleic acid 5 Acetic acid 2 Water 8 Wernco C transformer oil 65 Example N0. 22

Powder 5O Ferrous soap of castor oil acids 11 Sodium oleate 1 Water ..v 4 1,3 di-isopropyl benzene Deo-Base 20 Example N0. 23

Powder 68.5 Ferrous soap of castor oil acids 11 Sodium oleate 5 Water 7 Di-butyl sebacate 40 x-Tri-isopropyl benzene Example No. 24

Powder 34 Ferrous soap of castor oil acids 9 Sylon RD-602 1 Oleic acid 1 Water 3 Wernco C 22 Dibutyl sebacate 1 Example N0. 25

Powder 38 Ferrous soap of castor oil acids 4 Lead soap of castor oil acids 4 Sodium soap of linseed oil acids 4 Water a 3 Wernco C 28 Example No. 26

Powder I 100 Sodium soap of linseed oil acids 15 Lead laurate 15 Water 8 Type 500 silicone, 5 cp. 80

Example N0. 27

Powder 100 l Arlacel c 30 l Water 5 Type 500 silicone, 5 cp 70 Example N0. 28

Powder 100 Lead soap of castor oil acids 10 Sodium soap of linseed oil acids Water 8 Wernco C. 72

Example No. 29

Powder 100 Sodium soap of linseed oil acids 15 Lead naphthenate 5 Lead oleate 5 Lead laurate 5 Water 7 Wernco C 75 Example No. 30

Powder 100 Sodium soap of linseed oil acids 15 Lead naphthenate 5 Lead oleate 5 Lead laurate 5 Water 7 Wernco C 45 Type 500 silicone, 5 cp. 30

Example N0. 31

Powder 100 Sodium naphthenate 30 Water 8 Wemco C It has been found that finely ground synthetic ion exchange resins may be substituted for the silica gel in most of the above examples to give substantially comparable fluids if the proportion of solids is reduced from parts to 60 parts by weight of the total composition. Substitution of the solids of the type named hereinabove in compositions of the foregoing type has uniformly resulted in fluids demonstrating electro-viscosity.

, This application is a continuation-in-part of my copending application for Letters Patent, Serial No. 597,481, filed July 12, 1956 (now abandoned), which itself was a continuation-in-part of my application for Letters Patent, Serial No. 205,010, filed January 8, 1951, now abandoned. Serial No. 205,010 was a continuation-in-part of my application for Letters Patent, Serial No. 716,626, filed December 16, 1946 (now abandoned), which in turn was a continuation-in-part of my application for Letters Patent, Serial No. 438,967, filed April 14, 1942, which is now Letters Patent No. 2,417,850.

I claim:

1. A composition which is substantially unresponsive to a magnetic field but which readily responds to an electrical field by a significant increase in viscosity and in resistance to shear, which composition comprises; at least about 15% by weight of a liquid oleaginous vehicle having a viscosity not greater than a lubricating oil and having a dielectric constant of from about 2 to about 5.5; from about 30 to about 70% by weight of finely divided solids selected from the group consisting of silica gel, barium titanate and magnesium silicate having an average diameter not greater than about 5 microns; from about 0.75% to about 15 by Weight of water; and from about 5% to about 15 by weight of surface active dispersing agent.

2. A composition which is substantially unresponsive to a magnetic field but which readily responds to an electrical field by a significant increase in viscosity and in resistance to shear, which composition comprises at least about 15% by Weight of a liquid oleaginous vehicle having a viscosity not greater than a lubricating oil and having a dielectric constant of from about 2 to about 5.5; from about 30 to about 70% by weight of finely divided, silica gel having an average diameter not greater than about 5 microns; from about 0.75% to about 15% by weight of water; and from about 5% to about 15% by weight of surface active dispersing agent.

3. The composition of claim 2 wherein the oleaginous vehicle is a petroleum-derived hydrocarbon liquid.

4. The composition of claim 2 wherein the surface active dispersing agent comprises a non-ionic surfactant.

5. The composition of claim 2 wherein the surface active dispersing agent comprises a cationic surfactant.

6. The composition of claim 2 wherein the surface active dispersing agent comprises an anionic surfactant.

7. The composition of claim 2 wherein the oleaginous vehicle comprises a refined petroleum oil in the lubricating oil viscosity range and the surface active dispersing agent comprises a cationic surfactant.

8. The composition of claim 7 wherein the cationic surfactant comprises 4,4 bis hydroxymethyl-Z-heptadecenyl-2-oxaza1ine.

9. The composition of claim 7 wherein the oleaginous vehicle comprises about and the solids about 50% by weight of the total composition.

10. The composition of claim 2 which also includes an amount of at least one water miscible aliphatic hydroxycontaining compound not greater than about 15% by weight.

11. The composition of claim 10 wherein the aliphatic hydroxy compound is an alcohol.

12. The composition of claim 10 wherein the aliphatic hydroxy compound is a polyhydroxy compound.

13. A composition which is substantially unresponsive to a magnetic field but which readily responds to an electrical field by significant increase in viscosity and resistance to shear, which composition comprises at least about 15% by weight of a liquid oleaginous vehicle having a dielectric constant of from about 2 to about 5.5; from about to about 70% by weight of finely divided silica Xerogel having an average diameter not greater than about 5 microns; from 0.75% to about 15% by weight of water; and from about 5% to about 15 by weight of surface active agent.

14. The composition of claim 13 wherein the oleaginous vehicle is a petroleum derived hydrocarbon liquid.

15. The process of preparing a composition capable of substantial increase in resistance to shear when subjected to an electric field which comprises milling until homogeneous an admixture of liquid oleaginous vehicle having a dielectric constant of from 2 to about 5.5, an amount of finely divided silica gel having an average diameter not greater than about 5 microns, in the range of from about 30 to about 70% by weight, from about 0.75% to about 15% by weight of water, at least a portion of which is bound on the silica gel prior to admixing the silica gel with the vehicle, and from about 5% to about 15% by weight of surface active dispersing agent, the percentages of components of the admixture being based on the total composition.

16. The process of claim 15 wherein the silica gel consists essentially of silica Xerogel.

17. The composition of claim 2 which includes an amount of a compound selected from the group consisting of the hydroxides of sodium, lithium, potassium and calcium not greater than about 15% by weight.

18. The composition of claim 2 which includes an amount not greater than about 15% by weight of an amine.

'19. A composition which is substantially unresponsive to a magnetic field but which readily responds to an elec- 5 trical field by a significant increase in viscosity and in resistance to shear, which composition comprises: at least about 15% by weight of a liquid oleaginous vehicle having a viscosity not greater than a lubricating oil and having a dielectric constant of from about 2 to about 5.5; from about 30 to about 70% by weight of finely divided barium titanate having an average diameter not greater than about 5 microns; from about 0.75 to about 15 by weight of water; and from about 5% to about 15 by weight of surface active dispersing agent.

20. A composition which is substantially unresponsive to a magnetic field but which readily responds to an electrical field by a significant increase in viscosity and in resistance to shear, which composition comprises: at least about 15% by weight of a liquid oleaginous vehicle having a viscosity not greater than a lubricating oil and having a dielectric constant of from about 2 to about 5.5; from about 30 to about 70% by weight of finely divided magnesium silicate having an average diameter not greater than about 5 microns; from about 0.75 to about 15% by weight of water; and from about 5% to about 15 by Weight of surface active dispersing agent.

21. The composition of claim 1 wherein the finely divided solid is silica gel having an aver-age diameter of at least 0.1 micron.

22. The composition of claim 2, wherein the silica gel has an average diameter of at least 0.1 micron.

23. The process of claim 15 wherein the silica gel has an average diameter in the range of 0.1 to 1.0 micron.

References Cited in the file of this patent UNITED STATES PATENTS Stamberg Nov. 11, Brunstrum Feb. 15, Montgomery Mar 29, Kistler Oct. 28, Winslow Mar. 25, Small et al. July 4, Savage et al. Apr. 30, Fischer May 21, Haden et al. May 5,

OTHER REFERENCES Von A. Passynski: Kol'lid Z., 70, 1 80-8 (1935), see 50 Pasuinskii, Chem. Abstracts, vol. 29, p. 4236 (1935).

Kolloid 2., vol. 78, pp. 258-72 (1937). Ki-mura: Bull. Chem. Soc. Japan, 14, 243-9, see Chem. Abstracts, vol. 33, -p. 7647 (1939).

Physical Review, vol. 56, p. 847, article by A. A. Dixon, 55 Chem. Abstracts, vol. 45, p. 4645 (1941).

Dirty Oil Magic, Lindberg, Rocky Mountain Empire Magazine, Oct. 24, 1948, page 3.

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