GB1570234A

GB1570234A - Electric field responsive fluids

Info

Publication number: GB1570234A
Application number: GB392777A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1974-07-09
Filing date: 1978-01-24
Publication date: 1980-06-25

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC HELD RESPONSIVE FLUIDS (71) I, THE SECRETARY OF STATE FOR DEFENCE, London, a British Corporation Sole, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electroviscous !(EV) fluid compositions and is a modification of the fluid compositions which are disclosed and claimed in my earlier UK Application No. 30456/74 (Serial No.

1,501,635).

That application described and claims a normally fluid composition capable of exhibiting an increase in apparent viscosity under the influence of an electric field, which composition comprises at least one polyhydric alcohol in solid particulate form, which polyhydric alcohol contains acid groups and has a structure in which water is adsorbed, and an electrically non-conducting oleaginous vehicle in which the solid particles are dispersed.

It has now been found that a much broader range of polymeric materials can be used as the solid medium for an electroviscous fluid, the characterising feature of such polymeric materials being that they should contain acid groups. Polymers which have acid groups have been found to exhibit improved performance compared to those without. This improved performance is exemplified by an increase in electroviscous effect response of up to three times compared with that of fluids using corresponding non-acid group-containing solids.

Accordingly, the present invention provides an electroviscous fluid composition which comprises, in dispersion in an elec tricaily non-conducting oleaginous vehicle, water-containing particles of a polymer having free or neutralized acid groups, said polymer having a water absorbency as herein defined and a density of not greater than 1.8 gcmS, provided that said polymer is not a polyhydric alcohol. This present invention therefore provides alternative polymeric materials to the polyhydric alcohols previously described.

The introduction of acid groups into a neutral solid material has been found to lead to enhanced electroviscous effects in fluids containing such solids. This is true whether the acid groups are in free form or are in neutralized form as a salt and the enhancement produced results in EV fluids which are superior to those known heretofore. The use of solid phase materials which give an inherently better EV response, as in the fluids of this invention, permits either the use of a lower water content in the polymer whilst maintaining a good EV response, or, with the same water content in the solid phase, a much stronger EV response may be obtained. In the former instance, the use of lower water content can be advantageous; for example the no-field properties are improved, and generally current flows are lower with lower water content in the solid.

It is true that as the acid groups themselves promote a higher current flow, the advantage of lower water content is to some extent offset in respect of this property, and that in the case where water content is maintained to get a better EV response, this is achieved at the expense of a somewhat higher current flow than occurs with the corresponding neutral solid, but the improvement in EV response is such as to more than outweigh these disadvantages. Broadly speaking it will in general be necessary to design an EV fluid to obtain an optimum performance in any given duty, and the solids of the present invention greatly increase the scope for optimization apart from providing positive inherent advantages over such prior art solids as starch, silica or algizic acid.

The acid group or groups in the polymer may be of any type, that is to say any group which is capable of increasing the hydrogen ion concentration in a medium into which the polymer is placed, but preferred acid groups are the carboxylic acid group and the sulphur containing acid groups, eg sulphate, sulphonic or sulphonous acid groups.

It is preferred that the acid groups be neutralized, and for this purpose metal cations, for example of alkali mentals, can be introduced into the polymers All, or only a proportion of the acid groups may be neutralized by metal cations, and more than one type of cation may be introduced into a given polymeric material. Certain organic cations can also be used for neutralizing the polymer acid group.

As regards the polymeric backbone, a particularly preferred class of polymer is an addition polymer containing at least one monomer which has at least one acid group and/or at least one group which is convertible to an acid group after polymerization of the monomer. Exemplary of such monomers are acrylic acid, methacrylic acid, methyl acrylate and methyl methacrylate.

The polymer may comprise just one such monomer, or alternatively more than one such monomer may be co-polymerized together in any combination. In either case, these monomers may be co-polymerized with further unsaturated monomers, eg olefins or other vinyl monomers, such as vinyl acetate, maleic acid, maleic anhydride or N-vinyl pyrrolidone.

In general terms, it is important that the polymer to be used in the electroviscous fluid of this invention should be at least somewhat hydrophilic in character, in the sense of having a water-retaining character.

This is because it is essential for the production of an electroviscous effect that the polymer should contain water. Moreover the amount of water contained by the polymer is a significant factor in establishing the E.V. properties which a fluid containing the polymer will demonstrate. A further factor of importance, as far as the E.V. properties are concerned, is the environment of the water contained within the polymer. Although the invention is in no way limited by the following explanation as to the factors which govern the EV response of the fluids of the invention, basically it would appear that the function of the polymer is to provide the right environment for the water which must be present in an electroviscous fluid. More specifically, it seems that it may be necessary for a certain amount of water to be absorbed into sites of a given energy in the polymer. The energy produced on absorption of a water molecule into the polymer structure will be dependent on the environment in which that molecule finds itself. This relationship of water to the polymer can be viewed conveniently in terms of the hydrophilic or water-retaining character of the polymer.

One factor affecting the absorption site energy for a polymer in a fluid of this invention will be the extent of the charge on the polymer in the vicinity of the site. The likely function of acid groups in the polymer is that by their ionization the polymer attains a negative charge with respect to water molecules absorbed therein and as a result when an electric field is applied to the polymer, it acts to displace the water therefrom by electro-osmosis. The result is that when the polymer comprises the solid phase of an electroviscous fluid, the shear properties of the fluid are altered by the application of an electric field. From this it will be appreciated that the number and strength of the acid groups in the polymer is important in that it determines to a considerable extent the environment of the water present in the polymer.

The properties of the polymer per se which govern the E.V. response of the fluids of the invention may be conveniently viewed together in terms of the single parameter of the hydrophilic-oleophilic balance of the polymer. As stated, the polymer must have some hydrophilic character. However it should preferably not be so hydrophilic in nature as to be water-soluble since such materials suffer from the disadvantages of being difficult to free of extraneous materials and of tending, when in use, to deposit as a compact layer on the electrodes, necessitating frequent stripping down and cleaning.

Such soluble solids are also very troublesome to process since they tend to become "sticky" by absorbing atmospheric moisture and thus tend to clog instead of running freely. It is though, important also that the polymer should not be too oleophilic in character since this will make the no-field properties of any electroviscous fluid containing the polymer very poor due to the polymer swelling in the oleaginous vehicle in which it is suspended.

Within the limit set by the water absorbency of the polymer, the water content of the polymer can be varied much as desired in order to optimise the E.V. properties of a fluid containing any given polymer.

The water absorbency of the polymer is a measure of its openess to water penetration and sets an upper limit on the possible water content of the polymer. Because the optimum performance of an electroviscous fluid according to the invention is strongly dependent on the water content of the solid phase, the limits on this parameter can be very significant in performance terms and it has been found that the water absorbency of the polymer should be such that, after being allowed to equilibrate with moist air, a sample of the polymer should lose at least 8% of its weight on heating to constant weight under vacuum at a temperature which does not cause decomposition thereof and should eventually regain at least 40% of this loss in weight when exposed to air at an ambient relative humidity of about 60% and a temperature of 20"C. The heating stage is normally carried out at a temperature of about 50 to 600C but for many polymers higher temperatures may safely be used.

In terms of the EV effect demonstrated by a fluid containing a polymer, the effect of the actual water content of that polymer is as follows. The higher the water content the higher is the EV response (change in yield point per unit voltage applied) up to a limit, the lower the threshold voltage and the higher the current flowing. Such a fluid moreover has excellent dynamic properties, ie it will effectively retard a system in which the fluid is already being sheared When the voltage is applied. A fluid with a high water content also responds rapidly to changes in the applied field and in general gives repro ducible results. With too high a water con tent, however, the no-field characteristics are poor, the working range of voltage is reduced, since arcing occurs at low-field strengths, and in some cases the re ponse becomes poor at the higher volt ages, so that an increase in voltage does not lead to an increase in shear resistance. With fluids of low water content, on the other hand, the currents are low, and the working range of voltage wide, although the threshold voltage is high also. No-field properties are usually good. In use, however, although such fluids show good static properties, ie they will lock two initially stationary components together effectively, their dynamic properties are poor. As a result of having a higher static than dynamic yield-point such fluids tend to suffer from hysteresis effects and forms of 'stick-slip'. Low water content EV fluids also respond relatively slowly to changes in the applied field.

It will be seen therefore that in all EV fluids, the water content is an important parameter in optimising a fluid for a given application. With EV fluids of this invention, the range of water contents over which both good eleotro-viscous response and an acceptable level of current may be obtained is much greater than with the fluids pre viously known to the art. Typically for EV fluids of this invention, where the application is a static one, the water content of the polymer should be such that, at 300 C, the equilibrium relative humidity (ERH; defined as the R.H. of air in equilibrium with the solid at a known temperature is about 1% whilst for dynamic applications further water up to about 5% by weight of the polymer should be added to the polymer. This will increase the ERH of the polymer up to a maximum of about 60%, though even higher water contents may be appropriate in some cases.

From the point of view of the structure of the polymer those properties which effect its behaviour in an electro-viscous fluid are influenced by such factors as the inclusion in the polymer of non-ionizing hydrophilic groups, for example pyrrolidone groups, by cross-linking of the polymer and by neutralization of the acid groups in the polymer to form salts. All of these modifying factors have an effect upon the energy distribution of the water absorption sites in the polymer and thus, in the terms of the preceding discussion, bring about changes in the EV response of a fluid composition containing a polymer which is so modified.

As well as affecting the EV response cross-linking of the polymer also has a profound effect on the no-field properties of the final fluid. It appears that if a polymer is not cross-linked, or is only slightly crosslinked, the solid will contain an appreciable amount of low molecular weight material which will be soluble in the absorbed water and, to an extent therefore, mobile within the polymer. Such mobile material has a "flocculating effect" on the polymer particles which leads to an increase in the no-field flow resistance of the fluid. In line with this it is found that removal of these mobile elements from the solid, by exhaustive washing with water, leads to greatly improved no-field properties of the final fluid but such washing is generally impracticable with completely un-cross-linked polymers, since these will be soluble. With more completely cross-linked polymers, this problem is avoided and hence the no-field properties of fluids containing such polymers are better than for the other types of polymer.

Cross-linking of the polymer may be achieved by the use of cross-linking agents in the conventional manner but attention must be paid to the character of the agent in that it should preferably be hydrophilic in character, especially where the polymer is not itself particularly hydrophilic. Cross-linking agents such as N,N1-methylene bis acrylamide or diallyl ether are therefore generally suitable, though more hydrophobic agents such as divinyl benzene can be used where the polymer is itself stronglythydro- philic, eg with acrylate polymers.

Another method of cross-linking which avoids introduction of further materials is to irradiate the polymer, provided this does not damage the polymer backbone, and formation of salts with a polyvalent cation may also be effective to cross-link the polymer chains.

Formation of salts of the polymers with metal or organic base cations can be used to further control the EV properties of fluids containing the polymers and also tends to enhance the stability of the polymers. The presence of cations and correspondingly the existence of charged groups on the polymer backbones, may considerably affect the polarity of the polymer and hence its water absorption characteristics. Where the cations have a high charge density, as with calcium, aluminium and chronium cations for example, charge separation is minimal, the solid appears to be non-polar and may well be too oleophilic in character Such cations are however useful to effect X-linking of the polymer chains, but to offset the oleophilic character, the polymer should also contain cations of low or moderate charge density.

At the other extreme to the cations of high charge density, ions of low charge density, such as organic base cations, are relatively loosely held at the charged sites on the polymer chains, and the resultant polymer salts are very hydrophilic or even deliquescent and as such are unsuitable for use in EV fluids. In general, therefore, polymer salts with cations of moderate charge density most suitably combine the properties of good water penetration towards charge centres, as required for strong EV effects, with controlled water absorption. Suitable cations of this type include those of metals in Groups IA and IB of the Periodic Table, especially those of low atomic weight such as lithium, sodium, potassium and copper (II). Instead of using simple salts with cations of a single metal, similar results can also be achieved by using mixed salts in which two or more different metal cations are present at least one of which may be of high charge density and at least a further one of low charge density. In this way the effect of the different cations may be averaged out to give a polymer salt having properties similar to those of a salt with a cation of moderate charge density. In fact the use of mixed salts is advantageous in that it permits a more precise control of polymer properties through appropriate choice of the different cations and their respective prop or tions in the salt. Use of mixed salts also means, as indicated previously, that crosslinking of the polymer chains can be achieved simultaneously by use of at least a proportion of a polyvalent cation in the salt.

From all points of view, a mixture of uni- and trivalent cations has been found to be suitable, eg a mixture of cations of metals from Groups I and III of the Periodic Table.

The use of the hydrophilic-hydrophobic characteristic of the polymer as an indicator of useful EV properties may be illustrated by reference, by way of example, to a series of salts of poly(methacrylic acid) using guanidinium and lithium monovalent cations and aluminium and chromium trivalent cations. Electroviscous fluids containing these salts can be arranged in order of increasing no-field shear resistance thus : - lithium; guanidinium I chromium; guanidinium/aluminium lithium/chromium lithium / aluminium chormium; aluminium (highest) This order parallels the order of increasingly covalent bonding of the metal cation to the polymer acid groups, ie of increasing hydrophobicity of the polymer salt. Correspondingly to this order the no-field viscosities also increase with increasing hydrophobicity so that the "base line" for the EV response in the case of fluids containing such polymer salts will be relatively high. Unless therefore the fluid has a large EV response capability, either as a result of an inherently high response of the fluid composition and/or as a result of using a large volume fraction of the solid phase in the fluid, the proportionate change in viscosity on application of an electric field will be rather low and the fluid will not be very useful.

The use of mixed metal polymer salts is also advantageous since incorporation in a salt of at least a quantity of highly charged cation means that less metal is present in the salt and thus the density of the salt is minimised. This is important in that it allows the use, at least where density matching is to be achieved, of less dense oleaginous vehicles, or at least a lower proportion of a high density oil, which is significant both from the economic point of view and also because the high density oils tend to be highly viscous. As more of a dense oil is used, the volume fraction of the solid phase material has to be progressively lowered in order to keep the llo-field viscosity of the fluid down to a reasonable level. When the volume fraction of solid is low the absolute level of the EV response will likewise be rather low and certainly it will then be necessary to use a solid having a good EV response capability. In practice suitable oleaginous vehicles with densities in excess of 1.8 ,ocm3 are not available and hence the polymer or polymer salt should not have a higher density than this: the practical lower limit is about 1.2 g cm3.

As regards other general properties of the polymers, these should be chemically stable and resistant to chemical attack or degradation by heat or light. The transition temperature of the polymer may be important in certain contexts of use and may dictate the choice of polymer in these cases.

Polymers suitable for fluid compositions of this invention may, in some cases, be commercially available or may in general be prepared by radical polymerization processes when these monomers are water soluble, or from the corresponding ester monomers by the conventional process of suspension or emulsion polymerization in an aqueous medium, followed by hydrolysis of the ester groups when some at least of the monomers are not water soluble. Since this step of hydrolysis of the polyesters to get a polymer having acid groups is difficult to carry out requiring very vigorous conditions, an easier method of obtaining acid group-containing polymers from such monomers has been sought.

Accordingly, I have devised a means of preparing the poly(acid) polymers directly from acid group-containing monomers, where these are not all soluble in water. This novel preparative method involves suspending or emulsifying acid group-containing monomers (with co-monomers if required) in a perfluoropolyether, in which the mono mers (and the resultant polymer) are in soluble, effecting polymerization of the monomers and finally separating off the insoluble polymer formed. It will be appreciated that this procedure provides a very simple and direct route to the desired poly (acid) polymers. As the polymer tends to entrain some of the perfluorinated material and in view of the considerable expense of this material, it is most advantageous to extract the polymer, after filtering it off from the polymerization medium, with a material which will dissolve out the perfiuorinated polyether and which can be readily separated therefrom subsequently. A particularly suitable material for this function is trichiorotri- fluoroethane.

An alternative approach to carrying out either conventional polymerization of an ester monomer or direct preparation of an acid group-containing polymer according to the novel methol herein described is to use radiation polymerization where this is not destructive of the material.

As regards the polymer salts for use as the solid phase of EV fluids, in designing these I have found it convenient to start from a salt of a polymer with a monovalent cation and to add a sufficient amount of a trivalent metal to render the salt water-insoluble. In general the salts may be prepared in the following ways.

For metals having water-soluble hydroxides, an aqueous solution of the appropriate hydroxide may be added to a well stirred polymeric acid solution. A titration curve can be plotted to find the end point. In other cases a suitable method is to first prepare a salt of the above type and then to add a solution of a soluble salt of the metal whose polymer salt it is desired to obtain, whereupon the latter will be precipitated from solution. An at least stoichiometric amount of the polymer should be added except where mixed salts are to be prepared.

In order to avoid the trapping of globules of soluble polymer solution inside a skin of the insoluble polymer it is advisable to honiogenise the solution whilst carrying out this precipitation. Also it is recommended that the precipitate be washed entirely free of residual salts by washing with distilled water, A further method of preparing simple metal salts involves titration of a solution of the polymeric acid with a strong base to determine the amount of base required to neutralize the acid groups in the polymer, With this knowledge the amount of metal salt which it is necessary to add can readily be calculated. The mixed metal salts can be formed by double decomposition from a simple metal salt, eg for the lithium/ chromium polymethacrylate, chromium chloride may be added to a sample of lithium polymethacrylate.

In making up the electroviscous fluids of the invention, the polymer is dispersed, in particulate form, into an oleaginous vehicle.

It has been found to be advantageous to avoid the use of particles of polymer which are too small (roughly below 1 micron in size) since these lead to undesirable no-field properties in a fluid, and it is also preferable that the particles should not be too large in relation to the working gap in whatever apparatus the fluid is being used. The reason for the latter is that if particles larger than about one tenth of the smallest gap in the flow circuit are used, the fluid in the gap might be inhomogeneous leading to anomalies in operation. Thus where the working gap is 0.5mm, an upper limit on the particle size of about 50 microns has been found to be best adhered to. Within the limits indicated, the particle size of the solid phase material does not appear to be especially significant.

As regards oleaginous vehicles for the EV fluids of the invention, if the vehicle is to be density matched to the solid phase material, a mixture of two or more oleaginous liquids will be required. -Factors affecting density matching of liquid and solid phase materials are discussed in my earlier UK Patent Application No. 30456/74 (Serial No.

1,501,635).

The viscosity of the oleaginous vehicle determines how much of the solid polymer can be used in an EV fluid designed for any particular application. Thus, by way of example only, using a vehicle which is a mixture of Aroclor 1242 -(registered Trade Mark) and Fluorolube FS-5 (registered Trade Mark), a volume fraction of solid of about 30% will give a fluid which is still pumpable, having a viscosity of around 200 mPa.s. For some applications higher viscosities and hence higher volume fractions of solid may be tolerated, whilst for other applications lower viscosities will be necessary.

Some examples of the preparation of, and results obtained with, electroviscous fluids according to this invention will now be described by way of further illustration thereof. Measurements of electroviscous fluid behaviour are carried out in the following manner.

A concentric cylinder apparatus provides for an electroviscous fluid sample to be held in the annular gap between a fixed earthed outer cylinder and a cylindrical rotor mounted within it. A field can be applied across the gap and increasing torque can be applied to the rotor, both voltage and torque being measurable. The current which flows through the fluid sample is also measured. The whole apparatus is kept at a constant temperature during testing, since the characteristic parameters of the fluid are temperature dependent, especially the current.

As regards the significant parameters of a fluid, the 'yield-point' is taken as that value of the torque at which sustained rotation is observed. In a plot of yield point against voltage applied, the curve is basically linear in form but there is usually a threshold voltage, VO, below which there is no electroviscous effect. The mechanical behaviour of an electro-viscous fluid can be described by two parameters, viz, the threshold voltage V0 and the slope of the linear portion of the yield-point/voltage curve, designated S/v.

For so-called 'dynamic' measurements, the fluid is sheared as measurements are being made and three dimensional plots must be made to relate all the variables.

To obtain further parameters which are characteristic of a given fluid, the dc current passed by the fluid can be measured and related to the voltage applied across the working gap. The relationship of current to voltage takes the form : - Current/Unit area = P(V) + Q(V)2 This relationship tends to break down at low voltage values and this leads to negative values of P, but for all practical purposes the law is sufficiently accurate over the working range of the fluids.

The constants P and Q may be readily determined from experimental results of current and voltage by carrying out a linear regression of conductivity against voltage; the slope of the regression line gives Q and the intercept on the conductivity axis gives P.

It appears that the values of P and Q are not greatly affected by shearing of the fluid.

P is virtually temperature independent, but Q has a Boltzmann-type temperature dependence.

The characteristic data are highly dependent upon the water content of the polymer and this has not been optimised in every case. Where the polymer used was in 'as made' form after drying, the water content is stated as being 'ambient water content'.

In some cases further water has been added in the vapour phase and the amount so added is stated where appropriate as a percentage by weight of the "dry" polymer weight, ie 'ambient water content + X%.

The polymers were tested after being ballmilled and sieved to remove particl vessel. This mixture ',as then maintained at 60"C for eight hours, with constant stirring, under reflux and a slight positive pressure of nitrogen. (This is to exclude oxygen which inhibits the polymerization). At the end of this period, the mixture was cooled, and the solid removed and washed with water.

The solid was then resuspended in a 5:1 acetone: caustic soda mixture and heated under reflux in a water-bath for approximately 30 hrs. The solid, which had not apparently swelled or changed in appearance at all, was then recovered by decantation, washed several times by decantation with distilled water to remove fines and gummy material, and finally filtered and washed on the filter with more distilled water. The material was then resuspended in 11 of 3% aqueous caustic potash, and boiled under reflux with stirring for a further 20 hrs. It was again recovered and washed by decantation with distilled water; again no obvious change in its physical appearance was apparent. The material was then suspended in a 4:1 dilution of concentrated 'hydrochloric acid to water, and stirred with a fresh batch of acid for a further 4 hrs. It was finally thoroughly washed with distilled water and dried under vacuum. The material prepared by this sequence was a white, granular material, very similar in appearance to caster sugar. On drying, the water regain was 0.9 (gwater/g weight of solid); as expected, the cross-linking had greatly reduced the water holding ability of the material. The solid was milled for three hours, and a sample taken for density tests. At the end of this time, the material had been reduced in size to about twice the radius of starch, as judged under the microscope, ie about 10 microns particle radius. The density was 1.36 g cm-3, which should be compared with 1.42 g cm-3 for polyacrylic acid and 1.35 g cm3 for polymethacrylic acid; as expected the introduction of the hydrocarbon monomer decreases the density. Unlike commercial polyacrylic acid, the cross-linked polymer was not soluble, and as a consequence, was not at all 'sticky'.

After milling overnight, the majority of the material had been reduced in size to about 3-5 microns (as judged under the microscope) although some coarser particles remained; the latter could be removed by rough sieving through bolting-cloth.

Static methods were used to investigate the E.V. properties of the solid. A batch was dried down to an arbitrary level, and then a series of samples rehydrated under vacuum to known extents and tested as fluids at 30% by volume in Aroclor (registered Trade Mark). Measurements made of the characteristic values for the fluids are given for each sample in Table 1.

TABLE 1 Water added % by weight S/v '(Pa/v) V,0 (KV) P1 1.3 ... ... 1.95 + 0.17 1.68 + 0.29 0.66 + 0.51 0.62 + 0.20 3.2 ... ... 2.14 + 0.11 0.61 ~ 0.10 0.49 + 0.23 3.43 + 0.12 5 ... ... 1.58 + 0.08 0.29 + 0.07 0.62 + 1.67 23.6 f 1.21 1The units of P and Q are respectively amps/v X 7.87 X 106 and amps/volt2 X 7.87 X 109.

A clear trend is established, in that a rise in water content is associated with a fall in V,, a rise in the current passed, but only rather slight changes in S/v. It is apparent that V0 falls to useable values before the current becomes unacceptably high and therefore this solid would provide practically useful fluids.

No-field property measurements give a yield point of 92 Pa and a plastic viscosity of 0.22 Pa.s.

Example 2 The procedure of Example 1 was repeated but using 135 ml of methyl acrylate with 15 ml (10%) of divinyl benzene (50-60% w/w in ethyl vinyl benzene). The product had a density cf 1.34 g cm3 and was a white granular material. After milling to 3-5 microns size, a fluid was made up in Aroclor (registered Trade Mark) (30% v/v) the water content of the polymer being 'ambient' + 6.2% by weight. The static test data obtained were as follows: S/V 1.38 + 0.007 Pa/v VO 0.33 KV P 8.75 + 67.7 amps/v x7.87 x 106 Q 23.56 + 100 amps/v2 X 7.87 X 109 The no-field property values were : - yield point: 33 Pa, plastic viscosity 0.26 Pa.s.

Example 3 142.5 ml of acrylic acid was mixed with divinyl benzene (7.5 ml) and benzoyl peroxide (2g) and the whole added to 1 Kg (530 ml) of Fomblin (registered Trade Mark), vigorously stirred in a creased flask and maintained at 70"C. at should be noted that it was not possible to easily remove the inhibitor (p-methoxyphenol) from the acrylic acid; this was therefore 'swamped' by using a double quantity of benzoyl peroxide). As usual, nitrogen was passed over the surface of the reaction mixture to remove oxygen; the reaction vessel was provided with a thermometer and a reflux condenser.

After stirring the mixture for about an hour, the whole mass suddenly solidified, stopping the stirrer (it is essential to connect the latter to the drive motor by a slipping coupling to prevent breakage). The mass was allowed to cool, and lifted out above the reaction vessel and allowed to drain. The material was white and friable.

With some difficulty, the polymer was removed from the stirrer, roughly broken up, and washed on a filter with trichiorotri- fluoroethane (TCFFE). It was then exhaustively extracted in batches in a Soxhlet apparatus with the same solvent, finally being dried and milled.

It should be mentioned that roughly one third of the Fomblin (registered Trade Mark) had been entrained in the solid mass. After filtration, it was found that less than 10g. of Fomblin had been lost overall, a very satisfactory rate of recovery.

The solid was obtained as a pure white powder, density 1.37 g cm-3 (cf. the value of 1.36 gem-3 for material produced in Example 1 by conventional procedure).

When placed in water, there was some evidence of swelling; the initially hard particles swelled slightly, became more translucent and rubbery.

Following previous experiments, the solid was tested for E.V. activity as a 30 percent suspension in Aroclon (registered Trade Mark), after a known weight of solid had been allowed to pick up an amount of water equivalent to 3.5% by weight of the polymer. The static test results obtained are given below: S/V = 1.90 Pa/v Va =0.52 KV P = 3.59 + 0.63 amp/volt X7.87X106 Q = G.4 + 0.48 amp/volt2X7.87X100 Example 4 The procedure of Example 3 was repeated but using instead 148.5 ml of acrylic acid and 1.5 ml (1%) of divinyl benzene solution.

The product had a density of 1.39 g cm-3.

Static test data was obtained for a 30% v/v solution in Aroclor (registered Trade Mark) of the product with 'ambient' water content, as follows: S/V = 200 + 0.16 Pa/v V, = 0.48 + 0.18 KV P = 0.44 + 0.82 amp/volt X7.87X106 Q = 13.30 ~ 0.58 amp/volt2X7.8?X 10' tumple 5 Çommercially available pure samples of polyacrylic and polymethacrylic acids were statically tested for E.V. activity as fluids in 30% v/v Aroclor (registered Trade Mark).

The results obtained are given in Table 2.

No field property data are also given for polymethacrylic acid.

Example 6 4.1 Kg of lithium hydroxide (BDH Ltd) was made up in the minimum quantity of distilled water (about 16 1) and added slowly, with stirring to 411 of a 20% aqueous polymethacrylic acid solution {BDH Ltd.). As the mixing took place the pH was continuously monitored and after an initial rapid change, the pH shows a period of slow change followed by a further sudden rapid change. At this point addition of lithium was stopped: the pH was about 9.

The solution was then evaporated down at a temperature of not more than 80"C using the lowest practicable vacuum, As drying proceeds it is recommended to break up the surface at intervals to prevent "skinning" which retards evaporation. Also, to ensure that the solid is thoroughly dry it is preferred to heat it to 80"C in a vacuum of about 4 mm Hg at least overnight.

After drying the solid obtained, it was milled in a vibratory ball-mill and then, prior to grading, dried again to remove moisture which may have been absorbed during milling. The solid was then graded, the fraction below 20 microns being retained and the larger particles returned to the mill (after again drying) for remilling. The fine particles where then hydrated in the following manner.

The solid was kept at 30"C and a stream of air of controlled humidity (R.H. about 1% of static measurements, about 60% for dynamic measurements) passed through the solid until no further change in the relative humidity of the efflux was observed.

After hydration the solid (yield 8.82 Kg) was admixed immediately with the base liquid which comprised 2.66 Kg of Fluorolube FS-5 (registered Trade Mark) (Hocker Chemicals Plastics Corporation) and 9.45 Kg of Aroclor 1242 (registered Trade Mark) (Monsanto). The requirement for immediate mixing of the hydrated solid is because the solid will rapidly pick up water if exposed without the base oil.

Samples of the fluid composition were then tested for no-field properties and statically for EV activity, with the results set out in Table 2.

Example 7 109g of 20% aqueous polymethacrylic acid solution (BDH Ltd) was diluted to 200 ml with distilled water. The solution was titrated to pH 9.0 with 6.2% lithium hydroxide solution (solid supplied by BDH Ltd).

186 ml of the lithium hydroxide solution was required. The solution of lithium polymethacrylate was continuously homogenised using an 'Ilado' homogeniser and in this state an aqueous solution (29% w/v) of chromic chloride (green form supplied by Hopkin & Williams Ltd) added slowly.

When the ratio moles of Cr "+ added moles of Li+ required for initial neutralization had reached a value of 0.294, the entire solution gelled completely and at this stage the addition of the chromium chloride solu tion was stopped. The gel was dispersed in distilled water, filtered and washed with further portions of distilled water, dried on a fluid bed drier, ground and then sieved to remove particles greater in size than 50 microns. After this the dark grey-green solid obtained was made up as a 30% v/v suspension in Aroclor (registered Trade Mark) and tested both for no-field properties and for EV activity statically. The results are given in Table 2.

Example 8 A chemically cross-linked lithium polymethacrylate polymer salt was prepared by the following method.

1 L distilled water, 100g of methacrylic acid (Koch-Light Laboratories), 48.8g of lithium hydroxide monohydrate (BDH Ltd.) and 35.8g of methylene bisacrylamide (Koch Light Laboratories) were added to a 5 L beaker and stirred magnetically. The contents were warmed to 70"C and when no further solution was taking place, 1.5g of solid potassium persuiphate was added to the solution. Warming and stirring was continued and after about 15 minutes the liquid rapidly gelled, the temperature simultaneously rising by about 20"C. (The final temperature at this point should be kept below 100"C in order to prevent the contents being blown out of the beaker by steam pressure, or alternatively a closed reactor could be used and the process conducted under pressure).

After gelling of the contents, the beaker was removed from direct heat and placed in a boiling water bath, where it was kept until the gel is sufficiently stiff to crack as it shrinks, which is after about 2 hours in the water bath.

The white, opaque gel was removed from the beaker, cooled and roughly chopping into small pieces. About 3 L of distilled water were then added and the suspension homogenised in an Ilado homogeniser at about 10,000 rpm. The finely-divided polymer was then washed free of soluble materials by passing a slow stream of distilled water upwards through the suspension held in a tube of about 10cm diameter and having a glass sintered disc across its bottom. Distilled water was pumped in to the bottom of the tube through the disc using a peristaltic pump, at a rate of about 500 ml/hour. (The rate is chosen so that the polymer in suspension neither sinks to the bottom of the tube nor is carried out of the top of the tube.) Washing was continued for at least 24 hours, after which the solid was recovered by tiltra- tion, drained and dried in a fluid-bed drier at 60"C for about 3 hours or until dry.

About 130g of a white, friable powder was obtained, and after vacuum drying and milling, was suspended at 30% v/v in Aroclor 1242 (registered Trade Mark). Results of static tests for this material and for a further hydrated sample are given in Table 2.

Example 9 The same procedure as in Example 8 was followed, except that the initial neutralization of the methacrylic acid was done using guanidinium carbonate, using the method described in Example 6. The product was obtained as a white friable solid which, when dried over silica gel, gave the static test results shown in Table 2 when tested as a 30% v/v suspension in Aroclor 1242 (registered Trade Mark). Two samples were further hydrated by the addition, respectively, of 2.5% and 4.9% of water (% of polymer weight) and these results are also shown in Table 2.

TABLE 2 Yield Plastics 1 density of Water S/V Vo Point Viscosity EX No. Solid Material polymer Content (Pa/V) (KV) P Q (Pa) (mPas.s) 5 polyacrylic acid ambient 3.63 # 0.31 0.75 # 0.19 2.33 # 0.20 1.89 # 0.10 5 polymethacrylic 1.35 ambient 2.13 # 0.02 1.03 # 0.09 #0.45 # 0.11 1.27 # 0.05 58 330 acid g cm-3 ambient + 2.2% 2.14 # 0.21 1.29 # 0.27 -2.05 # 0.81 15.98 # 0.42 ambient + 3.7% 2.17 # 0.03 0.27 # 0.06 -9.00 # 7.32 2.00 # 5.4 6 lithium polymethacrylate 1.47 optimum 3.6 # 0.43 # 0.52 # 110 # (uncross-linked) g cm-3 7 lithium/chromium 1.51 ambient 3.92 # 0.49 0.97 # 0.29 0.507 # 0.075 0.456 # 0.056 0.65 236 polymethacrylate g cm-3 8 lithium polymethacrylate 1.43 ambient 3.95 # 0.045 0.52 # 0.16 -2.50 # 1.12 20.3 # 1.0 (cross-linked g cm-3 with methylene ambient + 4.2% 3.33 # 0.21 0.35 # 0.08 -25.7 # 8.30 27/ # 7.3 bis-acrylamide) 9 guanidinium dried over 2.78 # 0.38 1.49 # 0.39 ca.20 0 polymethacrylate 1.36 silica gel (cross-linked g cm-3) 2.5% water added 4.02 # 0.27 0.78 # 0.14 39.5 # 3.46 16.4 # 2.0 with methylene 4.9% water added 2.66 + 0.13 bis-acrylamide) 0.38 # 0.06 15.2 # 10.3 468 # 91 1 The solids were tested in each case as 30% v/v suspensions in Aroclor 1242 (registered trade Mark) (Monsanto).

2 The units of P and Q are respectively amp/volt x 7.87 x 106 and amp/volt x 7.87 x 109.

From the results in Table 2, it can be seen firstly that the 12% of sites occupied by lithium in the mixed salt (Example 7) are sufficient to reduce the no-field shear resistance completely to that of the simple equivalent salt (Example 6), at least when the trivalent ion is chromium. Very similar results are obtained when the lithium polymethacrylate salt is cross-linked using the added agent N,N1-methylene bis-acrylamide rather than by means of trivalent chromium ions. However the cross-linked polymer salt has the advantage of a lower density than the mixed salt (and also lower than the density of the simple uncross-linked lithium polymethacrylate) as reference to Table 2 shows. Furthermore the cross-linked lithium salt is easier to handle than the uncrosslined salt with no tendency to absorb moisture. With a density of 1.43, this polymer salt would require the presence, in a density-matching mixed Aroclor (registered Trade Mark) -Fluorolube (registered Trade Mark) oleaginous vehicle, of only 13% of Fluorolube (registered Trade Mark), which the more expensive and more viscous component. By contrast the uncross-linked lithium polymethacrylate salt would require a vehicle having 22% of Fluorolube (registered Trade Mark) and the lithium/ chromium mixed salt, one having 31% Fluorolube (registered Trade Mark).

The guanidinium salt of Example 9 would require an even less dense fluid vehicle than the lithium salt of Example 8, but was found to be otherwise somewhat less satisfactory than the latter on account of its much greater sensitivity to water content. On balance the preferred salt is the lithium polymethacrylate in which the polymer has been cross-linked with N,Nl-methylene bis-acrylamide to the least extent possible concomitant with obtaining a solid material which is readily washed and handled.

Claims

WHAT I CLAIM IS:-

1. An electroviscous fluid composition which comprises, in dispersion in an electrically non-conducting oleaginous vehicle, water-containing particles of a polymer having free or neutralized acid groups, a density of not greater than 1.8 g cm-3 and a water absorbency such that a sample thereof. after being allowed to equilibrate with moist air, loses at least 8% of its weight when heated to constant weight under vacuum at a temperature which does not cause decomposition of the polymer, and regains at least 40% of this loss in weight when exposed to air at an ambient relative humidity of about 60% and a temperature of 200 C, provided that said polymer is not a polyhydric alcohol.

2. An electroviscous fluid composition according to claim 1, wherein the acid group or groups comprise carboxylic acid, sulphate, suiphonic acid or sulphinic acid groups.

3. An electroviscous fluid composition according to claim 1 or claim 2, wherein said polymer is an addition polymer.

4. An electroviscous fluid composition according to claim 3, wherein said addition polymer is derived from at least one monomer which contains at least one acid group and/or at least one group which is convertible to an acid group after polymerization of the monomer.

5. An electroviscous fluid composition according to claim 3 or claim 4, wherein said polymer is an addition polymer of one or more monomers selected from acrylic acid and substituted acrylic acids.

6. An electroviscous fluid composition according to claim 5, wherein the substituted acrylic acid is methacrylic acid.

7. An electroviscous fluid composition according to claim 4, wherein said group convertible to an acid group is an ester group.

8. An electroviscous fluid composition according to claim 7, wherein said monomer is an alkyl acrylate.

9. An electroviscous fluid composition according to claim 8, wherein said monomer is methyl acrylate or ethyl acrylate.

10. An electroviscous fluid composition according to any of claims 3 to 9, wherein the polymer contains at least one other monomeric constituent selected from an olefin, maleic acid, maleic anyhydride, vinyl acetate or N-vinyl pyrrolidone.

11. An electroviscous fluid composition according to any of the preceding claims, wherein the polymer is a cross-linked polymer.

12. An electroviscous fluid composition according to claim 11, wherein the polymer is cross-linked by use of divinyl benzene, diallyl ether or N,N1-methylene bis-acrylamide.

13. An electroviscous fluid composition according to any of the preceding claims wherein at least a proportion of the acid groups of the polymer are neutralized to form salts.

14. An electroviscous fluid composition according to claim 13, wherein the acid groups are neutralized by metal or organic cations.

15. An electroviscous fluid composition according to claim 14, wherein the metal or metals is or are selected from Groups I, II and Ill of the Periodic Table.

16. An eleotroviscous fluid composition according to claim 15, wherein the metal or metals is or are selected from lithium, sodium, potassium, copper, magnesium, aluminium or chromium.

17. An electroviscous fluid composition according to claim 14, wherein the polymer is a salt of polymethacrylic acid.

18. An electroviscous fluid composition according to claim 17, wherein the salt is a lithium, guanidinium or mixed lithiumchromium polymethacrylate.

19. An electroviscous fluid composition according to claim 17, wherein the polymer has an equilibrium relative humidity (as hereinbefore defined) of between 1 and 65%.

20. An electroviscous fluid composition according to any preceding claim, wherein the polymer particles have a size of between 1 and 50 microns.

21. An electroviscous fluid composition according to any preceding claim, wherein the fluid contains from 20 to 40% v/v of the polymer.

22. An electroviscous fluid composition according to any of the preceding claims, wherein the oleaginous vehicle is density matched to the polymers

23. An electroviscous fluid composition according to any of the preceding claims, wherein the oleaginous vehicle is an admixture of at least two compounds, of which one compound has a density greater than that of the polymer and another compound has a density lower than that of the polymer.

24. An electroviscous fluid composition according to claim 23, wherein the oleaginous vehicle is a mixture of two compounds, one of which is a polychlorinated biphenyl fraction and the other of which is a polymer of trifiuorovinyl chloride.

25. An electroviscous fluid compobition comprising particles of lithium polymethacrylate having a water content such that the equilibrium relative humidity of the solid is between 1 and 65%, said particles constituting between 25 and 35% v/v cf the fluid, the balance of the fluid comprising an electrically non-conductive oleaginous liquid of density between 1.40 tand 1.50.

26. An electroviscous fluid composition according to claim 25, wherein the polymethacrylate is cross-linked with N,N1methylene bis-acrylamide and has a density of less than 1.45.

27. An electroviscous fluid composition according to claim 25 or claim 26, wherein the oleaginous vehicle is density matched to the lithium polymethacrylate and comprises a mixture of a polychlorinated biphenyl fraction and a polymer of trifluorovinyl chloride.

28. An electroviscous fluid composition substantially as hereinbefore described and with particular reference to the Examples.