GB2249553A - Electro-rheological fluids - Google Patents

Abstract

An electro-rheological fluid comprises a continuous phase comprising at least one liquid crystal material and, suspended therein, a dispersed phase comprising solid microscopic glass beads having an uncoated surface.

Description

ELECTRO-RHEOLOGICAL FLUIDS This invention relates to electro-rheological fluids.

The term 'electro-rheological fluid' is now commonly used to describe a fluid which demonstrates an increase in flow resistance resulting from the application of an electric field.

The effect was first demonstrated over forty years ago by Winslow (U.S. Patent No. 2417850), who disclosed that certain suspensions, composed of a finely divided solid such as starch, limestone or its derivatives, gypsum, flour, gelatin and carbon, dispersed in a non-conducting liquid, for example lightweight transformer oil, transformer insulating fluids, olive oil or mineral oil, will manifest an increase in flow resistance as long as an electrical potential difference is applied thereto. The effect, often referred to as the "Winslow effect", was originally interpreted as an increase in viscosity and such materials were referred to as "electroviscous fluids". Subsequent investigations have however shown that the increase in flow resistance may be due not only to an increase in viscosity in the Newtonian sense, but also to an applied electric field induced Bingham plasticity.Consequently the term "electro-rheological fluid" has come into general use.

Research has continued with a view to improving the dispersed and continuous phases of electro-rheological fluids and, as the mechanisms by which the phenomena occur are still not well understood, differing approaches have evolved. Thus some fluids rely on the presence of water associated with the dispersed phase - see for example UK Patent Nos. 1501635 and 1570234, while other fluids are water-independent - see for example UK Patent No.

2170510. A review of the variety of dispersed and continuous phases employed so far is to be found in a review article by Block et al., J. Phys. D. Appl. Phys. 21 (1988), 1661-1677. An even more recently published U.K. Patent Application No. 2208515 proposes an electro-rheological fluid comprising fluid which itself acts as the active material and which is preferably a liquid crystal material. If desired, dry microscopic glass bubbles may be suspended within the fluid, the majority of the bubbles preferably having diameters between 20 and 100 microns and wall thicknesses of between 0.5 and 2 microns. Two applications of such fluids are described, in the operation of a piston and in the operation of a printed circuit board test apparatus.In both instances the operation depends on a sharp increase in the yield stress of the fluid with application of an electric field so as to effect a rapid change in the fluid to a state approaching that of a solid.

However, not all the potential applications for electro-rheological fluids require a sharp increase in yield stress with increasing voltage. The present invention, therefore, seeks to provide novel electro-rheological fluids which extend the range of usefulness of such fluids, for example, into the area of robotics.

Accordingly, the present invention provides an electrorheological fluid comprising a continuous phase comprising at least one liquid crystal material and, suspended therein, a dispersed phase comprising solid microscopic glass beads having an uncoated surface.

Preferably the glass beads have a particle size of from 20 to 60m. Particularly suitable are beads of particle size of from 30 to 50m described as "specially selected soda lime glass beads (density about 2.48)" and manufactured by (Polysciences Ltd., Northampton, UK). The beads must be uncoated. Beads having a surface coating, for example of a silicone or other polymeric material, although readily available, are not suitable for use in the present invention. Although not wishing to be bound by any particular theory, it is postulated that the surface interaction between glass and the liquid crystal material is critical in the development of a satisfactory electro-rheological effect.

The continuous phase comprises a liquid crystal material.

Liquid crystals are a widely accepted class of materials which can exist in a mesophase (or "liquid crystal state") at temperatures between those at which the material is a solid and those at which it becomes an isotropic liquid. In the mesophase the material maintains some degree of molecular order but also has some fluidity. Compounds conventionally exhibiting this behaviour have a lath- or rod-like structure.

The continuous phase may comprise a well known commercially available liquid crystal material such as the biphenyl-based room temperature nematic mixture known as E7 (available from BDH Ltd., Poole, U.K.). However, other liquid crystal materials are suitable such as those known as E44 and E31LV (also available from BDH Ltd.). E44 is a wide temperature range and high birefringent mixture for display devices and E31LV is a low threshold mixture for display devices.

Additionally, combinations of liquid crystal materials, which themselves are mixtures, may be employed. Thus a commercially available mixture such as E7 may be blended with a second liquid crystal mixture, such as Merck Mixture N5 (which is a dynamic scattering mixture, available from E. Merck, Darmstadt, Germany). As an alternative, or in addition, various single compounds may be added to the existing liquid crystal mixture to modify its properties when used in an electro-rheological fluid.

Such added compounds, (known as dopants) may conveniently be molecules of lath- or rod-like structure known to exhibit themselves liquid crystal behaviour. Examples of such materials are phenyl and biphenyl benzoates substituted at least at the para positions of the ring systems. A particularly suitable dopant has been found to be a substituted phenyl biphenyl-carboxylate known as PG 495 which is of the type described in EP-A-110299, which is preferably employed together with the above-mentioned liquid crystal mixture E7. An alternative suitable dopant has been found to be a polysiloxane bearing long chain alkyl substituents, for example of chain length 15 to 20 carbon atoms.

The amount of dispersed phase present in the continuous phase can vary dependent on the desired flow properties of the resulting fluid and the intended application. However, it has been found preferable to employ from 30 to 75% of glass beads by weight of the composition so as to give a fluid with an adequate electro-rheological effect but which fluid does not exhibit too great a viscosity in the absence of an applied field for practical use. It has been found particularly suitable to employ from 55 to 65% by weight of glass beads dispersed in from 45 to 35% by weight of liquid crystal material. When a dopant is employed as part of the continuous phase, the amount thereof can vary widely but will usually be in the range of from 5 to 15% by weight of the overall composition.

It will be appreciated that minor amounts of other components may be employed in the fluids. Thus, for example, a surfactant may be included to aid dispersability and to maintain shelf life of the fluid. However, it is preferred to exclude water as far as is possible, as this avoids the problems of fluid deterioration, corrosion and increased electrical conductivity associated with the use of water.

Fluids in accordance with the present invention have surprisingly been found to exhibit a slow increase in yield stress with increase in DC voltage when compared with the sharp increase obtained with a typical commercial electro-rheological fluid. This slower and relatively steady increase still however leads to a sufficiently high yield stress at voltages considered feasible for commercial applications, thus making these fluids potentially suitable for use in areas such as robotics where a more delicate on/off control mechanism is required.

The invention will now be further described by way of example.

Measurement of Yield Stress Yield stress of the fluids investigated was measured as a function of voltage stress as follows: The test fluid was placed in a 0.5mm gap between the bottom of a trough and the base of a horizontally slidable trolley supported by the walls of the trough. The trolley was adapted so that it could be subjected to a known load via the provision of a fully suspended loadable container linked to the trolley via a cable passing over a pulley. A known DC voltage was applied to the fluid across the gap and load (water) added to the container at a given voltage until the load became sufficient to drag the trolley a set distance (2mm) along the trough. The yield stress for a given voltage stress was then calculated from the weight of water added to the container to effect trolley movement.

Example 1. Selection of Bead Type Samples of different commercially available glass beads were tested for production of an electro-rheological effect at differing concentrations (25, 40, 50, 60 and 70neo by weight) in commercially available E7 liquid crystal material at a voltage stress of 1.42 kV/mm'l The beads tested were: A) Spheriglass beads 5000 CP 000. These are solid uncoated glass beads available from Croxton and Garry Ltd., Dorking, U.K.

B) Polysciences beads (30-SOm). These are solid uncoated glass beads described as specially selected soda lime glass beads (density about 2.46) available from Polysciences Ltd., Northampton, U.K.

C) Spheriglass beads 5000 CP 100. These are coated glass beads, available as for A).

D) Spheriglass beads 3000 CP 100. These are coated glass beads, available as for A).

E) Whatman ODS beads (37-53m). These are silicone-coated glass beads available from Whatman Lab. Sales Ltd., Maidstone, U.K.

Beads of Type A gave good results. Beads of Type B gave excellent results. Beads of Types C, D and E were totally unsatisfactory and did not give compositions exhibiting an electro-rheological effect.

Example 2. Selection of Bead Concentration Differing quantities of Type B beads as described above were dispersed in E7 as the continuous phase and the yield stress measured for application of a voltage stress of 1.42 kV mm-1 The results are shown in Table 1 below.

Table 1 Composition (% by wt.) Yield Stress Beads E7 (kPa) 20 80 0.10 40 60 0.38 60 40 0.60 70 30 0.65 80 20 Note 1 Note 1 : The mixture was too viscous to handle.

During this investigation it was noticed that when the yield stress measurements were repeated on the same sample, the values steadily decreased. Thus for 60% w/w beads in 40% w/w E7 the following was observed: Yield Stress (kPa) at 1.42 kV mm#1 Run 1 0.60, 0.53, 0.48 Run 2 0.67, 0.55, 0.47 Example 3. Alternative Liquid Crystal Continuous Phases.

Using in each case a mixture of 60neo by weight Type B beads dispersed in 40% by weight liquid crystal continuous phase, yield stresses were determined for a number of alternative commercially available liquid crystal mixtures at a voltage stress of 1.42 kV mm#1. The liquid crystal materials tested were: a) E7 b) E44 (described as a wide temperature range and high birefringent mixture for display devices, available from BDH Ltd., Poole, UK.) c) E31LV (described as a low threshold mixture for display devices, available from BDH Ltd.) d) E7 plus Merck Mixture N5 (described as a dynamic scattering mixture), available from E. Merck, Darmstadt, Germany).

The fluid employing mixture d) contained 30% by weight of E7 and 10% by weight of N5. The results are shown in Table 2 below.

Table 2 Liquid Crystal Material Yield Stress (kPa) 1 2 3 4 a) 0.60 0.53 0.48 b) 0.40 0.28 0.20 c) Run 1 0.34 0.43 0.42 0.43 Run 2 0.41 0.42 0.43 0.45 d) 0.79 0.62 0.54 Columns 1-4 represent the yield stresses obtained by repeating the measurement on the same sample. It will be noted that, in contrast to the other materials, E31 LV gave an increasing yield stress on repeating the measurement on the same sample.

Example 4. Voltage Dependence of Yield Stress The variation of yield stress with applied voltage stress was measured for: a) A fluid composition consisting of 60% by weight Type B beads in 40% by weight E7.

b) A commercially available ER fluid described as "Standard ER fluid" (available from Air-Log Ltd., London, U.K.).

For each fluid, measurements were taken as follows: i) starting at low voltage stress and steadily increasing the value; ii) starting at high voltage stress and steadily decreasing the value.

The results are shown graphically in Fig. 1 for fluid a) and in Fig. 2 for fluid b). The values given are the first reading of the yield stress taken at each voltage stress.

The marked difference in behaviour of the conventional fluid b) from the fluids of the invention was further demonstrated using fluids a) and b) above plus: c) A fluid composition consisting of 60% by weight Type B beads in 40% by weight E31LV.

The change in yield stress with voltage stress was measured for a), b) and c) starting at high voltage stress and steadily decreasing the value. The results are shown graphically in Fig.

3. The values given are the first reading of the yield stress taken at each voltage stress.

Example 5. Addition of Dopants Several commercially available dopant materials were added to replace part of the standard 40% by weight E7 content of fluid compositions containing 60% by weight of Type B beads. The yield stresses in each case were measured at 1.42 kV mm-l In all cases except Experiment 14, the dopant was used in an amount of 10% by weight of the fluid composition. In Experiment 14, 5% was used. The dopants used were of the following types:

The results are shown in Table 3 below: Table 3 Expt. No.Dopant Yield Stress (kPa) 1 2 3 1. 0.60 0.53 0.48 2. (I) n=6; X=CH3; Y=CN 0.29 0.37 0.23 3. (I) n=6; X=H; Y=CN 0.39 0.26 0.31 4. (I) n=6; X=H; Y=OCH3 0.33 0.35 0.36 5. (I) n=4; X=H; Y=CF3 0.07 0.04 0.05 6. (II) 0.04 0.04 0.05 7. (III) 0.04 0.06 0.04 8. (V) 0.46 0.40 0.30 9. (VI) 0.51 0.43 0.36 10. (IV) R=C4Hg; X=C1; Y=H; R'=C4H9 0.49 0.41 0.38 11. (IV) R=C5H11; X=F; Y=H; R'=C2H5 0.33 0.32 0.30 12. (IV) R=C3H7; X=F;Y=F; R'=C5H11 0.40 0.26 0.20 13. (VII) 0.50 0.43 0.38 14. (VIII) 0.65 0.84 0.79 Columns 1 to 3 represent the yield stress obtained by repeating the measurement on the same sample. Two further measurements were taken in experiment 5: the yield stresses were 0.42 and 0.45 kPa.

While not wishing to be bound by any particular theory, the following general trends were observed.

For the benzoate and biphenylyl esters of formulae I, II and III, it can be seen by comparing experiments 2 and 3 that a laterally-positioned methyl group on a polar dopant had very little effect on yield stress while a large laterally-positioned group does have an adverse effect (compare experiments 2, 3, 4 and 7). While experiments 2 and 3, which incorporated polar dopants, gave reasonable yield stress, experiment 6, which used a highly polar dopant (dipole moment = 8D; Ac = 69.5) gave very low yield stress. This observation, coupled with the fact that the fluid of experiment 4, which incorporates a non-polar dopant, gave reasonable yield stress, strongly suggests that factors based on steric hindrance and not dipolar interactions play an important role in determining the yield stress values of liquid crystal ER fluids.

It is also to be noted that the fluid of experiment 4, which contained the non-polar dopant, gave an increase in yield stress on repeated application of the applied voltage, as found in Example 3 for liquid crystal mixture E31 LV. The following explanation is postulated for this observation.

The liquid crystal mixture E7 only contains mesogens with terminally positioned cyano-groups and it is a well known fact that in such systems the phenomenon of anti-parallel correlation can exist, i.e. a proportion of the mesogenic molecules exist as dimers which reduces the expected ae and the dipole moment of the system based on the structure of the mesogens in the liquid crystal mixture. The phenomenon of anti-parallel correlation is minimised in many commercial liquid crystal mixtures by the addition of a small amount of a non-cyanated ester, which breaks up the dimers and thus gives the expected Ac or dipole moment, i.e. the addition of the non-cyanated ester has effectively 'increased' the as and dipole moment of the liquid crystal mixture.

Both the liquid crystal mixture E31LV used in Examples 3 and 4 and the ER fluid of experiment 4 contain mainly mesogens with terminally positioned cyano-groups, but also a small amount of a non-cyanated ester. Thus, in the case of ER fluids based on E31LV and the ER fluid of experiment 4 the breaking up of the anti-parallel correlation in the continuous phase to give the 'higher' dipole moment has not led to an increase in the yield stress of the fluid, but has manifested an ER fluid system in which the yield stress is relatively constant (slightly increases) on numerous applications of the voltage stress. This property may prove important for certain applications of ER fluids.

Experiments 8 to 13 employ compounds of formulae IV to VII which are examples of "two frequency liquid crystal materials" (which change their sign of ae when the frequency of an applied a.c. field is increased). They are of high molecular weight and are lath-like in shape and contain terminally positioned polar or non-polar groups. In general they gave higher yield stress then the benzoate and biphenylyl esters of experiments 2 to 7. There was little difference in yield stress for terminally-positioned cyano- or alkyl substituents which again suggests that it is steric factors, rather than dipolar interactions which are more important in liquid crystal ER fluids. The dopant VIII (PG495) used in experiment 14 again shows this influence of steric factors on yield stress.

Example 6. Addition of Polysiloxane Dopants Alternative dopants using side chain liquid crystal polymers were employed in admixture with E7 substantially as described in Example 5 except that 2% dopant was used by weight of the fluid composition. The dopants were of general formula (IX):

Polysiloxanes of this general type are available from Fluorochem, Old Glossop, Derbyshire, UK. The polysiloxanes used in the example had A) X = H,

Polysiloxane B) was made as follows:

Step 2 Step 2 Step 1 was carried out by reaction with CH2=CH(CH2)4 Br in the presence of potassium carbonate and butanone to give a product condensed in step 2 with polysiloxane A) (X=H) in the presence of toluene and platinum divinyltetramethyldisiloxane.

Polysiloxane C) was prepared by condensation of polysiloxane A) with CH2-CH(CH2)15CH3 in the presence of toluene and platinum divinyltetramethyidisiloxane.

The results using polysiloxanes A), B) and C) are shown in Table 4 below.

Table 4 Experiment No. Polysiloxane Yield Stress (kPa) Dopant 1 2 3 1. A 0.04 0.04 0.04 2. B 0.05 0.09 0.09 3. C 0.66 0.52 0.48 While not wishing to be bound by any particular theory, these results appear to confirm that steric factors are more important to liquid crystal ER fluids than dipolar interactions.

Claims (15)

1. An electro-rheological fluid comprising a continuous phase comprising at least one liquid crystal material and, suspended therein, a dispersed phase comprising solid microscopic glass beads having an uncoated surface.

2. A fluid according to Claim 1 wherein the glass beads have a particle size of from 20 to 60m.

3. A fluid according to Claim 1 wherein the beads are soda lime glass beads having a particle size of from 30 to 50m.

4. A fluid according to any one of Claims 1 to 3 wherein the liquid crystal material is the biphenyl-based room temperature nematic mixture known as E7.

5. A fluid according to any one of Claims 1 to 3 wherein the liquid crystal material is a wide temperature range and high birefringent mixture known as E31LV.

6. A fluid according to any one of Claims 1 to 3 wherein the continuous phase comprises a mixture of liquid crystal materials.

7. A fluid according to Claim 6 wherein the mixture comprises E7 and a second liquid crystal material which is a dynamic scattering mixture known as N5.

8. A fluid according to any of Claims 1 to 7 wherein the continuous phase comprises a liquid crystal dopant.

9. A fluid according to Claim 8 wherein the dopant is of any one of formulae (I) or (IV) to (VIII) as defined herein.

10. A fluid according to Claim 8 or 9 wherein the dopant is employed in an amount from 5 to 15% by weight of the fluid.

11. A fluid according to any one of the preceding claims wherein the liquid crystal material comprises a compound of formula (IX) as defined herein wherein X is a long chain alkyl group.

12. A fluid according to any one of the preceding claims comprising from 30 to 75% by weight of glass beads dispersed in from 70 to 25% by weight of liquid crystal material.

13. A fluid according to Claim 11 comprising from 55 to 65% by weight of glass beads dispersed in from 45 to 35% by weight of liquid crystal material.

14. A fluid according to anyone of the preceding claims which is substantially anhydrous.

15. An electro-rheological fluid in accordance with claim 1 substantially as described herein with reference to anyone of Examples 1 to 6.