Electric Field Responsive Fluids

A sample of polyacrylic acid cross-linked with divinyl benzene was prepared as follows.

142.5 ml of methyl acrylate was taken and freed from stabilizer by by washing with dilute alkali in a separating funnel, followed by washing with water. The divinyl benzene (7.5 ml of 50-60% w/w in ethyl vinyl benzene) was treated similarly, and the monomers were then mixed and to the mixture 1.5g of benzoyl peroxide was added. This solution was then repidly added to a well-stirred aqueous solution of 1g potassium stearate in 200 ml water in the reaction vessel. This mixture was then maintained at 60.degree. 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 1l 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 (g water/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, which should be compared with 1.42 for polyacrylic acid and 1.35 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. Measurements made of the characteristic values for the fluid are given for each sample in Table 1.

A clear trend is established, in that a rise in water content is associated with a fall in V.sub.o, a rise in the current passed, but only rather slight changes in S/.sub.V. It is apparent that V.sub.o 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.

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 of 1.34 and was a white granular material. After milling to 3-5 microns size, a fluid was made up in Aroclor (30% v/v) the water content of the polymer being `ambient` + 6.2% by weight. The static test data obtained were as follows:

S/.sub.v = 1.38 .+-. 0.07 pa/v

V.sub.o = 0.33 KV

P = 8.75 .+-. 67.7 amps/v .times. 7.87 .times. 10.sup.6

Q = 23.56 .+-. 100 amps/v.sup.2 .times. 7.87 .times. 10.sup.9

The no-field property values were: yield point: 33 Pa, plastic viscosity: 0.26 Pa.s.

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, vigorously stirred in a creased flask and maintained at 70.degree. C. (It 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 trichlorotrifluoroethane (TCTFE). 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 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 (cf. the value of 1.36 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 Aroclor, 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/.sub.v = 1.90 pa/v

V.sub.o = 0.52 KV

P = 3.59 .+-. 0.63 amp/volt .times. 7.87 .times. 10.sup.6

Q = 6.4 .+-. 0.48 amp/volt.sup.2 .times. 7.87 .times. 10.sup.9

The procedure of Example 3 was repeated but using instead 148.5 ml of acrylic acid and 1.5 ml (1%) of the divinyl benzene solution. The product had a density of 1.39. Static test data was obtained for a 30% v/v solution in Aroclor of the product with `ambient` water content, as follows:

S/.sub.v = 2.00 .+-. 0.16 pa/v

V.sub.o = 0.48 .+-. 0.18 KV

P = 0.44 .+-. 0.82 amp/volt .times. 7.87 .times. 10.sup.6

Q = 13.30 .+-. 0.58 amp/volt.sup.2 .times. 7.87 .times. 10.sup.9

Commercially available pure samples of polyacrylic and polymethacrylic acids were statically tested for E.V activity as fluids in 30% v/v Aroclor. The results obtained are given in Table 2. No field property data are also given for polymethacrylic acid.

Characteristic data for electroviscous fluids made with prior art solids are presented in Table 3, for comparison with the like data set out previously for fluids of the present invention. All measurements were made on 30% v/v suspensions in Aroclor.