Light valve containing an improved suspension and liquid therefor.

0001The present invention relates to light valves and more particularly to improvements in the liquid suspension of particles contained or adapted to be contained within the light valve cell. Still more particularly, the present invention relates to liquids which may be included in such a liquid suspension of particles.

0002Light valves have been known for more than fifty years for modulation of light. In Edwin Land's U.S. Patent No. 1,955,923 the light valve was defined as a cell formed of two transparent sheets of insulating material spaced apart a small distance and containing a suspension of small particles in a liquid suspending medium. As a practical matter, the suspension also in­cludes a polymeric stabilizer dissolved in the liquid suspending medium to prevent agglomeration of the particles. Nitrocellulose was proposed as a polymeric stabilizer in the beginning of the development of light valves. More recently, U.S. Patents Nos. 4,164,365 and 4,273,422 disclosed polymeric stabilizers in the form of copolymers or a mixture of polymers.

0003In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the nature and concentration of the particles and the energy content of the light. When an electrical field is applied through the suspension in the light valve, the particles become aligned and for many suspensions a sub­stantial part of the light can pass through the cell.

0004Light valves have been extensively described in the literature. See U.S. Patent Nos. 1,955,923, 1,963,496, 3,512,876 and 3,773,684. In the early days of television, Donal, Langmuir and Goldmark thoroughly investigated the use of light valves in black and white as well as color television. See U.S. Patent Nos. 2,290,582, 2,481,621, 2,528,510 and 2,645,976. A more modern use of the light valve is as an alpha-numeric display. Also, active matrix addressed high information content light valve displays including flat color television set replacements for the cathode ray tube have been proposed by the assignee of the present invention. Such displays could function in a manner partly analogous to commercially available flat color TV sets which use liquid crystals to produce images.

0005Light valves have also been proposed for use in windows, eye-glasses and the like to control the amount of sun-­light passing therethrough.

0006A wide variety of organic and inorganic particles have been proposed for use in light valves including mica, graphite, numerous metals, and halogen-containing light-­polarizing crystals including, for example, certain metal halides and perhalides of alkaloid acid salts. Likewise, a wide variety of liquids have been suggested as all or part of the light valve liquid suspending medium to sus­pend the small particles and dissolve the polymeric stabilizer, such as certain esters, nitrobenzene, oils and other liquids. See U.S. Patent Nos. 1,961,664, 2,290,582 and others.

0007Isopentyl acetate and isopentyl isobutyrate have been proposed for use as a suspending liquid in a light valve in U.S. Patent 4,442,019 to Marks. The suspending liquids of the present invention have markedly improved stability over the esters proposed by Marks.

0008Dibutyl phthalate has been proposed for use as a suspending liquid in a light valve in U.S. Patents 1,951,664 and 1,955,923 to Land and 3,625,869 to Marks. U. S. Patents 4,025,163 and 4,113,362 to Saxe et al. propose the use of aromatic esters derived by reaction between an aromatic alcohol and an aliphatic acid.

0009U. S. Patent No. 4,407,565 proposed to bring a light valve suspension into substantial gravitational equilibrium by using as the liquid suspending medium therefor an electrically resistive, inert, low molecular weight, liquid fluorocarbon polymer having a specific gravity at room temperature of at least about 1.5 and having at least about 50% of its atoms constituted by halogen atoms, at least 60% of said halogen atoms being fluorine and the balance chlorine and/or bromine, and an electrically resistive organic liquid miscible there­with. A wide variety of organic liquids can be used as the aforesaid miscible organic liquid. These include esters of aliphatic and aromatic acids and alcohols, e.g., alkyl acetates, such as isopentyl acetate and alkylphenyl acetates, such as p-nonylphenyl acetate, dioctylphthalate, diisodecyl adipate, dioctyl sebacate, as well as aromatic hydrocarbons, e.g., benzene and toluene, and silicones.

0010The choice of the miscible organic liquid is im­portant. Its boiling point should preferably be well above 100°C so as to keep its vapor pressure relatively low in the normal operating temperature range for a light valve, which might be -40°C to +85°C.

0011The liquid should also have a very high electrical resistivity, preferably at least 10¹¹ ohm-cm and more preferably 10¹² ohm-cm or higher. The higher its electri­cal resistivity, the lower the voltage required to orient the suspended particles and the lower the power losses in the suspension.

0012For many purposes, for example suspensions for use in displays, rapid response time is important. Be­cause response time is related to the suspension vis­cosity, the lower the viscosity of the liquid, the better when fast speed of response is desired.

0013Finally, the liquid should be compatible with other materials in the suspension and not degrade or be degraded by them or any of the cell components.

0014The present invention provides liquids for use in the suspending medium of a light valve that meet these criteria. In particular, the present invention provides a light valve in which the liquid suspending medium comprises a liquid ester or ether of formula (I): R¹ - X - R²      (I) wherein R¹ is branched chain alkyl of at least 8 carbon atoms, such as from 8 to about 20 carbon atoms, or the group (III) <chemistry id="chem0001" num="0001"><img file="EP0342299A1_D0001.tif" /></chemistry> wherein Y and Z are independently a direct bond or straight or branched chain alkylene, such as from 1 to 8 carbon atoms, and R³ and R⁴ are lower alkyl, such as from 1 to 6, preferably 1 to 3,carbon atoms, R² is branched chain alkyl or cycloalkyl, such as from 3 to about 20 carbon atoms; and X is -O-,-O-<img file="EP0342299A1_D0002.tif" />- or -<img file="EP0342299A1_D0003.tif" />-O-. Preferably, R¹ and R² contain from 3 to about 12 carbon atoms. Suitably, Y is straight or branched chain alkylene of from 1 to about 6 carbon atoms and Z is a direct bond.

0015Embraced within formula (I) are the presently preferred esters and ethers: R⁵ - X - R⁶      (II) wherein R⁵ is tertiary butyl, neopentyl, 2-ethylhexyl or 3,5,5-trimethylhexyl, and R⁴ is isopropyl, tertiary butyl, 2-ethylhexyl, 3,5,5-trimethylhexyl or neopentyl.

0016The liquids used in the present invention are branched low polarity monoesters and monoethers in which the ester or ether linkage, as the case may be,is at least partially sterically hindered on both sides of the linkage with suitable groups, preferably branched alkyl of 3 to 12 carbon atoms. The aforesaid criteria of low polarity and hindrance of the linkage serve to reduce or eliminate any tendency of the liquid: (a) to desorb polymer from particles suspended in a light valve suspension or (b) in suspensions which employ a first polymer bonded to the suspended particles and a second polymer bonded to or associated with the first polymer, to weaken or destroy the bond or association between said first and second polymers. Any such desorption and/or weakening or de­struction of interpolymer bonds or associations will usually reduce the physical stability of the suspension and can often cause agglomeration and/or precipitation of the suspended particles.

0017The steric hindrance of the ester or ether also prevents oxidation reactions between the oxygen of the ester or ether and the particles, a problem that prior art esters and ethers frequently caused when polyiodide particles were suspended in the suspension.

0018Thus, the presence of the liquids used in the present invention in a light valve suspension does not chemically or physically degrade the quality of the sus­pension, as prior art liquids often did.

0019The liquids used in the present invention can have a wide range of boiling points and freezing points. However, the boiling point is generally above 150°C and preferably above 175°C. The freezing point is usually below 22°C and preferably below -40°C.

0020The viscosity of the liquids of the present invention can likewise vary substantially but where fast responding suspensions are desired, should preferably be 25 cps or less at room temperature.

0021When a liquid of the present invention is used in a light valve suspension which employs only one type of polymer, it should preferably be a solvent for said polymer. When used in a light valve suspension which employs two polymers as described above, it should prefer­ably be a solvent for the second polymer.

0022Examples of liquid esters of the present in­vention include 3,5,5-trimethylhexyl isobutyrate, 3,5,5-trimethylhexyl neopentanoate and neopentyl neo­pentanoate. Electrical resistivities of these three liquids are respectively about 3.8 x 10¹¹ ohm-cm (undistilled), 1.2 x 10¹² ohm-cm (distilled), and 2.8 x 10¹¹ ohm-cm (distilled).

0023Examples of liquid ethers of the present invention include di-2-ethylhexyl ether, di-3,5,5-trimethylhexyl ether and dineopentyl ether. Non-symmetrical branched ethers are also useable.

0024Where low viscosity is especially important, the ethers have an advantage over the corresponding esters. For example, it is known from the prior art that the viscosity of benzyl benzoate at 25°C is 8.292 cps whereas the viscosity of dibenzyl ether at 20°C is 5.333 cps. Also, the viscosity of ethyl acetate at 15°C is 0.473 cps whereas the viscosity of diethyl ether at 15°C is 0.247 cps. Assuming that this well-known relationship remains valid for the branched esters and ethers of the present invention, one may predict a lower viscosity for a branched ether than for the corresponding ester. A reason why this may be true is that in general an ester linkage is relatively inflexible compared to an ether linkage.

0025The use of the liquids of the present invention in suspensions for light valves is described in the following Examples:

EXAMPLE 1

Formulation For Making A Paste Of Crystals Of Dihydrocinchonidine Sulfate Periodide (DCSI

)

0026-A typical formulation which can be used to prepare colloidal particles of dihydrocinchonidine sulfate periodide is as follows:

Solution A

0027A solution consisting of: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">0.65 gm.</entry><entry namest="col2" nameend="col2" align="left">cesium iodide (CsI)</entry></row><row><entry namest="col1" nameend="col1" align="right">2.48 gm.</entry><entry namest="col2" nameend="col2" align="left">dihydrocinchonidine sulfate (DCS)</entry></row><row><entry namest="col1" nameend="col1" align="right">10.00 gm.</entry><entry namest="col2" nameend="col2" align="left">2-ethoxyethanol</entry></row><row><entry namest="col1" nameend="col1" align="right">5.00 gm.</entry><entry namest="col2" nameend="col2" align="left">H₂O</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">3.00 gm.</entry><entry namest="col2" nameend="col2" align="left">methanol</entry></row></tbody></tgroup></table></tables> is combined with <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="right">22.00 gm.</entry><entry namest="col2" nameend="col2" align="left">of a 33 1/3 solution of nitrocellulose in 2-ethyoxyethanol. The nitrocellulose should be a mixture of low viscosity (18.6 cps) and high viscosity (15 seconds) types, 50% each.</entry></row></tbody></tgroup></table></tables>

Solution B

0028<tables id="tabl0003" num="0003"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">3.00 gm.</entry><entry namest="col2" nameend="col2" align="left">chloroform</entry></row><row><entry namest="col1" nameend="col1" align="right">10.00 gm.</entry><entry namest="col2" nameend="col2" align="left">n-propanol</entry></row><row><entry namest="col1" nameend="col1" align="right">1.38 gm.</entry><entry namest="col2" nameend="col2" align="left">I₂</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">22.00 gm.</entry><entry namest="col2" nameend="col2" align="left">Tricresylphosphate</entry></row></tbody></tgroup></table></tables> Shake well for 15 minutes. Pour solution A into a plastic blender cup and add Solution B while blender is at high speed. In 3-5 minutes product will form having a deep blue color in a gel-like wet paste. The resulting wet paste should be spread on a glass plate at an 8 mil thickness and allowed to dry at room tempera­ture for at least one hour.

0029The unsulfated starting material for the afore­said reaction, dihydrocinchonidine alkaloid (sometimes called hydrocinchonidine) is commercially available. The alkaloid material is then sulfated by dissolving it in a mixture of 1.5 parts of methanol and 0.32 parts of 97% H₂SO₄ and then evaporating to dryness. The resulting dihydro­cinchonidine sulfate is in the form of a light tan colored powder.

Procedure for Processing a DCSI Paste

00301. In a suitable jar, place the dried paste which has been prepared in accordance with the method described in Example 1. Add isopentyl acetate (IPA) and hexanes so that the final percentage totals will be: <tables id="tabl0004" num="0004"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Paste</entry><entry namest="col2" nameend="col2" align="right">11%</entry></row><row><entry namest="col1" nameend="col1" align="left">IPA</entry><entry namest="col2" nameend="col2" align="right">59%</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Hexanes</entry><entry namest="col2" nameend="col2" align="right">30%</entry></row></tbody></tgroup></table></tables> Shake the above mixture for about 15 minutes and then place the jar into an ultrasonic generator for 10 hours. 2. After ultrasonicing, centrifuge the jar con­tents for eight hours at 2,500 RPM and discard the super­natant. 3. Spread the sediments from the centrifuge tubes onto a glass plate and record the weight. Hereafter these sediments will be called SMP (Solvent Moist Paste). 4. Add to the SMP one-eighth (1/8) the amount of SMP by weight of dioctyl adipate (DOA), and dry off at room temperature the remaining IPA in the SMP. 5. Grind the paste materials for 20 to 30 seconds. Collect the ground paste material and add enough IPA to form a suspension. This suspension is then rapidly stirred for one hour. After stirring,ultrasonically agitate for another ten hours. 6. Centrifuge the ultrasoniced material for one-­half (1/2) hour and save the supernatant. 7. Centrifuge the supernatant material for ten hours and pour its supernatant off, leaving only the sediment. The sediment (or SMP) is then resuspended one part SMP and four parts isopentyl acetate. 8. After thoroughly mixing, ultrasonic sus­pension for ten hours. 9. Add to the above suspension enough dioctyl adipate so that for each 2.6 gms. of solid paste material, there will be 7.4 gms. of DOA. The isopentyl acetate should now be vacuumed off leaving only a 26% paste/DOA concentrate. This concentrate is now ready to be dispersed in the final suspending medium.

Dispersion Into The Final Suspending Medium

003110. A copolymer consisting of 97% neopentyl acrylate/­3% methylol acrylamide (NPA-MOAM), by weight, and having a molecular weight of approximately 25,000 is dissolved in dioctyl adipate, neopentyl neopentanoate or type 0.8 Halo­carbon Oil in the following weight percentages to form three solutions: <tables id="tabl0005" num="0005"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Dioctyl adipate/NPA-MOAM -</entry><entry namest="col2" nameend="col2" align="right">50/50%</entry></row><row><entry namest="col1" nameend="col1" align="left">Neopentyl neopentanoate/NPA-MOAM -</entry><entry namest="col2" nameend="col2" align="right">75/25%</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Type 0.8 Halocarbon Oil/NPA-MOAM -</entry><entry namest="col2" nameend="col2" align="right">85/15%</entry></row></tbody></tgroup></table></tables> To reduce their viscosity, the above solutions are ultrasoniced for at least 50 hours. 11. The dilution factor for a concentrate depends upon the cell spacing and whether the material is to be used in a transmissive, transflective or reflective mode. I have found that much less copolymer is needed to stabilize a suspension which has a relatively high percentage of paste, and that preferably such paste percentage should be at least 2.6%. To take advantage of this information and to also avoid too dark an off-state for the suspension, as well as to minimize the voltage needed to activate the suspension in a light valve, the cell spacing should preferably be 2-mils or less. 12. Using the DOA/NPA-MOAM 50/50% solution, add enough to the concentrate so that the final dilution re­sult will show 2.6% paste and 10% for DOA. Be sure to mix well. To this is added the 75/25% Neopentyl neo­pentanoate /NPA-MOAM so that the final dilution result will show 10% for neopentyl neopentanoate. Mix well. The 15/85% Halocarbon Oil/NPA-MOAM solution should now be added and mixed in well. The amount of this solution is determined by the total percentage of copolymer needed in the final suspension. Halocarbon Oil alone is now added to bring the total percentage up to 100%. The final suspension is now ultrasoniced for at least one hour. <tables id="tabl0006" num="0006"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><thead valign="top"><row><entry namest="col1" nameend="col2" align="center"><u style="single">Final Suspension Percentages For A Cell Having A Distance Of 2-mils Between Its Electrodes</u></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Paste -</entry><entry namest="col2" nameend="col2" align="char" char=".">2.6%</entry></row><row><entry namest="col1" nameend="col1" align="left">NPA-MOAM Polymer -</entry><entry namest="col2" nameend="col2" align="char" char=".">15.0</entry></row><row><entry namest="col1" nameend="col1" align="left">DOA -</entry><entry namest="col2" nameend="col2" align="char" char=".">10.0</entry></row><row><entry namest="col1" nameend="col1" align="left">Neopentyl neopentanoate -</entry><entry namest="col2" nameend="col2" align="char" char=".">10.0</entry></row><row><entry namest="col1" nameend="col1" align="left">0.8 Halocarbon Oil -</entry><entry namest="col2" nameend="col2" align="char" char="."><u style="single">62.4</u></entry></row><row rowsep="1"><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="char" char=".">100.0%</entry></row></tbody></tgroup></table></tables> Type 0.8 Halocarbon Oil is described in Saxe, U.S. Patent 4,407,565.

EXAMPLES 2 - 7

0032Example 1 is repeated in each case except that the neopentyl neopentanoate is replaced: (a) in Example 2 with 3,5,5-trimethylhexyl isobutyrate (b) in Example 3 with 3,5,5-trimethylhexyl neopentanoate (c) in Example 4 with dineopentyl ether (d) in Example 5 with di-3,5,5-trimethylhexyl ether; (e) in Example 6 with di-2-ethylhexyl ether, and (f) in Example 7 with isobutyl neopentanoate.

EXAMPLE 8

0033Liquid suspending media used in the present in­vention were compared to isopentyl acetate and isopentyl isobutyrate as follows.

0034The procedure of Example 1 was followed to prepare five suspensions of DCSI in the candidate liquid, with the following modifications. In step 9, the amount of dioctyl adipate was 7.0 grams. In step 10, the NPA-MOAM copolymer was added to the candidate liquid suspending medium and in step 12 the concentrate from step 9 is mixed with the NPA-MOAM copolymer/candidate liquid. No halocarbon oil was used in order to provide a direct comparison between the five suspending liquids tested. The final suspensions contained: <tables id="tabl0007" num="0007"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="center"><u style="single">PERCENT</u></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">DCSI paste</entry><entry namest="col2" nameend="col2" align="char" char=".">2.6</entry></row><row><entry namest="col1" nameend="col1" align="left">NPA-MOAM Polymer</entry><entry namest="col2" nameend="col2" align="char" char=".">12.0</entry></row><row><entry namest="col1" nameend="col1" align="left">DOA</entry><entry namest="col2" nameend="col2" align="char" char=".">7.0</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Candidate liquid</entry><entry namest="col2" nameend="col2" align="char" char=".">78.4</entry></row></tbody></tgroup></table></tables>

0035The candidate liquids were neopentyl neopentanoate (Example 1), 3,5,5-trimethylhexyl isobutyrate (Example 2) and isobutyl neopentanoate (Example 7) according to the present invention, and isopentyl acetate and isopentyl isobutyrate according to Marks U.S. Patent 4,442,019.

0036Each of the final suspensions was subjected to accelerated aging by storage at 85°C for 66 hours. The properties of the suspensions were measured after storage. The results of the tests are reported below: <tables id="tabl0008" num="0008"><table frame="all"><tgroup cols="7" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="22.50mm" /><colspec colnum="2" colname="col2" colwidth="22.50mm" /><colspec colnum="3" colname="col3" colwidth="22.50mm" /><colspec colnum="4" colname="col4" colwidth="22.50mm" /><colspec colnum="5" colname="col5" colwidth="22.50mm" /><colspec colnum="6" colname="col6" colwidth="22.50mm" /><colspec colnum="7" colname="col7" colwidth="22.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col7" align="center"><u style="single">Properties Of Suspensions Before and After Storage At 85°C for 66 Hours</u></entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="center"><u style="single">Color of Suspension</u></entry><entry namest="col4" nameend="col5" align="center"><u style="single">Optical Density (Off)</u></entry><entry namest="col6" nameend="col7" align="center"><u style="single">Optical Density Ratio (OFF/ON)</u></entry></row><row><entry namest="col1" nameend="col1" align="center"><u style="single">Liquid Suspending Medium</u></entry><entry namest="col2" nameend="col2" align="center"><u style="single">Before</u></entry><entry namest="col3" nameend="col3" align="center"><u style="single">After</u></entry><entry namest="col4" nameend="col4" align="center"><u style="single">Before</u></entry><entry namest="col5" nameend="col5" align="center"><u style="single">After</u></entry><entry namest="col6" nameend="col6" align="center"><u style="single">Before</u></entry><entry namest="col7" nameend="col7" align="center"><u style="single">After</u></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Example 1</entry><entry namest="col2" nameend="col2" align="left">Blue</entry><entry namest="col3" nameend="col3" align="left">Blue</entry><entry namest="col4" nameend="col4" align="char" char=".">3.0</entry><entry namest="col5" nameend="col5" align="char" char=".">3.0</entry><entry namest="col6" nameend="col6" align="char" char=".">1.3</entry><entry namest="col7" nameend="col7" align="char" char=".">1.3</entry></row><row><entry namest="col1" nameend="col1" align="left">Example 2</entry><entry namest="col2" nameend="col2" align="left">Blue</entry><entry namest="col3" nameend="col3" align="left">Blue</entry><entry namest="col4" nameend="col4" align="char" char=".">3.3</entry><entry namest="col5" nameend="col5" align="char" char=".">3.1</entry><entry namest="col6" nameend="col6" align="char" char=".">1.3</entry><entry namest="col7" nameend="col7" align="char" char=".">1.2</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Example 7</entry><entry namest="col2" nameend="col2" align="left">Blue</entry><entry namest="col3" nameend="col3" align="left">Blue</entry><entry namest="col4" nameend="col4" align="char" char=".">2.4</entry><entry namest="col5" nameend="col5" align="char" char=".">2.4</entry><entry namest="col6" nameend="col6" align="char" char=".">1.25</entry><entry namest="col7" nameend="col7" align="char" char=".">1.2</entry></row><row><entry namest="col1" nameend="col1" align="left">Isopentyl acetate</entry><entry namest="col2" nameend="col2" align="left">Blue</entry><entry namest="col3" nameend="col3" align="left">Amber</entry><entry namest="col4" nameend="col4" align="char" char=".">2.0</entry><entry namest="col5" nameend="col5" align="char" char=".">&lt;0.6</entry><entry namest="col6" nameend="col6" align="char" char=".">1.3</entry><entry namest="col7" nameend="col7" align="char" char=".">1.0</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Isopentyl isobutyrate</entry><entry namest="col2" nameend="col2" align="left">Blue</entry><entry namest="col3" nameend="col3" align="left">Yellow</entry><entry namest="col4" nameend="col4" align="char" char=".">2.0</entry><entry namest="col5" nameend="col5" align="char" char=".">&lt;0.6</entry><entry namest="col6" nameend="col6" align="char" char=".">1.3</entry><entry namest="col7" nameend="col7" align="char" char=".">1.0</entry></row></tbody></tgroup></table></tables>

0037After storage, the prior art suspensions changed color, which indicates that degradation products had formed. However, the suspensions according to the invention did not degrade and showed no change in color.

0038The optical density of each suspension was measured at a field strength of approximately 11 volts/mil RMS (ON) and at no applied voltage (OFF), and the ratio of optical densities at the OFF and ON states was calculated.

0039All five suspensions functioned satisfactorily be­fore storage at 85°C for 66 hours, but after storage the two prior art suspensions had degraded, as seen by comparing their optical density and optical density ratio before and after storage. Indeed, an optical density ratio of 1.0, after storage, for the prior art suspensions means that there is no significant change in the optical density of the suspension between the ON and OFF states, and hence the prior art sus­pensions, after storage, had degraded to the point where they were inoperative as light valve suspensions.

0040In contrast, the suspensions prepared according to the present invention showed no significant change in their excellent optical densities or optical density ratios after storage. These data demonstrate the superior stability of the liquid suspending media used in the present invention.

0041While specific embodiments of my invention have been illustrated, it will be appreciated that the in­vention is not limited thereto, since many modifications may be made by one skilled in the art which fall within the true spirit and scope of the invention.