Electrophoretic particles and processes for the production thereof

Abstract

Polymer shells similar to those described in U.S. Pat. No. 6,822,782 can be formed on pigment particles by (a) physi-sorping a reagent comprising polymerizable groups on to the pigment particles by treating the particle with a reagent having a polymerizable or polymerization-initiating group, such that the reagent will not desorb from the particle surface when the particle is placed in a hydrocarbon medium; or (b) treating pigment particles bearing nucleophilic groups with a reagent having a polymerizable or polymerization-initiating group, and an electrophilic group, thus attaching the polymerizable or polymerization-initiating groups to the particle surface. The zeta potential of the pigment particles can be varied by a process similar to (b) but using a reagent which does not have a polymerizable or polymerization-initiating group.

Claims

1. A process for treatment of pigment particles, the process comprising physi-sorping a reagent comprising polymerizable groups on to the surfaces of the pigment particles by treating the particle with a solution of a reagent having a polymerizable or polymerization-initiating group, thereby causing the reagent to become physi-sorbed on to the particle surface such that the reagent will not desorb from the particle surface when the particle is placed in a hydrocarbon medium, wherein the pigment is negatively charged and the reagent contains a quaternary ammonium salt.

2. A process according to claim 1 wherein the pigment particle comprises an inorganic pigment.

3. A process according to claim 1 further comprising reacting the pigment particle with the reagent physi-sorbed thereon with at least one monomer or oligomer under conditions effective to cause reaction between the polymerizable or polymerization-initiating group on the particle and the at least one monomer or oligomer, thereby causing the formation of polymer on the particle.

4. A process according to claim 1 wherein the quaternary ammonium salt comprises at least one of [3-(methacryloyloxy)ethyl]trimethylammonium chloride, [3-(methacryloyloxy)-ethyl]trimethylammonium methyl sulfate, and [3-(methacryloylamino)-propyl]trimethylammonium chloride.

5. An electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field, wherein at least one of the charged particles is produced by the process of claim 1.

6. A process for treatment of pigment particles, the process comprising physi-sorping a reagent comprising polymerizable groups on to the surfaces of the pigment particles by treating the particle with a solution of a reagent having a polymerizable or polymerization-initiating group, thereby causing the reagent to become physi-sorbed on to the particle surface such that the reagent will not desorb from the particle surface when the particle is placed in a hydrocarbon medium, wherein the pigment is positively charged and the reagent contains an anionic functional group.

7. A process according to claim 6 wherein the reagent comprises at least one of 3-sulfopropyl methacrylate potassium salt and sodium 4-vinylbenzenesulfonate.

8. A process according to claim 6 wherein the pigment particle comprises an inorganic pigment.

9. A process according to claim 6 further comprising reacting the pigment particle with the reagent physi-sorbed thereon with at least one monomer or oligomer under conditions effective to cause reaction between the polymerizable or polymerization-initiating group on the particle and the at least one monomer or oligomer, thereby causing the formation of polymer on the particle.

10. An electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field, wherein at least one of the charged particles is produced by the process of claim 6.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 of the accompanying drawings is a graph showing the variation of zeta potential with amount of charge control agent in experiments described in Example 6 below.

(2) FIG. 2 is a reaction scheme of a passivation process of the present invention.

(3) FIG. 3 is a graph, generally similar to that of FIG. 1, showing the variation of zeta potential with amount of charge control agent in experiments described in Example 8 below.

DETAILED DESCRIPTION

(4) As indicated above, the present invention provides three different processes for treatment of pigment particles. Although these processes will mainly be described separately below, as already indicated, more than one process of the invention may be used in the synthesis of a single pigment particle; for example, a pigment particle which has been functionalized using either the first or second process of the invention may, before or after formation of polymer on the functionalized pigment, be treated with the third process of the invention.

(5) Part A: Absorption Process of the Invention

(6) As already mentioned, the absorption process of the present invention provides a process for treatment of pigment particles by physi-sorption of a reagent comprising polymerizable groups on to the surfaces of the pigment particles by treating the particle with a solution of a reagent having a polymerizable or polymerization-initiating group, thereby causing the reagent to become physi-sorbed on to the particle surface such that the reagent will not desorb from the particle surface when the particle is placed in a hydrocarbon medium. The absorption process avoids the vacuum drying step which, as previously mentioned, is normally required when pigment particles are functionalized with silanes, and provides a different surface chemistry which may lead to the ability to obtain pigments in zeta potential ranges which cannot be achieved by the processes described in the aforementioned U.S. Pat. No. 6,822,782. Elimination of the vacuum drying step is expected to improve the dispersion of the pigment at the polymerization stage and thereby reduce large aggregates in the resulting polymer-coated product. Experimentally it has been found that when the absorption process of the present invention is applied to a commercial silica/alumina coated titania, the proportion of polymer in the final polymer-coated pigment (as measured by thermogravimetric analysis) is similar to that is achieved after a silane functionalization, but the final polymer-coated pigment charges positively in electrophoretic media. This perhaps might be expected for cationic forms of the absorption process, but somewhat surprisingly has been found to be the case also for anionic forms.

(7) The following Examples are now given, though by way of illustration only, to show details of particularly preferred reagents, conditions and techniques used in the absorption process of the present invention.

Example 1: Surface Modification Using Anionic Modifying Agents

(8) The pigment being treated was dispersed at approximately 25% (by weight) in ethanol, and dilute hydrochloric acid was added to ensure the solution was well below the isoelectric point of the pigment. 3-Sulfopropyl methacrylate potassium salt (SPMK) (about 50-100 mg for each gram of pigment) was dissolved in water and added to the pigment dispersion. The solution is allowed to mix for several hours, then centrifuged and the solid residue washed twice with ethanol. The resulting pigment may be allowed to dry (this is not essential) and then dispersed into toluene for polymerization substantially as described in Example 28 of the aforementioned U.S. Pat. No. 6,822,782.

Example 2: Surface Modification Using Cationic Modifying Agents

(9) The pigment being treated was dispersed at approximately 25% (by weight) in ethanol, and aqueous ammonia was added to ensure the solution was well above the isoelectric point of the pigment. [3-(Methacryloyloxy)ethyl]trimethylammonium chloride (MAETAC) (about 50-100 mg for each gram of pigment) was dissolved in water and added to the pigment dispersion. The solution is allowed to mix for several hours, then centrifuged and the solid residue washed twice with ethanol. The resulting pigment may be allowed to dry (this is not essential) and then dispersed into toluene for polymerization substantially as described in Example 28 of the aforementioned U.S. Pat. No. 6,822,782. If the pigment was not dried, a solvent switch procedure can be effected to yield a pigment dispersion in toluene.

Example 3: Analysis of Various Pigments Produced by the Absorption Process

(10) Various pigments were treated by the absorption process of the invention with either SPMK or MAETAC as described in Examples 1 and 2 above, then washed, dispersed in toluene and a coating of poly(lauryl methacrylate) substantially as described in Example 28 of the aforementioned U.S. Pat. No. 6,822,782. The resultant polymer-coated pigments were tested by thermogravimetric analysis and suspended in Isopar E (a commercial hydrocarbon solvent) with the addition of 25 mg per gram of pigment of Solsperse 17K (a charge control agent), and their zeta potentials measured. The TGA values of the raw pigment, the functionalized pigment and the polymer-coated pigment are shown in Table 1 below.

(11) TABLE-US-00001 TABLE 1 Surface Notebook # Pigment Functionalization TGA.sub.raw TGA.sub.sf TGA.sub.poly (mV) 662-75-A Dupont R-794 MAETAC 1.187% 1.56% 4.95% +65.5 662-74-A Dupont R-794 SPMK 1.187% 1.55% 6.78% +46.3 662-60 Shepherd Blue 385 MAETAC 0.623% 2.01% 5.61% +85.8 662-72 Shepherd Blue 385 SPMK 0.623% 1.46% 13.22% +47.1 662-77 Shepherd Blue 300591 MAETAC 0.476% 1.87% 9.78% +80.3 662-76 Shepherd Blue 300591 SPMK 0.476% 0.75% 9.03% +14.7 662-61 Shepherd Green 260 MAETAC 0.07% 0.36% 1.44% +47

(12) From the foregoing, it will be seen that functionalization of the pigments by the absorption process of the present invention produced final pigments have satisfactory amounts of polymer and good positive zeta potentials.

(13) From the foregoing, it will be seen that the absorption process of the present invention can provide a simplification of the prior art silane functionalization process with elimination of a drying step. The process may also result better reproducibility in terms of the average particle size of the dispersion of functionalized pigment for the polymerization stage. Both effects would be potential cost savers, the former through process simplification, the latter by a potential yield increase.

(14) Part B: Nucleophilic Process of the Invention

(15) As already mentioned, the nucleophilic process of the present invention provides a process for treatment of pigment particles (which may be organic or inorganic pigment particles) bearing nucleophilic groups on their surfaces by treatment of the pigment particles by with a reagent having a polymerizable or polymerization-initiating group, and also comprising at least one electrophilic group. The electrophilic groups on the reagent react with the nucleophilic groups on the particle surfaces, thus attaching the polymerizable or polymerization-initiating groups to the particle surface.

(16) The nucleophilic process of the present invention can produce organic pigment particles which are readily dispersed in the hydrocarbon fluids typically used in electrophoretic media. The process can also produce organic and inorganic pigments having zeta potentials which are substantially independent of the charge control agents used, and this constant zeta potential may contribute to improved optical states in electrophoretic displays.

(17) The following Examples are now given, though by way of illustration only, to show details of particularly preferred reagents, conditions and techniques used in the nucleophilic process of the present invention.

Example 4: Nucleophilic Process Using Dimethyl Quinacridone

(18) Dimethyl quinacridone (Ink Jet Magenta E 02, 15 g) and toluene (135 g) were mixed and subjected to a high performance disperser for 1 minute. The resultant dispersion was transferred to a round-bottomed flask equipped with a magnetic stir bar and the flask was placed into a preheated 42 C. silicone oil bath and placed under a nitrogen atmosphere. Triethylamine (12 mL, 86 mmole,) was added; after 1 hour 4-vinylbenzyl chloride (VBC, 5.0 mL, 36 mmole) was added by syringe in a single addition. The reaction mixture was then allowed to stir at 42 C. under a nitrogen atmosphere overnight.

(19) The reaction mixture was poured into a plastic centrifuge bottle, diluted with toluene and centrifuged. The supernatant was decanted, the pigment washed with toluene, and the mixture again centrifuged. The washing procedure was repeated, then the supernatant was decanted and the treated pigment dried in a 70 C. vacuum oven overnight.

Example 5: Polymer Coating of Pigment Produced by Nucleophilic Process

(20) The dried pigment from Example 4 above was ground with a mortar and pestle. A sample was removed for TGA and the remaining pigment was dispersed in toluene (10 wt % pigment dispersion) with sonication and rolling. The resultant pigment dispersion was transferred to a round-bottomed flask equipped with a magnetic stir bar and the flask was placed into a preheated 65 C. silicone oil bath. Lauryl methacrylate (20 g) was added to the reaction mixture, a Vigreux distilling column was attached as an air condenser, and the flask was purged with nitrogen for at least 1 hour. A solution of 2,2-azobis(2-methylpropionitrile) (AIBN) in toluene (0.20 g AIBN in 5 mL toluene) was syringed into the reaction flask all at once, and the reaction mixture was stirred vigorously at 65 C. overnight.

(21) The reaction mixture was then poured into a centrifuge bottle, diluted with toluene and centrifuged for 30 minutes; the supernatant was decanted and submitted for GPC analysis. The pigment was washed once with toluene and centrifuged for 30 minutes, then the supernatant was decanted and the pigment was dried in a 70 C. vacuum oven overnight.

Example 6: Testing of Polymer Coated Pigment

(22) The polymer-coated pigment produced in Example 5 above was ground with a mortar and pestle, and it was dispersed to form a 20 wt % dispersion in Isopar E. This dispersion was sonicated and rolled for at least 24 hours, then filtered through a fabric mesh to remove any large particles. A sample of the dispersion was removed and its percent solid measured, and the dry pigment from that measurement was submitted for TGA and density by pycnometer. The TGA value for the treated pigment was 3.5%, whereas the raw pigment had a TGA value of 2.1%. The remaining dispersion was used to make 25 g of a 5% pigment dispersion with 0.5 g Solsperse 17000/g coated pigment for zeta potential measurement.

(23) Samples of the dispersion thus prepared were mixed with varying amounts of Aluminum tris[3,5-di-tert-butylsalicylate], available commercially as Bontron E88, a very acidic charge control agent, and the zeta potential of the pigment measured. To provide controls, samples of the raw pigment (without the treatment with VBC and subsequent polymerization were similarly. The results are shown in FIG. 1 of the accompanying drawings. From the data in FIG. 1, it will be seen that the raw pigment exhibits a sharp rise in zeta potential with concentration of the acidic charge control agent, whereas the zero potential of the pigment produced by the nucleophilic process of the present invention is substantially insensitive to concentration of charge control agent. Interestingly, the zeta potential of the pigment of the invention in OLOA 371 (a very basic charge control agent) is essentially identical to that Solsperse, when both charge control agents were present in an amount of 0.5 g/g of pigment; that is, the zeta potential of the pigment of the present invention is essentially the same in the presence of either a basic (OLOA) or acidic (Solsperse 17k/Bontron) charge control agent. For comparison, a white titania pigment (prepared substantially as described in Example 28 of the aforementioned U.S. Pat. No. 6,822,782) dispersed with OLOA was much more negative than under similar conditions in the presence of Solsperse 17k, while the addition of small quantities of Bontron E88 had a dramatic effect on the zeta potential of this white pigment, moving it in the more positive direction.

Example 7: Additional Pigments Produced by Nucleophilic Process of the Invention

(24) Additional samples of Ink Jet Magenta E 02 and of other pigments were functionalized, polymer-coated and tested in the same manner as in Examples 4-6 above. The results are shown in Table 2 below, which also includes data for the raw pigments.

(25) TABLE-US-00002 TABLE 2 Zeta Average Amount potential particle Pigment CCA CCA (mV) size (nm) Inkjet Mag EO2 (invention) S17k 0.5 g/g 68.6 165 Inkjet Mag EO2 (invention) S17k 0.5 g/g 72.4 177 Inkjet Mag EO2 (invention) S17k 0.5 g/g 68.0 188 Inkjet Mag EO2 (invention) OLOA 0.5 g/g 84.5 203 Inkjet Mag EO2 (invention) OLOA 0.5 g/g 75.2 193 Inkjet Mag EO2 Raw S17k 0.5 g/g 29.8 194 (control) Inkjet Yellow 4GC (control) S17k 0.53 g/g 57.2 210 Inkjet Yellow 4GC S17k 0.54 g/g 10.3 100 (invention) Inkjet Yellow H4G S17k 0.54 g/g 27.5 230 (control) Inkjet Yellow H4G S17k 0.54 g/g 14.6 205 (invention) PV Fast Yellow HG01 S17k 0.55 g/g 1.9 250 (control) PV Fast Yellow HG01 S17k 0.54 g/g 10.9 162 (invention) Toner Yellow HG (control) S17k 0.54 g/g 8.1 234 Toner Yellow HG S17k 0.54 g/g 8.3 151 (invention)

(26) From the foregoing, it will be seen that the nucleophilic process of the present invention provides a process capable of functionalizing a wide variety of pigments to enable the formation of polymer coatings thereon; the process is especially useful for attachment of a polymer shell to organic pigments, which lack the silica or metal oxide surfaces common to many inorganic pigments and capable of reacting with silanes. The process is very simple and relies on well established chemistry in which the equilibrium strongly favors the coupled state so the yield of the nucleophilic reaction is essentially quantitative. The reagents used can be selected to be very reactive species which react readily with even weakly nucleophilic groups on the pigment particles. The nucleophilic groups on the pigments particles can be either part of the actual crystal structure of the pigment or can arise from an additive.

(27) The ability of the nucleophilic process of the present invention to make the zeta potential of the polymer-coated pigment essentially independent of the choice of charging agent (as demonstrated in the Examples above) provides great latitude in the development on new electrophoretic internal phases, and has been shown to be of potential advantage in terms of optical states accessible with typical driving voltages and pulse lengths.

(28) Part C: Passivation Process of the Invention

(29) As already mentioned, the passivation process of the present invention provides a process in which pigment particles bearing nucleophilic groups on their surfaces are treated with a reagent having an electrophilic group but not bearing a polymerizable or polymerization-initiating group so that a residue of the reagent is chemically bonded to the pigment particle. The reagent is chosen so that the treatment of the pigment particle therewith affects the zeta potential of the pigment particle. The preferred reagents are typically alkyl halides (a term which is used herein to include aralkyl halides), especially benzyl chloride.

(30) In prior art polymer-coated electrophoretic pigment particles which have been subjected to the silane/polymerization-treatment described above, in the final pigment particles the silane groups on the pigment particle surface having been found to be the dominant moieties for generating surface charge and zeta potential of the pigment. Thus, modification of pigment particle charging can be effected through incorporation of functional silanes and/or by incorporation of functional monomers in the polymer shell that change the inherent charge generation character of a pigment. However, varying the charging of the pigment particles by these two methods also affects other important pigment properties, such as dispersability in nonpolar solvents, polymer grafting density, and pigment dispersion viscosity. These highly interdependent properties make it difficult to vary a pigment's zeta potential without affecting other important properties. Accordingly, to allow optimization of various pigment properties independently of one another, it is desirable to synthesize a pigment with the appropriate silane and polymer shell and use post-polymerization modification of the pigment surface in accordance with the passivation process of the present invention in order to control zeta potential and thus electrophoretic mobility.

(31) As already noted, the preferred reagents for use in the passivation process of the present invention are alkyl halides, especially benzyl chloride, preferably in the presence of triethylamine, as illustrated in FIG. 2 of the accompanying drawings. Typically, the benzyl chloride should be used in an amount of about 30 molecules per square nanometer of pigment surface, which is estimated to be about a fivefold excess relative to the nucleophilic hydroxyl groups available on a typical metal oxide pigment surface; the extent of benzylation may be inferred through the shift in the maximum zeta potential of the pigment. Pigment zeta potential is typically determined in Isopar E with the charge control agent Solsperse 17000; the zeta potential shift increases with increasing reactivity for the alkylation reaction, and such increased reactivity can be achieved through increasing the reaction temperature or time, solvent polarity, and strength or hindrance of the non-nucleophilic base. Typical positive zeta shift magnitudes range from a net +10 to +60 mV. Recommended alkylation conditions for reaction with benzyl chloride are a slightly polar organic solvent (toluene or tetrahydrofuran) in the presence of a non-nucleophilic organic base (triethylamine or diisopropylethylamine) at about 66 C. The reaction can be performed immediately treatment of the raw pigment with silane or after polymer coating.

(32) For example, in one series of experiments a raw silica/alumina coated titania pigment (R794 sold by du Pont) was found to have a weight loss of 1.08% during TGA. After treatment with vinylbenzylchloride (VBC, an electrophile similar to benzyl chloride) in the presence of triethylamine, the weight loss increased to 1.23%, corresponding to the addition of 1.03 alkyl group per square nanometer of pigment surface. Subsequent polymerization of the VBC treated pigment with lauryl methacrylate increased weight loss to 3.90%. This subsequent grafting of polymer to the VBC-treated titania verifies the covalent surface-functionalization of the metal oxide.

(33) The passivation process of the present invention works through the modification of pigment zeta potential through covalent attachment of alkyl (or other) groups to the pigment surface. The attachment of benzyl groups to the surface of white titania pigments with inherently negative zeta potential values before modification serves to shift the zeta potential to a more positive value. The functionality attached to the electrophilic alkyl group will serve to determine the sign and magnitude of the zeta potential modification. Using 4-fluorobenzylchloride or 4-nitrobenzylchloride will tend to induce a more negative zeta potential through fluorinated and acidic surface-functionalization. Conversely, alkylation with basic alkyl groups like 4-(chloromethyl)pyridine or 4-(dimethylamino)benzoyl chloride would modify the zeta potential to more robust positive values. Finally, incorporation of tert-butyl benzyl chloride or long chain alkyl halides like 1-bromooctane could be useful to provide additional steric hindrance to exclude diffusion of molecules to the pigment surface.

(34) The following Examples are now given, though by way of illustration only, to show details of particularly preferred reagents, conditions and techniques used in the passivation process of the present invention.

Example 8: Passivation Process of the Invention Applied to Spinel-Based Black Pigment

(35) This Example reports the results of preliminary experiments in which a spinel-based polymer-coated black pigment was treated with benzyl chloride and benzyl bromide to test the hypothesis that amine groups present in the pigment could be quaternized and hence permanently positively charged, thus producing a pigment which is insensitive to the choice of charging agent.

(36) A polymer-coated black pigment based on Shepherd BK444 and produced substantially as described in Example 1 of U.S. Pat. No. 8,270,064 was treated with benzyl chloride in substantially the same manner as in Example 4 above except that the reaction was conducted in Isopar E. The polymer-coated black pigment (24 g), benzyl chloride (4 g), triethylamine (4.7 g) and Isopar E were mixed at room temperature for 24 hours. The resultant modified pigment was repeatedly centrifuged and washed with Isopar E. Dispersions of the pigment were made with Bontron E88 and variable amounts of OLOA 371, and their zeta potentials measured. To provide controls, similar dispersions of the untreated polymer-coated black pigment were prepared and their zeta potentials measured. The results are shown in FIG. 3.

(37) From FIG. 3 it will be seen that the alkylated pigment is much less sensitive to the presence of OLOA than the non-alkylated pigment and yields a nearly constant surface charge.

Example 9: Passivation Process of the Invention Applied to Titania-Based White Pigments

(38) Several titania-based white pigments were treated with benzyl chloride either after silane functionalization or after formation of polymer on the pigment. Both the raw and the treated pigments were tested by TGA, and their maximum zeta potentials were measured in Isopar-E. The results are shown in Table 3 below.

(39) The procedure used for treatment with benzyl chloride after formation of a polymer layer on the white pigment was as follows. The pigment (300 g) was added to a 1 L plastic bottle to which tetrahydrofuran (THF500 mL) was also added. The plastic bottle was rolled on a roll mill and then sonicated. The resultant dispersion was placed in a jacketed reactor equipped with a four-necked reactor top equipped with an overhead mechanical stirrer, a condenser capped with a nitrogen gas inlet, a thermometer or thermocouple and a septum. The dispersion was rinsed into the reactor with a small amount of THF and heated to reflux and stirred vigorously. The headspace of the reactor was purged with nitrogen and kept under positive pressure of nitrogen for the remaining stages of the reaction. Triethylamine was added by syringe to the reactor and the resultant mixture stirred for 30 minutes, then benzyl chloride was added by syringe to the reactor, and the resultant reaction mixture stirred overnight at reflux. To isolate the product, the reactor was drained into two 1 L centrifuge bottles and the dispersion diluted to 1000 g total solvent and then centrifuged. The supernatant was decanted and the pigment redispersed by rolling with 1000 g total THF for 90 minute on a roll mill, after which the pigment dispersion was again centrifuged and the supernatant decanted. The wet pigment pack was then dried at 70 C. overnight in a vacuum oven.

(40) TABLE-US-00003 TABLE 3 Pigment Zeta Potential No. Pigment Base Surface Treatment TGA/washTGA Max 1 Essentially as U.S. Pat. No. 6,822,782, None 1.98%/1.73% Nd Example 28, Part A 2 As Pigment 1 Benzyl chloride, 1.87%/1.81% Nd triethylamine 3 As Pigment 2 after benzyl chloride Poly(lauryl 9.22%/9.16% 3.4 mV treatment methacrylate) 4 As Pigment 1 (with process None 8.69%/7.87% 56 mV modifications) 5 As Pigment 4. BzCl, TEA 7.56%/7.91% 25 mV 6 Essentially as U.S. Pat. No. 8,582,196, None 8.93%/7.77% 91 mV Example 1 7 As Pigment 6 BzCl, TEA 7.43%/7.77% 52 mV 8 As Pigment 6 BzCl, TEA 7.63%/7.77% 11 mV

(41) Following the various surface modifications, Pigments 6 and 7 were converted into experimental single pixel displays as described in U.S. Pat. No. 8,582,196, Example 2 using the same spinel-based black pigment there described, and the resultant experimental displays were subjected to electro-optical tests as described in Example 3 of this patent. The results are shown in Table 4 below. Prior to the electro-optical tests, the single pixel displays were switched repeatedly to their extreme black and white states, then finally switched to black or white, and the L* value measured 3 seconds after the end of the final drive pulse to allow transient effects to dissipate. The image stability figures are measured by allowing the display to remain in a black or white extreme state for a dwell time of 10 seconds, driving it to its opposite optical state, measuring the L* value of this state immediately (20 milliseconds) after the end of the drive pulse and 30 seconds later, and taking the difference. The DSD (dwell state dependency) values are similarly measured by allowing the display to remain in a black or white extreme state for a dwell time of 20 seconds, driving it to its opposite optical state, measuring the L* value of this state immediately after the end of the drive pulse and 30 seconds later, and taking the difference.

(42) TABLE-US-00004 TABLE 4 Formulation White/Black WS DS WS DS W:K Ratio Pigment Pigment L* WS L* DS is is DSD DSD wt % wt % 6 (no surface 73.9 21.2 0.0 3.3 0.8 1.4 4.8:1 47.5% 39.25/8.2 treatment) 6 (no surface 75.1 22.3 0.0 4.0 0.7 2.7 6.1:1 47.5% 40.8/6.7 treatment) 6 (no surface 73.3 25.8 0.1 1.9 0.2 9.0 8.5:1 47.5% wt 42.5/5.0 treatment) 7 (BzCl surface 72.3 16.0 0.2 1.0 0.3 2.9 4.8:1 47.5% 39.25/8.2 treatment) 7 (BzCl surface 73.9 18.0 0.1 1.4 0.5 5.1 6.1:1 47.5% 40.8/6.7 treatment) 7 (BzCl surface 74.9 24.1 0.0 2.2 0.1 5.0 8.5:1 47.5% 42.5/5.0 treatment)

(43) From the data in Table 4, it will be seen that the benzyl chloride surface treatment did not significantly affect the white state of the displays (the change following the benzyl chloride treatment is not more than 1-2 L*) but did result in a substantial decrease (about 5 L*) in the L* value of the dark state; thus, the benzyl chloride treatment produced a useful increase (about 3-4 L*) in the dynamic range of the displays. Statistical analysis indicates no significant changes in image stability and DSD data between the untreated and the benzyl-treated pigments.

(44) From the foregoing, it will be seen that the passivation process of the present invention allows surface functionalization of pigment particles, and consequent modification of pigment zeta potential, through covalent attachment of alkyl groups to the pigment surface. Specifically, benzyl group attachment to the surface of white pigments with inherently negative zeta potential values before modification serves to shift their zeta potential to a more positive value. Although this is demonstrated above for titania-based white pigments, it may reasonably be assumed to apply to any inorganic pigment with a nucleophilic metal oxide surface. The sign and magnitude of the zeta potential modification may be controlled by functional groups attached with an electrophilic alkyl group. For example, 4-fluorobenzyl chloride and 4-nitrobenzyl chloride should induce a more negative zeta potential, since fluorinated and acidic surface-functionalization are documented to effect zeta potential modification of colloids in nonpolar liquids. Conversely, alkylation with basic alkyl groups such 4-(chloromethyl)pyridine or 4-(dimethylamino)benzoyl chloride would serve to modify the zeta potential to more positive values. Finally, treatment with tert-butyl benzyl chloride or long chain alkyl halides, such as 1-bromooctane may be useful to provide additional steric hindrance to exclude diffusion of molecules to the pigment surface.

(45) It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.