HK1225751B - Electrophoretic particles and processes for the production thereof - Google Patents

Description

Electrophoretic particles and preparation method thereof

This application relates to:

(a) U.S. Patent No. 6,822,782;

(b) U.S. Patent No. 7,411,720;

(c) U.S. Patent No. 8,582,196;

(d) U.S. Patent Application Publication No. 2009/0009852 (currently abandoned); and

(e) U.S. Patent Application Publication No. 2014/0340430.

The present invention relates to electrophoretic particles and methods for their preparation. More particularly, the present invention relates to surface modification of electrophoretic particles, especially for the purpose of controlling the charge on the electrophoretic particles when they are present with other components typically present in an electrophoretic medium.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to a display comprising display cells having first and second display states that differ in at least one optical property such that upon driving any given cell with an addressing pulse of finite duration, it assumes either its first or second display state, which state persists after termination of the addressing pulse for at least several times, e.g., at least four times, the minimum duration of the addressing pulse required to change the state of the display cell. U.S. Patent No. 7,170,670 shows that some particle-based electrophoretic displays capable of grayscale are stable not only in their extreme black and white states but also in intermediate gray states, and some other types of electro-optical displays are similarly stable. This type of display is properly referred to as "multistable" rather than bistable, but for convenience, the term "bistable" may be used herein to cover both bistable and multistable displays.

Electrophoretic displays (EPDs) have been the subject of intensive research and development for several years. Compared to LCDs, EPDs offer the following attributes: good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, issues related to the long-term image quality of these displays have hindered their widespread adoption. For example, the particles that make up EPDs tend to settle, resulting in a limited lifetime for these displays.

Electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but electrophoretic media can be prepared using a gaseous fluid; see, for example, Kitamura, T. et al., "Elec trical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y. et al., "Toner display using insulative part icles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4. See also U.S. Patents Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be prone to the same types of problems as liquid-based electrophoretic media due to particle settling when the media is used in an orientation that allows such settling, such as in signs where the media is set in a vertical plane. In fact, particle sedimentation appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media, since the lower viscosity of the gaseous suspending fluid compared to the liquid fluid allows for faster sedimentation of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies for encapsulated electrophoretic and other electro-optical media. Such encapsulated media include: a plurality of small capsules, each of which contains an internal phase comprising electrophoretically mobile particles in a fluid medium; and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymer binder to form a bonding layer between two electrodes. The technologies described in these patents and applications include:

(a) Electrophoretic particles, fluids, and fluid additives; see, e.g., U.S. Pat. Nos. 5,961,804, 6,017,584, 6,120,588, 6,120,839, 6,262,706, 6,262,833, 6,300,932, 6,323,989, 6,377,387, 6,515,649, 6,538,801, 6, No. 580,545, No. 6,652,075, No. 6,693,620, No. 6,721,083, No. 6,727,881, No. 6,822,782, No. 6,870,661, No. 7,002,728, No. 7,038,655, No. 7,170,670, No. 7,180,649, No. 7,230,750, No. 7,230,751, No. 7, No. 236,290, No. 7,247,379, No. 7,312,916, No. 7,375,875, No. 7,411,720, No. 7,532,388, No. 7,679,814, No. 7,746,544, No. 7,848,006, No. 7,903,319, No. 8,018,640, No. 8,115,729, No. 8,199,395, No. 8, Nos. 270,064 and 8,305,341, and U.S. Patent Application Publication Nos. 2005/0012980, 2008/0266245, 2009/0009852, 2009/0206499, 2009/0225398, 2010/0148385, 2010/0207073, and 2011/0012825;

(b) Capsules, binders, and encapsulation methods; see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719;

(c) Films and subassemblies comprising electro-optical materials, see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

(d) Backplanes, adhesive layers, and other auxiliary layers and methods for use in displays; see, e.g., U.S. Patent Nos. 7,116,318 and 7,535,624;

(e) color formation and color adjustment; see, e.g., U.S. Pat. Nos. 7,075,502 and 7,839,564;

(f) Methods for driving displays; see, for example, U.S. Patent Nos. 7,012,600 and 7,453,445;

(g) Display applications; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348.

Both encapsulated and unencapsulated electrophoretic media are known and can be divided into two main types, which for convenience will be referred to as "single particle" and "dual particle" respectively in the following text. A single particle medium has only a single type of electrophoretic particle suspended in a colored fluid, at least one optical characteristic of the colored fluid being different from the optical characteristic of the particles. (When referring to a single type of particle, we do not imply that all particles of that type are absolutely identical. For example, if all particles of that type have essentially the same optical characteristics and an electric charge of the same polarity, considerable variations in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) The optical characteristic is typically a color visible to the human eye, alternatively or in addition, any one or more of: reflectivity, retroreflectivity, luminescence, fluorescence, phosphorescence or color in a broader sense meaning differences in absorption or reflectance at non-visible wavelengths. When such a medium is placed between a pair of electrodes, at least one of which is transparent, the medium can display the optical characteristics of the particle (when the particle is adjacent to the electrode closer to the observer, referred to as the "front" electrode) or the optical characteristics of the fluid (when the particle is adjacent to the electrode farther from the observer, referred to as the "back" electrode, so that the particle is hidden by the colored suspension medium) depending on the relative potential of the two electrodes.

A two-particle medium comprises two different types of particles that differ in at least one optical characteristic, and a fluid that can be uncolored or colored, but is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility can be in polarity (this type may be referred to below as an "oppositely charged two-particle" medium) and/or in magnitude. When such a two-particle medium is placed between the aforementioned pair of electrodes, the medium can exhibit the optical characteristics of either set of particles, depending on the relative potentials of the two electrodes, but the exact manner in which this is achieved depends on whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particle is black and the other type is white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles are negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the back electrode, the black particles will be attracted to the front electrode and the white particles will be attracted to the back electrode, so that the medium will appear black to an observer. Conversely, if the front electrode is positive relative to the back electrode, the white particles will be attracted to the front electrode and the black particles will be attracted to the back electrode, so that the medium will appear white to an observer.

However, if the two types of particles have charges of the same polarity but different electrophoretic mobilities (this type of medium may be referred to hereinafter as "same polarity two-particle" medium), the two types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer is different depending on the electrode to which the particles are attracted. For example, suppose the previous description is changed so that both the black particles and the white particles are positively charged, but the black particles have a higher electrophoretic mobility. If the front electrode is now negative relative to the back electrode, both the black particles and the white particles will be attracted to the front electrode, but the black particles will reach the front electrode first due to their higher mobility, so that a layer of black particles will cover the front electrode, and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the back electrode, both the black and white particles will be attracted to the back electrode, but the black particles, due to their higher mobility, will reach the back electrode first, so that a layer of black particles will cover the back electrode, leaving a layer of white particles away from the back electrode and towards the viewer, so that the medium will appear white to the viewer: Note that this type of two-particle medium requires the suspending fluid to be sufficiently transparent to allow this layer of white particles away from the back electrode to be easily visible to the viewer. Typically, the suspending fluid in such displays is not colored at all, but some color may be added for the purpose of correcting any undesirable tint in the white particles seen through it.

Both single particle electrophoretic displays and dual particle electrophoretic displays may be capable of an intermediate grey state having optical characteristics between the two extreme optical states already described.

Some of the aforementioned patents and published applications disclose encapsulated electrophoretic media having three or more different types of particles within each capsule.For the purposes of this application, such multi-particle media are considered a subspecies of two-particle media.

Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete electrophoretic fluid droplets and a continuous phase of polymer material, and that the discrete electrophoretic fluid droplets within such a polymer-dispersed electrophoretic display can be considered capsules or microcapsules, even though there is no discrete capsule membrane associated with each individual droplet; see, for example, U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, the charged particles and fluid are not encapsulated in microcapsules, but rather are retained within multiple cavities formed within a carrier medium, typically a polymer film. See, for example, U.S. Patents Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be prepared to operate in a so-called "shutter mode," in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Patents Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346. Electrophoretic media operating in a shutter mode can be used in a multilayer structure for a full-color display; in such a structure, at least one layer adjacent to a viewing surface of the display operates in a shutter mode to expose or conceal a second layer further from the viewing surface.

Encapsulated electrophoretic displays generally do not have the disadvantages of clustering and resolving failure modes of conventional electrophoretic devices, and offer additional advantages, such as the ability to print or coat the display on a variety of flexible and rigid substrates. (The use of the word "printing" is intended to include all forms of printing and coating, including but not limited to: pre-metered coating, such as patch die coating, slot or extrusion coating, slide or waterfall coating, curtain coating; roll coating, such as knife roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; screen printing process; electrostatic printing process; thermal printing process; inkjet printing process; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar technologies.) Therefore, the resulting display can be flexible. In addition, because the display medium can be printed (using various methods), the display itself can be prepared inexpensively. However, the service life of encapsulated electrophoretic displays of both the single particle type and the dual particle type is still not entirely satisfactory. This useful life appears to be limited by factors (although the invention is in no way limited by any theory as to these matters), such as adhesion of the electrophoretic particles to the capsule walls, and the tendency of the particles to aggregate into clusters, which prevent the particles from completing the movement necessary to switch the display between its optical states. In this regard, oppositely charged two-particle electrophoretic displays present a particularly difficult problem, since the intrinsically oppositely charged particles in close proximity will be electrostatically attracted to each other and will exhibit a strong tendency to form stable aggregates. Experimentally, it has been found that if attempts are made to prepare black/white encapsulated displays of this type using untreated commercially available titanium dioxide and carbon black pigments, the displays either fail to switch at all or have such a short useful life as to be undesirable for commercial use.

As discussed in detail in the aforementioned US Patent No. 6,822,782, it has long been known that the physical properties and surface characteristics of electrophoretic particles can be altered by adsorbing various materials onto the particle surfaces or chemically binding various materials to these surfaces.

The aforementioned U.S. Patent No. 6,822,782 teaches that the stability of an electrophoretic medium can be improved by using pigment particles having from about 1 to about 15% by weight, based on the weight of the pigment, of a polymer chemically bound to or cross-linked around the pigment particles as electrophoretic particles. These polymer-coated pigment particles are prepared by a process comprising: (a) reacting the particles with a reagent having a functional group capable of reacting with and binding to the particles and further having a polymerizable group or a polymerization-initiating group, thereby reacting the functional group with the particle surface and attaching the polymerizable group to the particle surface; and (b) reacting the product of step (a) with at least one monomer or oligomer under conditions effective to cause a reaction between the polymerizable group or polymerization-initiating group on the particles and at least one monomer or oligomer, thereby forming a polymer bound to the particles. Preferred reagents for step (a) of the process (the so-called "surface functionalization" step) are silanes, especially trialkoxysilanes with attached polymerizable groups (e.g., 3-(trimethoxysilyl)propyl methacrylate; and N-[3-(trimethoxysilyl)propyl]-N'-(4-vinylbenzyl)ethylenediamine hydrochloride). In practice, processes using trialkoxysilanes require a vacuum drying stage to concentrate the silanols formed on the particle surface and fully anchor the polymer shell to the bare pigment surface. This process works very well, but can produce pigments that are difficult to disperse to a consistent particle size after surface functionalization and before the polymerization step. Furthermore, the process cannot be used with certain organic pigments (the process relies on the presence of hydroxyl groups on the particle surface, or similar groups present on nearly all inorganic pigments), and such organic pigments may be required in full-color electrophoretic displays. Furthermore, it has been found difficult to adjust the zeta potential of the coated pigment particles prepared by this method; the zeta potential of the coated pigment particles tends to be independent of the type of charging agent used in the electrophoretic medium, and the ability to control the zeta potential is important in fixing the optimal optical state from the electrophoretic medium.

The present invention therefore seeks to provide an alternative process for the surface modification of pigments which overcomes the aforementioned disadvantages of the silane-based process described in the aforementioned US Patent No. 6,822,782.

Thus, in one aspect, the present invention provides a (first) method for treating pigment particles, particularly inorganic pigment particles, by treating the particles with a solution of a reagent having a polymerizable group or a polymerization-initiating group, thereby physically adsorbing the reagent containing the polymerizable group onto the surface of the pigment particles, thereby causing the reagent to become physically adsorbed onto the particle surface so that when the particles are placed in a hydrocarbon medium, the reagent will not desorb from the particle surface. The method may also include reacting the pigment particles having the reagent physically adsorbed thereon with at least one monomer or oligomer under conditions effective to cause a reaction between the polymerizable group or polymerization-initiating group on the particles and at least one monomer or oligomer, thereby causing a polymer to form on the particles. This method may hereinafter be referred to as the "absorption" method of the present invention.

In one form of this first method, the reagent can be dissolved in an ionic solvent or solvent mixture, such as a water/ethanol mixture. The method can include adjusting the pH of the reagent solvent to control the charge on the pigment particles, and selecting the reagent for physical adsorption based on the charge on the particles. As is well known to those skilled in the art of pigment chemistry, the charge on many pigment particles in ionic liquids depends on the pH of the liquid, and there is generally a pH (isoelectric point) at which the particles are uncharged. Above the isoelectric point of the pigment, the pigment is negatively charged, and the reagent can contain quaternary ammonium groups (largely in the manner of preparing organoclays), while below the isoelectric point, the positively charged pigment surface can be modified by adsorption of reagents containing anionic functional groups. Reagents with quaternary ammonium groups include: [3-(methacryloyloxy)ethyl]trimethylammonium chloride (MAETAC), [3-(methacryloyloxy)ethyl]trimethylammonium methyl sulfate, and [3-(methacryloylamino)propyl]trimethylammonium chloride. Reagents with anionic functional groups include potassium 3-sulfonatepropyl methacrylate (SPMK) and sodium 4-vinylbenzenesulfonate. The method is not limited to pigments with inorganic oxide surfaces and can, in principle, be extended to any pigment that can be sufficiently charged to promote adsorption of the functional reagent. The reagent that physically adsorbs to the surface of the pigment particles should desirably be selected so that it becomes essentially irreversibly bound to the particles in the low dielectric constant solvent (typically an aliphatic hydrocarbon) used in the polymerization step and the internal phase of the final electrophoretic display.

In another aspect, the present invention provides a (second) method for treating pigment particles (which can be organic or inorganic pigment particles) having nucleophilic groups (e.g., amine groups) on their surfaces by treating the pigment particles with a reagent having a polymerizable group or polymerization-initiating group and further comprising at least one electrophilic group. The electrophilic group on the reagent reacts with the nucleophilic group on the particle surface, thereby attaching the polymerizable group or polymerization-initiating group to the particle surface. The method can also include reacting the pigment particles having the polymerizable group or polymerization-initiating group thereon with at least one monomer or oligomer under conditions effective to cause a reaction between the polymerizable group or polymerization-initiating group on the particle and at least one monomer or oligomer, thereby resulting in the formation of a polymer on the particle. This method may hereinafter be referred to as the "nucleophilic" method of the present invention.

The second method of the present invention does not require strong nucleophilic groups on the pigment particles and can be used with organic pigments containing nitrogen-containing rings, even when the nitrogen is not strongly basic. For example, the second method is used with quinacridone dyes such as dimethylquinacridone (IUPAC: 5,12-dihydro-3,10-dimethyl-quinolino[2,3-b]acridine-7,14-dione).

Reagents used in this method include acyl halides such as 4-vinylbenzyl chloride, methacryloyl chloride, and 2-isocyanatoethyl methacrylate. Benzyl halides and acyl chlorides are very reactive toward nucleophilic substitution reactions. For example, a coupling reaction with 4-vinylbenzyl chloride yields styrene groups attached to the surface.

The present invention also provides a third method, which is essentially a variation of the second method. Again, pigment particles (particularly inorganic pigment particles) having nucleophilic groups on their surfaces are treated with a reagent having electrophilic groups but not polymerizable or polymerization-initiating groups, such that the residue of the reagent chemically binds to the pigment particles. The reagent is selected so that treatment of the pigment particles with it affects the zeta potential of the pigment particles. Preferred reagents for this method are alkyl halides, particularly benzyl chloride or benzyl bromide; these alkyl halides are particularly useful for treating titanium dioxide particles. Treatment with either benzyl chloride or benzyl bromide shifts the zeta potential of the pigment particles to a more positive value.

The third method of the present invention can be implemented on pigment particles that have been silane treated or have a polymer formed thereon by any of the methods described in the aforementioned U.S. Patent No. 6,822,782, or by the first or second methods of the present invention. Because the third method of the present invention does not require functionalization or changes in the polymer forming reaction on the pigment, the third method of the present invention is easy to adopt and beneficial. It is believed (but the present invention is in no way limited to this concept) that the third method of the present invention acts to passivate the pigment surface (especially the inorganic high-energy metal oxide surface used in many pigments in electrophoretic displays) by covalent surface functionalization with alkyl chains. Therefore, the third method of the present invention may be referred to as the "passivation" method of the present invention hereinafter.

The present invention extends to an electrophoretic material comprising a plurality of 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 particles is prepared by the method of the present invention. The charged particles and the fluid may be confined within a plurality of capsules or microcells. Alternatively, the charged particles and the fluid may exist as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material. The fluid may be liquid or gaseous. The present invention also extends to an electrophoretic display comprising a layer of the electrophoretic material of the present invention and at least one electrode arranged to apply an electric field to the layer of electrophoretic material.

The displays of the present invention can be used in any application in which prior art electro-optical displays have been used. Thus, for example, the displays of the present invention can be used in e-book readers, portable computers, tablet computers, mobile phones, smart cards, signage, watches, shelf labels, variable transmission windows, and flash drives.

FIG1 of the accompanying drawings is a graph showing changes in zeta potential, wherein the amount of charge control agent in the experiment is as described in Example 6 below.

FIG2 is a reaction flow chart of the passivation method of the present invention.

3 is a graph generally similar to FIG. 1 showing the change in zeta potential, wherein the amount of charge modifier in the experiment is as described in Example 8 below.

As described above, the present invention provides three different methods for treating pigment particles. Although these methods will be primarily described separately below, as already indicated, more than one method of the present invention can be used to synthesize a single pigment particle; for example, a pigment particle that has been functionalized using the first or second method of the present invention can be treated using the third method of the present invention, either before or after polymer formation on the functionalized pigment.

Part A: Absorption Method of the Invention

As already mentioned, the absorption method of the present invention provides a method for treating pigment particles by treating the particles with a solution of a reagent having a polymerizable group or a polymerization initiating group, physically adsorbing the reagent containing the polymerizable group onto the surface of the pigment particles, thereby causing the reagent to become physically adsorbed onto the particle surface so that when the particles are placed in a hydrocarbon medium, the reagent will not desorb from the particle surface. The absorption method avoids the vacuum drying step, which, as previously mentioned, is typically required when using silane-functionalized pigment particles and provides a different surface chemistry that can result in the ability to obtain pigments in a zeta potential range that is not achievable by the method described in the aforementioned U.S. Patent No. 6,822,782. The omission of the vacuum drying step is expected to improve the dispersion of the pigment during the polymerization stage and thereby reduce large aggregates in the resulting polymer-coated product. Experimentally, it has been found that when the adsorption method of the present invention is applied to commercially available silica/alumina coated titanium dioxide, the proportion of polymer in the final polymer-coated pigment (as measured by thermogravimetric analysis) is similar to the proportion of polymer achieved after silane functionalization, but the final polymer-coated pigment is positively charged in the electrophoretic medium. This might be expected for the cationic form of the adsorption method, but it was somewhat surprising to find that this was also true for the anionic form.

The following examples are now given, but are given by way of illustration only, to show details of particularly preferred reagents, conditions and techniques for use in the absorption method of the present invention.

Example 1: Surface modification using anionic modifiers

The pigment being treated is dispersed in ethanol at approximately 25% by weight, and dilute hydrochloric acid is added to ensure that the solution is well below the isoelectric point of the pigment. 3-Sulfopropyl methacrylate potassium salt (SPMK) (approximately 50-100 mg per gram of pigment) is 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 can be allowed to dry (this is not necessary) and then dispersed in toluene for polymerization essentially as described in Example 28 of the aforementioned U.S. Patent No. 6,822,782.

Example 2: Surface modification using cationic modifiers

The pigment being treated is dispersed in ethanol at approximately 25% by weight, and aqueous ammonia is added to ensure that the solution is well above the isoelectric point of the pigment. [3-(Methacryloyloxy)ethyl]trimethylammonium chloride (MA ETAC) (approximately 50-100 mg per gram of pigment) is dissolved in water and added to the pigment dispersion. The solution mixture is allowed to stand for several hours, then centrifuged and the solid residue washed twice with ethanol. The resulting pigment can be allowed to dry (this is not required) and then dispersed into toluene for polymerization essentially as described in Example 28 of the aforementioned U.S. Patent No. 6,822,782. If the pigment has not been dried, a solvent switch procedure can be performed to produce a pigment dispersion in toluene.

Example 3: Analysis of various pigments prepared by the absorption method

As described in Examples 1 and 2 above, various pigments were treated by the absorption process of the present invention using either SPMK or MAETAC, then washed and dispersed in a coating of toluene and poly(lauryl methacrylate) essentially as described in Example 28 of U.S. Patent No. 6,822,782 above. The resulting polymer-coated pigments were tested by thermogravimetric analysis and the zeta potential of the resulting polymer-coated pigments was measured by suspending them in Isopar E (a commercial hydrocarbon solvent) with 25 mg of Solsp erse 17K (a charge modifier) added per gram of pigment. The TGA values for the original pigment, the functionalized pigment, and the polymer-coated pigment are shown in Table 1 below.

Table 1

From the foregoing, it will be seen that functionalization of the pigment by the absorption method of the present invention produces a final pigment having a satisfactory amount of polymer and a good positive zeta potential.

From the foregoing, it will be seen that the absorption method of the present invention can provide a simplification of prior art silane functionalization methods by omitting the drying step. The method can also result in better reproducibility of the average particle size of the functionalized pigment dispersion during the polymerization stage. Both effects can potentially lead to cost savings, the former through process simplification and the latter through potential yield improvements.

Part B: Nucleophilic Methods of the Invention

As already mentioned, the nucleophilic method of the present invention provides a method for treating pigment particles (which may be organic or inorganic pigment particles) having nucleophilic groups on their surfaces by treating the pigment particles with a reagent having a polymerizable group or a polymerization-initiating group and further comprising at least one electrophilic group. The electrophilic group on the reagent reacts with the nucleophilic group on the particle surface, thereby attaching the polymerizable group or polymerization-initiating group to the particle surface.

The nucleophilic method of the present invention can produce organic pigment particles that are easily dispersed in hydrocarbon fluids commonly used in electrophoretic media. The method can also produce organic and inorganic pigments with a zeta potential that is essentially independent of the charge modifier used, and this constant zeta potential can help improve the optical state in electrophoretic displays.

The following examples are now given, but are given by way of illustration only, to show details of particularly preferred reagents, conditions and techniques for use in the nucleophilic methods of the present invention.

Example 4: Nucleophilic method using dimethylquinacridone

Dimethylquinacridone (Ink Jet Magenta E 02,15g) and toluene (135g) are mixed and subjected to a high-efficiency disperser for 1 minute. The resulting dispersion is transferred to a round-bottom flask equipped with a magnetic stirring bar, and the flask is placed in a preheated 42°C silicone oil bath and placed under a nitrogen atmosphere. Triethylamine (12mL, 86mmol) is added; 4-vinylbenzyl chloride (VBC, 5.0mL, 36mmol) is added in a single portion by a syringe after 1 hour. The reaction mixture is then allowed to stir at 42°C under a nitrogen atmosphere and spend the night.

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

Example 5: Polymer coating of pigments prepared by a nucleophilic method

The dried pigment from Example 4 above was ground using a mortar and pestle. A sample was removed for TGA measurement and the remaining pigment was dispersed in toluene (10 wt% pigment dispersion) under ultrasound and rotation. The resulting pigment dispersion was transferred to a round-bottom flask equipped with a magnetic stirring bar and the flask was placed in a preheated 65°C silicone oil bath. Lauryl methacrylate (20 g) was added to the reaction mixture, a Vigreux distillation column was connected as an air condenser, and the flask was purged with nitrogen for at least 1 hour. A toluene solution of 2,2'-azobis(2-methylpropionitrile) (AIBN) (0.20 g AIBN in 5 mL toluene) was added to the reaction flask in one go using a syringe, and the reaction mixture was vigorously stirred at 65°C overnight.

The reaction mixture was poured into a centrifuge bottle, diluted with toluene and centrifuged for 30 minutes; the supernatant was decanted and subjected to 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 vacuum oven at 70°C overnight.

Example 6: Testing of polymer-coated pigments

The polymer-coated pigment prepared in Example 5 above was ground using a mortar and pestle and dispersed to form a 20 wt% dispersion in Isopar E. The dispersion was sonicated and rotated for at least 24 hours and then filtered through a fabric mesh to remove any large particles. A sample of the dispersion was removed and its percent solids measured, and the dried pigment from this measurement was submitted for TGA and density measurement by a pycnometer. The TGA value of the treated pigment was 3.5%, while the original pigment had a TGA value of 2.1%. The remaining dispersion was used to prepare 25 g of a 5% pigment dispersion of pigment coated with 0.5 g Solsperse 17000/g for zeta potential measurement.

Samples of the dispersion thus prepared were mixed with varying amounts of aluminum tris[3,5-di-tert-butyl-butylsalicylate], a very acidic charge modifier commercially available as Bontron E88, and the zeta potential of the pigment was measured. To provide a control, a similar sample of the original pigment, but without treatment with VBC and subsequent polymerization, was obtained. The results are shown in Figure 1 of the accompanying drawings. As can be seen from the data in Figure 1 , the original pigment exhibits a sharp increase in zeta potential with the concentration of the acidic charge modifier, while the zero potential of the pigment prepared by the nucleophilic method of the present invention is essentially insensitive to the concentration of the charge modifier. Interestingly, the zeta potential of the pigment of the present invention in OLOA371 (a very basic charge modifier) is essentially the same as that of Solsperse when both charge modifiers are present at 0.5 g/g pigment; that is, the zeta potential of the pigment of the present invention is essentially the same in the presence of either the basic (OLOA) or acidic (Solsperse 17k/Bontron) charge modifier. For comparison, white titanium dioxide pigment dispersed with OLOA (essentially as described in Example 28 of aforementioned U.S. Patent No. 6,822,782) is much more negative than in the presence of Solsperse 17k under similar conditions, and the addition of a small amount of Bontron E88 has a significant effect on the zeta potential of this white pigment, shifting it in a more positive direction.

Example 7: Additional Pigments Prepared by the Nucleophilic Method of the Invention

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

Table 2

From the foregoing it will be seen that the nucleophilic method of the present invention provides a method for functionalizing a wide variety of pigments to enable the formation of polymer coatings thereon; the method is particularly useful for attaching polymer shells to organic pigments that lack the silica or metal oxide surfaces common to many inorganic pigments and are capable of reacting with silanes. The method is very simple and relies on a well-established chemistry in which the equilibrium strongly favors the coupled state so that the yield of the nucleophilic reaction is essentially quantitative. The reagents used can be selected to be highly reactive species that readily react with even weak nucleophilic groups on the pigment particles. The nucleophilic groups on the pigment particles can be part of the actual crystal structure of the pigment or can be generated by additives.

The ability of the present nucleophilic approach to render the zeta potential of polymer-coated pigments essentially independent of the choice of charging agent (as demonstrated in the examples above) provides great breadth for the development of new electrophoretic internal phases and has been shown to have potential advantages in terms of the optical states achievable using typical drive voltages and pulse lengths.

Part C: Passivation Method of the Invention

As already mentioned, the passivation method of the present invention provides a method in which pigment particles having nucleophilic groups on their surface are treated with a reagent having electrophilic groups but not having polymerizable groups or polymerization initiating groups so that the residues of the reagent are chemically bound to the pigment particles. The reagent is selected so that the treatment of the pigment particles with it affects the zeta potential of the pigment particles. Preferred reagents are generally alkyl halides (this term is used herein to include aralkyl halides), especially benzyl chloride.

In the prior art polymer-coated electrophoretic pigment particles that have been subjected to the above-mentioned silane/polymerization treatment, it is found that the silane groups on the pigment particle surface are the main component for generating the surface charge and zeta potential of the pigment in the final pigment particles. Therefore, the modification of the charged pigment particles can be achieved by adding functional silanes and/or by adding functional monomers to the polymer shell, which changes the internal charge generation characteristics of the pigment. However, changing the charge of the pigment particles by these two methods also affects other important pigment properties, such as dispersibility in non-polar solvents, polymer graft density and pigment dispersion viscosity. These highly interdependent properties make it difficult to change the zeta potential of the pigment without affecting other important properties. Therefore, in order to allow various pigment properties to be optimized independently of each other, it is desirable to use suitable silanes and polymer shells to synthesize the pigment and use post-polymerization modification of the pigment surface according to the passivation method of the present invention to control the zeta potential and thereby control the electrophoretic mobility.

As already noted, the preferred reagent for use in the passivation method of the present invention is an alkyl halide, particularly benzyl chloride, preferably in the presence of triethylamine, as shown in Figure 2 of the accompanying drawings. Typically, benzyl chloride should be used in an amount of about 30 molecules per square nanometer of pigment surface, estimated to be approximately a five-fold excess relative to the available nucleophilic hydroxyl groups on the surface of typical metal oxide pigments; the degree of benzylation can be inferred from the maximum zeta potential shift of the pigment. The pigment zeta potential is typically measured in Isopar E with the charge modifier Solsperse 17000; the zeta potential shift increases with increasing reactivity of the alkylation reaction, and such increased reactivity can be achieved by increasing the reaction temperature or time, solvent polarity, and the strength or steric hindrance of the non-nucleophilic base. Typical positive zeta shifts are a net of +10 to +60 mV. Recommended alkylation conditions for the 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 approximately 66°C. The reaction can be carried out either immediately after treatment of the raw pigment with the silane or after polymer coating.

For example, in a series of experiments, it was found that the original silica/alumina coated titanium dioxide pigment (R794 sold by duPont) had a weight loss of 1.08% during TGA. After treatment with vinylbenzyl chloride (VBC, an electrophilic reagent similar to benzyl chloride) in the presence of triethylamine, the weight loss increased to 1.23%, equivalent to an increase of 1.03 alkyl groups per square nanometer of pigment surface. Subsequent polymerization of the VBC-treated pigment with lauryl methacrylate increased the weight loss to 3.90%. This subsequent grafting of the polymer to the VBC-treated titanium dioxide demonstrates covalent surface functionalization of the metal oxide.

The passivation method of the present invention is covalently attached to the pigment surface by an alkyl (or other) group, and is acted upon by a change in the pigment zeta potential. The benzyl group is attached to the surface of the white titanium dioxide pigment with an inherent negative zeta potential value before the change and plays a role in migrating the zeta potential to a more positive value. The functional group attached to the electrophilic alkyl group will be used to determine the sign and amplitude of the zeta potential change. Using 4-fluorobenzyl chloride or 4-nitrobenzyl chloride will tend to induce a more negative zeta potential by fluorination and acidic surface functionalization. On the contrary, using the alkylation of basic alkyl groups such as 4-(chloromethyl) pyridine or 4-(dimethylamino) benzoyl chloride will change the zeta potential to a stronger positive value. Finally, adding tert-butylbenzyl chloride or long-chain alkyl halides such as 1-bromooctane can be used to provide additional steric hindrance to prevent molecular diffusion into the pigment surface.

The following examples are now given, but are given by way of illustration only, to show details of particularly preferred reagents, conditions and techniques for use in the passivation method of the present invention.

Example 8: Passivation method of the invention applied to spinel-based black pigments

This example reports the results of preliminary experiments in which spinel-based polymer-coated black pigments were treated with benzyl chloride and benzyl bromide to test the hypothesis that amine groups present in the pigments can be quaternized and thereby permanently positively charged, resulting in pigments that are insensitive to the choice of charging agent.

Except that the reaction is carried out in Isopar E, benzyl chloride is used to treat the black pigment of the polymer coating based on Shepherd BK444 and the preparation described in Example 1 of No. 8,270,064 United States Patent (USP) substantially in the same manner as in Example 4 above. The black pigment of polymer coating (24g), benzyl chloride (4g), triethylamine (4.7g) and Isopar E are mixed at room temperature for 24 hours. The pigment of the modification obtained is repeatedly centrifuged and washed with Isopar E. Dispersions of the pigment are prepared using Bontron E88 and variable amounts of OLOA 371, and their zeta potential is measured. In order to provide a control, similar dispersions of untreated polymer coating black pigment are prepared and their zeta potential is measured. The results are shown in Figure 3.

It will be seen from Figure 3 that the alkylated pigments are much less sensitive to the presence of OLOA than the non-alkylated pigments and produce a nearly constant surface charge.

Example 9: Passivation method of the invention applied to white pigments based on titanium dioxide

Several titanium dioxide-based white pigments were treated with benzyl chloride after silane functionalization or after polymer formation on the pigment. Both the original and treated pigments were tested by TGA and their maximum zeta potential was measured in Isopar-E. The results are shown in Table 3 below.

The program for using benzyl chloride to process after forming polymer layer on white pigment is as follows. Pigment (300g) is added to 1L plastic bottle, tetrahydrofuran (THF-500mL) is also added thereto. Rolling plastic bottle in roller mill, then ultrasonic. The dispersion obtained is placed in the jacketed reactor equipped with four-neck reactor, and the four-neck reactor top is equipped with overhead mechanical stirrer, condenser with nitrogen inlet, thermometer or thermocouple and diaphragm. Use a small amount of THF that dispersion is rinsed into reactor, reflux and vigorously stirred. Use the head space of nitrogen purging reactor and remain under positive nitrogen pressure in the remaining stage of reaction. By syringe, triethylamine is added in reactor and stirred the mixture produced 30 minutes, subsequently by syringe, benzyl chloride is added to reactor, then under reflux, the reaction mixture stirred obtained spends the night. In order to separate product, reactor is discharged in two 1L centrifuge bottles, and dispersion is diluted to 1000g total solvent, then centrifugal. The supernatant was decanted and the pigment was redispersed by rolling on a roller mill using a total of 1000 g THF for 90 minutes, after which the pigment dispersion was centrifuged again and the supernatant decanted. The wet pigment pack was then dried in a vacuum oven at 70°C overnight.

Table 3

After various surface modifications, pigments 6 and 7 were converted into experimental single pixel displays as described in Example 2 of U.S. Patent No. 8,582,196 using the same spinel-based black pigments described therein, and the resulting experimental displays were electro-optically tested as described in Example 3 of the patent. The results are shown in Table 4 below. Prior to electro-optical testing, the single pixel displays were repeatedly switched to their extreme black and white states, then finally switched to black or white, and the L* value was measured 3 seconds after the end of the final drive pulse to allow transient effects to dissipate. The image stability graph was measured by allowing the display to remain in the black or white extreme state for a dwell time of 10 seconds, driving it to its opposite optical state, measuring the L* value of that state immediately after the end of the drive pulse (20 milliseconds) and after 30 seconds, and taking the difference. The DSD (dwell state dependence) value was similarly measured by allowing the display to remain in the black or white extreme state for a dwell time of 20 seconds, driving it to its opposite optical state, measuring the L* value of that state immediately after the end of the drive pulse and after 30 seconds, and taking the difference.

Table 4

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 display (the change after benzyl chloride treatment was no more than 1-2 L*), but did result in a substantial decrease in the L* value of the dark state (about 5 L*); therefore, the benzyl chloride treatment produced a useful increase in the dynamic range of the display (about 3-4 L*). Statistical analysis showed no significant changes in image stability and DSD data between the untreated and benzyl treated pigments.

From the foregoing, it will be seen that the passivation method of the present invention allows the surface functionalization of the pigment particles by covalently attaching an alkyl group to the pigment surface, thereby allowing the pigment zeta potential to change. Specifically, benzyl is attached to the surface of a white pigment with an intrinsic negative zeta potential value before modification to effect its zeta potential migration to a more positive value. Although this has been shown above for a white pigment based on titanium dioxide, it is reasonable to assume that it is applicable to any inorganic pigment with a nucleophilic metal oxide surface. The signs and amplitude of the zeta potential change can be controlled by the functional group connected to the electrophilic alkyl group. For example, owing to the description that fluorination and acidic surface functionalization realize the zeta potential change of colloids in non-polar liquids, 4-fluorobenzyl chloride and 4-nitrobenzyl chloride should induce a more negative zeta potential. On the contrary, the alkylation using basic alkyl groups such as 4-(chloromethyl)pyridine or 4-(dimethylamino)benzoyl chloride will effect the zeta potential change to a more positive value. Finally, treatment with tert-butylbenzyl chloride or long chain alkyl halides such as 1-bromooctane can be used to provide additional steric hindrance to prevent diffusion of molecules to the pigment surface.

Claims (4)

1. A method for treating pigment particles having nucleophilic groups on their surface, the method comprising: treating the pigment particles with the reagent having a polymerizable group or a polymerization initiating group and further comprising at least one of the electrophilic groups under conditions that cause an electrophilic group on a reagent to react with the nucleophilic group on the particle surface, thereby attaching the polymerizable group or polymerization initiating group to the particle surface, wherein the pigment is a quinacridone dye. 2. The method of claim 1, further comprising: reacting the pigment particles having the polymerizable group or polymerization initiating group on the particles with the at least one monomer or oligomer under conditions that effectively induce a reaction between the polymerizable group or polymerization initiating group on the particles and at least one monomer or oligomer, thereby resulting in the formation of a polymer on the particles. 3. The method according to claim 1, wherein the pigment is dimethylquinacridone. 4. An electrophoretic material comprising a plurality of 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 prepared by the method of claim 1.