WO2009100803A2

WO2009100803A2 - Fluid compositions for colour electrophoretic displays

Info

Publication number: WO2009100803A2
Application number: PCT/EP2009/000289
Authority: WO
Inventors: Mark John Goulding; Ashley Nathan Smith; Johannes Canisius; Richard Vidal; Sihame Khoukh; Matthias Koch
Original assignee: Merck Patent Gmbh
Priority date: 2008-02-11
Filing date: 2009-01-19
Publication date: 2009-08-20
Also published as: EP2240518A2; TW200951146A; WO2009100803A3

Abstract

This invention relates to coloured polymer nanoparticles with surface functionality for charge retention, a process for their preparation, the use of these particles for the preparation of an electrophoretic device, and to colour electrophoretic displays comprising such particles.

Description

Fluid Compositions for Colour Electrophoretic Displays

Electrophoretic displays (EPDs) generally comprise an electric double layer produced in an interface between a solid (charged particle) and a liquid (dispersion medium), in which a charged particle migrates to an electrode having polarity opposite to the charge possessed by the charged particle by using, as motive power, the force exerted by an electric field.

The velocity of a particle in a unit electric field is known as the electrophoretic mobility, and can be calculated using the Henry equation:

where ε is the dielectric constant of the dispersion medium, η is dynamic viscosity of the dispersion medium (Pa s), and ζ is zeta potential (i.e., the electrokinetic potential of the slipping plane in the double layer). f(Ka) is 'Henrys function' and is related to the ratio of particle radius to double layer thickness.

For small particles in low dielectric constant media, f(Ka) becomes 1.0 and thus the electrophoretic mobility is given by:

3/7 This is the Huckel approximation as disclosed in Huckel, E., Physik.Z., 25,

204 (1924).

Commercially available electrophoretic displays typically comprise a front plane visual element made from a film comprising a layer of microcapsules about 50 μm in diameter bound with an isotropic polymer matrix to an electrically conducting Indium Tin Oxide (ITO) coated transparent plastic substrate. Within the microcapsules is a fluid composition, comprising an organic dielectric fluid and black and white nanoparticles, typically made from surface modified Titanium dioxide. The individual particles are typically

200 - 300 nm in diameter and hold on their surface by means of a double layer, an electric charge. In the film the black particles typically possess a permanent negative charge and the white particles possess a permanent positive charge. When this film is attached to a conducting backplane, typically with a patterned electrode structure, typically a direct drive backplane or active matrix TFT backplane, it forms an electrophoretic display element.

When a potential difference of sufficient magnitude is applied across the two electrodes, migration of the charged nanoparticles occurs, resulting in their separation and collection at one or other side of the film. The macroscopic effect of this is to effect a colour change to the film from gray to black or white depending upon the polarity of the electric field applied to the frontplane and backplane electrodes. There are many parameters which affect the electro-optical response of such a display device. Particle size, particle zeta potential, particle density, the magnitude of the applied electrical field, viscosity of the dielectric media are some but not all of the influencing parameters.

A limitation of electrophoretic displays which contain such films is their monochrome appearance. The display is limited to black, white and a small range of intermediate gray shades. It is not possible to show multiple colours with such films except by means of an overlying colour filter array. This is colour generation by the additive colour principle. Sub-pixellation using a colour filter array means that the non-active sub-pixels show black when not in use and absorb light, rather than reflecting it. This reduces the overall display brightness. Other electrophoretic display technologies exist, such as the microcup technology which comprises a dispersion of charged white particles in a dyed dielectric fluid. The fluid composition is constrained by a micro replicated cell structure on the display substrate which provides mechanical stability to the display element and prevents wide area flow of the fluid composition. This display can show a colour to white electro-optical transition, but not a full colour gamut. Again, this electrophoretic display principle uses the additive colour principle and will suffer from the disadvantages detailed above.

Subtractive colour techniques have also been used for the production of full- colour electro-optical display media. This involves the superposition of one coloured layer over another, as commonly employed in printed media. Examples of this are found in both liquid crystal displays and electrophoretic displays (WO2004051353 (A1 ).

The use of different coloured particles in a single pixel has been exemplified in recent patent literature (US7304634 (B2), (GB2438436 (A1 ), ((US2007 0268244), but all of these approaches require the use of complex cell structures and drive schemes.

Special coloured particles for EPDs and processes for their preparation are disclosed in US 2007/0297038), US 2008/0013156), US 6,822,782), WO 2007/048721 ), WO 2008/003619), WO 2008/003604), US 2005/0267263),

WO 2006/126120), and J. Nanosci. Nanotechn. 2006, Vol. 6, No. 11 , p. 3450 - 3454. Two particles system comprising inorganic and resin particles are also known (EP 1491941). These coloured particles are only achievable by complicated processes and/or they are only suitable for specific applications. Similar coloured particles and their preparation processes are known for analytical techniques (US 5,607,864 and US 5,716,855) and as toner particles for ink jet printing (US 4,613,559). Furthermore, in the emerging field of "e-paper" and flexible displays especially those for mobile use; there is a requirement for low power consumption to maximize device battery life. A well reported technique to reduce display power consumption is the use of bistable display media.

Other electrophoretic display principles and descriptions are given in "E- Paper for Displays", C. M. Lampert, Displays; 2004, 25(5) published by Elsevier B.V., Amsterdam.

It can be seen from the display principles above, that a need exists for electrophoretic displays with enhanced properties. Furthermore, starting from the coloured particles and their preparation described above, there is also a need for coloured particles for use in EPDs and an easy way to prepare them.

Therefore, the object of this invention is to provide electro-optically active media for colour electrophoretic displays with specifically engineered properties and coloured particles for use in such media.

This object is solved by a process for the preparation of coloured polymer nanoparticles for use in electrophoretic devices comprising the steps of a) preparing polymer particles comprising sites of charging and optionally of stabilization, b) colouring the polymer particles, and c) washing and drying the coloured polymer particles, by the use of these particles for the preparation of an electrophoretic device, and by colour electrophoretic displays comprising such particles.

The subject matter of this invention relates specifically to the use of specifically engineered polymer nanoparticles and their dispersion in dielectric organic media to produce a composition preferably suitable as the electrically switchable component of a full colour e-paper or electrophoretic display. It relates more specifically to the synthesis of polymer nanoparticles, their surface modification with covalently bonded substituents to promote dispersability and the holding of a charge. It also relates specifically to the physical and irreversible entrapment of an optionally chemically bonded dye moiety to give colour to the nanoparticles.

It also relates specifically to dispersions of the aforementioned polymer nanoparticles in dielectric organic media, which enable electrophoretic switching of the particles in an applied electric field.

An advantage of the present invention may also be that by varying the size of the particles, a controlled 'gradient switching' may be possible to allow greyscales to be achieved by selectively switching the particles in batches, and so control the particle density within the display.

Advantages of the polymer nanoparticles according to the invention may be, in particular:

- high control of particle size in the range of 20 - 500 nm diameter, with a monodisperse size distribution for image quality, and/or

- a glassy polymer nature for optical clarity and colour compatibility, and/or

- a homogeneous crosslinked network structure (i.e. intraparticles ) for solvent resistance, and/or

- a non-swelling nature when dispersed in solvent media, impact strength, hardness, and/or

- dispersible in a non polar continuous phase that is the most used media for EPD, and/or

- high electrophoretic mobility in dielectric media, and/or

- technique is universally applicable for dye incorporation across all colours, and/or

- accurate zeta potential is possible, and/or

- high stability to fading/bleaching, and/or - wide gamut of colours and colour matching capability, and/or

- all colours have same density (good for sedimentation / agglomeration performance), and/or

- excellent dispersability in benign solvents with minimal or no additives, and/or

- excellent switching behaviour, faster response times at comparable voltages.

Preferably, the coloured polymer particles according to the invention provide electrophoretic fluids showing response times which meet the usual requirements for such fluids of < 10 sec, preferably < 5 sec, especially < 2 sec. Advantageously, response times < 1 sec are possible.

So the present polymer nanoparticles preferably show the characteristics electrophoretic particles have to show to realize a high resolution electrophoretic display based on coloured electronic ink.

The basic steps to provide electrophoretic fluids according to the present invention are: - preparing, preferably colourless, resin particles by emulsion polymerisation, comprising sites of charging and optionally of stabilization,

- preferably grafting the polymer particles with a functionalized spacer,

- colouring the resin particles by a dye, and

- dispersing the particles into a dielectric fluid.

The selection of the polymerization conditions depends on the expected size and size distribution of the particles.

The most appropriate method to synthesise uniform sub-micronic particles is by emulsion polymerisation. Advantageously, the procedure by which an emulsion polymerisation is carried out has a profound effect upon the resulting particle size and polymer properties. Indeed, nanoparticles with quite different performance characteristics can be produced from the same reaction formulation by appropriate control of polymerisation process and conditions used. Comprehensive reviews of emulsion polymerisation conditions are given in "Emulsion polymerization"; van Herk, Alex; Gilbert, Bob; Department of Polymer Chemistry, Eindhoven University of

Technology, Eindhoven, Neth. Editor(s): Van Herk.

Preferably, a batch emulsion polymerization process is used wherein all reactants are completely added at the outset of the polymerisation process. In such a process only relatively few variables have to be adjusted for a given formulation. Preferred changes which can be made in such cases are to the reaction temperature, reactor design and the type and speed of stirring.

Thus, a batch emulsion polymerisation process is used for manufacture versus a semi-continuous batch process because of limited versatility and simple evaluations of reaction formulation.

To meet the required characteristics of the nanoparticles for EPD, a surfactant-free emulsion copolymerisation using batch process is preferred. Protective colloids (water-soluble polymers) and surfactants are usually key formulation variables in emulsion polymerisation because of their impact on the intraparticle stability and particle size control but they may have a detrimental effect on the electrophoretic response.

Preferably the polymerization according to the invention is a free radical polymerization.

Preferably a monomer composition according to the invention comprises a structure monomer, a crosslinking monomer, an ionic monomer and an initiator. As for the polymerizable monomer which is used as backbone units for the physico-chemical structure of the nanoparticles it is possible to use various compounds comprising -CH=CHR groups, wherein R can be H, a linear or branched alkyl group or an aryl group, which groups may also be substituted. Preferably, R is H or substituted or unsubstituted alkyl with 1 - 6 carbon atoms. The following compounds can be used: styrene-based monomers, such as styrene, or o-, m- or p-alkyl-styrenes, bis alkyl substituted styrenes, vinyl ether monomers, vinyl ketone monomers, vinyl acetates, or acrylic and methacrylic acids and their derivatives. Combination of two or more monomers may be used, too.

Preferably, esters of acrylic and/or methacrylic acid are used, especially, methyl acrylate (MA), ethyl acrylate (EA), n-butyl acrylate (BA), 2-ethylhexyl acrylate, methyl methacrylate (MMA), ethyl methacrylate (EMA), n-butyl methacrylate (BMA).

The following compounds comprising two or more -CH=CHR groups as described in the foregoing can be used as intraparticle crosslinking monomers for solubility control and solvent swelling resistance: styrene- based monomers, such as divinylbenzene, divinylether, or compounds comprising at least two acrylic or methacrylic groups. Combination of two or more monomers may be used, too.

Preferably derivatives of acrylic and methacrylic acids are used, especially ethylene glycol di(meth)acrylate, di- or triethylenglycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, or allyl (meth)acrylate. Advantageously, ethylene glycol dimethacrylate (EGDMA), and allyl methacrylate (ALMA) may be used.

As for the ionic monomer, cationic as well as anionic monomers may be used. Examples for cationic monomers, preferably for particle stability and particle size control, are monomers as described above, wherein these monomers comprise a cationic functional group, such as an ammonium group, a pyridinium group, a sulfonium group, or a phosphonium group. Preferred compounds are such comprising an ammonium group. Especially preferred are 2-methacryloxy ethyl trimethyl ammonium chloride (MOTAC), and acryloxy ethyl trimethyl ammonium chloride (AOTAC).

Examples for anionic monomers are monomers as described above, wherein these monomers comprise an anionic functional group, such as a carboxylate group, a sulfate group, or a phosphate group. Preferred compounds are such comprising a carboxylate group. Especially preferred are sodium acrylate (NaA) and sodium-2-methylpropanesulfonate (NaAMPS).

Preferably an oil soluble initiator is used in the surfactant-free emulsion copolymerisation in order to control size, particle morphology and to reduce the residual monomers at the end of the reaction. Examples are azo compounds or peroxide compounds, hydroperoxides or peracid esters. Preferably azo compounds are used, especially azobis(isobutylamidine) hydrochloride (AIBA) and similar compounds.

The polymerizable composition of the invention usually comprises 50 - 95 %, preferably 70 - 90 %, by weight of monomer, 1 - 40 %, preferably 1 - 10 %, by weight of crosslinking monomer, 1 - 30 %, preferably 1 - 10 %, by weight of ionic monomer and 0.1 - 10 %, preferably 0.1 - 5 %, by weight of initiator, all percentages are based on the polymerizable composition.

Cross-linked copolymer nanoparticles can preferably be prepared by emulsifier-free copolymerization of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDMA), and a cationic comonomer, methacryloxy ethyl trimethyl ammonium chloride (MOTAC) using azobis(isobutylamidine) hydrochloride (AIBA) as an initiator. Preferably emulsifier-free emulsion polymerizations are conducted using a batch process, Polymer particles prepared according to the present invention preferably have a particle size of 50 - 500 nm, especially 50 - 400 nm, preferably 100 - 400 nm. Especially preferred are particles having a particle size of 150 - 350 nm.

To enhance the surface charge of the polymeric nanoparticles in a non- polar continuous phase, a spacer is grafted onto the surface of the particles. When cationic monomers are used in the polymerization step, preferably an alkyl amine spacer is used, preferably by nucleophilic reaction with the carbonyl carbon of a methyl (meth)acrylate unit. The aminolysis reaction leads to formation of the amino alkyl modified polymer nanoparticles. Preferably, alkyl-amine spacer lengths from diethylene diamine to octaethylene diamine can be grafted onto the surface of the particles. When an anionic monomer is used in the polymerization step, preferably an alkyl carbonic acid is used. Preferably, alkyl dicarboxylic spacers with an alkyl chain of 2 to 8 carbon atoms may be used. The spacer group gives 1 to 20 % by weight of the particle, especially 5 to 10 % by weight.

Coloration of nano and micro particles can be done by several dyeing technologies such as disperse dyeing or "solvent swelling of particles".

The "solvent swelling of particles" technology is preferred because of it is a post-polymerisation process with a large number of possibly absorbable dyes such as azo dyes, anthraquinone dyes, triarylmethane dyes, acridine dyes, cyanine dyes, oxazine dyes, polymethine dyes, or thiazine dyes. Azo- based dyes, anthraquinone-based dyes, and triarylmethane-based dyes are preferred examples. Suitable dyes are preferably soluble in the particle swelling solvent and insoluble in water. This feature allows various dyes to be driven by the solvent within the nanoparticles and retained inside for.

Preferred dyes are Waxoline blue APFW from Lubrizol (chemical category : anthraquinone), mixture of solvent yellow (colour index : 11021 ) + solvent blue (colour index: 61556) distributed by Europhtal - France, organol red distributed by Europhtal France (chemical category: p. Phenylazoaniline), macrolex blue RR GRAN from Bayer (chemical category: anthraquinone), macrolex red violet from Bayer (chemical category: anthraquinone), solvent yellow 16 (colour index 12700) distributed by Europhtal France, Waxoline black OBP [solvent yellow 14 (anthraquinone) + carbon black)] from Lubrizol.

The amounts in which the dyes are mixed with the polymeric nanoparticles can vary within wide limits depending on their solubility in the solvent and the desired depth of shade; in general, amounts from 1 to 30 % by weight, preferably 2 to 20 % by weight, based on the material to be coloured, have proved to be advantageous. Especially preferred are amounts in a range from 4% to 15% by weight, preferably from 6% to 12% by weight. Preferred solvents that may be used are for example butanone, cyclohexanone, toluene, xylene, methylene chloride, ethylene chloride, dioxane, or acetone.

Also a mixture that is made up of 80 - 95 % by volume of one or more of these solvents and an alkyl alcohol of 3 - 5 carbon atoms can be used. Especially acetone, can advantageously be used. Advantageously, the dyeing process can be run without the addition of emulsifiers. Preferably the dyeing process is conducted in a temperature range of from room temperature to 80°C, preferably from 30⁰C to 70⁰C, especially preferred in a range from 30⁰C to 60°C, especially from 40°C to 60⁰C, especially from 45°C to 55°C. The dyeing process is preferably carried out by mixing the polymer particles and the dye in a solvent and stirring the mixture at a predetermined temperature for 0.5 - 4 hours, preferably 1 - 3 hours. A preferred stirring speed of 30 - 300 rpm may be used. For water miscible solvents like acetone the stirring speed is preferably 30 - 50 rpm, for water non-miscible solvent it is 50 - 200 rpm. The solvent is then removed by distillation under vacuum and the particles are optionally washed and dried by common techniques, preferably by freeze drying. This process gives dyed nanoparticles without altering their initial size and morphology. The electrophoretic particles according to the present invention are suitable for all known electrophoretic media and electrophoretic displays, e.g. flexible displays, one particle systems, two particle systems, dyed fluids, systems comprising microcapsules, microcup systems, air gap systems and others as described in C. M. Lampert, Displays; 2004, 25(5) published by Elsevier B. V., Amsterdam. Examples of flexible displays are dynamic keypads, e- paper watches, dynamic pricing and advertising, e-readers, reliable displays, smart card media, product packaging, mobile phones, lab tops, display card, digital signage.

Usually electrophoretic fluids comprise a charged inorganic nanoparticle such as titania, alumina or barium sulphate, coated with a surface layer to promote good dispersibility in dielectric media and a dielectric fluid media. The solvents and additives used to disperse the particles are not limited to those used within the examples of this invention and many other solvents and/or dispersants can be used. Lists of suitable solvents and dispersants for electrophoretic displays can be found in existing literature, in particular WO 99/10767) and WO 2005/017046) The Electrophoretic fluid is then incorporated into an Electrophoretic display element by a variety of pixel architectures, such as can be found in C. M. Lampert, Displays; 2004, 25(5) published by Elsevier B.V., Amsterdam.

Electrophoretic displays comprise typically, the electrophoretic display media in close combination with a monolithic or patterned backplane electrode structure, suitable for switching the pixels or patterned elements between the black and white optical states or their intermediate greyscale states.

Preferably the coloured particles according to the invention are used in combination with white particles. Apart from the preferred compounds mentioned in the description, the use thereof, compositions and processes, the claims disclose further preferred combinations of the subject-matters according to the invention.

The disclosures in the cited references are thus expressly also part of the disclosure content of the present application.

The following examples explain the present invention in greater detail without restricting the scope of protection. In particular, the features, properties and advantages, described in the examples, of the compounds on which the relevant examples are based can also be applied to other substances and compounds which are not described in detail, but fall within the scope of protection, unless stated otherwise elsewhere. In addition, the invention can be carried out throughout the range claimed and is not restricted to the examples mentioned here.

Examples

Example 1: Synthesis of the polymethylmethacrylate (PMMA) nanoparticles

The emulsifier-free emulsion polymerization is conducted using a batch process, in a three-neck round-bottomed flask equipped with mechanical stirrer, a nitrogen inlet and a water-cooled reflux condenser. A monomer mixture of 95 g of methyl methacrylate (MMA), 5 g of ethylene glycol dimethacrylate (EGDMA) and 7 g of methacryloxy ethyl trimethyl ammonium chloride (MOTAC) is dispersed in water. Then, the 0.7 g azobis(isobutylamidine) hydrochloride (AIBA) initiator is added to the reactor with the stirring speed fixed at 300 rpm and the polymerization is allowed to proceed for 20 h at 70⁰C.

Uniform smooth single particles of 300 nm are obtained.

After polymerization, the nanoparticles are purified by centrifugation in deionised water.

Example 2 Synthesis of amido modified polymethylmethacrylate nanoparticles

Surface modification of PMMA nanoparticles by aminolysis

0,6 g of 1 ,6 hexanediamine is added to 10 g of polymeric nanoparticles that are dispersed in water at 10 % by weight and the nucleophilic reaction is continued with stirring at 150 rpm at 50⁰C for 12 h. Amino-modified particles are purified with deionised water by repeated washing and centrifugation.

Example 3 Coloration of amido modified polymethylethacrylate nanoparticles

Dyeing of NH₂-PMMA nanoparticles

5 g of NH₂-PMMA nanoparticles are dyed in a 250 ml round flask. The dye is dispersed in acetone and then added to the polymeric particles. The volume of acetone is twice this of water in which the polymeric particles concentration is of 10 % by weight. Dyeing is carried out at 48°C for 2 h using a Bϋchi rotation setup in order to allow the particles swelling and a gently dye incorporation into the nanoparticles.

Acetone is finally distilled under vacuum. Dyed particles are filtered to remove residues of dye molecules that are not incorporated into particles and any aggregated dyed particles. Then, the coloured nanoparticles are washed with deionised water by centrifugation until clear supernatant is obtained.

The final polymeric nanoparticles are dried in a freeze-drier in order to be re-dispersed in a dielectric suspension medium.

The following dyes have been used:

Example 3a: Waxoline blue APFW from Lubrizol (chemical category: anthraquinone)

Example 3b: mixture of solvent yellow (colour index: 11021 ) + solvent blue

(colour index: 61556) distributed by Europhtal - France

Example 3c: organol red distributed by Europhtal France (chemical category : p. Phenylazoaniline)

Example 3d: macrolex blue RR GRAN from Bayer (chemical category: anthraquinone)

Example 3e: macrolex red violet from Bayer (chemical category: anthraquinone)

Example 3f: solvent yellow 16 (colour index 12700) distributed by Europhtal France

Example 3g: Waxoline black OBP [solvent yellow 14 (anthraquinone) + carbon black)] from Lubrizol Example 4 Characterization of colored NHg-PMMA nanoparticles

Particle size distribution is measured by light scattering Malvern instrument and by transmission electron microscopy (TEM). The diameters of the monodisperse polymeric nanoparticles are found out in a 300-400 nm range.

The replacing of the ester groups by amino groups after aminolysis is proved by FT-IR analysis. The zeta potential measurements of the polymeric nanoparticles are performed in a water solution of 10^~3 mole/l NaCI in accordance with pH variation on Malvern Zetasizer.

The result for the raw polymeric nanoparticles is close to + 50 mV for pH from 2 to 11 and it is slightly enhanced to over 50 mV for the amido- modified nanoparticles and the coloured material as well.

Example 5: Formulation of Electrophoretic display fluid, containing coloured polymer nanoparticles.

A number of different formulations can be made which exhibit electrophoretic behaviour in a display environment. The following are 3 examples of the way in which the coloured polymer nanoparticles can be used to illustrate optical effects within a display, and is not intended to be an exhaustive list. Identical methods are used for each of the 7 coloured particles of Examples 3a - g. The organic solvents used for the examples within this invention are lsopar G (Multisol Ltd) and Halocarbon oil 0.8

(Chemical Raw Materials Ltd). Dispersants OLOA 11 K(Chevron Oronite) , Solsperse 17K(Lubrizol Ltd), and affinity surfactant Sorbitan trioleate Span 85(Sigma Aldrich) are included in some formulations to improve dispersion stability and charge. All formulations are weighed as described below, then sonicated for a minimum of 1 hour to facilitate dispersion of the particles.

1 ) red polymer nanoparticles + a dyed solvent 0.010 g of Oil Blue N (Sigma-Aldrich) is dissolved into a solvent mixture containing 8.343 g of Halocarbon oil 0.8 and 1.471 g lsopar G. 0.634 g of this mixture is then combined with 0.045 g of red polymer nanoparticles, 0.010 g of OLOA11 K, and 0.002 g of Span 85.

2) red polymer nanoparticles + solvent.

0.103 g of red polymer nanoparticles is dispersed into 0.454 g of lsopar G, 0.555 g of Halocarbon oil 0.8, 0.052 g of Span 85 and 0.060 g of Solsperse 17K.

3) red polymer nanoparticles + oppositely charged contrast particle. 0.193 g of red polymer nanoparticles is dispersed into 1.789 g of lsopar G, 2.186 g of Halocarbon oil 0.8, 0.002 g of Span 85 and 0.004 g of Solsperse 17K. 0.066 g of a white contrast particle (100 nm diameter styrene - divinyl benzene polymer nanoparticle, synthesised via miniemulsion polymerisation; with a measured zeta potential of -9 mV) is added to the dispersion.

The above compositions were replicated using the green, blue, black, yellow, cyan and magenta particles also.

Example 6: Colour measurement of inventive polymer nanoparticles and their definition in CIE1931 (or other) colour space.

All colour data is measured on a MINOLTA CR-300 connected to a Minolta DP-301 data processor.

A suspension of coloured polymer nanoparticles in a non-swelling organic solvent, such as lsopar G (Multisol Ltd), is painted onto a standard white background and the solvent allowed to evaporate before measurements are taken. CIE 1931 xyY coordinates are disclosed in Table I. Table

Example 7: Electro-optical measurement of Electrophoretic display fluid, containing inventive polymer nanoparticles.

All measurements are carried out on a DMS-301 display measurement system, using formulations similar to those described in example 6, filled into 100 micron glass cells. The cells consist of two ITO coated plates of glass separated to 100 micron distance. The movement of the particles from one ITO electrode to the other yields a difference in luminance which is measured by the DMS-301 as shown.

The electro-optic data are disclosed in Table II. Discussion of results concerning electro-optic data: The contrast is taken as (relative reflectance (ON) - relative reflectance (OFF)), i.e. the height of the electro-optic curve as shown. VO is the voltage at which the reflectance starts to change, V10 is the voltage at which reflectance reaches 10% of its maximum reflectance value. V90 is the voltage at which reflectance reaches 90% of its maximum reflectance value. Thus, V90-V10 gives the 'steepness' of the curve. It is preferable for VO, V10 and V90 to be as low as possible. It is expected that the voltages required to switch the cell will differ for different cell thicknesses due to the extra distance required for the particles to travel, and this appears to be the case. The red polymer nanoparticles measured in a 50 micron cell within a dye formulation, show the lowest of all

OFF voltages, and a very low V90-V10(OFF) of 2.3 V. In terms of the steepness of the electro-optic curves, the coloured polymer nanoparticles compare favourably with the state of the art. The dual particle systems show significantly higher voltage switching because of the unoptimised white contrast particle used. This is demonstrated by the high values obtained when the white particles are measured in a dye, in comparison to the particles of this invention.

The response times are shown in Tables III, IV and V Discussion of results concerning response times: The contrast is taken as (relative reflectance (ON) - relative reflectance

(OFF)), i.e. the height of the curve at t=0. T10 is the amount of time taken for the reflectance to increase (ON) or decrease (OFF) to 10% of its final value. T90 is the amount of time taken for the reflectance to increase (ON) or decrease (OFF) to 90% of its final value. T90-T10 indicates the length of time taken to switch the cell (from 10% to 90% relative reflectance) - also known as the response time. At higher voltages, the response times are lower, and greatest contrast is reached. At low (sub 5V) voltages, there is a drop in contrast and the response times are increased.

The Blue, Green, Red, black, yellow, cyan and magenta particles according to the invention show acceptable response times when compared in the 100 micron cell setup. The red polymer nanoparticles of this invention measured in 50 micron cells compare favourably to response times of the state of the art. Table

O

Table

ro

Table IV Inventive polymer nanoparticle + contrast particle systems.

N) K)

Table V

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Claims

1. A process for the preparation of coloured polymer particles for use in electrophoretic devices, comprising the steps of a) preparing polymer particles comprising sites of charging and optionally of stabilization, b) colouring the polymer particles, and optionally c) washing and drying the coloured polymer particles.

2. Process according to claim 1 , characterized in that the polymer particles are grafted with a functionalized spacer before step c).

3. Process according to any one of claims 1 to 2, characterized in that the polymer particles are prepared by surfactant-free emulsion.

4. Process according to any one of claims 1 to 3, characterized in that the polymer particles are prepared by surfactant-free emulsion copolymerisation in a batch process.

5. Process according to any one of claims 1 to 4, characterized in that the polymer particles are prepared from a composition comprising a monomer, a crosslinker, an ionic monomer, and an initiator.

6. Process according to any one of claims 1 to 5, characterized in that bi or tri functional acrylates and/or methacrylates are used.

7. Process according to any one of claims 1 to 6, characterized in that anionic acrylates and/or methacrylates are used.

8. Process according to any one of claims 1 to 7, characterized in that cationic acrylates and/or methacrylates are used.

9. Process according to any one of claims 1 to 8, characterized in that the colouring of the polymer particles is done by solvent swelling with a dye at a temperature range from 30 - 60 ⁰C.

10. Process according to any one of claims 1 to 9, characterized in that the polymer particles have a diameter of 50 - 400 nm.

11.Coloured polymer particles obtainable by a process according to any one of claims 1 to 10.

12. Use of a coloured polymer particles according to claim 11 for the preparation of a mono, bi or polychromal electrophoretic device.

13. Colour electrophoretic display device comprising coloured polymer particles according to claim 11.