ELECTROPHORETIC INK

Abstract

Some embodiments relate to an electrophoretic ink including particles that may have a negative charge, dispersed in a non-polar organic solvent. The ink includes a trialkylamine charge-control agent, selected from the following charge-control agents: tributylamine, triisobutylamine, tripentylamine, trihexylamine, tri(2-ethylhexyl)amine, trioctylamine, triisooctylamine, tridodecylamine, triisododecylamine. The particles have a hydrophobic surface and an isoelectric point (IEP) or a point of zero charge (PZC) that is lower than the pKa of the charge-control agent.

Claims

1. An electrophoretic ink, comprising: particles configurable to be negatively charged, dispersed in an apolar organic solvent; and a charge control agent of trialkylamine type, chosen from the following charge control agents: tributylamine, triisobutylamine, tripentylamine, trihexylamine, tris(2-ethylhexyl)amine, trioctylamine, triisooctylamine, tridodecylamine, triisododecylamine, wherein the particles have a hydrophobic surface and an isoelectric point (IEP) or a point of zero charge (PZC) lower than the pKa of the charge control agent.

2. The ink according to claim 1, wherein the charge control agent is a trialkylamine with carbon-based chains, the number of carbons of which is greater than 8.

3. The ink according to claim 1, wherein the charge control agent is tridodecylamine.

4. The ink according to claim 3, wherein the concentration of tridodecylamine in the apolar solvent is between 0.1 and 250 mmol/l.

5. The ink according to claim 1, wherein the particles are modified pigments or hybrid particles including a modified pigment and a surface polymer.

6. The ink according to claim 5, wherein the hybrid particles are more particularly particles including a modified pigment core at the surface of which polymer particles have precipitated.

7. The ink according to claim 5, wherein the pigments are modified by silanization with coupling agents chosen from methyltrimethoxysilane, ethyltrimethoxysilane, butyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane (OTS), decyltrimethoxysilane, dodecyltrimethoxysilane (DTS), hexadecyltrimethoxysilane or octadecyltrimethoxysilane; and preferably OTS or DTS.

8. The ink according to claim 7, wherein the grafting density of the alkyl chains, derived from the coupling agents, at the surface of the pigments is between 3 and 6 mol/m.sup.2.

9. The ink according to claim 5, wherein the pigments are firstly covered with a silica shell, before their modification by silanization.

10. The ink according to claim 1, wherein the apolar solvent is selected from at least one of the following solvents: hydrocarbon-based oils, halocarbon oils, or silicone oils.

11. The ink according to claim 10, wherein the apolar solvent is a hydrocarbon-based oil.

12. A process for manufacturing the electrophoretic ink according to claim 1, comprising: synthesizing particles having a hydrophobic surface, the isoelectric point (IEP) or point of zero charge (PZC) of which is lower than the pKa of the charge control agent, dispersing the synthesized particles in an apolar solvent, adding the charge control agent to the apolar medium in order to negatively charge the hydrophobic particles, the one charge control agent being of trialkylamine type and chosen from the following charge control agents: tributylamine, triisobutylamine, tripentylamine, trihexylamine, tris(2-ethylhexyl)amine, trioctylamine, triisooctylamine, tridodecylamine, triisododecylamine.

13. An electrophoretic display device, comprising: the electrophoretic ink of claim 1; an integrated current having a transistor; a plurality of cells filled with the electrophoretic ink, each cell being in fluidic communication with its neighbour and defining a pixel; a surface electrode and a bottom electrode including a contact pad under each pixel, each pad being connected to the transistor of the integrated circuit configured for controlling the application of an electrostatic force to each pixel.

14. (canceled)

15. The ink according to claim 3, wherein the concentration of tridodecylamine in the apolar solvent is between 1 and 100 mmol/l.

16. A process for manufacturing the electrophoretic ink according to claim 2, comprising: synthesizing particles having a hydrophobic surface, the isoelectric point (IEP) or point of zero charge (PZC) of which is lower than the pKa of the charge control agent, dispersing the synthesized particles in an apolar solvent, adding the charge control agent to the apolar medium in order to negatively charge the hydrophobic particles, the one charge control agent being of trialkylamine type and chosen from the following charge control agents: tributylamine, triisobutylamine, tripentylamine, trihexylamine, tris(2-ethylhexyl)amine, trioctylamine, triisooctylamine, tridodecylamine, triisododecylamine.

17. The ink according to claim 3, wherein the particles are modified pigments or hybrid particles including a modified pigment and a surface polymer.

18. The ink according to claim 4, wherein the particles are modified pigments or hybrid particles including a modified pigment and a surface polymer.

19. The ink according to claim 6, wherein the pigments are modified by silanization with coupling agents chosen from methyltrimethoxysilane, ethyltrimethoxysilane, butyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane (OTS), decyltrimethoxysilane, dodecyltrimethoxysilane (DTS), hexadecyltrimethoxysilane or octadecyltrimethoxysilane; and preferably OTS or DTS.

20. The ink according to claim 6, wherein the grafting density of the alkyl chains, derived from the coupling agents, at the surface of the pigments is between 3 and 6 mol/m.sup.2.

21. The ink according to claim 10, wherein the apolar solvent is a paraffin oil.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] Other advantages and features of some embodiments will appear on reading the following examples given by way of illustrative and nonlimiting example, with reference to the appended figures which represent:

[0035] FIG. 1, graphs of the electrophoretic mobility of various charged particles with standard charge control agents on the one hand and with tridodecylamine on the other hand;

[0036] FIG. 2, a graph of the electrophoretic mobility of a TiO.sub.2@SiO.sub.2OTS modified hydrophobic pigment as a function of the concentration of tridodecylamine in the apolar solvent;

[0037] FIG. 3, a graph of the electrophoretic mobility of a Fe.sub.2O.sub.3-OTS modified hydrophobic pigment as a function of the concentration of tridodecylamine in the apolar solvent;

[0038] FIG. 4, a graph of the electrophoretic mobility of a hydrophobic hybrid particle, including, at the core, a Fe.sub.2O.sub.3-OTS modified pigment and, at the surface, particles of poly(4-vinyl pyridine-co-lauryl acrylate) polymer, as a function of the concentration of tridodecylamine in the apolar solvent;

[0039] FIG. 5, a graph of the surface tension of a drop of deionized water in Isopar-G measured at various concentrations of tridodecylamine in the medium in order to determine the critical micelle concentration CMC of tridodecylamine in the apolar medium.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0040] As a preamble, it is specified that the expressions between and/or less than and/or greater than used within the context of this description should be understood as including the limits mentioned.

[0041] The expression point of zero charge (acronym PZC) denotes the pH of a dispersion in which the charge density at the surface of the particles of the dispersion is equal to zero. The PZC characterizes the acidic or basic property of a particle. Similarly, the term isoelectric point (IEP) itself also denotes the pH of a dispersion in which the charge density at the surface of the particles of the dispersion is equal to zero. The IEP itself also characterizes the acidic or basic property of a particle. The difference between the PZC and the IEP is based on the phenomenon of specific adsorption. Thus, if the quantity measured does not depend on the solution used for measuring it (pH, concentration, nature of the ions), then it is a PZC. In the opposite case, it is an IEP that is measured.

[0042] Irrespective of the value measured, IEP or PZC of the particles, it is measured in water by varying the pH of the solution using a Malvern Nano ZS cell. More specifically, at each pH of the solution, the electrophoretic mobility of the particles is measured. The IEP, or the PZC, corresponds to the pH at which the electrophoretic mobility of the particles is zero.

[0043] The IEP or PZC measurements were carried out on unmodified pigments. The alkyl chains, originating from the OTS or DTS coupling agents used for modifying the surface of the pigments in order to render it hydrophobic, are neutral and do not vary the value of the IEP or PZC.

[0044] A dispersion is understood to mean a colloidal system having a continuous liquid phase and a discontinuous second phase that is distributed throughout the continuous phase.

[0045] The formulation of the electrophoretic ink according to some embodiments advantageously includes chargeable particles, dispersed in an apolar organic solvent, and a charge control agent of trialkylamine type, chosen from the following charge control agents: tributylamine, triisobutylamine, tripentylamine, trihexylamine, tris(2-ethylhexyl)amine, trioctylamine, triisooctylamine, tridodecylamine, triisododecylamine. Advantageously or preferably, this charge control agent is a trialkylamine with carbon-based chains, the number of carbons of which is greater than 8. More advantageously or preferably still, the charge control agent is tridodecylamine, also denoted by Dod.sub.3N in the remainder of the description. The chargeable particles are particles having an IEP or PZC lower than the pKa of the charge control agent used and having a hydrophobic surface.

Regarding the Charge Control Agent

[0046] Tridodecylamine is a strong organic base, the pKa of which is equal to 10.83. This amine reacts, by acid-base reaction, with hydroxyl groups present at the surface of the particles. Pairs of ions are then created with, on the one hand, a metal alcoholate for example, if the particle is a metal oxide, and an ammonium countercation on the other hand, soluble in the apolar solvent. In an apolar medium, the dissociated charges provide greater electrostatic forces than in a polar medium. A tiny portion of these pairs of ions dissociating in the apolar medium is then sufficient to make it possible to induce negative charges at the surface of the particles and thus render them electrophoretic. Typically, the dissociation of one pair of ions out of 1000 million pairs of ions is sufficient to negatively charge the particles. The article entitled Charge Generation in Low-Polarity Solvents: Poly(ionic liquid)-Functionalized Particles, published in the journal Langmuir, 2013, 29(13), p. 4204-4213, Hussain, G., A. Robinson, and P. Bartlett; describes such a dissociation of an ionic species, to give a [Dod.sub.4]N.sup.+ quaternary amine on the one hand and a [TPhB].sup. borate counteranion on the other hand, in an apolar medium.

[0047] Too low a concentration of Dod.sub.3N in the apolar solvent does not make it possible to correctly charge the particles and too high a concentration risks leading to the formation of reverse micelles, which it is precisely desired to avoid. Advantageously, the concentration of tridodecylamine in the apolar solvent is between 0.1 and 250 mmol/l, advantageously or preferably between 0.5 and 150 mmol/l, and more advantageously or preferably between 1 and 100 mmol/l.

Regarding the Particles

[0048] The particles suitable for being negatively charged are acidic or basic particles, which have an isoelectric point IEP or point of zero charge PZC lower than the pKa (of 10.8) of tridodecylamine, and having a hydrophobic surface. They have a size of between 250 nm and 2 m. They are chosen from any colored particle, which has hydroxyl groups at its surface and which is more acidic than tridodecylamine.

[0049] Thus, the particles may be chosen from inorganic particles, such as modified inorganic pigments for example or from hybrid particles including a modified inorganic pigment at the core and polymer particles at the surface.

[0050] The inorganic pigments may for example be chosen from metal oxides.

[0051] The more acidic the pigments are with respect to tridodecylamine, the more they can be negatively charged, and the more negatively charged they are, the greater their electrophoretic mobility.

[0052] When the inorganic pigments do not have a sufficient acidity, it is possible to adjust the acid-based interactions at the surface of the pigments by covering them with a silica shell, leading to core-shell type pigments being obtained, which are stable in an apolar organic medium and which have acidic properties.

[0053] When the inorganic pigments have a weakly hydrophobic or hydrophilic surface, they may advantageously be modified, by silanization, in order to render their surface hydrophobic. For this, coupling agents, chosen from methyltrimethoxysilane, ethyltrimethoxysilane, butyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane (OTS), decyltrimethoxysilane, dodecyltrimethoxysilane (DTS), hexadecyltrimethoxysilane or lastly octadecyltrimethoxysilane, are grafted to the surface of the particles. Advantageously or preferably, the coupling agents are octyltrimethoxysilane (OTS) or dodecyltrimethoxysilane (DTS). The degree of grafting of the alkyl groups, derived from these coupling agents, to the surface of the inorganic particles, makes it possible to quantify the transfer of hydrophobicity to the surface of the particles.

[0054] The degree of grafting is determined from elemental analysis of the carbon on the unmodified inorganic pigment and on the inorganic pigment silanized by OTS or DTS coupling agents. More particularly, the degree of grafting N is determined from the following formula:

[00001]$N\left(\mathrm{mol}.m^{-2}\right)=\frac{C\left(\%\right).Math.m_{\mathrm{part}.}}{1.Math.0.Math.01.Math.2N_C}\frac{1.Math.0^6}{S_{\mathrm{part}.}},$

[0055] in which C (%) is the carbon content of the modified pigment, determined by elemental analysis of the carbon, S.sub.part is the surface area of the modified pigment (m.sup.2), determined from the diameter of the pigment by electron microscopy, m.sub.part (g) is the mass of the particle, determined from the density and from the size of the pigment. N.sub.c is the number of carbon atoms forming the OTS or DTS groups. The diameter of the hydrophilic pigment is considered to be identical to that of the modified pigment. Indeed, the OTS and DTS groups are assumed not to affect the diameter of the pigment due to their size (of 10 ), which is negligible with respect to the diameters of the particles that range from 117 to 210 nm depending on the nature thereof.

[0056] Thus, the grafting density of the alkyl chains derived from the OTS and DTS groups at the surface of the pigments was determined and is between 3 and 6 mol/m.sup.2. Starting from the principle that the number of hydroxyl groups in the initial state, at the surface of the unmodified pigments, is 8 mol/m.sup.2, as described in the article entitled Encapsulation of Inorganic Particles by Dispersion Polymerization in Polar Media: 1. Silica Nanoparticles Encapsulated by Polystyrene, Bourgeat-Lami, E. and J. Lang, Journal of Colloid and Interface Science, 1998. 197(2): p. 293-308, a degree of grafting of between 35% and 75%, advantageously or preferably between 50% and 70% is obtained.

[0057] The size of the pigments before and after modification was also measured by the dynamic light scattering (DLS) technique. Before modification, the pigments are hydrophilic and aggregated. After modification, the pigments become hydrophobic and their size is of the order of a micrometre. The surface modification of the pigments by OTS or DTS groups therefore improves the dispersion of the particles in the apolar medium. The addition of tridodecylamine makes it possible, by electrostatic repulsion, to further reduce the size of the particles to between 300 and 600 nm, thus improving the dispersion of the particles in the medium.

[0058] The chargeable particles may also be hybrid particles, including a core including or consisting of inorganic pigment and a polymer surface. These hybrid particles may for example have morphologies of raspberry type or else core-crown type. These hybrid particles are stable in an apolar organic medium. They are synthesized by dispersion polymerization in an apolar medium using a macroinitiator. The polymer thus formed at the surface of the inorganic pigment makes it possible to reduce the density of the hybrid particles and promotes the dispersion thereof. This polymer surface is synthesized from functional monomers that may be chosen from 4-vinylpyridine or an acrylic or methacrylic acid and derivatives thereof, optionally copolymerized with another neutral monomer such as styrene or MMA (methyl methacrylate) for example.

Regarding the Apolar Solvent

[0059] The apolar solvent is advantageously chosen from liquid alkanes, liquid haloalkanes, or else liquid silicones. More particularly, it is chosen from halocarbon oils, hydrocarbon-based oils or silicone oils.

[0060] Among the halocarbon oils, mention may for example be made of chlorotrifluoroethylene, sold under the references halocarbon 1.8 or halocarbon 0.8, or else tetrafluorodibromoethylene, tetrachloroethylene, 1,2,4-trichlorobenzene, or else tetrachloromethane.

[0061] Among the hydrocarbon-based oils, mention may for example be made of paraffin oils, heptane, dodecane, tetradecane, etc.

[0062] Among the silicone oils, mention may for example be made of the fluid silicone oils sold by Dow Corning under the reference DOW 200, or else octamethylcyclosiloxane, poly(methylphenylsiloxane), hexamethyldisiloxane or polydimethylsiloxane.

[0063] Advantageously or preferably, the apolar solvent is chosen from hydrocarbon-based oils, and preferentially from paraffin oils. More advantageously or preferably, the apolar solvent is chosen from the paraffin oils manufactured and sold by Exxon under the commercial reference Isopar, and more particularly the oil sold under the reference Isopar G.

EXAMPLES

Example 1: Synthesis of Coloured Particles Capable of being Negatively Charged

a) Modified Inorganic Pigments

[0064] Various particles were synthesized in order to be compared with one another.

[0065] The inorganic pigments used are metal oxides, more particularly titanium dioxide TiO.sub.2 and ferric oxide Fe.sub.2O.sub.3.

[0066] The isoelectric points IEP of these two unmodified pigments were measured respectively at 7.6 and 8.4, expressing their basic nature. The isoelectric point of the unmodified pigments was measured in water with a Malvern Nano ZS cell while varying the pH of the solution. More specifically, at each pH, the electrophoretic mobility of the particles was measured. The IEP corresponds to the pH at which the electrophoretic mobility of the particles is zero.

[0067] In order to regulate the acid-base interactions at the surface of these pigments, they are covered with a silica shell. Core-shell type particles of TiO.sub.2@SiO.sub.2 were thus synthesized. The synthesis of the SiO.sub.2 shell is carried out by following the process developed by Stber W., A. Fink, and E. Bohn and described in the document entitled Controlled growth of monodisperse silica spheres in the micron size range, Journal of Colloid and Interface Science, 1968. 26(1): p. 62-69. The isoelectric point was measured for these particles at 3.10, expressing their acidic nature.

[0068] Since these pigments are pigments with a hydrophilic surface, they were modified by silanization carried out with octyltrimethoxysilane (OTS) or dodecyltrimethoxysilane (DTS). For this, the hydrophilic pigment is mixed with toluene, in an amount of 50 g/l and 3.86 mmol of OTS (0.907 mg), or 3.06 mmol of DTS (0.89 mg), then heated under reflux for 15 h. The pigments are subsequently washed by cycles of centrifugation/redispersion in toluene and then dried in the oven at 50 C. under vacuum.

[0069] Another method of silanization may be to carry out this modification in bulk by introducing the pigment directly into a solution of OTS or DTS (in an amount of 50 g/l).

[0070] Irrespective of the method used, the degree of grafting is of the same order of magnitude. The grafting density of the alkyl groups, derived from the coupling agents, at the surface of the inorganic particles was determined from the elemental analysis of the carbon on the unmodified and modified pigments, and as described above. The greater the grafting density, the more the particle has a hydrophobic surface. The particles thus modified are denoted by TiO.sub.2-OTS, TiO.sub.2-DTS, Fe.sub.2O.sub.3-OTS or Fe.sub.2O.sub.3-DTS or else TiO.sub.2@SiO.sub.2OTS and TiO.sub.2@SiO.sub.2-DTS when they are firstly covered with a silica shell.

[0071] Through these coupling agents, the grafting density of the alkyl chains derived from the OTS and DTS groups, at the surface of the pigments, is between 3 and 6 mol/m.sup.2. Such a density corresponds to a degree of between 35% and 75%, advantageously or preferably between 50% and 70%. The surface of the pigments is then rendered hydrophobic, and the higher the degree of grafting, the higher the hydrophobicity too. Hydroxyl groups remain however available at the surface of the pigments to enable the acid-base reaction with Dod.sub.3N and to thus enable the negative charging of the pigments.

b) Hybrid Particles

[0072] In order to synthesize hybrid particles, including an inorganic pigment at the core and polymer particles at the surface, a first step consists in synthesizing a macroinitiator. This macroinitiator will enable not only the polymerization of the polymer particles around the pigment, but also the stabilisation of the particles in the apolar organic medium and the control of their sizes so that they are all homogeneous.

[0073] A macroinitiator denotes an additive composed of a hydrophobic polymer chain, used for the stabilisation of the particles, and of an initiating portion which is used for starting the polymerization reaction and ultimately leads to the formation of a copolymer. The macroinitiator is advantageously synthesized by nitroxide-controlled free radical polymerization with an initiator manufactured and sold by Arkema under the Blocbuilder brand. After the initiation of the polymerization reaction on the macroinitiator, an amphiphilic copolymer is formed with a (stabilizing) hydrophobic block and a hydrophilic block which, via its precipitation, will be the source of nuclei. The latter will then, during the synthesis, coalesce and form particles. Thus, the hydrophobic polymer chains of the macroinitiator remain connected to the particles and may thus stabilize them in the apolar organic medium.

[0074] In order to synthesize the hybrid particles, by dispersion polymerization in an apolar medium, use is made of the macroinitiator, poly(lauryl acrylate), synthesized by nitroxide-controlled free radical polymerization with an initiator manufactured and sold by Arkema under the Blocbuilder brand, in toluene. Once the macroinitiator has been synthesized and purified, it is mixed in the apolar solvent, for example Isopar-G, with a hydrophilic monomer, for example chosen from 4-vinylpyridine, acrylic acid or methyl methacrylate for example, and the modified pigment, so as to synthesize the hybrid particles including a modified pigment core at the surface of which polymer particles have precipitated.

[0075] For this, 3 g of modified pigment (Fe.sub.2O.sub.3-OTS) are mixed with 3 g of macroinitiator, poly(lauryl acrylate), in 90 ml of Isopar-G. The macroinitiator assists with the stabilization of the pigment particles in the apolar solvent. This solution is mixed in an ultrasonic bath using an ultrasonic probe. It is then poured into a mechanically stirred reactor. 10 g of functional monomers, 4-vinyl pyridine, are then added along with 1.5 g of macroinitiator in order to initiate the reaction. The solution is then degassed under nitrogen for 1 hour, then heated at 120 C., with mechanical stirring at 300 rpm for 15 hours. Once synthesized, the (Fe.sub.2O.sub.3-OTS/Poly(4-VP-co-LA) particles obtained are washed by centrifugation and redispersion in Isopar-G.

Example 2: Formulation of the Electrophoretic Ink

[0076] The particles synthesized, whether they are in the form of modified pigments or hybrid particles, are dispersed in Isopar G. Next, between 1 and 100 mmol/1 of tridodecylamine are added in order to negatively charge the particles.

[0077] An ink is thus synthesized for each particle.

[0078] The electrophoretic mobility of the electrophoretic particles of the inks thus synthesized is measured by the PALS (acronym for Phase Analysis Light Scattering) technique using a Malvern Nano ZS cell designed for an apolar medium. A square-wave signal ranging from 2.5 to 20 kV/m is applied to the cell. This technique consists in measuring the phase shift between the incident wave and the wave reflected by a mobile electrophoretic particle in dispersion. The ink samples analyzed include 0.005% by weight of particles in Isopar G.

[0079] Since tridodecylamine is a strong base, with a pKa equal to 10.83, it provides more negative charges at the surface of the particles than the known charge control agents, of OLOA type for example. Thus, the electrophoretic mobility of the particles charged with tridodecylamine is higher (as an absolute value) than those which have been charged with OLOA 11000, Span 80 and AOT.

[0080] FIG. 1 illustrates this electrophoretic mobility of the charged hydrophilic particles with, on the one hand, particles charged with Span 80, OLOA 11000 and AOT and, on the other hand, hydrophobic particles charged with Dod.sub.3N (at a concentration of 16 mmol/1 in Isopar G). In this figure, the highest electrophoretic mobility of a particle, as an absolute value, is plotted as a function of its point of zero charge PZC. Thus, for example, the TiO.sub.2-OTS hydrophobic pigment charged with Dod.sub.3N has a PZC of 7.6 and an electrophoretic mobility of 0.10 mcm/Vs when 16 mmol/1 of tridodecylamine are added to Isopar G.

[0081] In this FIG. 1, it is observed that the hydrophilic pigment particles, represented by solid symbols, do not have a very high electrophoretic mobility, at most of the order of 0.075 mcm/Vs as an absolute value. The hydrophobic modified pigments, represented by open symbols in FIG. 1, and negatively charged with Dod.sub.3N (at a concentration of 16 mmol/1 in Isopar G), have a higher electrophoretic mobility, typically between 0.27 and 0.10 mcm/Vs as an absolute value.

[0082] In comparison, in the article entitled Effect of alkyl functionalization on charging of colloidal silica in apolar media, Journal of Colloid and Interface Science 351 (2010) p. 415-420, Saran Poovarodom, Sathin Poovarodom and John C. Berg studied the electrophoretic mobility of silica particles, the surface of which was modified by hexadecyltrimethoxysilane to render it hydrophobic, in Isopar-L and negatively charged with various standard charge control agents chosen from AOT, OLOA 11000 and zirconyl 2-ethyl hexanoate (ZrO(oct).sub.2). This article states that the modified silica particles, with a hydrophobic surface, have a higher electrophoretic mobility, as an absolute value, than the mobility of the same unmodified particles. However, irrespective of the charge control agent used from among the standard charge control agents for charging these particles, the electrophoretic mobility of the hydrophobic particles remains much lower than that measured when the particles are charged with tridodecylamine. Indeed, the electrophoretic mobility measured on particles charged with standard CCAs varies, as an absolute value, between 0.01 and 0.062 mcm/Vs depending on the charge control agents used, whereas it is between 0.1 and 0.4 mcm/Vs with Dod.sub.3N.

[0083] The mobility of each hydrophobic particle was also measured as a function of the concentration of tridodecylamine in the apolar solvent. FIG. 2 thus represents the graph of the electrophoretic mobility of the TiO.sub.2@SiO.sub.2OTS modified pigment as a function of the concentration of Dod.sub.3N in Isopar-G. It turns out that the mobility of this pigment is maximum, as an absolute value, with a concentration of Dod.sub.3N in Isopar-G of 16 mmol/l. In this case, the maximum mobility is equal to 0.38 mcm/Vs.

[0084] Similarly, FIG. 3 represents the graph of the electrophoretic mobility of the Fe.sub.2O.sub.3-OTS modified pigment as a function of the concentration of Dod.sub.3N in Isopar-G. It turns out that the mobility of this pigment is maximum, as an absolute value, with a concentration of Dod.sub.3N in Isopar-G of 16 mmol/l. In this case, the maximum mobility is equal to 0.33 mcm/Vs.

[0085] FIG. 4 represents the graph of the electrophoretic mobility of an Fe.sub.2O.sub.3-OTS hybrid particle with, at the surface, poly(4-vinylpyridine-co-lauryl acrylate) polymer particles, as a function of the concentration of Dod.sub.3N in Isopar-G. It turns out that the mobility of this pigment is maximum, as an absolute value, with a concentration of Dod.sub.3N in Isopar-G of 32 mmol/l. In this case, the maximum mobility is equal to 0.11 mcm/Vs.

[0086] Generally, the maximum mobility, as an absolute value, for each of the hydrophobic particles analyzed, was measured for a concentration of tridodecylamine of between 8 and 32 mmol/1 in Isopar G.

[0087] Tridodecylamine therefore makes it possible to negatively charge particles, the isoelectric point (IEP) or point of zero charge (PZC) of which is lower than the pKa of tridodecylamine and the surface of which is hydrophobic, and to obtain electrophoretic particles having a better electrophoretic mobility than with the standard charge control agents. Since the concentration of tridodecylamine in the ink is lower than the critical micelle concentration CMC, which was determined at 250 mmol/l, the electrophoretic ink obtained has no reverse micelle capable of degrading the mobility of the particles over time. The display devices including such an ink therefore have a significantly increased lifetime.

[0088] The CMC was determined by measuring the surface tension of a drop of deionized water, in Isopar-G, by the pendant drop method, at various concentrations of tridodecylamine in the apolar medium, using a Kruss FM3200 tensiometer. Each surface tension value plotted on the graph from FIG. 5 corresponds to an average of five measurements.