US3920532A - Process for electrodeposition of a dispersion of finely divided substances in an apolar dispersing agent - Google Patents

Abstract

A dispersion of finely divided substances in an apolar dispersing agent, to an electrophoretic method using such a dispersion and to an electrophotographic method using such a dispersion as a developer.

Description

United States Patent [191 Hansen et a1.

[11] 3,920,532 [451 Nov. 18, 1975 [54] PROCESS FOR ELECTRODEPOSITION OF A DISPERSION OF FINELY DIVIDED SUBSTANCES IN AN APOLAR DISPERSING AGENT [75] Inventors: Norbert Ernst Fritz Hansen;

Siegfried Stotz, both of Aachen,-

Germany [73] Assignee: U.S. Philips Corporation, New

York, NY.

[22} Filed: July 6, 1973 21 Appl. No.; 377,068

Related US. Application Data [62] Division of Ser. No. 43,223, June 3, 1970, Pat. No.

[30] Foreign Application Priority Data June 6, 1969 Germany 1928817 Primary Examiner-Howard S. Williams Attorney, Agent, or Firm-Frank R. Trifari; Norman N. Spain [57] ABSTRACT A dispersion of finely divided substances in an apolar dispersing agent, to an electrophoretic method using such a dispersion and to an electrophotographic method using such a dispersion as a developer.

2 Claims, No Drawings PROCESS FOR ELECTRODEPOSITION OF A DISPERSION OF FINELY DIVIDED SUBSTANCES IN AN APOLAR DISPERSING AGENT This is a division of application Ser. No. 43,223, filed June 3, 1970, and now U.S. Pat. No. 3,766,125.

Electrophoresis is understood to mean the transport of dispersed particles in an electric field, which transport is brought about because these particles carry an electric charge. Many uses of electrophoresis in polar dispersing'agents are known in technology. Charging of the particles is based on the formation of 'ions by electrolytic dissociation in polar dispersing agents having a high dielectric constant such as water or alcohols whose dipolar molecules surround the ion. The solvatation energy of this process is equal to or larger than the lattice energy of an ionic compound and 'renders the dissolution of such a compound in the dispersing agent possible. To obtain a charge the colloidal particle itself must include dissociable groups, for example, macromolecules having built-in carboxyl or hydroxyl groups, or such particle absorbing an ion of a low molecular weight electrolyte. The mechanism of electrophoresis in polar dispersing agents is based on methods of depositing finely divided substances on objects which for this purpose are arranged as electrodes and have a conducting surface. Known technical uses are among others coating oxide cathodes and lacquering motor bodies and other objects.

In certain cases there are great limitations in using the method for the electrophoretic deposition of particles. Substances which are soluble in water or react chemically in a different manner cannot be used in aqueous systems. The comparatively high natural con ductivity of these systems bars their use in those cases wherein only a limited quantity of the charges generating the field is available and wherein as great a possible mass for each transported charge must be deposited.

Furthermore it has already been proposed to use apolar organic liquids having a small dielectric constant and a high electric resistance as dispersing agents. The formation of ions by means of dipolar solvatation in the manner described above is not possible in these dispersing agents. Small solid-substance particles may, however, be charged by barrier layer effects, triboelectrical effects or contact potential differences, a particle having a dielectric constant which is higher than that of the dispersing agent being positively charged and being negatively charged in the converse case. This natural charge may be sufficient for small particles (approximately 0.1 pm) for transport in an electric field;

on the other hand larger particles cannot be deposited on a flat electrode. In this case the so-called dielectrophoresis in non-homogeneous electric fields occurs. This occurs in such a manner that an uncharged or a weakly charged particle in the electric field is polarized. In the non-homogeneous field a net attraction of gravity towards a higher field strength is obtained independent of the polarity of the electrode, when the dielectric constant or the polarisability is larger than that of the surrounding dispersing agent. The attractive force is proportional to the polarisability and to the volume of the particle and the gradient of the square of the field strength. Larger and insufficiently charged particles can thus be deposited on the edges of the electrodes and on irregularities and on corners and edges of charging profiles because non-homogeneous field distributions occur in these areas.

. 2 For a uniform electrophoretic deposition of particles larger than 0.1 pin, up to approximately 10 gm as occur in most cases in a homogeneous electric field between two electrodes in apolar media it must be attempted to find a possibility to sufficiently charge these particles. The same applies to the deposition of pigments which are proportional to the charge on differently charged dielectric surfaces as occur in the latent charge image of an image-reproducing layer in electrostatic electrophotography. In that case it must be possible to check and control the charge of the particles satisfactorily in order to deposit by means of an optimum ratio between charge and mass as many as possible particles per surface charge and to obtain a smooth electrophoretical behaviour.

German Pat. No. 1,047,616 describes a method of developing electrostatic charge images wherein a dispersion of pigment particles in apolar media such as cyclohexane is used. The pigment particles are electrically rendered selective for the image charge by adding a substance which controls the pigment. This substance gets to surround the particle and to influence charging by adjusting a given ratio between its dielectric constant and the dielectric constant of the solvent. Such a substance is, for example, an alkyd resin, an alkyd resin modified with linseed oil or boiled linseed oil itself. The two first-mentioned compounds must charge the pig ment particle positively while the last-mentioned comcess they are hardened. In that case a known hardening agent, namely a soap of a heavy metal such as lead or cobalt naphthenate is used. The agent is already added to the developer solution.

In addition more has become known about the stability of suspensions in apolar media. On the one hand the steric or entropic stabilization effect is known and on the other hand the stabilization by electrostatic repelling of particles charged at the same polarity. Thus, suspensions may be stabilized both with ionic and nonionic substances from the group of surface-active substances. It was found that solutions of ionic stabilizers 1 have an electric conductivity which is 10 times higher than that of the pure apolar solvents. Thus it must be possible to form ions also in apolar media, even at a very slight degree of dissociation. For an ionic sub stance in an apolar solvent, it must be assumed that as a result of the structure of the substances having these effects micelles are formed from a lyophilic organic acid radical and a lyophobic cation, which micelles may envelope and shield a few lyophobic cations, while a few lyophilic radicals are present in the solution. If additional particles are present in a suspension the lyohpobic cation can be adsorbed in polar groups at the surface of the particles. The charge may thus be regarded to be a dissociative adsorption wherein a certain type of ion is bound in an adsorption phase while the other ion is present in the solvent.

The experiments discussed in the preceding paragraphs were performed in comparatively strong solvents such as benzene or xylene on fairly small particles l um) having polar surfaces (oxides). However, it was found that no satisfactory electrophoretic deposition'in the sense of the invention could, be obtained under the said circumstances. The following difficulties occur among others:' due to still unexplained conditions on the electrodes it is difficult to obtain an adherent deposition to the electrodes with the aid of the particles transported by electrophoretic process.

For electrophotographic and electrographic techniques all particles must at an average carry the same charge of a certain polarity because otherwise the pigmentation proportional to the charge is not assured and deposition occurs in unwanted areas. In addition to charged particles, insufficiently charged particles frequently occur in comparatively large particles so that not only electrophoresis but also dielectrophoresis is observed. As a result strong potential differences as occur at the edges in the image are particularly strongly pigmented.

Ions of the polarity which are adsorbed at the solid particle may also occur in micelles so that the charge image for the deposition cannot be utilized optimally because the micelles neutralize the charge unused.

An object of the present invention is to provide a dispersion of solid particles in apolar solvents which does not give the above-mentioned drawbacks and wherein solid particles of an arbitrary material nature such as metals, semiconductors and dielectrics having a grain size of approximately 0.01-10 um and in special cases even larger particles are charged.

According to the invention the dispersion of a finely divided substance in an apolar dispersing agent having an electrical conductivity which is smaller than 10 9 cm' is characterized in that the dispersion includes surface-active ion-forming substances which are soluble in the dispersing agent and are dissociable in such a manner that their conductivity in a 10' molar solution is greater than 10" *cm and preferably between 1 X and l X 10 0 "cm and that these substances comprise bivalent or multivalent ions.

Such a multivalent compound dissociates into an ion consisting of the metal atom and the remaining lyophilic acid radical and into an oppositely charged lyophilic radical itself. A metal ion including a remaining lyophilic acid radical formed in this manner may be stabilised either into a micelle or into an adsorption phase built up similarly in an easier manner than a single alkali ion. Alternatively mixtures of several of these substances may yield particularly favourable results. The concentration of the substance to be added must lie between certain limits: When this concentration is toolow dielectrophoresis occurs and when it is too high the charges of a charge pattern are compensated by micelles of the same polarity as the pigment particles so that only a small deposit thickness can be obtained. A

. series of simple tests enables anyone skilled in the art to determine the active concentration range for each individual case.

4 acrylic acid esters having a molecular weight of 10 -10, polyacrylic acid esters, polyalkyl styrenes, polyvinyl alkyl ethers and copolymers or polymethacrylates having cyclic amides and fumarates.

By adding such a macromolecular compound which is denoted by the term intensifying and stabilising agent" it is possible to render potentially ionic substances usable by which a usable suspension as such for an electrophoretic deposition within the scope of the present invention cannot be manufactured. Dispersions can be stabilised in an optimum and reproducible manner with these substances. In this manner surface coatings of more than 2 mg/cm can be obtained by means of an electrophotographic technique. The usual quantities of electrophotographically deposited pigment are slightly more than 0.1 mg/cm It is alternatively possible tocharge one and the same pigment either unambiguously positively or unambiguously negatively by using different ionic substances.

The quantities to be used are dependent on the concentration of the particles, the nature and the size of their surface and on the manner of dispersing. As a result of the synergistic action of the two additions a quantity of ionic substance is used which is much smaller than when only the last-mentioned substance is used. The intensifying and stabilising agent is first dispersed with the pigment, optionally together with the pigment and the added ionic substance, The optimum activity frequently cannot be obtained if this substance is added to the suspension at a later stage.

Ionic substances which are soluble in isoparaffins are, for example,: allkaline earth alkyl sulfonates containing 24 32 carbon atoms such as Ca(SO C H basic alkaline earth alkyl sulfonates such as R-SO BaOCO+ BaSO R wherein R may be a mixture of different radicals, salts of fatty acids such as Mg, Ca and Ba oleates, Co-naphthenate, salts of alkyl salicylic acid such as calcium diisopropyl salicylate, alkaline earth and aluminum salts of alkyl esters of sulphosuccinic acid, such as Ca-dodecyl sulphosuccinate or cetyl-, octyl or stearyl titanate. Mixtures of these substances may alternatively be active.

Particularly active combinations of these ionic substances are mixtures of an alkyl salicylate including cetyl titanate or an alkyl salicylate including an alkyl sulphosuccinate.

A dispersion has already been described hereinbefore wherein pigment particles are dispersed in cyclohexane while adding an alkyd resin,,an alkyd resin modified with linseed oil or linseed oil itself including Pb or Co-naphthenate as a hardening agent, which, however, is not based on the recognition of the present invention. These combinations are therefore explicitlyexcluded from the rights applied for.

The dispersion according to the invention is described hereinafter with reference to a few Examples.

EXAMPLE 1 A mixture of solid substances having the following composition:

0.5 g of Zn-activated ZnCdS having a grain size of approximately 1.5 pm.

0.015 g of a mixture of Ca-alkyl salicylate and Cadodecyl sulphosuccinate 0.08 g of a copolymer of polymethacrylate including cyclic amides having a molecular weight of several times 10',

was dispersed in 100-300 g'ms' of isoparaffin (C ,C, having a boiling range of between 180 and 210C. After electrophoresis at a field strength 'of 50-100 V.cm this dispersion yielded very homogeneous, compact layers having a smooth surface These layers are suitable for luminescent screens having a high resolving power. v i

In this method the solid substance may be replaced by any other substance while the nature and the concentration of the additions may be changed and adapted to the envisaged object and to the condition of the solid substance. Luminescent substances which are soluble in water such as, for example, cesium iodide may be deposited advantageously in this manner. Metals, for example, silver may be deposited for intensifying metallic conductive paths. Ceramic and metallic pulverulent magnetic materials may be used for the manufacture of magnetic layers and structures.

EXAMPLE 2 An electrophographic developer was prepared by dispersing the following mixture:

1 g of gas soot which was superficially rendered oxygen-containing by post-oxidation and had a particle size of approximately 0.1 urn in the developer; primary particle size 0.03 pm,

0.8 g of basic Ba sulphonate of C l-I in 1000 gms of isoparaffin of the quality according to Example 1. Very fine-grained images were obtained with this very sensitive developer. Zinc oxide papers which are usually charged to several hundred Volts of surface potential by means of a corona discharge could be developed to full density even at 30-70 V. This technique is a condition for a high quality reproduction of half tones. Zinc oxide papers having a great charge produced stained and mottled images as a result of an non-homogeneous charge distribution and additionally showed a steep gradation.

When being used as a negative developer the exposed areas are pigmented on the negatively charged zinc oxide; thus this developer is suitable for a negative-positive process.

Positively charged soot developersare very difficult to manufacture. Usually soot-pigmented synthetic resins or pastes having a high content of synthetic resin are used as basic material for such developers. Such a developer may, however, be manufactured easily from pure soot having few additions with the aid of the stabilising agents. This developer has, for example, the following composition:

1 g flame soot 0.1 g Ca-diisopropyl salicylate I 0.4 g polymethacrylic acid alkyl ester having side chains of -20 C-atoms and a molecular weight of approximately 7% X 10" dispersed in 1000 gms of isoparaffin of the quality according to Example 1.

In many technological processs photographic methods are used to obtain patterns on carrier materials by means of additive or subtractive methods (photoetfrh-f ing, photohardening method). It is evident to obtain the pattern in the form ofa charge image for additive methods which image may be obtained electrophotographiabove-described method, strict requirements are imposed on the optimum controllable charging capacity of the particles.

An interesting use of the dispersion according to the invention consists in the manufacture of the luminescent layer on the inner side of television picture tubes and particularly of a pattern comprising three luminescent materials for the screens of colour television picture tubes. For this purpose the inner side of the picture screen is provided with a conducting layer and a photoconducting organic layer is provided on said layer; an electric charge is supplied by a corona discharge to the photoconducting layer whereafter the charged layer is exposed in accordance with the desired pattern and is treated with the liquid dispersion of luminescent materials according to the invention. This process is repeated for the two other luminescent materials. The conducting layer and the photoconducting layer are finally removed by heating while simultaneously the luminescent layers are compacted by sintering.

The following dispersions are described as Examples for the manufacture of screens for colour television picture tubes according to the above-mentioned methods.

A mixture having the following composition:

3 g activated ZnS (blue luminescent material) grain size approximately 5 pm,

0.03 g of a mixture of Ca-diisopropyl salicylate and cetyl titanate (l 1) 0.5 g of a polyacrylic acid alkyl ester, molecular weight several times 10 was first dispersed in:

g isoparaffin (C -C boiling range 4060C),

while using ultrasonic treatment, whereafter the dispersion obtained was dispersed in 250 gms of isoparaffin (C -C boiling range 2 l 0C) and was diluted to 1,500 mls. for use. The dispersing was carried out by an ultrasonic treatment so as to expose the luminescent material as little as possible to mechanical load which reduces its light output. To influence the cavitation for obtaining an effective irradiation in a favourable sense, a low boiling isoparaffin was used first.

A mixture of the following composition:

3 g activated YVO (red luminescent material) grain size approximately 5 um,

0.003 g of a mixture of Ca-alkylsalicylate and Ca-dodecylsulphonate.

0.2 g of a polymethacrylic acid alkyl ester having side chains of C H and a molecular weight of 8 X 10 was dispersed in isoparaffin likewise as the blue luminescent material. In this dispersion the particles were positively charged.

The following mixture was used for the dispersion of a negatively charged (green) luminescent material:

3 g activated ZnCdS, grain size approximately 5 pm,

0.2 g Ba-oleate 0.3 g of a polyalkyl styrene having alkyl side, chains of more than 4 carbon atoms.

What is claimed is:

7 1. ln an'electrophoretic method of manufacturing luminescent picture screens the improvement which comprises employing a dispersion in a mixture of isoparaffins apolar dispersing agents having an electrical conductivity of less than 10' cm. of finely divided particles capable of being deposited on an electrode by electrophoresis and having a particle size of about 0.01- am, at least one surface-active ionforming substance, said surface-active ion-forming substance being soluble in said dispersing agent, having a dissociation capacity such that its electrical conductivity in a l0 molecular solution lies between about I X l0 and 1 X 10 0 cm." and comprising bivalent and multivalent ions and a soluble nonionic macromolecular compound having at least one side chain of at least 4 carbon atoms.

2. In an electrophoretic method of depositing finely divided particles on an electrode in a homogeneous field, the improvement which comprises employing a dispersion in a mixture of isoparaffins apolar dispersing agents having an electrical conductivity of less than lO'"Q'cm. of finely divided particles capable of being deposited on an electrode by electrophoresis and having a particle size of about 0.0l-l0um, at least one surface-active ion-forming substance, said surface-active ion-forming substance being soluble in said dispersing agent, having a dissociation capacity such that its electrical conductivity in a 10 molecular solution lies between about l X l0- and l X IO OF cm. and comprising bivalent and multivalent ions and a soluble non-ionic macro-molecular compound having at least one side chain of at least 4 carbon atoms.

Claims (2)

1. IN AN ELECTROPHORETIC METHOD OF MANUFACTURING LUMINESCENT PICTURE SCREENS THE IMPROVEMENT WHICH COMPRISES EMPLOYING A DISPERSION IN A MIXTURE OF IOSPARAFFINS APOLAR DISPERSING AGENTS HAVING AN ELECTRICAL CONDUCTIVITY OF LESS THAN 10**-14 $-1 CM.-1 OF FINELY DIVIDED PARTICLES CAPABLE OF BEING DEPOSITED ON AN ELECTRODE BY ELECTROPHORESIS AND HAVING A PARTICLE SIZE OF ABOUT 0.01-10 UM, AT LEAST ONE SURFACE-ACTIVE IONFORMING SUBSTANCE, SAID SURFACE-ACTIVE ION-FORMING SUBSTANCE BEING SOLUBLE IN SAID DISPERSING AGENT, HAVING A DISSOCIATION CAPACITY SUCH THAT ITS ELECTRICAL CONDUCTIVITY IN A 10**-3 MOLECULAR SOLUTION LIES BETWEEN ABOUT 1 X 10**-10 AND 1 X 10**-1 $-1 CM.-1 AND COMPRISING BIVALENT AND MULTIVALENT IONS AND A SOLUBLE NONIONIC MACRO-MOLECULAR COMPOUND HAVING AT LEAST ONE SIDE CHAIN OF AT LEAST 4 CARBON ATOMS.

2. In an electrophoretic method of depositing finely divided particles on an electrode in a homogeneous field, the improvement which comprises employing a dispersion in a mixture of isoparaffins apolar dispersing agents having an electrical conductivity of less than 10 14 Omega 1cm. 1 of finely divided particles capable of being deposited on an electrode by electrophoresis and having a particle size of about 0.01-10 Mu m, at least one surface-active ion-forming substance, said surface-active ion-forming substance being soluble in said dispersing agent, having a dissociation capacity such that its electrical conductivity in a 10 3 molecular solution lies between about 1 X 10 10 and 1 X 10 1 Omega 1 cm. 1 and comprising bivalent and multivalent ions and a soluble non-ionic macro-molecular compound having at least one side chain of at least 4 carbon atoms.