US3909433A

US3909433A - Liquid electrophotographic developing compositions

Info

Publication number: US3909433A
Application number: US374774A
Authority: US
Inventors: Yvan Karel Gilliams; Engeland Jozef Leonard Van; Volder Noel Jozef De
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1972-07-12
Filing date: 1973-06-28
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30
Also published as: JPS4945744A; BE802171A; FR2192329A1; FR2192329B1; GB1438954A; CA1005671A; DE2334353A1; DE2334353C2

Abstract

An electrostatic liquid developer for the development of electrostatic charge patterns is provided wherein the said developer comprises an electrically insulating carrier liquid having a dielectric constant of less than 3 and a volume resistivity in excess of 109 Ohm.cm, resin-coated toner particles suspended in said carrier liquid and an alkylated polymer of a heterocyclic N-vinyl monomer. By the presence of the said polymer excellent developers of positive polarity can be obtained.

Description

United States Patent Gilliams et al.

1451 Sept. 30, 1975 [54] LIQUID ELECTROPHOTOGRAPI-IIC DEVELOPING COMPOSITIONS [75] Inventors: Yvan Karel Gilliams, Berchcm;

Jozef Leonard Van Engeland, St. Katelijne-Waver; Noe'l Jozef De Volder, Edegem, all of Belgium [73] Assignee: Agfa-Gevaert N.V., Mortsel, Belgium [22] Filed: June 28, 1973 [21] Appl. No.: 374,774

[30] Foreign Application Priority Data July 12, 1972 United Kingdom 32655/72 [52] US. Cl. 252/62.1; 117/37 LE; 96/1 LY [51] Int. Cl. G03G 9/00 [58] Field of Search 252/62.1 L; 117/37 LE [56] References Cited UNITED STATES PATENTS 3,432,329 3/1969 Van Den Heuvel et al. 252/621 L 3.542.682 ll/l970 Mutaffis 252/621 L 3,585,140 6/1971 Machida et all 252/621 L 3,657,130 4/1972 Machida et al. 252/621 L Primary E.\'uminerMaynard R. Wilbur Assistant E.\'umilzer-T. M. Blum Attorney, Agent, or Firm-A. W. Breiner resin-coated toner particles suspended in said carrier liquid and an alkylated polyrner of a heterocyclic N- vinyl monomer. By the presence of the said polymer excellent developers of positive polarity can be obtained.

38 Claims, No Drawings merit of electrostatic charge velopers used therefor.

LIQUID ELECTROPI-IOTOGRAPI-IIC" DEVELOPING COMPOSITIONS I The present invention relates to electrophotography, and more particularly to improvements in the developpatterns and to liquid dc- Known electrophotographic processes comprise the 'steps'of electrostatically charging in the dark a photoconductive surface, image-wise exposing the said surface whereby the irradiated areas become discharged in accordance with the intensity of radiation, thus forming a latent electrostatic image and developing the material to form a visible image by depositing on the image a finely divided 'electroscopic material known as toner. The thus developed image may be fixed to the surface of the photoconductor or transferred to another surface and fixed thereon. Instead of forming the latent electrostatic image by the steps described above it is also possible to directly charge the photoconductive layer in image configuration.

In one type of electrophotography, the latent electrostatic image is developed or rendered visible by use of liquid developers comprising coloured toner particles suspended in an insulating carrier liquid, which should have a volume resistivity in excess of IO Ohm.cm and a dielectric constant below 3. The suspended toner particles, usually finely divided pigments or dyes, are elec- I trostatically charged and develop the latent image under influence of the charge of the latent electrostatic image.

It is known to produce positive-working and negative-working liquid developers. A negative-working developer contains toner particles which are attracted by positive electrostatic charges, or which upon contact with a surface bearing latent image areas formed by negative electrostatic charges are repelled by such negative charges and deposit onto the non-image areas. A positive working developer behaves in the reverse sense and thus contains particles which are attracted by negative electrostatic charges. 7

Electrophotographic coatings containing zinc oxide are generally charged negatively with the result that the latent image formed by exposure is negative in polarity.

.Thus, a positive developer can be applied to a zinc oxide layer containing a negative latent image to produce a facsimile reproduction of the original radiation patternwhereas a negative developer can be applied to such layer to produce areversal reproduction of the original radiation pattern.

The polarity of the toner particles with respect to the electrostatic latent image to be developed is determined by the nature of the materials used in the preparation of the liquid developer and by so-called polarity control agents, which confer to the suspended toner particles either a negative or positive charge.

According to U.S. Pat. No. 3,542,681 alkylated polymers of heterocyclic N-vinyl monomers induce in liquid to'ner compositions strong negative polarity on the suspended toner particles. These polymeric materials are preferably materialsobtained by alkylation of homopolymers or copolymers of a N-vinyl lactam monomer, most preferably a N-vinyl pyrrolidone monomer with an a-olefin containing from 2 to 2000 carbon atoms, or obtained by simultaneously polymerizing and alkylating a mixture containing a heterocyclic N-vinyl monomer and an a-olefin.

Surprisingly it has now been found that these alkylated polymers of heterocyclic N-vinyl monomers give excellentdevelopers of positive polarity when the toner particles are suspended in an insulating liquid and the said toner particles are overcoated with a resin.

The present invention thus provides a positiveworking electrostatic liquid developing composition comprising an electrically insulating carrier liquid having a dielectric constant of less than 3 and a volume resistivity in excess of 10 Ohm.cm, resin-coated toner particles suspended in said carrier liquid and an alkylated polymer of a heterocyclic N-vinyl monomer.

Resin-coated toner particles are widely known in the art of liquid electrostatic developers. The resin functions as fixing agent to fix the toner particles where they are deposited e'lectrostatically. These resins.include natural as well as synthetic resins. Examples of resins that can be used in accordance with the present invention are esters of hydrogenated rosin and longoil, rosin-modified phenol formaldehyde resins, pentaerythritol esters of rosin, glycerol esters of hydrogenated rosin, short oil and medium oil alkyd resins, polyvinyl esters e.g. polyvinyl stearate, coumarone-indene resins, polyacrylates and polymethacrylates, e.g. polyisobutylmethacrylate, polystyrene, styrenebutadiene copolymers, cyclized rubber, etc. Preferred in accordance with the present invention are coumarone-indene resins e.g. PICCOVAR 440 (trade name of the Picco Company USA) cyclized rubber e.g. ALPEX 514 of Reinhold Albert Chemie A.G., W. Germany as well as rosin and synthetic resins derived from rosin e. g. the PENTALYN, POLYPALE ESTER, and STAYBE- LITE ESTER resins (registered trade marks of Hercules) such as PENTALYN A, a'pentaerythritol ester of wood rosin with acid No. 12, PENTALYN H, a pentaerythritol ester of hydrogenated wood rosin with acid No. 13, PENTALYN X, a pentaerythritol ester of hydrogenated wood rosin with acid No. 14, POLYPALE ESTER 10, a glycerin ester of polymerized wood rosin with acid No. 6 and STAYBELITE ESTER 10, a glycerin ester of hydrogenated wood rosin with acid NO. 8; AMBEROL M21 and M82 (registered trade marks of the Rohm & Haas Company), which are wood rosinmodified phenol formaldehyde resins. Other specific examples of suitable resins are POLYSTYRENE PS 3 marketed by Dow, LUVISKOL VA 64 of BASF, which is a co(vinylpyrrolidon-vinylacetate) (:40), POLYS- TYROL B of BASF, which is a co(styrenebutylacrylate), KYRAX A of AIRKO, USA, which is a polyvinyl stearate.

From the prior art it is known that these fixing agents need not necessarily be present as a coating'over the toner particles and may be included in the liquid composition as a separate addition. It is pointed out that when such fixing agents are added separately to the liquid composition and thus not coated on the pigment particles beforehand, the above alkylated polymers of heterocyclic N-vinyl monomers provide a negativeworking toner composition or a toner composition with both negative and positive characteristics, which is therefore unsuitable for use. Accordingly, it is surprising that an exceptionally positive-working toner composition can be provided with the above alkylated polymers of heterocyclic N-vinyl monomers when the toner particles are resin-coated.

The positive polarity increases with the concentration of the said alkylated polymers so that the polarity can be easily controlled and an optimum relationship obtained. However, exception must be made for resincoated carbon blacks .where the amount of said a1kyl ated polymers must be limited to at most by weight relative to the amountof resin-coated carbon black since'otherwise the polarity may become reversed. As is set forth hereinafter, this maximum percentage can be higher by the presence in the developing composition of other positively polarizing agents and/orthe use of a mixture of carbon black and a colour-improving dyestuff as colouring matter that is resin-coated Resin-coated toner particles are particularly advantageous in electrophotographic multicolour reproduction processes. In these processes images are produced by repeating for each colour separation image the successive operations of-charging the photoconductive element, exposing to a colour image through a filter absorbing the colour to be reproduced and developing by means of toner particles of the desired colour. In order to allow recharging of the photoconductive element for the formation of the subsequent colour separation image, the image areas of the toner first deposited should be capable-of carrying the appropriate charge and comprise'therefore an insulating material e.g. a resin coated over the, toner deposit. It is possible forthis purpose to incorporate the insulating resins in the liquid developing composition. However, large amounts are required which reduces the intensity .of colour deposit. Improved results are obtained when in the liquid developing composition, the toner particles are overcoated with the resin. In accordance with the present invention, the resin-coated toner particles allow proper recharging of the photoconductive element.

The alkylated polymers of heterocyclic N-vinyl monomers which are used in accordance with the present invention in combination with resin-coated toner particles are preferably derived: fron N-vinyllactam monomers, mostpreferably from N-vinylpyrrolidone and N Pat. No. 3,542,681, which is incorporated herein by reference; the alkylated polymers of the heterocyclic N-vinyl monomers suitable for use in accordance with the present invention can be prepared by alkylation of a homopolymer or copolymer of such heterocyclic N- vinyl monomer with an alpha-olefin containing from 2 carbon atoms to 2,000 carbon atoms, preferably in the range of from 2 to 200 carbon atoms, and most preferably in the range offrom 8 to 42 carbon atoms, said alkylation process beingmore fully described in U.S. Pat. No. 3,417,054, or by simultaneously polymerizing and alkylating a mixture containing a heterocyclic N-vinyl monomer and an alpha-olefin containing from 2 carbon atoms to 2,000 carbon atoms or a mixture of the same or. two different heterocyclic N-vinyl monomers and an alpha-olefin containing from 2 carbon atoms to 2,000

carbon atoms, said simultaneous polymerization and alkylation process being more fully described in U.S. Pat. No. 3,423,367.

Representative examples of heterocyclic N-vinyl monomers, the alkylated polymer derivatives of which can be used in accordance with the present invention can be found in the above U.S. Pat. No. 3,542,681 which also includes representative examples of monoethylenically unsaturated polymerizable monomers suitable for copolymerization with the said heterocyclic N-vinyl monomers to form copolymers which are readily alkylated as described above. More details as regards the a-olefins employed for producing the alkylated polymers can also be found in the said U.S. Pat. No. 3,542,681. It can also be learned from this U.S. Patent that the alkylated polymers have a K-value comprised between 10 and 140, preferably between 30 and 100. Particularly suitable for use in accordance with the present invention are GANEX 216, GANEX 220 and ANTARON V216 which are olefin-alkylated poly-N-vinylpyrrolidone with 20 vinylpyrrolidone residue commercially available from GAF.

By means of the alkylated polymers of the above heterocyclic N-vinyl monomers, the positive polarity of the resin-coated toner particles can be easily controlled. This makes possible to obtain an optimum relationship, as a function of the electrostatic latent image charge, between the developable density and the stability of the toner composition against settling of the toner particles. Moreover, it was found that the positiveworking toner compositions of the present invention are perfectly suitable for continuous-tone multicolour reproduction in that the gradation is dependent on the polarity of the toner particles; the more positive the polarity the softer the gradation that is obtained.

In addition to the alkylated polymer of the heterocyclic N-vinyl monomers, a liquid'electrophotographic developer according to the present invention can contain other positively polarizing agents, e.g. metal soaps of fatty acids such as aluminium stearate, zinc stearate, lead stearate, copper stearate, cadmium stearate, calcium stearate, zinc palmitate, aluminium palmitate, lithium octoate, aluminium dresinate (aluminium salt of abietic acid), vanadium dresinate, tin dresinate, copper linoleate, manganese linoleate, etc. and the bivalent or trivalent metal salts of an oxyacid derived from phosphorus containing at least one organic residue according to British Pat. No. 1,151,141.

As is described in the British Pat. No. 1,151,141, the bivalent or trivalent metal salts of a phosphorus oxyacid are preferably bivalent or trivalent metal salts of:

a. a monoester or diester of an oxyacid derived from phosphorus,

b. an oxyacid derived from phosphorus and containing one or two organic groups linked to the phosphorus atom by a carbon atom, or

c. an oxyacid derived from phosphorus and containing an ester group and an organic group linked by a carbon atom to the phosphorus atom.

The said organic group can be aliphatic, cycloaliphatic or aromatic.

The salt comprises an organic residue e.g. organic group(s) and/or ester group(s) rendering the salt substantially soluble in the electrically insulating carrier liquid. The organic residue preferably comprises a chain of at least 4 carbon atoms, most preferably from 10 to 18 carbon atoms, and such chain may be substituted and/or interrupted by heteroatom(s), e.g. oxygen, sulphur, or nitrogen atom(s).

The solubility in the electrically insulating carrier liquid of the metal salts can be promoted by the presence of one or more organic groups with branched structure, e.g. branched aliphatic groups, such as a 2-butyl-octyl group.

Excellent results are obtained where the metal of the metal salts of the phosphorus oxyacid is zinc. However,

other salts may also be used for example magnesium salts, calcium salts, strontium salts, barium salts, iron salts, cobalt salts, nickel salts, copper salts, cadmium salts, aluminium salts, and lead salts.

More details about these bivalent or trivalent metal salts, representative examples thereof, and methods of preparing these salts canbe found in the aboveBritish Patent, which is incorporated herein by reference.

By the use of the above supplemental positively polarizing agents it was found possible to obtain a liquid developing composition having a positive polarity exceeding that of the composition, with the alkylated poly-N-vinyl heterocycle alonefParticularly favourable results could be obtained when in addition to the alkylated poly-N-vinyl heterocycle, the zinc salt of mono-=2 butyl-octyl phosphate or aluminium dresinate is used.

When such supplemental positively polarizing agent is used in developing compositionscomprising resincoated carbon blacks the above limitation of maximum 10 ,7! by weight of alkylated heterocyclic N-vinyl polymer no longer stands.

The liquid developer composition of the present invention comprises as a base fluid any of the conventional electrically insulating carrier liquids generally employed in liquid developer compositions. Thus, for example the carrier liquid includes various hydrocarbon solvents; e.g. aromatic hydrocarbons such as benzene, toluene and xylenes, aliphatic hydrocarbons such as hexane, cyclohexane and heptane; fluorocarbons and silicone oils. The carrier liquid is preferably a commercial petroleum distillate e.g. mixtures of aliphatic hydrocarbons preferably havinga boiling point comprised between 150C and 220C such as the IS OPARS G, H, K and L (trade names) of the Esso Standard Oil Company, SHELL SOL T (trade name) of the Shell Oil Company, etc. In view of its low dielectric constant and high resistivity the carrier liquid is non-polar,

The, pigment or colouring agent used as toner may be any of the pigments and dyestuffs commonlyemployed in liquid electrostatic toner compositions providedthat they carry a resin-coating. Thus, forexample, the colouring agent or pigment can be carbon black and various analogous forms thereof e.g. lamp black, channel black and furnace black. In view of the above limitation of the maximum amount of alkylated heterocyclic N- vinyl polymer, it is preferred to use in developing com positions containing resin-coated carbon blacks, one of the above other positively polarizing agents in addition to the alkylated heterocyclic N-vinyl polymer. It is also possible to use a mixture of carbon black and a blue colour-improving dyestuff as toner that isresin-coated, in which case it is also possible to increase the amount of alkylated heterocyclic N-vinyl polymer beyond the above limit of 10 /z.

Suitable coloured pigments include azo dyes, xanthene dyes, phthalocyanine dyes, which may be in X- form e.g. as described in published German Patent (DOS) 1,944,021, triphenylmethane dyes, diphenylmethane dyes, stilbene dyes, acridine dyes, quinoline dyes, quinone imine dyes, thiazine dyes, azine dyes, etc.

A large variety. of dyes are known for use in liquid electrophotographic developers and therefore, the following dyes are given for illustration purposes only: Heliogen Blau LG (a metal free phthalocyanine blue pigment of BASF), Monastral Blue (acopperiphth alocyanine pigment Cl 74,160), Heliogen Blau B pulver (BASF), l-lelioecht Blau HG (Bayer), .Brillia nt Carr'nine 6B' (Cl115,850), Helioecht Rose (Bayer), Quinacridone Magenta (CI. Pigment Red 122), Thioindigo Magenta (e.g. C173,31 Fanalrosa B supra pulver (BASF), Lithol Rubine (BASF), lrgalith echt Gelb (Geigy), Benzidin Yellow (C.l. 21,090, 21,100), Hansa Yellow (e.g. Cl 11680), Permanentgelb GR 52 (Hoechst),-Monolit GelbGNS (1C1), Helioecht Gelb GRN (Bayer), etc.

.As is pointed out above, the pigments or dyes used in accordance withthe present invention should be resincoated, which can be achieved by any suitable means. This may be effected e.g. by. mixing the pigment or dye with the resin, heating the vmixture to melting, and kneading the melt. The kneaded mass is then cooled, broken, and ground to fine powder.

It is alsopossible to use commercially available resincoated dyes or pigments-e.g. those marketed by Ciba- Geigy for example Microlith CT, which is carbon black coated with a resinderived from rosin, and Microlithe Blue 4GT,,Microlithe Blue 22027T, Microlithe Yellow mixing. When the resin is merely added as a separate additive, the: alkylated heterocyclic N-vinyl polymer has no favourable polarizing effect. 7

The mannerin which the liquid composition is produced is by no way critical. As is illustrated by the Examplesnhereinafter, it is conventional to prepare by means ofsuitahle mixers, e.g. 3-rollmills, ball mills, colloid mills, homogenizers, high speed stirrers, etcl a concentrate in the insulating carrier liquid of the materials selected for the composition, and subsequently adding further insulating carrier liquid to provide the liquid toner composition ready for use in the electrostatic reproductionprocess.

The resin-coated pigment or dye is employed in the composition ready for use in the amount necessary to be capable of being deposited, when attracted to the electrostatic image,-so that the desired image density is reached. Ingeneral, .the pigment or colouring agent is used in an amount comprised between about 0.01 g and 10 g per litre, preferably between about 0.01 g and about 2 g per litre.

The ratio of resin to pigment'or dye may vary within .very wide limits The resin preferably outweighs the pigment or dye but should permit to obtain sufficient density. A preferred weight ratio is comprised between about 1:1 andaboutlzlO. l

The amount of the positively polarizing alkylated polymer of a heterocyclic N-vinyl monomer as well as of other possible positively polarizing agents e.g. the metal soaps and bivalent or trivalent metal salts of phosphorus oxyacids as referred to. hereinbefore, .will vary with the nature of the charge already on the resincoated toner particles and depends on the desiredieffect e.g. obtaining a developer yielding imagesof steep or soft gradation. Moreover, as is noted above it is possible to control by means of the amount of the above 2GT, Microlithe Yellow 22018T, Microlithe Bordeaux positively polarizing agents, the polarity of the toner particles so as to obtain optimum relation between image density and contrast and stability of the liquid composition against settling of the toner particles. An individual determination of the required quantity may be made for each new combination by some simple experiments. The amounts should, of course, be such that they do not lower the volume resistivity of the resulting composition below 10 Ohm.cm or raise the dielectric constant above 3.

The following examples illustrate the present invention.

EXAMPLE 1 200 g of STAYBELITE ESTER l (Hercules) and 100 g of Helioecht Blau HG (Bayer) were mixed at 6065C in a mixing and kneading apparatus of MEILI, Switzerland, type liliput O30 L.N. The kneaded mass was cooled and ground in a grinding apparatus l.K.A., model A (of Janke & Kunkel, W. Germany) to obtain a fine powder of resin-coated pigment.

In a ball-mill 2 g of the above resin-coated pigment powder were ground for 10 hours together with:

l 1.5 g of a 25 76 by weight solution of NEOCRYL B 702 (trade name for a copolymer of butylmethacrylate and stearyl methacrylate comprising about 1 7c of methacrylic acid marketed by Polyvinylchemie, The Netherlands) in ISOPAR G (trade name for an aliphatic hydrocarbon solvent having a boiling range of l60l75C and a KB value of 27, marketed by the E550 Standard Oil Company) 0.5 g of a 25 7c by weight solution of ANTARON V2l6 marketed by GAF in ISOPAR G 2 ml of a 0.2 (g/vol) solution in ISOPAR G of the zinc salt of mono-2-butyloctyl phosphate,

25 ml of ISOPAR G.

When 10 ml of the above concentrated liquid developing composition was diluted with 1 litre of ISOPAR G a stable positive-working electrophotographic liquid developer was formed for the development of a latent electrostatic image formed on a conventional electrophotographic recording element comprising paper coated with photoconductive zinc oxide in a resinous binder by negative charging and image-wise exposure to light. The developer was suitable for continuoustone reproduction.

Increasing amounts of ANTARON V 216 or of the above zinc salt improve the stability of the developer but at the cost of a reduction of the developable maximum density. For a given density region, however, the optimum stability can be determined.

EXAMPLE 3 Example I was repeated with the only difference that the pigment now used was Fanalrosa B supra pulver (BASF).

A stable positive-working electrophotographic liquid developer was formed suitable for continuous tone reproduction.

EXAMPLE 4 EXample l was repeated with the only difference that the pigment now used was carbon black (PRIN- TEX G marketed by Degussa A.G., W.-Germany) and that 0.1 g of the solution of ANTARON V2 l 6 was used and 10 ml of the zinc salt solution.

A very good positive-working developer was obtained.

EXAMPLE 5 Example 1 was repeated with the only difference that in the preparation of the resin-coated pigment, the 200 g of STAYBELITE ESTER l0 (Hercules) was replaced by 200 g of POLYPALE ESTER l0 (Hercules).

EXAMPLE 6 Example I was repeated with the only difference that in the preparation of the resin-coated pigment, the 200 g of STAYBELITE ESTER l0 (Hercules) was replaced by 200 g of PENTALYN A (Hercules).

EXAMPLE 7 Example I was repeated with the only difference that in the preparation of the resin-coated pigment, the 200 g of STAYBELITE ESTER 10 was replaced by 200 g of a coumarone-indene resin marketed by the PICCO Comp, USA, under the trade name PICCOVAR 440 and that in the preparation of the developer concentrate l g of the ANTARON V 216 solution was used.

An ideal positive-working electrophotographic liquid developer was obtained suitable for the reproduction of continuous tone images.

EXAMPLE 8 Example 1 was repeated with the only difference that in the preparation of the resin-coated pigment, part of the STAYBELITE ESTER 10 was replaced by PICCO' VAR 440.

When replacing 40 g, g or g of the STAYBE- LITE ESTER 10 by 40 g, 100 g or 160 g respectively of PICCOVAR 440 and further using the resin-coated pigment obtained as described in example 1 for the preparation of an electrophotographic liquid developer, positive-working developers were obtained suitable for continuous tone reproduction.

EXAMPLE 9 g of STAYBELITE ESTER l0 (Hercules), 20 g of the aluminium salt of abietic acid and 100 g of Helioecht Blau HG (Bayer) were mixed and molten together as described in example I. The melt was thorougly kneaded and then cooled whereupon it was ground to obtain a fine powder of resin-coated pigment.

The following ingredients were ground in a ball-mill for 12 hours.

10 g of a 30 7c by weight solution of NEOCRYL B 702 in ISOPAR G 0.5 g of a 25 70 by Weight solution of ANTARON V216 in ISOPAR G 2 g of the above resin-coated pigment powder, and

35 ml of ISOPAR G.

When 10 ml of the above concentrated liquid developing composition was diluted with 1 litre of ISOPAR G a very satisfactory positive-working electrophotographic liquid developer was formed suitable for continuous-tone reproduction.

EXAMPLE 10 form a positiveworking electrophotographic liquid developer giving high density images when used for the development of electrostatic charge patterns.

EXAMPLE 11 Example 10 was repeated with the difference that in the preparation of the developer concentrate 6 g of the NEOCRYL B 702 solution and 6 g of the ANTARON V 216 solution were used. 1

A highly stable positive-working electrophotographic liquid developer was obtained suitable for continuous tone reproduction. The maximum density obtainable was slightly lower than with the developer of example b 10.

EXAMPLE 12 Example 10 was repeated with the difference that in the preparation of the developer concentrate 1 1.5 g of the NEOCRYL B 702 solution and 0.5 g of the AN- TARON V 216 solution were used and that also 4 ml of a 0.2 7: (g/vol) solution in ISOPAR G of the zinc salt of mono-2-butyloctylphosphate was added.

The positive-working electrophotographic liquid developer obtained upon dilution had high-stability and yielded upon use for the development of electrostatic charge patterns images of favourable density.

EXAMPLE 13 Example 10 was repeated using MlCROLlTl-l CT (a resincoated carbon black of CIBA, the said resin being STAYBELITE ESTER 10) as pigment.

A positive-working liquid electrostatic developer suitable for continuous-tone reproduction was obtained.

EXAMPLE 14 Example 10 was repeated using MlCROLlTl-l Gelb 2GT (a resin-coated yellow pigment of CIBA) as pigment.

EXAMPLE 15 The liquid electrostatic developer compositions of examples 1. 2 and 3 can be used successfully for the reproduction of multicolour images.

A panchromatically sensitized photoconductive zinc oxide element having a paper support was negatively charged (about 400 V) and exposed through a blue filter (e.g. a Kodak Wratten filter 473 or an Agfa- Gevaert band filter V449) to a multicolour photographic silver halide transparency fitted in a conventional photographic enlarger. The selectively discharged photoconductive layer was developed by means of 'the yellow electrostatic liquid developer of example 2' to form the yellow separation image.

The photoconductive element carrying the yellow toner image was then recharged either in dry or wet condition and again exposed (in register) to the transparency, now through a green filter (e.g. a Kodak Wratten filter 61 or an Agfa-Gevaert band filter U531 The selectively discharged pip toconductive layer was developed by means of the magenta electrostatic liquid developer of example 3 to form the magenta separation image.

The steps of charging and exposure were then repeated using now a red filter (e.g. a Kodak Wratten filter 29 or an Agfa-Gevaert cut-off filter L622). Development occurred by means of the cyan electrostatic liquid developer of example 1 to form the cyan separation image.

The ultimate result was a very good continuous tone reproduction of the transparency.

The sequence according to which the separation images are formed by exposure and corresponding development can be altered at will.

What we claim is:

1. A positive-working electrostatic liquid developer which will develop electrostatic charge patterns comprising an electrically insulating carrier liquid having a dielectric constant of less than 3 and a volume resistivity in excess of 10 Ohm.cm, resin-coated toner particles suspended in said carrier liquid, and an alkylated polymer of a heterocyclic N-vinyl monomer.

2. A liquid developer according to claim 1 wherein said alkylated polymer of a heterocyclic N-vinyl monomer is an alkylated polymer of a N-vinyl lactam monomer.

3. A liquid developer according to claim 2, wherein said N-vinyl lactam monomer is a N-vinyl pyrrolidone or a N-vinylpiperidone.

4. A liquid developer according to claim 1 wherein said alkylated polymer is formed by alkylation with an alpha-olefin.

5. A liquid developer according to claim 4 wherein said alpha-olefin comprises from 8 to 42 carbon atoms.

6. A liquid developer according to claim 1 wherein the toner particles carry a coating of a synthetic resin.

7. A liquid developer according to claim 1 wherein the toner particles carry a coating of a natural resin.

8. A liquid developer according to claim 7 wherein said resin is rosin or a resin derived from rosin.

9. A liquid developer according to claim 7 wherein said resin is a coumarone-indene resin or cyclized rubber.

10. A liquid developer according to claim 8 wherein said resin derived from rosin is an ester of rosin or hydrogenated rosin.

1 1. A liquid developer according to claim 10 wherein said ester is a pentaerythritol ester or glycerol ester.

12. A liquid developer according to claim 1 wherein the resin-coated toner particles are resin-coated dyes.

13. A liquid developer according to claim 1 which also comprises a metal soap of a fatty acid.

14. A liquid developer according to claim 13. wherein said metal soap is the aluminium salt of abietic acid.

15. A liquid developer according to claim 1, which also comprises a bivalent or trivalent metal salt of oxyacid derived from phosphorus containing at least one organic residue.

16. A liquid developer according to claim wherein said oxyacid derived from phosphorus containing at least one'organic residue is a monoester or diester of an oxyacid derived from phosphorus.

17. A liquid developer according to claim 16, wherein said oxyacid derived from phosphorus containing at least one organic residue is a mono-2-butyloctyl phosphate.

18. A liquid developer according to claim 15 wherein said metal salt is a zinc salt.

19. A liquid developer according to claim 1 wherein the carrier liquid is an aliphatic hydrocarbon.

20. Method of preparing an electrostatic liquid developer for the development of electrostatic charge patterns by mixing toner particles with an electrically insulating carrier liquid having a dielectric constant of less than 3 and a volume resistivity in excess of 10 ohm.cm, wherein mixing occurs in the presence of an alkylated polymer of a heterocyclic N-vinyl monomer and the said toner particles are coated with a resin prior to mixing was to form a positive-working electrostatic liquid developer.

21. Method according to claim 20, wherein said alkylatedv polymer of a heterocyclic N-vinyl monomer is an alkylated'poly'mer of a N-vinyl lactam monomer.

22. Method according to claim 21, wherein said N- vinyl lactammonomer is a N-vinyl pyrrolidone or a N- vinylpiperidone.

23. Method according to claim 20, wherein said alkylated polymer is formed by alkylation with an alphaolefin.

24. Method according to claim 23, wherein said alpha-olefin comprises from 8 to 42 carbon atoms.

25. Method according to claim 20, wherein the said resin is a synthetic resin.

26. Method according to claim 20, wherein the said resin is a natural resin.

27. Method according to claim 26, wherein said resin is rosin or a resin derived from rosin.

28. Method according to claim 26 wherein said resin is a'coumarone-indene resin or cyclized rubber.

29. Method according to claim 27, wherein said resin derived from rosin is an ester of rosin or hydrogenated rosin.

30. Method according to claim 29, wherein said ester is a pentaerythritol ester or glycerol ester.

31. Method according to claim 20 wherein the toner is a dye.

32. Method according to claim 20, wherein mixing also occurs in the presence of a metal soap of a fatty acid.

33. Method according to claim 32 wherein said metal soap is the aluminium salt of abietic acid.

34. Method according to claim 20, wherein mixing also occurs in the presence of a bivalent or trivalent metalsalt of an oxyacid derived from phosphorus containing at least one organic residue.

35. Method according to claim 34, wherein said oxyacid derived from phosphorus containing at least one organic residue is a monoester or diester of an oxyacid derived from phosphorus.

36. Method according to claim 34, wherein said oxyacid derived from phosphorus containing at least one organic residue is a mono-2-butylocytl phosphate.

37. Method according to claim 34, wherein said metal salt is a zinc salt.

38. Method according to claim 20, wherein the carrier liquid is an aliphatic hydrocarbon.

Claims

1. A POSITIVE-WORKING ELECTROSTATIC LIQUID EDVELOPE WHICH WILL DEVELOP ELECTROSTATIC CHARGE PATTERMS COMPRISING EN ELECTRICALLY INSULATING CARRIER LIQUID HAVING A DIELECTRIC CONSTANT OF LESS THAN 3 AND A VOLUME RESISTIVITY IN EXCESS OF 109 OHNM.CM, RESIN-COATED TONER PARICLES SUSPENDED IN SAID CARRIER LIQUID, AND AN ALKYLATED POLYMER OF A HETHEROCYCLIC N-VINYL MONOMER.

11. A liquid developer according to claim 10 wherein said ester is a pentaerythritol ester or glycerol ester.

14. A liquid developer according to claim 13, wherein said metal soap is the aluminium salt of abietic acid.

16. A liquid developer according to claim 15 wherein said oxyacid derived from phosphorus containing at least one organic residue is a monoester or diester of an oxyacid derived from phosphorus.

20. Method of preparing an electrostatic liquid developer for the development of electrostatic charge patterns by mixing toner particles with an electrically insulating carrier liquid having a dielectric constant of less than 3 and a volume resistivity in excess of 109 ohm.cm, wherein mixing occurs in the presence of an alkylated polymer of a heterocyclic N-vinyl monomer and the said toner particles are coated with a resin prior to mixing so as to form a positive-working electrostatic liquid developer.

21. Method according to claim 20, wherein said alkylated polymer of a heterocyclic N-vinyl monomer is an alkylated polymer of a N-vinyl laCtam monomer.

22. Method according to claim 21, wherein said N-vinyl lactam monomer is a N-vinyl pyrrolidone or a N-vinylpiperidone.

23. Method according to claim 20, wherein said alkylated polymer is formed by alkylation with an alpha-olefin.

25. Method according to claim 20, wherein the said resin is a synthetic resin.

26. Method according to claim 20, wherein the said resin is a natural resin.

28. Method according to claim 26, wherein said resin is a coumarone-indene resin or cyclized rubber.

31. Method according to claim 20 wherein the toner is a dye.

34. Method according to claim 20, wherein mixing also occurs in the presence of a bivalent or trivalent metal salt of an oxyacid derived from phosphorus containing at least one organic residue.

37. Method according to claim 34, wherein said metal salt is a zinc salt.