EP0114300B1 - Liquid developer for the development of electrostatic charge images, and process for its production - Google Patents

Description

The invention relates to a liquid developer with negatively charged toner particles for the development of electrostatic charge images, from a carrier liquid of high electrical resistance and low dielectric constant, pigment or dye, polymer containing N-vinylpyrrolidone, waxy substance and conventional additives, and a process for its preparation.

Liquid developers of the type mentioned are used in electrographic or electrophotographic copying processes to make latent electrostatic charge images visible. In principle, they consist of colored particles which are dispersed in the carrier liquid in a solution of binder and charge control agent, optionally together with dispersant. In the electric field of the charge image, the colored particles with binder and charge control agent are separated electrophoretically. In the presence of N-vinylpyrrolidone-containing polymer, the dispersant, binder and charge control agent can also be identical.

It is known (DE-OS 27 40 870, corresponding to US-PS 4 243 736 or DE-A 3 011 193, corresponding to EP-OS 0037 475). in liquid developers containing an N-vinylpyrrolidone-containing polymer as a dispersant and binder, which also serves as a charge control agent, to use waxes in the pigment content in appropriate amounts. They serve the better sedimentation behavior of the liquid developer or improve the smudge resistance of the developed images. Polyethylene wax is used as the wax. Halogenated paraffins can also be present in smaller quantities. However, it has been shown that these liquid developers make solid areas visible that do not yet meet the highest demands.

The addition of waxes to liquid developers is also evident from other publications. A suspension developer is known (DE-OS 25 32 282, corresponding to US Pat. No. 3,992,342), which additionally contains polyethylene, polyethylene wax and / or paraffin wax with a softening point of 60 ° to 130 ° C. The addition influences the stability, the sedimentation behavior and the viscosity of the liquid developer, the gloss, the smudge resistance of the copies made and the degree of device contamination. Also known from DE-AS 25 38 581 (corresponding to US Pat. No. 4,081,391), DE-AS 29 36 042 (corresponding to US Pat. No. 4,250,241) are electrophotographic suspension developers which additionally contain a wax or polyolefin with a softening point of 60.degree up to 130 ° C included. The additives are similar in terms of specific gravity to that of the carrier liquid. They are soluble in the heated liquid developer and separate out in the form of fine particles when they cool down.

DE-OS 30 46 654 (corresponding to US Pat. No. 4,306,009) discloses a liquid developer with a special gel latex in addition to the usual components, which can contain a wax and a tree resin.

When studying the corresponding descriptions, however, it is noticeable that only polyethylene, polyethylene wax or paraffin wax and beeswax are used, such as a polyethylene with a softening point of 115 ° C. and a molecular weight of 1,500, low-molecular polyethylene with a softening point of 107 ° C. or 108 ° C, and paraffin waxes with softening points in the range of 60 ° to 110 ° C. With the well-known liquid developers, copies with a large circulation can be obtained in part. However, the copies show restless, inhomogeneously colored solid areas. This is particularly disadvantageous when using copy paper that has a low surface smoothness.

It was therefore an object of the invention to provide a liquid developer with the known good properties, which provides copies with good area coverage in electrographic or electrophotographic copying processes, even when copy papers with low surface smoothness are used.

The solution to the problem is based on a liquid developer of the type mentioned, which is characterized in that the liquid developer selected at least one wax which is slightly soluble in the carrier liquid with increasing temperature, but poorly soluble at room temperature and lowers the specific electrical conductivity of the liquid developer based on montan wax produced acid or ester wax or a wax-like substance such as hydrogenated castor oil or polyoctadecene, with a softening point in the range from 50 ° C to 120 ° C and an acid number from zero to about 200.

A wax is preferably used which, when 1% by weight is added to the toner concentrate with about 7% by weight solids content, lowers the specific electrical conductivity of the liquid developer by at least 15 percent.

The liquid developer according to the invention preferably contains 0.1 to 1.5 parts by weight of wax per 1 part by weight of pigment or dye.

This ensures that better copies can be obtained in commercially available copying machines than with the liquid developers known hitherto. Solid areas are developed stronger and homogeneously. Fine graphic symbols are reproduced true to the original on copy paper with a rough surface. The liquid developer according to the invention enables the use of relatively undemanding, inexpensive copy paper.

The cause of the effect achieved is still unknown. However, it can be assumed that the amount of charge on the toner particles is reduced by adding the wax according to the invention, and toner and wax particles are thereby deposited more onto the latent image, resulting in a voluminous, massive toner image leads, which can be easily transferred to the copy paper.

On the other hand, it is assumed that the wax contained in the moist, not yet fixed toner image is melted during the fixing process and / or redissolved in the remaining carrier liquid. As a result, the inhomogeneous image areas become more uniform.

According to the invention, waxes which can be used are those which correspond to the required conditions of solubility (poor solubility at room temperature with more than about 95%), the melting or softening range and the electrostatic chargeability of the toner particles or the properties which lower the specific electrical conductivity of the liquid developer. They are usually commercially available. These include the acid or ester waxes based on montan wax. Wax-like substances such as. B. hydrogenated castor oil or polyoctadecene. The waxes preferably have a softening point in the range between 70 ° and 90 ° C. The acid numbers of the waxes are in the range from zero to about 200, preferably in the range from zero to about 160. Good results are also obtained with acid numbers in the range from 10 to 160. Mixtures of the waxes with one another or mixtures with polyethylene or paraffin wax can also be used be used. Waxes have proven to be particularly suitable, such as ester wax, with a softening point in the range from 82 ° to 88 ° C., an acid number from 78 to 88 and a saponification number from 120 to 135 (1), ester wax with a softening point from 76 ° to 82 _° C, an acid number of 15 to 20 and a saponification number of 125 to 155 (2) and acid wax with a softening point of 78 ° to 84 ° C and an acid number of 135 to 155 (3). Hydrogenated castor oil with a softening point of 86 ° to 88 ° C and an acid number of 2 can also be used (4). Further waxes that can be used, for example, can be found in the following table.

The waxes according to the invention reduce the specific electrical conductivity of the liquid developer produced by at least 15% with an addition of 1% to the toner concentrate with a solid fraction of about 7% by weight. The reduction in the specific electrical conductivity is a measure of the reduction in the toner charge.

The conductivity-reducing effect of a wax can be determined in the following way: A hot solution of 2.5 g of wax in 30 ml of a liquid aliphatic hydrocarbon, boiling range 170 ° to 190 ° C., is stirred into 250 g of the toner concentrate described in Comparative Example 2. After standing for about 24 hours, the mixture is diluted with 2 of the aliphatic hydrocarbon and the specific electrical conductivity of the ready-to-use developer thus obtained is measured in a DC measuring cell under the same conditions as a liquid developer without added wax. Table 2 shows the values obtained with waxes according to the invention.

(See table 2 page 4 f.)

Suitable carrier liquids are, in particular, the known aliphatic hydrocarbons used in liquid developers with boiling points between 150 ° and 190 ° C. It should be mentioned here as non-exclusive examples of petroleum distillation products (petroleum fractions) which are essentially odorless, relatively cheap and commercially available. These products represent various hydrocarbon mixtures with about 8 to 16 carbon atoms. They have a high electrical resistance of more than 10 ⁹ ohm × cm and a low dielectric constant, about less than 3.

The developer liquid must be vaporizable sufficiently quickly, namely at evaporation temperatures which are lower than the temperature at which the copy paper chars. The developer liquid is preferably free of aromatic liquids and other, excessively toxic or corrosive components. The developer liquid also has a sufficiently low viscosity that rapid migration of the particles attracted by the electrostatically charged image areas to be developed is made possible. The viscosity of the carrier liquid is between about 0.5 and 2.5 mPa.s at room temperature.

The pigments used according to the invention are known. Synthetic carbon blacks are generally used to produce black developers. To improve the color, the carbon blacks can be mixed with blue pigments, for example.

In principle, all organic and inorganic pigments and dyes can be used. Examples include: carbon black (CINr. 77 266), oil blue (CINr. 61 555), alkali blue (CINr. 42 750), phthalocyanine blue (CINr. 74 160), phthalocyanine green (CINr. 74 260 or 42 040 ), Spirit Black (CINr. 50 415), Oil Violet (CINr. 60 725), Benzidine Yellow (CINr. 21 090 or 21 100), Methyl Orange (CINr. 13 025), Brilliant Carmine (CINr. 15 850 ) or true red (CINr. 15 865).

To generate a negative charge on the pigment particles, polymers containing N-vinylpyrrolidone are used which are soluble in aliphatic solvent. These are copolymers of a solubilizing methacrylic acid ester with an alkyl radical having 6 or more carbon atoms and 15 to 40 percent by weight of N-vinylpyrrolidone-2 or graft polymers that are obtained by polymerizing 2.5 to 10 percent by weight of N-vinylpyrrolidone-2 on soluble homopolymers or copolymers of methacrylic acid esters. However, other known negative control means can also be used, e.g. B. phospholipoids such as lecithin or metal salts of organic sulfonic acids soluble in aliphates, such as the sodium salt of dioctylsulfosuccinic acid. The same time It is possible to use several tax funds.

Various additives can be influenced by the additives that are not absolutely necessary, e.g. B. charge level, sedimentation behavior, service life, redispersibility of agglomerates, transferability and smudge resistance on the copy paper.

• a. halogenhaltige Wachse, z. B. Chlorparaffin mit einem Gehalt von 70 % Chlor,
• b. in der Trägerflüssigkeit lösliche Polymerisate wie Styrolbutadien-Copolymerisat, Polyvinylalkylether, Polyisobutylene, Polyvinylstearat und Polyacrylate bzw. Polymethacrylate mit höherem Alkoholrest,
• c. in der Trägerflüssigkeit lösliche Harze wie Kohlenwasserstoffharze und Polyterpenharze,
• d. Weichmacher, z. B. Dialkylphthalate.

Occasional additions can be considered as usual additives

• a. halogen-containing waxes, e.g. B. chlorinated paraffin containing 70% chlorine,
• b. polymers soluble in the carrier liquid such as styrene-butadiene copolymer, polyvinyl alkyl ether, polyisobutylene, polyvinyl stearate and polyacrylates or polymethacrylates with a higher alcohol residue,
• c. resins soluble in the carrier liquid, such as hydrocarbon resins and polyterpene resins,
• d. Plasticizers, e.g. B. dialkyl phthalates.

Der.Gatt the liquid developer on the waxes to be added according to the invention can vary within wide limits. 0.1 to 1.5 parts by weight, in particular 0.3 to 0.8 parts by weight, of wax are preferably used per part by weight of pigment.

The weight ratio of polymer to colorant is not particularly critical. Based on 1 part by weight of colorant, i.e. H. Pigment or dye, 0.2 to 3 parts by weight of polymer are generally used.

The invention also relates to a method for producing the liquid developer.

When producing the liquid developer, care must be taken that the wax according to the invention is brought into solution in some stage of the production process by heating in the carrier liquid. When it cools down, it then flocculates in fine particles or microcrystalline on the order of about 0.1 to a few 10 f.lm.

In order not to burden the production with an additional work step, the wax is generally added to the raw material dough. You can use wax powder or a wax dispersion made by dissolving the wax in hot carrier liquid and cooling the solution. During the grinding of the raw material dough, the ground material has to be heated so high that the wax is in solution.

The inventive method for producing a liquid developer with negatively charged toner particles for the development of electrostatic charge images from a carrier liquid of high electrical resistance and low dielectric constant, pigment or dye, polymer containing N-vinylpyrrolidone, wax-like substance and conventional additives consists of the pasting of the pigment or Dye with the N-vinylpyrrolidone-containing polymer and the other additives with the carrier liquid, grinding the raw material dough at temperatures in the range from 20 ° to 100 ° C, diluting the ground material with carrier liquid, in the N-vinylpyrrolidone-containing polymer can be solved, producing a toner concentrate and filling the same with 5 to 20 times the amount of carrier liquid, and it is characterized in that one optionally the raw material dough, the regrind or the toner concentrate at least one in the carrier liquid mi t Rising temperature slightly wax, but sparingly soluble at room temperature, which lowers the specific electrical conductivity of the liquid developer, selected from acid wax or ester wax based on montan wax or from a wax-like substance such as hydrogenated castor oil or polyoctadecene, with a softening point in the range of 50 ° to 120 ° C and an acid number from zero to about 200 supplies. The wax is preferably fed to the raw material dough in finely divided solid form or dispersed in the carrier liquid.

Another process for producing the liquid developer is characterized in that the wax is added to the toner concentrate in the form of a hot solution in the carrier liquid with vigorous stirring.

Three-roll mills, attritors, ball mills, agitator ball mills and the like are suitable for dispersing the raw material dough. The ground material is diluted with propeller stirrers, intensive stirrers or laboratory dissolvers.

The invention is described in more detail with reference to the following examples, without being restricted thereto.

Comparative Example 1 a. Production of the toner concentrate (analogous to the teaching of DE-OS 30 11 193)

• 60 soot
• 18 g copper phthalocyanine
• 24 g of powdered polyethylene wax with an average molecular weight of 1,500 were pasted with
• 200 g of a commercially available 20% solution of a graft polymer, which by grafting 4 parts by weight of N-vinylpyrrolidone-2 to 96 parts by weight of the copolymer dodecyl methacrylate / methyl methacrylate (76:20) dissolved in an aliphatic hydrocarbon, boiling range 170 ° to 190 ° C., with an average molecular weight of approx. 450,000, and
• 420 g of the aliphatic hydrocarbon, boiling range 170 ° to 190 ° C.

• 200 g der obengenannten 20-%igen Pfropfpolymerisatlösung und
• 1 900 g des obengenannten Kohlenwasserstoffs.

The raw material dough was ground in a stirred ball mill at 80 ° C. for 3 hours and diluted with a mixture

• 200 g of the above 20% graft polymer solution and
• 1 900 g of the above hydrocarbon.

b. Manufacture of the liquid developer

The toner concentrate prepared under a) was diluted with about nine times the amount of an aliphatic hydrocarbon, boiling range 160 ° to 180 ° C., and a well copying, long-lasting liquid developer with negatively charged toner particles was obtained. However, the copies showed inhomogeneously colored solid areas.

Examples 1 to 4

In each case, 10 g of wax (as indicated in Table 3) were dissolved in 150 g of aliphatic hydrocarbon, boiling range 170 ° to 190 ° C, at temperatures from 50 ° to 80 ° C. The warm solution was then slowly added to 500 g of the toner concentrate prepared according to Comparative Example 1 with vigorous stirring using a laboratory dissolver. The mixture was then stirred with an intensive mixer for 21/2 minutes.

Ready-to-use liquid developers were obtained by dilution with about nine times the amount of an aliphatic hydrocarbon, boiling range 160 ° to 180 ° C. The results obtained in a conventional copier are summarized in Table 3.

Example 5

30 g of ester wax, softening point 82 ° to 88 ° C, and 4 g of chlorinated paraffin (70% chlorine content) were in 420 g of an aliphatic hydrocarbon, boiling range 170 ° to 180 ° C, and 200 g of the N-vinylpyrrolidone mentioned in Comparative Example 1 -Copolymer solution dissolved warm, the solution then cooled. 60 g of carbon black and 18 g of copper phthalocyanine were introduced into the resulting wax dispersion. The raw material dough was, as stated in Comparative Example 1, ground and then with 300 g of the specified copolymer solution and 1 900 g of the aliphatic Hydrocarbon diluted to the toner concentrate.

The ready-to-use liquid developer, produced by dilution with seven times the amount of an aliphatic hydrocarbon, boiling range 160 ° to 180 ° C., delivered high-contrast copies with excellent homogeneous full-tone areas in a commercially available copier.

Good copies were also obtained using a paper suitable for dry development, which normally cannot be used for liquid toner development.

Example 6

Example 5 was repeated using an acid wax, softening point 78 to 84 ° C, acid number 135 to 155.

The toner concentrate formed was stirred with an intensive mixer.

When tested in a copier, the results were as good as in Example 5.

Example 7

Example 5 was repeated using a polyoctadecene with a melt viscosity (140 ° C) of 605 mPa · s.

In a commercially available copier, the toner delivered copies with excellent full-tone areas, even when using rough copy papers.

Example 8

• 60 g Ruß
• 20 g Kupferphthalocyanin
• 50 g Esterwachs, Erweichungspunkt 82° bis 88 °C, Säurezahl 78 bis 88,
• 4 g Chlorparaffin
• 200 g Pfropfpolymerisatlösung nach Vergleichsbeispiel 1
• 420 g eines aliphatischen Kohlenwasserstoffes, Siedebereich 170° bis 190 °C,

wurde nach den Angaben im Vergleichsbeispiel 1 vermahlen. Das durch Verdünnen mit 300 g Pfropfpolymerisatlösung und 1 900 g des Kohlenwasserstoffes gewonnene Tonerkonzentrat wurde mit einem Intensivmischer nachgerührt.A raw material dough

• 60 g of carbon black
• 20 g copper phthalocyanine
• 50 g ester wax, softening point 82 ° to 88 ° C, acid number 78 to 88,
• 4 g chlorine paraffin
• 200 g of graft polymer solution according to Comparative Example 1
• 420 g of an aliphatic hydrocarbon, boiling range 170 ° to 190 ° C,

was ground according to the information in Comparative Example 1. The toner concentrate obtained by dilution with 300 g of graft polymer solution and 1 900 g of the hydrocarbon was stirred in with an intensive mixer.

A liquid developer suitable for conventional copying machines was prepared by diluting with 6.5 times the amount of the aliphatic hydrocarbon and then subjected to the test. Copies with very good solid areas were obtained.

Example 9

• 60 g Ruß
• 24 g Kupferphthalocyanin
• 30 g Esterwachs wie in Beispiel 8
• 10 g Polyethylenwachs (E.P. 103° bis 107 °C)
• 4 g Chlorparaffin
• 200 g Copolymerisatlösung nach Vergleichsbeispiel 1
• 420 g eines aliphatischen Kohlenwasserstoffes, Siedebereich 170⁰ bis 190 °C.

Example 8 was repeated with the following raw material dough:

• 60 g of carbon black
• 24 g copper phthalocyanine
• 30 g of ester wax as in Example 8
• 10 g polyethylene wax (EP 103 ° to 107 ° C)
• 4 g chlorine paraffin
• 200 g copolymer solution according to Comparative Example 1
• 420 g of an aliphatic hydrocarbon, boiling range 170 ⁰ to 190 ° C.

After appropriate dilution as in Example 8, a liquid developer was obtained which was used in a conventional copying machine. The resulting copies showed excellent homogeneous image areas.

Example 10

• 60 g Ruß
• 18 g Kupferphthalocyanin
• 40 g Polyoctadecen, Schmelzviskosität (140 °C) von 960 mPa · s.
• 200 g Copolymerisatlösung nach Vergleichsbeispiel 1
• 420 g aliphatischem Kohlenwasserstoff, Siedebereich 170 bis 190 °C

wurde entsprechend den Angaben von Vergleichsbeispiel 1 vermahlen.A raw material dough

• 60 g of carbon black
• 18 g copper phthalocyanine
• 40 g polyoctadecene, melt viscosity (140 ° C) of 960 mPa · s.
• 200 g copolymer solution according to Comparative Example 1
• 420 g aliphatic hydrocarbon, boiling range 170 to 190 ° C

was ground in accordance with the information from comparative example 1.

750 g of the ground material were diluted with vigorous stirring using a laboratory dissolver with a mixture of 600 g of aliphatic hydrocarbon, boiling range 160 to 180 ° C., and 210 g of the copolymer solution mentioned in Comparative Example 1.

The resulting toner concentrate was mixed with 6 times the amount of aliphatic hydrocarbon, boiling range 160 to 180 ° C, to a ready-to-use liquid developer. In a commercial copier, copies of excellent quality could be obtained, even when using dry toner copy paper.

Examples 11 to 20

• 60 g Ruß
• 18 g Kupferphthalocyanin
• 8 g Chlorparaffin
• 200 g Copolymerisatlösung nach Vergleichsbeispiel 1
• 420 g eines aliphatischen Kohlenwasserstoffes, Siedebereich 170° bis 190 °C,

wurde, wie im Vergleichsbeispiel 1 angegeben, vermahlen. Je 140 g des Mahlgutes wurden verdünnt mit einem Gemisch aus 72 g Copolymerisatlösung nach Vergleichsbeispiel 1 und 350 g eines aliphatischen Kohlenwasserstoffes, Siedebereich 170° bis 190 °C, und dann unter starkem Rühren mittels eines Labor-Dissolvers versetzt mit einer heißen Lösung von jeweils 8 g Wachs (wie in Tabelle 4 angegeben) in 75 g des aliphatischen Kohlenwasserstoffes.A raw material dough

• 60 g of carbon black
• 18 g copper phthalocyanine
• 8 g chlorine paraffin
• 200 g copolymer solution according to Comparative Example 1
• 420 g of an aliphatic hydrocarbon, boiling range 170 ° to 190 ° C,

was ground as indicated in Comparative Example 1. 140 g of the millbase were diluted with a mixture of 72 g of copolymer solution according to Comparative Example 1 and 350 g of an aliphatic hydrocarbon, boiling range 170 ° to 190 ° C, and then, with vigorous stirring using a laboratory dissolver, a hot solution of 8 was added g wax (as indicated in Table 4) in 75 g of the aliphatic hydrocarbon.

The liquid developers obtained by dilution with 8 times the amount of hydrocarbon, boiling range 160 ° to 180 ° C, gave the following result:

• 60 g Ruß
• 18 g Kupferphthalocyanin
• 200 g der in Beispiel 1 genannten Pfropfpolymerisatlösung und
• 420 g eines aliphatischen Kohlenwasserstoffes, Siedebereich 170° bis 190 °C

wurde nach den Angaben in Vergleichsbeispiel 1 vermahlen.A raw material dough

• 60 g of carbon black
• 18 g copper phthalocyanine
• 200 g of the graft polymer solution mentioned in Example 1 and
• 420 g of an aliphatic hydrocarbon, boiling range 170 ° to 190 ° C

was ground according to the information in Comparative Example 1.

• 380 g der obengenannten Pfropfpolymerisatlösung und
• 1 660 g des aliphatischen Kohlenwasserstoffes

zum Tonerkonzentrat verdünnt.The grist was made with a mixture

• 380 g of the above graft polymer solution and
• 1,660 g of the aliphatic hydrocarbon

diluted to toner concentrate.

Dilution with 7 times the amount of an aliphatic hydrocarbon, boiling range 160 ° to 180 ° C., gave a liquid developer which gave copies with inhomogeneously colored full-tone areas.

Examples 21 to 32

A solution of 2.5 g of wax in 22.5 g of hot aliphatic hydrocarbon, boiling range 170 ° to 190 ° C., was run in a thin jet into 230 g of the toner concentrate prepared according to Comparative Example 2 with vigorous stirring using a laboratory dissolver. The mixture was stirred intensively for 5 minutes.

Ready-to-use liquid developers were obtained by dilution with 7 times the amount of an aliphatic hydrocarbon, boiling range 160 ° to 180 ° C. When the liquid developers were tested in a commercial copier, the results given in Table 5 were achieved.

Claims (15)

1. Liquid developer containing negatively charged toner particles for developing electrostatic charge images, which comprises a carrier liquid of high electric resistivity and low dielectric constant, a pigment or dye, an N-vinylpyrrolidone-containing polymer, a waxy substance and conventional admixtures, characterized in that said liquid developer contains at least one wax, which is easily soluble in the carrier liquid at rising temperatures, but difficulty soluble at room temperature, which reduces the specific electric conductivity of the liquid developer and which has a softening point within a range of 50 to 120 °C and an acid number of 0 to about 200, said wax being selected from acid wax or ester wax prepared on a basis of montan wax or from a waxy substance, such as hydrogenated castor oil or polyoctadecene.

2. Liquid developer as claimed in Claim 1, wherein the wax reduces the specific electric conductivity of said liquid developer by at least 15 percent, when 1 percent by weight of wax is added to the toner concentrate having a solids content of about 7 percent by weight.

3. Liquid developer as claimed in Claim 1, wherein the wax has a softening point within a range of 60 to 90 °C.

4. Liquid developer as claimed in Claim 1, wherein the wax has an acid number of 0 to 160.

5. Liquid developer as claimed in Claim 1, wherein the wax has an acid number of 10 to 160.

6. Liquid developer as claimed in Claim 1, wherein between 0.1 and 1.5 parts by weight of wax are contained per 1 part by weight of pigment or dye.

7. Liquid developer as claimed in Claims 1 to 6, wherein the wax is an ester wax which is derived from montan wax and has a softening point within a range of 82 to 88 °C, an acid number of 78 to 88, and a saponification number of 120 to 135.

8. Liquid developer as claimed in Claims 1 to 6, wherein the wax is an ester wax which is derived from montan wax and has a softening point within a range of 76 to 82 °C, an acid number of 15 to 20, and a saponification number of 125 to 155.

9. Liquid developer as claimed in Claims 1 to 6, wherein the wax is an ester wax which is derived from montan wax and has a softening point within a range of 78 to 86 °C, an acid number of 15 to 30 and a saponification number of 130 to 150.

10. Liquid developer as claimed in Claims 1 to 6, wherein the wax is an acid wax which is derived from montan wax and has a softening point within a range of 78 to 84 °C and an acid number of 135 to 155.

11. Liquid developer as claimed in Claims 1 to 6, wherein the wax is a hydrogenated castor oil which has a softening point of 86 to 88 °C and an acid number of 2.

12. Liquid developer as claimed in Claims 1 to 4 and 6, wherein the wax is a polyoctadecene which has a softening point of about 70 °C and an acid number of 0.

13. Process for the preparation of a liquid developer containing negatively charged toner particles for developing electrostatic charge images, from a carrier liquid of high electric resistivity and low dielectric constant, a pigment or dye, an N-vinylpyrrolidone-containing polymer, a waxy substance and conventional admixtures, by forming a paste of the pigment or dye, the N-vinylpyrrolidone-containing polymer and the other additives in the carrier liquid, grinding the paste of raw material at temperatures of 20 to 100 °C, diluting the ground material with the carrier liquid in which N-vinylpyrrolidone-containing polymer may be dissolved whereby a toner concentrate is obtained, and filling up said toner concentrate with 5 to 20 times its quantity of carrier liquid, said process being characterized in that at least one wax, which is easily soluble in the carrier liquid at increasing temperatures, but difficulty soluble at room temperature, which reduces the specific electric conductivity of the liquid developer, and which has a softening point within a range of 50 to 120 °C and an acid number of 0 to about 200, is added either to the paste of raw material, to the ground material or to the toner concentrate, said wax being selected from acid wax or ester wax prepared on a basis of montan wax or from a waxy substance, such as hydrogenated castor oil or polyoctadecene.

14. Process as claimed in Claim 13, wherein the wax is added to the paste of raw material in the form of finely divided solid particles or dispersed in the carrier liquid.

15. Process as claimed in Claim 13, wherein the wax, in the form of a very hot solution in carrier liquid, is stirred into the toner concentrate.