CA2137502A1 - Use of metal oxide pigment as charge stabilizers in electrostatic toners - Google Patents

Abstract

Metal oxide pigments are useful as charge stabilizers in electrostatic toners.

Description

2 1~7SQ2 Use of metal oxide pigment as charge stabilizers in electrostatic toners 5 The present invention relates to the novel use of metal oxide pigments as charge stabilizers in electrostatic toners.

Electrophotography involves selective irradiation of an electros-tatically charged photoconductor drum with light reflected by the 10 original to be copied to produce a latent electrostatic image. In a laser printer, this is done by a laser beam.

The electrostatic image is developed by transporting toner particles to the photoconductor drum via a magnetic brush, ie.
15 carrier particles aligned along the field lines of a sector mag-net. The toner particles cling electrostatically to the carrier particles and, in the course of transport in the magnetic field, receive due to friction an electrostatic charge opposite to that of the carrier particles. The toner particles thus transferred 20 from the magnetic brush to the photoconductor drum produce a toner image, which is subsequently transferred to, and fixed on, paper or film.

To obtain strong, crisp images, the toner is admixed with com-25 pounds to stabilize its electrostatic charge which are known as charge stabilizers or as charge controlling agents.

Charge stabilizers have to meet a number of requirements: they must be able to develop the latent electrostatic image into a 30 strong visible image; they must be readily dispersible in the toner preparation in order that fault-free, crisp and uniform images may be produced; and not least they must be impervious to moisture and possess a high thermal stability.

35 These requirements are very difficult to meet at one and the same time. Prior art charge stabilizers therefore frequently have de-fects in their property profile.

It is an object of the present invention to provide novel charge 40 stabilizers having advantageous application properties.

We have found that this object is achieved by the use of metal oxide pigments as charge stabilizers in electrostatic toners.

45 Metal oxide pigments for the purposes of the present invention are the metal oxides themselves and also metal oxide hydrates and mixtures of metal oxides and metal oxide hydrates. Of course, it - 2137~02 is also possible for mixed oxides and mixed oxide hydrates, ie.
oxides or oxide hydrates which contain various metals, to be present.

5 Preference is given to oxide pigments contA;n;ng metals of the subgroup 8 of the periodic table, particularly cobalt or nickel and very particularly iron.

Suitable mixed oxides include not only oxides formed of various 10 metals of subgroup 8 but also oxides which contain not only these metals but other metals as well.

Examples of suitable metal oxides and oxide hydrates include titanium dioxide, zinc oxide, antimony(III) oxide, chromium(III) 15 oxide, chromium(III) oxide hydrate, cobalt(II) oxide, -~cobalt(II,III) oxide Co304, lead(II,III) oxide Pb304 and also in particular iron(III) oxide, especially a-Fe203 (haematite; C.I.
Pigment Red 101; C.I. 77491) and iron(III) oxide hydrates FeO(OH) xH20 (x from about 1 to 7; C.I. Pigment Yellow 42;
20 C.I. 77492), especially a-FeOOH (goethite), and also mixtures thereof.

Examples of suitable mixed oxides include rutile types such as FeTiO3 and especially spinel types such as ZnCo204 and especially 25 CoAl204 (cobalt spinel; C.I. Pigment Blue 28) and (Co,Ni)Al204.

Of these, those metal oxide pigments which are present in trans-parent (highly transparent to semitransparent) form (eg. finely divided a-Fe203 and a-FeO(OH)-xH20) are particularly preferred.
30 These iron oxide pigments are generally known. Information on their preparation can be gleaned for example from Rompps Chemie-Lexikon, 8th edition, volume 2, 1066-1067 (1981).

The metal oxide pigments of the invention can advantageously be 35 used as charge stabilizers in the preparation of electrostatic toners for one- and especially two-component developers. The average particle diameter of the pigments is generally s 1 ~m, preferably from 0.005 to 0.1 ~m, particularly preferably from 0.01 to 0.05 ~m. The values mentioned relate in the case of acicular 40 pigment particles to the diameter perpendicular to the longitudi-nal extension.

The most important constituents of an electrostatic toner are generally the binder and the charge stabilizer, which usually 45 accounts for from 0.01 to 10 % by weight, in particular from 0.01 to 5 % by weight, of the ready-prepared toner.

2131~2 The binders in toners are usually, as will be known, thermoplas-tic polymers having softening points from 40 to 200 C, preferably from 50 to 130 C, particularly preferably from 65 to 115 C.

5 Examples of suitable binders include polystyrene, copolymers of styrene and an acrylate or methacrylate, copolymers of styrene and butadiene and/or acrylonitrile, polyacrylates, polymethacry-lates, copolymers of an acrylate or methacrylate and vinyl chloride or vinyl acetate, polyvinylchloride, copolymers of vinyl -10 chloride and vinylidene chloride or vinyl acetate, polyesterresins, epoxy resins, polyamides and polyurethanes.

If desired, the electrostatic toners may contain further ingredi-ents such as waxes, flow control agents, colorants and magneti-15 cally attractable materials. -, Colorants may be selected from organic dyes or pigments, such as nigrosine, An;l;ne blue, 2,9-dimethylquinacridone, C.I. Disperse Red 15 (C.I. 60 710), C.I. Solvent Red 19 (C.I. 26 050), C.I.
20 Pigment Blue 15 (C.I. 74 160), C.I. Pigment Blue 22 (C.I. 69 810) or C.I. Solvent Yellow 16 (C.I. 12 700) or inorganic pigments, such as carbon black, lead red, yellow lead oxide or chromium yellow. Generally, the proportion of colorant present in the toner does not exceed 15 % by weight, based on the weight of the 25 toner.

The magnetically attractable materials can be for example iron, nickel, chromium oxide, iron oxide or a ferrite of the formula MeFe2O4, where Me is a bivalent metal, eg. iron, cobalt, zinc, 30 nickel or manganese.

Toners can be prepared with the charge stabilizers of the inven-tion in a conventional manner, for example by mixing the ingredi-ents in solid form in a kneader and subsequently pulverizing or 35 by dispersing the rest of the ingredients in the molten binder using known mixing or kneading ~ch;nes, subsequently cooling the melt to form a solid mass, and grinding the solid mass to par-ticles of the desired size (generally from 0.1 to 50 ~m).

40 It is also possible to dissolve the binder in a suitable solvent and to disperse the charge stabilizer finely in this solution.
The toner preparation thus obtA;ne~ can be used directly, for example in a xerographic image recording system, or first be sub-jected to a drying process, for example spray drying, freeze-45 drying or evaporation of the solvent, with subsequent grinding tothe desired particle size.

21~750'~

The metal oxide pigments to be used according to the invention as charge stabilizers are notable for altogether good application properties. They are impervious to moisture, thermally stable up to 180 C, and in particular they confer a favorable electrostatic 5 charging profile on a toner preparation, ie. the toners can be charged up rapidly and to a high level. Moreover, they also have the effect that this charge is kept constant at a high level.

Examples I. Preparation of electrostatic toners cont~;n;ng charge stabi-lizers according to the invention The binders used were Resin A:an uncrosslinked styrene/butyl acrylate resin, or Resin B:a linear uncrossl;nked polyester resin.

The toner preparation was either by - freeze drying (Method F) by dispersing 0.2 g of metal oxide pigment in a solution of 10 g of the respective resin in 100 ml of p-xylene and subsequently freeze-drying the resulting suspension, or by - kneading (Method K) by intensively mixing 0.2 g of metal oxide pigment and 10 g of the respective resin in a mixer, kneading at 120 C, extruding and grinding, producing toner particles having an average particle size of 50 ~m.

The table summarizes details of the toners and of the metal oxide pigments used as charge stabilizers.

II. Preparation and testing of developers To prepare the developers, the toners thus prepared were each mixed in a weight ratio of 1:99 with a steel carrier having an average particle size of 100 ~m and activated on a roll stand.

Samples were taken after 10, 30, 60 and 120 min and measured in respect of their electrostatic charge in a q/m meter (from Epping, Neufahrn, Germany).

- 21~7502 For this purpose in each case 5 g of the developer were weighed into a hard blowoff cell which was coupled to an electrometer and which contained sieves of mesh size 63 ~m. A
fast air stream (about 4000 cm3/min) with simultaneous aspiration was employed to remove the toner particles almost completely from the carrier particles with the latter being held back in the measuring cell by the sieves. The charge on the carrier, which corresponds to the charge on the toner particles except for the sign being opposite, was read off on the electrometer; the measuring cell was weighed back to determine the weight of the blown-off toner; and in this way the electrostatic charge q/m [~C/g] on the toner was deter-mined.

The measurements obtained are summarized in the table, Table Ex. Metal oxide pigment Resin Method Charge q/m [~C/g]
(average particle diameter [~m]) following activation for 10 min 30 min 60 min 120 min 1 a-Fe2O3 (C.I. Pigment Red 101; C.I. 77491; B F -10.9-11.3-12.7 -11.8 0,014 ~m) 2 a-Fe2O3 (as for Ex. 1) B K -4.7 -4.4-4.7 -4.8 3 a-Fe2O3 (as for Ex. 1) A F -17.8 -18.8-19.2-19.3 4 a-Fe2O3 (as for Ex. 1) A K -8.5 -8.1-7.7 -7-9 a-FeOOH xH2O (C.I. Pigment Yellow 42; B F -6.8-8.6 -9.1 -9.7 C.I. 77492; 0,012 ~m) 6 a-FeOOH . xH2O (as for Ex. 5) B K -5.1 -5.9-5.6 -5.5 7 a-FeOOH . xH2O (as for Ex. 5) A K -16.3 -16.3-17.6-16.9 8 a-FeOOH . xH2O (as for Ex. 5) A K -11.1 -11.5-11.7-11.2

Claims (6)

1. A process for preparing electrostatic toners, which comprises using metal oxide pigments as charge stabilizers.

2. A process as claimed in claim 1, wherein the oxide pigments used contain metals of subgroup 8 of the periodic table.

3. A process as claimed in claim 1, wherein the oxide pigments used contain iron, cobalt and/or nickel.

4. A process as claimed in claim 1, wherein iron(III) oxide, iron(III) oxide hydrate or mixtures thereof are used.

5. A process as claimed in claim 1, wherein transparent oxide pigments are used.

6. Electrostatic toners comprising metal oxide pigments as charge stabilizers.