Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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
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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.
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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.
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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).
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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
