EP0668542A2 - Metaloxide- and metalcoated carrier for electrophotography - Google Patents

Abstract

Carriers for electrophotography based on magnetic cores coated with metal oxide and with metal or magnetite, and the production and use thereof for two-component electrophotographic developers.

Description

The present invention relates to new carriers for electrophotography based on magnetic cores coated with metal oxide and with metal or magnetite.

The invention further relates to new carriers coated with molybdenum and / or tungsten.

The invention also relates to the production of these carriers and their use for the production of two-component electrophotographic developers.

Two-component developers are used in electrophotographic copiers and laser printers for developing an electrophotographically generated latent image and usually consist of carrier particles and toner particles. The carrier particles are magnetizable particles with sizes of generally 20 to 1000 μm. The toner particles consist essentially of a coloring component and binder and are about 5 to 30 microns in size.

The electrostatic, latent image is generated in the copying process by selective exposure of an electrostatically charged photoconductor roller with light reflected from the original. In the laser printer, this is done by a laser beam.

To develop the electrostatic image, toner particles are transported to the photoconductor roller via a "magnetic brush", that is carrier particles aligned along the field lines of a sector magnet. The toner particles adhere electrostatically to the carrier particles and receive an electrostatic charge opposite to the carrier particles when they are transported in the magnetic field by friction. The toner particles thus transferred from the magnetic brush to the photoconductor roller form a "toner image" which is then transferred to electrostatically charged paper and fixed.

The carrier particles used have to meet a number of requirements: They should be magnetizable and thus enable the magnetic brush to be assembled quickly.

Furthermore, their surface should have a conductivity that is low enough on the one hand to prevent a short circuit between the sector magnet and the photoconductor roller, but on the other hand should be high enough especially for fast-working systems such as high-performance laser printers in order to build up a so-called conductive magnetic brush and thus also to enable sufficient large blackening (or other coloring; "solid area development") in the finished image. Favorable resistance values are usually in the range of 10³ to 10⁸ Ohm for this purpose.

The conductivity should remain constant over long operating times of the carrier in order to maintain the optimal working area of the magnetic brush.

Last but not least, the carrier particles should also be flowable and not clump in the developer reservoir.

In order to meet these requirements, the carrier particles consisting of magnetic material generally have to be coated.

In EP-A-303 918 and DE-A-41 40 900 metal oxide coated carriers are described with which any, especially high, positive toner charges are made possible. However, depending on the thickness of the metal oxide layer applied in each case for sufficient toner charging, in particular for fast systems, the conductivities often have conductivities which are too low (resistance values of typically> 10⁸ ohms).

Metal-coated carriers are known from US Pat. Nos. 3,632,512 and 3,736,257 which have extremely high conductivities, but with which the desired toner charges cannot be set.

The object of the invention was therefore to provide carriers for electrophotography which have a satisfactory property profile.

Accordingly, carriers for electrophotography have been found based on magnetic cores coated with metal oxide and with metal or magnetite.

In addition, a process for the production of these carriers by means of gas phase coating of moving core particles was found, which is characterized in that the metal oxide layers are passed through Hydrolysis of volatile metal alcoholates or halides or by oxidation of metal carbonyls or organyls and the metal layers by inert gas phase decomposition of metal carbonyls or organyls.

In addition, a process for the production of carriers having an inner molybdenum and / or tungsten layer and an outer molybdenum oxide and / or tungsten oxide layer has been found, which is characterized in that the moving core particles are initially reacted by inert gas phase decomposition of molybdenum and / or tungsten carbonyls or -arylene coated with a metal layer and then oxidized on the surface by heating in an oxidizing atmosphere.

Furthermore, carriers for electrophotography on the basis of magnetic cores coated with molybdenum and / or tungsten and a process for their production were found, which is characterized in that the moving core particles by inert gas phase decomposition of molybdenum and / or tungsten carbonyls or -arylene coated with molybdenum and / or tungsten.

Last but not least, the use of the carriers mentioned was found for the production of two-component electrophotographic developers.

The cores of the carriers according to the invention can consist of the usual soft magnetic materials such as iron, steel, magnetite, ferrites (for example nickel / zinc, manganese / zinc and barium / zinc ferrites), cobalt and nickel or of hard magnetic materials such as BaFe₁₂O₁₉ or SrFe₁₂O₁₉ and as spherical or irregularly shaped particles or in sponge form. So-called composite carriers, i.e. particles of these metals or metal compounds embedded in polymer resin, are also suitable.

For the metal oxide coating of the carriers according to the invention, preference is given to those metal oxides which can be deposited on the substrate to be coated from the gas phase by decomposing suitable volatile metal compounds.

Among these, molybdenum oxide (MoO₃), tungsten oxide (WO₃) and tin oxide (SnO₂) and mixtures thereof are particularly preferred, since they enable high positive charges, as are required for most laser printers, also of negative-charging polyester resin toners, which due to their good fixing properties are particularly suitable for high copying speeds.

The thickness of the layer containing metal oxide is generally 1 to 500 nm, preferably 5 to 200 nm, depending on the desired application properties (more or less high toner charge).

Also suitable for the metal coating according to the invention are in particular those metals which can be deposited by gas phase decomposition of corresponding starting compounds.

Chromium, manganese, cobalt, nickel, zinc, especially tungsten and iron and very particularly molybdenum may be mentioned as preferred examples.

Depending on the desired conductivity of the carriers, the thickness of the metal-containing layer is generally 1 to 500 nm, preferably 2 to 50 nm.

Instead of the metals, more conductive metal oxides such as magnetite can of course also be applied.

Carriers in which the metal oxide layer is present as the inner layer and the metal or magnetite layer as the outer layer will be preferred for most applications.

If the coatings are molybdenum and tungsten and their oxides, the reverse layer sequence is also possible. These carriers can be produced very simply by oxidizing the applied metal layer to the desired extent on the surface by targeted heating (generally 100 to 800 ° C.) in an oxidizing atmosphere, preferably with oxygen, in particular in the form of air.

In the production of the coated carriers according to the invention, the metal oxide layers and the metal layers (with the exception of the above-mentioned variant) are applied to the moving (fluidized) carrier cores by hydrolytic or oxidative or inert decomposition of volatile compounds of the corresponding metals ("chemical vapor deposition" ", CVD).

Suitable starting compounds for this are the metal alcoholates, metal halides, metal carbonyls and metal organyls.

The following are examples of preferred compounds: chromium carbonyls, in particular chromium hexacarbonyl, also chromaryls such as dibenzene chromium, molybdenum carbonyls, in particular molybdenum hexacarbonyl, also molybdenum aryls such as dibenzene molybdenum, also tungsten aryls such as zibenzene halotrimide, Iron carbonyls, especially iron pentacarbonyl, cobalt carbonyls, especially dicobalt octacarbonyl, nickel carbonyls, especially nickel tetracarbonyl, zinc dialkyls, especially zinc diethyl, and manganese carbonyls, especially dimanganecacarbonyl.

Suitable tin compounds include above all tin organyls, which are essentially non-decomposable under inert conditions and are oxidative in the gas phase, e.g. can be decomposed to tin dioxide by reaction with oxygen or air or other oxygen / inert gas mixtures, since they enable a particularly gentle coating of the carrier cores.

Particularly suitable are especially compounds of the formula SnR₄ in which the radicals R are identical or different and are alkyl, alkenyl or aryl, e.g. Tin tetraalkyls, tin tetraalkenyls and tin tetraaryls as well as mixed binary binary alkyls and tin alkylalkenyls.

In principle, the number of carbon atoms in the alkyl, alkenyl and aryl radicals is irrelevant, but preference is given to those compounds which have a sufficiently high vapor pressure at temperatures up to about 200 ° C. to ensure easy evaporation.

Accordingly, in the case of tin organyls with 4 identical radicals R, in particular C₁-C₆, especially C₁-C₄ alkyl, C₂-C₆ alkenyl, especially allyl, and phenyl are preferred.

Finally, bi- or polynuclear tin organyls, which can be bridged, for example, via oxygen atoms, can also be used.

Examples of suitable organotin compounds are tin diallyldibutyl, tin tetraazyl, tin tetra-n-propyl, bis (tri-n-butyltin) oxide and especially tin tetra-n-butyl and tin tetramethyl.

The decomposition temperatures for the tin organyls are generally 200 to 1000 ° C., preferably 300 to 500 ° C.

The temperature and also the amount of oxygen are expediently chosen so that the oxidation of the organic residues to carbon dioxide and water is complete and no carbon is incorporated into the tin dioxide layer. If less oxygen is introduced than is stoichiometrically required, depending on the temperature selected, either the tin organyl is only partially decomposed and then condenses in the exhaust gas area, or soot and other decomposition products are formed.

Furthermore, the evaporator gas stream containing the tin organyl should expediently be set so that the gaseous tin organyl does not make up more than about 10% by volume of the total amount of gas in the reactor in order to avoid the formation of finely divided, particulate tin dioxide. Favorable tin organyl concentrations in the carrier stream itself are usually ≦ 5% by volume.

The oxidative decomposition of the metal carbonyls to the corresponding metal oxides preferably also takes place with oxygen or air or other oxygen / inert gas mixtures, reaction temperatures of generally 100 to 400 ° C. being suitable. Magnetite-containing layers are usually applied by decomposing iron carbonyls in the presence of water vapor.

The hydrolysis of the metal halides or alcoholates with steam to form the metal oxides is usually carried out at 100 to 600 ° C., the halides generally requiring the higher temperatures.

The decomposition of the metal carbonyls and organyls for the deposition of metal layers is carried out under an inert gas, such as especially nitrogen. Suitable decomposition temperatures are generally 100 to 400 ° C for the carbonyls and 150 to 400 ° C for the organyls.

In the case of the suitable zinc alkylenes of the formula ZnR₂, the number of carbon atoms in the alkyl radicals is in principle not important, but preference is again given to those compounds which have a sufficiently high vapor pressure at temperatures up to 200.degree. Accordingly, C₁-C₄ alkyl radicals are particularly suitable.

The cooling process after the coating has ended should also be carried out under inert gas. Nevertheless, a passivation of the surface of the metal layer, whereby a passivation film of usually <2 nm thickness is formed, cannot usually be ruled out. In the case In order to increase stability, an outer iron layer is even desired to be passivated, which is why air is preferably blown into the reactor even during cooling.

Suitable reactors for the production processes according to the invention are fixed or rotating tubes or moving mixing units in which there is a moving fixed bed or a fluidized bed of the carrier cores to be coated. The movement of the carrier cores can take place by fluidization with a gas stream, by free-fall mixing, by the action of gravity or with the aid of stirring elements in the reactor.

In terms of process engineering, it is expedient to proceed as follows:
The volatile metal compounds are transferred with the aid of an inert carrier gas stream, for example nitrogen or argon, from an evaporator vessel through a nozzle into the reactor in which the carrier cores, which have been heated to the desired reaction temperature and are fluidized with inert gas, are located. The metal compound is generally introduced as a pure substance in the evaporator vessel, but can also be introduced in the form of a solution in an inert, high-boiling (boiling point about 180 to 200 ° C.) solvent (for example 30 to 50% by weight zinc diethyl solution in petroleum).

If a metal oxide layer is to be deposited, then the corresponding reaction gas - either oxygen or hydrogen - is preferably also supplied with the aid of an inert carrier gas such as nitrogen via a separate feed line.

In the production of the carrier according to the invention, which has an inner metal oxide layer and an outer metal layer, the metal deposition can be connected directly to the metal oxide deposition, of course first the supply of the reaction gas has to be switched off and, if necessary, the evaporator template has to be replaced and the temperature has to be regulated .

In the production of the carriers, which also have an inner molybdenum and / or tungsten layer and an outer layer which essentially consists of the oxides of these metals, the oxide layer can also be passed directly to the metal deposition with addition of oxygen / inert gas mixtures, if necessary after regulation the temperature.

With the CVD coating, the concentration of the vaporized metal compound (as well as the reaction gases) in the carrier gas should preferably be ≦ 5% by volume in order to ensure a uniform coating of the carrier. The evaporation rates and the reaction temperatures should, as already described above for the tin organyls, also be chosen in such a way that the most complete possible conversion takes place and no finely divided metal oxide or metal is formed which would be discharged with the exhaust gas stream.

The thickness of the layers formed naturally depends on the amount of metal compound supplied and can thus be controlled over the duration of the coating. Both very thin and very thick layers can be applied.

The coating of the carriers via the gas phase decomposition of corresponding metal compounds is the preferred procedure for producing the carriers according to the invention. In principle, however, the metal oxide layers can also be applied by precipitation of the metal oxide or hydroxide from an aqueous metal salt solution or from an organic solvent and subsequent heat treatment, and the metal layers can be applied by electroless chemical metal deposition.

The carriers according to the invention have homogeneous, abrasion-resistant metal oxide and metal layers and a surface conductivity in the desired range (about 10³ to 10⁸ ohm resistance). In addition, they have long lifetimes and can therefore be used overall advantageously with the commercially available toners for the production of electrophotographic two-component developers, the carriers which are distinguished by high positive toner charges and are coated with molybdenum, tungsten and / or tin oxide being particularly emphasized.

Examples Production and testing of carriers according to the invention

The coating of the carrier cores according to the invention was carried out in an electrically heated fluidized bed reactor with an inner diameter of 150 mm and a height of 130 cm with cyclone and carrier return.

To examine the coated carriers, their electrical resistance and the electrostatic chargeability of a toner were determined.

The electrical resistance of the carriers was measured using the C-meter from the PES laboratory (Dr. R. Epping, Neufahrn). For this purpose, the carrier particles were moved in a magnetic field of 600 Gauss at a voltage U _o of 10 V for 30 s. The capacitance C was 1 nF as standard, capacitors with capacitances of 10 or 100 nF were switched on with resistors <10⁷ Ohm.

Dabei bedeuten:

R:

Widerstand [Ohm];

t:

Zeit der Messung [s];

C:

Kapazität [F];

U_o:

Spannung zu Beginn der Messung [V];

U:

Spannung am Ende der Messung [V].

The resistance R can be calculated according to the following formula from the voltage drop over time after the applied electrical field has been switched off: $${\text{R = t / [C (ln (U}}_{\text{O}}\text{/ U)]}$$

Here mean:

R:

Resistance [ohm];

t:

Time of measurement [s];

C:

Capacity [F];

U _o :

Voltage at the beginning of the measurement [V];

U:

Voltage at the end of the measurement [V].

The resistance R is usually specified in logarithmic values (log R [log ohms]).

To determine the electrostatic chargeability, the carriers were mixed with a polyester resin toner suitable for commercial laser printers (crosslinked fumaric acid / propoxylated bisphenol A resin with an average particle size of 11 μm and a particle size distribution of 6 to 17 μm) in a weight ratio of 97: 3 and for activation in a 30 ml glass jar for 10 min in a tumble mixer at 200 rpm.

2.5 g of the developer produced in this way were placed in a hard blow-off cell coupled to an electrometer (Q / M meter from the PES laboratory, Dr. R. Epping, Neufahrn), into the sieves with a mesh size of 32 μm were inserted, weighed. The toner powder was almost completely removed by blowing out with a strong air flow (approx. 3000 cm³ / min) and simultaneously sucking off, while the carrier particles were retained in the measuring cell by the sieves.

Then the voltage generated by charge separation was read on the electrometer and from this the charge of the carrier was determined (Q = C · U, C = 1 nF), which corresponds to the charge of the toner with the opposite sign, and by weighing the measuring cell back to the weight of the blown out Toner obtained and thus its electrostatic charge Q / m [µC / g] determined.

example 1

4 kg of a sponge-shaped steel carrier with an average particle size of 40 to 120 μm (type XCS 40-120 NOD from Höganäs, Sweden) were heated to 350 ° C. in a fluidized bed reactor with fluidization using 1,800 l / h nitrogen.

148 g (100 ml) of tin tetrabutyl were transferred into the reactor in 11 h using a 400 l / h nitrogen stream from an upstream evaporator vessel heated to 125 ° C.

At the same time, 400 l / h of air were passed into the reactor for oxidation via the fluidizing gas.

The tin dioxide-coated carrier obtained was then cooled to 200 ° C. in the reactor with fluidization with nitrogen.

Then 30 ml of iron pentacarbonyl were transferred into the reactor in 4 h with the aid of a nitrogen stream of 100 l / h from an evaporator vessel at a temperature of 22 ° C.

After the iron coating had been completed, the carrier was cooled to 80 ° C. with further fluidization. An air flow of 200 l / h was then introduced into the reactor for passivation of the iron surface for 30 minutes.

After cooling to room temperature, the coated carrier was removed.

A tin content of the carrier of 0.7% by weight was determined by means of atomic absorption spectroscopy.

During the further investigation of the carrier, the following resistance and charge values were determined: log R [log ohm] Q / m [µC / g] Raw carrier / SnO₂ / Fe 6.0 + 4.0 Rohcarrier / SnO₂ (compare) 9.53 + 5.0 Raw carrier (see comparison) <3.0 (outside the measuring range) - 2.5

Example 2

4 kg of an irregularly shaped iron powder with average particle sizes of 60 to 150 μm (Steel Powder, Höganäs, Sweden) were heated to 220 ° C. in a fluidized bed reactor with fluidization using 1,800 l / h nitrogen.

30 g of molybdenum hexacarbonyl were transferred into the reactor in 3 hours with the aid of a nitrogen flow of 400 l / h from an evaporator vessel heated to 60 ° C.

At the same time, 400 l / h of air were passed into the reactor for oxidation via the fluidizing gas.

The molybdenum oxide-coated carrier obtained was then additionally coated with metallic molybdenum by supplying a further 5 g of molybdenum hexacarbonyl in 400 l / h of nitrogen from the evaporator vessel now heated to 50 ° C. and its inert decomposition at 220 ° C. in 1 h.

The carrier, cooled and removed under nitrogen to room temperature, had a molybdenum content of 0.2% by weight (AAS).

Resistance and charge values: log R [log ohm] Q / m [µC / g] Rohcarrier / MoO₃ / Mo 5.69 + 16.6 Rohcarrier / MoO₃ (see comparison) 9.10 + 22.6 Raw carrier (see comparison) 8.50 + 3.8

Example 3

• a) 3 kg of the raw carrier from Example 2 were heated to 230 ° C. in a fluidized bed reactor with fluidization using 1,800 l / h of nitrogen.
  75 g of molybdenum hexacarbonyl were transferred with the aid of a nitrogen stream of 400 l / h from an evaporator vessel heated to 60 ° C. into the reactor in 5 h and were inertly decomposed there.
  The molybdenum-coated carrier obtained was removed after cooling under nitrogen.
• b) 500 g of the molybdenum-coated carrier were annealed in a rotary ball oven with movement and an air inlet of 100 l / h for 1 hour at 1) 100 ° C., 2) 200 ° C. or 3) 300 ° C.

The removed after cooling to room temperature, molybdenum and molybdenum oxide-coated carriers b₁) - b₃), like the only molybdenum-coated carrier a), had a molybdenum content of 0.7% by weight.

Resistance and charge values: log R [log ohm] Q / m [µC / g] a) Rohcarrier / Mo <3.00 + 24.0 b₁) Rohcarrier / Mo / MoO₃ 4.73 + 21.1 b₂) Rohcarrier / Mo / MoO₃ 6.92 + 23.1 b₃) Rohcarrier / Mo / MoO₃ 9.02 + 20.6 Raw carrier 8.50 + 3.8

Claims (10)

Carrier for electrophotography based on magnetic cores coated with metal oxide and with metal or magnetite. Carrier according to claim 1, wherein the carrier cores with A) a first layer containing metal oxide and B) a second layer containing metal or magnetite is covered. Carrier according to claim 1 or 2, wherein the metal oxide layer contains molybdenum oxide, tungsten oxide and / or tin oxide. Carrier according to claims 1 to 3, in which the metal layer contains iron, cobalt, nickel, chromium, molybdenum, tungsten, zinc and / or manganese. The carrier of claim 1, wherein the carrier cores are coated with an inner layer containing molybdenum and / or tungsten and an outer layer containing molybdenum oxide and / or tungsten oxide. Carrier for electrophotography based on magnetic cores coated with molybdenum and / or tungsten. Process for the production of carriers according to Claims 1 to 5 of core particles moved by gas phase coating, characterized in that the metal oxide layers are applied by hydrolysis of volatile metal alcoholates or halides or by oxidation of metal carbonyls or organyls and the metal layers by inert gas phase decomposition of metal carbonyls or organyls . Process for the production of carriers according to claim 5, characterized in that the moving core particles are first coated with a metal layer by inert gas phase decomposition of molybdenum and / or tungsten carbonyls or aryls and then oxidized on the surface by heating in an oxidizing atmosphere. Process for the production of carriers according to claim 6, characterized in that the moving core particles are coated with molybdenum and / or tungsten by inert gas phase decomposition of molybdenum and / or tungsten carbonyls or -arylene. Use of carriers according to claims 1 to 6 for the production of electrophotographic two-component developers.