EP0674238A2 - Carrier for electrophotography having a double metaloxide coating - Google Patents

Abstract

Electrophotographic carrier is based on magnetic core coated with various metal oxides, comprising (a) a first layer of electrically insulating metal oxide and (b) a second layer of metal oxide which controls electrostatic charging of the toner and does not cause a significant redn. in the resistance of the carrier provided by layer (a). Also claimed are methods for producing these carriers and a method of coating carrier cores with Al2O3.

Description

• A) eine erste, im wesentlichen aus elektrisch isolierendem Metalloxid bestehende Schicht und
• B) eine zweite, im wesentlichen aus die elektrostatische Aufladung des Toners steuerndem Metalloxid bestehende Schicht, welche den durch die Schicht (A) bewirkten elektrischen Widerstand der Carrier nicht wesentlich erniedrigt,

aufweisen.The present invention relates to new carriers for electrophotography based on magnetic cores coated with various metal oxides

• A) a first layer consisting essentially of electrically insulating metal oxide and
• B) a second layer, consisting essentially of the metal oxide controlling the electrostatic charging of the toner, which does not significantly reduce the electrical resistance of the carriers caused by the layer (A),

exhibit.

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 to develop 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 result in 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 low conductivity in order to prevent a short circuit between the sector magnet and the photoconductor roller. This conductivity should remain constant over long operating times of the carrier in order to keep the triboelectric charging of the developer constant for a long time. 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.

From EP-A-303 918 and DE-A-41 40 900 simple metal oxide coated carriers are described, with which any toner charges are made possible, but a simultaneous control of the electrical resistance of the carriers is not possible.

Finally, the older German patent application P 44 03 678.7 also describes two carriers, coated with a metal and a metal oxide layer, which have low resistances of generally 10³ to 10⁸ ohms.

Carriers that cause both strong, in particular positive, toner charging and at the same time have an electrically insulating effect (i.e. have resistances> 10¹⁰ ohms) are not yet known. Such carriers are of particular interest for office copiers and other slower-working systems.

The invention was therefore based on the object of providing carriers for electrophotography which correspond to this property profile.

• A) eine erste, im wesentlichen aus elektrisch isolierendem Metalloxid bestehende Schicht und
• B) eine zweite, im wesentlichen aus die elektrostatische Aufladung des Toners steuerndem Metalloxid bestehende Schicht, welche den durch die Schicht (A) bewirkten elektrischen Widerstand der Carrier nicht wesentlich erniedrigt,

aufweisen.Accordingly, carriers for electrophotography based on magnetic cores coated with various metal oxides have been found

• A) a first layer consisting essentially of electrically insulating metal oxide and
• B) a second layer, consisting essentially of the metal oxide controlling the electrostatic charging of the toner, which does not significantly reduce the electrical resistance of the carriers caused by the layer (A),

exhibit.

In addition, a process for the production of these carriers has been found, which is characterized in that the metal oxide layers are wet-chemically by hydrolysis of organic metal compounds in which the organic radicals are bonded to the metals via oxygen atoms, in the presence of an organic solvent in which the metal compounds are soluble are, or by gas phase decomposition of volatile metal compounds in the presence of oxygen and / or water vapor onto the carrier cores.

In addition, a process has been found for the production of carrier cores coated with aluminum oxide, which is characterized in that aluminum alkyls are decomposed in the gas phase in the presence of oxygen and moving carrier cores.

We have also found the use of these carriers 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.

Titanium dioxide, aluminum oxide, iron oxide and above all silicon dioxide and also mixtures thereof are particularly suitable for the first metal oxide layer (A), which has an electrically insulating effect.

The thickness of layer (A) depends on the desired level of electrical resistance of the carrier and is generally 10 to 500 nm, preferably 30 to 300 nm and particularly preferably 50 to 200 nm.

For the second metal oxide layer (B) which controls the electrostatic charging of the toner, metal oxides such as molybdenum oxide, tungsten oxide and tin dioxide causing strong positive toner charging are particularly preferred.

Depending on the electrical conductivity of the metal oxides used, layer (B) should be chosen to be more or less thin. Conductive layers (B) that are too thick reduce the electrical resistance of the carrier caused by layer (A); those layers (B) which reduce the resistance by no more than about 1.5 powers of ten are particularly suitable. As a rule, the layer (B) will therefore be 1 to 50 nm, preferably 2 to 20 nm thick.

In the process according to the invention for producing the coated carriers, the metal oxide layers are either wet-chemically by hydrolysis of organic metal compounds in which the organic radicals are bonded to the metals via oxygen atoms, in the presence of an organic solvent or by gas phase decomposition of volatile metal compounds in the presence of oxygen and / or chemical vapor deposition (CVD) is applied to the carrier cores.

The wet chemical process variant is particularly suitable for coating with silicon oxide. Of course, the other metal oxides can also be applied by precipitation from aqueous solutions or from solutions in organic solvents.

Suitable organic solvents for this purpose are both aprotic solvents such as ketones, β-diketones, ethers, especially cyclic ethers, and nitrogen-containing solvents, e.g. amidic solvents, as well as protic solvents such as mono- or polyhydric alcohols, preferably having 1 to 6 carbon atoms, which are miscible with water.

Examples of preferred solvents are acetone, tetrahydrofuran, ethanol and n- and iso-propanol as well as diethyl ketone, acetylacetone, dioxane, trioxane, ethylene glycol, propylene glycol, glycerol, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridone and acetonitrile.

Suitable metal-containing starting compounds in the organic solvents mentioned are organic compounds in which the organic radicals are bonded to the metals via oxygen atoms. Preferred examples are the acetylacetonates and in particular alcoholates, especially C₁-C₄ alkanolates, for example tetraethoxysilane and aluminum triisopropanolate.

The hydrolysis is preferably carried out in the presence of a base or an acid as a catalyst. For this, e.g. in addition to alkali solutions such as sodium hydroxide solution, especially aqueous ammonia solutions. Suitable acidic catalysts are, for example, phosphoric acid and organic acids such as acetic acid and oxalic acid.

Water should be present at least in the stoichiometric amount required for the hydrolysis, but 2 to 100 times, in particular 5 to 20 times, is preferred.

Based on the amount of water used, 3 to 40% by volume, preferably 5 to 30% by volume, of a 25% by weight aqueous ammonia solution are generally added.

For the temperature control, it has proven advantageous to gradually heat the reaction mixture to reflux temperature within 10 to 48 h. If isopropanol is used as the solvent, the mixture is preferably stirred, for example, first for 4 to 20 hours at 40 ° C., then for 4 to 20 hours at 60 ° C. and finally for 2 to 8 hours at 80 ° C.

In terms of process engineering, it is expedient to proceed as follows:
The carrier cores, organic solvent, water and base are introduced and the metal compound to be hydrolyzed, pure or dissolved, for example as a 30 to 70, preferably 40 to 60,% by volume solution in the organic solvent. If the metal compound is added in one step, the suspension is then heated with stirring as described above. However, the metal compound can also be metered in continuously at elevated temperature, the water preferably not being initially introduced, but also metered in continuously. After the coating has ended, the reaction mixture is cooled again to room temperature.

In order to prevent agglomerate formation during the coating process, the suspension can be subjected to strong mechanical stress such as pumping, vigorous stirring or the action of ultrasound.

The coating step can be repeated if desired, but this will generally not be necessary. If the mother liquor looks milky, it is advisable to replace it before applying another coating.

The carrier cores coated with layer (A) can be isolated in a simple manner by filtration, washing with organic solvent, preferably the alcohols also used as solvent, and subsequent drying (usually 1 to 5 h at 100 to 250 ° C.).

In particular, the metal alcoholates, metal halides, metal carbonyls and metal organyls are suitable as volatile metal compounds for the CVD process variant.

The following are examples of preferred compounds:
Titanium alcoholates, especially titanium tetraisopropanolate, also silicon halides such as silicon tetrachloride, iron carbonyls, especially iron pentacarbonyl, molybdenum carbonyls, especially molybdenum hexacarbonyl, also molybdenum aryls, especially dibenzene molybdenum, tungsten carbonyls, especially tungsten hexacarbonyl, in particular zirconium halide, such as zamfromhalohalogenol, such as C₁-C₆ alkyls such as aluminum trimethyl, triethyl and triisobutyl.

As already described in the older German patent application P 44 03 679.5, tin organyls should also be mentioned as suitable tin compounds, which are essentially non-decomposable under inert conditions and which are oxidative, 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 not important, but preference is given to those compounds which unite at temperatures up to about 200.degree have sufficiently high vapor pressure 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 tetraamyl, 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 concentration in the carrier stream itself is usually ≦ 5% by volume.

The oxidative decomposition of the metal carbonyls and the further metal organyls to give the corresponding metal oxides is preferably likewise carried out using oxygen or air or other oxygen / inert gas mixtures. Reaction temperatures of 100 to 400 ° C. are generally suitable for this. The decomposition of the aluminum alkyl is usually carried out at 200 to 1000 ° C, preferably 300 to 500 ° C.

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.

Suitable reactors for gas phase coating 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.

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 so that the most complete possible conversion takes place and no fine-particle metal oxide is formed which would be discharged with the exhaust gas stream. Further details can be found in DE-A-41 40 900.

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 carriers according to the invention are notable for the high quality of the metal oxide layers applied (homogeneous, film-like and abrasion-resistant) and have a resistance in the desired range of> 10 11 ohms, that is to say they have an electrically insulating effect.

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.

example Production and testing of a carrier according to the invention

180 ml of a 25% strength by weight aqueous ammonia solution were added to a suspension of 4.5 kg of a ferrite carrier (barium / zinc ferrite, particle size 45 to 105 μm, type KBN 100 from Hitachi, Japan) in 2250 ml of isopropanol. After the mixture had been heated to 40 ° C., 720 ml (669.6 g) of tetraethoxysilane were added dropwise in the course of 10 minutes.

After stirring for a further four hours at 40 ° C. and stirring for one hour at 60 ° C. and 80 ° C., the supernatant, milky, cloudy liquid phase was decanted off. The carrier coated with SiO₂ or SiO₂ hydrates was washed three times with 1500 ml of isopropanol, filtered off and dried at 100 ° C. for 1 h.

A silicon content of 0.42% by weight was determined by means of atomic absorption spectroscopy.

Then 4 kg of the SiO₂-coated carrier in an electrically heated fluidized bed reactor (150 mm inner diameter, 130 cm high, with cyclone and carrier return) were heated to 230 ° C. with fluidization using a total of 1800 l / h of nitrogen.

13.2 g of molybdenum hexacarbonyl were transferred into the reactor in 3 hours with the aid of a nitrogen stream of 400 l / h of nitrogen from an upstream evaporator vessel heated to 50 ° C.

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

After completion of the molybdenum oxide coating, the carrier was cooled to room temperature with further fluidization with nitrogen.

A molybdenum content of 0.08% by weight was determined by means of atomic absorption spectroscopy.

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

The electrical resistance of the carrier was measured with the C-meter from the PES laboratory (Dr. R. Epping, Neufahrn). The carrier particles were subjected to this for 30 s in a magnetic field of 600 gauss a voltage U _o of 10 V. The capacitance C was 1 nF.

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 using 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 carrier was 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 mixed in a 30 ml glass vessel for 10 minutes in a tumble mixer at 200 rpm.

2.5 g of the developer thus produced 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 weighed in. 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.

, C = 1 nF), die der Aufladung des Toners mit umgekehrtem Vorzeichen entspricht, und durch Zurückwägung der Meßzelle auf das Gewicht des ausgeblasenen Toners bezogen und so dessen elektrostatische Aufladung Q/m [µC/g] bestimmt.The voltage generated by charge separation was then read on the electrometer and the charge on the carrier was determined ( $\text{Q = CU}$

, C = 1 nF), which corresponds to the charging of the toner with the opposite sign, and based on the weight of the blown-out toner by weighing the measuring cell and thus determining its electrostatic charging Q / m [µC / g].

The following results were obtained in these investigations: log R [log ohm] Q / m [µC / g] Rohcarrier / SiO₂ / MoO₃ 10.28 + 20.7 Raw carrier / SiO₂ (see comparison) 11.48 + 8.9 Raw carrier (see comparison) 10.51 - 9.5

Claims (10)

Carrier for electrophotography based on magnetic cores coated with various metal oxides A) a first layer consisting essentially of electrically insulating metal oxide and B) a second layer, consisting essentially of the metal oxide controlling the electrostatic charging of the toner, which does not significantly reduce the electrical resistance of the carriers caused by the layer (A), exhibit. Carrier according to claim 1, in which the layer (A) consists essentially of silicon oxide, aluminum oxide, titanium oxide, iron oxide or mixtures thereof. Carrier according to claim 1 or 2, in which the layer (B) consists essentially of molybdenum oxide, tungsten oxide, tin oxide or mixtures thereof. Carrier according to Claims 1 to 3, in which the layer (A) has a layer thickness of 10 to 500 nm. Carrier according to Claims 1 to 4, in which the layer (B) has a layer thickness of 1 to 50 nm. Process for the production of carriers according to Claims 1 to 5, characterized in that the metal oxide layers are wet-chemically by hydrolysis of organic metal compounds in which the organic radicals are bonded to the metals via oxygen atoms, in the presence of an organic solvent in which the metal compounds are soluble are, or by gas phase decomposition of volatile metal compounds in the presence of oxygen and / or water vapor onto the carrier cores. Process according to Claim 6, characterized in that the C₁-C₄ alcoholates or acetylacetonates are used as the organic metal compounds. A method according to claim 6, characterized in that the alcoholates, halides, carbonyls or organyls are used as volatile metal compounds. Process for the production of carrier cores coated with aluminum oxide, characterized in that aluminum alkyls are decomposed in the gas phase in the presence of oxygen and moving carrier cores. Use of carriers according to claims 1 to 5 for the production of two-component electrophotographic developers.