EP0359041A2 - Oxide-coated carrier, its manufacturing process and its use - Google Patents

Abstract

The novel carriers consist of steel cores which have an iron oxide- containing coating of the formula (FeO)x . Fe2O3 where x = 0.1 - 1. The carriers are stable and effective over long cycle times (6 .10<6> prints). …<??>The carriers are obtained by treating steel cores (steel spheres) with defined small amounts of sulphuric acid of defined concentration, partial oxidation of the treated cores and drying at 60-150 DEG C at </= 100 mbar.

Description

Electrophotographically generated images are today mainly developed with dry toners according to the one-component or the two-component system. The one-component system consists of a toner that can be magnetized. The developer of two-component systems usually consists of magnetic carrier and non-magnetic toner particles.

In electrophotography, an invisible, latent image is generated by selective exposure of a photoconductor loaded with charge carriers. In order to make this charge pattern visible, it has to be developed. This is done by supplying toner powder, which in the two-component system essentially consists of a coloring component and a binder and has particle sizes between 5 and 30 μm. The toner powder is over the so-called "magnetic brush" - d.s. chains of carriers aligning on a sector magnet along the electrical field lines - transported to the photoconductor. The carrier is loaded with toner and is evenly brought up to the photoconductor. This transport causes a controlled, electrostatic charging of the toner powder, which can now be transferred to the photoconductor. The magnetic brush made of carriers wipes off excess toner from the photoconductive layer and conveys it back into the storage container. The developed toner image is then transferred to paper and fixed. The functioning of the development process in two-component systems is well known and z. B. in DE-C 2 404 982 described in detail.

The carriers typically consist of a core whose material can be magnetized. The material can for example be made of iron, nickel, magnetite, γ-Fe₂0₃ or certain ferrites. Steel carriers are still widely used today due to their excellent soft magnetic properties.

The carrier particles usually have a surface coating in order to set the required electrical and electrostatic properties. These coatings also affect the mechanical properties. Spherical particles are particularly flowable. Irregular carrier shapes are used when high electrostatic charges are desired. The toner particles are charged to the desired extent by electron exchange processes or also by ion transitions [KL Birkett and P. Gregory, Dyes and Pigments, 7 , 341 (1986)], which are characterized by the Friction of the toner particles and carriers against each other are induced (triboelectric effect). Since the toner particles interact intensively with the carrier surface, undesirable side effects such as abrasion and sticking of the surfaces occur in addition to the desired charge exchange processes.

Due to the intense friction, both the toner and carrier surfaces are subject to abrasion. The abrasion of tiny particles of the toner leads to sticking on the carrier surface, which reduces the activity of the carrier. This manifests itself in a steady decrease in the carrier's ability to charge the toner particles to a certain extent, and as a result, the printed image deteriorates.

The adhesion of toner resin particles to the carrier surface and the breakdown of the carrier activity has become known under the terms "exhaustion phenomenon" or "toner impaction".

In order to reduce this sticking of the carrier surface, plastics with low surface energies, e.g. B. silicone resins (e.g. US-A 3 562 533) or fluorocarbon-containing polymers (e.g. US-A 3 553 835). However, the mechanical resistance of such carrier coatings left much to be desired. It has therefore become known to improve the abrasion resistance by means of fillers such as silicon carbide, potassium titanate (DE-A 3 312 741), chromium oxide or iron oxide (US Pat. No. 3 798 167) or other metal oxide compounds. Most of the polymers also had to be added with conductive components because of their high electrical resistance. This measure ensures that the surface is mechanically stable, but the toner particles form abrasion during the transport process, which covers the surface of the carrier and glues the latter after appropriate compaction, thereby reducing the activity of the carrier. In order to eliminate this disadvantage, attempts have been made to compensate for the decreasing activity of the carrier by means of a coating which, for. B. contains organotin compounds with concentration gradients within the layer (DE-A3 511 171). The latter serves as a catalyst in the hardening of the silicone resin and compensates to a certain extent for the carrier's decreasing activity with a certain toner. However, the production of such layers can only be achieved via a complicated process and must be matched to the viscoelastic behavior of the corresponding toner.

Fundamental studies on the subject of exhaustion and triboelectricity were carried out. The phenomenon of "toner impaction" was dependent on the toner particle size, the carrier particle size, the occupancy of the toner and the loading of the Carriers examined with toner (RJ Nash and JT Bickmore, "Toner impaction and triboelectric Aging", Paper Summeries of the 4th Congress on Advance in Non-Impact-Printing Technologies, p. 84, March 1988, New Orleans). As a result of these investigations, it can be said that smaller toner particles, smaller carrier particles and surface coating of the toner with hydrophobized silica extend the life of the developer.

Steel carriers with certain electrical properties are known: according to US Pat. No. 3,632,512, steel balls are oxidized in a defined manner by treatment with 2N sulfuric acid or oxidized by tempering according to CA-A-1 103 079. These carriers have an oxide layer on their surface. The treatment of steel balls according to US-A-3 632 512 with 2N sulfuric acid is associated with a considerable waste water load and a technical implementation is difficult and expensive due to the complicated drying. The carriers obtained by this process have very homogeneous coatings and improve the load distribution and ensure a better print image.

The aim of these surface coatings is to produce the most abrasion-resistant coatings possible, in addition to the good electrical properties (average resistivities from 10⁻¹ to 10 möglichst Ω · cm⁻¹). The decrease in carrier activity can be delayed due to the low affinity of the iron oxide layer for the toner resin. Nevertheless, a steady decline in carrier activity is to be expected, since the flaked-off toner resin particles initially remain on the carrier surface due to the electrostatic charge and are increasingly compressed and bonded to one another by the rotational movement of the carrier particles. However, the question arises whether the exhaustion phenomenon cannot be delayed in a fundamentally different way.

A fundamental disadvantage of all known carrier developments is that carrier activity is steadily decreasing, i. H. the printed image changes constantly over the life of the developer. To prevent this, the carrier surface must be constantly regenerated in order to maintain its original character over many thousands of copying cycles.

It was an object of the present invention to produce steel carriers with an oxidic surface which is constantly regenerated when used, so that a long service life should result with consistently good print quality. Furthermore, the process should be inexpensive and environmentally friendly.

This object is achieved by the carriers of the invention.

The invention relates to carriers which have, on steel cores one containing iron oxide surface coating of the formula (Fe0) _x · Fe₂0₃ (x = 0.1 s -1) and which are obtainable by treating the steel cores (balls) with aqueous sulfuric acid, per m² of ball surface 5 · 10⁻⁵ to 2.5 · 10⁻⁴ mol of sulfuric acid are used and the acid concentration at the beginning of the treatment is 10⁻² to 10⁻⁶ mol / l, oxidation of the balls treated with sulfuric acid with oxygen or an oxidizing agent in one Amount that corresponds to 5 · 10⁻⁵ to 5 · 10⁻⁴ oxidation equivalent / m² sphere surface and drying the spheres at 60 to 150 ° C under a pressure of ≦ 100 mbar.

The carriers of the invention have a surface which corresponds to the material composition (Fe0) _x Fe₂0₃. The new carriers have a surface in which the abrasion of the carrier surface takes on the important function of cleaning and renewing the carrier particle surface.

The surface of the carrier according to the invention consists of an approximately 0.3 µm thick, mostly X-ray amorphous iron oxide layer, the composition of which was determined with (Fe0) _x Fe₂0₃ - for which x: 0.1 ≦ x ≦ 1 - by wet chemical analysis of collected abrasion samples . When creating concentration depth profiles by removing the carrier surface with argon plasma, the decrease in the oxygen concentration from outside to inside was determined with a scanning Auger microsonde. The results were compared with those of carriers that have an artificially vapor-deposited iron oxide film of a defined thickness. The layer thickness could be determined to be around 0.3 ± 0.1 µm. Weak X-ray lines indicate that the oxidic surface has a spinel structure.

In the carriers of the invention, the surface layer consists of intergrown, predominantly platelet-shaped oxidation products of the iron surface, the platelets on average having particle sizes of 0.05-0.1 μm and platelet thicknesses of approximately 10-50 nm. The platelets are only fused together over the edges, so that individual particles can break out under mechanical stress.

The developer of toner and carrier according to the invention can, so to speak, be described as a three-component system of toner, carrier and abrasion. Through the special coating technology according to the invention it is possible to produce an oxidic surface layer that emits abrasive iron oxide particles in small amounts during the copying process.

The iron oxide particles of 0.05 - 0.1 µm released from the carrier surface initially remain on the carrier surface as abrasion due to the large adhesive forces. There they can combine with the toner abrasion and thus facilitate its detachment from the carrier surface.

The new carrier is manufactured by subjecting the uncoated steel carrier to a special treatment with aqueous sulfuric acid, oxidizing it and finally drying it. For this, 0.05-0.25 mmol of the acid is used per m² surface of the steel carrier, the acid concentration at the beginning of the treatment being 1 × 10 -2 to 1 × 10 -4 mol / l, i.e. the pH must not fall below a value of 2. A mode of operation with an initial pH value of 3.5-4.5 is particularly advantageous. It was found that 5 10⁻⁵ to 2.5 10⁻⁴ mol of sulfuric acid are required per m² of carrier surface in order to produce an optimal layer thickness of the surface covering. When treated with smaller amounts of acid, only a slight effect on the electrostatic charge distribution is observed compared to the uncoated material.

Too large amounts of acid lead to products that are not stable in storage, i.e. H. the coating is too brittle and the carriers may corrode.

Sulfuric acid is preferred because sulfate ions do not reduce the storage stability of the steel balls. The use of other mineral acids is possible, but leads e.g. B. in the case of hydrochloric acid to corrosion problems. When using dilute nitric acid solution, the iron (II) ions formed are oxidized in an uncontrolled manner.

The sulfuric acid treatment and the partial oxidation of the Fe (II) ions can be carried out either sequentially or simultaneously. The partial oxidation can e.g. B. with oxygen-saturated water or acid solution or by adding alkali metal permanganate in a normality of 5 10⁻⁵ to 5 10⁻⁴ mol per m² surface. However, the oxidation can also be carried out with other oxidizing agents such as hydrogen peroxide, ammonium peroxodisulfate.

Acid treatment and oxidation are preferably carried out simultaneously, in particular with oxygen-saturated sulfuric acid or sulfuric acid containing permanganate. However, the oxidation of the iron (II) hydroxide formed can also be carried out after the sulfuric acid treatment with oxygen-containing gases, preferably with air.

The amount of oxidizing agent is 5 · 10⁻⁵ to 5 · 10⁻⁴ oxidation equivalents per m² steel carrier surface. The carrier, which is coated with an oxide layer, is dried at temperatures of 60 - 150 ° C and pressures ≦ 100 mbar. If the product is dried at around 70 ° C, the product changes its own color after a few days. However, the effect remains (see example 3). Carriers that have been dried at temperatures above 100 ° C. are preferred. Due to the extremely low sulfuric acid concentration, the process is very environmentally friendly.

As raw material (steel carrier) z. B. steel balls from Metallurgica Toniolo S.p.A., Maerne, Italy, which are commercially available under the name TC 100. The latter consist of 98.5% Fe, 0.4% Mn, 0.4% Si and Ni, Cr, Cu with 0.1% each and traces of Co, Zn, Mg and Ca. But raw carriers with irregular particle shapes can also be used. Steel carriers which have been produced by spray atomization are particularly preferred.

The studies on carriers with perfect functionality show that a carrier always delivers a good print image and can be regarded as fully functional if the electrostatic chargeability of the toner particles in the developer has a narrow distribution (q / d). The electrostatic chargeability distribution is determined using a q / d meter (Epping GmbH, Neufahrn). The measurement method uses the different deposition rates of toner particles with different q / d values (q: charge of a toner particle, d: diameter of a toner particle) on an electrode in the electric field. Regardless of this, the toner concentration in the developer must not change; H. the toner particles on the carrier should largely remain the same over their service life; apart from small deviations, they should neither accumulate nor accumulate.

The stress test (life test) for the full functionality of the carrier was carried out in a practical way in a laser printer ND2 (Siemens AG, Müchen). This printer consumed an average of 350 g / h of toner with 8 kg of developer. The specific toner consumption was 43.8 g toner / kg developer / h.

With 3 million prints, the carrier in the developer had an operating time of around 600 hours. During this time, approximately 210 kg of toner are used, ie 26.3 kg of toner per kg of developer.

Even after an operating time of approximately 1200 hours of the carrier according to the invention contained in the developer, there were no impairments in the printed image, i. H. after over 6 million prints, the carrier according to the invention was still fully effective.

In comparison, a carrier produced according to US-A-3,632,512, example 1, showed significant fatigue after only three million prints, which manifested itself in a noticeable deterioration in the printed image and a disproportionate accumulation of toner in the developer. If one determines the q / d distribution of the toner particles present in this exhausted developer, then compared to the toner in the still fully functional developer with the carrier according to the invention, after 3 million prints, a significantly broader charge distribution is found.

In order to underpin the concept of the renewing carrier surface and the important role of abrasion, uncoated steel carriers were mixed with finely divided, mostly amorphous iron oxide and then a developer was created.

In the initial phase (approx. 6 hours) this developer worked excellently in the laser printer. The print image turned out to be completely flawless and according to the above test the toner particles in the developer showed a narrow charge distribution (q / d measurement). In the course, however, and quite obviously after the discharge of the artificially added iron oxide abrasion, the developer soon showed exhaustion. The printed image deteriorated and the toner particles in the developer had a wide charge distribution. The analytical examination of the iron content in the toner was able to show that even wear over the entire life of the developer is released with the carriers according to the invention.

The invention is further illustrated by the following examples.

example 1

1000 g of steel powder (TC 100 steel powder, Toniolo, Maerne, Italy) with a particle size distribution of 75-175 µm, a particle size of 105 µm are placed in a 1000 ml stirred kettle with a pH electrode, blade stirrer, sieve plate, inlet and outlet (Weight average) and a surface area of 36 cm² / g. In a receptacle 4 are a sulfuric acid solution, which has a pH of 4 by introducing an air flow of 100 l / h saturated with air (0.0205 vol .-% O₂ in water at 15 ° C). The solution is then pumped through the steel powder filling at a volume flow of 20 l / h. The running solution is in the template returned to the vessel, the pH value in the supply vessel and in the reactor being measured continuously. Air is blown into the reservoir with a volume flow of 100 l / h. After about 20 minutes, the pH in the storage vessel rose to 8 and no longer differs from the pH in the reaction vessel.

The slightly yellow colored solution is drained from the reactor. The reactor vessel is then heated to 135 ° C. with 4 bar steam while the vacuum pump is running and dried for 4 hours at a pressure of 55 mbar. Then the very free-flowing, slightly yellowish-colored steel powder is discharged from the reactor and can be used to prepare the developer.

Comparative Example 1

According to the information in US Pat. No. 3,632,512, Example 1, a carrier was produced from the steel balls used in Example 1. For this purpose, the steel balls were treated with 2N sulfuric acid, then washed with water and methanol as described in the example and then dried under air at 80 ° C. using an IR radiator.

Example 2

1000 g of steel powder (TC 100 steel powder, Toniolo, Maerne, Italy) with a particle size distribution of 75-175 µm and an average particle size of 105 µm are placed in a 1000 ml stirred kettle with pH electrode, blade stirrer, sieve plate, inlet and outlet (Weight average) and a surface area of 36 cm² / g. Then 4 l of a sulfuric acid solution, which has a pH of 4 and in which 1.3 10⁻⁵ mol / l of potassium permanganate are dissolved, are pumped through the steel powder filling with a volume flow of 20 l / h while stirring. The running solution is returned to the storage vessel and the pH value in the storage vessel and in the reactor is constantly measured. After about 15 minutes the pH in the storage vessel has risen to 8 and no longer differs from the pH in the reaction vessel.

The slightly brown colored solution is drained from the reactor. Then the reactor with the steel balls remaining therein is evacuated (55 mbar) and the reactor vessel is heated to 120 ° C. while the vacuum pump is running and the product is dried for 4 hours. Then the very free-flowing, slightly yellowish-colored steel powder is discharged from the reactor and can be used directly to prepare the developer.

Example 3

1000 g of steel powder (TC 100 steel powder, Toniolo company, Maerne, Italy) with a particle size distribution of 75 - 175 µm and a particle size of 105 µm (in a 1000 ml stirred kettle with pH electrode, blade stirrer, sieve plate, inlet and outlet Weight average) and a surface area of 36 cm² / g. 4 l of a sulfuric acid solution which has a pH of 3 are provided in a receptacle. The solution is then pumped through the steel powder filling at a volume flow of 20 l / h. The running solution is returned to the storage vessel, the pH value in the storage vessel and in the reactor being continuously measured. After about 17 minutes, the pH in the storage vessel rose to 8 and no longer differs from the pH in the reaction vessel.

The slightly yellow colored solution is drained from the reactor. The reactor vessel is then evacuated. Thereafter, with the vacuum pump running, within 5 min. 100 ml of air passed through the damp iron powder filling. The air supply is then stopped and the moist carriers are discharged from the reaction vessel under a nitrogen blanket. One third of the moist carrier was dried at 70, 100 and 130 ° C in an evacuable drying cabinet for 4 hours.

The cooled samples were stored in a climatic chamber at 85% relative humidity at 25 ° C for 1 week.

The color of the carrier dried at 70 ° C changed from yellow to rust red. However, the electrostatic charge corresponds to that of carriers dried at 130 ° C. The samples dried at 110 and 130 ° C show no change in color and the electrostatic charge distribution corresponds to that of Example 2.

Developer 1

The developer is obtained by precisely weighing 988 g (98.8% by weight) of the carrier produced according to Example 1 and 12 g (1.2% by weight) of original toner for Siemens laser printers ND2 / ND3 (Siemens AG, Munich) and subsequent activation. For this, the mixture is placed in a 500 ml glass bottle on a roller stand at 60 rpm for 5 minutes. emotional.

Developer 2

For comparison, a developer was produced from 98.8% by weight of the steel carrier obtained according to Comparative Example 1 and 1.2% by weight of toner for Siemens laser printer ND2 / ND3 and activated as for Developer 1.

Developer 3

For comparison, a developer is made from 98.8% by weight of uncoated steel balls (TC 100) and 1.2% by weight of toner for Siemens laser printers ND2 / ND3. Activation is the same as for Developer 1.

Developer 4

Uncoated carrier (TC 100) 0.005% by weight of a finely divided iron oxide (Sicotransorange L 2515, BASF AG, Ludwigshafen) is mixed and shaken in the Red Devil for 15 minutes. A developer is then produced by mixing 98.8% by weight of the carrier thus prepared and 1.2% by weight of toner for Siemens laser printers ND2 / ND3.

Determination of the electrostatic chargeability q / m

The activated developer (charge separation by triboelectricity) is weighed exactly and poured into a measuring cell, which is sealed with sieve inserts at the top and bottom.

The mesh size of 50 µm is selected so that all toner particles can pass through, but the carrier (75 - 175 µm) remains completely inside the measuring cell, the measuring cylinder is set up in isolation and coupled to an electrometer (q / m meter, Epping GmbH, Neufahrn). With a powerful air flow of approx. 4000 cm³ / min. and simultaneous suction, the toner, which adheres electrostatically to the carrier, is completely removed from the carrier particles and blown out of the cell. The charge can be read on the electrometer. The amount of the charge with the opposite sign then corresponds to the charge of the blown out toner, the mass of which is determined by weighing the measuring cell back. In the printer, the magnetic brush development is activated by the toner particles sliding along the carrier chains. The degree of charge separation depends on the materials used and the duration and intensity of the activation. A developer can be destroyed by very strong shaking movements, since either the coatings are rubbed off or the toner sticks to the surface of the carrier.

Trial 1 Determination of q / m

600 g of Developer 1 are filled in an LD tester (Epping, GmbH, Neufahrn near Munich). Toner for Siemens ND2 / ND3 laser printers is filled into the reservoir. The speed of the magnetic brush is 15 cm / sec. The distance to the photoconductor is 2.0 mm. The speed of the semiconductor drum is 7 cm / sec. and the potential between the semiconductor and the developer roller 300 V. The amount of toner transferred is sucked off on the other side of the photoconductor. After a few minutes, the development process is interrupted and a sample of developer 1 is taken. A q / m measurement is carried out. The q / m value is 15.5 ± 1.0 µC / g (Table 1).

As in the other tests, the deviation was determined by the arithmetic mean of 10 tests.

The q / m values of the comparison developers 2, 3 and 4 were determined in the same way. The measured values are summarized in Table 1.

Results:

The mean, electrostatic chargeability of developers 1, 2 and 4 are the same within the range of errors. The Developer 3 with an uncoated carrier has a very high charge compared to the other developers.

Trial 2

20 g of Developer 1 are placed in a 50 ml glass bottle on a trolley at 60 rpm. Activated for 10 minutes. Then a q / d measurement (q / d meter, Epping GmbH, Neufahrn) is carried out. The mean q / d value is 6.9 ± 3.6 fC / 10 µm, the standard deviation 4.0 ± 0.5. In the same way, 2, 3 and 4 q / d measurements were carried out on the developers. The results are summarized in Table 1.

Trial 3 Determining the amount of toner in Developer 1 under operating conditions

8000 g of Developer 1 were filled into a laser printer ND2 (Siemens AG, Munich) and operated under normal operating conditions. The blackness and quality of the printed image was checked.

After every 500,000 prints, the iron content of the toner transferred to paper was analyzed. A sample of the developer was drawn after 3 million prints and the total toner concentration determined. It was 1.8%. After 6 million prints, the developer was removed from the device and the total toner concentration determined. It was 3.6%.

Trial 4 Determining the amount of toner in Developer 2 under operating conditions

Analogous to experiment 3, 8000 g of developer 2 were filled into a laser printer ND2 and the device was operated as in experiment 3. After every 500,000 prints, toner samples were taken and the iron content determined. After three million prints, the total toner concentration was already 5.6%.

The attempt was then stopped. The results are summarized in Table 2.

Trial 5 Determining the amount of toner in Developer 3 under operating conditions

Analogous to experiment 3, developer 3 was tested in a laser printer. The experiment was terminated after only a few thousand prints due to an insufficient print image. The results are summarized in Table 2.

Trial 6 Determining the amount of toner in Developer 4 under operating conditions

Analogous to experiment 3, developer 4 was tested in a laser printer. The developer supplied more than 5000 prints, clean, flawless prints. After that, the printed image deteriorates drastically, so that the experiment was stopped. The results are summarized in Table 2. Table 1 Electrostatic chargeability of the developers q / m in µC / g middle q / d in fC / 10 µm Standard deviation Developer 1 (according to the invention) 15.5 6.9 3.6 Developer 2 (carrier according to US PS) 16.0 7.2 4.0 Developer 3 (uncoated steel carrier) 36.5 13.9 5.9 Developer 4 (steel carrier coated with finely divided iron oxide) 14 7.0 3.4 Results of the printing tests Developer 1 Developer 2 Developer 3 Developer 4 Image quality + blackness through densitometric measurement of a reference pattern after 1000 prints a good 0.48 a good 0.47 too strong 0.53 a good 0.48 after 10,000 prints normal 0.53 normal 0.51 insufficient insufficient after 100,000 prints normal 0.52 normal 0.55 - - after that normal 0.53 normal 0.52 - - Iron content in the toner after 1000 prints 1.0 ppm 0.5 ppm - 1 ppm after 500,000 prints 1.06 ppm <0.1 ppm - - 1 million prints 0.8 ppm <0.1 ppm - - 1.5 million prints 1.5 ppm <0.1 ppm - - 2 million prints 0.9 ppm <0.1 ppm - - 2.5 million prints 0.7 ppm <0.1 ppm - - 3 million prints 0.9 ppm <0.1 ppm - - 3.5 million prints 1.0 ppm <0.1 ppm - - 4.0 million prints 0.7 ppm <0.1 ppm - - 4.5 million prints 0.6 ppm <0.1 ppm - - 5 million prints 0.5 ppm <0.1 ppm - - 5.5 million prints 0.7 ppm <0.1 ppm - - 6 million prints 0.8 ppm <0.1 ppm - - Total toner concentration begin 1.2 1.2 1.2 1.2 after 3 million prints 1.8 5.6 - - after 6 million prints 3.6 - - -

Claims (6)

1. Carrier, which have an iron oxide-containing surface coating of the formula (Fe0) _x · Fe₂0₃ (x = 0, 1 - 1) on steel cores, obtainable by treating the steel cores (spheres) with aqueous sulfuric acid, with 5 · 10⁻ per m² spherical surface ⁵ to 2.5 · 10⁻⁴ mol of sulfuric acid are used and the acid concentration at the start of the treatment is 10⁻² to 10⁻⁶ mol / l, oxidation of the balls treated with sulfuric acid with oxygen or an oxidizing agent in an amount that 5 · 10⁻⁵ - 5 · 10⁻⁴ oxidation equivalent / m² spherical surface and drying the spheres at 60 to 150 ° C under a pressure of ≦ 100 mbar. 2. A process for the preparation of the Garrier according to claim 1 by treating steel balls with aqueous sulfuric acid and drying the treated balls, characterized in that the steel balls per m² ball surface are treated with 5 · 10 · to 2.5 · 10⁻⁴ mol of sulfuric acid , at the beginning of the treatment the concentration is 10⁻² to 10⁻⁶ mol / l, the balls treated with sulfuric acid with oxygen or an oxidizing agent in an amount corresponding to 5 · 10⁻⁵ to 5 · 10⁻⁴ oxidation equivalents , partially oxidized and the oxidized balls dried at 60 to 150 ° C under a pressure of ≦ 100 mbar. 3. The method according to claim 2, characterized in that the treatment with the sulfuric acid and the oxidation take place simultaneously. 4. The method according to claim 2 or 3, characterized in that the oxidation is carried out with atmospheric oxygen or with alkali metal permanganate. 5. The method according to claim 2, 3 or 4, characterized in that steel cores (balls) are treated, which have been produced by the spray atomization method. 6. Use of the carrier according to claim 1 in developers for laser printers.