CA2141486A1 - Tin dioxide-coated carriers for electrophotography - Google Patents

Abstract

Carriers for electrophotography, based on tin dioxide-coated magnetic cores, are obtainable by oxidative decomposition of organotin compounds in the gas phase in the presence of agitated cores.

Description

Tin dioxide-coated carriers for electrophotography The present invention relates to novel carriers for electrophoto-5 graphy, based on tin dioxide-coated magnetic cores, obtainable by oxidative decomposition of organotin compounds in the gas phase in the presence of agitated cores.

The present invention furthermore relates to the preparation of 10 these carriers and their use for the preparation of electrophoto-graphic two-component developers, and electrophotographic two-component developers which contain these carriers.

Two-component developers are used in electrophotographic copiers 15 and laser printers for developing an electrophotographically pro-duced, latent image and usually consist of carrier particles and toner particles. The carrier particles are magnetizable particles having sizes of, as a rule, from 20 to 1000 ~m. The toner par-ticles consist essentially of a color-imparting component and 20 binder and have a size of from about 5 to 30 ~m.

In the copying process, the electrostatic, latent image is pro-duced by selective exposure of an electrostatically charged photoconductor drum to light reflected from the original. In the 25 laser printer, this is effected by a laser beam.

For the development of the electrostatic image, toner particles are transported to the photoconductor drum by means of a magnetic brush, ie. carrier particles oriented along the field lines of a 30 sector magnet. The toner particles adhere electrostatically to the carrier particles and acquire an electrostatic charge opposite to that of the carrier particles as a result of friction during transport in the magnetic field. The toner particles thus transferred from the magnetic brush to the photoconductor drum 35 give a toner image, which is then transferred to electro-statically charged paper and fixed.
The carrier particles used have to meet a number of requirements:
they should be magnetizable and thus permit rapid build-up of the 40 magnetic brush. Furthermore, their surface should have low con-ductivity in order to prevent a short-circuit between the sector magnet and the photoconductor drum. This conductivity should remain constant over long operating times of the carrier, in order also to keep the triboelectric charging of the developer 45 constant over a long period. Not least, the carrier particles ~ 2141486 `_ should also be free-flowing and should not agglomerate in the developer reservoir.

In order to meet these requirements, the carrier particles 5 consisting of magnetic material must as a rule be coated.

Thus, DE-A-41 40 900 discloses, inter alia, tin dioxide-coated carriers which are of particular interest owing to their ability to impart highly positive charge to toners. They even permit 10 positive charging of polyester resin toners which are par-ticularly suitable for high copying speeds owing to their good fixing properties but can usually be only negatively charged.

However, the carriers described in DE-A-41 40 900 and coated with 15 tin dioxide by hydrolytic decomposition of tin tetrachloride have the following disadvantages. Particularly in the case of steel carriers, a poor shelf life is observed owing to rusting of the carrier cores, caused by the water vapor present during the production and the hydrogen chloride formed. In the case of 20 ferrite carriers, too, the attack by hydrogen chloride results in a change in the carrier surface with formation of iron chlorides, which change may reduce the adhesive strength of the metal oxide coating.

25 It is an object of the present invention to provide tin dioxide-coated carriers which do not have the stated deficiencies.

We have found that this object is achieved by carriers for 30 electrophotography, based on tin dioxide-coated magnetic cores, which are obtainable by oxidative decomposition of organotin compounds in the gas phase in the presence of agitated cores.

We have also found a process for the preparation of these 35 carriers, wherein organotin compounds are decomposed in the gas phase by reaction with an oxygen-containing gas in the presence of agitated cores.

We have furthermore found the use of these carriers for the 40 preparation of electrophotographic two-component developers, and electrophotographic two-component developers which contain the carriers.

The cores of the novel carriers may consist of the conventional 45 magnetically soft materials, such as iron, steel, magnetite, ferrites (for example nickel/zinc, manganese/zinc and barium/zinc ferrites), cobalt and nickel or of magnetically hard materials, such as BaFe12O1g or SrFe12O1g, and may be in the form of spherical or irregularly shaped particles or in sponge form. Composite carriers, ie. particles of these metals or metal compounds embedded in polymer resin, are also suitable.

The novel tin dioxide-coated carriers are advantageously obtain-able by the novel preparation process by decomposition of organotin compounds by reaction with an oxidizing gas in the presence of agitated carrier particles.
Suitable organotin compounds are in particular those compounds which can be vaporized essentially without decomposition under inert conditions and can be decomposed oxidatively, ie. by reaction with oxygen or air or other oxygen/inert gas mixtures, 15 to give tin dioxide.

Compounds of the formula SnR4, where the radicals R are identical or different and are each alkyl, alkenyl or aryl, for example tetraalkyltins, tetraalkenyltins or tetraaryltins or mixed aryl-20 alkyltins or alkylalkenyltins, are particularly suitable.

The number of carbon atoms in the alkyl, alkenyl and arylradicals is in principle unimportant, but compounds which have a sufficiently high vapor pressure at up to about 200C are 25 preferred, in order to ensure easy vaporization.

Accordingly, in the case of tin organyls having 4 identical radicals R, in particular C1-C6-alkyl, especially C1-Cg-alkyl, and C2-C6-alkenyl, especially allyl and phenyl, are preferred.
Finally, dinuclear and polynuclear tin organyls which may be bridged, for example by oxygen atoms, can also be used.

Examples of suitable organotin compounds are diallyldibutyltin, 35 tetraamyltin, tetra-n-propyltin, bis(tri-n-butyltin~ oxide and especially tetra-n-butyltin and tetramethyltin.
The process for the novel preparation of the tin dioxide-coated carriers is advantageously as follows:
The tin organyls are transferred with the aid of an inert carrier gas, such as nitrogen or argon, from an evaporator vessel kept at from 20C to the boiling point of the particular tin organyl via a nozzle into the heated reactor, in which a fluidized bed or an 45 agitated fixed bed of the carrier cores is present. The oxygen-containing gas is fed in via a separate inlet line and decomposes the organotin compound to give tin dioxide, which is deposited directly on the surface of the carrier particles.

Suitable reactors are stationary or rotating pipes or agitated 5 mixing apparatuses. The agitation of the carrier cores may be effected by fluidization with a gas stream, by free-fall mixing, by the action of gravitational force or the aid of stirring elements in the reactor.

10 The decomposition temperatures are as a rule from 200 to 1000C, preferably from 300 to 500C.

The temperature and also the amount of oxygen are advantageously chosen so that the oxidation of the organic radicals to carbon 15 dioxide and water is complete and no carbon is incorporated in the tin dioxide layer. If in fact the amount of oxygen passed in is smaller than the stoichiometrically required amount, depending on the chosen temperature, either the tin organyl is only partially decomposed and then condenses in the exit gas region or 20 formation of carbon black and other decomposition products takes place.

Furthermore, the evaporator gas stream containing the tin organyl should advantageously be adjusted so that the gaseous tin organyl 25 accounts for not 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. An advantageous tin organyl concentration in the carrier gas stream itself is usually ~ 5% by volume.
The novel process can be used for applying in a specific manner both very thin and very thick tin dioxide layers to the carrier cores. Usually, the layer thicknesses typical for conventional applications are from 1 to 500 nm.
The novel carriers have homogeneous, abrasion-resistant tin dioxide coatings and the desired low surface conductivity. More-over, the carriers and the developers produced from them have a virtually unlimited shelf life.

Examples A. Preparation of novel tin dioxide-coated carriers Example 1 The coating of 1800 g of a spherical steel carrier having a mean particle size of from 75 to 180 ~m (type TC 100 from Pometon S.p.A., Maerne, Italy) with tin dioxide was carried out in a 500 ml quartz flask, which was connected to the shaft of a rotary evaporator drive, was rotated for thorough mixing of the carrier and was present in a hinge-type electric oven for heating. A thermostatable metal nozzle which contained two separate gas feeds for air and tin organyl-laden nitrogen passed through the shaft and the flask neck into the carrier bed.

8.7 g (5.9 ml) of tetrabutyltin were transferred, with the aid of a nitrogen stream of 50 l/h from the upstream evaporator vessel heated to 180C, in the course of 2 hours, via a feedline thermostatted at 185C and the metal nozzle which was likewise heated, into the reactor heated at 400C.

Heating of the carrier bed was carried out while passing in nitrogen. After the desired carrier and evaporator temperature had been reached, the tetrabutyltin was introduced into the evaporator and the second gas stream was changed to 50 l/h of air.

The coated carrier was then cooled while passing in nitrogen and was discharged.

The tin content of the carrier was determined as 0.12% by weight by means of atomic absorption spectroscopy.

Example 2 The coating of 3. 5 kg of a sponge-like carrier having a particle size of from 40 to 120 ~m (type XCS 40-120 NOD from Hoganas, Sweden) was carried out in an electrically heated, vertical quartz glass tubular reactor (internal diameter 60 mm, length 80 cm) having a lower end conically tapering to an internal diameter of 10 mm and capable of being closed by a ball valve. The carrier trickled out of the reactor through the lower orifice and was transported pneumatically by means of a nitrogen stream of 900 l/h through a thermostatable glass tube (internal diameter 15 mm) into the top of the reactor again.

A thermostatable metal nozzle having two separate gas feed-lines for air and tin organyl-laden nitrogen from the evapo-rator was immersed in the carrier bed in the middle of the reactor.

After the carrier had been heated to 350C while passing in nitrogen and the evaporator and the gas feedline and the nozzle had been heated to 150C, 59.2 g (40 ml) of tetrabutyl-tin were introduced into the evaporator vessel and trans-ferred into the reactor by means of a nitrogen stream of 100 l/h in the course of 5 hours. At the same time, 100 l/h of air were passed into the reactor.

After subsequent cooling under nitrogen, a coated carrier having a tin content of 0.31% by weight (AAS) was obtained.

20 B. Preparation of developers and testing For the preparation of the developers, the carriers thus coated were mixed with a polyester resin toner suitable for commercial laser printers (crosslinked fumaric acid/propoxy-lated bisphenol A resin having a mean particle size of 11 ~mand a particle size distribution of from 6 to 17 ~m), in each case in a weight ratio of ~7:3, and the mixture was activated by thorough mixing in a 30 ml glass vessel for 10 min in a tumbler mixer at 200 rpm.
In order to determine the electrostatic chargeability Q/m [~C/g], 2.5 g of each developer were weighed into a hard blow-off cell (Q/M meter from PES-Laboratorium, Dr. R.
Epping, Neufahrn) which was coupled to an electrometer and in which screens of mesh size 32 ~m had been inserted. By blow-ing off with a vigorous airstream (about 3000 cm3/min) and simultaneous air extraction, the toner powder was virtually completely removed while the carrier particles were kept back in the measuring cell by the screens.
Thereafter, the voltage generated by the charge separation was read from the electrometer and was used to determine the charge build-up on the carrier (Q=CU, C=lnF), which corresponds to the charge build-up on the toner with the opposite sign, and by reweighing the measuring cell, said charge build-up on the carrier was related to the weight of 21~1486 the blown-off toner and its electrostatic charge Q/m [~C/g]
was thus determined.

The results obtained in the measurements are summarized in the Table below, the measured values (V1 and V2) obtained in each case using uncoated carriers also being stated for comparison.

Table ExampleElectrostatic charge Q/M [~C/g]
1 - 0.6 V1 + 5.9 2 - 11.4 V2 + 15.2

Claims (5)

1. A carrier for electrophotography, based on tin dioxide-coated magnetic cores, obtainable by oxidative decomposition of organotin compounds in the gas phase in the presence of agitated cores.

2. A carrier as claimed in claim 1, obtainable by oxidative decomposition of an alkyltin or an aryltin.

3. A process for the preparation of carriers as claimed in claim 1, wherein organotin compounds are decomposed in the gas phase by reaction with an oxygen-containing gas in the presence of agitated cores.

4. A method for the preparation of electrophotographic two-component developers, which comprises using a carrier as claimed in claim 1.

5. An electrophotographic two-component developer containing the carrier as claimed in claim 1.