Note: Descriptions are shown in the official language in which they were submitted.
2141486
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
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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,
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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
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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.
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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
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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
