Carbon black, process for the production thereof, as well-as
Carbon blacks are known from Ullroanns Enzyklopädie der
technischen Chemie, 4th Edition (1977), Vol. 14, pp. 633 to
648.
The most important processes for producing carbon blacks are
cased on the oxidative pyrolysis of carbon-containing carbon
black raw materials. In these processes the carbon black raw
materials are incompletely burnt at high temperatures in the
presence of oxygen. These carbon black production processes
include for example the furnace carbon black process, the gas
carbon black process and the flame carbon black process.
Predominantly polynuclear aromatic carbon black oils are used
as carbon black raw materials.
Carbon blacks are used as fillers and as reinforcing agents
in the preparation of rubber mixtures for the tyre industry.
Typical rubber mixtures include, in addition to natural
and/or synthetic rubber, also carbon black, mineral oil and
further auxiliaries as well as sulfur as vulcanisation agent.
Carbon blacks influence the abrasion resistance, rolling
resistance as well as the wet skidding behaviour of tyres
produced from these rubber mixtures. For rubber mixtures
that serve as tyre tread, so-called tread mixtures, a high
abrasion resistance with at the same time as low a rolling
resistance as possible combined with a good wet skidding
behaviour are required. A low rolling resistance leads to a
low fuel consumption of the vehicle.
Rolling resistance and wet skidding properties are influenced
by the viscoelascic behaviour of the tread mixture. With
periodic deformation the viscoelastic behaviour can be
described by the mechanical loss factor tand and, in the case
of stretching or compression, by the dynamic modulus of
elasticity |E*|. Both quantities are strongly temperature-
dependent.
The wet skidding behaviour of the tread mixture is correlated
with the loss factor tand0 at 0°C, while the rolling
resistance is correlated with the loss factor tand60 at 60°C.
The higher the loss factor at the low temperature, the better
usually is the wet skidding behaviour of the tyre mixture.
In order to reduce the rolling resistance as small a loss
factor as possible at the high temperature is required
however.
The abrasion resistance and the viscoelastic properties, and
thus also the loss factor of the tread mixtures, are
essentially determined by the properties of the reinforcing
carbon blacks that are used.
An important index for the rubber-active surface proportion
of the carbon black is the specific surface, in particular
the CTA surface on STSA surface. With increasing CTAB
surface or STSA surface both the abrasion resistance and tand
increase.
Further important carbon black parameters are the DBP
absorption as a quantitative measure of the initial
structure, and the 24M4-DBP absorption as a measure of the
residual structure still remaining after the carbon black has
been subjected to mechanical stress.
For tread mixtures carbon blacks are suitable that have CTAB
surfaces between 80 and 180 m2/g and 24M4-DBP absorption
values between 80 and 140 ml/100 g.
It is known that ASTM carbon blacks are unable to influence
the temperature dependence of the loss factor tanS in such a
way that the tread mixture has a lower rolling resistance
with the same or better wet skidding behaviour. As is known,
the desired reduction of the rolling resistance is directly
coupled to a deterioration of the wet skidding behaviour.
Carbon blacks that have a low rolling resistance are termed
so-called "low hysteresis" carbon blacks.
It is furthermore known that the rolling resistance of tyres
can be reduced by replacing the carbon black by silica
In order to bind the silica to the
polymer building blocks of the rubber, silane coupling
reactants are used. Silica-containing rubber mixtures have a
loss factor tand60 that is reduced by up to 50%.
The object of the present invention is to provide carbon
blacks that impart to rubber mixtures of natural rubber or
synthetic rubber or mixtures thereof a reduced rolling
resistance with at the same time the same r an improved wet
skidding behaviour and abrasion resistance.
The present invention provides a carbon black that has a STSA
surface of between 20 and 180 m2/g, a 24M4-DBP absorption of
between 40 and 140 ml/100 g, a specific BET surface of
between 20 and 250 m2/g and a content: of 0.01 to 20 wt. % cf
silicon, referred to its overall weight, which is
characterised in that in rubber mixtures it has a tand0/tand60
ratio of greater than 3.37 - 0.0068 . STSA.
In one embodiment of the invention, the carbon black can also
contain 0.01 to 1 wt.% nitrogen in addition—to silicon.
The silicon is incorporated into the carbon black aggregates
during the production process. For this purpose silicon-
containing compounds may for example be mixed into the carbon .
black raw material. Suitable silicon-containing compounds
may be organosilicon compounds such as organosilanes,
organochlorosilanes, siloxanes and silazanes. In particular
silicone oils, silicon tetrachloride, siloxanes and silasanes
may be used. Silanes and silicone oils may preferably be
used.
The starting compound has only a slight influence on the
incorporation of the silicon atoms into the carbon black
aggregates. It can be shown by X-ray photoelectxon
spectrometry (XPS) and secondary ion mass spectrometry (SIMS)
that the silicon atoms are oxidically bound and distributed
in the carbon black aggregates. The oxidic bonding consists
predominantly of silicon dioxide. Other silicon atoms form
silanol groups. Whereas the silanol groups are mainly
located on the surface of the carbon black aggregates,
silicon dioxide is distributed uniformly over the cross-
section of the aggregates.
In one embodiment of the invention the silicon may be
concentrated in the sub-surface regions of the carbon black
aggregates.
The silicon-containing groups on the surface of the carbon
black aggregates influence, after incorporation into rubber
mixtures the interaction of the filler with the rubber
polymer components. To effect a covalent bonding of the
silanol groups of the carbon blacks to the mixture polymers
bifunctional silanes, for example Si69 (Bis(3-
triethoxysilylpropyl)-tetrasulfane) from Degussa, may be
added as silane coupling reagent to the rubber mixtures.
The tread mixtures produced with the silicon-containing
carbon blacks according to the invention exhibit an increased
value of tand0 and a reduced value of tand60 compared to known
carbon blacks having the same specific surface and structure,
without the need to add a coupling reagent. These values
correspond to a substantially improved wet skidding behaviour
combined with a substantially reduced rolling resistance of
the tread. The rolling resistance of the rubber mixtures can
be improved still more, i.e. reduced further, by adding
bifunctional silanes.
The carbon blacks according to the invention may be produced
by the furnace carbon black process according to
DE 195 21 565 A1.
According to the furnace carbon black process the oxidative
pyrolysis of the carbon black raw material is carried out in
a reactor lined with highly refractory material. In such a
reactor three zones, lying one after the other along the
reactor axis and through which the reaction media flow in
succession, may be distinguished.
The first zone, the so-called combustion zone, essentially
comprises the combustion chamber of the reactor. A hot
process gas is produced in this zone by burning a fuel, as a
rule hydrocarbons, with an excess of preheated combustion air
or other oxygen-containing gases. Natural gas may be used as
fuel. Liquid hydrocarbons such as light and heavy heating
oil may also be used.
In a preferred embodiment of the invention carbon black raw
material (carbon black oil) may also be used as fuel.
The combustion of the fuel normally takes place with an
excess of oxygen. The excess air promotes the complete
conversion of the fuel and serves to control the quality of
the carbon black. The fuel is normally introduced by means
of one or more burner lances into the combustion chamber.
The formation of the carbon black takes place in the second
zone of the carbon black reactor, the so-called reaction zone
or pyrolysis zone. To this end the carbon black raw
material, in general a so-called carbon black oil, is
injected into and mixed in with the stream of hot process
gas. The amount of hydrocarbons introduced into the reaction
zone is in excess referred to the incompletely reacted amount
of oxygen in the combustion zone. For this reason the
formation of carbon black normally takes place here.
If the carbon black oil is also used as fuel, the formation
of carbon black may take place already in the combustion
zone. In the reaction zone further carbon black may then be
applied to the carbon black particles formed in the
combustion zone.
Carbon black oil may be injected in various ways into the
reactor. For example, an axial oil injection lance or one or
more radial oil lances, arranged on the circumference of the
reactor in a plane vertical to the flow direction, are
suitable. A reactor may contain several planes with radial
oil lances along the flow direction. Spray or injection
nozzles are arranged on the head of the oil lances, by means
of which the carbon black is mixed into the flow of process
gas.
With the simultaneous use of carbon black oil and gaseous
hydrocarbons, for example methane, as carbon black raw
material, the gaseous hydrocarbons may be injected separately
from the carbon black oil via their own set of gas lances
into the flow of the hot waste gas.
In the third zone of the carbon black reactor, the so-called
termination zone (quench zone), the carbon black formation is
terminated by rapid cooling of the carbon black-containing
process gas. In this way undesired after-reactions are
avoided. The reaction is normally terminated by spraying in
water through suitable spray nozzles. The carbon black
reactor generally includes several places along the reactor
for spraying in water, i.e. "quenching", so that the
residence time of the carbon black in the reaction zone may
be varied. In a heat exchanger connected downstream the
residual heat of the pressure gas is utilised to preheat the
combustion air and the carbon black oil.
Whereas the aim of the known furnace carbon black processes
is to achieve as complete a combustion as possible of the
fuel in the combustion chamber, or in the combustion zone,
the process according to the invention for producing carbon
black is based on the fact that carbon seeds are formed in
the combustion zone as a result of the incomplete combustion
of the fuel, which seeds are transported with the flow of hot
waste gas into the reaction zone, where they initiate a seed-
induced carbon black formation with the added carbon black
raw material. The sought-after incomplete combustion of the
fuel does not mean however that the fuel is burnt in a
deficit of oxygen. Rather, the process according to the
invention too employs an excess of air or oxygen-containing
gases in the combustion chamber. K factors of between 0.3
and 1.2 may be employed as with conventional carbon blacks.
The process is preferably operated however with K factors of
between 0.6 and 0.7.
Various methods may be adopted in order to produce carbon
black seeds despite the excess air. In a preferred variant
of the process according to the invention liquid hydrocarbons
are used as fuel, which are burnt instead of natural gas in
the combustion chamber of the reactor with an excess of air
or oxygen-containing gases. Liquid hydrocarbons burn more
slowly than gaseous hydrocarbons since they first have to be
converted into the gaseous form, i.e. have to be evaporated.
Despite the excess oxygen, in addition to the combustion
there may also be produced with liquid hydrocarbons carbon
seeds which, if sufficient time is available and the
temperature is sufficiently high, also continue to burn or,
if rapid cooling is effected, can grow into larger carbon
black particles. The seed-induced carbon black formation is
based on the fact that the seeds formed in the combustion of
liquid hydrocarbons with excess oxygen are brought into
contact directly with the carbon black oil and thus initiate
the seed growth.
Another variant of the process according to the invention
uses natural gas as fuel. A seed formation is achieved if
the outflow speed of the gas from the burner lance or lances
is chosen sufficiently low so as intentionally to achieve a
poor intermixing of the natural gas with the hot flow of the
combustion air. The formation of carbon black seeds with
poorly mixed flames is known, in which connection on account
of the glow of the formed particles one also speaks of
glowing flames. With this procedure it is likewise
important, as with the combustion of liquid hydrocarbons, to
bring the resultant seeds immediately after their formation
into contact with the cerbon black c I. If an attempt is
made by means of a larger combustion chamber or combustion
zone to react ne seeds with the oxygen present in excess in
the combustion zone so as to achieve a complete combustion in
the combustion zone of the carbon black reactor, then no
seed-induced formation of carbon black takes place.
The carbon blacks according to the invention may be produced
by mixing the aforedescribed silicon-containing compounds
into the carbon black raw materials or spraying them
separately into the combustion chamber or the pyrolysis zone,
of the carbon black reactor. The mixing of the silicon-
containing compounds into the carbon black oil may be
effected in the form of a solution if the compounds are
soluble in the carbon black oil, or in the form of an
emulsion. An incorporation of the silicon atoms into the
carbon black primary particles is achieved by means of these
measures. One or more of the oil lances normally employed
for spraying in the carbon black raw material may be used for
the separate spraying of the silicon-containing compounds
into the pyrolysis zone of the carbon black reactor.
The furnace carbon black process is modified for the
production of inversion carbon black. Whereas the object of
the conventional furnace carbon black processes is to achieve
as complete a combustion as possible of the fuel in the
combustion chamber or in the combustion zone, the process
according to DE 195 21 5j65_for producing inversion carbon
blacks is based on the fact that carbon seeds are formed by
incomplete combustion of the fuel in the combustion zone,
which seeds are transported with the flow of hot waste gas
into the reaction zone and there initiate a seed-induced
formation of carbon black with the added carbon black raw
material. The sought-after incomplete combustion of the fuel
does not mean however that the fuel is burnt in a deficit of
oxygen. Rather, the process according to the inven ion too
operates with an excess of air or oxygen-containing gases in
the combustion chamber. K factors of between 0.3 and 0.9 may
be employed as with conventional carbon black.
In order to produce carbon black seeds despite the excess
air, various measures may be adopted according to
DE 195 21 565. In a preferred variant of tne process liquid
hydrocarbons are used as fuel, which are burnt with an excess
of air or oxygen-containing gases instead of natural gas in
the combustion chamber of the reactor. Liquid hydrocarbons
burn more slowly than gaseous hydrocarbons since they first
have to be converted into the gaseous form, i.e. have to be
evaporated. Despite the excess oxygen, in addition to the
combustion there may thus also be produced with liquid
hydrocarbons carbon seeds which, if there is sufficient time
and the temperature is sufficiently high, will also continue
to burn, or if rapid cooling is effected can grow to form
larger carbon black particles. The seed-induced formation of
carbon black is based on the fact that the seeds formed in
the combustion of liquid hydrocarbons with excess oxygen are
brought into contact directly with the carbon black oil and
thus initiate the seed growth.
Another variant of the process according to DE 195 21 565
uses natural gas as fuel. A seed formation is achieved if
the outflow speed of the gas from the burner lance or lances
is chosen sufficiently low so as intentionally to achieve a
poor intermixing of the natural gas with the hot flow of the
combustion air. The formation of carbon black seeds with
poorly mixed flames is known, in which connection on account
of the glow of the formed particles one also speaks of
glowing flames. With this procedure it is likewise
important, as with the combustion of liquid hydrocarbons, to
bring the resultant seeds immediately after their formation
into contact with the carbon black oil. If an attempt is
made by means of a larger combustion chamber or combustion
zone to react the seeds with the oxygen present in excess in
the combustion zone so as to achieve a complete combustion in
the combustion zone of the carbon black reactor, then no
seed-induced formation of carbon black takes place.
The two aforedescribed variants may also be combined with one
another. In this case the liquid hydrocarbons and natural
gas or other gaseous fuels are added simultaneously in
suitable ratios to the combustion zone. Oils, for example
the carbon black oil itself, are preferably used as liquid
hydrocarbons.
The process according to DE 195 21 565 thus comprises using
in the combustion zone, in which the oxygen is present in
excess referred to the hydrocarbons that are used, liquid
and/or gaseous hydrocarbons as fuel and thereby ensuring that
carbon black seeds are formed for example by an insufficient
residence time of the liquid hydrocarbons or by an
insufficient intermixing of the gaseous hydrocarbons with the
combustion air, which carbon seeds immediately after their
formation are brought into contact in the reaction zone with
the carbon black material, which is used in excess referred
to the amount of oxygen, the resultant carbon black/reaction
gas mixture is cooled by spraying water into the termination
zone, and the carbon black that is thus formed is worked up
in the conventional way.
According to DE 195 21 565 the fuel contributes decisively to
the carbon black formation and is therefore hereinafter
termed primary carbon black raw material. The carbon black
raw material that is to be mixed into the reaction zone is
accordingly termed secondary carbon black raw material and
contributes most quantitatively to the carbon black that is
formed.
The inversion carbon blacks according to DE 195 21 565 impart
to carbon black mixtures a reduced rolling resistance and a
comparable wet adhesion compared to corresponding
conventional carbon blacks. Furthermore, it has been found
by AFM investigations (AFM = Atomic Force Microscopy) that
the inversion carbon blacks have a significantly rougher
surface than corresponding standard ASTM carbon blacks and
thereby enable an improved binding of the rubber polymer to
the carbon black particles (see W Gronski et al. "NMR
Relaxation - A Method Relevant for Technical Properties of
Carbon Black Filled Rubbers; International rubber conference
1997, Nuremberg, p. 107). The improved bonding of the rubber
polymer leads to the reduced rolling resistance.
Investigations on abrasion of rubber mixtures using inversion
carbon blacks have shown that these carbon blacks impart an
improved abrasion resistance to the rubber mixtures at low
loads. At high loads, such as occur in the case of lorry
tyres, these rubber mixtures exhibit an increased abrasion.
In one embodiment of the invention improved inversion carbon
olacks can be used that are characterised in particular by a
reduced abrasion at high loads.
Thus it is possible to use a furnace carbon black having CTAB
values of between 20 and 190 m2/g and 24M4-DBP absorption of
between 40 and 140 ml/100 g with a tand0/tand60 ratio which,
on incorporation into a SSBR/BR rubber mixture, satisfies the
relationship

wherein the value of tand60 is always lower than the value for
ASTM carbon black having the same CTAB surface and 24M4-DBP
absorption. This carbon black is accordingly characterised
by the fact that the distribution curve of the particle
diameters of the carbon black aggregates have an absolute
skewness of less than 400 000 nm3.
These carbon blacks that can be used according to the
invention satisfy the same requirements as regards the
tand0/tand60 ratio as the known inversion carbon blacks, and
accordingly when incorporated into rubber mixtures impart a
reduced rolling resistance to the tyres produced therefrom.
However, they are characterised by a narrower aggregate size
distribution compared to the known inversion carbon blacks.
The mathematical quantity "absolute skewness" known from
statistics is used to describe the aggregate size
distribution (see: Lothar Sachs: "Statistische
Auswertungsmethoden", Springer-Verlag Berlin, 3rd Edition,
pp. 81 to 83). This quantity provides a description of the
shape of the aggregate size distribution curve that can be
applied to the present problem in the form of a restriction
on the aggregate sizes by means of maximum and minimum
values.
The term "absolute skewness" is understood to be the
deviation from a symmetrical aggregate size distribution. A
skew distribution curve exists when one of the two descending
branches of the distribution curve is extended. If the left-
hand part of the curve is extended, one speaks of negative
skewness, i.e. the determination of the absolute skewness
provides values less than zero. If the right-hand section of
the curve is extended, a positive skewness exists with values
greater than zero. The known ASTM carbon blacks as well as
the inversion carbon blacks and the carbc. blacks according
to the invention all have a positive skewness of differing
magnitudes.
It was surprisingly found that the accepted opinion in the
prior art that a broader aggregate size distribution of the
reinforcing carbon black imparts a reduced rolling resistance
to the rubber mixtures does not have any general validity.
The improvement in the rolling resistance of rubber mixtures
that is observed with inversion carbon-blacks is obviously
not dependent on the width of the aggregate size
distribution, but is essentially due to the greater surface
roughness of the inversion carbon blacks and the associated
better bonding of the rubber polymer to the carbon black
surface.
With regard to the known inversion carbon blacks with their
relatively broad aggregate size distribution, their abrasion
resistance can now be improved according to the invention by
restricting the width of the aggregate distribution. In
particular, the proportion of carbon black aggregates with
large particle diameters must be reduced if the carbon blacks
are to impart to the rubber mixtures an improved abrasion
resistance combined at the same time with a reduced rolling
resistance. This is the case if the absolute skewness of the
aggregate size distribution is less than 400 000, preferably
less than 200 000 nm3. The absolute skewness of the
inversion carbon blacks known from DE 195 21 565 is above
400 000 nm3, whereas the absolute skewness of standard ASTM
carbon blacks is below 100 000 nm.
The absolute skewness of the aggregate size distribution of a
carbon black can be determined by means of a disc centrifuge
and corresponding evaluation of the measurement values. The
carbon black sample to be investigated is dispersed in an
aqueous solution and separated in a disc centrifuge according
to its particle size: the larger the particles, the greater
their mass and the more rapidly the carbon black particles
move outwardly in the aqueous solution as a result of the
centrifugal force. The particles traverse a light barrier by
means of which the extinction is recorded as a function of
time. The aggregate size distribution, in other words the
frequency as a function of the particle diameter, is
calculated from these data. The absolute skewness AS can be
determined from this distribution as follows:

In the above expression Hi denotes the frequency with which
the particle size diameter Xi occurs. x is the particle size
diameter of the particles whose mass corresponds to the mean
particle mass of the carbon black aggregates. x is also
calculated with the aid of the aggregate size distribution.
The summations in the above formula must be performed in the
range from 1 nm to 3 000 nm at equidistant intervals of in
each case one nanometer. Any missing measurement values are
calculated by linear interpolation.
The inversion carbon blacks according to the invention can be
produced by the generic process described in DE 195 21 565.
According to this process the inversion carbon black is
produced in a carbon black reactor that contains along the
reactor axis a combustion zone, a reaction zone and a
termination zone. In the combustion zone a stream of hot
waste gases is produced by combustion of a primary carbon
black raw material in oxygen-containing gases. This hot gas
stream is passed from the combustion zone through the
reaction zone to the termination zone. In the reaction zone
a secondary carbon black raw material is mixed in with the
hot waste gas. The formation of carbon black is stopped in
the termination zone by spraying in water. In the above
process oil, and oil/natural gas mixture or natural gas alone
is used as primary carbon black raw material. The combustion
of the primary carbon black raw material in the combustion
zone is carried out in such a way that carbon black seeds are
formed, with which the secondary carbon black raw mat rial is
brought into direct contact.
In order to obtain the carbon blacks according to the
invention this process must now be carried out in such a way
that the carbon black that is formed has an aggregate size
distribution with an absolute skewness of less than
400 000 nm3. This can be achieved for example by increasing
the addition of combustion air, or primary and secondary
carbon black raw material.
The described process is not restricted to a specific reactor
geometry. Indeed, it can be adapted to various types and
sizes of reactors. The person skilled in the art can effect
the desired seed formation in the combustion zone by various
measures. Possible influencing factors for optimising the
seed formation when using oil as fuel are the combustion
air/oil weight ratio, the type of fuel atomiser that is used,
and the size of the atomised oil droplets. Pure pressure
atomisers (single-substance atomisers) as well as two-
substance atomisers with internal or external mixing can be
used as fuel atomisers, in which connection compressed air,
steam, hydrogen, an inert gas or also a hydrocarbon gas can
be used as atomising medium. The aforedescribed combination
of a liquid and a gaseous fuel can thus be realised for
example by using the gaseous fuel as atomising medium for the
liquid fuel.
The invention is now illustrated in more detail with the aid
of the accompanying drawing, in which:
Fig. 1 is a longitudinal section through the reactor used to
produce the carbon blacks according to the invention.
Examples
A carbon black according to the invention is produced in the
carbon black reactor 1 illustrated in Fig. 1. This carbon
black reactor 1 has a combustion chamber 2 in which the hot
waste gas for the pyrolysis of the carbon black oil is
generated by burning oil under the addition of an excess of
atmospheric oxygen. The fuel is added to the combustion
chamber through the axial burner lance 3. The burner lance
can be displaced axially in order to optimise the seed-
induced formation of carbon black.
The combustion air is added through the opening 4 in the
front wall of the combustion chamber. The combustion chamber
capers conically to the constriction 5. After passing
through the constriction the reaction gas expands into the
reaction chamber 6.
Various positions for the injection of the carbon black oil
into the hot process gas by means of the oil lances 7 are
denoted by A, B and C. The oil lances are provided at their
head with suitable spray nozzles. At each injection position
four injectors are distributed over the circumference of the
reactor.
The combustion zone, reaction zone and termination zone,
which are important for the process according to the
invention, are denoted in Fig. 1 by the Roman numerals I to
III. They cannot be sharply differentiated from one another.
Their axial length depends on the relative positions of the
burner lance, cllances and quenching water lance 8.
The dimensions of the reactor that is used are given in the
following list:
Largest diameter of the combustion 530 mm
chamber:
Length of the combustion chamber to the 1525 mm
constriction:

1) Measured from the entry to the constriction (+: after
entry -: before entry)
All carbon blacks produced in the described reactor are
formed into beads according to known processes before their
characterisation and incorporation into the rubber mixtures.
Natural gas and a carbon black oil with a carbon content of
91.4 wt.% and a hydrogen content of 6.1 wt.% are used as fuel
for producing the carbon blacks according to the invention.
The reactor parameters for the production of the carbon
blacks according to the invention are listed in Table 1.
Carbon blacks R1, R2 and R3 as well as the comparison carbon
black A4496 are produced. For the production, silicone oil
is admixed as silicon-containing compound with the carbon
black oil.
For the carbon blacks R1 to R3 according to the invention the
relevant quantities are metered so that the finished carbon
black contains 5.6 wt.% of silicon.
The analytical data of the produced carbon blacks are
determined according to the following Norms and are listed in
Table 2:
STSA surface: ASTM D-5816
DBP absorption: ASTM D-2414
24M4-DBP Absorption: ASTM D-3493

Application example
The carbon blacks R1, R2 and R3 as well as the comparison
carbon blacks N220 and A4496 are used to produce rubber
mixtures. Among other properties, the viscoelastic
properties of the rubber mixtures are determined.
The viscoelastic properties of the rubber mixtures reinforced
with these carbon blacks are determined according to
DIN 53513. The loss factors tan8 at 0°C and at 60°C are in
particular determined. The test formulation used for the
rubber mixtures is itemised in Table 4.

The SSBR rubber component is a SBR copolymer polymerised in
solution, with a styrene content of 25 wt.% and a butadiene
content of 75 wt.%. The vinyl content of the butadiene is
67%. The copolymer contains 37.5 phr oil and is marketed
under the trade name Buna VSL 5025-1 by Bayer AG. Its Mooney
viscosity (ML l+4/100°C) is about 50.
The BR rubber component is a cis 1,4-polybutadiene (Neodym
type) with a cis 1,4- content of at least 96 wt.%, a trans
1,4-content of 2 wt.%, a 1,2-content of 1 wt.%, and a Mooney
viscosity of 44 ± 5. This component is marketed under the
trade name Buna CB 24 by Bayer AG.
Naftolen 7D from Chemetall is used as aromatic oil. The PPD
component of the test formulation is Vulkanox 4020 and the
CBS component is Vulkacit CZ, DPG is Vulkacit D and TMTD is
Vulkacit Thiuram, all from Bayer AG. Protector G35 from HB-
Fuller GmbH is used as wax.
The carbon blacks are incorporated into the rubber mixture in
three stages corresponding to the following tabular
description:

The subsequent determination of the rubber properties, i.e.
Shore hardness, tensile stress values M100 and M300, rebound
at 0° and 60°C as well as loss factor
tan8 at 0° and 60°C and the dynamic modulus of elasticity
?E*? at 0°C, are all measured according to the specified
Norms. The measurement conditions for the viscoelastic
properties are summarised in Table 4.
Table 4: Determination of the viscoelastic properties
according to DIN 53513

In each case the median value of the measurements on the five
test bodies is used.
The results of the rubber investigations are listed in
Table 5. Compared to the comparison carbon black, the carbon
blacks according to the invention impart to the rubber
mixtures a reduced loss factor at 60°C and an increased loss
factor at 0°C without a coupling agent. The loss factor at
60°C can be reduced further by adding Si69. Tyres that are
produced from such rubber mixtures may therefore be expected
to have an improved wet skidding behaviour with at the same
time a reduced rolling resistance.
The dry beaded carbon black R3 leads to a further drop in
tan8 60 °C compared to the wet beaded carbon black R1.
The advantageous behaviour of the carbon blacks according to
the invention is shown graphically in Fig. 2.
In Fig. 2 the tand0/tand60 ratio is plotted against the STSA
surface for these carbon blacks. The two carbon blacks
according to the invention have a significantly larger tan8
ratio for the same STSA surface, i.e. a steeper temperature
profile of the loss factors.
The region of the carbon blacks according to the invention
can be clearly demarcated from from that of the conventional
carbon blacks. It lies above the boundary straight line
shown in Fig. 2, which is given by the calculation
tand0/tand60 = 3.37 -0.0068 •STSA.
We Claim:
1. A process for producing a carbon black comprising the step of
oxidative pyrolysis of carbon-containing carbon black raw
materials, wherein silicon-containing compounds such as herein
described are mixed into the carbon-containing carbon black raw
materials.
2. A process for producing a carbon black as claimed in claim 1 by
oxidative pyrolysis of carbon-containing carbon black raw
materials, wherein silicon-containing compounds are sprayed into
the reaction chamber or reaction chambers of the carbon black
reactor.
3. A process as claimed in one of claims 1 and 2, wherein
organosilicon compounds such as organosilanes,
organochlorosilanes, silicic acid esters, siloxanes or silazanes are
used as silicon-containing compounds.
4. A process as claimed in claims 1 to 3, wherein carbon black oil,
oil, an oil/natural gas mixture or natural gas alone is used as fuel
and the combustion of the fuel is carried out in such a way that
seeds are formed and the carbon black raw material is brought into
direct contact with these carbon black seeds.
5. A process as claimed in claims 1 to 4, wherein the silicon
distribution in the carbon black is influenced by varying the mixing
of the silicon-containing compound in the carbon black oil.

A carbon black with a STSA surface of between 20 and 180 m2/g, a 24M4-
OBP absorption of between 40 and 140 ml/100 g and a specific BET surface
of between 20 and 250 m2/g and a content of 0.01 to 20 wt.% of silicon,
referred to its overall weight, wherein in rubber mixtures it has a tanδ0/tanδ60
ratio greater than 3.37-0.0068 • STSA.