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The effect of pressure on carbon-blackelastomer powders. Part I Density measurements on haf-blacksbr-powders at pressures up to 19 kbar

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Die Angewandte Makromolekulare Chemie 35 (1974) 131-146 ( N r . 496)
From the Dunlop-Forschungslaboratorium, 645 Hanau, Germany
The Effect of Pressure
on Carbon-Black/Elastomer Powders
Part I: Density Measurements on HAF-Black/SBR-Powders at Pressures up to 19 kbar
(Received 12 June 1973)
Carbon-black/elastomer powders were prepared by spray-drying mixtures of aqueous
dispersions of high abrasion furnace (HAF) black with styrene-butadiene rubber (SBR)
latex. The HAF/SBR ratio was varied in the range 100/0, 9515, 75/25, 67/33, 50150,
33/67 and 0/100 parts by weight. The powders were subjected to pressures up to 19
kbar in a piston-cylinder apparatus.
The density of the powders was measured at different pressures and it was found
that an interaction between carbon-black and polymer occurs which results in the
formation of coherent samples. They exhibit a pressure dependent degree of porosity
the magnitude of which was calculated from their density and compared to the porosity
determined experimentally from the methanol uptake of the samples.
At higher carbon-black loadings the specimen become increasingly porous with a
corresponding decrease in density. Density measurements on the three-phase system
(HAF/SBR/pores) permitted to determine the density of the HAF-black at 19 kbar
as 1.97 g/cm3.
Density and porosity at 0 kbar of the pressed specimen are a function of the pressure
at which they were prepared. The density increases with increasing pressure whereas
the porosity decreases due to stronger interaction between carbon-black and elastomer.
Aus Mischungen waI3riger Dispersionen von hochabriebfestem 01-RUB (HAF) und
Styrol-Butadien-Kautschuk-Latex(SBR) wurden durch Spruhtrocknung RuB/ElastomerPulver hergestellt. Das Verhaltnis von HAF/SBR wurde zu 100/0, 95/5, 75/25, 67/33,
50/50,33/67 und 0/100 Gewichtsteilen gewahlt. Die Pulver wurden in einer Kolben/Zylinder-Apparatur Driicken bis zu 19 kbar unterworfen.
Unter dern EinfluB des Druckes bildeten sich kompakte Priifkorper, deren Dichte
bei verschiedenen Driicken bestirnmt wurde. Unter bestimmten Bedingungen sind diese
Priifkorper poros; der Grad ihrer Porositat wurde sowohl aus ihrer Dichte berechnet
als auch aus der Methanolaufnahme experimentell errnittelt.
Bei hoheren RuRgehalten werden die Priifkorper zunehmend poroser, wobei ihre Dichte
entsprechend abnimmt. Aus Dichtemessungen an dem 3-Phasen-System HAF/SBR/Poren
wurde die Dichte des HAF-RuRes bei 19 kbar zu 1,97 g/cm3 bestimmt.
T. S. NG, and H. WOLF
Dichte und Porositat der Priifkorper bei 0 kbar sind eine Funktion des Herstellungsdrukkes. Mit steigendem Herstellungsdruck steigt infolge starkerer Wechselwirkung zwischen
RUB und Polymer die Dichte an, wahrend die Porositat abnimmt.
The nature of the interaction between carbon-black and elastomers has
been studied for more than 50 years and the question whether physical
or chemical bonds are responsible for the effect of reinforcement has been
discussed in numerous publications. A comprehensive literature survey on
these efforts was recently given by KRUSE'.
Most of the work carried out in the past dealt with vulcanized or unvulcanized carbon-black loaded rubbers prepared at low pressure; however only
a few workers studied the influence of high pressure on the system
carbon-black/elastomer. In 1949 WILKINSON
and GEHMANinvestigated the
influence of high pressures up to 10 kbar on the vulcanization behaviour
ofcarbon-black loaded NR, SBR and IIR compounds. O K H R I M E N Kstudied
the crosslinking of natural and synthetic rubbers with and without vulcanizing
agents and/or carbon-black at pressures between 3 kbar and 10 kbar and
temperatures between 120 and 200°C. These papers put the main emphasis
on the physical properties of the vulcanizates and not so much on the carbonblack/polymer or the carbon-black/carbon-black interaction.
The latter was studied by P A Y N E
~ prepared carbon-black/paraffin-oil
mixtures as model materials which contained up to 32 % by volume of carbonblack. The samples were obtained by mixing black together with a low viscosity
paraffin oil. Subsequently the mixtures were pressed in a metal mould to
form the specimen shape. The pressures were generated by a small laboratory
press and must have been fairly lows. The properties of the samples thus
obtained varied from very soft to extremely hard depending on the concentration and the type of carbon-black in the mixture. PAYNE4 found that in
his model system the carbon-black forms a three dimensional network which
is held together by VAN DER WAALSforces between the particles and which
behaves very much like carbon-black structures in vulcanizates. This could
be shown by dynamic measurements and creep experiments.
The morphology of carbon-black particles is of great importance with
respect to their interaction with polymers. Particles of HAF-black, e. g. consist
of elementary spheres of 100 to 300A diameter which during the production
process are fused together to so-called primary aggregates. The latter are
rigid bodies of irregular shape similar to branched chains which, in the
Density Measurements on HAF-BlackISBR-Powders
average, extend to about 2500A in their main axis and to about l W A
in the perpendicular direction. The morphology of primary aggregates was
extensively studied by HECKMAN
and M E D A L I Aand
al.’. These authors investigated the number of elementary spheres per primary
aggregate by means of electron microscopy and established the corresponding
distribution functions. Similar studies by means of light scattering techniques
were carried out by RAVEY
et al.*.
The work described in the present paper deals with the system HAFblack/SBR in the range of compositions of 100/0, 95/5, 75/25, 67/33, 50/50,
33/67 and 0/100 parts by weight HAF/SBR. The samples were prepared
by spray-drying mixtures of HAF-black dispersions with SBR-latex. In this
way fine powders were obtained which were put in a pressure cell and subjected
to static pressures up to 19 kbar. Thus coherent samples were obtained
on which density and porosity measurements were carried out.
Apparatus and Preparation of Samples
Calibration of the Pressure Cell
The pressure cell used was a conventional potassium bromide press mould (PerkinElmer) for the preparation of cylindrical samples of 13 mm diameter and 1 to 4 mm
height. A cross-section of this cell is shown in Fig. 1.
1 3 m m pI
Cy I i nde r
Fig. 1. Pressure cell.
T. S. NG, and H. WOLF
The press in which this cell was used was a 25 ton laboratory press (Ring press
Type 00-25 from Research & Industrial Instruments Company). Maximum pressure
on the sample was 19 kbar.
For the determination of the density of carbon-black/polymer systems under pressure,
which will be described later, it was necessary to take into consideration the geometrical
changes of the cell under the influence of pressure. Fig. 2 shows the load-compression
curve of the cylindrical cell. The compression of the piston and of the anvil was measured
with a cathetometer with an accuracy of better than 1/100 mm. The pressure was
measured with a standard pressure gauge.
Pressure (kbar)
Fig. 2. Load-compression curve of the pressure cell.
The increase in cross-section of the piston due to the influence of the pressure is
only 0,5 % at the highest pressure applied and was therefore neglected in the calculations
of the sample volume. However, the increase must be taken into account with respect
to the wall friction between the piston and the cylinder. Calculations based on WASSILEFF’s9 work about the forces between steel shafts and hollow cylinders lead to the
conclusion that the frictional forces which are generated between the inner wall of
our pressure cell and the outer surface of the piston when the latter is pressed into
the cylinder should be not higher than a few kbar. This value is only a first approximation,
however, and in order to confirm it the following experiment was carried out:
Brass washers of 7,00mm diameter, 0,50mm thickness and a central bore of 2,60mm
were subjected to forces up to 0,2MN. This treatment leads to a permanent reduction
of their thickness due to plastic deformation of the brass. Fig. 3 shows the result
Density Measurements on HAF-BlacklSBR-Powders
of the experiments. The dimensionless quantity (ho- hp)/ho is plotted versus the applied
force, where h, original thickness of the sample and h, thickness of the sample after
application of pressure p. One set of measurements (broken curve) was obtained with
the brass washers in the cavity of the cylinder and one set (solid curve) was obtained
with the washers between the piston and the anvil with the cylinder removed. Each
point on the curve is the mean value of 5 measurements.
In a separate experiment it was found that the quantity (h,-h,)/h, depends on the
original thickness h, and hence only such washers were used for the calibration which
had equal thicknesses within the range of *0,05 mm.
In Fig. 3 the curve for the measurements with the washers in the cavity of the
cylinder lies below the one for the measurements with the washers between the free
pistons, thus indicating less deformation in the first case. The distance between the
two curves parallel to the abscissa is indicative for the frictional forces in the cell.
They are in the order of one kbar at forces above 0,l MN in fair agreement with
the work of WASSILEFF9 cited above.
0,O 5
h -h
Force (MN)
Fig. 3. Deformation behaviour of brass-washers in the pressure-cell with and without
However, in our case the work of WASSILEFF9 can only be applied with a number
of assumptions, e.g. about the coefficient of dynamic friction, and because our own
measurements plotted in Fig. 3 lack a higher degree of precision, we did not correct
our pressure figures with respect to friction. This can be tolerated because such a
correction would be in the order of 10% at the highest pressures applied.
On the other hand all density measurements reported in this study were corrected
for the compression of the piston and the anvil as shown in Fig. 2 whereby the hysteresis
of the calibration curve was also taken into account. In spite of the fairly high pressures
up to 19 kbar no plastic deformation of the components of the pressure cell was observed
after more than 100 loading and unloading cycles.
Preparation of HAF-BlcicklSBR Powders
20 pts. of fluffy HAF-black (Corax 3 from Degussa) were dispersed in 78.6 pts. of
water by means of 1.4 pts. of the sodium-salt of the naphthalene-sulfonic acid as emulsifier
(Vultamol from Bayer AG). In order to obtain a good dispersion the above components
were treated in a ball mill for 24 hrs.
The SBR-latex used was a non-agglomerated latex which was taken directly from
the polymerization unit (special grade of INTEX F-28 from the International Synthetic
Rubber Company Ltd.). The latex had a solids content of 32 wt.-%. The number average
molecular weight of the polymer was 670000 and the MOONEYviscosity MS 4 (100'C)
was 100. The polymer was completely soluble in toluene.
The carbon-black dispersion and the latex were poured into a beaker and mixed
by means of a high speed stirrer (24.000 r.p.m.) for some minutes. By altering the
mixing ratio of the carbon-black dispersion and the SBR-latex different HAF/SBR ratios
were obtained.
The aqueous HAF/SBR dispersions were dried in a nozzle type spray dryer (1 k g h
Laboratory Spray Dryer from Nubilosa) under the following conditions:
Temperature at top of the tower 145-150 C; temperature at bottom of the tower
65-75 C ; feed rate 1 l/h; air pressure on the nozzle 2.5 bar.
The yield of the drying process depends on the HAFjSBR ratio as can be seen
from Table I.
Table 1.
Dependence of the powder yield from the HAF/SBR ratio of the dispersion.
HAF/SBR ratio
Powder yield (%)
The powder particles have diameters in the region of 5pm and the powders as such
are free flowing with the exception of compositions between 0: 100 and 50: SO HAF/SBR
which show a certain tackiness. The preparation of HAF/SBR powders in the full
range of composition from 0 to 100% SBR is not possible by means of conventional
mechanical mixing techniques as they are applied in the rubber industry because the
receptivity of the polymer towards carbon black is limited with these techniques.
Expwimental und Results
Coherent samples were prepared from different powders by compressing
the latter in the above described cylindrical cell. In order to determine the
density of the powders in the cell as function of the pressure a known quantity
of powders was introduced into the cell, the piston was inserted and the
displacement of the latter at different pressures was measured by means
of a cathetometer. From the position of the piston relative to the bottom
Density Measurements on HAF-BlacklSBR-Powders
of the cell the height of the sample was determined after correction for
the compression of the piston and the anvil. From the height of the sample
and the known cross-section of the cylinder the volume of the samples can
be calculated and with the mass of the powder being known the density
can easily be found.
All pressing experiments were carried out at room temperature. No means
were provided to measure the temperature in the samples during the compression cycles but the stepwise pressurization was carried out in such a way
that isothermal conditions can be assumed.
D e n s i t y (g-cni3)
Pressure (kbar)
Fig. 4.
Pressure dependence of the density of HAF-black
The first experiments were carried out with carbon-black which was used
in its unpelletized (fluffy) form. Fig. 4 shows the pressure dependence of
the density of HAF-black. Because our apparatus did not permit to determine
the density at pressures near zero, this value was taken as 0,83 g/cm3 from
et al. l o . With increasing pressure the density of HAF-black
increases and reaches 1,74 g/cm3 at 19 kbar. When the pressure is released,
the density is decreasing again, but it is obvious from Fig. 4 that the descending
part of the density-pressure curve does not coincide with the ascending part.
Under the influence of pressure a permanent increase in density has occurred
for the following reason: At the beginning of the compression cycle an orientation is taking place by which the primary aggregates of the black are moved
in such positions that a close packing is achieved. With further increase
of pressure primary aggregates are elastically deformed resulting in a still
closer packing and a higher density of the whole system. This process is
partly irreversible and hence the permanent increase in density is observed.
From our measurements we cannot conclude that primary aggregates are
partly broken at the pressures applied but this seems likely according to
the observations of HECKMAN
Unfortunately carbon-black does not form tablets without the addition
of binders and therefore the density of HAF-black at zero pressure could
not be determined after the unloading cycle. Similar observations were reported
by FUHRER'^ who studied the compression behaviour of organic powders
D e n s i t y (g.cG3)
Fig. 5. Pressure dependence of the density of HAF/SBR-powder of composition 75 :25
pts. by weight (two successive runs).
When the loading and unloading cycles are repeated several times the
loading curve approaches the unloading curve more and more until the
two finally coincide. This means that a permanent state of order has been
Density Measurements on HAF-BlackJSBR-Powders
achieved. An illustration of this effect is given in Fig. 5 where the compression
behaviour of 75:25 parts by weight HAF/SBR powder is shown. The 2nd
run upwards (increasing pressure) almost coincides with the 1st run downwards
(falling pressure) and the 2nd run downwards is already fully coinciding
with the 1st run downwards. With a few further runs the steady state will
be reached.
When comparing Fig. 5 to Fig. 4 one realizes that the area between the
ascending and the descending part of the density-pressure curves is
reduced with increasing proportion of SBR in the powders. From 50: 50
parts by weight of HAF/SBR onwards there is no difference at all between
ascending and descending curve as can be seen from Fig. 6. The reason
for this is that the proportion of carbon-black per unit volume is reduced
by the addition of polymer which as an homogeneous elastic material does
not show any permanent density changes after the pressure on the sample
has been released (cf. bottom curve in Fig. 6).
Pressure ( kbar)
Fig. 6. Pressuredependence of the density of HAF-black, HAF-SBR-powder of composition 50:50 pts. by weight, and SBR-powder.
Density-pressure curves were obtained for all HAF/SBR-powders listed
in Table 1. They all have the same character as the ones shown in Fig.
T. S. NG, and H. WOLF
5 and Fig. 6. It should be mentioned in this place that it is very likely
that the shape of the density-pressure curves will depend on the rate at
et al.13 who
which the pressure is applied. This was shown by YURCHENKO
investigated the pressure dependence of the density of asbestos material with
a rubber binder in the region of 1 kbar by means of a tensile testing machine
which permitted to apply the load at rates between 2 mm/min and 100
mm/min. This was not possible with our hand-operated press, but we applied
identical stepwise pressurization procedures with all experimental runs.
Density (g.cni3)
10 SBR
r a t i o HAF: SBR
Fig. 7. Density of HAF-SBR-powders at pressures of 19 kbar and 0 kbar as function
of their composition.
From the measurements shown in Figs. 4, 5 and 6 and the other ones
mentioned above it is possible to plot the density of the powders at fixed
pressures versus their compositions. This is shown in Fig. 7 for two typical
pressures, namely 19 kbar and 0 kbar. The values of the density at 19 kbar
were taken from the experimental density-pressure curves whereas the densities
Density Measurements on HAF-BlacklSBR-Powders
at 0 kbar were determined on samples which were taken out of the pressure
cell after they had been subjected to pressures of 19 kbar. From their weight
and their geometrical dimensions the density was calculated.
The density d of a system consisting of two components with the densities
d l and d 2 resp. depends linearly on the volume fractions c1 and c2 of the
d=cldi + ~ 2 d 2
c1 +c2 = 1, hence
d=ci(di -d2)+d2
In order to make use of this relation the weight ratios of our HAF:SBR
powders had to be converted into volume ratios by means of the density
values for the particular pressure ranges.
a) Pressure 0 k b a r : Density of HAF-black: d l = 1,86g/cm3, density of
SBR: d 2=0,94g/cm3. The value for HAF-black was taken from a publication by VOETet a1.14,whereas thedensity of SBR was determined experimentally on SBR in bulk form.
b) Pressure 19 k b a r : Density of SBR (19 kbar): d2= 1,22g/cm3.This value
was taken from the density-pressure curve for SBR in Fig. 6.
The density of HAF-black, however, could not be taken from Fig. 4 because
at 19 kbar we still have porosity present in the compressed carbon-black.
For this reason the density of HAF-black at 19 kbar was calculated from
the experimentally determined densities d at 19 kbar of HAF-SBR-powders
which are porosity-free in the composition range of 0: 100 to about 60:40
parts by weight. In equation (1) the volume fractions c1 and c2 can be
substituted by the weight fractions w1 and w2.
and by rearrangement
T. S. NG, and H. WOLF
From this equation results: Density of HAF-black (19 kbar) is dl=1,97
g/cm3. This figure agrees well with the values published by V O E T ' ~who
determined the density of HAF-black in helium.
With these values it is possible to plot the density of the powders versus
their volume composition. This is shown in Fig. 7. It was found that there
exists a linear relationship between density and volume ratio according to
equation (2) at least in certain ranges. For the 19 kbar curve this is true
up to about 70: 30 parts by volume of HAF : SBR. At higher carbon-black
concentrations the density curve deviates from linearity due to the appearance
of porosity in the samples. There is not enough polymer present to fill out
completely the free space between the carbon-black particles. At a pressure
of 0 kbar the deviation from linearity begins already at an WAF-SBR ratio
of 20 :80. Due to the absence of an external pressure the carbon-black particles
are not forced together as close as possible and hence porosity develops
at low HAF concentrations already.
In Fig. 7 the right linear part of the 0 kbar density curve was connected
by a broken line with the corresponding density value of HAF-black (1,86
g/cm3 according to VOET et al.I4)whereas in the case of the 19 kbar measurements the well pronounced linear part of the curve was extrapolated graphically
to 100/0 HAF/SBR. From the differences between the broken lines and the
experimental curves it is easily possible to calculate the porosity of the samples
which is given by the equation
Relative porosity
= -= 1 - -
where VP volume of pores
Vs volume of solid matter (HAF + SBR)
d experimentally determined density
d, extrapolated density (broken lines in Fig 7).
This calculation is based on the assumption that the system carbon-black/polymer/pores is a true 3-phase system. The possibility that a minor part of
the polymer in the carbon-black/polymer interphase has a higher degree
of order as suggested by WESTLINNING'' and hence a higher density is
neglected in the following considerations. The result of these calculations
is shown in Fig. 8. From the 19 kbar curve it follows that the porosity
of HAF-black at this pressure is about 15 v01.-YO. Materials which contain
less than 70 parts by volume of HAF-black do not have any porosity at
Density Measurements on HAF-BlacklSBR-Powders
P o r o s i t y (Vol. %)
Volume r a t i o H A F : S B R
Fig. 8. Porosity of HAF-SBR-powders at pressures of 19 kbar and 0 kbar as function
of their composition.
A 0 kbar HAF-black has a porosity of about 55 v01.-YO. The porosity
curve is almost linearly decreasing and reaches zero at about 20 v01.-%
of HAF-black. In the case of the 0 kbar measurement it is possible to determine
the porosity experimentally. For this purpose samples of known mass and
geometrical dimensions were suspended in methanol, a poor swelling agent
for SBR. This solvent quickly penetrates into the pores of the samples, thereby
increasing their weight. From this increase in weight and the density of
methanol (0,79 g/cm3) the volume of the pores can be calculated. It must
be mentioned, however, that the samples show a certain degree of volume
increase when being suspended in methanol (e.g. 7 v01.-% with
HAF : SBR = 33 :77 and 12 v01.-YO with HAF : SBR = 91 : 9). This effect was
taken into account and corresponding corrections were applied. The results
of these measurements are incorporated in Fig. 8 and it can be seen that
measured and calculated porosity values are in good agreement. For obvious
reasons the porosity cannot be determined experimentally at pressures above
Fig. 9. Volume concentrations of WAF-black, SBR, and porosity in HAF-SBR-powders
at pressures of 19 kbar and 0 kbar.
It must be mentioned that the volume ratio of HAF-black/SBR on the
abscissa of Fig. 8 is only related to the composition of the solid phase
of the samples investigated.With the appearance of porosity a three-component
system is formed which consists of HAF-black, SBR and unoccupied space.
The true concentrations of all three components at 19 kbar and at 0 kbar
are shown in Fig. 9 which was derived from Fig. 8 by simple calculation.
The graph shows that the maximally achievable volume concentration of
HAF-black alone is about 45% at 0 kbar and 87% at 19 kbar.
The density and porosity measurements which were discussed so far were
all obtained with samples which had been subjected to pressures of 19 kbar.
It seemed interesting to study the effect of the maximum pressure applied
to the samples during the densification process in the pressure cell on their
remaining density at zero pressure outside the cell. For this purpose a series
of 6 samples of HAF:SBR powder of composition 60:40 parts by volume
was subjected to 6 different maximum pressures between 1 and 19 kbar.
The densified powders were taken out of the pressure cell and their density
and porosity was determined as described above. The results of this study
Density Measurements on HAF-BlacklSBR-Powders
are compiled in Fig. 10. It is obvious that the density increases with increasing
densification pressure whereas the porosity decreases at the same time. The
reason for this is a better penetration of the polymer into the voids between
the carbon-black particles when the pressure is increasing.
Fig. 10. Density and porosity of 6 samples of HAF-SBR-powder of composition 60:40
pts. by volume prepared at 6 different pressures.
The porosity was also determined experimentally by the methanol uptake
of the samples. The results are plotted in the upper part of Fig. 10. They
do not coincide completely with the calculated curve but one must keep
in mind that the ordinate is spreaded in the drawing.
of THE INTERNAThe authors would like to thank Dr. B. J. RIDGEWELL
COMPANY, LTD.,for the supply of special grade
T. S. NG, and H. WOLF
INTEX-latex, and Dip1.-Ing. S. WOLFF of DEGUSSA
for the supply of fluffy
’ J. KRUSE,Gummi, Asbest, Kunstst. 25 (1972) 548 and 646
and S. D. GEHMAN,
Ind. Eng. Chem. 41 (1949) 841
Rubber Chem. Technol. 33 (1960) 1019
A. R. PAYNE,Trans. Instn. Rubber Ind. 40 (1964) 135; Rubber Chem. Technol.
38 (1965) 387
A. R. PAYNE,Private Communication, Oct. 1972
and A. I. MEDALIA,
J. Inst. Rubber Ind. 3 (1969) 66
and A. I. MEDALIA,
Rev. gen. Caoutchouc
Plastiques 47 (1970) 1307
and P. HORN,Eur. Polym. J. 6 (1970) 1527
VDI-Forschungsheft 390, Berlin 1938
Rubber Chem.
Technol. 40 (1967) 1311
Rubber Chem. Technol. 43 (1970) 943
Chem. Ing. Tech. 43 (1971) 849
and V. V. UTKIN,Sov. Rubber Technol. 30
(1971) 21
A. VOET, F. R. COOK, and R. HOGUE,Rubber Chem. Technol. 43 (1970) 969
Deutsche Kautschuk-Gesellschaft, Vortragstagung Freiburg 1962,
p. 61
A. VOET,Paper at the Conference “Physico-Chimie du Noir de Carbone”, Mulhouse,
27-28 Sept. 1963
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measurements, powder, effect, blackelastomer, part, blacksbr, kbar, haf, pressure, carbon, density
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